Petrology and Geochemistry including Platinum Group Element Abundances of the Mesoproterozoic Ultramafic (Lamproite) Rocks of Krishna District, Southern India: Implications for Source Rock Characteristics and Petrogenesis

A suite of potassic ultramafic rocks occurs in the Krishna district in the southern Indian craton along with clusters of kimberlite. The principal minerals include olivine, pyroxene, amphibole and phlogopite. Compositional variation exists in the phlogopite of the different occurrences of the region. Mg number varies from 0.37 to 0.56 and the groundmass phlogopites are enriched in Fe and Ti and poor in Al compared to the phenocrysts. Major and trace element data indicate enrichment of LILE and LREE (La/Yb = 24 - 104; Gd/Yb = 3.8 - 8.02). Ba contents vary from 132 to 5327 ppm, which is rather low compared to literature values for lamproite. However, the abundances of Zr (> 500 ppm), Sr (> 1000 ppm) and La (>200 ppm) are similar to those in the lamproite. The potassic ultramafic rocks have variable but high U/Pb (2-37, average 17.2) and Th/U (4.6-10.7) ratios. Concentrations of Platinum Group Elements (PGE) are quite variable with Ir, Pt and Pd varying by >9, 21 and 15 times respectively over the suite. Significant features include differences in fractionation between PGE and enrichment of Cu relative to the mantle source region. Some of the samples are not ultrapotassic and peralkaline. However, combining their mineralogical and geochemical properties, these rocks of the Krishna region may be considered as a member of the lamproite clan. There is no evidence of significant crustal contamination, but fractional crystallisation of olivine and perovskite is suggested from the geochemical relations. Available isotopic age data indicate that the ca. 1220 Ma old Krishna lamproite preceded kimberlite emplacement in the region by about 160 million years. Partial melting of a mantle metasomatically enriched in Ti and Fe would yield the Krishna lamproite magma. Amphibole contributed to the melt, but the HREE abundance data preclude garnet involvement. The depth of melt formation was likely to be shallower than that of kimberlites.