Petrogenesis of the Kunavaram alkaline complex and the tectonothermal evolution of the neighboring Eastern Ghats Belt granulites, SE India

Geochemical and geochronological study of the Kunavaram alkaline rocks near the craton–Eastern Ghats Belt (EGB) contact in SE India indicates that alkaline magma was emplaced at ∼1384 Ma along a NE–SW Mesoproterozoic rift at the cratonic margin. This paleo-rift can be delineated by the several Mesoproterozoic alkaline intrusives at the cratonic margin (e.g. Khariar, Kunavaram, Jojuru, Elchuru and others). The geochemical variations in the Kunavaram alkaline suite can be explained by an assimilation-fractional crystallization (AFC) process dominated by the fractionation of clinopyroxene–amphibole, apatite, titanite, zircon and allanite and the simultaneous assimilation of bulk upper crustal material from the craton. The alkaline complex and other craton margin rocks and the marginal granulites of the EGB were overprinted by an amphibolite facies tectonothermal event during the Neoproterozoic–Paleozoic (611–484 Ma) when Pan-African tectonism thrust the EGB granulites westward over the cratonic foreland along a number of northwest-vergent shear zones. The Pan-African thrust stacking has the cratonic boundary and the alkaline rocks in the footwall and the EGB granulites in the hanging wall. The continuous nature of the Pan-African shear fabric across the craton–EGB contact near Kunavaram implies that the two crustal segments shared a common boundary during the Neoproterozoic–Paleozoic. Prior to the Pan-African event, the evolutionary history of the EGB is characterized by the presence of an early Neoproterozoic (1017–1087 Ma) granulite-facies tectonometamorphic overprint during the Grenvillian orogeny, evidence for which is lacking in the alkaline and other craton margin rocks. The craton–EGB contact thus represents a rifted Mesoproterozoic margin deformed during Pan-African collisional tectonics.