Neoproterozoic arc magmatism in the southern ...

Gondwana Research 45 (2017) 1–42

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Neoproterozoic arc magmatism in the southern Madurai Block, India: Subduction, relamination, continental outbuilding, and the growth of Gondwana M. Santosh a,b,c,⁎, Chao-Nan Hu a, Xiao-Fang He a,b, Shan-Shan Li a, T. Tsunogae d,e, E. Shaji f, G. Indu f a

School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China Centre for Tectonics, Exploration and Research, University of Adelaide, Adelaide, SA 5005, Australia c Department of Geology, Northwest University, Northern Taibai Str. 229, Xi'an 710069, China d Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japan e Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa f Department of Geology, University of Kerala, Kariyavattom Campus, Trivandrum 695 581, India b

a r t i c l e

i n f o

Article history: Received 5 October 2016 Received in revised form 12 December 2016 Accepted 27 December 2016 Available online 27 January 2017 Handling Editor: S. Kwon Keywords: Continental outbuilding Petrology and geochemistry Zircon geochronology and Lu–Hf isotopes Southern Madurai Block, Gondwana

a b s t r a c t The Madurai Block in southern India is a composite collage of at least three sub-blocks, with Neoarchean– Paleoproterozoic segments in the north and central domains, and a Neoproterozoic segment in the south. Here we investigate a suite of rocks with magmatic protoliths that constitute the basement in the southern margin of the Madurai Block including alkali granites, charnockites, enderbites and gabbros. The alkali granites are dominantly composed of perthitic K-feldspar, minor plagioclase and quartz, with hornblende as the main mafic mineral suggesting a calc-alkaline nature. The enderbites and charnockites have a broadly similar mineralogical constitution except for the variation in the modal content of plagioclase, K-feldspar and quartz, as well as the additional presence of clinopyroxene in some of the enderbites. The high modal content of hornblende in the gabbros suggests crystallization from hydrous basaltic melts. The geochemical features of this suite are identical to those of arc magmatic rocks, with distinct Nb, Ta, and Ti depletion suggesting magmatism in a subduction-related environment. We envisage that the underplating of basaltic magmas within a convergent margin setting provided the heat input for lower crustal melting generating the charnockitic suite of rocks. The intrusion of the underplated mafic melts as gabbroic dykes and sills into the crystallizing felsic magmas resulted in magma mixing and mingling generating the widespread enclaves of gabbroic rocks. The alkali granites were derived from the differentiation of lower crustal melts. Zircon U–Pb data from the alkali granites yield weighted mean 206 Pb/238U ages of 786 ± 10 to 772 ± 11 Ma for the oldest and the most dominant group of magmatic grains, with a 662 ± 20 Ma subordinate group. The oldest group of magmatic zircons in the charnockite samples shows ages of 938 ± 27 Ma, 896 ± 12 Ma, and 786 ± 9 Ma, suggesting multiple magmatic pulses during early and mid-Neoproterozoic. A subordinate population of magmatic zircons with ages of 661 ± 9 Ma and 632 ± 7 Ma is also present. In the enderbites, the magmatic zircon population yields weighted mean ages of 926 ± 22 Ma, 923 ± 36 Ma, 889 ± 13 Ma, 803 ± 10 Ma, 787 ± 23 Ma, 786 ± 10 Ma, 748 ± 27 Ma, 742 ± 11 Ma, 717 ± 8 Ma and 692 ± 10 Ma suggesting continuous and multiple pulses of magmas emplaced throughout early to mid-Neoproterozoic. Magmatic zircons from the gabbros show weighted mean 206Pb/238U ages of 903 ± 13 Ma, 777 ± 10 Ma, 729 ± 10 Ma and 639 ± 27 Ma. Metamorphic zircons from all the rock types show latest Neoproterozoic-Cambrian ages in the range of 567 ± 19 Ma to 510 ± 8 Ma suggesting prolonged heating. Zircon Lu–Hf data show that the alkali granite-charnockite-enderbite suite has depleted mantle ages (TDM) in the range of 1164–2172 Ma and crustal residence ages (TCDM) of 1227–3023 Ma. These spots show both negative εHf(t) and positive εHf(t) values (− 22.1 to 10.6), suggesting magma derivation from mixed juvenile mid- to late-Mesoproterozoic components and reworked Mesoarchean to mid-Mesoproterozoic components. Zircon grains from the gabbroic rocks show depleted mantle ages and (TDM) in the range of 1112–2046 Ma, crustal residence ages (TCDM) of 1306–2816 Ma, and both negative and positive εHf(t) values (−17.8 to 7.9), suggesting that the magmas were dominantly derived from juvenile mid-Mesoproterozoic to Neoproterozoic components as well as reworked Mesoarchean to mid-Mesoproterozoic sources. Our data clearly reveal multiple arc magmatism along the southern Madurai Block during distinct pulses throughout early to late Neoproterozoic, suggesting an active convergent margin along this zone at this time.

⁎ Corresponding author at: School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China. E-mail addresses: [email protected], [email protected] (M. Santosh).

http://dx.doi.org/10.1016/j.gr.2016.12.009 1342-937X/© 2017 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

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M. Santosh et al. / Gondwana Research 45 (2017) 1–42

Crustal thickening occurred through relamination by mafic magmas associated with slab melting. Continental outbuilding and southward growth of the Madurai Block were associated with the lateral accretion of the vast sedimentary belt of Trivandrum Block, culminating in collisional metamorphism during latest Neoproterozoic– Cambrian associated with Gondwana assembly. © 2017 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction The Southern Granulite Terrain (SGT) of India, south of the Dharwar Craton, is a collage of crustal blocks including the Coorg, Nilgiri, Salem, Madras, Madurai, Trivandrum and Nagercoil Blocks, the basement rocks of which range in age from Mesoarchean to Neoproterozoic (Santosh et al., 2009, 2015, 2016; Clark et al., 2009; Collins et al., 2014; Plavsa et al., 2014; Praveen et al., 2014; Shaji et al., 2014; Samuel et al., 2014; Yang et al., 2015; Amaldev et al., 2016) (Fig. 1). These blocks are dissected by shear/suture zones, also ranging in age from Mesoarchean to latest Neoproterozoic–Cambrian. The crustal blocks to the north of the Palghat-Cauvery Suture Zone (PCSZ) amalgamated during Mesoarchean to Neoarchean and were accreted onto the Dharwar Craton during Neoarchean (Samuel et al., 2014; Amaldev et al., 2016). Those to the south were mostly accreted during Neoproterozoic–Cambrian associated with the final amalgamation of the Gondwana supercontinent (Santosh et al., 2009; Collins et al., 2014; Li et al., 2016). The Madurai Block (MB), bounded between PCSZ in the north and Achankovil Suture Zone (AKSZ) in the south, is mainly composed of Neoarchean to Meso-Neoproterozoic charnockite-granite-migmatite gneisses intercalated with metasedimentary rocks (Plavsa et al., 2012, 2014). The Madurai Block is considered to have amalgamated to the south of the Salem Block along the PCSZ during late-NeoproterozoicCambrian (Santosh et al., 2003, 2006, 2009; Collins et al., 2007a, 2007b; Clark et al., 2009). The Madurai Block (MB) is the largest among the various crustal blocks in the SGT and has been the focus of various investigations relating to crustal evolution including the various magmatic records and ultrahigh-temperature metamorphism (Harris et al., 1994; Brown and Raith, 1996; Sajeev et al., 2004; Tateishi et al., 2004; Santosh et al., 2009; Tsunogae and Santosh, 2011; Plavsa et al., 2012, 2014, among others). Although earlier studies considered the MB as a single crustal block, recent investigations have shown that the block is a collage of at least three sub-blocks, the Northern Madurai Block (NMB), the Central Madurai Block (CMB) and the Southern Madurai Block (SMB) as inferred from distinct age provinces (Plavsa et al., 2012, 2014; Li et al., 2016). However, the formation and evolution of these sub-blocks and their assembly into the unified MB remain unclear. Also, the relationship of the MB and its sub-blocks with those in other fragments of the Gondwana assembly are critical to understand the Proterozoic crustal evolution and the processes leading to the final assembly of the late Neoproterozoic supercontinent Gondwana. In this study, we investigate a suite of magmatic rocks including alkali granites, charnockites, enderbites and gabbros along the southern margin of the Madurai Block through field investigations, petrology, geochemistry, zircon U–Pb and Lu–Hf isotopes. Our data reveal prominent and multiple magmatic events during Neoproterozoic in this region and continental outbuilding through both juvenile and recycled components in the southern Madurai Block. We evaluate the results in reconstructing the petrogenetic history and tectonic significance. 2. Geological framework The Madurai Block is among the largest crustal blocks in the Southern Granulite Terrane of India, and is also one of the largest discrete

crustal domains bound by shear/suture zones on both sides within the Gondwana supercontinent assembly. To the north, this block is bound by the E-W trending Palghat-Cauvery Suture Zone (considered to be trace of the Mozambique Ocean suture; Collins et al., 2007a), and to the south by the NW-SE trending Achankovil Suture Zone. Although the MB was considered as a single crustal block in previous studies, complex Archean to Neoproterozoic magmatic suites and metasedimentary belts with polyphase deformation history and evidence for multiple thermal events have been recognized within the block in recent studies, leading to various models. These include the proposition that the MB is a collage of magmatic arcs developed through protracted subduction events and accreted together with intervening oceanic and supracrustal lithologies, with subsequent regional metamorphism during late Neoproterozoic–Cambrian associated with the final stages of assembly of the Gondwana supercontinent (Meert and Van Der Voo, 1997; Yoshida and Upreti, 2006). A distinct isotopic boundary has also been demarcated within the block represented by the Karur–Kambam–Painavu–Trichur (KKPT) shear zone (Bhaskar Rao et al., 2003; Ghosh et al., 2004). Recent studies by Plavsa et al. (2012, 2014) led to a sub-division of the block into Neoarchean to Paleoproterozoic Northern Madurai Block (NMB), Paleo-Mesoproterozoic Central Madurai Block (CMB) and largely Mesoproterozoic to Neoproterozoic Southern Madurai Block (SMB) (Fig. 1). The NMB is dominated by Neoarchean charnockitic massifs and associated TTG gneisses (Harris et al., 1994; Plavsa et al., 2012; Rajesh, 2012; Brandt et al., 2014) with the southeastern domain accreted by metasediments including quartzites, calc-silicates and high- to ultrahigh-temperature metapelites (Sajeev et al., 2006; Santosh et al., 2006, 2009). Brandt et al. (2014) reported late-Neoarchean (2.53–2.46 Ga) subduction-related, magnesian charno-enderbites, which were reworked in the early-Paleoproterozoic (2.47–2.43 Ga) from the western part of the Madurai Block whereas the supracrustal sequence in the east was deposited in a late-Paleoproterozoic (1.74–1.62 Ga) basement. Following this, both domains were intruded by voluminous mid-Neoproterozoic A-type charnockites and felsic intrusions. Other studies have also recorded a dominant Archean to Paleoproterozoic signature in detrital zircons in the metasediments from this region (Collins et al., 2007b; Kooijman et al., 2011) as well as the intrusion of felsic and mafic magmatic suites during early Neoproterozoic (ca. 800 Ma; e.g., Teale et al., 2011). In contrast, the SMB is dominantly composed of latest Mesoproterozoic to Neoproterozoic magmatic complexes (e.g., Santosh et al., 2005, 2009; Sato et al., 2010; Ghosh et al., 2004) and intercalated with metasedimentary belts. Previous studies have recorded distinctly younger Nd model ages of ca. 1.6–1.3 Ga from the SMB rocks (e.g., Harris et al., 1994; Bartlett et al., 1998; Tomson et al., 2013), as compared to older ages (ca. 2.0 to 3.1 Ga) in the CMB and NMB (Bhaskar Rao et al., 2003; Ghosh et al., 2004), revealing major Neoproterozoic juvenile crustal addition in the southern domain of this crustal block. The dominant magmatic lithologies in the MB are hornblendebiotite (TTG) gneisses, charnockites and their intermediate enderbitic rocks, diorites, minor gabbros (including mafic granulites), pink and gray granitoids, pinkish and grayish granites and pegmatites. The metasedimentary units are represented by garnet-biotite gneiss, garnet-biotite-sillimanite gneiss, quartzites, and metacarbonates


(e.g., Santosh et al., 2009). At several places within the metapelites, high Mg–Al domains with diagnostic mineral assemblages representing ultra-high temperature (UHT) metamorphism have been recorded (e.g. Brown and Raith, 1996; Satish-Kumar, 2000; Tateishi et al., 2004; Sajeev et al., 2004; Tsunogae and Santosh, 2011; Prakash et al., 2007; Tsunogae et al., 2008). The peak UHT metamorphism has been confirmed by the occurrence of some key mineral assemblages such as orthopyroxene + sillimanite + quartz, spinel + quartz, Al-rich orthopyroxene, and mesoperthite as well as sapphirine + quartz in pelitic granulites and Mg–Al-rich rocks (e.g. Mohan and Windley, 1993; Tamashiro et al., 2004: Tateishi et al., 2004; Sajeev et al., 2004; Tsunogae and Santosh, 2006, 2010; Tadokoro et al., 2007; Kondou et al., 2009, among others). Most of the reports on UHT metamorphism are from the central Madurai Block, such as the well-known sapphirine-bearing UHT granulites of Ganguvarpatti, although one classic locality at Rajapalaiyam occurs in the southern Madurai Block from where Tateishi et al. (2004) first reported fine-grained intergrowth of sapphirine + quartz within porphyroblastic garnet in pelitic and quartzo-feldspathic granulites with peak metamorphism at N1000 °C. Sapphirine-absent, cordierite-, orthopyroxene- and spinel-bearing high Mg–Al granulite metasediments have also been reported from various localities from the northern part of the Achankovil Suture Zone, bounding the southern margin of the MB (e.g., Ishii et al., 2006). In a recent study, Li et al. (2016) analyzed detrital zircon grains from a suite of quartzites accreted along the southern part of the Madurai Block. Their data reveal multiple populations of magmatic zircons, among which the oldest group ranges in age from Mesoarchean to Paleoproterozoic (ca. 2980–1670 Ma, with peaks at 2900–2800 Ma, 2700–2600 Ma, 2500–2300 Ma, 2100–2000 Ma). Importantly, zircon grains in two samples show magmatic zircons with exclusively Neoproterozoic (950–550 Ma) ages. The metamorphic zircons from these rocks show a range of 580–500 Ma, correlating with the timing of metamorphism. The Mesoarchean to Neoproterozoic age range suggests distal source regions implying an open ocean environment whereas rocks with an exclusive Neoproterozoic detrital zircon population in the absence of older zircons indicate proximal sources in the southern Madurai Block.

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3.2. Charnockites Charnockites are the dominant rocks in the southern Madurai Block. Although the geological map prepared by the Geological Survey of India, a modified version of which is shown in Fig. 3, shows hornblende-biotite gneisses as the major basement rocks, our detailed field studies revealed that most of these exposures are weathered charnockites carrying orthopyroxene- and K-feldspar bearing charnockitic rocks and their more intermediate enderbitic variants. This was further confirmed from many locations where fresh exposures of these rocks can be studied in sub-surface quarry sections where greenish medium to coarse grained charnockitic rocks with orthopyroxene predominate. Since the extent of these rocks and their inter-relationship with the other lithologies are obscure due to intense surface weathering and lateritisation, we have shown their occurrence in Fig. 3 only in the localities investigated. In this study, we sampled charnockites from three localities: Kuttalam, Alamkulam and Veerakerala Puthur. The high hills surrounding Kuttalam are composed of massive medium to coarse grained charnockites. The sample collected in this study is from a freshly quarried exposure and shows a slightly foliated, greasy green texture with clots of orthopyroxene. The rock is felsic and rich in greenish feldspar and quartz; garnet is absent. At Alamkulam, medium grained charnockitic rocks are exposed along the ground and are associated with folded veins of ferruginous meta-quartzite alternating with altered fine grained pelitic layers and blocks of calc-silicate rock. From field relations, the metasediments (quartzites, pelites and calc-silicates) appear as bands accreted onto the charnockitic basement. The quartzite is dominantly composed of coarse recrystallized quartz, ferruginous and brownish. The calc-silicate blocks are greenish with clinopyroxene, plagioclase, quartz, calcite and biotite. The charnockites are composed of greenish K-feldspar, plagioclase and quartz with minor orthopyroxene grains. At the third locality, coarse greasy greenish massive-textured felsic charnockites are exposed beside the Tirunelveli-Tenkasi road. Clots of orthopyroxene grains and biotite flakes occurring along foliation planes are also observed. Several well cuttings nearby also expose similar massive charnockitic rocks confirming that the basement is dominantly composed of these rocks.

3. Sampling and field observations

3.3. Enderbites

The sample localities, rock types, GPS readings and representative mineralogy of the different rock types analyzed in this study are given in Table 1. A brief description of the field settings of these rocks is given below and representative field photographs are shown in Fig. 4.

