Paleo-Mesoarchean crustal growth and reworking in the western Dharwar Craton, southwestern India: Evidence from trondhjemitic gneiss and granitic gneiss

Paleo-Mesoarchean felsic orthogneisses are widely exposed in the southern part of the western Dharwar Craton (WDC), southern India shield. The felsic orthogneisses studied in this paper from the WDC can be divided into two groups based on their geological, geochronological, and geochemical data: the 3.4-3.3 Ga trondhjemitic gneiss (including trondhjemitic veins) and the similar to 3.1-3.0 Ga granitic gneiss. The 3.4-3.3 Ga trondhjemitic gneiss is characterized by high contents of SiO2 (70.4-73.1 wt%), Na2O (5.49-5.91 wt%) and Sr (647-740 ppm), high ratios of Na2O/K2O (4.7-7.7) and Sr/Y (53-106), but relatively lower Mg# values (32-39), and Cr (9.5-28.7 ppm) and Ni (1.34-5.33 ppm) concentrations, as well as insignificant Eu anomalies, fractionated REE patterns and mostly positive zircon epsilon(Hf)(t) values (+0.8-+5.9). These data indicate that the trondhjemitic gneiss was derived from partial melting of juvenile mafic crust in a relatively moderate-shallow depth, with residual mineral assemblage of garnet, amphibole and clinopyroxene, implying crustal growth event at 3.4-3.3 Ga. By contrast, the similar to 3.1-3.0 Ga granitic gneiss is characterized by relatively lower contents of Na2O (3.82-4.63 wt%) and Sr (96.9-291 ppm), low ratios of Na2O/K2O (1.4-1.7) and Sr/Y (3.02-8.65), as well as significantly negative Eu anomalies and negative to positive zircon epsilon(Hf)(t) values (-5.6-+4.5). Therefore, the granitic gneiss was probably derived from partial melting of pre-existing crustal rocks, suggesting a prominent crustal reworking event at similar to 3.1-3.0 Ga. This study emphasizes that the western Dharwar Craton is dominated by Paleoarchean (3.4-3.2 Ga) crustal growth event and a strong crustal reworking event in early Mesoarchean (similar to 3.1-3.0 Ga), though the Mesoarchean crustal growth event is discernible, evidenced by regional similar to 3.0 Ga TTG rocks. The early Mesoarchean (similar to 3.1-3.0 Ga) crustal reworking event, represented by series of potassium-rich granites, is also reported in other Archean cratons, and thus it may have global significances.