Accepted Manuscript Petrogenesis of carbonatitic lamproitic dykes from Sidhi gneissic complex, Central India M. Satyanarayanan, D.V. Subba Rao, M.L. Renjith, S.P. Singh, E.V.S.S.K. Babu, M.M. Korakoppa PII:
S1674-9871(17)30097-X
DOI:
10.1016/j.gsf.2017.04.011
Reference:
GSF 563
To appear in:
Geoscience Frontiers
Received Date: 20 May 2016 Revised Date:
12 April 2017
Accepted Date: 28 April 2017
Please cite this article as: Satyanarayanan, M., Rao, D.V.S., Renjith, M.L., Singh, S.P., Babu, E.V.S.S.K., Korakoppa, M.M., Petrogenesis of carbonatitic lamproitic dykes from Sidhi gneissic complex, Central India, Geoscience Frontiers (2017), doi: 10.1016/j.gsf.2017.04.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Petrogenesis of carbonatitic lamproitic dykes from Sidhi gneissic complex, Central India
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CSIR-National Geophysical Research Institute, Hyderabad 500007, India
b
Geological Survey of India, Hyderabad 500068, India
c
Department of Geology, Bundelkhand University, Jhansi 284128, India
d
NCEGR, Geological Survey of India, Bengaluru 560070, India
*Corresponding author. E-mail address:
[email protected]
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Abstract
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M. Satyanarayanana, D.V. Subba Raoa,*, M.L. Renjithb, S.P. Singhc, E.V.S.S.K. Babua, M.M. Korakoppad
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Petrographic, mineral chemical and whole-rock geochemical characteristics of two newly discovered lamproitic dykes (Dyke 1 and Dyke 2) from the Sidhi Gneissic Complex (SGC), Central India are presented here. Both these dykes have almost similar sequence of mineral-textural patterns indicative of : (i) an early cumulate forming event in a deeper magma chamber resulting in megacrystic/large size phenocrysts of phlogopites with subordinate amount of olivine and clinopyroxene; (ii) crystallization at shallow crustal levels promoted fine-grained phlogopite, K-feldspar, calcite and Fe-Ti oxides in the groundmass; (iii) dyke
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emplacement related quench texture (plumose K-feldspar, acicular phlogopites) and finally (iv) post emplacement autometasomatism by hydrothermal fluids which percolated as micro-veins and also altered the mafic phases. Phlogopite phenocrysts often display resorption textures together with growth zoning indicating that during their crystallization equilibrium at the crystal-melt interface they were fluctuated multiple times likely due to incremental addition or chaotic
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dynamic self mixing of the lamproitic magma. Carbonate aggregates as late stage melt segregation are common in both these dykes, though their micro-xenolithic forms suggest assimilation with a plutonic carbonatite body thus enhanced the
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carbonatitic nature of these dykes. Geochemically both dykes are ultrapotassic (K2O/Na2O: 3.0–9.4) with low CaO, Al2O3 and Na2O content and high SiO2 (53.3–55.6 wt.%) and K2O/Al2O3 ratio (0.51–0.89) characterizing them as high-silica lamproites. Inspite of these similarities, many other features indicate that both these dykes appear to have evolved independently from two distinct magmas. In dyke 1, phlogopite composition has evolved towards the minette trend (Al-enrichment) from a differentiated parental magma having low MgO, Ni and Cr contents; whereas in dyke 2, phlogopite composition shows an evolutionary affinity towards the lamproite trend (Al-depletion) from a more primitive magma having high MgO, Ni and Cr content. Whole-rock trace-elements signatures like enriched LREE, LILE, negative Nb-Ta and positive Pb anomalies; high Rb/Sr, Th/La, Ba/Nb, and low Ba/Rb, Sm/La, Nb/U ratios in both dykes indicate that their parental magmas were sourced from a subduction modified garnet facies mantle containing phlogopite. From the various evidences it is concluded that the
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studied lamproitic dykes from Sidhi area stand out an example for lamproite magma attained a carbonatitic character and undergone diverse chemical evolution as consequences of parental melt character, storage at deep crustal level and post emplacement autometasomatism.
