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 portion of zircons of both types show positive ɛHft values (varying from -3 to +9), indicating a relatively juvenile parent ( 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 ∼2560-2500 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 resulted in the formation of the dominantly N-S trending structures in the basement.