Nature of charnockite and Closepet granite in the Dharwar Craton: Implications for the architecture of the Archean crust

Abstract

The architecture of the Archean crust is crucial in understanding the geodynamics of cratonization. The Dharwar Craton in the Indian peninsula is well known for its continuous change of metamorphic grades from granulite facies in the south to amphibolite facies in the middle and greenschist facies in the north. Here the genetic relationship of different units is re-analyzed to decipher the crustal architecture of the craton. The major lithologic unit – the granitoids can be subdivided into TTG, charnockite, Closepet granite and anatectic granites/migmatites. The charnockite near Salem has two types of occurrence: one is massive, garnet-free and plutonic, crystallized at ~2560 Ma, and the other is gneissic, garnet-bearing and incipient, metamorphosed at ~2490 Ma. Both types are compositionally close to tonalite-granodiorite, and generally show positive Eu-anomalies. A large proportion of zircons of both types show positive ɛHft values (varying from −3 to +9), indicating a relatively juvenile parent (<2800 Ma). The garnet-bearing charnockite was probably transformed in situ from the lower crust at granulite facies; whereas the garnet-free charnockite represents exotic magmas originated from the lower crust. The Closepet granite is potassium-rich and high in Mg#, and shows negative Eu-anomalies. It was emplaced at 2520–2510 Ma (simultaneous with the garnet-bearing charnockite), and the igneous zircons show negative ɛHft values (−9 to +1). It likely originated from relatively ancient (>2900 Ma) crust at medium-pressure (in the middle crustal level) with significant mantle contribution. The genetic relationship of different units, the spatial age difference of greenstone belts and the spatial variation of metamorphic grade in the Dharwar craton suggest a trinity of Archean crustal architecture (‘sandwich’ structure), with 2700–2500 Ma-dominated uppercrust, 3400–2900 Ma-dominated middle crust, and ~2560 to 2500 Ma anatectic lower crust. This ‘sandwich’ structure possibly indicates vertical growth of the crust in the Archean. The whole craton was tilted at ~2500 Ma with the southern end uplifted the most by an intra-cratonic process, and then it was shaped by later orogenic events resulting in the formation of the dominantly N-S trending structures in the basement.