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Key words: Lamproite; Phlogopite; Carbonate aggregates; Metasomatized mantle; Sidhi Gneissic Complex; Central India 1. Introduction
Lamproites are rare mantle derived potassium-rich igneous rocks (Jaques et al., 1986; Foley et al., 1987; Mitchell and Bergman, 1991; Krmicek, et al., 2016) closely associated with intra-continental tectonic settings or post-orogenic collapse,
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post-dating convergent tectonics and active margin processes (Mitchell and Bergman, 1991). Starting from partial melting, there are several complex magma processes are involved in the lamproite petrogenesis such as magma mixing (O'Brien et al.,
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1988, Meyer et al., 1994; Prelevic et al., 2004), crustal assimilation (Carlier et al., 1997), xenolith incorporation (Morin and Corriveau, 1996), differentiation into more felsic or potassic melts (Esperanca and Holloway, 1987; Recently they are termed as "leucolamproites" by Krmicek et al., 2016), liquid immiscibility (Foley, 1984; Mitchell, 1991), formation of various segregation features (Sato, 1997) and late stage autometasomatism (Rock, 1991). Sequence of such complex processes involved in their petrogenesis is systematically recorded in the form of grain textures, mineral chemistry and geochemical
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compositions. Here we document petrography, mineral chemistry and whole-rock geochemistry of two newly discovered lamproite dykes (Dyke 1 and Dyke 2) from the Sidhi Gneissic Complex, Central India and discuss the possible roles played by
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various magma processes involved in their petrogenesis.
2. Geological setting and field relations
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The Central Indian Tectonic Zone (CITZ) (Fig. 1a), a ENE–WSW trending Mesoproterozoic orogenic belt (Radhakrishna and Naqvi, 1986; Radhakrishna, 1989; Acharyya, 2003), has been considered as a collision zone along which the Northern Indian Block (NIB, comprising of the Aravalli–Bundelkhand Craton) and the Southern Indian Block (SIB, comprising the Singhbhum-, Bastar- and Dharwar-Cratons) were amalgamated during the Paleozoic and formed the Indian subcontinent (Yedekar et al., 1990; Jain et al., 1991; Eriksson et al., 1999; Mishra et al., 2000). The CITZ is bound by the Son-Narmada north- and south-faults in the northern part and the Central Indian Shear/Suture (CIS) in the southern part (Fig. 1b). The major geological framework within the CITZ includes metamorphosed volcano-sedimentary supracrustal sequences in which the Sausar, Sakoli, Dongargarh and Betul groups occur spatially closer toward the CIS, whereas the Mahakoshal Group is confined between the two Son-Narmada faults (Fig.1b). Older Gneissic complexes along Balaghat-Bhandara, Tirodi, Ramakona-Katangi, Chotanagpur, Harda and Sidhi form the basement for these supracrustal volcano-sedimentary sequences.
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Both the supracrustal rocks as well as the gneissic complexes have been intruded by post-collisional younger granitoids (Acharyya, 2003; Bhowmik and Roy, 2003; Bhowmik et al., 2005; Bhandari et al., 2011). All these units are overlain by the late Cretaceous Deccan Trap lava flows and the Permo-Carboniferous Gondwana sedimentary sequences. Present study area, the Sidhi Gneissic Complex (SGC), is a ca. 2.5 Ga old tectonic block (Roy and Bandyopadhyay,
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1988) situated in the northern boundary of CITZ and comprising of a granite, gneiss-migmatitic domain (Gneissic Complex) and forming an inlier surrounded by the unconformably overlying supracrustal rocks of Mahakoshal Group and the Vindhyan Supergroup toward the southern and northern parts respectively (Fig. 1c). The dominant lithounits in SGC include the older high-grade metamorphic rocks (gneisses, migmatites, amphibolites, talc-chlorite schist); dykes (metabasite and metadolerite)
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in close association with episodic, multiple pulses of younger intrusives (granite, pegmatite, syenite and lamprophyre dykes). Due to lack of radiometric ages, geological evolution of the poorly studied SGC is deduced mainly from cross-cutting field
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relationships and order of superposition. In general, the gneisses of SGC show NE–SW trend which is dissected by numerous ENE–WSW and NNW–SSE trending faults and shear zones traversing both the Sidhi and Mahakoshal Group of rocks, suggesting that the terrane has been subjected to multiple deformation. Younger intrusions comprising of lamprophyre, syenite and mica bearing ultramafic rocks are undeformed and unmetamorphosed. Bhattacharya et al. (2014) and Bhattacharya and Khonglah (2014) studied these alkaline dykes (Lamprophyre and syenite) in detail. According to them these dykes are
