Cryogenian volcanic arc in the NW Indian Shield

GR-00746; No of Pages 18 Gondwana Research xxx (2011) xxx–xxx

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Cryogenian volcanic arc in the NW Indian Shield: Zircon SHRIMP U–Pb geochronology of felsic tuffs and implications for Gondwana assembly C.V. Dharma Rao a,⁎, M. Santosh b, c, Sung Won Kim d a

National Disaster Management Authority, A-1, Safdarjung Enclave, New Delhi, India Department of Interdisciplinary Science, Faculty of Science, Kochi University, Akebono-cho, Kochi 780-8520, Japan Geoscience Frontiers, China University of Geosciences, Xueyuan Road, Haidian District, Beijing 100083, China d Geological Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, South Korea b c

a r t i c l e

i n f o

Article history: Received 4 August 2011 Received in revised form 22 October 2011 Accepted 29 October 2011 Available online xxxx Handling Editor: Z.M. Zhang Keywords: Ocean plate stratigraphy Petrology Geochemistry SHRIMP zircon geochronology Cryogenian arc magmatism Gondwana supercontinent

a b s t r a c t The NW domain of the Indian lithosphere witnessed multiple cycles of the birth and demise of ocean basins associated with the history of Proterozoic supercontinents. Here we present field, petrological and geochemical evidence for imbricated ocean plate stratigraphy from the Sindreth Group in Rajasthan characterized by pillow basalt, bedded chert, rhyolite, sandstone and conglomerate suggesting the accretion of oceanic sediments together with continental detritus. The petrographic and geochemical characteristics of basalts and associated rhyolites indicate a volcanic arc setting for the Sindreth Group. The SHRIMP U–Pb analyses of zircons from a siliceous tuff and a rhyolitic tuff intercalated with chert yield well-defined 206Pb/238U concordia ages of 765.1 ± 7.2 (MSWD = 0.76) and 768.2 ± 6.9 (MSWD = 2.3), constraining the timing of arc volcanism associated with mid Neoproterozoic subduction. A younger population of zircons in the rhyolitic tuff yield ages in the range of 666 to 644 Ma, suggesting that the arc magmatism continued to late Cryogenian. Our results correlate the Sindreth Group with the Malani volcanics and we interpret the tuff beds to have been derived from ash flows from a volcanic arc associated with mid Neoproterozoic subduction realm. The data from this study together with those from the recent works in other regions including the Syechelles, Madagascar, southern India and Sri Lanka suggest that Cryogenian magmatic arcs were widely distributed along the margins of the East African Orogen associated with the subduction history of the Mozambique Ocean lithosphere, prior to the final amalgamation of the Gondwana supercontinent in the latest Neoproterozoic–Cambrian. © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction The Neoproterozoic globe witnessed a dynamic period in terms of tectonic, biologic and environmental history of the Earth, associated with the final assembly of the Gondwana supercontinent involving the closure of ocean basins and subduction of oceanic lithosphere along a number of convergent margins (Collins et al., 2007; Meert and Lieberman, 2008; Santosh et al., 2009a, b; Boger, 2011; Johnson et al., 2011). The multiple subduction zones pulled together widely dispersed crustal blocks into a tight assembly during the Neoproterozoic–Cambrian (Santosh et al., 2009a, b). Many of the crustal fragments incorporated within the Gondwana assembly also preserve the history of extensive Paleo- and Mesoproterozoic continental growth (Kaur et al., 2009; Dharma Rao et al., 2010, 2011; Boger, 2011; Santosh, in press). This was followed by Neoproterozoic rifting, formation of wide ocean basins and their eventual closure during a prolonged subduction–

⁎ Corresponding author. E-mail address: [email protected] (C.V.D. Rao).

accretion history prior to the Cambrian collisional event (Santosh et al., 2009b; Boger, 2011; Johnson et al., 2011). The NW margin of Indian plate consists of an Archean basement (Banded Gneiss Complex – Bundelkhand craton) flanked in the west by the Paleoproterozoic Aravalli terrane and Neoproterozoic South Delhi terrane (Fig. 1a). Geological and geophysical studies have identified subduction–accretion history in this region associated with various cycles of the birth and demise of oceanic lithosphere, with distinct evidence for a Late Paleoproterozoic Andean-type continental arc margin (Kaur et al., 2009; Naganjaneyulu and Santosh, 2010; Santosh, in press). Geologic, geochronologic and paleomagnetic data suggest that the Aravalli terrane in Rajasthan (Fig. 1a) bears the signature of a Neoproterozoic subduction–collision orogen (Torsvik et al., 2001; Ashwal, et al., 2002; Gregory et al., 2009; van Lente, et al., 2009). In the Gondwana reconstruction, NW India lies proximal to the East African Orogen (EAO). Magmatism at around 750 Ma is represented in India (Sindreth Volcanics, Malani Volcanics), Madagascar and the Seychelles (Tucker et al., 2001; Ashwal et al., 2002; Kochhar, 2008; Thomas et al., 2009; van Lente et al., 2009), as well as in the Arabian–Nubian Shield (ANS) (Stern and Dawoud, 1991; Johnson et al., 2011).

1342-937X/$ – see front matter © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2011.10.014

Please cite this article as: Rao, C.V.D., et al., Cryogenian volcanic arc in the NW Indian Shield: Zircon SHRIMP U–Pb geochronology of felsic tuffs and implications for Gondwana assembly, Gondwana Res. (2011), doi:10.1016/j.gr.2011.10.014

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C.V.D. Rao et al. / Gondwana Research xxx (2011) xxx–xxx

Fig. 1. (a) Generalized regional geological map showing the Precambrian stratigraphic units of the Aravalli region in NW India with the present study area shown in box (after van Lente et al., 2009). (b) Interpreted geological cross-section corresponding to various geological units of the Aravalli terrane Geological cross section of the Aravalli belt (after Sharma, 1995). Note the high grade metamorphic orogen of Sandmata Complex with westerly dips, incorporating the Phulad ophiolites, correlating with a westward subduction and subsequent extrusion of the metamorphic belt.

Based on the similarity in geochemistry of the ~750 Ma granites, Kochhar (2008) proposed a Malani supercontinent consisting of India, ANS, Madagascar and China. Torsvik et al. (1999) indicated a close similarity in the palaeo-pole position between NW India and the Seychelles during this period. An Achaean basement has been identified in the Affif-Abas terrane of the Arabian–Nubian shield and central Madagascar, and these Archaean components could either be micro-continental blocks (e.g. Azania, after Collins and Pisarevsky, 2005) or the remnant of extensions to the Tanzania, Dharwar or the Marwar Cratons that formed part of the Malani supercontinent. However, geochemical and isotopic data from Madagascar and the Seychelles (Tucker et al., 1999; Thomas et al., 2009) clearly demonstrate these to represent juvenile (oceanic) arcs. The late Cryogenian–Ediacaran (650–542 Ma) history of the Arabian–Nubian Shield (ANS) as synthesized in recent works (e.g., Johnson et al., 2011) also show accretion of island arc terranes prior to the final assembly and amalgamation of the ANS with the

Saharan Metacraton concurrent with the assembly of eastern and western Gondwana. The Aravalli succession in NW India incorporates extensive volcanics such as lavas and pyroclastics including tuffs. The existence of a Neoarchean ocean in the Aravalli Terrane has been speculated, with sedimentation in a shallow water stromatolite-bearing facies in the east and deep water carbonate-pelite facies in the west (Purohit et al., 2010 and references therein). The facies-domains are separated by the ophiolitebearing Rakhabdev shear zone that defines a subduction zone along which the Aravalli basin is considered to have closed at 1.8 Ga (Deb et al., 1989; Sarkar et al., 1989; Sugden et al., 1990; Verma and Greiling, 1995). Typical ophiolite and blueschist association has been reported from the Basantgarh and Phulad areas in the South Delhi terrane (Sinha-Roy, 1984; Khan et al., 2005) and is considered as an indication for a convergent plate boundary. Crustal convergence initiated ophiolite obduction over the arc setting with an east-directed subduction inferred



