Petrogenesis and tectonic significance of ∼1.6 Ga and ∼ 0.5 Ga A-type granite magmatism in the Assam-Meghalaya Gneissic Complex (NE India): Insights from geochronology, geochemistry and Sr-Nd isotopes

Abstract

The widespread granite magmatism in the Assam-Meghalaya Gneissic Complex (AMGC) is crucial for understanding its tectono-magmatic evolution of NE India. This study presents geochemical and Sr-Nd isotopic data along with U-Pb zircon and U-Th-total Pb monazite ages of granites from western, central and eastern parts of the AMGC. Zircon U-Pb dating reveals two distinct episodes of granite magmatism during 1617–1603 Ma and 540–497 Ma. Geochemically, the ∼1.6 Ga and ∼ 0.5 Ga granites exhibit high SiO₂, Na₂O + K₂O, and moderate to low CaO and MgO contents. These granites show high Ga/Al ratios and display enrichment in large-ion lithophile elements (LILEs) relative to high field strength elements (HFSE). The ∼1.6 Ga and ∼ 0.5 Ga granites are classified as A-type granites that were emplaced in post-collisional extensional tectonic setting. The ∼1.6 Ga granites show variable Nd composition [εNd(t) = 0.4 to −3.9] with Nd model ages (T_DM) of 2.6–2.2 Ga, indicating that these granites were derived from the partial melting of juvenile underplated mafic lower crust. In contrast, the ∼0.5 Ga granites show uniform negative εNd(t) values of −10.8 to −12.1 and younger Nd model ages (T_DM) of 2.2–1.7 Ga, suggesting that these granites were derived from the partial melting of the Paleoproterozoic to Mesoproterozoic continental crust beneath the AMGC. The two episodes of A-type granite magmatism in AMGC at ∼1.6 Ga and ∼ 0.5 Ga mark the collisional imprints of Columbia and East Gondwana assemblies, respectively.