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trending ENE–WSW, NE–SW and rarely along NW–SE and N–S. The present study is focus onto these younger dykes and reports the occurrence of two lamproite dykes near Sidhi town for the first time (Dyke 1 located at UTM E 582209.67, N 2699768.63; Dyke 2 located at E 584470.32, N 2707902.36; both in UTM Zone 44R). Dyke 1 is ~ 2 m wide and ~ 50 m in length, while Dyke 2 is ~5 m wide × 100 m long in dimensions. Dyke 1 (Fig. 2a–c) is dark coloured and medium- to fine-
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grained, and devoid of macrocrystic phases whereas Dyke 2 (Fig. 2d,e) is grey and medium- to coarse-grained rock with
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abundant megacrystic phlogopite. 3. Petrography
Both lamproite dykes from Sidhi area have almost similar mineral-textural pattern like phenocrysts of coarse-grained phlogopite, clinopyroxene (Cpx) and olivine are embedded in a fine- to medium-grained panidiomorphic groundmass containing K-feldspar, phlogopite, calcite, plagioclase, magnetite, apatite, dolomite and quartz (in decreasing modal content) (Fig. 2). Phlogopite, the most abundant phase (70–85 by vol.%) is found in three populations: (i) large crystals (Fig. 2b) or megacrysts (~1 cm) (Fig. 2e); (ii) medium grained crystals (Fig. 2f) and (iii) fine-grained acicular grains in the groundmass. Megacryst size phlogopites are abundant in Dyke 2 (Fig. 2e) than Dyke 1. Megacrystic and/or medium size phlogopites from both dykes frequently show oscillatory zoning (Fig. 3a,b), patchy zoning (Fig. 3c), dissolution/resorption texture (Fig. 3c–f) and broken nature (Fig. 3g). In megacrystic phlogopites inclusions are almost absent. However, rarely in Dyke 2, phlogopites
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carry inclusions of Cpx and olivine toward core and grain margins respectively; wherein in Dyke 1, inclusions of apatite along with dolomites are rarely noticed in phlogopites (Fig. 4a). Large apatite grains (~900 µm) from Dyke 1 carry inclusions of calcite (Fig.4b). Micro-veins of quartz, barite (Fig. 4c) and magnetite are found frequently along the cleavage fractures of large phlogopites from both dykes. In the groundmass, phlogopites are mostly acicular, bladed or tabular in habits, though a
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few pseudohexagonal equant grains are also present. Magnetites are euhedral to subhedral (200–600 µm) and disseminated uniformly. They are often found as glomerocrysts having cluster of 3–5 grains. SEM image shows that their individual grains have fritted/dusty core and a clear rim (Fig. 4d). Olivine and Cpx are mostly altered to pseudomorphs of calcite, serpentine and talc preserving their euhedral shape (Fig. 4f,g). Olivine pseudomorphs exhibit ocellar-like texture due to mantling of
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acicular/tabular phlogopites around them (Fig. 4g). Altered olivines show extensive development of patchy secondary magnetites (Fig. 4e). In both dykes, calcite occur in two distinct forms such as interstitial groundmass (Fig. 2c,f) and aggregate
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(Fig. 5a,b). Based on modal content and texture two types of calcite aggregate are recognized: (i) Type 1 aggregate (Fig. 5a) mainly composed of calcite with subordinate amount of K-feldspar, magnetite and phlogopite in equigranular allotriomorphic texture. Medium-grained phlogopites mantling around these aggregates is a characteristic feature. (ii) Type 2 aggregate (Fig. 5a,b) mainly composed of coarse grained calcite, dolomite and Cpx with equigranular cumulate texture. They are rounded to sub rounded in shape with well defined boundary. It is rarely noticed as micro-inclusion in megacrystic phlogopites (Fig. 5d). A few of these aggregates developed irregular boundary of corona-like structure in which magnetites are pervasively noticed
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(Fig. 6). Another significant textural feature noticed in both dykes is the aggregate of K-feldspars in plumose form (Fig. 7). Association of phlogopite with these aggregate structure is unique and that observed in various textural patterns such as (i) phlogopite mantles around the aggregate (Fig. 7a); (ii) acicular/bladed phlogopites within the aggregate (Fig. 7b) and (iii)
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4. Analytical methods
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aggregate without any phlogopites (Fig. 7c,d).
Mineral compositions were determined using well polished and carbon coated thin sections by Cameca® SX-100 electron microprobe at CSIR-NGRI, Hyderabad and NCEGR, Geological Survey of India, Bangalore, equipped with wavelengthdispersive spectrometers, using 15 kV acceleration voltage and a 20 nA beam current. Standards include a variety of natural minerals and oxides. SEM images (Scanning Electron Microscope) of minerals were taken using HITACHI® S3400 N SEM coupled with energy dispersive spectrometry (EDS) at CSIR-NGRI. Whole-rock major elements were analyzed in pressed pellet samples by XRF method (Phillips® MAGIX PRO Model 2440) with a relative standard deviation of