from the zoning in the ophiolite complex (Khan et al., 2005). The basin closed through subduction along the Kaliguman shear/suture zone running along the contact between the Delhi and Aravalli Terranes (Biswal, 1988; Sugden et al., 1990). The Aravalli terrane, along the western flanks of Aravalli mountain region and further to the west, contains volcano-sedimentary sequences including the Punagarh and Sindreth Groups (Fig. 1a,b) (Chore and Mohanty, 1998). These Groups are considered to represent the final phase of magmatism of the Delhi Supergroup, and define the stratigraphic boundary of the subsequent Malani magmatism. However some authors (e.g., Bose, 1989; Chore and Mohanty, 1998) attribute the Punagarh and Sindreth Groups, presumed to be coeval due to their similar lithologies and structural positioning, to possibly span the time of felsic magmatism of Malani. Roy and Sharma (1999) recognized the close resemblance of Sindreth volcanics with the Malani event in terms of lithology, field relations and the absence of any major deformation. Sharma (1995) reported a co-genetic carapace of felsic volcanics (welded tuff, trachyte, explosion breccia, and rhyolite) and chert inter-bedded with pillow basalt from the Sindreth Group suggesting a convergent margin along which ocean closure occurred. Based on the presence of pillow basalts, possible sea floor alteration of rocks, and the geochemical data that compare well with known back arc basin basalts, the Punagarh and Sindreth groups have been interpreted in the context of a back arc basin (van Lente et al., 2009) closely related to the nearby arc activity observed in the Seychelles islands and northeastern Madagascar, which lay along NW India at about 750 Ma. New U–Pb ages from the Malani rhyolites and the Jalore granite (Torsvik et al., 2001) range between 767 and 748 Ma. It appears that these represent a single pulse of granitic magmatism in the Rajasthan region that is consistent with a continental arc setting (Torsvik et al., 2001). Magmatic activity in the Seychelles and north-eastern Madagascar is attributed to the north-eastward subduction of the Mozambique Ocean (Handke et al., 1999; Tucker et al., 1999; Meert, 2003). Drawing a similarity between the South Delhi Terrane and the components of the EAO, Singh et al. (2010) proposed that the South Delhi Terrane marks a suture zone between the western components including East Africa, Madagascar and the ANS, intervening oceanic arcs such as the Bemarivo Belt of northern Madagascar and the Seychelles, and eastern components including the Dharwar-Marwar Craton and the Aravalli Mobile belt-Bundelkhand Protocontinent. Thus, the Sindreth group and coeval Malani volcanics might be related to the subduction history of the proto-Mozambique Ocean in NW India. The basin closed through subduction, sediments were metamorphosed to granulite facies and the metamorphic orogen was exhumed during the Neoproterozoic collision. The Neoproterozoic terranes, namely the South Delhi Terrane and Sindreth group might extend south-westward and join with similar terranes of the EAO (Vijaya Rao et al., 2000). Elsewhere in Peninsular India, recent studies on the Eastern Ghats Mobile Belt, Southern Granulite Terrane and Aravalli Mobile Belt have recorded various arc-generated crustal blocks during a prolonged subduction history in the Mesoproterozoic prior to the final amalgamation of these blocks during the Neoproterozoic (Biswal et al., 2007; Gregory et al., 2009; Dharma Rao et al., 2010; 2011; Naganjaneyulu and Santosh, 2010). In the north-western region of the Indian plate, the suturing of the Africa-ANS with the Indian Peninsula along the South Delhi Terrane marks a significant event in the build up of Greater India during the Neoproterozoic. In this paper, we provide new evidence for ocean plate stratigraphy associated with Neoproterozoic subduction–accretion tectonics in the NW Indian region. We also present new geochemical data as well as precise SHRIMP U–Pb ages of zircons from volcanic tuffs inter-bedded with cherts from the Sindreth Group in Rajasthan. The data offer important insights into the Cryogenian subduction history of the Mozambique ocean lithosphere along the NW Indian margin, prior to the final amalgamation of the Gondwana supercontinent. Our study has important

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implications on the making of Gondwana supercontinent through subduction–accretion tectonics in the Neoproterozoic prior to the final amalgamation in the latest Neoproterozoic–Cambrian.

2. Geological framework 2.1. Geological background The Sindreth Group (Table 1) of mafic (mainly pillowed) and felsic volcano-sedimentary rocks overlie the Delhi Supergroup and occur along linear isolated basins in the Trans-Aravalli region in Rajasthan (Chore and Mohanty, 1998). The Sindreth Group unconformably overlies the metapelites of the Sirohi Group and is intruded by a set of rhyolite dykes. Gupta et al. (1980) considered the volcanic sequence of the Punagarh and Bambholai region to be part of the Punagarh Group (the youngest component of Delhi Supergroup) and equated them with the Sindreth Group (Table 1). The rocks of the Punagarh and Sindreth Groups comprise subaqueous bimodal volcanics with pillow lava structures, cherts, jaspelites and hydroclastic deposits. Bose (1989) confirmed the bimodal character of the Sindreth and Bambholai rocks and their strong resemblance to the Malani volcanics. Sinha-Roy et al. (1998) considered that Malani volcanism resulted due to a low angle subduction of the Delhi oceanic/transitional crust below the western Marwar craton under distensional tectonic regime. Similarly, Chore and Mohanty (1998) based on their study on rocks of Sindreth group proposed the development of retro-arc basins as a consequence of the subduction of the South Delhi oceanic transitional crust under the Western Marwar continental crust. Roy and Sharma (1999) believed that the Sindreth Group of rocks is related to the Malani event of SW Rajasthan. The deposition of the volcano-sedimentary rocks over Sirohi Group was presumed by these authors to have occurred at ~780 Ma, coinciding with the basin opening marked by basaltic flows and basal conglomerates. Felsic volcanics and arenites overlie these, and the entire assemblage forms a conformable sequence. Chore and Mohanty (1998) suggested that although the Punagarh and Sindreth Groups strongly resemble the adjacent Malani Igneous Province, the two Groups indicate a subaqueous depositional environment unlike the sub-aerial environment of the Malani volcanics. Bhushan (2000) described two continuous episodes of Neoproterozoic magmatism: the first one represented by the syn- to late-tectonic magmatism resulting in Erinpura, Mt. Abu granitoids and Sindreth–Punagarh bimodal volcanics at 900–800 Ma, and the second represented by the ‘anorogenic’ bimodal Malani magmatism, at 750–730 Ma. Sharma (1995) described the undeformed and unmetamorphosed character of the Sindreth rocks and the absence of any penetrative deformation in the Sindreth units. However, Chore and Mohanty (1998), on the basis of minor flow warps and flow fold bands in basalts, assumed a single phase of deformation. In a more recent study, van Lente et al. (2009) interpreted the magmatism in Punagarh and Sindreth Groups in the context of a back arc basin. The similar age data on Trans-Aravalli region rhyolites and granites (770–750 Ma; Torsvik et al., 2001; Gregory et al., 2009; van Lente et al., 2009) and the Mt. Abu granitoids (800–780 Ma; Sivaraman and Raval, 1995) may thus indicate a major Cryogenian magmatic arc in

Table 1 The stratigraphy of Sindreth group.

f

Angor formation

f

Khamal formation

f

Dolerite dykes Porphyry, quartz–granite Conglomerate, bimodal volcanics Vesicular basalt, Ash flows, tuffs Basalt, mud-stone, siltstone Chert, bedded-tuff, pillow basalt Conglomerate, Arkose