In many places within the southern Madurai Block, the charnockites are slightly more intermediate in composition and are therefore designed as enderbites, as also confirmed from their geochemical data discussed in a later section in this paper. We sampled enderbitic charnockites from six locations in this study (Table 1). The first locality is a large chain of working quarries near the Chenkottai railway station exposing mostly garnet-, sillimanite- and cordierite-bearing granulite facies metapelites. The silica-rich domains of these metasediments show development of coarse patches, veins and clots of incipient charnockites with orthopyroxene and cordierite. In addition, the metapelites incorporate bands and blocks of medium grained massive greenish enderbitic rocks. The field relations suggest that the metasedimentary sequence was accreted along the southernmost part of the Madurai Block dominated by charnockitic basement rocks and fragments of these were incorporated within the sedimentary mélange sequence, co-folded and metamorphosed. Subsequent to peak metamorphism and development of the dominant foliation, the incipient charnockites were overprinted as coarse patches through local dehydration reactions. The next location is in Sivagiri, a quarried rock hillock near a small temple adjacent to a paddy field where the dominant rock type is grayish medium to coarse-grained enderbitic charnockite veined by orthopyroxene-rich partial melt zones. Several melanocratic, medium to fine grained enclaves of various sizes, ranging from few tens of cm to a meter occur within this rock. These gabbroic/dioritic enclaves

3.1. Alkali granites Alkali granites occur as minor plutons and sheets mostly intruding into charnockitic rocks in the study area. We sampled alkali granites from two localities, one at Sitaparappanallur and the other at Sitaparappanallur. The first locality is a large working quarry where the major rock is greasy, green, medium to coarse grained charnockite showing feeble foliation. The charnockite carries mafic magmatic enclaves of melanocratic gabbroic rocks and both charnockites and gabbros are invaded by sheets of pink granite and coarse grained pink K-feldspar bearing pegmatite. Surrounding these intrusions, the charnockite is retrogressed generating a ‘bleached’ zone. The alkali granite is medium grained and K-feldspar rich, slightly foliated and contains greenish clots of hornblende and brown biotite. The second locality is a hillock exposure across a paddy field along the side of the Tirunelveli road where medium grained and slightly foliated pink granites are exposed. Hornblende and biotite occur along thin compositional bands or as scattered grains in the matrix.

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typically resemble MMEs (mafic magmatic enclaves) produced through mixing and mingling during the intrusion of mafic magmas into felsic magma chambers as reported elsewhere (Kumar and Rino, 2006; He et al., 2016a). At Karumpuliyuthu, ground level exposures of intercalated medium grained brownish ferruginous quartzites occur intercalated with thin layers of pelitic rocks. The basement rocks exposed about 200 m north are mostly medium to coarse grained, slightly foliated greasy greenish enderbitic rocks with visible clots of orthopyroxene and laths of biotite oriented parallel to the foliation. Garnet is absent in these rocks. The

enderbitic host rocks carry medium to fine grained enclaves of dioritic rocks and thin stretched layers and lenses of hornblendite. Around 500 m west of Sitaprappanallur, several well cuttings expose the basement rocks which are mostly medium grained, massive greenish enderbites with clots and layers of orthopyroxene. At places the rock is slightly foliated. The next location is at Thalayuthu, around 10 km from Tirunelveli along the Kovilpatti road diversion, near marble mines, where several active quarries in the sub-surface level expose enderbitic rocks. In the weathered surface, they appear as hornblende-biotite bearing gneisses,

Fig. 1. Generalised geological and tectonic framework of the Southern Granulite Terrane of India showing the major crustal blocks and intervening suture zones (after Collins et al., 2014; Santosh et al., 2015, 2016). The tectonic sub-division of the Madurai Block is modified after Plavsa et al. (2012, 2014). The area covered by a dotted line box is shown in Fig. 2.

Table 1 Sample numbers, rock types, localities, GPS readings and mineral assemblages of the rocks from the Southern Madurai Block analyzed in this study. Sample No.

Rock type

Sample location

Co-ordinates

Mineralogy

TRM 2-1 TRM 2-3 TRM 3-1 TRM 3-2 TRM 3-3 TRM 3-4 TRM 5-1 TRM 5-2 TRM 6-2 TRM 6-3 TRM 7-2 TRM 7-4 TRM 8-2 TRM 9-1 TRM 9-2 TRM 10-1 TRM 10-2 TRM 11-2 TRM 13-1 TRM 13-2 TRM 13-3 TRM 14-1 TRM 14-2 TRM 14-3 TRM 15-1

Enderbite Enderbite Enderbite Enderbite Gabbro Gabbro Charnockite Charnockite Alkali-granite Gabbro Gabbro Charnockite Enderbite Alkali-granite Alkali-granite Enderbite End erbite Gabbro Enderbite Enderbite Gabbro Enderbite Gabbro Enderbite Charnockite

Chenkottai railway station Chenkottai railway station Sivagiri Sivagiri Sivagiri Sivagiri Kuttalam Kuttalam Valiban Pothai Valiban Pothai Alangulam Alangulam Karumpuliyuthu Sitaparappanallur Sitaparappanallur 500 m West of Sitaparappanallur 500 m West of Sitaparappanallur Rettiyarpatti Thalayuth Thalayuth Thalayuth Kaanarpetti Kaanarpetti Kaanarpetti Veerakerala Puthur

08°59′27.56″, 77°14′15.10″ 08°59′27.56″, 77°14′15.10″ 09°21′04.06″, 77°26′25.55″ 09°21′04.06″, 77°26′25.55″ 09°21′04.06″, 77°26′25.55″ 09°21′04.06″, 77°26′25.55″ 08°55′52.65″, 77°14′19.41″ 08°55′52.65″, 77°14′19.41″ 08°57′46.34″, 77°18′03.55″ 08°57′46.34″, 77°18′03.55″ 08°52′19.70″, 77°27′58.46″ 08°52′19.70″, 77°27′58.46″ 08°50′07.71″, 77°31′53.74″ 08°47′15.86″, 77°36′57.54″ 08°47″15.86″, 77°36′57.54″ 08°46′53.81″, 77°36′57.67″ 08°46′53.81″, 77°36′57.67″ 08°40′36.20″, 77°45′25.30″ 08°48′09.52″, 77°43′20.57″ 08°48′09.52″, 77°43′20.57″ 08°48′09.52″, 77°43′20.57″ 08°53′18.48″, 77°38′28.15″ 08°53′18.48″, 77°38′28.15″ 08°53′18.48″, 77°38′28.15″ 08°58′48.96″, 77°26′15.99″

Pl + Kfs + Qtz + Opx + Bt + Ap + Zr Pl + Kfs + Qtz + Opx + Bt + Ap + Zr Pl + Kfs + Qtz + Opx + Bt + Ap + Zr Pl + Kfs + Qtz + Opx + Bt + Ap + Zr Pl + Hbl + Opx + Bt + Il + Zr Pl + Opx + Cpx + Hbl + Zr Kfs + Qtz + Pl + Opx + Bt + Ap + Zr Kfs + Qtz + Pl + Opx + Hbl + Ap + Zr Kfs + Qtz + Pl + Hbl + Zr Pl + Hbl + Opx + Bt + Mt. + Zr Pl + Cpx + Opx + Mt. + Zr Kfs + Qtz + Pl + Opx + Hbl + Bt + Ap + Zr Pl + Kfs + Qtz + Hbl + Il + Ap + Bt Kfs + Qtz + Pl + Hbl + Ap + Zr Kfs + Qtz + Pl + Hbl + Bt + Zr Pl + Kfs + Qtz + Hbl + Bt + Ap + Il + Zr Pl + Kfs + Qtz + Hbl + Bt + Il + Ap + Zr Pl + Opx + Hbl + Mt. + Zr Pl + Kfs + Qtz + Opx + Cpx + Hbl + Il + Ap + Zr Pl + Kfs + Qtz + Opx + Cpx + Hbl + Il + Ap + Zr Pl + Kfs + Qtz + Opx + Hbl + Ap + Zr Pl + Kfs + Qtz + Opx + Hbl + Mt. + Ap + Zr Pl + Cpx + Opx + Hbl + Mt. + Zr Pl + Opx + Hbl + Bt + Il + Ap + Zr Kfs + Qtz + Pl + Opx + Hbl + Bt + Ap + Zr


Serial. No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Mineral abbreviations: Qtz — quartz; Kfs — K-feldspar including perthite; Pl — plagioclase; Opx — orthopyroxene; Cpx — clinopyroxene; Hbl — hornblende; Bt — biotite; Mt — magnetite; Il — ilmenite; Ap — apatite; Zr — zircon.

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but the fresh exposures clearly show a greasy green texture, intermediate composition and presence of visible clots and layers of orthopyroxene. The rocks carry melanocratic gabbroic enclaves of various dimensions similar to mafic magmatic enclaves. From our regional studies in this area, it is evident that the charnockitic basement rocks are overlain metasediments, dominated by metacarbonates, and the basement rocks are exposed as windows where the metasedimentary cover has eroded. A track road going west from the Kanarapatti electrical substation leads to a large quarry below surface level. The homogenous massivelooking greasy green enderbitic charnockites here carry biotite-rich domains and also visible clots and domains rich in orthopyroxene. 3.4. Gabbros Gabbroic rocks in the southern Madurai Block constitute only a minor component and mostly occur as few meter-sized bands, blocks and boudins of various dimensions and as small enclaves within charnockitic and enderbitic rocks. In this study, we sampled gabbroic rocks from five localities (Table 1). At Sivagiri, these rocks occur as sub-angular to rounded enclaves of various sizes ranging from few tens of cm up to 1 m within enderbitic charnockites. The margins of these enclaves show reaction relation and gradational contact with the host enderbites. The gabbroic enclaves at Valiban Pothai within charnockitic rocks are also similar to those in Sivagiri described above and resemble typical MMEs and suggest mingling of bimodal magmas. At Alamkulam, they occur as disrupted boudins and layers ranging up to 2 m in size within charnockite. A new highway cutting across a large hillock in the TirunelveliTrivandrum road near Rettiyarpatti exposes few hundred meters of fresh exposure where these quartzite bands intercalated with pelitic layers are exposed. Within these bands disrupted layers and boudinaged blocks of medium to fine grained melanocratic metagabbroic rocks occur. A series of such boudinaged blocks and bands are incorporated within the metasedimentary sequence all along the exposure and appear to be fragments from the basement rocks incorporated within the accreted sediments, and all the rocks underwent common metamorphism. The final location is at Taliyuthu, where gabbroic rocks occur as mafic magmatic enclaves within enderbitic rocks. Their field relations are similar to those from other localities such as Sivagiri, with gradational margins and interaction between the host rocks and partial digestion during the high temperature magmatic stage, resembling magma mixing and mingling process. 4. Analytical methods 4.1. Petrography and mineral chemistry Polished thin sections were prepared for petrographic study at the University of Tsukuba, Japan. Mineral chemical analyses were carried out using an electron microprobe analyzer (JEOL JXA8530F) at the Chemical Analysis Division of the Research Facility Center for Science and Technology, the University of Tsukuba. The analyses were performed under conditions of 15 kV accelerating voltage and 10 nA sample current for all minerals, and the data were regressed using an oxide-ZAF correction program supplied by JEOL. 4.2. Whole rock geochemistry The least altered and homogeneous portions of 24 whole rock samples were crushed and powdered to 200 mill for geochemical analyses after petrographic studies. Major and trace (including rare earth elements) elements analyses were conducted in the National Research Center for Geoanalysis, Beijing. The major elements were determined

by X-ray fluorescence (XRF model PW 4400), with analytical uncertainties ranging from 1 to 3%. Loss on ignition was obtained using about 1 g of sample powder heated at 980 °C for 30 min. The trace elements were analyzed by Agilent 7500ce inductively coupled plasma mass spectrometry (ICP-MS). About 50 mg of powder was dissolved for about 7 days at ca. 100 °C using HF–HNO3 (10:1) mixtures in screw-top Teflon beakers, followed by evaporation to dryness. The material was dissolved in 7 N HNO3 and taken to incipient dryness again, and then was re-dissolved in 2% HNO3 to a sample/solution weight ratio of 1:1000. The analytical errors vary from 5 to 10% depending on the concentration of any given element. An internal standard was used for monitoring drift during analysis. Trace and rare earth elements were analyzed with analytical uncertainties 10% for elements with abundances b10 ppm and approximately 5% for those N10 ppm (Gao et al., 2008). 4.3. Zircon separation, U–Pb geochronology and trace element geochemistry Zircon grains were separated using standard procedures for U–Pb dating at the Yu'neng Geological and Mineral Separation Survey Centre, Langfang City, Hebei Province, China. The CL imaging was carried out at the Beijing Geoanalysis Centre. Individual grains were mounted onto double-side adhesive tape and enclosed in epoxy resin disks. The disks were polished to a certain depth to expose the cores of the grains and gold coated for cathodoluminescence (CL) imaging and U–Pb isotope analysis. Zircon morphology and internal structure were imaged by a JSM-6510 Scanning Electron Microscope (SEM) equipped with a backscatter probe and a Chroma CL probe. The zircon grains were also examined under transmitted and reflected light images using a petrological microscope. U–Pb dating and trace element analyses of zircon were conducted synchronously by LA–ICP-MS at the Wuhan Sample Solution Analytical Technology Co., Ltd. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as described by Liu et al. (2008, 2010a, 2010b). Laser sampling was performed using a GeoLas 2005. An Agilent 7500a ICP-MS instrument was used to acquire ion-signal intensities. Helium was applied as a carrier gas. Argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP. Nitrogen was added into the central gas flow (Ar + He) of the Ar plasma to decrease the detection limit and improve precision (Hu et al., 2008; Liu et al., 2010b). Each analysis incorporated a background acquisition of approximately 20–30 s (gas blank) followed by 50 s data acquisition from the sample. The Agilent Chemstation was utilized for the acquisition of each individual analysis. Off-line selection and integration of background and analyte signals, and time-drift correction and quantitative calibration for trace element analyses and U–Pb dating were performed by ICPMSDataCal (Liu et al., 2008, 2010a). Zircon 91500 was used as external standard for U–Pb dating, and was analyzed twice every 5 analyses. Time-dependent drifts of U–Th– Pb isotopic ratios were corrected using a linear interpolation (with time) for every five analyses according to the variations of 91500 (Liu et al., 2010a). Preferred U–Th–Pb isotopic ratios used for 91500 are from Wiedenbeck et al. (1995). Uncertainty of preferred values for the external standard 91500 was propagated to the ultimate results of the samples. Concordia diagrams and weighted mean calculations were made using Isoplot/Ex_ver3 (Ludwig, 2003). Trace element compositions of zircons were calibrated against multiple-reference materials (BCR-2G and BIR-1G) combined with internal standardization (Liu et al., 2010a). 4.4. Zircon Lu–Hf isotopes Lu–Hf analyses of zircon were conducted at the same domains or immediately adjacent domains where the U–Pb data were analyzed using


the Laser Ablation System (MC–ICP-MS) at the Wuhan Sample Solution Analytical Technology Co., Ltd. Laser sampling was performed using a Geolas 193. A Neptune Plus MC–ICP-MS instrument was used to acquire ion-signal intensities, with a beam diameter of 44 μm and a repetition rate of 10 Hz. Zircon 91500 and Mud Tank was used as standard for U–Pb dating, and was measured during every 8 analyses. The detailed analytical procedure and correction for interferences are similar to those described by Wu et al. (2006). The 176Hf/177Hf ratios of the standard zircon (Mud Tank) and 91500 during analysis were 0.282500 ± 0.000030 (2σ, n = 200) and 0.282300 ± 0.000030 (2σ, n = 200), respectively. The 176Hf/177Hf ratio of Mud Tank is almost identical to the values based on LA–MC–ICP-MS analyses, which are 0.282523 ± 0.000043 (2σ, n = 2190; Griffin et al., 2006) and 0.282504 ± 0.000044 (2σ, n = 158; Woodhead and Hergt, 2005), respectively. 5. Results 5.1. Petrology The petrographic features of the representative rocks analyzed for geochemistry and zircon geochronology are summarized below, and representative photomicrographs are presented in Fig. 5a–l. In the sample descriptions below and in other sections, the prefix ‘meta’ is not added, although all the rocks in this study have undergone latest Neoproterozoic–Cambrian metamorphism. 5.1.1. Alkali granites The alkali granites (samples TRM 6-2, 9-1, 9-2) are characterized by the presence of coarse-grained (ca. 3.6 mm) and abundant antiperthite (60–70%) with quartz (25–35%), calcic amphibole (2–5%), apatite, and zircon (Fig. 5e). The antiperthite shows an irregular grain margin suggesting recrystallization during metamorphism. The host:lamella ratio is about 2:1 to 3:1, and the volume of lamella decreases toward the recrystallized rim. The matrix quartz and calcic amphibole are medium grained (0.2–2.0 mm and 0.3–1.1 mm, respectively) and subhedral. In sample TRM 6-2, K-feldspar and plagioclase occur as discrete phases with less of perthitic intergrowths. The modal abundances of Fe–Ti oxide (1–2%) and biotite (2–3%) are also higher in this sample. 5.1.2. Charnockites Charnockites from the study area are massive, coarse-grained, greenish gray, and orthopyroxene-bearing quartzo-feldspathic rock without obvious foliation or migmatization, which are typical characteristics of massive charnockite in high-grade metamorphic terranes worldwide (e.g., Rajesh and Santosh, 2012). Samples TRM 5-1 and 5-2 are typical medium-grained quartz-rich charnockites, composed of quartz (30–40%), anti-perthitic plagioclase (20–30%), K-feldspar (20–30%), and orthopyroxene (2–5%) with accessory biotite, apatite, and zircon. The rock is characterized by the semi-granoblastic texture of plagioclase (ca. 3.8 mm), K-feldspar (ca. 2.2 mm), and quartz (ca. 2.8 mm). Subhedral to rounded orthopyroxene (0.6–1.5 mm) is present along grain boundaries of feldspars and quartz. Coarse-grained K-feldspar (ca. 5.2 mm) also occurs in the matrix of medium-grained quartz and feldspars, and it often contains fine-grained rounded quartz grains. Fine-grained (0.2–1.4 mm) reddish-brown biotite mostly occurs around magnetite and orthopyroxene as a retrograde mineral. Myrmekite is often present as interstitial minerals around K-feldspar. Samples TRM 7-4 and 15-1 are amphibole-bearing varieties with quartz (30–40%), plagioclase (30–40%), perthitic K-feldspar (15–25%), orthopyroxene (1–2%), calcic amphibole (1–2%) and biotite (b 1%). Matrix plagioclase is a medium grained (0.5–3.3 mm), subhedral, and inclusion-free mineral, although coarse-grained (ca. 5.8 mm) plagioclase is poikilitic and contains numerous inclusions of quartz, apatite, biotite, and calcic amphibole. K-feldspar (0.3–3.5 mm) is subhedral and perthitic (host:lamella ratio is about 3:1 to 5:1). Quartz is medium grained

