Xenoliths in Proterozoic kimberlites from southern India: petrology and geophysical implications.

There are no published records of mantle xenoliths in basalts from India. A few relatively fresh ultrabasic and eclogite xenoliths have been recovered from the kimberlite pipes of the Wajrakarur-Lattavaram area of southern India. The mineralogy of the ultrabasic xenoliths is described; these include garnet harzburgites, lherzolites, wehrlites and olivine clinopyroxenites. Some of the garnet-bearing nodules have spinel, and most have phlogopite. The geotherms defined by the calculated P-T conditions of mineral equilibria in these ultrabasic xenoliths and in the Cretaceous xenoliths from N Lesotho, southern Africa, are very similar. This similarity implies nearly steady-state thermal conditions in the upper mantle beneath continental shield area over at least 1000 m.y. The inferred P-T regime of the Indian xenoliths also suggests that the lithosphere was = or >185 km thick during the mid-Proterozoic. The ultrabasic and eclogite xenoliths have been derived from depths of 100-180 km and 75-150 km, respectively. The chemical compositions of garnet, which include a low Ca variety, seem to suggest that the Proterozoic mantle beneath the Indian Shield was progressively more 'fertile' with increasing depth at the time of kimberlite eruption. -J.M.H.