Late Neoproterozoic to early Cambrian high-grade metamorphism from Mikir Hills(Assam-Meghalaya gneissic Complex,northeast India):Implications for eastern Gondwana assembly  

作　　者：Rahul Nag H.Hrushikesh Nathan Cogné N.Prabhakar 

机构地区：[1]Department of Earth Sciences,Indian Institute of Technology Bombay,Powai,Mumbai,Maharastra 400076,India [2]Geological Survey of India,Northeastern Region,Guwahati,Assam 781005,India [3]Géosciences Rennes,UMR6118,CNRS Universitéde Rennes,France

出　　处：《Geoscience Frontiers》2024年第5期256-282,共27页地学前缘（英文版）

基　　金：supported by the Prime Minister’s Research Fellows(PMRF)scheme from the Ministry of Education,New Delhi,Government of India(PMRF ID:1300753);Fieldwork and laboratory studies were financially supported by the Science and Engineering Research Board(SERB,Govt.of India)core research grant(CRG/2019/000812)to N.Prabhakar;Mineral and monazite analyses were performed using the SERB-funded EPMA National Facility at the Department of Earth Sciences,IIT Bombay(IRPHA grant no.IR/S4/ESF-16/2009);Prabhakar acknowledges the core research grant(file no.CRG/2019/000812)for the financial support given for laboratory analysis。

摘　　要：Mikir Hills region,which represents the eastern segment of the Assam-Meghalaya Gneissic Complex(AMGC)in northeast India,constitutes part of the Eastern Gondwana.The Mikir Hills preserves multiple metamorphic and magmatic events ranging from Early Mesoproterozoic to Early Cambrian.Out of these events,documenting the late Neoproterozoic to early Cambrian tectonothermal events is helpful in correlating the continental blocks of Eastern Gondwana.We present an integrated study involving field relations,petrology,P-T history and zircon-monazite geochronology of hitherto poorly studied pelitic and quartzo-feldspathic gneisses from the Mikir Hills region.These gneisses have experienced at least three deformation events(D_(1),D_(2) and D_(3))with dominant foliation indicated by ENE-WSW striking and shallow-moderately dipping(<40°)S_(2) gneissic foliation.The peak metamorphism in pelitic and quartzo-feldspathic gneisses is characterized by garnet(core)-K-feldspar-sillimanite-plagioclase-bio tite-rutile-quartz-ilmenite-melt and garnet-plagioclase-K-feldspar-biotite-quartz-ilmenite-melt assemblages,respectively.The application of thermobarometric methods constrains the peak P-T conditions of 7.5-8.4 kbar at 674-778℃and 6.7-7.4 kbar at 601-618℃for pelitic and quartzofeldspathic gneisses,respectively.These results are consistent with the values estimated using phase equilibria modelling and melt reintegration approach.The results of pseudosection modelling suggests a clockwise P-T path for pelitic gneisses involving migmatisation during peak metamorphism followed by near isothermal decompression from 8.0 to 8.6 kbar at 768-780℃to 4.0-5.0 kbar at 720-765℃.In contrast,quartzo-feldspathic gneisses preserved slightly lower peak P-T conditions at 3.8-4.6 kbar and 590-650℃.The U-Pb zircon dating of migmatised pelitic and quartzo-feldspathic gneisses yielded concordant ages of 1647±11 Ma and 1590±7 Ma,respectively.These dates represent the inherited igneous protolith components,possibly equivalent to the Mesoproterozoic

关 键 词：GONDWANA Assam-Meghalaya Gneissic Complex Migmatisation NEOPROTEROZOIC Geochronology P-T-t history 

分 类 号：P588.3[天文地球—岩石学]