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(0.3–2.5 mm), subhedral to anhedral, and often fills the matrix of feldspars, whereas coarse-grained (ca. 6 mm) quartz is subhedral and weakly elongated. Calcic amphibole is dark greenish brown, subhedral to anhedral, and medium grained (0.4–2.2 mm). Orthopyroxene (0.5– 2.0 mm) occurs as subhedral aggregates in an amphibole-absent portion of the rock. Reddish-brown biotite is fine grained (b0.5 mm) and occurs as interstitial mineral. 5.1.3. Enderbites Among the enderbites, sample TRM 2-3 contains abundant plagioclase (40–50%) and quartz (20–30%) with subordinate orthopyroxene (5–10%), K-feldspar (5–10%), biotite (2–5%), zircon (b 1%), and apatite (b1%) (Fig. 5a). It is characterized by coarse-grained (0.6–4.1 mm), subidioblastic to xenoblastic, pleochroic, and inclusion-free orthopyroxene surrounded by rounded plagioclase (ca. 2.2 mm) and quartz (ca. 3.5 mm). K-feldspar (b0.2 mm) is present only as interstitial grains mostly between matrix plagioclase and quartz. Medium to finegrained (0.2–1.1 mm) reddish-brown flakes of biotite occur adjacent to orthopyroxene grains, possibly formed by the following hydration reaction (1).

Opx þ Kfs þ H2 O ¼ NBt þ Qtz

ð1Þ

Sample TRM 3-1 carries very coarse-grained (ca. 7.5 mm) subhedral orthopyroxene. It shows a typical granoblastic texture of plagioclase (60–70%), quartz (20–30%) and orthopyroxene (5–10%) (Fig. 5b). Interstitial K-feldspar is also present, similar to the occurrence in sample TRM 2-3. Samples TRM 13-1 and TRM 14-1 are similar plagioclaserich and clinopyroxene-bearing enderbites with plagioclase (55–65%), K-feldspar (10–25%), orthopyroxene (10–15%), clinopyroxene (2–5%), quartz (2–5%), and Fe–Ti oxide (1–2%) (Fig. 5g,h). Minor calcic amphibole is also present in sample TRM 14-1 (b 1%). Both orthopyroxene (0.3–1.4 mm) and clinopyroxene (0.2–0.9 mm) are subhedral to rounded, rarely anhedral, and occur as aggregates forming pyroxene-rich domains. Plagioclase (0.4–2.3 mm) and quartz (0.2–1.8 mm) are semigranoblastic and subhedral, while K-feldspar occurs as fine- to medium-grained (b0.3 mm) interstitial grains. Mineral composition of the other enderbite samples is also broadly similar to those described above. 5.1.4. Gabbro The gabbroic samples (broadly equivalent to mafic granulites) from this study show a medium-grained semi-equigranular granoblastic texture. Representative samples (such as TRM 3-3, TRM 6-3, and TRM 13-3; Fig. 5j–l) contain broad mineral assemblages of plagioclase (50–60%), calcic amphibole (10–30%), orthopyroxene (2–10%), biotite (5–20%), and Fe–Ti oxide (2–10%), corresponding to hornblende gabbro. Greenish to brownish calcic amphiboles (0.3–1.2 mm) and reddish-brown flakes of biotite (0.4–2.2 mm) occur as aggregates with subhedral to rounded orthopyroxene (0.2–1.4 mm) and clinopyroxene (0.2– 0.8 mm). The pyroxenes are often mantled by biotite, suggesting retrograde hydration. Coarse-grained (ca. 2 mm) calcic amphibole in sample TRM 6-3 contains fine-grained inclusions of apatite, plagioclase, and magnetite. Plagioclase (0.3–1.8 mm) is subhedral to anhedral and fills the matrix of the ferromagnesian minerals. Sample TRM 11-2 is orthopyroxene-rich noritic variety, comprising semi-equigranular plagioclase (50–60%), orthopyroxene (20–30%), and Fe–Ti oxides (ilmenite and magnetite; 5–10%). The minerals are medium-grained (mostly 0.3– 0.7 mm) and granoblastic. Plagioclase and orthopyroxene are subhedral. Fine-grained (b0.3 mm) Fe–Ti oxide is scattered throughout the thin section. The other gabbroic samples also show similar mineralogical composition.

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5.2. Mineral chemistry Representative compositions of minerals obtained by electron microprobe analyses are summarized in Supplementary Table 2 and briefly discussed below. 5.2.1. Pyroxenes Orthopyroxene in charnockites and enderbites shows consistent enstatite-rich compositions of XMg = Mg/(Fe + Mg) = 0.52–0.61, and in some cases (e.g., TRM 3-1 and TRM 15-1) it displays slightly Fe-rich compositions of XMg = 0.45–0.47 (Fig. 6a). Compositional variation between the core and rim of a single grain is absent. Orthopyroxene in the gabbroic rocks (samples TRM 3-3 and TRM 6-3) also shows Fe-rich compositions of XMg = 0.44–0.48. The CaO and MnO contents in the mineral are generally low (CaO = 0.06–1.51 wt.%, MnO = 0.27–1.69 wt.%). Al2O3 content in orthopyroxene shows significant variation (0.48– 7.01 wt.%) with the highest Al2O3 content of 6.66–7.01 wt.% found in the orthopyroxene in the enderbite sample TRM 2-3, which corresponds to yOpx (= Si + Al − 2) of 0.12 to 0.13 pfu, plotted near the array of orthopyroxene in metamorphic charnockite (Rajesh et al., 2011). Orthopyroxene in samples TRM 5-1, TRM 11-2, and TRM 15-1 shows an intermediate yOpx of 0.05–0.07, 0.03–0.05, and 0.06–0.08, respectively (Fig. 6b). Orthopyroxene in metadiorite and other charnockite samples show a lower yOpx of 0–0.03, distributed along a magmatic orthopyroxene array (Rajesh et al., 2011). Clinopyroxene in the examined samples are all classified as augite, and there is no significant compositional gap between clinopyroxenes in charnockite and metadiorite, although clinopyroxene in the enderbite sample TRM 3-1 is slightly Fe-rich (XMg = 0.60–0.61) than that of other charnockite and metadiorite (XMg = 0.68–0.72) (Fig. 6c). Na2O and Al2O3 contents in the mineral are low, Na2O = 0.31– 0.68 wt.% and Al2O3 = 1.2–2.6 wt.%. 5.2.2. Calcic amphibole Calcic amphibole from enderbites, charnockites and gabbros are enriched in Ca (1.75–1.92 pfu) and Na + K (0.65–0.93 pfu), depleted in Ti (0.18–0.35 pfu), and is classified mostly as edenite, pargasite, or ferro-pargasite (Fig. 6c). Calcic amphibole in the charnockite is enriched in XMg (0.59–0.65) and Si (6.56–6.80 pfu) as edenite, whereas that in sample TRM 15-1 is slightly depleted in XMg (0.48) and Si (6.35– 6.42 pfu) and classified as ferropargasite. The latter compositions are nearly consistent with the values of metadiorite samples (XMg = 0.47–0.57, Si = 6.25–6.50 pfu). 5.2.3. Feldspars Plagioclase in the samples show notable compositional variation depending on the sample. Samples TRM 11-2 (gabbro) and TRM 9-1 (alkali granite) contain plagioclase with the highest and lowest anorthite contents of An16–18 and An77–76, respectively (Fig. 6d), probably reflecting high and low CaO contents of the rocks, whereas plagioclase in other charnockites and enderbites shows consistent anorthite contents of An24–34. Plagioclase in the gabbros shows generally higher anorthite content of An36–41. K-feldspar shows orthoclase-rich compositions of Or89–96, whereas grain-boundary K-feldspar in the gabbroic sample TRM 6-3 that possibly recrystallized during metamorphism shows albite-rich composition of Or67-72Ab26–31. Coarse-grained K-feldspar with plagioclase lamellae in sample TRM 15-1 (charnockite) shows intermediate compositions of Or79–81. 5.2.4. Biotite Biotite composition also varies depending on rock type (Fig. 6e). Biotite in sample TRM 13-3 (gabbro) shows the highest TiO2 content of 5.9–6.1 wt.% with intermediate XMg of 0.59–0.60. That in sample TRM 3-1 (enderbite) is depleted in TiO2 and XMg as 3.0–3.1 wt.% and 0.48– 0.49, respectively. Other charnockites contain biotite with intermediate

TiO2 content (4.5–5.0 wt.%) and higher XMg (0.64–0.70). Biotite in sample TRM 6-3 (gabbro) is also Mg-rich (XMg = 0.67–0.68), although its TiO2 content is low (3.3–3.4 wt.%). 5.3. Geochemistry The locations of 24 samples from the charnockitic suite of rocks in the Southern Madurai Block analyzed for geochemistry in this study are shown in Figs. 2 and 3. Major and trace elements analyses of these samples are given in Supplementary Table 3. 5.3.1. Major elements Charnockites, like granitoids, is a term applied to a suite of compositionally diverse rocks. Charnockitic rocks have been broadly divided into intermediate and felsic types (Rajesh, 2004, 2012; Rajesh and Santosh, 2012). The samples in this study can be divided into four groups based on their field relationships, mineral assemblages and geochemical features. The gabbro group comprises seven samples (TRM 33, 3-4, 6-3, 7-2, 11-2, 13-3 and 14-2); the enderbite group includes eleven samples (TRM 2-1, 2-3, 3-1, 3-2, 8-2, 10-1, 10-2, 13-1, 13-2, 14-1 and 14-3); the charnockite group is composed of four samples (TRM 5-1, 5-2 and 15-1) and the alkali granite group comprises three samples (TRM 62, 9-1 and 9-2). Major element data show that one charnockite sample and those of the alkali granite group (particularly samples TRM 5-1, 5-2, 15-1, 6-2, 91 and 9-2) have the highest SiO2 content (65–71 wt.%). The enderbite samples have intermediate SiO2 content in the range of 54–60 wt.%. The lowest SiO2 content is displayed by the gabbroic samples (48– 52 wt.%). In the TAS diagram (Fig. 7a), the enderbite, charnockite and alkali granite vary from dioritic to granitic in composition. The mafic magmatic enclaves (MMEs) in these rocks are plotted in the gabbroic field. The rocks follow a calc-alkali trend in the TAS diagram (Fig. 7a). In terms of alumina saturation index (A/CNK) as indicated by the A/CNK–A/NK diagram (Fig. 7b), the studied samples show a continuous trend from peralkaline through metaluminous to peraluminous. All the charnockitic rocks (enderbite and charnockite) are metaluminous to peraluminous as indicated by their plots in Fig. 7b. In the normative An–Ab–Or and K–Na–Ca diagrams (Fig. 8a, b), the samples define a continuous range from tonalite/trondhjemite to granite. Based on the QAP (quartz–alkali feldspar–plagioclase) classification diagram of charnockite, it is appropriate to refer the intermediate rocks as “enderbite” (tonalite/trondhjemite composition) and felsic rocks as “charnockite” (granite composition). The rocks display a range of TiO2 values from 0.1 to 2.83 wt.%, with the mafic enclaves (gabbroic samples) having higher TiO2 contents whereas the charnockite and alkali granite samples show the lowest TiO2 content (0.1–0.85 wt.%). In general, the samples with higher TiO2 also show relatively higher P2O5, FeOT, total alkali (Na2O + K2O) and lower Mg number (Mg# = MgO/(MgO + FeOT) (molar)). In the AFM (Na2O + K2O–total FeO–MgO) diagram, most samples show a calc-alkaline nature whereas a few samples show tholeiitic affinity (Fig. 9). In plots of SiO2 versus FeOt/(FeOt + MgO) (Fig. 10a) and modified alkali lime index (Na2O + K2O–CaO) (Fig. 10b), the enderbites and charnockites display both ferroan and magnesian features, and straddle the fields from calcic to alkalic. Compared to the charnockites in the highland massifs of the Southern Granulite Terrane summarized in Rajesh and Santosh (2004), the samples of the present study show a wider range of FeOt/(FeOt + MgO) (0.53–0.83) and modified alkali lime index (Na2O + K2O–CaO) (−10.8–6.7). In major elements binary variation (Harker) diagrams (Fig. 11), TiO2, FeOt, CaO and P2O5 show decreasing trends with increasing SiO2 whereas K2O shows a broad scatter and Al2O3 content shows a limited variation. The empirical thermometers based on the relationship between TiO2 and P2O5 with SiO2 (Fig. 11a,e) show a temperature range of 800–1000 °C for the crystallization of these rocks.


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Fig. 2. Geological map of the southern part of the Madurai Block, together with Trivandrum and Nagercoil Blocks to the south. The sample locations of the present study are also shown.

The wide range of K2O content (0.1 to 5.3 wt.%) attributes calc-alkaline to shoshonitic affinity for these rocks as indicated by plots in the K2O vs. Na2O diagram (Fig. 12a). Because of the high Al2O3 content of most samples, the enderbites, charnockites and granites are plotted within the adakitic field in the Al2O3 vs. SiO2 diagram (Fig. 12b). 5.3.2. Trace elements The rocks display a wide range of Sr and Y content and can be grouped into high Sr/Y and low Sr/Y groups (Fig. 12c,d). The samples with higher Sr also have relatively higher Al2O3, Na2O and Mg#, with lower Y and Yb content. Their high Sr and low Y indicate adakite or TTG affinities (Fig. 12c). Broadly, the studied samples cross the fields of normal island arc andesite–dacite–rhyolite series and adakite–TTG field. These features are consistent with those of charnockitic rocks reported in previous studies in this region (e.g., Rajesh and Santosh, 2004). In the trace elements variation binary diagram (Harker diagram) (Fig. 13), the Nb, Ga and Ni contents show clear negative correlation with SiO2 whereas Th shows decreasing trend with increasing SiO2. The Rb and Ce contents are similar in the different samples. The REE and trace element concentrations of the mafic enclaves (gabbroic samples) show high Ni (13–131 ppm), Cr (2–238 ppm) and V concentrations (101–353 ppm) (Supplementary Table 3) whereas those in the charnockites and alkali granites show the lowest contents of these ‘mantle’ elements. Among the large ion lithophile elements (LILE) Ba and Sr show a large variation ranging between 81 and 2318 ppm and 162–1119 ppm. The mafic enclaves (gabbroic samples) show relatively lower total REE concentration and slightly LREE enriched patterns (Supplementary Table 3) (Fig. 14a). The (La/Yb)N ratio (4.8–33.3) and (La/ Sm)N ratio (1.7–4.8) show moderate range, with negative, positive or no Eu anomalies (Eu/Eu* = (EuN)/√[(SmN) × (GdN)] = 0.5–1.5). The

ratios of (Gd/Yb)N (1.2–3.8), (Nb/Y)N (1.9–5.6) show limited variations whereas the (Th/Yb)N (0.6–34.4) shows marked variation. In the primitive mantle-normalized (Sun and McDonough, 1989) trace elements diagram (Fig. 14b), the rocks display obvious LILE (K, Rb, Sr, Ba) enrichment relative to HFSE (Nb, Ta, Zr, Hf) coupled with distinct Nb–Ta depletion and negative Zr–Hf, Ti anomalies. A prominent Pb enrichment with variable Sr contents is also noted. The enderbite samples show REE distribution patterns (Fig. 14c) similar to those of the gabbro samples with (La/Yb)N ratios of 7.7–33.5 and (La/Sm)N ratio (3.0–7.3). They also show prominent and highly variable positive or negative Eu anomalies (Eu/Eu* = 0.6–1.9). In the primitive mantle-normalized trace elements diagram (Fig. 14d), the patterns are similar for both rocks except for prominent P depletion in the gabbros. In chondrite-normalized REE plots (Fig. 14e,g), the charnockites and alkali granites are characterized by high concentration of light rare earth elements (LREEs) and relatively low contents of heavy rare earth elements (HREEs), with significant LREE/HREE fractionation ((La/Yb)N = 12.9–90.7), (La/Sm)N ratio (3.6–13.4), resulting in steep patterns between LREE and HREE. These samples are characterized by highly fractionated patterns both in terms of total REE and HREE. They also show positive Eu anomalies or lack of Eu anomalies. Primitive mantle normalized multi-trace element diagrams of the charnockites and alkali granites (Fig. 14f,h) show distinct Th–U, Nb–Ta, P, Ti depletion, Pb enrichment and relative Zr-Hf enrichment. A general characteristic of the studied charnockitic rocks is their Ba enriched feature, with all granitic samples plotting in the high Ba–Sr granitoids field as indicated from the Sr–Rb–Ba ternary plot (Fig. 15). This is consistent with the data on charnockites reported by Rajesh, (2004) and Tomson et al. (2006) from other localities from southern India.

