V016-0002
A Review of Geochemistry of Granitoids of Shillong Plateau, Northeast India

Wednesday, 9 December 2020
Poster
Faizan Ahmad Khan, Indian Institute of Science Education and Research Bhopal, Earth and Environmental Science Department, Bhopal, India, Gaurav Kumar, Indian Institute of Science Education and Research Bhopal, Bhopal, India and Arundhuti Ghatak, Indian Institute of Science Education and Research Bhopal, Department of Earth & Environmental Sciences, Bhopal, India
Abstract:
The Shillong Craton is situated in Meghalaya and parts of Assam, Northeast India. Like other cratons, it has a basement of archean granitic gneisses, overlain by the Shillong Group of meta-volcanic and meta-sedimentary rocks. Together they have faced multiple episodes of magmatism consisting of a wide range of chemical and mineralogical assemblages. Amongst them are late Neoproterozoic (Seindhuli (~881 ma) and Rongjeng granites (~788 ma)) to Cambrian (South Khasi (~516 ma), Kyrdem (~512 ma) and Nongpoh granites (~535-506 ma)) and Ordovician (Mylliem granites (~480-443 ma) granitic intrusions. The whole region is bounded and traversed by a number of faults and lineaments including Brahmaputra graben in the north, Dauki fault in the south and NE-SW trending Barapani-Tyrsad shear zone along which Umsning Schist Belt is situated.

The granitic batholiths are composed largely of pink porphyritic granites with phencrysts of K-feldspar. Almost all of these intrusions are highly felsic except for South Khasi and Nongpoh granites which are intermediate to felsic in composition. These two plutons are situated on the Barapani-Tyrsad shear zone. Large noritic enclaves are also abundant in these two intrusions. Traditionally all these plutons are believed to be metaluminous I-type granites associated with the amalgamation of Gondwana supercontinent.

Here we make a first attempt to constrain the geochemistry of all these granites collectively and suggest the origin and evolution of these intrusive rocks in the light of global tectonics. This study is based on exisitng literature and data of all these granitic plutons. All these samples show calc-alkaline, meta-aluminous to peraluminous nature. The chondrite normalized REE pattern shows fractionated trends (LaN/LuN = 20.9 - 33.3) with strong enrichment of LREEs (LaN /SmN = 2.4 - 7.4), a strong negative Eu anomaly (Eu* =0.004 - 0.034), and depletion in HREEs (GdN/LuN = 1.8 - 6.8). The depletion of Nb, Pb and Sr and enrichment of Th and U is apparent in spider plots.

Therefrom, we propose that greater degree of partial melting has occurred along Barapani-Tyrsad shear zone leading to less evolved geochemistry and presence of mafic bodies within plutons sitting on this shear zone. The granitic plutons further away are more evolved and indicating of being derived from much smaller melts.

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