Age and origin of proterozoic kimberlites and lamproites from the Dharwar Craton, southern India

Osborne, Ian (2012). Age and origin of proterozoic kimberlites and lamproites from the Dharwar Craton, southern India. PhD thesis The Open University.



This is a study of kimberlite and lamproite intrusions located in southern India within the Eastern Dharwar craton (EDC). Bulk-rock major-element, trace-element, REE- and Nd- isotopic data support previous studies that have classified the ultramafic magrnatism located in the EDC as either Group I kimberlite or lamproite. This study reports new high-precision Ar-Ar age data for southern Indian ultrapotassic rocks (kimberlites and lamproites). Previously, the Chelima lamproite (ca. 1400 Ma) was considered to be one of the oldest lamproites in the world. However, our age data suggest that at least one lamproite (Pochampalle from Krishna lamproite field) was generated in the same region 100 Ma before the Chelima event. The Pochampalle lamproite was emplaced around ~ 1500 Ma as shown by the Ar-Ar data in this study, roughly 250 Ma before the other Krishna lamproites. It would seem that the Pochampalle lamproite was also derived from an isotopically distinct source region with a lower 143Nd/144Nd ratio than other Krishna lamproites. These findings not only have implications for regional ultramafic/ultrapotassic magrnatism, but also demonstrate that the mantle processes for producing lamproitic melts existed earlier than previously thought. This study also presents the first Hf isotopic analyses of perovskites from Indian kimberlites. Perovskites fall into two distinct groups in Hf isotope space, suggesting a degree of heterogeneity in the source of the kimberlites in the EDC. The kimberlites are all found in such close proximity that lateral variations in the source are unlikely, but this does not preclude vertical heterogeneity in the source region(s). I propose a model of kimberlite generation with a vertically heterogeneous source that has undergone separate periods of enrichment and depletion before kimberlite melt generation and emplacement at ~ 11 00 Ma.

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