Petrology, geochemistry and genesis of newly discovered mesoproterozoic highly magnesian, calcite-rich kimberlites from Siddanpalli, Eastern Dharwar Craton, southern India: products of subduction-related magmatic sources?

Abstract

The Siddanpalli kimberlites constitute a newly discovered cluster (SKC) of Mesoproterozoic (1090 Ma) dykes occurring in the granite-greenstone terrain of the Gadwal area in the Eastern Dharwar Craton (EDC), Southern India. They belong to coherent facies and contain serpentinized olivines (two generations), phlogopite, spinel, perovskite, ilmenite, apatite, carbonate and garnet xenocrysts. A peculiar feature of these kimberlites is the abundance of carbonate and limestone xenoliths of the eroded platformal Proterozoic (Purana) sedimentary cover of Kurnool/Bhima age. Chemically, the Siddanpalli dykes are the most magnesium-rich (up to 35 wt.% MgO) and silica-undersaturated (SiO₂ < 35 wt.%) of all kimberlites described so far from the Eastern Dharwar Craton. The La/Yb ratio in the Siddanpalli kimberlites (64–105) is considerably lower than that in the other EDC kimberlites (108–145), primarily owing to their much higher HREE abundances. Since there is no evidence of any crustal contamination by granitic rocks we infer this to be a specific character of the magmatic source. A comparison of the REE geochemistry of the Siddanpalli kimberlites with petrogenetic models for southern African kimberlites suggests that they display involvement of a wide range in the degree of melting in their genesis. The different geochemical signatures of the SKC compared to the other known kimberlites in the EDC can be explained by a combination of factors involving: (i) higher degrees of partial melting; (ii) relatively shallower depths of derivation; (iii) possible involvement of subducted component in their mantle source region; and (iv) previous extraction of boninitic magmas from their geological domain.