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Fig. 3. Detailed geological map of the study area (after Geological Survey of India, 2005) showing sample locations of present study.

In addition, the studied charnockitic rocks show geochemical features common in felsic igneous rocks, including unmetamorphosed charnockites as indicated by the K/Rb vs. K plots (Fig. 16a). In terms of Rb vs. Sr binary diagram (Fig. 16b), most of the samples show Rb/Sr ratios in the range of 0.03 to 0.3, lying between the typical value of upper crustal and mantle Rb/Sr ratios. In the La/Th vs. Th/U diagram (Fig. 17a), the samples are plotted above the field of typical igneous rocks and appears to have experienced U depletion with little Th depletion as also inferred from the spider diagrams (Fig. 15). For those samples with higher La/Th values, both Th depletion and U depletion are indicated, but generally more Th than U as suggested by relatively lower Th/U ratios (b 5). In the Nb/Yb vs. Th/Yb diagram (Fig. 17b), the samples show a subduction zone enrichment trend, and also suggest that the magma was derived from enriched sources. The plots of alkali granites, charnockites, and enderbites in relation to the compositional field of partial melts produced from crustal sources, such as amphibolites, pelites, greywackes, and charnockites, are illustrated in Fig. 18, where the majority of plots fall in the field of amphibolites. The implications of this for the source rocks will be evaluated under the Discussion section. 5.4. Zircon U–Pb geochronology and REE geochemistry Representative cathodoluminesence (CL) images of zircons from the diorite samples TRM 2-3, TRM 3-1, TRM 5-1, TRM 6-2, TRM 7-4, TRM 8-

2, TRM 9-1, TRM 10-1, TRM 11-2, TRM 13-1, TRM14-1, TRM 15-1 are shown in Figs. 19–22, together with the analytical spots. The U–Pb age data are presented in Supplementary Table 4, which are plotted in concordia diagrams together with bar charts and histograms (Figs. 23–38). Zircon trace element data are reported in Supplementary Table 5 and the REE distribution patterns are plotted in Figs. 39–41. The zircon characteristics and results in individual samples are discussed below. 5.4.1. Alkali granites 5.4.1.1. TRM 6-2. Zircons in this sample are colorless, transparent, and euhedral to subhedral. They show prismatic to sub-rounded morphology with lengths varying from 80 to 250 μm and a length to width ratio of 3:1. Needle-like apatite inclusions can be observed in many grains. In CL images, most of the zircons display distinct core-rim structures, with the magmatic cores displaying oscillatory zoning, patchy zoning or light banding, and a metamorphic rim of up to 50 μm in width. A few grains are also homogeneous, corresponding to discrete structureless metamorphic zircons. A total of 30 analyses were performed on both magmatic core domains and metamorphic rim domains from 27 zircon grains. The Th and U contents are in the range of 198 to 740 ppm and 332 to 2047 ppm respectively with Th/U ratios of 0.34 to 1.86. Excluding 1 analysis (spot 30) which is discordant, 22 of the thirty analyses on


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Fig. 4. Representative field photographs. (a) Alkali granite of Sitaparappanallur. (b) Charnockite at Veerakerala Puthur. (c) Enderbite at Sivagiri carrying mafic magmatic enclaves (MMEs). (d) Gabbroic MMEs within the enderbites at Sivagiri. (e) Enderbite in working quarries below surface level at Thalayuth. (f) Gabbroic enclaves (MMEs) within enderbite at Thalayuth.

magmatic zircons form two coherent age groups within analytical error and a Gaussian-style age distribution pattern. The remaining analyses of metamorphic rims define a prominent age peak (Fig. 23). The oldest group shows ages in the range of 720 to 841 Ma with most of them concentrated in the range of 757 to 818 Ma and yielding a 206Pb/238U weighted mean age of 722 ± 11 Ma (MSWD = 2.7) (Fig. 23b). The second group includes four analyses that show midNeoproterozoic ages ranging from 647 to 677 Ma with a weighted mean age of 662 ± 20 Ma (MSWD = 2.6). (Fig. 23a). The youngest group are from zircon rim domains and homogeneous zircons, and shows Cambrian ages ranging from 512 to 581 Ma and (mostly between 512 and 534 Ma) with a weighted mean age of 528 ± 12 Ma (MSWD = 1.9). Twenty-nine spots were analyzed for trace elements including seven rims and twenty-two cores. The normalized REE patterns of all spots show enriched HREE abundance increasing from ca. 10 2 (Tb) to 104 (Lu) times chondrite (Fig. 39). However they display a salient difference in LREE patterns between rims and cores. Seven spots from rims display a distinct positive Ce anomaly (Ce/Ce* = 47–95) and negative Eu-anomaly (Eu/Eu* = 0.08–0.12). The REE patterns of cores (SmN /La N = 1.2–425) show scattered Ce and Eu anomalies (Ce/Ce* = 1–131, Eu/Eu* = 0.09–0.33). In addition, there are two cores with ages of 751 Ma and 854 Ma that display a concave REE pattern with a distinct Eu-anomaly and no significant Ce anomaly.

5.4.1.2. TRM 9-1. The zircon grains from this sample are colorless, transparent, and euhedral to subhedral, with some grains carrying tiny inclusions. They show long prismatic to stumpy grain morphology with lengths varying from 100 to 300 μm and a length to width ratio of 3:1. In CL image, most of them display dark cores with typical oscillatory zoning surrounded by a narrow rim, and the rest show cores with a distinctly bright rim. A total of 35 analyses on both rim and cores in zircons from 30 zircon grains were carried out. The Th contents range from 48 to 720 ppm and U contents range from 21 to 844 ppm, with Th/U ratios in the range of 0.19–7.37. Excluding the data from 2 grains (spots 31 and 35) which are discordant, 33 of the 35 analyses form two coherent age groups within analytical error with two major peaks. The older group yields spot ages ranging from 660 to 777 Ma with a weighted mean 206Pb/238U age of 724 ± 9 Ma (MSWD = 4.1) corresponding to the timing of magma emplacement (Fig. 24). The younger group from metamorphic domains yields ages ranging from 461 to 556 Ma with a weighted mean age of 523 ± 12 Ma (MSWD = 4.2). Thirty-one spots in this sample were analyzed for trace elements including twenty-one rims and ten cores. The chondrite-normalized REE patterns are characterized by HREE enrichment relative to LREE with (Lu/Gd)N ranging from 3 to 96. Except for one spot that exhibits concave pattern, the cores display scattered LREE patterns (SmN/LaN = 0.6–326) with positive Ce anomalies (Ce/Ce* = 0.8–142) and negative Eu

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Fig. 5. Photomicrographs showing textures and mineral assemblages of representative samples. (a) Coarse-grained orthopyroxene in the matrix of abundant plagioclase and quartz in enderbite sample TRM 2-3. (b) Subhedral orthopyroxene in the matrix of coarse-grained plagioclase in enderbite sample TRM 3-1. (c) Medium-grained charnockite with a semigranoblastic texture of plagioclase, K-feldspar, and quartz (sample TRM 5-1). (d) Plagioclase-rich charnockite (sample TRM 7-4). (e) Coarse perthitic feldspar in alkali granite (sample TRM 9-1). (f) Orthopyroxene-rich domain in noritic gabbro comprising semi-equigranular plagioclase, orthopyroxene, clinopyroxene and Fe–Ti oxide (sample TRM 11-2). (g) Plagioclase-rich and clinopyroxene-bearing granoblastic enderbite (sample TRM 13-1). (h) Two-pyroxene + plagioclase + quartz assemblage in enderbite (samples TRM 14-1). (i) Subhedral orthopyroxene in the matrix of semi-granoblastic quartz and feldspars in charnockite sample TRM 15-1. (j) Subhedral orthopyroxene mantled by aggregates of biotite and hornblende in gabbro (sample TRM 3-3). (k) Granoblastic hornblende and plagioclase in gabbro (sample TRM 6-3). (l) Coarse-grained hornblende and associated orthopyroxene and biotite in gabbro (sample TRM 13-3). Qtz: quartz, Opx: orthopyroxene, Pl: plagioclase, Mag: magnetite, Apth: anti-perthite, Hbl: hornblende, Cpx: clinopyroxene, Bt: biotite.

anomalies (Eu/Eu* = 0.09–0.71). The rims show two groups and from La to Lu, the REE contents of most spots in the first group range from 100 × to 103 × chondrite (Fig. 39). Four spots (with ages of 529 Ma, 533 Ma, 507 Ma, and 515 Ma) from metamorphic zircons compose the other group which show relatively flat patterns and higher REE abundances (102 to 104 × chondrite) relative to the first group, and without positive Ce anomalies.

5.4.2. Charnockites 5.4.2.1. TRM 5-1. The zircon grains in this sample are transparent and dark brownish with a length range of 80–200 μm and aspect ratios of 3:1–1:1. Most of the grains show prismatic to stumpy morphology, whereas some grains have irregular and angular shape. They display distinct core-rim structures and overgrowth rims, with most of the


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Fig. 6. Compositional diagrams showing chemistry of representative minerals in various rock types from the southern Madurai Block. (a) Triangular diagram showing pyroxene chemistry. (b) yOpx (=Si + Al − 2) versus XMg diagram showing compositions of orthopyroxene. Arrays of magmatic and metamorphic charnockites are after Rajesh et al. (2011). (c) Si (pfu) versus XMg diagram showing compositions of calcic amphibole. (d) Triangular diagram showing feldspar chemistry. (e) XMg versus TiO2 (wt.%) diagram showing biotite chemistry.

cores possessing clear magmatic oscillatory zoning surrounded by a thin bright rim which ranges in width up to 50 μm. A total of 30 analyses were carried out on the different domains from 27 zircon grains. The Th and U contents range from 3.9 to 298.4 ppm and 69.9 to 334.3 ppm respectively with Th/U ratios ranging from 0.23 to 2.21. Excluding 2 analyses (spots 5 and 21) which are discordant, 28 of the thirty analyses form 5 major age groups (Fig. 25). The oldest age group shows spot ages in the range of 922 to 958 Ma with 206Pb/238U weighted mean age of 938 ± 27 Ma (MSWD = 2.1) (Fig. 25b). The second group yields slightly younger mid-Neoproterozoic ages ranging from 888 to 902 Ma with a weighted mean age of 896 ± 12 Ma (MSWD = 0.26). The third group yields slighter mid-Neoproterozoic ages ranging from 655 to 668 Ma with a weighted mean age of 661 ± 9 Ma (MSWD = 0.81). The data above from the three groups are mainly

from magmatic zircon cores, and indicate multiple magmatic events. The spots of the other two groups are on metamorphic zircon rims, and five analyses yield late Neoproterozoic ages ranging from 544 to 596 Ma with a weighted mean age of 567 ± 19 Ma (MSWD = 1.9) (Fig. 25c). Trace element data from twenty-seven spots including seventeen cores and eleven rims show a steeply-rising slope from LREE to HREE with positive Ce-anomaly and negative Eu-anomaly (Fig. 39). Few spots show a flat pattern of LREE distribution. The cores display scattered La content and high contents of HREE. The HREE abundance of rims in these zircons are diverse, some of them even lower than 103 × chondrite. Nevertheless the LREE patterns broadly overlap with markedly low La contents, and display scattered patterns in HREE, especially from Dy to Lu.

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Fig. 7. (a) Plots of SiO2 vs. Na2O + K2O, (b) alumina saturation index (ASI) diagram: A/NK [molar ratio Al2O3/(Na2O + K2O)] vs. A/CNK [molar ratio Al2O3/(CaO + Na2O + K2O)]. Fields in (a) are after Wilson (1989).

5.4.2.2. TRM 7-4. Zircons in this sample are transparent to dark brownish, and subhedral. They show grain size varying from 50 to 100 μm and length to width ratio of nearly 1:1. In CL image, the grains display distinct recrystallization features without the typical core-rim texture. A total of 30 analyses were performed on different domains of 30 zircon grains. The Th contents range from 56.6 to 434.0 ppm and U contents range from 710.6 to 1265.7 ppm, with Th/U ratios in the range of 0.06–0.39, with values lower than 0.3, suggesting that these are mostly of metamorphic origin. Excluding the data from 1 grain (spot 1) which is discordant, 29 of the 30 analyses form a coherent age group within analytical error, and define a Gaussian-style distribution pattern on the probability density plot (Fig. 26). The 25 analyses yield 206Pb/238U ages varying from 487 to 551 Ma with a weighted mean age of 515 ± 5 Ma (MSWD = 3.5) marking the time of metamorphism. Twenty-nine spots were analyzed on zircon grains from this sample for trace element data. The chondrite-normalized REE patterns are more smooth as compared to other samples with (LuN/GdN) = 0.66–2.8. The data show a distinct positive Ce anomaly (Ce/Ce* = 10.87–156.3) and less pronounced Eu anomaly (Eu/Eu* = 0.51–1.02) (Fig. 39). Compared to other samples, the abundance of HREE is obviously low (b102 × chondrite), suggesting partitioning in metamorphic garnets coexisting with zircon.

5.4.2.3. TRM 15-1. Most of the zircon grains from this sample are brownish, transparent to translucent and euhedral to subhedral (Fig. 20). They show long prismatic to stumpy grain morphology with lengths varying from 110 to 220 μm and a length to width ratio of 3:1, including inclusions. In the CL image, most of the grains display a clear core-rim texture. Some of them possess a dark core with oscillatory zoning or patchy zoning surrounded by a luminescent rim, and some show a dark core with weak zoning surrounded by a thick bright rim. A total of 30 analyses were made on both magmatic core domains and metamorphic rim domains from 30 zircon grains. Excluding 1 analysis (spot 24) which is discordant, 29 of thirty analyses form three groups. Eight analyses yield ages ranging from 753 to 817 Ma with a weighted mean 206Pb/238U age of 786 ± 9 Ma (MSWD = 1.9), and another three spots belonging to the second group show a weighted mean age of 632 ± 7 Ma (MSWD = 0.92), suggesting multiple pulses of magmatism during mid-Neoproterozoic and late Neoproterozoic (Fig. 27). The youngest group yields ages ranging from 467 to 555 Ma with a weighted mean age of 537 ± 14 Ma (MSWD = 4) corresponding to the timing of metamorphism. Twenty-six spots were analyzed for trace element data including sixteen cores and ten rims. The plots show HREE enriched pattern with average (Lu/Gd)N ratios of 29 and 21 respectively for the cores and rims, with no significant difference in the HREE distribution

Fig. 8. (a) Normative Ab–An–Or ternary plot and classification of charnockitic rocks of the study area with reference to the scheme after Barker (1979). (b) Na2O–K2O–CaO ternary plot for studied charnockitic rocks from Southern Madurai Block. Dashed curves correspond to trends of calc–alkaline (CA) and trondjhemitic (TR) differentiation, from Barker (1979).


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groups yielding ages in the range of 623 to 1057 Ma. These groups show weighted mean 206Pb/238U ages of 692 ± 10 Ma (MSWD = 3, n = 0.9), 742 ± 11 Ma (MSWD = 0.46, n = 3), 803 ± 10 Ma (MSWD = 1.2, n = 3), and 923 ± 36 Ma (MSWD = 1.4, n = 3) suggesting multiple magmatic pulses during Neoproterozoic (Fig. 28). Nine metamorphic spots yield a weighted mean 206Pb/238U age of 515 ± 10 Ma (MSWD = 2.9), suggesting Cambrian metamorphism. Trace element data from twenty-seven spots including sixteen cores and eleven rims show an HREE enriched pattern relative to the LREE (Fig. 40) with positive Ce and negative Eu anomalies (Sm/La = 1.32– 900, Lu/Gd = 5.92–60.83). Few grains show a relatively flat pattern with higher LREE abundance. Compared with the cores, the rims display a lower LREE abundance, particularly low La contents.