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the Trans-Aravalli region during Neoproterozoic. The Cryogenian Neoproterozoic magmatism in NW India can be compared with the equivalent Neoproterozoic arc magmatic provinces in the nearby continental fragments. According to Torsvik et al. (2001), India, Seychelles and northern Madagascar formed a tectonic-trio at least since the assembly of Gondwana (~550 Ma) but their pre-Gondwanan history is less constrained. Ashwal et al. (2002) have proposed an east-directed (present-day coordinates) subduction beneath and associated magmatism into and onto the western margin with resultant magmatism represented in India and Madagascar. Torsvik et al. (2001) and Pandit et al. (2001) have also suggested an Andean-type arc on the western margin of the continental assembly. Paleomagnetic data from Seychelles and Malani yield local paleolatitude of 30°N and 40°N, and a new Malani– Seychelles fit places Seychelles only 600 km away from Malani during mid-Neoproterozoic The sequence of rocks on Seychelles is broadly similar to the Sindreth Group of rocks, and it has been postulated that the Seychelles rocks were sourced from the Archean Banded Gneiss Complex in Rajasthan (Ashwal et al., 2002). Overlapping ages (754–715 Ma) from the Daraina sequence in northern Madagascar are also correlated to the igneous activity in the Seychelles and India (Tucker et al., 1999). Both Madagascar and Seychelles record igneous activity that is attributed to the subduction of the Mozambique Ocean (Fig. 1 inset) Handke et al., 1999; Torsvik et al., 2001; Tucker et al., 2001; Ashwal et al., 2002). This is again interpreted as an Andean-type active margin, closely related to the nearby coeval arc activity observed in the Seychelles islands and north-eastern Madagascar. In a recent study on the Erinpura granite terrane that occurs to the east of Malani in the foreland of the Grenvillian Delhi Fold Belt, Just et al. (2011a, b) identified a close link between shearing and anatectic melt generation (~775 Ma; Th–U–Pb dating of monazites) coeval with Sindreth magmatism (770–750 Ma Torsvik et al., 2001; Gregory et al., 2009). 2.2. Geological background of the Sindreth Group Fig.1a shows a generalized geological framework of the Aravalli– Delhi domain outlining the major geological units and Fig. 1b is a geological cross section of the Aravalli terrane. The lithological association and their nature of distribution are in accordance with a Paleoproterozoic Pacific-type orogeny in the Aravalli region (see Santosh, in press) with a westward subduction of the Archean cratonic margin and development of a wide accretionary belt, imbricated ocean plate stratigraphy including blueschists, ophiolites and the extrusion of a high grade regional metamorphic belt at the orogenic core following the final collision. The NE-SW trending Aravalli terrane is flanked by the Marwar and Mewar Cratons in the west and east, respectively (Fig. 1a). The Mewar Craton forms a promontory structure on the western edge of the Bundelkhand Proto-continent and the Aravalli Mobile Belt warps around it, forming a westerly closing flexure as a result of indentation tectonics, a process comparable to the Himalayan tectonics along the northern edge of the Indian Peninsula. Among the several models of tectonic settings proposed for the Neoproterozoic magmatism in NW India, it has been suggested (see Bhushan, 2000) that the first stage of the associated basaltic and felsic flows was generated by a hot spot source or lithospheric thinning and melting at the base of the crust. However, the available age data on the magmatic suites including the Malani igneous suite (ca. 818–793 Ma, Bhushan, 2000; Murao et al., 2000; 827.0 ± 8.8 Ma, Pradhan et al., 2010) and Sindreth volcanics (765.9 ± 1.6 Ma, van Lente et al., 2009, and our present study) correlate them with Neoproterozoic arc magmatic suites elsewhere in the Gondwana fragments. Paleomagnetic data juxtapose India alongside the Seychelles, and north-eastern Madagascar is also placed along the India margin based on temporal and geological similarities (Ashwal et al., 2002). Both Madagascar and Seychelles record mid Neoproterozoic igneous activity that is attributed to subduction (Fig. 5; Torsvik et al., 2001b, Tucker et al., 2001; Ashwal et al., 2002). The paleoposition of India (at

the western margin of Rodinia) in relation to the ancient supercontinent Rodinia and proto-East Gondwana, based on previous studies, conflicts with the hypothesis of rift tectonics and is more indicative of an Andean-type arc environment resulting from the subduction of the eastern Mozambique Ocean (Meert and Torsvik, 2003). In a more recent study, de Wall et al. (2011) proposed that tectono-magmatic events in the foreland of the Aravalli region (Mt Abu and the present study area of Sindreth) are middle Neoproterozoic (770–750 Ma) in age and the transpressional forces were interpreted to be induced by far field stress transfer related plate convergence and closure of the Mozambique Ocean. 3. Lithological description and petrographic characteristics The Sindreth Group (Fig. 2, Table 1) consists of conglomerate, basalt and pyroclasts, rhyolite, rhyolitic tuffs, volcanic silicic ash, bedded chert, conglomerate and arkosic sediments. On the basis of rock associations, the Sindreth Group is divided into Khamal and Angor formations (Fig. 2) that are inferred to uncomfortably overlie the Sirohi group. The sequence of rock types in both the formations in the study area is briefly described below. 3.1. Khamal formation The Khamal formation consists of two basaltic flows separated by bedded chert and sandstone. 3.1.1. Conglomerate This basal unit forms a north–south trending hill near Sindreth. It is a red colored, polymictic conglomerate (comprising unsorted angular pebbles of carbonaceous phyllite, phyllite, quartz mica schist, quartzite and granite, cemented in siliceous and ferruginous matrix. Fine to medium grained lithic matrix is observed in a few cases. The pebbles are unsorted in the lower part. Upward in the succession, the rock changes to grit, and exhibits graded bedding, a feature that indicates a westward younging direction. The quartz porphyry dykes, intrusive into the conglomerate, are observed at various places. The pebbles in the conglomerate have been drawn from the rocks of the Sirohi Group as well as from the basement granites. The unsorted and sub-angular character of pebbles signifies that the provenance was close by. 3.1.2. Bedded rhyolitic tuff The basaltic flows are separated by green cherty bedded tuff (Fig. 3a) containing angular clasts of quartz and feldspar in chlorite matrix. 3.1.3. Basalt and pyroclastics Basalt occurs in the valley between hills composed of the Sindreth conglomerates and Pamta rhyolites. The basalt lies conformably over the Sindreth conglomerate, but also includes a few lenses of conglomerate as inter-trappean beds. The basalt is fine grained, pillowed (Fig. 3b) with amygdaloidal and vesicular structure with color varying from almost black to dark green; the latter corresponding to epidotisation. Augite and plagioclase feldspar are typical constituents, with glass and ore-minerals forming the accessories. Epidote and chlorite form minor constituents. In some samples, the modal of epidote–chlorite is around 5%. The amygdules vary in size, and are chiefly made up of epidote, calcite, quartz, chalcedony and zeolite. Augite is fine grained and surrounded by plagioclase laths. Opaque minerals, mainly iron sulfides are euhedral to anhedral in shape. Chlorite and epidote occur as alteration products. Field study suggests several phases of volcanic activity. The periods of quiescence are marked by chunks of cherts and jaspelite (Fig. 3c). There are also beds of pyroclastics interlayered with sediments. The pyroclastic rocks are observed in some small stream cuttings and are mostly volcanic tuffs and agglomerates. The tuff is



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Fig. 2. Geological map of the study region showing the linear rift basin structural setting and ocean plate lithologies of the Sindreth group (after Sharma, 1996).

light green in color, fine grained and shows a finely bedded character. Beds strike north south and have a moderate westerly dip. Agglomerates are observed locally to the west of Khamal. The rock is fine to coarse grained, red colored and includes angular fragments of basalts. 3.1.4. Rhyolite The rhyolite at Pamta, constitutes the highest physiographic feature in the form of a north–south running hill, known as Pamta hill. The rhyolite conformably overlies the basalt. The rock is fleshy red in color, finegrained, showing phenocrysts of quartz and plagioclase. At places, it shows ignimbritic texture of quartz and feldspar, indicating welding of these grains at high temperature. In thin sections, the rock shows a fine grained microcrystalline or glassy groundmass enclosing phenocrysts of quartz and plagioclase feldspar. Quartz is anhedral in shape and sometimes exhibits well rounded corroded margins and fracturing. Feldspar (oligoclase) grains are anhedral and subhedral in shape, and shows lamellar twinning. Chlorite, epidote and calcite occur as secondary minerals. In the southern part of the Pamta hill, light pink colored variety of the rhyolites is exposed. 3.1.5. Felsic rhyolitic tuff The felsic rhyolitic tuff is banded, fine grained (maximum grain size less than 1 mm) (Fig. 4a) and consists of rounded and embayed quartz, chlorite, abundant opaque oxides and minor plagioclase and K-feldspar in a fine grained quartz-rich ground mass matrix. Primary magmatic textures are well preserved in the tuffs. 3.1.6. Bedded chert and siliceous tuff Bedded chert and siliceous tuff interstratified with siltstone (Fig. 4b) overlie the basic-volcanic flows towards northwest of Sindreth. These basic volcanic flows contain pillow lava with conjugate pair of open cracks partly filled with quartz and chert and slickenside surfaces (Fig. 4c). The siliceous tuff is gray colored (color index ~ 10) fine-grained (grain size b 1 mm). The rock contains plagioclase, quartz, K-feldspar fragments and abundant carbonates and opaque oxides in a fine grained

matrix. Layers of lighter and darker minerals are generally noticed. The average grain size of the minerals is 1 mm. 3.1.7. Arkose Well-bedded sandstone, arkose and mudstones showing sedimentary features are exposed in the western part of the study area. The total thickness of the unit is about 100 m, with individual beds up to 15 cm or more in thickness. Graded bedding has been observed in some thicker beds of sandstone having coarser clasts at the base, which gradually become finer at the top. In some cases, flat pebbly conglomerates occur at the top of beds. There are partings of shales separating folded turbidity beds of arkose (Fig. 5a). The arkose is pink colored, consists chiefly of quartz, altered feldspars, and fragments of granite. In some cases, smaller fragments of these minerals and rocks make up the matrix. The grains are generally well rounded and sorting is relatively poor. 3.2. Angor formation The Angor formation consists mainly of felsic volcanics and bedded chert, ash flow tuff, rhyolite, ignimbrite and conglomerate that are restricted to N–S trending ridges. 3.2.1. Rhyolite Rhyolite in Angor forms a small outcrop at the northern extremity of the Pamta hill near the village Angor, conformably overlying the Pamta rhyolite. It is a green colored, fine grained, silica-rich ultrapotassic rock in which quartz and feldspar occur as phenocrysts and appear as large anhedral grains with smoothly corroded margins. Orthoclase feldspar is subhedral to anhedral. The groundmass is made up of cryptocrystalline to microcrystalline material as well as glass. 3.2.2. Bedded chert-mudstone A sedimentary sequence of layered chert alternating with mudstone (Fig. 5b) with cracks is present at Angor. Laminated mudstone shows sub-angular silt-grade quartz, muscovite flakes oriented parallel to lamination and organic-clay.


