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<p>The Assam-Meghalaya Gneissic Complex (AMGC) in northeast India represents the eastern fringe of the Precambrian Indian Shield and comprises Meso- to Neoproterozoic migmatitic quartzofeldspathic gneisses, foliated granitoids, metapelites and Ediacaran-Cambrian granites. Our work focuses on the petrogenesis of garnet-bearing and garnet-absent granites of the West Garo Hills (WGH), in the westernmost AMGC, that are intrusive into basement quartzofeldspathic gneisses (QFG) and foliated granitoids. We use phase equilibria modelling to evaluate whether partial melting of basement rocks could have produced the Ediacaran-Cambrian granites in the area. Textural and chemical characteristics of garnet support a magmatic origin. Phase equilibria modelling, carried out on garnet-bearing granite, indicates that the garnet core composition equilibrated at ∼7.5&#xa0;kb/830°C, while rim composition record ∼5.3&#xa0;kb/730°C. Phase equilibria modelling suggests that the QFG and metapelite basement rocks were metamorphosed at ∼5.5&#xa0;kb/750°C and ~ 5.7&#xa0;kb/740°C, respectively. U-Pb dating of monazite and xenotime in the granite, the host QFG, and pegmatites yielded comparable 591 − 542&#xa0;Ma, 582 − 540&#xa0;Ma and 562 − 460&#xa0;Ma ages, respectively, which date the timing of high-grade metamorphism and granite emplacement during Pan-African orogenesis. Our melt-reintegrated phase equilibria modelling suggests that partial melting of basement QFG and associated metapelite with varying degrees of fractional crystallization could produce melts having composition similar to those of the granites in the WGH. A comparison of the modelled melt compositions with the compiled granite data from the entire AMGC suggests that basement QFG-derived melts can largely explain the composition of the Ediacaran-Cambrian granites.</p>

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Phase equilibria modelling and geochronology of Ediacaran-Cambrian granites in West Garo Hills, Assam-Meghalaya Gneissic Complex, NE India: implications for East Gondwana assembly

  • 期刊论文

  • 发表日期：

    2026年1月10日

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  • Rhituparna Gogoi,
  • Pranjit Hazarika,
  • Dewashish Upadhyay

摘要

The Assam-Meghalaya Gneissic Complex (AMGC) in northeast India represents the eastern fringe of the Precambrian Indian Shield and comprises Meso- to Neoproterozoic migmatitic quartzofeldspathic gneisses, foliated granitoids, metapelites and Ediacaran-Cambrian granites. Our work focuses on the petrogenesis of garnet-bearing and garnet-absent granites of the West Garo Hills (WGH), in the westernmost AMGC, that are intrusive into basement quartzofeldspathic gneisses (QFG) and foliated granitoids. We use phase equilibria modelling to evaluate whether partial melting of basement rocks could have produced the Ediacaran-Cambrian granites in the area. Textural and chemical characteristics of garnet support a magmatic origin. Phase equilibria modelling, carried out on garnet-bearing granite, indicates that the garnet core composition equilibrated at ∼7.5 kb/830°C, while rim composition record ∼5.3 kb/730°C. Phase equilibria modelling suggests that the QFG and metapelite basement rocks were metamorphosed at ∼5.5 kb/750°C and ~ 5.7 kb/740°C, respectively. U-Pb dating of monazite and xenotime in the granite, the host QFG, and pegmatites yielded comparable 591 − 542 Ma, 582 − 540 Ma and 562 − 460 Ma ages, respectively, which date the timing of high-grade metamorphism and granite emplacement during Pan-African orogenesis. Our melt-reintegrated phase equilibria modelling suggests that partial melting of basement QFG and associated metapelite with varying degrees of fractional crystallization could produce melts having composition similar to those of the granites in the WGH. A comparison of the modelled melt compositions with the compiled granite data from the entire AMGC suggests that basement QFG-derived melts can largely explain the composition of the Ediacaran-Cambrian granites.