Fig. 9. Ternary AFM (total alkali (A) − total FeO (F) − MgO (M)) plot for studied rocks of the study area. The field discrimination of calc–alkaline and tholeiitic rocks are after Irvine and Baragar (1971).

between cores and rims except for a spot among the youngest rim domains (486 Ma) that shows unusually high REE contents (Fig. 39). There is, however, a significant scatter in the LREE with (Sm/La)N ratios varying 0.17–398.5 and 3.56–64.99 respectively. The Ce anomaly is also variable resulting from the scatter of (Ce/Ce*) in the range of 0.77–35.09 and 1.45–35.03 for the cores and rims respectively. 5.4.3. Enderbites 5.4.3.1. TRM 2-3. Zircons in this sample are transparent or dark brownish in color and show a size range of 80–200 μm with aspect ratios of 2:1 to 1:1. In CL images, the zircon grains display euhedral to subhedral and stumpy to sub-rounded morphology. Most grains show a core-rim texture with the magmatic core showing clear oscillatory zoning or patchy zoning and a bright overgrowth rim with a width of up to 80 μm (Fig. 20). A total of 31 analyses on both magmatic core domains and metamorphic rim domains from 28 zircon grains were carried out. The Th and U values show a range of 5.8 to 564.3 ppm and 74.2 to 1002.2 ppm with Th/U ratios of 0.03 to 1.35. Excluding 3 analyses (spots 14, 30, and 31) which are discordant, 28 analyses can be divided into magmatic and metamorphic zircons (Fig. 28). The magmatic population forms 4

5.4.3.2. TRM 3-1. Most of the zircon grains from this sample are colorless and transparent and display long prismatic to stumpy morphology with length varying from 50 to 220 μm, and length to width ratio of 4:1. Most grains show a core-rim texture with a magmatic core showing clear oscillatory zoning or patchy zoning and an overgrowth rim of up to 30 μm in width. A few grains also show a homogeneous texture (Fig. 20). A total of 30 analyses were carried out on both magmatic core domains and metamorphic rim domains from 30 zircon grains. Their Th and U contents range from 4.4 to 1214.6 ppm and 102 to 1727.5 ppm respectively. The Th/U ratios range from 0.03 to 1.21. Excluding 1 analysis (spot 30) which is discordant, 22 of the thirty analyses of magmatic zircons form distinct age groups within analytical error, and the metamorphic rims define a prominent age peak (Fig. 29). The magmatic population shows ages ranging from 710 to 945 Ma, suggesting multiple pluses of early Neoproterozoic to mid-Neoproterozoic magmatism. The metamorphic zircons yield ages ranging from 471 to 612 Ma but are mostly distributed between 535 and 612 Ma with a 206Pb/238U weighted mean age of 563 ± 13 Ma (MSWD = 3.8). The youngest group includes three spots with Late Cambrian to Early Ordovician ages ranging from 471 to 501 Ma, suggesting a thermal event at this time. Trace elements were analyzed from twenty-seven spots including sixteen cores and eleven rims. The chondrite normalized patterns show an HREE enriched pattern with (Lu/Gd)N ratios ranging from 13 to 23, and no significant difference in patterns between rims and cores. In the case of LREE, however, most cores and rims exhibit diverse patterns with scattered positive Ce-anomaly and negative Eu-anomaly (Fig. 40). Few cores even display a negatively sloped REE pattern with a negative Eu-anomaly and the absence of a positive Ce-anomaly. Compared to rims, the cores display more obvious Eu anomalies (Eu/Eu* = 0.02–0.56) and higher HREE abundance.

Fig. 10. (a) Fe number (FeOt/(FeOt + MgO)) vs. SiO2 and (b) modified alkali lime index (Na2O + K2O–CaO) vs. SiO2 plots illustrating the general grouping of studied charnockitic rocks from Southern Madurai Block. Boundaries are after Frost et al. (2001). Fields for different magma types are from Frost et al. (2001).

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Fig. 11. Selected Harker diagrams of major elements plotted against SiO2 illustrating geochemical variations of studied rocks from Southern Madurai Block. Dashed isothermal curves in the TiO2–SiO2 and P2O5–SiO2 plot show Fe–Ti oxide saturation temperatures and apatite saturation temperature at 7.5 kbar respectively (Watson and Harrison, 1984; Green and Pearson, 1986).

5.4.3.3. TRM 8-2. The zircon grains from the sample are transparent and dark brownish, and euhedral to subhedral in shape with a length range of 80–150 μm and aspect ratios of 3:1–1:1. They show prismatic to subrounded morphology. In CL image, they display a distinct core-rim texture, and 60% of zircons show dark cores with typical oscillatory zoning or patchy zoning and a narrow luminescent rim. The rest of the grains show smaller cores and thick bright rims (Fig. 20). A total of 30 analyses were carried out on the different domains of zircons from 27 grains. The Th contents range from 94 to 1933 ppm and U contents range from 101 to 2714 ppm, with Th/U ratios in the range of 0.5–1.35. Excluding the data from 5 grains (spots 01, 06, 25, 29, and 30) which are discordant, 25 of the 30 analyses form distinct age groups within analytical error with three peaks from magmatic zircons and one major peak from metamorphic domains (Fig. 30). The magmatic data range from 663 to 956 with 206Pb/238U weighted mean age peaks at 926 ± 22 Ma (MSWD = 3.9), 787 ± 23 Ma (MSWD = 3.7), 748 ± 27 Ma (MSWD = 1.3), suggesting continuous pulses of magmatism from early to mid-Neoproterozoic. The metamorphic zircons show spot ages in the range of 445 to 528 Ma with a weighted mean 206Pb/238U age of 493 ± 15 Ma (MSWD = 2.9). Trace element data were obtained from twenty-five spots including seventeen cores and eight rims and the results display obvious fractionated REE patterns between cores and rims. The enrichment of HREE is

obvious with average (Lu/Gd)N ratios of 24 and 30 respectively for the cores and rims, with no significant difference in the HREE distribution pattern (Fig. 40). However, LREE shows a significant scatter in the cores with (Sm/La)N ratios varying from 0.4 to 64, and some grains display a concave pattern with the absence of a positive Ce-anomaly. The rest of the patterns of the cores are similar with those of the rims which show a positive Ce-anomaly and negative Eu-anomaly. In addition, compared to cores, the rims show low total REE contents especially in LREE. 5.4.3.4. TRM 10-1. The zircon grains in this sample are transparent and dark brownish with a length range of 100–300 μm and aspect ratios of 3:1–1:1. Most of the grains show prismatic to sub-rounded morphology, and some grains have an irregular and angular shape. As revealed by CL images, about half of the grains display a core-rim texture with dark cores and bright overgrowth rims which range in widths of possibly up to 100 μm and represent metamorphic overgrowth. The core domains are mostly weakly zoned but some of them show patchy zoning and magmatic texture. The other half of the zircon grains show patchy zoning with an extremely narrow rim or without a rim (Fig. 21). A total of 33 analyses were done on different domains of zircons from 26 zircon grains. The Th contents range from 92 to 448 ppm and U contents from 79 to 519 ppm, with Th/U ratios in the range of 0.23–


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Fig. 12. (a) Na2O vs. K2O plot showing the potassium content of studied rocks from the Southern Madurai Block; (b) Al2O3 vs. SiO2 plot; (c) Sr/Y vs. Y plot and (d) La/Yb vs. Yb plot illustrating the adakitic feature of studied rocks from the Southern Madurai Block. The fields in (c) and (d) are from Drummond and Defant (1990) and Drummond et al. (1996).

2.33. Excluding the data from 2 grains (spots 12 and 13) which are discordant, 31 of the 33 analyses form two coherent age groups within analytical error and define two major age peaks. The older group yields a continuous range from 592 to 840 Ma, mostly concentrated between 720 and 850 Ma, and yield a 206Pb/238U weighted mean age of 786 ± 10 Ma (MSWD = 3.5) (Fig. 31). Six spots from the overgrowth rims yield Cambrian ages with a weighted mean age of 537 ± 9 Ma (MSWD = 1.03), representing the timing of metamorphism. The trace element data were analyzed on thirty spots including twenty-three cores and seven rims. Their chondrite-normalized REE patterns show similar features for rims and cores, with both showing a steeply-rising slope from LREE to the HREE, positive Ce-anomaly (Ce/Ce* = 5.1–65) and negative Eu-anomaly (Eu/Eu* = 0.13–0.24) (Fig. 40). However, compared to cores, the rims show relatively low total REE contents. 5.4.3.5. TRM 13-1. Zircons in this sample are colorless, transparent, and euhedral to subhedral. They show prismatic to sub-rounded grain morphology with lengths varying from 80 to 200 μm and a length to width ratio of 1 to 3. Needle shaped apatite inclusions as well as other inclusions can be observed in some grains. In CL images, most of the zircons display distinct core-rim structures, with 60% of the grains showing typical oscillatory zoning or patchy zoning surrounded by a thin luminescent rim, and the rest showing a thick metamorphic rim around the magmatic core (Fig. 21). A total of 32 analyses were performed on different domains of zircons from 29 grains. The Th and U contents are in the range of 20 to 928 ppm and 54 to 721 ppm and Th/U ratios show a large range from 0.08 to 2.15. Excluding the data from 5 grains (spots 2, 9, 11, 16, and 19) which are discordant, 27 of the 32 analyses can be divided into two groups. The magmatic zircons yield ages ranging from 640 to

817 Ma. The data are concordant within analytical error and display a Gaussian-style distribution pattern with a major age peak at 717 ± 8 Ma (MSWD = 0.56) (Fig. 32). The metamorphic zircons yield ages ranging from 468 to 602 Ma with two major peaks at 551 ± 15 and 464 ± 13 Ma respectively. Trace element data from twenty-seven spots including fourteen cores and thirteen rims display obvious fractionated REE patterns between the two domains. The grains are HREE enriched with average (Lu/Gd)N ratios of 16 and 20 respectively for cores and rims, with no significant difference of HREE distributions between the two domains except that rims have lower HREE contents. The major difference is that most of the cores have flat (SmN/LaN = 1–25) patterns in LREE with prominent negative Eu anomalies (Eu/Eu* = 0.018–0.27) and mild positive Ce anomalies (Ce/Ce* = 1–17), whereas the rims show a dentate shape in LREE with distinct positive Ce anomalies (Ce/Ce* = 6–54) and negative Eu anomalies (Eu/Eu* = 0.05–0.16) (Fig. 40).

5.4.3.6. TRM 14-1. The zircon grains in this sample are transparent and dark brownish in color with a length range of 80–150 μm and aspect ratios of 3:1–1:1. Most of the zircons show prismatic to stumpy morphology with tiny ilmenite needles and fluid inclusions. As revealed by CL images, most of the grains display a distinct corn-rim texture, with some of them displaying a dark core with oscillatory zoning or patchy zoning surrounded by a luminescent rim, and some grains showing a dark core with weak zoning surrounded by a thick bright rim. The remaining grains are homogeneous without a core-rim structure and appear to be of metamorphic origin. Few of the grains in this sample display dark cores with clear oscillatory zoning surrounded by two rims, one bright overgrowth around the core and the other a dark thin rim at the periphery (Fig. 21).

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Fig. 13. Selected Harker diagrams of trace elements plotted against SiO2 illustrating geochemical variations of studied rocks from the Southern Madurai Block.

A total of 34 analyses were performed on different domains of 33 zircon grains. The Th contents range from 13.5 to 1095.2 ppm and U from 83.4 to 710.6 ppm, with Th/U ratios in the range of 0.05–1.73 (Supplementary Table 4). Excluding the data from 1 grain (spot 12) which is discordant, 33 of the 34 analyses form two coherent age groups within analytical error. The older age group of magmatic zircons yield ages in the range of 857 to 932 Ma with a weighted mean 206Pb/238U age of 889 ± 13 Ma (MSWD = 3.5) representing the timing of magmatism (Fig. 33). The younger group yields ages ranging from 475 to 527 Ma with a weighted mean age of 510 ± 8 Ma (MSWD = 3.3) corresponding to metamorphism. Thirty-six spots were analyzed for trace element data including twenty cores and sixteen rims of zircon grains. In general, both cores and rims display HREE enrichment and LREE depletion, although the fractionated REE patterns are obvious between the two domains. The rims exhibit almost overlapping REE patterns with positive Ce anomalies (Ce/Ce* = 5–67) and negative Eu anomalies (Eu/Eu* = 0.17– 0.46), with only a slight scatter in La contents (ranging from 10−1 to 101 × chondrite) (Fig. 40). Based on their REE abundance, the REE patterns of cores can be divided into two groups, the first group ranges from 100 to N103 × chondrite with positive Ce anomalies (Ce/Ce* = 1.1–43) and negative Eu anomalies (Eu/Eu* = 0.16–0.58), whereas the second group ranges from 10− 2 to 103 × chondrite with more

pronounced Ce anomalies (Ce/Ce* = 3–29) and less prominent Eu anomalies (Eu/Eu* = 0.52–0.82). 5.4.4. Gabbros 5.4.4.1. TRM 3-3. Zircon grains in this sample are transparent or dark brownish, and range from euhedral to anhedral. They show a size range of 80–200 μm and aspect ratios of 3:1–1:1. In CL images, few of the grains show a core-rim structure. Most of the zircons display weak zoning but some display a distinct banded or patchy zoning and magmatic texture (Fig. 21). A total of 30 analyses were performed on both magmatic core domains and metamorphic rim domains from 29 zircon grains. The Th contents range from 33 to 251 ppm and U contents range from 67 to 335 ppm, with Th/U values in the range of 0.41–0.78. Excluding the data from 1 grain (spot 1) which is discordant, 29 spots show ages ranging from 626 Ma to 965 Ma. Among these, 15 spots form a coherent age group within analytical error (Fig. 34) with 206Pb/238U ages varying from 840 to 965 Ma and yielding a weighted mean age of 903 ± 13 Ma (MSWD = 3.5) corresponding to early Neoproterozoic magma crystallization. Twenty-nine spots were analyzed for trace elements and the chondrite-normalized REE patterns show steeply rising patterns from LREE


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Fig. 14. (a), (c), (e), (g) Chondrite-normalized REE spider plots for samples from the Southern Madurai Block, India. (b), (d), (f), (h) Primitive mantle-normalized multi-element variation diagrams for samples from the Southern Madurai Block, India. Normalizing average chondrite values are from McDonough and Sun (1995), and primitive mantle normalizing values are from Sun and McDonough (1989).

to HREE with (Sm/La)N = 10–600 and (Lu/Gd)N = 14–28. The zircon grains show scattered positive Ce anomalies (Ce/Ce* = 2.2–26.4) and negative Eu (Eu/Eu* = 0.25–0.41) anomalies (Fig. 41). 5.4.4.2. TRM 6-3. Zircons in this sample display euhedral to subhedral morphology and are elliptical to rounded crystals, ranging in length from 80 to 200 μm with aspect ratios of 3:1–1:1. Most grains are transparent or dark brownish. In CL images, the magmatic cores are

surrounded by bright metamorphic rims. The cores generally display oscillatory zoning or patchy zoning. The metamorphic rims are bright and structureless and range up to 50 μm in width (Fig. 22). A total of thirty spots were analyzed in both magmatic cores and overgrowth rims in twenty-seven zircons. The Th contents range from 57 to 2800 ppm and U contents range from 133 to 1199 ppm. The Th/U ratios are in the range of 0.05–2.34. Thirty analyses form three coherent age groups within analytical error with three major

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Fig. 15. Sr–Rb–Ba ternary plot showing the Ba-enriched nature of studied granitic rocks from the Southern Madurai Block, India. Fields of high Ba–Sr and low Ba–Sr granitoids are from Fowler et al. (2001).

peaks (Fig. 35). The oldest group shows ages ranging from 705 to 823 Ma with a weighted mean 206Pb/238U age of 777.6 ± 9.4 Ma (MSWD = 1.2). Another three spots form the second group yielding a weighted mean age of 639 ± 27 Ma (MSWD = 1.8). Both these groups belong to magmatic zircons. The youngest group is defined by metamorphic zircon domains with ages ranging from 510 to 547 Ma and a weighted mean 206Pb/238U age of 532 ± 9 Ma (MSWD = 3.1). Twenty-nine spots in this sample were analyzed for trace elements including ten cores and nineteen rims. The REE patterns increase steeply from LREE to HREE with negative Eu anomalies and positive Ce anomalies especially in the cores (Fig. 41). Compared to the rims whose REE abundance range from 10−2 to 103 × chondrite (Eu/Eu* = 0.26–0.59, Ce/Ce* = 6.48–87.15), the REE patterns of cores exhibit distinctly high total REE contents (range from 100 to 104 × chondrite) with more pronounced Eu anomalies (Eu/Eu* = 0.09–0.47) and weak Ce anomalies (Ce/Ce* = 1.16–10.82) (Fig. 41).

5.4.4.3. TRM 7-2. Zircon grains in this sample are euhedral to anhedral and are transparent or dark brownish. Most zircons display an elliptical to round morphology with a length range from 50 μm to 120 μm and aspect ratios of 2:1–1:1. Some of the grains show a clear core-rim structure, with the magmatic cores possessing patchy zoning and metamorphic rims ranging in width up to 80 μm (Fig. 22).

