Petrology and (U-Th)/He Thermochronology of Mesoproterozoic Kimberlites from the eastern Dharwar Craton, southern India

Abstract

Apatite (U-Th)/He thermochronometry data from two different kimberlite clusters of the Dharwar Craton is used, together with geologic constraints, to develop a model for the burial, uplift, and unroofing history of southern India (i.e. Peninsular India). The apatite helium (AHe) dates for the CC-5 kimberlite at a present-day elevation of 398 m range between 128-235 Ma, with a mean age of 164.9 ± 21.2 Ma (1 S.D.). The AHe dates for the SK-2 kimberlites at a present-day elevation of 289 m range from 121.1-170.7 Ma, with a mean age of 166.3 ± 25.2 Ma (1 sigma standard deviation). The mean AHe ages for the CC-5 and SK-2 kimberlites from the approximately 150 km apart Wajrakarur and Raichur kimberlite fields, respectively, are indistinguishable within their uncertainties. This suggests a similar uplift and erosion history of the two areas on the eastern Dharwar craton. All these dates are younger than kimberlite pipe emplacement (ca. 1100 Ma) signalling a major post-emplacement erosion during the Mesozoic (Middle Jurassic). We use the data to link not only uplift and erosion to elevation gain, but to show how this corresponds to deep mantle processes and continental formation or breakup. Our HeFTy time-temperature modelling results indicate heating to temperatures of ~175 °C suggesting burial, which was followed by an unroofing (cooling) event. Plate tectonic modelling using GPlates software package is also consistent with t-T models, in that the signals observed are also recorded by plate movements (e.g. the 175 Ma). Burial is restricted to a depth of 3 km after pipe emplacement. Erosion estimates show that the Dharwar Craton underwent ≥ 1.5-4 km of erosion during the Jurassic, most likely related to Gondwana break-up that commenced at ~180 Ma. The fast drift of the Indian plate (18 cm/a) towards the northern hemisphere during the Cretaceous is related to the removal of the once thick cratonic keel beneath the Indian Craton. Whether parts of the mantle lithosphere were delaminated/removed during or post Gondwana is controversially debated. Herein, we propose a model for cratonic root delamination, related to deep mantle processes (i.e., dynamic topography) that may have played a role or aided in lithospheric thinning of the Indian tectonic plate.