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Aqueous conditions during ash deposition caused chertification of the siliceous material. Under microscope the rock shows a fine groundmass of glassy to microcrystalline materials with a few phenocrysts of quartz, orthoclase and plagioclase. In some cases, the individual grains are angular. The ash-flow tuff is coarsely porphyritic and contains large tabular phenyocrysts of quartz, perthite and feldspar in a glassy ground mass. 3.2.5. Angor conglomerate This is a transgressive conglomerate deposited during the concluding stage of the Sindreth basin. It is a polymictic, yellowish colored conglomerate, and in no way has any resemblance with the Sindreth conglomerate. It is composed of poorly sorted, quartz pebbles and granitic fragments. 3.2.6. Porphyry Feldspar porphyry is light pink to gray colored dyke and has feldspar phenocrysts in fine grained groundmass. These rocks consist of plagioclase feldspar, orthoclase and quartz as essential minerals. Chlorite and calcite occur as secondary minerals. These dykes vary in thickness from 2 to 15 m. The maximum length is approximately 4 km. Quartz porphyry is a greenish looking fine grained rock, showing phenocrysts of quartz. The silica percentage is higher (~80wt.%) in these rocks, which gives them compactness. The dyke intrudes the Sindreth conglomerate and Angor rhyolite.

Fig. 3. Field photographs of the major rock types in Sindreth. (a) Two aggregated tuff beds inter-layered with chert beds and shale. Note the difference in rheology between the relatively competent chert which is resistant to weathering, and the relatively incompetent (squishy) broken layers of shale that is more easily etched away (pen for scale). (b) Typical sea-floor pillowed basalt showing finer rinds around the pillowed center thought to have been caused by chilling and solidification at the contact with seawater when the rocks erupted on the floor of the ocean. This outcrop example of pillow basalt is part of the large mass of pillow basalts that wrap around the Bambholi formation in Punagarh. Pencil for scale. (c) Chunks of red chert nodule cut by quartz and jaspelite veins (hammer for scale). Note the slickensides on chert bedding surface.

3.2.3. Composite felsic and mafic volcanics This sequence of felsic and mafic volcanics consists predominantly of vesicular basalt and basic tuff with corroded plagioclase crystals in altered glassy groundmass. 3.2.4. Ash-flow silicic tuff Conformably overlying the Angor rhyolite is a thick (about 400 m) formation of ash flow silicic tuff. The outcrop runs for several kilometers northward from Angor. The formation comprises a number of units and the thickness of individual units varies from a fraction of a cm to about 20 cm. These are well bedded rocks separated from one other by thin layers of clayey material. Penecontemporaneous structures like folding, faulting, disrupted bedding, intraformational cherty conglomerates etc. are well preserved in these rocks.

3.2.7. Petrography The Sindreth basalts are typically dark greenish to black fine grained rocks characterized by the presence of vesicular structures. In thin sections, these rocks are composed of plagioclase laths mostly altered to sericite, clinopyroxene (altered to chlorite), chlorite, epidote and iron oxides (mostly ilmenite and magnetite) in a fine grained ground mass consisting sometimes of devitrified glass. Relict olivine occurs as rounded grains along with a mixture of micro-phenyocrysts and large lathshaped phenocrysts of plagioclase (Fig. 6a). No K-feldspar has been observed in the studied sections. The vesicles commonly observed in the basalts are filled with quartz and calcite and minor amounts of chlorite. The Sindreth rhyolites are light brown colored, coarse grained and consists mainly of quartz, plagioclase, K-feldspar, secondary chlorite and opaues. Some samples show embayed quartz and K-feldspar subhedral phenocrysts in fine-grained groundmass (Fig. 6b). Bedded chert at Angor is mixture of fine-grained to cryptocrystalline silica and granular carbonate, with traces of iron oxide. Microcrystalline quartz is the main pervasive component in virtually all of the studied chert samples, but angular fragments of accessory minerals that are disseminated within the siliceous groundmass are distinctive (Fig. 6c). The siliceous tuff is gray colored, fine grained, usually bedded and consists of quartz, plagioclase and K-feldspar in a fine grained quartz matrix. Greenish chert tuff consists of angular clasts of quartz and feldspar in chlorite-rich matrix (Fig. 6d). Under microscope arkose shows a fine groundmass of glassy to microcrystalline materials with a few phenocrysts of quartz, orthoclase and plagioclase. In some cases, the individual grains are angular. The ash-flow tuff is coarsely porphyritic and contains large tabular phenocrysts of quartz, perthite and feldspar in a glassy ground mass. 4. Whole rock geochemistry 4.1. Analytical methods Typically 5–10 kg of rock samples (Fig. 2) from Angor formation showing the widest possible variation in modal mineralogy have been analyzed for major and trace element. Cleaned chips were powdered to ~200 meshes using a steel jaw crusher and a ring mill. Atomic absorption spectrometry determined SiO2, Al2O3, TiO2, Na2O, and K2O, while FeO was determined by a volumetric method. Analytical precisions for major and minor oxides range between 2 and 4% RSD, while those for



7

Fig. 4. (a) Planar stratified rhyolitic tuff and lapilli tuffs. Note thin-tuff beds indicating suspension deposits (hammer for scale). (b) Siliceous tuff interstratified with chert beds overlying altered mafic pillow units. (c) Pillow lava with conjugate pair of open cracks partly filled with quartz and chert (hammer for scale).

trace elements range up to 7%. Whole rock chemical analysis including major and trace elements of 5 basalts, 7 rhyolites and 6 feldspar porphyry are given in Table 2a. Combining data from the present study and those from a previous study (van Lente et al., 2009) the rock compositions are plotted in variations diagrams (Figs. 7 and 8). In addition 3 most representative tuff samples were analyzed at ACME Analytical

Labs Ltd in Vancouver, BC, Canada using inductively coupled plasmamass spectrometry (ICP-MS). Major, trace and rare earth element data was obtained with an analytical uncertainty of less than 1%. International standards were used to monitor the quality of analyses throughout the analytical processes for ICP-MS. The whole rock analyses of the tuffs are presented in Table 2b. 4.2. Results

Fig. 5. (a) Turbidite layers–coherent layers of arkose and sandstone that have been compressed into an anticlinal fold. (b) Quartzite clasts and cracked siliceous mudstone beds inter-stratified with chert.

All the analyzed samples are relatively fresh as examined under the microscope. The SiO2 content in Sindreth basalts range from 51.34 to 52.69 wt.% (average 52.03 wt.%, n = 5). In the Harker plots most of the major elements (MgO, Fe2O3t, TiO2, MnO, CaO and P2O5) show an inverse relationship with SiO2 except K2O and Na2O which show an increase (Fig. 7). The normative compositions of Sindreth basalts are dominantly olivine tholeiite, although a few are nepheline and quartz normative. The intrusive rhyolites show much higher SiO2 and wider range (71.97 to 81.86 wt.%; average 76.57 wt.%, n = 7). The samples show systematic decrease in MgO, Fe2O3t, TiO2, MnO, and CaO with increasing SiO2, whereas Na2O and K2O exhibit covariance (Fig. 7). The feldspar porphyry shows moderate SiO2 ranging from 65.43 to 72.90 wt.% (average 71.18 wt.%, n = 6). The samples show systematic decrease in MgO, Fe2O3t, TiO2, MnO and CaO against increasing SiO2, whereas Na2O and K2O exhibit covariance Fig. 7). The trace element variations of Sindreth basalts are as follows (in ppm): Rb= 7–56 ppm, Sr = 164–371, Ba= 29–433 Li = 20–80 and Zn = 87–152. The Harker diagrams show covariance for SiO2 and Rb and no apparent relationship with other trace elements (Fig. 8). The trace element variations of Sindreth rhyolites are as follows (in ppm): Rb = 237–374, Sr= 8–35, Ba= 79–450, Li= 2–17 and Zn = 2–29. The Harker diagrams show covariance for SiO2 and Rb and no apparent relationship with other trace elements (Fig. 8). The trace element variations of Sindreth porphyry rocks are as follows (in ppm): Rb = 186–450, Sr = 5–132, Ba= 14–943, Li = 4–30 and Zn = 8–94. The Harker diagrams show covariance for SiO2 and Rb and no apparent relationship with other trace elements (Fig. 8). The geochemical characteristics of