A total of 30 analyses on both magmatic core domains and metamorphic rim domains from 27 zircon grains show Th and U contents ranging from 41 to 168 ppm and 49 to 692 ppm respectively. The Th/U ratios show a wide range from 0.15 to 1.4. Excluding 8 analyses (spots 10, 11, 14, 21, 22, 27, 29, and 30) which are discordant, 13 of the 30 analyses form an age group within analytical error and a Gaussian-style distribution pattern (Fig. 36). The group yields a weighted mean 206Pb/238U age of 525 ± 11 Ma (MSWD = 3.2). They are identified as metamorphic zircons from their CL images, and represent zircon growth during Cambrian high grade metamorphism in the area. Trace element data were acquired from twenty-nine spots including ten cores and nineteen rims. Their chondrite-normalized REE patterns exhibit positive Ce anomalies, negative Eu anomalies and enrichment in HREE relative to LREE with markedly low La contents. Compared to cores (Ce/Ce* = 2.9–94, Eu/Eu* = 0.27–0.77), the rims show broadly overlapping REE patterns with more prominent Eu anomalies (Eu/Eu* = 0.05–0.18) and weaker Ce anomalies (Ce/Ce* = 3–21) (Fig. 41). The rims are also characterized by relatively high LREE abundance. 5.4.4.4. TRM 11-2. Zircons in this sample are colorless to dark, transparent, and subhedral to anhedral. Most of them display sub-rounded morphology with a length range of 80–100 μm and aspect ratios close to 1:1. In CL image, the grains display weak patchy zoning and an hour glass like texture without any distinct core-rim structure. Most of the grains display distinct homogeneous recrystallization features and only few grains have a core-rim texture (Fig. 22). A total of 30 analyses were performed on different domains of zircons from 30 grains. The Th contents range from 78.4 to 412.3 ppm and U contents range from 418.9 to 884.7 ppm, with Th/U ratios in the range of 0.18–0.52 (Supplementary Table 4). Most of the values are lower than 0.3, suggesting that these grains are mainly of metamorphic origin. Excluding the data from 1 grain (spot 19) which is discordant, 29 of the 30 analyses form a coherent age group within analytical error, and have a Gaussian-style distribution pattern on a probability density plot (Fig. 37). The data yield 206Pb/238U ages varying from 464 to 565 Ma with most ages concentrating between 501 and 565 Ma and yielding a 206 Pb/238U weighted mean age of 534 ± 5 Ma (MSWD = 3.5) corresponding to the timing of metamorphism. Twenty-nine spots of different domains of zircons in this sample were analyzed for trace element data. The chondrite-normalized REE patterns of most grains are characterized by LREE depletion and HREE enrichment with positive Ce-anomaly and negative Eu-anomaly, with a slight scatter in HREE and (Sm/La)N ratios varying from 53 to 85 (Fig. 41). Two spots from metamorphic domains (518 Ma, 528 Ma) display unusual patterns with relatively low total REE contents and absence of negative Eu-anomaly.

Fig. 16. (a) K (wt.%) vs. K/Rb distribution of charnockites from the study area. Samples with K lower than 1.5% are granulites with negligible K-feldspar, which in many cases show Rb depletion (K/Rb N 500). Field of common igneous rocks are after Rudnick and Presper (1990). (b) Sr vs. Rb distribution in the studied samples. Field of common igneous rocks are after Rudnick and Presper (1990).


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Fig. 17. (a) Th/U vs. La/Th distribution of charnockites from the study area showing varying effects of U and Th; (b) Th/Yb versus Nb/Yb discrimination diagrams illustrating the petrogenetic characteristics of the Southern Madurai charnockitic rocks (fields after Pearce, 2008). SZE, CC and WPE indicate subduction, collision and within-plate trends respectively, FC — fractional crystallization. The yellowish color background is given according to the distribution of sample spots. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

5.4.4.5. TRM13-3. Zircon grains from the sample are mostly subhedral to anhedral, with elliptical to round morphology. Some large grains have an irregular shape and anhedral grain boundary. They are colorless to dark brown, transparent to translucent, and range in length from 80 to 200 μm. As revealed by CL images, about half of the grains display a core-rim texture, and the core domains are mostly weak zoned but some of them show patchy zoning and some grains display a bright domain in the core. The other half are homogeneous without a core and rim texture, and they are mainly metamorphic overgrowths displaying bright luminescence domain (Fig. 22). A total of 30 analyses from 30 zircon grains were carried out. Excluding 1 analysis (spot 23) which is discordant, 29 data form four coherent age groups within analytical error and a Gaussian-style distribution pattern with four major peaks (Fig. 38). The first group includes four spots with a weighted mean 206Pb/238U age of 729 ± 10 Ma (MSWD = 0.76). Their Th contents range from 243.6 to 649.1 ppm and U contents show a range of 142.4–357.0 ppm. The Th/U ratios are in the range of 1.61–1.81, suggesting magmatic crystallization. The second group includes five analyses with a weighted mean 206Pb/238U age of 661 ± 21 Ma (MSWD = 3.5). Their Th contents range from 131.1 ppm to

1268.3 ppm and U contents show a range of 123.1–587.7 ppm. The Th/U ratios are in the range of 1.07–2.16, corresponding to magmatic crystallization. The third group includes 7 analyses with a weighted mean 206Pb/238U age of 519 ± 7 Ma (MSWD = 0.79). The Th and U show variable contents ranging from 148.4 to 414.4 ppm and 289.0 to 948.5 ppm respectively. The Th/U ratios show a wide range from 0.2980 to 0.6412. The last group includes 4 analyses with a weighted mean 206Pb/238U age of 482 ± 6 Ma (MSWD = 0.77). The Th and U show variable contents ranging from 244.1 to 307.1 ppm and 356.7 to 718.6 ppm respectively. The Th/U ratios show a wide range from 0.37 to 0.57. The spots belonging to the last two groups represent metamorphic grains. Twenty-nine spots were analyzed in zircons from this sample for trace elements including sixteen rims and thirteen cores. The normalized patterns are characterized by a steeply-rising slope from the LREE to the HREE with positive Ce anomalies and negative Eu anomalies, especially in the cores. Compared to rims whose Eu anomalies vary from 0.12 to 0.39, and (Sm/La)N is in the range of 4.6–118, the cores display more steep patterns in LREE (SmN/LaN = 95–472) with more distinct Eu anomalies (Eu/Eu* = 0.39–0.81) and higher total REE contents (Fig. 41).

Fig. 18. Plots of major element variations for studied rocks in the Madurai Block, India. Each diagram plots the sum of two or more major oxides versus their product. The fields correspond to compositions of melts generated experimentally by dehydration melting of different bulk compositions (explained in the legend) from Patino Douce (1999). (a) Plot of CaO/Al2O3 vs. CaO + Al2O3 (wt.%); (b) plot of (Na2O + K2O)/(FeO + MgO + TiO2) vs. Na2O + K2O + FeO + MgO + TiO2 (wt.%).

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Fig. 19. Cathodoluminescence (CL) images of representative zircons from alkali granite (TRM 6-2, TRM 9-1) and charnockite (TRM 5-1 and TRM 7-4). Small white circle: 207 Pb/206Pb age in Ma; large red circle: εHf(t) value. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

5.5. Zircon Lu–Hf analysis The Lu–Hf analytical data on zircon grains from the magmatic suite of southern Madurai Block are given in Supplementary Table 6. The

Fig. 20. Cathodoluminescence (CL) images of representative zircons from charnockite (TRM 15-1) and enderbite (TRM 2-3, TRM 3-1, and TRM 8-2). Small white circle: 207 Pb/206Pb age in Ma; large red circle: εHf(t) value. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 21. Cathodoluminescence (CL) images of representative zircons from enderbite (TRM 10-1, TRM 13-1, and TRM 14-1) and gabbro (TRM 3-3). Small white circle: 207Pb/206Pb age in Ma; large red circle: εHf(t) value. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

analyses were made on the same spots or immediately adjacent similar domains from where the U–Pb data were gathered. The results from each sample are briefly discussed below.

Fig. 22. Cathodoluminescence (CL) images of representative zircons from gabbro. Small white circle: 207Pb/206Pb age in Ma; large red circle: εHf(t) value. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


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Fig. 23. Zircon U–Pb concordia plots (a) (b) (c) and age data histograms with probability curves (d) for the sample TRM 6-2 alkali granite.

5.5.1. Alkali granites

5.5.2. Charnockites

5.5.1.1. TRM 6-2. Eight zircons were analyzed from this sample and the results can be divided into two groups. One shows initial 176Hf/177Hf values of 0.281752 with a depleted mantle age (TDM) of 2083 Ma and crustal residence age (TCDM) of 2949 Ma, when calculated by U–Pb age of 677 Ma. Their εHf(t) values show highly negative values of −21.4, indicating Mesoarchean reworked components (Fig. 42). Another group of the dominant population shows initial 176Hf/177Hf values of 0.281695–0.281799 with depleted mantle (TDM) in the range of 2020 Ma to 2168 Ma and crustal residence ages (TCDM) in the range of 2790–3018 Ma, when calculated by U–Pb ages of 720–814 Ma. The εHf(t) values show highly negative values of up to − 21.4 suggesting magma derivation from reworked Mesoarchean to Neoarchean components.

5.5.2.1. TRM 5-1. The analytical data from eight zircons in this sample can be divided into two groups. One group shows initial 176Hf/177Hf values of 0.282327–0.282367 with depleted model ages (TDM) ranging from 1253 to 1312 Ma and crustal residence ages (TCDM) of 1588–1701 Ma, when calculated by U–Pb ages of 655–690 Ma. The εHf(t) shows both positive and slightly negative values of −1.8 to 0.4 with an average of − 0.76 (Fig. 42), suggesting mixed juvenile and reworked Paleo- to Mesoproterozoic components. Another five zircon grains show initial 176 Hf/177Hf values of 0.282265–0.282363 with depleted model ages (TDM) ranging from 1252 to 1403 Ma and crustal residence ages (TCDM) of 1543–1724 Ma, when calculated by U–Pb ages of 853–922 Ma. Their εHf(t) values are dominantly positive (0.3 to 5.4), suggesting juvenile Mesoproterozoic source components.

5.5.1.2. TRM 9-1. Eight zircons were analyzed from this sample and the results show initial 176Hf/177Hf values of 0.281714–0.281819, depleted mantle ages (TDM) in the range of 2011–2172 Ma and crustal residence ages (TCDM) of 2764–3023 Ma, when calculated using the U–Pb age of 683–777 Ma. The εHf(t) values are highly negative (−22.1 to −17.6) (Fig. 42), suggesting reworked Mesoarchean to Neoarchean crustal components as the magma source.

5.5.2.2. TRM 7-4. All the eight zircon grains from this sample show initial Hf/177Hf values of 0.282165–0282416. Their εHf(t) values are negative (−10.1 to −1.7; Fig. 42) with an average of −5.29. 176

5.5.2.3. TRM 15-1. Eight zircons from this sample can be grouped into two. The first group of three zircons shows initial 176Hf/177Hf values of 0.282140–0.282205. The εHf(t) values are negative (average − 9.15). Another group of zircons is magmatic and shows initial 176Hf/177Hf

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Fig. 24. Zircon U–Pb concordia plots (a) (b) (c) and age data histograms with probability curves (d) for the sample TRM 9-1 alkali granite.

values of 0.281979–0.282149, depleted model ages (TDM) of 1566– 1778 Ma and crustal residence ages (TCDM) of 2021–2383 Ma, when calculated by U–Pb ages of 770–805 Ma. The εHf(t) values are negative with an average of −7.08 (Fig. 42).

shows both positive and negative values (− 13.1 to 6.7; Fig. 42) with an average of − 0.19, suggesting both mid-Mesoproterozoic juvenile source and reworked early-Paleoproterozoic components.

5.5.3. Enderbites

5.5.3.3. TRM 10-1. Eight zircons from this sample show initial 176Hf/177Hf values of 0.282079–0.282154, depleted mantle ages (TDM) of 1567– 1656 Ma and crustal residence ages (TCDM) of 2009–2187 Ma, when calculated by U–Pb age of 692–840 Ma. The εHf(t) shows negative values in the range of − 8.7 to − 5.0 (Fig. 42; average − 6.50), suggesting that the sources were derived from reworked mid-Mesoproterozoic components.

5.5.3.1. TRM 2-3. Seven zircons from this sample show two groups. One group has initial 176Hf/177Hf values of 0.282367 and the other shows initial 176Hf/177Hf values in the range of 0.282251–0.282438, with depleted model ages (TDM) of 1164–1404 Ma and crustal residence ages (TCDM) of 1227–1695 Ma, when calculated by U–Pb ages of 698–1057 Ma. The εHf(t) shows both positive (up to 10.6) and negative values (Fig. 42) with an average of 4.27, suggesting magma derivation dominantly from both juvenile Neoproterozoic sources with minor reworked early- to mid-Mesoproterozoic components. 5.5.3.2. TRM 3-1. Eight zircons from this sample can be grouped into two. One spot shows initial 176Hf/177Hf values of 0.282120 and the εHf(t) value is negative at −10.4. Another seven magmatic zircons show initial 176 Hf/177Hf values of 0.281968–0.282404, depleted model ages (TDM) in the range of 1211–1821 Ma and crustal residence ages (TCDM) of 1379– 2423 Ma, when calculated by U–Pb ages of 612–945 Ma. The εHf(t)

5.5.3.4. TRM 13-1. Eight zircons from this sample can be grouped into two. One spot has initial 176Hf/177Hf values of 0.282195. The εHf(t) value for this spot is negative (−9.7). The other seven spots from magmatic zircons show initial 176Hf/177Hf values of 0.282122–0.282239, depleted model ages (TDM) of 1468–1648 Ma and crustal residence ages (TCDM) of 1889–2197 Ma, when calculated by U–Pb age of 640–753 Ma. The εHf(t) shows negative values in the range of − 9.9 to − 4.1 (Fig. 42) with an average of −6.78, indicating that the sources were derived from reworked mid-Paleoproterozoic components as the magma source.


Fig. 25. Zircon U–Pb concordia plots (a) (b) (c) and age data histograms with probability curves (d) for the sample TRM 5-1 charnockite.

Fig. 26. Zircon U–Pb concordia plots (a) and age data histograms with probability curves (b) for the sample TRM 7-4 charnockite.

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Fig. 27. Zircon U–Pb concordia plots (a) (b) (c) and age data histograms with probability curves (d) for the sample TRM 15-1 charnockite.

5.5.3.5. TRM 14-1. Eight zircons from this sample fall into two groups. One group includes spots which have initial 176Hf/177Hf values of 0.282170 and 0.282210, together with negative εHf(t) values of −9.7 and − 8.8. The other six zircons from magmatic grains have initial 176 Hf/177Hf values of 0.282125–0.282388, depleted model ages (TDM) of 1241–1572 Ma and crustal residence ages (TCDM) of 1449–2013 Ma, when calculated by U–Pb age of 737–904 Ma. Their εHf(t) shows both positive and negative values in the range of −5.8 to 5.2 (Fig. 42; average of −0.92), suggesting both juvenile mid-Mesoproterozoic sources and reworked mid-Paleoproterozoic components.

in the range of −17.8 to −3.9 (Fig. 42) with an average of −13.14, suggesting reworked Neoarchean to mid-Paleoproterozoic components. 5.5.4.3. TRM 7-2. Seven zircons from this sample show two age groups. One group has initial 176Hf/177Hf values of 0.282188 whereas the other shows a range of 0.282167–0.282235, with depleted model ages (TDM) of 1406–1503 Ma and crustal residence ages (TCDM) of 1845– 1959 Ma, when calculated by U–Pb ages of 694–777 Ma. The εHf(t) shows negative values of − 4.7 to − 3.1 (Fig. 42) with an average of − 4.0, suggesting magma derivation from Paleoproterozoic crustal components.

