Petrochronology and geochemistry of migmatites from the Assam-Meghalaya gneissic complex (NE India): Implications for the crustal anatexis and reworking during Gondwana assembly

Abstract

The extensive Proterozoic migmatisation in the Assam-Meghalaya Gneissic Complex (AMGC) is critical for understanding its Precambrian tectonic evolution. The present study reports the occurrence of distinct lithologies in Guwahati and adjoining regions, providing new insights into the Proterozoic high-grade migmatisation events in the central AMGC. This is achieved through an integrated analysis of structures, mineral chemistry, phase equilibrium modelling, whole-rock geochemistry and zircon-monazite geochronology. The gneisses and amphibolites experienced three deformation episodes (D₁, D₂ and D₃), and migmatisation occurred during the D₂ deformation. Phase equilibrium modelling results indicate that the quartzo-feldspathic gneisses and amphibolites experienced peak P-T metamorphism at 5.3–8.0 kbar/720–730 °C and 5.8–7.0 kbar/690–700 °C, respectively. The melt re-integration approach for the quartzo-feldspathic gneisses reveals that the prograde metamorphism occurred through the sub-solidus breakdown of muscovite, followed by the breakdown of biotite during the peak metamorphic event. U-Pb zircon geochronology yielded concordant upper intercept ages of 1674–1610 Ma, representing a Mesoproterozoic igneous protolith for the migmatites. Discordant lower intercept ages (512–426 Ma) and widespread U-Th-total Pb monazite ages (499–470 Ma) constrain the timing of high-grade metamorphism and partial melting in the central AMGC. Geochemically, the leucosomes developed in quartzo-feldspathic gneisses and amphibolites show K-rich and Ca-rich compositions. According to trace element and REE patterns, the K-rich and Ca-rich leucosomes in the quartzo-feldspathic gneisses and amphibolites were formed by partial melting of granitic or metasedimentary protoliths and mafic protoliths, respectively. Combining P-T and geochronological results presented in this study with the existing data, we propose that the central AMGC largely evolved in the hinterland region during the Cambrian-Ordovician (499–470 Ma) collision event. The correlation of widespread Late Neoproterozoic partial melting, high-grade metamorphism and magmatism in the Guwahati region with similar events in western Australia and Antarctica indicate that the central and eastern AMGC represents a convergent margin evolved during the assembly of the Gondwana supercontinent.