Zircon U-Pb ages and Hf and O isotope systematics of crustal zircons from Mesoproterozoic kimberlites of the Dharwar craton, India: Implications for Neoarchean craton assembly

Plate tectonic models for the amalgamation of the dominantly Paleo-Mesoarchean Western Dharwar Craton (WDC) and Neoarchean Eastern Dharwar Craton (EDC) remain inconclusive on the polarity of Neoarchean subduction. Here we present insitu U-Pb age and Hf-O isotopic compositions of xenocrystic zircons in ca. 1.4-1.1 Ga kimberlites and lamproites emplaced within the ca. 2.8-2.5 Ga EDC as deep probes of the crustal architecture across the WDC-EDC boundary. The different zircon populations yield wide range of ages extending back to ca. 3.6 Ga, including distinctly 'kimberlitic zircons' with a mean 206Pb/238U age of 1112 +/- 42 Ma, consistent with previous age estimates for kimberlite magmatism. The presence of zircons with ages > 3.33 Ga is surprising as this age group of zircons is atypical of magmatic rocks in the EDC, but wide-spread in the WDC. Also, the highest concentration of zircons with ages > 3.0 Ga is found in the kimberlites closest to the notional boundary shear zone between the WDC and EDC. The Hf isotope systematics of a majority of magmatic zircons with ages between 2.75 and 2.50 Ga require melting of older (3.6-3.3 Ga) source rocks. O-isotope data for these Neoarchean zircons show a wide scatter, lie both above (818O to 13.5 %o) and well below mantle values (818O to < 2 %o). 818O values outside the mantle range are closely associated with low epsilon Hf. The data are interpreted as evidence for trans-portation of older upper crustal rocks to lower crust and mantle depths consistent with tectonic models invoking collisional under thrusting of WDC beneath EDC.