5.5.4. Gabbros 5.5.4.1. TRM 3-3. Seven zircons from this sample show initial 176Hf/177Hf values of 0.282375–0.282456, depleted model ages (TDM) of 1112– 1217 Ma and crustal residence ages (TCDM) of 1302–1456 Ma, when calculated by U–Pb age of 626–945 Ma. The εHf(t) shows positive values in the range of 1.7 to 7.9 (Fig. 42; average 5.02), suggesting dominantly juvenile late Mesoproterozoic components. 5.5.4.2. TRM 6-3. Seven zircons from this rock show initial 176Hf/177Hf values of 0.281777–0.282243, depleted model ages (TDM) of 1486– 2046 Ma and crustal residence ages (TCDM) of 1889–2816 Ma, when calculated by U–Pb age of 631–823 Ma. The εHf(t) shows negative values

5.5.4.4. TRM 11-2. Eight zircons from this sample show initial 176Hf/177Hf values of 0.282152–0.282517 and εHf(t) shows both positive and negative values (−11.2 to 2.4, Fig. 42; average −6.94). 5.5.4.5. TRM 13-3. Eight zircons from this sample show two age groups. The first group includes four zircons with initial 176Hf/177Hf values ranging from 0.282229 to 0.282339. The second group shows initial 176 Hf/177Hf values in the range of 0.282259–0.282480, with depleted model ages (TDM) of 1213–1401 Ma and crustal residence ages (TCDM) of 1456–1970 Ma, when calculated by U–Pb ages of 619–782 Ma. The εHf(t) shows both negative and positive values of − 4.9–3.6 (Fig. 42) with an average of 0.25, suggesting magma derivation dominantly


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Fig. 28. Zircon U–Pb concordia plots (a) (b) (c) and age data histograms with probability curves (d) for the sample TRM 2-3 enderbite.

from both juvenile Neoproterozoic sources with minor reworked earlyto mid-Mesoproterozoic components. 6. Discussion 6.1. Petrogenesis and source characteristics: inferences from field relations, petrology and geochemistry Our field studies in the southern Madurai Block have revealed a suite of magmatic rocks including gabbros, enderbites, charnockites and alkali granites which, despite high grade metamorphism, preserve their igneous textures and mineral assemblages. Among these, the enderbites and charnockites constitute the dominant basement rocks exposed in this area. Bands and layers of metasedimentary rocks including garnet-, cordierite-, orthopyroxene-, spinel- and biotite-bearing metapelites, garnet-biotite gneisses and quartzites, along with minor calc-silicate rocks occur as accreted remnants mostly along the southern part of the study, which extend into the larger belt of similar rocks in the Achankovil Shear Zone and further into the vast accretionary belt of the Trivandrum Block. The gradational field relations of the enderbites and charnockites suggest that these rocks represent a differentiated suite from the same parent magma, whereas the alkali granites and gabbros seem to have originated from distinct parental melts, with the granites intruding the charnockitic basement and the gabbroic rocks occurring

as enclaves within the enderbite-charnockite suite. In localities like Sivagiri, the relationship between the gabbroic enclaves and host enderbites can be studied well where the former occurs as mafic magmatic enclaves (MMEs) of various sizes and with partly digested boundaries within the more felsic host rock suggesting invasion of mafic magmas into crystallizing felsic magma chambers and resultant magma mixing and mingling as described elsewhere (e.g., Renjith et al., 2014; He et al., 2016a, 2016b). The mineralogical composition of the gabbros with common occurrence of high modal content of hornblende suggests that they crystallized from hydrous mafic melts. Both orthopyroxene and clinopyroxene are present in these rocks in variable proportions. The enderbites and charnockites have broadly similar mineralogical constitution except for the variation in the modal content of plagioclase, K-feldspar and quartz, as well as the presence of clinopyroxene in some of the enderbite samples in addition to orthopyroxene. The alkali granites dominantly composed of perthitic K-feldspar with minor plagioclase and quartz, and the presence of hornblende in these rocks confirm their calc-alkaline nature. The general tholeiitic–calc-alkaline affinity of the charnockitic suite of rocks from the southern Madurai Block are similar to those reported for charnockites and associated rock suites in subduction-related settings (e.g., Rajesh, 2012; Rajesh and Santosh, 2012; He et al., 2016a, 2016b; Yang et al., 2015, 2016; Yang and Santosh, 2015). A common

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feature of the charnockitic rocks is their distinct Nb, Ta, and Ti depletion in mantle-normalized primitive mantle multi-element diagrams, which has been considered to be one of the potential geochemical imprints of continental margin magmatism in a subduction-related environment (e.g., Manikyamba et al., 2014; Yang and Santosh, 2015). However, the Nb and Ta depletion is unlikely to have been produced through a simple crustal melting process and might have been inherited from a subduction process within a continental arc. The charnockites from the southern Madurai Block commonly carry CO2-rich fluids inclusions in quartz and feldspars (our unpublished data). According to Eggler (1987), CO2rich melts are usually enriched in Ba, K and Zr, and this is consistent with the trace element characteristics of the charnockitic rocks investigated in this study. The Ba–Sr contents of our rocks are similar to the Ba–Sr enriched granitoids reported in Tarney and Jones (1994), and broadly akin to adakites. In this respect, the charnockites are different from the traditional I, S and A-type granitoids that usually show low-Ba and Sr concentrations. As in a typical MORB, the precursor always has a deficiency of Ba and Sr, and it is reasonable to envisage a more ‘enriched’ mafic source. The geochemical features may suggest involvement of subducted ocean-island basalts, or continental intraplate basalts, which may reflect more crustal involvement on the one hand, or perhaps mixed phlogopite and hornblende dominated sources on the other.

Geochemical modelling and studies on experimental petrological studies have suggested that the processes that govern the formation of TTGs and their modern analogues of adakites include (i) partial melting of subducted, hydrous, basaltic oceanic crust leaving a garnet-bearing or an eclogitic residue and (ii) melting of basalt at the base of a thickened continental crust, basalt underplating through magmatic processes or underthrust during flat subduction (e.g., Condie, 2005). The adakitic signature and associated geochemical features in the studied rocks are comparable to those generated through low angle, flat Chilean-type subduction and melting of hot, young subducted oceanic slab at an active continental margin setting. This subduction-derived magma underplated the thickened older continental crust and the magmatic underplating supplied heat for melting of lower crustal rocks. The alkali granite-charnockite-enderbite rocks inherited their subductionprocessed, adakitic geochemical attributes through a geochemical exchange between the underplated subduction-derived, adakitic slab melts and lower crustal rocks at a continental arc setting. In general, all the Proterozoic charnockitic rocks show a similar range of enrichment and fractionation of LREE and HREE (e.g., Rajesh, 2012). Variable HREE-depletion may reflect mineralogical differences in the source, where patterns with low-HREE fractionation represent melts from garnet-poor sources at lower pressures and those showing higher-HREE fractionation patterns reflect increasing pressure and proportion of garnet. As indicated by the chondrite normalized REE


29


patterns (Fig. 14), an increasing pressure of the source region from intermediate charnockite to felsic charnockite can be envisaged. The pronounced LREE/HREE and LREE/MREE fractionation trends for the studied rocks suggest melting with garnet in the restite as residual phase. Compared to the Archean charnockites, our Neoproterozoic charnockitic rocks show a wider range of Ti, P and Zr compositions (also see Rajesh and Santosh, 2004; Tomson et al., 2006; Rajesh, 2008) and high TiO2 and/or P2O5 contents of the least evolved samples. At the higher end, these indicate relatively higher melting temperatures (800–1080 °C) (Fig. 11) for the Neoproterozoic charnockites considering the higher saturation temperature for Fe–Ti oxide and apatite. According to Rollinson (1996), the depletion in Rb, as indicated by K/ Rb ratios, is a common feature of granulite-facies rocks. Despite the high-grade metamorphic overprint, the K/Rb ratios (Fig. 16a) of studied samples are similar to common igneous rocks. This indicates that the effect of metamorphic overprint for these samples did not significantly alert the whole rock geochemistry. Thus, the geochemical features of the rocks analyzed in this study reflect the compositions of a protolith before metamorphism. Therefore, source rock characterization of the studied charnockitic rocks can be attempted. A wide range of potential source compositions have been proposed for generating charnockites from melting experiments, including felsic pelites, mafic pelites, metagreywackes, basalts, andesites and amphibolites (Patino Douce, 1999; Kemp and Hawkesworth, 2003). The compositional difference

of partial melts produced from crustal sources, such as amphibolites, pelites, greywackes, and charnockites, are illustrated in Fig. 18. The potential source rocks for the charnockites in the study region and surrounding areas include pelitic gneiss, garnet-biotite gneiss and mafic granulite. Melting of a pelitic source usually produces peraluminous melts (Patino Douce, 1999). However, the studied charnockitic samples from the Madurai Blocks do not display strong peraluminous affinity and most of them are metaluminous. Montel and Vielzeuf (1997) showed that the K2O/Na2O ratio of melts generated from pelitic protoliths is in the range of 4 to 24, which is much higher than that of the studied rocks (0.5–2) (Fig. 12a). Therefore, the melting of felsic or mafic pelites or greywackes is unlikely to have generated the charnockitic suite of rocks in the Southern Madurai Block. Based on amphibolite dehydration melting experiments conducted at 750–1000 °C, over a wide range of pressures, Rapp et al. (1991), and Springer and Seck (1997) have reported that amphibolite melting can produce magmas with composition in the range of tonalite-granodiorite-granite. The charnockitic samples studied are depleted in total alkalis and enriched in CaO, comparable with amphibolite-derived melts; they also plot in the fields related to amphibolite melting (Fig. 18a, b). Thus, the melting of protoliths with basaltic composition corresponding to subducted slab is a reasonable assumption for the formation of the parent magmas of the enderbites and charnockites in the southern Madurai Block. This is also consistent with the report of

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charnockites from some other regions in southern India as reported in Rajesh and Santosh (2004, 2012). The underplated mafic melts intruded as gabbroic dykes and sills during the crystallization of the felsic magmas in higher crustal levels resulting in magma mixing and mingling generating the widespread bands and enclaves of gabbroic rocks within the charnockite suite. The alkali granites were also derived from the differentiation of lower crustal melts, and similar alkali granites from other parts of the Southern Granulite Terrain in India were considered to have formed from the melting of charnockitic lower crustal rocks (e.g., Rajesh, 2004). Therefore, we envisage a possible model for the generation of the studied gabbro-enderbite-charnockite-alkali granite series as follows. At the first stage, the partial melting of subducted slab forms enriched mafic magma, and leads to basaltic magma underplating and vertical crustal accretion. The underplating of basaltic magmas provided the heat input for lower crustal melting generating the charnockitic suite of rocks. In the second stage when the lower crust is partially melted, intermediate charnockites with flat HREE patterns were formed, and subsequently after the underplating has thickened the crust through the garnet-in transition, the more felsic charnockites with garnet as residual phase were formed, resulting in fractionated HREE patterns. The zircon U–Pb data of the alkali granites in our study with the oldest population of magmatic zircons in these rocks showing ages slightly younger than

those in the charnockitic suite are also consistent with the melting of earlier formed charnockites to produce the alkali granites. This is a viable example of crustal accretion and continental growth by subduction process and relamination of subducted slab melts beneath lower arc crust. Crustal thickening can result from mafic magma relamination during subduction (e.g., Maunder et al., 2016), whereas continental outbuilding through crustal accretion is attributed to melting of the lower crust and generation of alkali granite-charnockiteenderbite rocks. 6.2. Zircon geochronology, trace elements and Lu–Hf isotopes: implications and correlations Zircon U–Pb data from the alkali granites yield weighted mean Pb/238U ages of 772 ± 11 Ma and 786 ± 10 Ma for the oldest and the most dominant group of magmatic grains, both ages overlapping within error limits and confirming mid-Neoproterozoic magmatism. A second group of zircons yields a weighted mean age at 662 ± 20 Ma suggesting another distinct thermal event at this time. Both samples carry metamorphic zircons with weighted mean ages of 528 ± 12 Ma and 523 ± 12 Ma marking Cambrian metamorphism. The oldest group of magmatic zircons in the charnockite samples yields 206Pb/238U weighted mean ages of 938 ± 27 Ma, 896 ± 12 Ma, and 786 ± 9 Ma, suggesting multiple magmatic pulses during early 206


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and mid-Neoproterozoic, with the second pulse coinciding with the time of alkali granite formation. The youngest magmatic population shows weighted mean ages of 661 ± 9 Ma and 632 ± 7 Ma, broadly comparable with the ages recorded by the second group of zircons in the granitoids. Metamorphic zircons in the charnockites record weighted mean ages of 567 ± 19 Ma, 537 ± 14 Ma and 515 ± 5 Ma marking high temperatures retained for a period of over 50 m.y. In the enderbites, the magmatic zircon populations yield weighted mean 206Pb/238U ages of 926 ± 22 Ma, 923 ± 36 Ma, 889 ± 13 Ma, 803 ± 10 Ma, 787 ± 23, 786 ± 10 Ma, 748 ± 27 Ma, 742 ± 11 Ma, 717 ± 8 Ma and 692 ± 10 Ma suggesting continuous and multiple pulses of magmas emplaced throughout early to mid-Neoproterozoic. Some of these age peaks also coincide with those recorded from charnockites and alkali granites. The metamorphic zircons in these rocks show 206Pb/238U weighted mean ages of 563 ± 13 Ma, 551 ± 15, 537 ± 9 Ma, 515 ± 10 Ma and 510 ± 8 Ma. One sample (TRM 141) also preserves zircon growth at 464 ± 13 Ma. The majority of metamorphic ages from the enderbites are identical to those from granites and charnockites. Magmatic zircons from the gabbros show weighted mean 206Pb/238U ages of 903 ± 13 Ma, 777 ± 10 Ma, 729 ± 10 Ma and 639 ± 27 Ma, broadly correlating with the multiple magmatic pulses recorded by the other magmatic suites from the SMB. The slight shift to younger

ages with each corresponding pulse of the felsic equivalents is also in accordance with the intrusion of mafic magmas from pulses of underplated basalts through slab melting, subsequent to the construction of the felsic magma chambers and initial crystallization. We envisage that the heat input from underplated mafic magmas would first cause crustal melting and formation of felsic magma chambers, subsequent to which the mafic magmas intrude. Metamorphic zircons in the gabbroic rocks show weighted mean 206Pb/238U ages of 534 ± 5 Ma, 532 ± 9 Ma, and 525 ± 11 Ma, with one younger age at 482 ± 6 Ma similar to that in an enderbite sample mentioned above. An evaluation of the zircon Lu–Hf data from zircons shows that the alkali granite-charnockite-enderbite suite has depleted mantle ages (TDM) in the range of 1164–2172 Ma and crustal residence ages (TCDM) of 1227–3023 Ma, when calculated by their U–Pb magmatic age range of 612–1057 Ma. These spots show both negative εHf(t) and positive εHf(t) values (− 22.1 to 10.6), suggesting magma derivation from mixed juvenile mid- to late Mesoproterozoic components and reworked Mesoarchean to mid-Mesoproterozoic materials. Zircons from gabbroic rocks show depleted mantle ages and (TDM) in the range of 1112–2046 Ma, with crustal residence ages (TCDM) of 1306– 2816 Ma, when calculated by their U–Pb magmatic age range of 619– 945 Ma. These zircons show both negative and positive εHf(t) values (−17.8 to 7.9), suggesting that the magmas were dominantly derived

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Fig. 34. Zircon U–Pb concordia plots (a) and age data histograms with probability curves (b) for the sample TRM 3-3 enderbite.


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Fig. 35. Zircon U–Pb concordia plots (a) (b) (c) and age data histograms with probability curves (d) for the sample TRM 6-3 gabbro.

from juvenile mid-Mesoproterozoic to Neoproterozoic components as well as reworked Mesoarchean to mid-Mesoproterozoic sources. A compilation of the age data from this study is shown in Fig. 43 where three distinct age peaks are seen at ca. 920 Ma and 780 Ma corresponding to major magmatic pulses and at 535 Ma corresponding to metamorphism. Previous studies also reported Neoproterozoic (ca. 1.0 Ga–ca. 0.8 Ga) magmatic ages and latest Neoproterozoic–Cambrian metamorphic ages from both southern and northern domains of the Madurai Block (Fig. 44), with ages from the northern domain dominated by Neoarchean–Paleoproterozoic (2.6–1.9 Ga) basement (Teale et al., 2011; Sato et al., 2011; Plavsa et al., 2012; Santosh et al., 2003; Ghosh et al., 2004). Plavsa et al. (2012) reported charnockite crystallization age of Neoarchean (ca. 2.7–2.5 Ga) and metamorphic age of ca. 535 Ma from the north and north-west domains. However, they observed that the south-east Madurai Block is dominantly Neoproterozoic (ca. 1000 Ma– ca. 780 Ma) with metamorphism during ca. 576–ca.499 Ma. The Kadavur gabbro-anorthosite complex in the northern part of the Madurai Block show similar crystallization age of 829 ± 14 Ma and Paleoproterozoic TDM model age of 2.5–2.3, with the isotopic data suggesting that the magma was sourced from juvenile Neoproterozoic mantle contaminated Mesoarchean crustal components. Sato et al. (2011) obtained a crystallization age of 817 ± 16 Ma from zircons in a plagiogranite. In an earlier study, Santosh et al. (2003) reported ca.

1.7 Ga for the cores of zircon grains and ca. 0.82 Ga for their overgrowth rim and surrounded by younger ages of ca. 0.58 Ga from a biotite gneiss, suggesting at least two distinct major tectonothermal event in Madurai Block. Ghosh et al. (2004) obtained ca. 925 Ma magmatic ages from the xenolith for a granitic gneiss. They also reported ca. 800 Ma for the syntectonic granite in the southern part of the Palghat-Cauvery Suture Zone. Santosh et al. (2009) noted that the wide belt of granitic plutons and igneous charnockite massifs in the Madurai Block shows ages of ca. 750–560 Ma. They proposed a long-lived Neoproterozoic magmatic arc formed through southward subduction, which was further corroborated from interpretation of seismic reflection data and the imbricate layers and mega duplexes, representing an arc-accretionary complex (Naganjaneyulu and Santosh, 2011). Tomson et al. (2006) studied the geochemistry of charnockites from the Madurai Block and suggested crustal contamination through intraplate melting at 0.8 Ga. In a recent study, Li et al. (2016) reported detrital zircon grains in several quartzite samples from the southern Madurai Block, and their results show that apart from the Mesoarchean to Paleoproterozoic cores, the major zircon population shows a wide distribution of Neoproterozoic crystallization ages in the range 950–550 Ma, consistent with the observations in previous studies (Collins et al., 2007b; Kooijman et al., 2011; Teale et al., 2011; Plavsa et al., 2014). Strong evidence was supported by our investigation of the U–Pb age of ca. 920–ca. 780 Ma for the charnockite,

34 M. Santosh et al. / Gondwana Research 45 (2017) 1–42

Fig. 36. Zircon U–Pb concordia plots (a) and age data histograms with probability curves (b) for the sample TRM 7-2 gabbro.