8


Fig. 6. Photomicrographs of thin sections of main rock types of the Ocean Plate Stratigraphy in Sindreth. All photos in crossed nicols. (a) Khambal formation basalt. (b) Angor formation rhyolite. (c) Bedded siliceous chert from Angor formation. (d) Siliceous tuff showing angular grains.

the Sindreth basalts are similar to those reported from Sindreth by van Lente et al. (2009) (Figs. 7 and 8) and are also comparable with the supra-subduction zone basalts reported from fore-arc settings. The petrological and geochemical characteristics of the major lithological units in Sindreth suggest that these rocks represent the remnants of arc magmas probably within a supra-subduction zone tectonic setting. In the SiO2 versus FeO⁎/MgO plot of Miyashiro (1974) all the basalt samples plot in the tholeiite field (Fig. 9a). In the Al–Fet + Ti–Mg cation

triangular plot, all the samples except one plot in the high-Fe tholeiite basalt field (Fig. 9b). In the TiO2–10xMnO–10xP2O5 tectonic discrimination diagram, the majority of basalts plot within the Oceanic Island Arc Field (OIA) field (Fig. 9c). In the FeO–K2O + Na2O–CaO plot (Fig. 9d) the Sindreth basalts plot into the fields of subduction-related arc magmas. Tuffs from Sindreth may be classified as rhyolites on TAS (Fig. 10a, Middlemost, 1994) and Nb/Y–Zr/Ti (Fig. 10b, Pearce, 1996) classification diagrams. In general, these felsic rocks are less altered than the

Table 2a Whole-rock major (wt.%) and trace elements (ppm) of Sindreth rocks. Source: Sharma, 2005. Sample

1

Rock

Basalt

SiO2 TiO2 Al2O3 Fe2 O3 FeO MnO MgO CaO Na2O K2O P2O5 Sum Sr Rb Ni Co Cr Li Ba Cu Zn

51.3 1.4 15.1 4.62 7.48 0.16 5.38 4.75 3.51 0.73 0.78 95.2 296 37 57 17 25 54 148 16 130

2

3

4

5

6

7

8

9

10

11

12

Rhyolite 52.3 1.45 15.2 6.76 6.12 0.11 4.89 6.1 2.6 0.7 0.79 97 371 35 65 16 26 52 168 25 97

51.9 1.25 14.9 2.54 9.4 0.17 5.28 6.93 1.66 1.19 1.03 96.2 217 54 68 14 26 70 169 17 133

52 1.42 14.1 2.53 10.6 0.16 5.29 7.54 1.8 0.83 0.69 96.9 321 56 84 15 31 80 433 24 152

52.7 1.01 16.1 3.07 9.6 0.13 3.65 4.79 6.26 0.02 0.38 97.7 164 7 37 13 23 20 29 25 87

68.18 0.29 13.92 4.39 1.8 0.01 0.64 1.54 3 6.73 0.18 100.68 51 154 15 7 9 8 154 4 27

13

14

15

16

17

18

66.2 0.18 14.3 3.22 2.21 0.04 0.42 1.01 4.31 7.86 0.07 99.9 47 270 32 11 3 5 181 9 48

71.12 0.18 13.78 2.07 2.16 0.03 0.27 0.61 2.47 7.17 0.16 100 65 290 15 8 5 16 220 8 94

71.46 0.19 13.86 2.2 1.84 0.05 0.11 0.81 3.65 6.01 0.05 100.2 41 276 17 10 18 11 92 9 65

80 0.05 13.1 1.05 0.16 0.38 0.38 0 0 3.88 0.13 99.1 5 450 17 5 10 23 14 9 8

65.43 0.46 14.27 3.59 4.8 0.07 0.54 3.06 2.19 6.03 0.31 100.8 132 186 25 10 15 30 943 12 69

Porphyry 71.97 0.15 12.6 2.94 0.6 0.01 0.12 1.69 3.28 7.01 0.04 100.4 21 282 15 6 15 7 382 5 9

74.6 0.1 12.7 2.05 0.32 0.01 0.14 1.26 2.85 6.33 0.02 100 14 374 9 5 12 7 79 9 2

75.24 0.1 12.84 1.68 0.2 0 0.2 1.26 3.12 5.63 0.04 100.3 35 237 12 8 16 2 293 18 10

77.9 0.1 10.8 1.84 0.4 0.05 0.08 0.13 0.11 7.75 0.0 99.1 10 242 15 12 15 13 450 7 15

81.9 0.14 10.3 0.98 0.56 0.01 0.1 0.04 0.04 5.1 0.0 99.1 9 245 19 7 19 16 276 7 17

77.9 0.09 11 2.49 0.56 0.02 0.31 0.36 0.05 6.35 0.0 99.1 8 295 11 4 18 17 258 8 29

72.9 0.17 14.18 1.54 1.64 0.01 0.32 0.71 3.58 5.79 0.09 100.93 32 238 25 9 2 4 102 6 21



9

Fig. 7. Harker diagrams showing variations of major oxides with silica in Sindreth basalts, rhyolites and porphyry.

basalts and rhyolites having LOI values less than 1.5 wt.%. For the most part, the REE show uniform pattern shapes, with variable but prominent negative Eu anomalies (Fig. 10c); the patterns are strikingly similar to the REE patterns felsic volcanics from Sindreth (Fig. 10c). Normalized

trace element concentrations for the tuffs show high variability with Ba, Nb, Sr and Ti troughs (Fig. 10 d), and cannot be clearly correlated with any of the major types of basalt or rhyolites. For this and other reasons we choose not to attempt tectonic discrimination based on

Fig. 8. Harker diagrams showing variations of selected trace elements with silica in Sindreth basalts, rhyolites and porphyry.


10


Table 2b Whole rock major (wt.%) elements of Sindreth rocks. Sample # KK-17 KK-18 KK-19

SiO2 80.52 82.02 80.44

TiO2 0.14 0.15 0.13

Al2O3 9.56 9.04 9.67

Whole rock trace elements (ppm) of Sindreth rocks Sr Rb Ni Co 11.6 296.5 20 9.2 12.6 251.3 20 15.1 12.3 300.0 20 10.7

Fe2O3 1.64 1.39 1.72

Ba 317 356 318

Whole rock rare earth elements (ppm) of Sindreth rocks La Ce Pr Nd Sm 89.4 155.7 20.07 74.6 14.13 85.4 159.4 19.03 68.1 12.61 101.5 175.3 22.64 82.1 15.82

EeO

MnO

MgO

0.00 0.00 0.00

0.01 0.01 0.01

0.28 0.19 0.28

CaO 0.30 0.09 0.30

Na2O 0.23 0.32 0.23

K2O 5.66 5.66 5.71

P2O5 0.01 0.02 b0.01

LOl 1.5 1.0 1.4

Sum 99.85 99.89 99.89

Pb 4.0 3.5 4.1

Cs 1.8 1.4 1.8

Hf 6.2 6.6 7.7

Nb 18.3 18.7 16.8

Ta 2.0 2.4 2.2

Th 43.8 45.4 47.2

U 5.1 4.1 5.7

Zr 181.0 191.5 203.0

V 66.3 62.9 71.6

Eu 0.81 0.71 0.90

Gd 12.20 10.83 13.71

Tb 2.03 1.86 2.25

Dy 11.98 10.60 12.70

Ho 2.24 2.03 2.40

Er 6.36 5.98 7.02

Tm 0.97 0.92 1.04

Yb 6.13 5.66 6.51

Lu 0.86 0.80 0.91

geochemistry. Fig. 11 shows comparative major versus trace element plots of Sindreth basalts of present study with arc-related Sindreth basalts and coeval Punagarh basalts of previous study (van Lente et al., 2009). The comparable plots of Sindreth basalts of the present study with that of arc-related basalts of Sindreth and Punagarh further confirm the arc nature of the Sindreth basalts.

