2.61 Ga potassic granites and crustal reworking in the western Dharwar craton, southern India: tectonic, geochronologic and geochemical constraints

Abstract

We present combined field, structural data and spot image analysis, petrographic data, U–Pb zircon ages, Nd isotopes and whole-rock geochemical data for the late Archaean granite plutons from Arsikere–Banavara (AB suite) and Chitradurga–Jampalnaikankote–Hosdurga (CJH suite), in the western Dharwar craton (WDC). AB plutons are syn-kinematic with respect to the development of dome-and-basin patterns resulting from partial crustal diapiric overturn (D1), whilst CJH suite plutons show deformation patterns resulting from the superimposition of large-scale regional strike-slip shearing (D2) onto D1 syn-emplacement fabrics. SIMS U–Pb zircon ages of 2614 ± 10 Ma for the Chitradurga pluton (CJH) and of 2617 ± 3 Ma for the Arsikere pluton (AB) define the minimum age of the D1 strain pattern. The CJH and AB suites are mainly high-K granites whose origin is to be sought in partial melting of an old (>3.0 Ga) depleted lower crust of intermediate composition. The CJH and AB suites correspond mainly to the CA1- and CA2-types of Archaean calc-alkaline granites and some facies to the transitional TTG. Their heterogeneous chemical compositions indicate involvement of various sources and also significantly different depths of partial melting to produce these granites. These results allow documenting regional partial reworking of the WDC 2615 Ma ago, requiring an HT event to have taken place before 2.61 Ga and thus different from the 2.55 to 2.51 Ga granulite facies metamorphic episode associated to regional-scale strike-slip shearing (D2) that affected the entire craton. Partial reworking of the older lower crust at 2.61 Ga and associated deformation would result from heat advection and/or crustal thickening that may relate to voluminous mafic greenstone volcanism and moderate shortening at that time.