Fig. 37. Zircon U–Pb concordia plots (a) and age data histograms with probability curves (b) for the sample TRM 11-2 gabbro.

Fig. 38. Zircon U–Pb concordia plots (a) (b) (c) and age data histograms with probability curves (d) for the sample TRM 13-3 gabbro.

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Fig. 39. Chondrite-normalized REE patterns of zircons in charnockites and alkali granites.

enderbite, gabbro and alkaline-granite rocks and their geochemistry features. Neoproterozoic magmatic events are also widely reported from the adjacent Gondwana fragment of Sri Lanka from the Wanni Complex (ca. 886 Ma), Vijayan Complex (ca. 960 Ma), Highland Complex (1073–748 Ma) and Kadugannawa Complex (ca. 910 Ma) (Fig. 45) (Hölzl et al., 1994; Santosh et al., 2014; Kröner et al., 1994; He et al., 2016a, 2016b; Baur et al., 1991). Kröner et al. (1994) reported crystallization ages of ca. 1100 Ma–1000 Ma and granulite metamorphism constraint at 580–540 Ma from the Wanni Complex. Similar ages were also identified from the ortho- and paragneisses of ca. 1033–ca. 1016 Ma in the Vijayan Complex (Hölzl et al., 1994). Santosh et al. (2014) reported Neoproterozoic ages of ca. 980 Ma and ca. 916 Ma, and a metamorphic age of 532 Ma from metadiorite and metagranodiorite of the Kadugannawa Complex. The metagranodiorite also carries imprints of two major thermal events at ca. 805 Ma and 734 Ma and metamorphism at ca. 546 Ma in the Wanni Complex. The arc related setting invoked by them for these rocks were substantiated by He et al. (2016a) who studied a suite of hornblende biotite gneiss, charnockites, metagabbro and metadiorite from the Kadugannawa and Wanni Complexes with crystallization ages of 1000–973 Ma, and metamorphism at 570 Ma. In another study, magmatic ages of ca. 966–924 Ma for granitoid gneisses, ca. 772 Ma for clinopyroxenite and ca. 617 Ma for garnet-bearing mafic granulite, with metamorphism at 580–550 Ma, were recorded by He et al. (2016b). The metasediments from Sri Lanka also carry abundant Neoproterozoic magmatic detrital zircons (Takamura et al., 2016; Kröner et al., 1987; Sajeev et al., 2010).

In contrast, the adjacent crustal fragments of Trivandrum Block (ca. 2.5 Ga, ca. 2.0–1.8 Ga), Madras Block (ca. 2.6–ca. 2.5 Ga), and Nilgiri Block (ca. 2.7 Ga, ca. 2.5–2.45 Ga) show distinctly older ages (Bartlett et al., 1998; Santosh et al., 2003, 2006; Ghosh et al., 2004; Samuel et al., 2014; Yang et al., 2015). However, the ca. 550 Ma metamorphism is common in all these terranes. Previously published Nd model ages (TDM) (Fig. 46) show a range of Mesoarchean to late Mesoproterozoic ages for the different crustal blocks in the Southern Granulite Terrane such as in the northern block (ca. 3.5–2.6 Ga), Madurai Block (3.2–2, 1.7, 1.5, 1.3 Ga), and Trivandrum Block (2.9–2.0, 1.7, 1.5–1.2 Ga) (Bernard-Griffiths et al., 1987; Brandon and Meen, 1995; Choudhary et al., 1992; Jayananda et al., 1995; Bartlett et al., 1998; Bhaskar Rao et al., 2003; Cenki et al., 2004; Plavsa et al., 2012; Tomson et al., 2013). The Hf model ages (TDM) from the southern Madurai Block in this study are distinctly younger (2.2– 1.1 Ga), suggesting a magma source from both reworked and juvenile Paleo to Neoproterozoic sources. Brandon and Meen (1995) reported Nd model ages in charnockites from Cardamom Hills in the Madurai Block which show 2.8–2.1 Ga. Nd model ages from cordierite-bearing metasediments of the Kerala Khondalite Belt (Achankovil Unit) are ranging from 3.0 Ga to 2.0 Ga and 1.6–1.3 Ga, and widely spread along this unit (Cenki et al., 2004). Teale et al. (2011) investigated the Kadavur gabbro-anorthosite complex of the northern Madurai Block which shows crystallization ages of 829 ± 14 Ma and Hf model age of 2.5– 2.3 Ga, which is compatible with the Nd model age of 2287 Ma, suggesting that the magma was derived from reworked sources. Charnockites from the northern and north-western domains of the Madurai Block

Fig. 40. Chondrite-normalized REE patterns of zircons in enderbites.


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Fig. 41. Chondrite-normalized REE patterns of zircons in gabbros.

preserve Nd depleted mantle model ages (2.5–3.0 Ga) corresponding to their Neoarchean (ca. 2.7–2.5 Ga) U–Pb magmatic ages. In contrast, magmatic lithologies in the southeastern part of the block possess younger model ages of (1.7–1.4 Ga) and crystallization ages of 1007 ± 23 Ma and 784 ± 18 Ma as reported in the study of Plavsa et al. (2012). The distinction between the northern and southern domains of the block is consistent with magma derivation from a different-aged basement source (Plavsa et al., 2012). The data presented in our study provide additional evidence for both juvenile and reworked components in a continental arc setting. 6.3. Tectonic implications The results from our study reveal multiple magmatic events during early to late Neoproterozoic and extensive crust building and recycling along the southern Madurai Block in India, attesting to significant crustal outgrowth from the earlier Archean-Paleoproterozoic mosaic of the central and northern domains of this large crustal block. Similar Neoproterozoic arc magmatism and crustal growth during the Neoproterozoic have been recorded from the northern margin of the Madurai Block (Teale et al., 2011) as well as in the adjacent Gondwana fragment of Sri Lanka and in East Antarctica (e.g., He et al., 2016a, 2016b; Tsunogae et al., 2015), thus confirming extensive Neoproterozoic subduction-related magmatic activity associated with

Fig. 42. Zircon Hf isotopic evolution diagram of studied rocks from the Southern Madurai Block. CHUR, chondritic uniform reservoir. The corresponding lines of crustal extraction are calculated by using the 176Lu/177Hf ratio of 0.015 for the average continental crust (Griffin et al., 2006).

Fig. 43. Compiled age data histograms of alkali granites, charnockites, enderbites and gabbros from the southern Madurai Block.

Fig. 44. Compiled age data from the northern and southern domains of the Madurai Block from published works (Teale et al., 2011; Sato et al., 2011; Plavsa et al., 2012) and this study. Neoproterozoic crust building events are commonly seen in both the northern and southern domains of the Madurai Block, although Archean and Paleoproterozoic ages dominate in the northern domain.

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the assembly of crustal fragments during the making of the Gondwana supercontinent. In the present case, the occurrence of a widespread Neoproterozoic arc magmatic suite at the southern margin of the Madurai Block adjacent to a wide accretionary belt of quartzites, psammites and pelites along the Achankovil Suture Zone and Trivandrum Block to the south suggests lateral accretion during northward subduction of the Paleoproterozoic basement of the Trivandrum and Nagercoil Blocks. Recent studies have confirmed that the latter two blocks are dominantly composed of Paleoproterozoic orthogneisses with some records of Neoarchean inheritance (Kröner et al., 2015; Liu et al., 2016). This model proposed in this study is in deviation to the previously proposed

southward subduction models (e.g., Rajesh et al., 2013), because such a southward subduction process should have built a Neoproterozoic arc on the southern margin of the accretionary sequence in the Trivandrum Block, adjacent to the northern margin of the Nagercoil Block. However, the identification of suprasubduction zone assemblages from the Achankovil Suture Zone, along the southern margin of the southern Madurai Block by Rajesh et al. (2013) remains as an important and valid finding, and fits well with the northward subduction model proposed in this study. It is possible that the northern and central domains of the Madurai Block were once contiguous with their equivalents in the Trivandrum and Nagercoil Blocks during the Paleoproterozoic as a

Fig. 45. Compiled age data histograms and probability density curves from zircon records in the southern Madurai Block in this study compared with those from the various complexes in the adjacent Sri Lankan crustal block (data compiled from Santosh et al., 2014; He et al., 2016a, 2016b).


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with Neoproterozoic magmatic zircon crystallization ages have also been reported from the southern margin of the Palghat-Cauvery Suture Zone, adjacent to the northern Madurai arc (Santosh et al., 2012). The arc magmatism started during subduction initiation in early Neoproterozoic and continued until ocean closure in late Neoproterozoic, outbuilding the Madurai Block along both its southern and northern margins through both vertical and lateral accretion (Fig. 47). The orogeny culminated with collision of the blocks and high- to ultrahigh-temperature metamorphism during latest Neoproterozoic–Cambrian associated with the final assembly of the Gondwana supercontinent, evidence for which have been reported in various studies including the metamorphic zircons in the present study. 7. Conclusions

Fig. 46. Histograms of crustal residence ages (TDM) for the various crustal blocks in the Southern Granulite Terrane, compiled from the present study and references cited in the text.

unified block within the Columbia supercontinent (Rogers and Santosh, 2002). Rifting during latest Mesoproterozoic–earliest Neoproterozoic resulted in the opening of an ocean basin that possibly stretched from the northern margin of the present Achankovil Suture Zone to the southern margin of the sedimentary belt in the Trivandrum Block, similar to the branch of the Mozambique ocean that divided the northern Archean blocks with the Madurai Block in the Neoproterozoic (Collins et al., 2014). Suprasubduction zone ophiolite sequences

We report widespread magmatic suites in the southern part of the Madurai Block composed of alkali granites, charnockites, enderbites and gabbros. The Nd, Ta and Ti depletion and other geochemical features of these rocks are consistent with arc magmatism in a subduction-related setting. Zircon U–Pb data reveal multiple magmatic pulses from early to late Neoproterozoic, culminating in latest Neoproterozoic–Cambrian metamorphism. Lu–Hf data show that the magmas were derived from both Neoproterozoic juvenile material as well as a range of reworked Mesoarchean to Mesoproterozoic sources. Our data reveal continental outbuilding in the southern domain of the Madurai Block with both vertical addition by magmas through

Fig. 47. Simplified cartoon model to illustrate the Neoproterozoic tectonic evolution of the Southern Granulite Terrane in India. The model envisages that simultaneous rifting occurred between the northern Archean domain and the northern part of the Archean-Paleoproterozoic collage of the Madurai Block, as well as between the southern part of the ArcheanPaleoproterozoic Madurai Block and equivalent basement in the Trivandrum and Nagercoil Blocks at N1.0 Ga opening two branches of the Mozambique ocean. Southward subduction and accretion in the north built the accretionary complex of the Palghat-Cauvery Suture Zone and arc complex in the north (e.g., Teale et al., 2011) and the Achankovil and Trivandrum accretionary sequence and arc in the south (Li et al., 2016; this study). Evidence for Neoproterozoic ophiolites and suprasubduction zone complexes have been reported from the southern part of the Palghat-Cauvery Suture Zone adjacent to the northern margin of the Madurai Block (Santosh et al., 2012), and from the Achankovil Suture Zone adjacent to the southern margin of the Madurai Block (Rajesh et al., 2013), supporting the proposed model.

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relamination and lateral accretion of sediments. We propose a new model of northward subduction of the composite Paleoproterozoic mosaic of the Trivandrum and Nagercoil Blocks to account for the extensive arc-accretionary complex built along the southern margin of the Madurai Block. We correlate these magmatic episodes to the subduction-accretion events associated with the assembly of the Gondwana supercontinent. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.gr.2016.12.009. Acknowledgements We thank Gondwana Research Associate Editor Prof. Sanghoon Kwon and two anonymous referees for their helpful comments. Santosh received support as Foreign Expert at the Northwest University, Xi'an and China University of Geosciences Beijing, China and on a Professorial position at the University of Adelaide, Australia. Partial funding for this project was produced by a Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS) (no. 26302009) to Tsunogae. References Amaldev, T., Santosh, M., Li, T., Baiju, K.R., Tsunogae, T., Satyanarayanan, M., 2016. Mesoarchean convergent margin processes and crustal evolution: petrologic, geochemical and zircon U–Pb and Lu–Hf data from the Mercara Suture Zone, southern India. Gondwana Res. 37, 182–204. Barker, F., 1979. Trondhjemite: definition, environment and hypotheses of origin. In: Barker, F. (Ed.), Trondhjemites, Dacites and Related Rocks. Elsevier, Amsterdam, pp. 1–12. Bartlett, J.M., Dougherty-Page, J.S., Harris, N.B.W., Hawkesworth, C.J., Santosh, M., 1998. The application of single zircon evaporation and model Nd ages to the interpretation of polymetamorphic terrains: an example from the Proterozoic mobile belt of south India. Contrib. Mineral. Petrol. 131, 181–195. Baur, N., Kröner, A., Liew, T.C., Todt, W., Williams, S., Hofmann, A.W., 1991. U–Pb isotopic systematics of zircons from prograde and retrograde transition zones in high-grade orthogneisses, Sri Lanka. J. Geol. 99, 527–545. Bernard-Griffiths, J., Jahn, B.-M., Sen, S.K., 1987. Sm-Nd isotopes and REE geochemistry of Madras granulites, India: an introductory statement. Precambrian Res. 37, 343–355. Bhaskar Rao, Y.J., Janardhan, A.S., Vijaya Kumar, T., Narayana, B.L., Dayal, A.M., Taylor, P.N., Chetty, T.R.K., 2003. Sm-Nd model ages and Rb-Sr isotopic systematics of charnockite gneisses across the Cauvery shear zone, south India, implication for Archean, Neoproterozoic terrane boundary in the Granulite Terrain. In: Ramkrishnan, M. (Ed.), Tectonics of the Southern Granulite Terrain, Kuppam-Palani Geotransect. Geological Society of India, Memoir 50, pp. 297–317. Brandon, A.D., Meen, J.K., 1995. Nd isotopic evidence for the position of southernmost Indian terranes within East Gondwana. Precambrian Res. 70, 269–280. Brandt, S., Raith, M., Schenk, V., Sengupta, P., Srikantappa, C., Gerdes, A., 2014. Crustal evolution of the Southern Granulite Terrane, south India: new geochronological and geochemical data for felsic orthogneisses and granites. Precambrian Res. 246, 92–122. Brown, M., Raith, M., 1996. First evidence of ultrahigh-temperature decompression from the granulite province of southern India. J. Geol. Soc. Lond. 153, 819–822. Cenki, B., Braun, I., Brocker, M., 2004. Evolution of the continental crust in the Kerala Khondalite Belt, southernmost India, evidence from Nd isotope mapping, U–Pb and Rb-Sr geochronology. Precambrian Res. 134, 275–292. Choudhary, A.K., Harris, N.B.W., Van Calsteren, P., Hawkesworth, C.J., 1992. Pan-African charnockite formation in Kerala, South India. Geol. Mag. 129, 257–264. Clark, C., Collins, A.S., Kinny, P.D., Timms, N.E., Chetty, T.R.K., 2009. SHRIMP U–Pb age constraints on the age of charnockite magmatism and metamorphism in the Salem Block, southern India. Gondwana Res. 16, 27–36. Collins, A.S., Clark, C., Sajeev, K., Santosh, M., Kelsey, D.E., Hand, M., 2007a. Passage through India: the Mozambique Ocean suture, high-pressure granulites and the Palghat-Cauvery shear zone system. Terra Nova 19, 141–147. Collins, A.S., Santosh, M., Braun, I., Clark, C., 2007b. Age and sedimentary provenance of the Southern Granulites, South India: U-Th-Pb SHRIMP secondary ion mass spectrometry. Precambrian Res. 155, 125–138. Collins, A.S., Clark, C., Plavsa, D., 2014. Peninsular India in Gondwana: the tectonothermal evolution of the Southern Granulite Terrain and its Gondwanan counterparts. Gondwana Res. 25, 190–203. Condie, K.C., 2005. TTGs and adakites: are they both slab melts? Lithos 80, 33–44. Drummond, M.S., Defant, M.J., 1990. A model for trondhjemite–tonalite dacite genesis and crustal growth via slab melting: Archaean to modern comparisons. J. Geophys. Res. 95, 21503–21521. Drummond, M.S., Defant, M.J., Kepezhinskas, P.K., 1996. Petrogenesis of slab-derived trondhjemite–tonalite-dacite/adakite magmas. Geol. Soc. Am. Spec. Pap. 315, 205–215.

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