5. SHRIMP zircon U–Pb geochronology 5.1. Methods Zircons were separated from two samples (siliceous tuff SS-22 and rhyolitic tuff SS-4) using a water table, heavy liquids and a

Fig. 9. Geochemical classification diagrams and tectonic discrimination diagrams of Sindreth basalts. (a) SiO2 versus FeOt/MgO plot of (Miyashiro, 1974). (b) FeO–MgO–Al2O3 diagram. (c) TiO2–10xMnO–10xP2O5 diagram showing the majority of basalts plotting within the Oceanic Island Arc field. (d) FeO–K2O + Na2O–MgO plot showing the majority of the analyzed samples plot in the non-cumulate Arc field. Symbols are the same as in Fig. 6.



11

Fig. 10. Geochemical classification diagrams and tectonic discrimination diagrams of Sindreth tuffs. (a) TAS diagram (Middlemost, 1994) showing plot of tuff samples in the field of rhyolite. (b) Nb/Y–Zr/Ti plot (Pearce, 1996) classification diagram showing the plot of tuff samples in the rhyolite field. (c) Chondrite normalized REE patterns of tuffs showing uniform patterns and a prominent negative Eu anomaly (normalization values from Boyonton, 1984). Also shown are the field REE patterns of felsic volcanics from Sindreth from previous study (van Lente, et al., 2009). (d) Normalized multi-element diagram of Sindreth tuffs.

magnetic separator. The grains were mounted in epoxy and polished to approximately half thickness. The hand-picked zircon grains were mounted in epoxy with zircon standards SL13 (U = 238 ppm) and Temora ( 206Pb*/ 238U = 0.06683) or FC1 ( 206Pb*/ 238U = 0.1859) and polished to expose cross sections for analysis. Before analysis, the grains were photographed under an optical microscope, and their internal zoning was imaged by a cathodoluminescence (CL) using a JEOL 6610LV scanning electron microscope at the Korea Basic Science Institute (KBSI), Ochang Campus, South Korea. The zircon was analyzed for Pb– Th–U isotopes using the SHRIMP II ion microprobe at the KBSI. The instrumental conditions and data acquisition procedures were similar to those described by Williams and Claesson (1987) and Williams (1998). The Pb isotopic compositions were measured directly, without correction for the small (ca. 2‰/amu) mass-dependent fractionation. Corrections for much larger inter-element fractionations were made by reference to the Temora or FC1 standard using a power-law relationship between Pb+/U+and UO+/U+. For most analyses plotted, common Pb contents were estimated using 204Pb. Concentrations of Pb, U, and Th were calculated with reference to SL13. Each analysis consisted of five scans through the Zr, Pb, U, and Th species of interest and took about 9 min. Uncertainties are listed in the data table and are plotted on the concordia diagrams; they are 1σ and include the measurement errors and those in the common Pb corrections. The uncertainties in the mean 206Pb/238U ages are the 95% confidence limits (tσ, where ‘t’ is Student's t), and include

those in the Pb/U calibration for each analytical session (0.25–0.40%). Ages were calculated using the constants recommended by the International Union of Geosciences (IUGS) Sub-commission on Geochronology (Steiger and Jager, 1977). CL images of zircons from the two samples are shown in (Figs.12 and 14) SHRIMP U–Pb analytical results are listed in Table 3 and are illustrated in concordia plots in (Figs.13 and 15). 5.2. Results 19 zircons grains recovered from the siliceous tuff (sample SS-22) are typically medium grained (50–200 μm diameter), and most of them show euhedral crystal morphology preserving faces and interfacial edges. The prismatic grains and grain fragments display internal structure characterized by oscillatory or sector zoning (Fig. 12) consistent with a magmatic crystallization history (e.g., Rubatto, 2002; Corfu et al., 2003). No overgrowths or metamorphic zircons with internal structures were observed. The zircons show moderate U values (57–753 ppm), and variable Th (5–267 ppm) contents. Except for a single grain with low Th and younger age (spot 9.1, Table 3), all other zircon grains in this sample show high Th/U values consistent with a magmatic origin (>0.5). Nineteen spot analyses were made on 19 zircon crystals, among which seven are concordant and define a weighted mean 206Pb/238U age of 765.1 ± 7.2 (MSWD= 0.76; Fig. 13a,b), which we interpret as the timing of magmatic crystallization of the zircons


12


Fig. 11. Comparative SiO2 versus Rb, Ni, Ba and Sr plots of Sindreth basalts of present study with Sindreth basalts and coeval Punagarh basalts of previous study (van Lente et al., 2009).

and broadly corresponding to the deposition of the rhyolite tuff. Concordant Neoarchean ages are displayed by few grains which have relatively CL bright cores. These, and the highly discordant Paleo- and Mesoproterozoic ages from some of the grains reflecting variable Pb loss during subsequent thermal events, are considered to be xenocrysts entrained by the magmas from the country rocks during their ascent and eruption. 17 zircons from the unfoliated rhyolite tuff (SS-4) are also generally euhedral prismatic grains or grain fragments typical of those from volcanic rocks. They range in size from 50 to 200 μm in diameter, and are relatively less CL-bright as compared to those in sample SS-22. The grains display either fine-scale concentric zoning or broad bands typical of magmatic crystallization (Fig. 14). Un-zoned domains locally occur at grain terminations, where they truncate banding. No overgrowth textures were observed in any of the grains. The zircons from sample SS-4, in comparison with those from sample SS-22, show higher and variable U (177–3367 ppm), and Th (115–1360 ppm) contents. The Th/U values are also higher with a lesser variation (0.34–0.67; average 0.50; Table 3) suggesting a magmatic crystallization history. Seventeen spot analyses were made on 17 zircon crystals. Fourteen of them define a tight cluster on the concordia and define a weighted mean 206Pb/238U age of 768.2±6.9 (n=14, MSWD=2.3) (Fig. 15). This age is interpreted as the timing of magmatic crystallization and deposition of the rhyolite tuff. The remaining three analyses show slightly younger (666–644 Ma) spot ages, and also fall on the concordia. All the zircons in this sample yield mid-late Neoproterozoic ages, with no xenocrystic discordant zircons of older ages. The U–Pb age data from the two samples correspond to the timing of zircon crystallization in the felsic tuffs, which in turn provide the timing of arc magmatism associated with the Neoproterozoic

subduction system in this region. The younger population of zircons in the rhyolitic tuff yielding ages in the range of 666 to 644 Ma suggests that the arc magmatism continued until late Cryogenian. 5.3. Previous geochronology of MIS Previous geochronologic results from the Malani felsic volcanics show ages in the span of about 100 million years (Table 1). Crawford and Compston (1970) reported a pioneering Rb–Sr age of 730 Ma for rhyolites, whereas Dhar et al. (1996) and Rathore et al. (1999) reported whole-rock Rb–Sr isochron ages for felsic volcanic rocks and granite plutons, emplaced during the first two stages of activity in the Malani Igneous Suite (MIS; first and second stages, respectively), ranging from 779 to 681 Ma. The wide distribution of ages is partially a result of incorporation of the so-called ultrapotassic rhyolites found in Sindreth (Sharma, 2004). The youngest Rb–Sr isochron age of 681 Ma (Rathore et al., 1999) comes from a solitary occurrence of the “ultra-potassic” rhyolite). This age is closely comparable with the age of the younger group of zircons from the rhyolitic tuff reported in the present study. Torsvik et al. (2001) cited precise U–Pb ages of 771 and 751 Ma for rhyolite magmatism in the MIS from data of Tucker et al. (1999) albeit without analytical details and sample descriptions. 6. Discussion 6.1. Reconstruction of ocean plate stratigraphy (OPS) The overall tectono-stratigraphic sequence around Sindreth as reported in our study represents a typical record of imbricated ocean plate stratigraphy associated with subduction–accretion history. Ocean



13

been described from Precambrian terranes in recent studies (e.g. Kawai et al., 2008) including from major suture zones associated with the assembly of late Neoproterozoic–Cambrian Gondwana (Santosh et al., 2009a, b) and Paleoproterozoic Columbia (Santosh, 2010a) supercontinents. OPS therefore, is a potential geological tool to identify subduction–accretion–collision tectonics associated with the formation of supercontinents (Santosh, 2010b). Movement of oceanic lithosphere, which provides key information for reconstruction of paleogeography, is recorded in active margins, and particularly at subduction zones. In the Sindreth area, the 20 m thick basal pillow basalt was originally capped by ~5 m thick bedded chert, limestone and quartzite, in turn covered and engulfed by a 10 m thick, arc-derived volcanoclastic mafic mudstone, and finally by about 50 m thick well-bedded sandstone turbidite deposited in a trench. Such quartzites associated with mafic mudstones might be derived from outbursts of siliceous volcanism from the arc. Slickensides on chert bedding surfaces provide evidence that they were formed by a tectonic process like thrusting, and not by slumping. The extensive pillow basalt and volcano-sedimentary sequences with the pillow lava overlain by a thick sequence of sandstone are suggestive of deposition in a shallow marine environment such as an intraoceanic seamount. The stratigraphy of the interlayered lithologies consisting of basalt pillow lavas overlain by thin chert succeeded by mudstone as observed in the study area is interpreted as ocean-floor type (Maruyama et al., 2010). The lithological association in Sindreth, thus, represents a typical imbricated OPS sequence and preserves important records of subduction–accretion history. The dominant population of zircon U–Pb isotopic ages from the two tuff beds reported in this study is similar. The tuff layers are interbedded with chert, which indicates that all these units were formed during a similar age span. These age relations further support the accretionary history according to which the tuffs, derived from a volcanic arc, were deposited over the cherts as the oceanic plate approached the trench adjacent to the arc in an active continental margin. The incorporation of the accretionary complex to the continental or arc margin must have been later than the depositional ages of the rocks.

