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.

Automated summary

This study reveals the characteristics of crustal structure and tectonic history of the Paleo-Mesoarchean Western Dharwar Craton (WDC) and Neoarchean Eastern Dharwar Craton (EDC) in India through the analysis of crustal composition and rock samples. The discovery of zircons with ages exceeding 3.6 billion years in the xenocrystic zircons suggests that ancient upper crustal rocks were transported to lower crust and mantle depths, supporting the collisional under thrusting of the WDC beneath the EDC.