Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

A. K. Chaudhuri ¹, D. Mukherjee ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, IN

Abstract

Near Kishangarh in Rajasthan, Delhi conglomeratic quartzite and quartz mica schist overlie unconformably Aravalli metasediments. The foliation in the concordant nepheline syenites is parallel to the bedding plane foliation of the Aravallis. The Aravalli metasediments and the nepheline syenites show exactly similar development of superposed folds, with the axial surface of early isoclinal, inclined to reclined folds by nearly N-S striking upright folds. Evidence of such superposition is seen in all scales from hand specimens to maps. Spectacular outcrop patterns characteristic of superposed folds are developed. The effects of both the deformations can be seen only in the older Aravalli rocks and the nepheline syenites, while the younger Delhi rocks, in contrast, show only the later folds of the Aravalli rocks, which can be tied up with the first folding of the Delhi rocks.

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H. Thomas ¹, D. Mukherjee ²

Affiliations
1 Department of Applied Geology Dr. H. S. Gour University, Sagar (M.P.), IN
2 Geological Survey of India, Agartala, Tripura, IN

Abstract

No Abstract.

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N. C. Ghose ¹, D. Mukherjee ², N. Chatterjee ³

Affiliations
1 G/608, Raheja Residency, Koramangala, Bangalore - 560 034, IN
2 Department of Geology, Patna University, Patna - 800 005, IN
3 Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, Massachusetts 02139, IN

Abstract

The Chotanagpur gneiss-Granulite complex (CGGC)-a mobile belt north of the Archaean Singhbhum cratonic nucleus and contiguous orogenic belt with the Singhbhum Proterozoic basin, is a vast tract of high-Grade rocks and gneisses with enclaves of granulite and metasedimentary rocks, and intrusive granites of Proterozoic age. Pervasive intrusions of mantle-Derived rocks of varied composition ranging from mafic-Ultramafic, sodic-Ultrapotassic alkaline rocks, massif anorthosite to younger tholentic basalts (Rajmahal) and dolerite at different geological periods ranging from Late Paleoproterozoic to Early Tertiary, give evidence of an active mantle in the prolonged history of evolution of this mobile belt. The present study is limited to metamorphosed mafic-Ultramafic rocks at the eastern sector of CGGC. The mafic-Ultramafic suite is represented by amphibolite, basic granulite and hornblendite. Compositions of the primary amphibole in these rocks range from hornblende to pargasitic hornblende, and plagioclase from An₄₀ to An₅₉. Positive correlation of Mg# between clinopyroxene and hornblende, and clinopyroxene and orthopyroxene, indicating that the Fe-Mg exchange K_D is constant between the ferromagnesian minerals, and the rocks to have attained equilibrium conditions of metamorphism. Calculated post-Peak metamorphic equilibrium pressures and temperatures are 3 8-5 4 kb and 643- 781°C, similar to conditions in the surrounding country rocks.

Chemically, the metabasic rocks studied are associated with the Bengal anorthosite massif at Saltora and have been grouped into low-Ti and high-Ti tholeiites. The former shows similarity with transitional basalts derived from T-MORB, while the latter is rich in incompatible elements and shows affinity with basalts derived from E-MORB or from recycled mantle fed by subducted oceanic crust. The nepheline normative ultramafic rock is the most depleted in incompatible elements and shows similarity in trace element contents with MORB. All these rocks show variable crustal contamination. Nevertheless, the bulk chemical compositions of the low-Ti rocks preserve evidence of low-Pressure fractional crystallisation involving olivine, plagioclase and clinopyroxene. The intra-cratonic Mesoproterozoic bi-Modal (tholeiitic-alkaline) magmatism in CGGC is analogous to Phanerozoic magma generation(Rajmahal tholentes-Ultrapotassic mafic-Ultramafic intrusions in the Gondwana basins) in a rift setting (Damodar graben /Shield margin faults) accompanied with crustal thinning. The trace element geochemistry of mafic-Ultramafic rocks gives evidence of plume-Generated magmatism in the Eastern Indian Shield margin during Mesoproterozoic time, which is correlatable with the global thermal event in the Precambrian shields.

Keywords

Mafic-Ultramafics, Mesoproterozoic, Intra Cratonic Magmatism, Mantle Plume, Chotanagpur Gneiss-Granulite Complex, Eastern India.

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D. K. Bhattacharya ¹, D. Mukherjee ², V. C. Barla ³

Affiliations
1 Post Graduate Department of Geology, Ranchi University, Ranchi - 834 008, IN
2 202, Manna Surti Complex, Lohianagar, Kankarbagh, Patna - 800 020, IN
3 P.G. Department of Geology, Vinoba Bhave University, Hazaribag - 821 305, IN

Abstract

Komatiite near Semra village, southwest of Daltonganj in Palamau District of Jharkhand, occurs within tremolite actinolite schist of ultramafic parentage. The fragmented olivine phenocrysts show mutually parallel as well as angular alignment, representing relict spinifex texture. Mineralogically the Semra ultramafic is lehrzolite in composition. The cumulates lack visible deformation suggesting original magmatic crystallization of these ultramafic rocks. The present occurrence of Spinifex Textured Peridotitic Komatiite (STPK) in Chhotanagpur Gneissic Complex (CGC) at northwestern part of Eastern Indian Shield is of great significance as it opens up a possibility of the presence of Archaean rock in CGC, which is yet to be established.

These ultramafics are geochemically characterised by distinctive high MgO, Ni, Cr and poor in alkali, TiO₂, Ba, Cs, Rb, Nb, Hf and Y contents. It has low abundance of incompatible elements and is LREE depleted [(La/Yb)_n=0.74 - 1.07] with enriched flat HREE profile, representing chondrite like composition. This shows diagnostic convex upward REE profile. All these together with relict but distinct spinifex texture confirm the komatiitic character of Semra ultramafic. The chemical data plots for ultramafics confine to komatiite field in discriminate diagrams. The Mg# (>74) of the Semra ultramafics are comparable to primitive upper mantle Mg# (89.8) and high enough for rocks derived from ultramagnesian liquid of the mantle derived source. Considerably high Zr concentration in Semra ultramafic is attributed to Zr-enriched mantle source. Geochemically Semra ultramafics is comparable with "Al-undepleted" Munro type komatiites. Depleted incompatible trace elements also point towards Al-undepleted nature for the investigated ultramafics, which are characterised by distinctive low SiO₂ content in comparison to many well known komatiites of the world. The low K₂O content indicate plume related magmatism for its generation. Tectonic setting of this STPK with distinctive cumulus nature is suggestive of its emplacement in an extensional tectonic regime.

Keywords

Komatiite, Plume Magmatism, Extensional Regime, Chhotanagpur Gneissic Complex, Semra, Palamau, Jharkhand.

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