6.2. Significance of SHRIMP geochronological data Fig. 12. Cathodoluminescence (CL) images of zircons from the from the siliceous tuff sample SS-22 showing the dated spots, and respective ages in Ma.

plate stratigraphy (OPS) is the fundamental, first-order structure of accretionary orogens as documented from the ongoing subduction–accretion and continent building process along the Pacific margins (Isozaki et al., 1990, 2010; Wakita and Metcalfe, 2005; Maruyama et al., 2010). The classic section of OPS is represented by mid-ocean-ridge basalt (MORB) chert–hemipelagic mudstone–sandstone/turbidite/conglomerate. Oceanic crust is commonly delaminated in a trench and the accreted basalts and sediments are typically imbricated by bedding parallel thrusts. The idealized travel history of an oceanic plate from mid oceanic ridge to subduction zone is illustrated in Fig. 16 (as compiled in Santosh, 2010b). At or near the mid oceanic ridge, pelagic chert is deposited on the base of MORB crust. If the ridge rises above carbonate compensation depth, the chert may be overlain by marine carbonates. As the ocean floor spreads, the basal chert is transported toward the subduction zone, during which period considerable thickness of chert layers are deposited. When the pelagic sediments reach a hemipelagic environment on the offshore side of a trench typically near a continental margin, the oceanic cherts are succeeded by hemipelagic siliceous shales and mudstones that consist of radiolaria and fine-grained continental-derived detritus. Finally, once all these units enter the trench, a voluminous cover of continent-derived detritus is deposited on the top, in the form of shales, sandstones, conglomerates and turbidites. The evidence for OPS has

The U–Pb ages of 765 ± 7 and 768 ± 7 Ma reported from magmatic zircons in rhyolitic and silicic tuffs in this study place robust temporal constraints on the timing of the first stage of magmatism in the MIS. This age is consistent with the oldest available Rb–Sr isochron age of 779 ± 10 Ma (Rathore et al., 1999), based on felsic volcanic rocks from widely spaced sampling sites. It is also consistent with the 771 ± 5 Ma for a rhyolitic tuff documented by Gregory et al. (2009) and also the two unpublished zircon dates of 771 ± 2 and 751 ± 3 Ma quoted by Torsvik et al. (2001a) for rhyolite arc magmatism during the first stage of MIS magmatism. The Sindreth group has long been attributed to a rift setting or a mantle plume (Bhushan, 2000; Singh et al., 2006), despite the lack of large volumes of basaltic rocks expected in the case of a mantle plume event. Also, previous studies on the Sindreth rocks attributed them to anorogenic-type (A-type) volcanism, but the mechanism for this eruption has not been explained. The undeformed magmatic units of Sindreth likely represent the inboard units of a subduction zone, with the deformed and eroded material largely buried or deep subducted. Madagascar is commonly placed alongside India in the Gondwana reconstructions, and Seychelles has been paleomagnetically placed adjacent to India's margin (Fig. 5, Torsvik et al., 2001). Both these continental fragments preserve the record of Neoproterozoic volcanism attributed to the subduction of an oceanic lithosphere (Torsvik et al., 2001; Tucker et al., 2001; Ashwal et al., 2002). Considering the nature and timing of magmatism in the Seychelles and western India, it appears that the bulk of the granitic and subsequent mafic magmatism in these regions can be constrained to the interval from ~771 to 751 Ma.


14


Table 3 SHRIMP zircon U–Pb data of tuffs from Sindreth region. Spot

ppm U

ppm Th

Th/U

204

P/206Pb

SS22_1.1 SS22_2.1 SS22_3.l SS22_4.1 SS22_5.1 SS22_6.1 SS22_7.l SS22_8.1 SS22_9.1 SS22_10.1 SS22_11.l SS22_12.1 SS22_13.1 SS22_14.1 SS22_I5.I SS22_16.1 SS22_17.1 SS22_18.1 SS22_19.1 SS4_l.1 SS4_2.1 SS4_31 SS4_4.1 SS4_5.1 SS4_6.1 SS4_7.1 SS4_8.1 SS4_9.1 SS4_10.1 SS4_11.1 SS4_12.1 SS4_13.1 SS4_14.1 SS4_15.1 SS4_16.1 SS4_17.1

93 61 330 238 402 145 204 460 753 589 335 281 175 209 173 101 75 257 57 177 634 1474 3369 1004 470 618 805 1543 532 725 1823 1456 SS4 1787 659 1209

126 94 181 246 95 163 70 165 5 247 70 1SS 137 267 227 43 86 133 64 115 224 616 1360 543 295 330 396 839 224 411 904 719 222 927 225 810

1.35 1.54 10.SS 1.03 0.24 112 0.34 0.36 0.01 042 0.21 0.55 018 1.28 1.31 0.43 115 0.52 112 0.65 0.35 042 04 0.54 0.63 0.53 049 0.54 0.42 0.57 0.5 0.49 04 0.52 0.34 07

6.2E-4 1.8E-3 1.OE-4 1.9E-4 1.3E-4 3.OE-4 6.2E-5 4.4E-4 SIE-4 3.3E-3 5.2E-5 2.9E-5 9.8E-5 3.4E-4 7.OE-5 1.3E-4 8.4E-4 8.3E-5 94E-4 5.8E-4 3.7E-4 4.SE-3 3.5E-3 4.4E-5 1.OE-4 6.4E-5 8.3E-5 2.2E-3 6.8E-5 8.SE-4 1.3E-3 2.6E-3 47E-4 4.8E-4 7.8E-5 5.2E-5

±%

207

Pb/206Pb

24 20 21 26 24 26 24 22 10 4 26 32 24 20 27 24 21 21 22 20 87 16 2 26 25 29 21 11 30 8 4 20 12 7 25 22

0.075 0.079 0.146 0.067 0.071 0.068 0.191 0.130 0.098 0.168 0.117 0.173 0.156 0.069 0.166 0.185 0.075 0.158 0.072 0.074 0.080 0.120 0.118 0.066 0.066 0.066 0.066 0.096 0.067 0.076 0.083 0.103 0.073 0.070 0.066 0.065

±%

206

Pb/238U

1.73 213 049 1.09 0.82 1.34 011 0.58 1.28 2.20 0.52 0.38 0.52 115 1.02 1.00 117 044 2.23 1.22 13 5.62 2.37 0.55 0.80 012 0.59 2.94 017 0.63 2.04 743 012 042 0.69 0.51

0.246 0.251 0.483 0.264 0.295 0.273 1.034 0.249 0.201 0.273 0.712 0.991 0.848 0.271 0.942 1.043 0.270 0.783 0.270 0.263 0.253 0.289 0.276 0.273 0.259 0.249 0.268 0.217 0.251 0.252 0.270 0.294 0.241 0.260 0.236 0.247

±%

206 Pb/238U age

207 Pb/206Pb age

208 Pb/232Th age

% discordant

Err corr

4.0 4.6 41 3.3 5.2 31 1.6 3.9 2.3 1.5 3.7 3.9 2.8 3.0 4.0 4.3 4.0 3.9 21 31 3.0 2.3 2.5 31 3.6 31 2.6 8.0 4.0 31 2.8 44 4.0 41 4.5 4.6

759 764 1431 773 888 755 2582 813 640 812 1837 2494 2252 761 2449 2678 764 2101 772 759 781 761 644 784 774 773 779 651 776 749 761 796 749 666 768 773

797 352 2280 760 892 738 2740 2016 1452 1997 1694 2587 2397 744 2507 2681 700 2420 530 804 1069 411 859 783 760 786 770 739 792 730 745 802 822 717 781 746

763 727 1297 796 924 750 2531 775 1880 890 1789 2503 2234 764 2468 2703 751 1962 722 779 949 760 799 787 752 798 776 754 786 779 771 865 795 671 798 795

+5 − 124 + 41 −2 +0 −2 +7 + 63 + 59 + 63 +3 +4 +7 −2 +3 +0 − 10 + 15 − 48 +6 + 29 − 90 + 26 −0 −2 +2 −1 + 12 +2 −3 −2 +1 +9 +8 +2 −4

0.3 0.2 1.0 0.6 0.7 0.5 0.8 0.9 0.5 0.3 1.0 1.0 0.9 0.5 0.7 0.8 0.3 1.0 0.2 0.4 0.1 0.1 0.2 0.8 0.7 0.8 0.8 0.5 0.7 0.5 0.3 0.1 0.6 0.7 0.8 0.9

± 10 ± 18 ± 35 ±8 ±9 ±8 ± 24 ± 19 ±6 ±8 ± 29 ± 48 ± 22 ±8 ± 24 ± 28 ± 10 ± 43 ± 11 ±8 ±9 ± 11 ±6 ±7 ±7 ±7 ±7 ± 24 ±7 ±7 ±7 ± 16 ±7 ±6 ±7 ±7

±81 ± 240 ±9 ±34 ±22 ±48 ±12 ±22 ±31 ±62 ±10 ±7 ±10 ±42 ±17 ±17 ± 100 ±8 ± 132 ±61 ± 310 ± 537 ±96 ±13 ±21 ±18 ±15 ± 160 ±19 ±37 ±63 ± 351 ±31 ±18 ±17 ±12

± 18 ± 30 ± 40 ± 13 ± 35 ± 14 ± 40 ± 33 ± 539 ± 37 ± 41 ± 62 ± 33 ± 17 ± 33 ± 53 ± 21 ± 62 ± 24 ± 21 ± 184 ± 166 ± 23 ± 10 ± 11 ± 11 ± 10 ± 59 ± 13 ± 14 ± 25 ± 121 ± 17 ±9 ± 13 ± 27

Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Standard calibration was 0.20% (not included in above errors but required when comparing data from different mounts). (1) Common Pb corrected using measured 204Pb. (2) Common Pb corrected by assuming 206Pb/238U–207Pb/235U age-concordance. (3) Common Pb corrected by assuming 206Pb/238U–208Pb/232Th age-concordance.

6.3. Implications for the assembly of Gondwana It has been suggested (Powell et al., 1993; Windley et al., 1994; Dalziel, 1997; Yoshida and Upreti, 2006) that a coherent East

Gondwana existed from the Mesoproterozoic through the bulk of the Precambrian and until the assembly of the Gondwana supercontinent at 550–530 Ma. However this is largely based on poorly controlled data with high flexibility of interpretation, with

Fig. 13. U–Pb concordia plots and age data bar charts for zircons from siliceous tuff sample SS-22.



15

Fig. 14. Cathodoluminescence (CL) images of zircons from the rhyolitic tuff sample SS-4 showing the dated spots, and respective ages in Ma.

no adequate supporting evidence for oceanic sutures with appropriate ages. Yoshida and Upreti (2006) discussed the evidence for Neoproterozoic juxtaposition of India and Australia–East Antarctica based on similarities in cratonic and orogenic detrital zircon and

neodymium isotopic signatures. However, recent models challenge the notion of a united East Gondwana throughout the Proterozoic (Meert et al., 1995; Fitzsimons, 2000; Meert, 2001; Torsvik et al., 2001; Boger, 2011; Johnson et al., 2011).

Fig. 15. U–Pb concordia plots and age data bar charts for zircons from rhyolitic tuff sample SS-4.


16


Fig. 16. Schematic illustration of an idealized travel history of ocean plate stratigraphy from mid-oceanic ridge to subduction zone (as compiled by Santosh, 2010b). See text for discussion.

For East Gondwana to be a coherent group at 750 Ma, it is necessary for India to be located along the paleoequator, according to its placement in the typically accepted Gondwana fit (de Wit et al., 1988) relative to the locations of Australia and Antarctica within Gondwana reconstructions. Younger-aged sutures between Gondwana components also indicate a more complex Gondwana amalgamation. The East African Orogen (Stern, 1994) is considered as an 800–650 Ma collision zone involving India, Madagascar, Sri Lanka, East Antarctica, and the Kalahari craton with the Congo craton and Arabian–Nubian shield (Johnson et al., 2011). The later Kuunga Orogen (Meert et al., 1995; Meert and Lieberman, 2008) places the final Gondwana assembly at about 550 Ma with Australia–Antarctica amalgamating to the above collage. Recent studies from the Palghat–Cauvery Suture Zone (PCSZ) in southern India, a trace of the Mozambique Ocean closure and Gondwana-forming suture (Collins et al., 2007; Santosh et al., 2009b) provide new insights into a Neoproterozoic Pacific-type subduction– accretion tectonics culminating in late Neoproterozoic–Cambrian Himalayan-style collision. Santosh et al. (in press) reported zircon U– Pb data on the ophiolite complex of Manamedu within the PCSZ which show 206Pb/238U magmatic crystallization ages of 737±23 to 782 ± 24 Ma from the plagiogranites and 786 ±7.1 to 744 ±11 Ma from the gabbros. In a previous study, Sato et al. (2011a) reported LA–ICPMS zircon U–Pb ages of 800 ±14 Ma from the plagiogranites of Manamedu. Their study also reported younger intercept age 759±41 Ma from zircons in a quartzite and metamorphosed banded iron formation accreted within the subduction complex. Zircons from the Kadavur gabbroanorthosite complex in Teale et al.'s (2011) study show 207Pb/206Pb ages of 825 ±17 (Teale et al., 2011). The U–Pb ages of 843± 23 Ma from metamorphic rims on zircons from the surrounding quartzites reported by them closely compare with Sato et al.'s (2011a) data. Teale et al. (2011) also obtained an age of 766±8 Ma from oscillatory-zoned euhedral crystals of magmatic zircons from the felsic gneisses of Kadavur. In another study, Sato et al. (2011c) reported 206Pb/238U ages of 819± 26 Ma from arc-related rapakivi granites at Tangalamvaripatti within the southern domain of the PCSZ. All these data suggest a prominent mid Cryogenian subduction system along the southern margin of the PCSZ. We correlate the mid Neoproterozoic arc-related magmatism within PCSZ and adjacent domains with those in NW India, Madagascar and Seychelles to a series of Cryogenian magmatic arcs developed during

the destruction of the Mozambique Ocean floor prior to the final assembly of Gondwana. The rocks described in this study represent the remnants of the OPS accreted onto the continental margin and the zircon age data constrain the timing of the early stage of subduction. In southern India, the subduction–accretion along the PCSZ culminated in a major collisional orogeny at around 540 Ma, coinciding with the final assembly of the Gondwana supercontinent. The results from our present study from NW India, together with those from the recent works in other regions discussed above thus suggest that Cryogenian magmatic arcs were widely distributed along the margins of the EAO and the ANS, associated with the subduction history of the Mozambique Ocean lithosphere, prior to the final amalgamation of the Gondwana supercontinent in the latest Neoproterozoic–Cambrian. 7. Conclusions (1) The presence of oceanic sediments together with continental detritus and the reconstructed ocean-plate stratigraphy in the Sindreth area confirms the existence of a Neoproterozoic accretionary complex. (2) The presence of pillow basalts and their geochemical data suggest an active convergent margin. (3) Zircons from two tuff beds intercalated with cherts from within the OPS sequence yielded similar U–Pb zircon ages of ca. 765 Ma. The ages are consistent with a Cryogenian volcanic arc in the Sindreth region. The youngest zircon population in the rhyolitic tuff shows ages in the range of 666 to 644 Ma, suggesting that the arc magmatism continued to late Cryogenian. (4) The data from our present study from NW India, together with those from the recent works in other regions including the Syechelles, Madagascar, southern India and Sri Lanka suggest that Cryogenian magmatic arcs were widely distributed along the margins of the East African Orogen. (5) Our study brings out important insights into Cryogenian subduction system in NW India and the imprints of subduction of the Mozambique Ocean lithosphere prior to the final amalgamation of the Gondwana supercontinent during the Late Neoproterozoic– Cambrian.



Acknowledgments We gratefully acknowledge the constructive comments from Editor, Prof Zeming Zhang and two anonymous reviewers during peer review that greatly improved the presentation of the manuscript. The field work for this study was carried out with K. K. Sharma and Ritesh Purohit who are gratefully acknowledged for all the support. We thank K. K. Sharma and Ritesh Purohit for stimulating discussions on geology of the Sirohi and Sindreth regions during field work. The authors thank Dr. N.V. Chalapathi Rao and Dr. B. Lehmann for help in obtaining the geochemical data.

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