Kimberlite and Lamproite Rocks of Vajrakarur Area, Andhra Pradesh

Kimberlite and lamproite diatremes of late Proterozoic age (840 to 1020 m.y.) within the Archaean granites and gneisses (Peninsular Gneiss) around Vajrakarur in Andhra Pradesh show unusual mineralogy, mantle and crustal xenoliths and variable diamond distribution. Two of the pipe rocks correspond to kimberlite breccias with around 40% crustal xenoliths. Two pipe rocks are identical in mineralogy, xenolith content and chemistry to the classical kimberlites of Kimberley in South Africa. These four kimberlite pipes are diamond-bearing with xenocrysts and megacrysts of forsteritic olivine, pyrope-garnet, chrome diopside and magnesian ilmenite. Mantle xenoliths identified include peridotite and eclogite. The other two pipe rocks rich in phlogopite vary in petrography, mineral chemistry and whole rock geochemistry from classical kimberlites and show closer affinity to lamproites of West Australia, Spain and Arkansas. These rocks contain two generations of olivine, sanidine, phlogopite in a fine-grained matrix composed of phlogopite, olivine, perovskite, sanidine, clinopyroxene, richterite, serpentine, carbonates. melilite and possible leucite and glass. Rarity of pyrope garnet, chrome diopside from the' indicator' suite, xenoliths from 'mantle' suite and the absence of diamond are characteristic. The six kimberlites generally show high values of MgO, AI₂O₃, CaO, K₂O and Na₂O. The two olivine lamproites described are characterised by significantly high values of TiO2, Fe₂O₃, K₂O, Na₂O and P₂O₅•

The petrology, mineral chemistry and rock geochemistry of mica-rich kimberlite are found to be transitional between kimberlite and lamproite rocks and suggest a separate petrogenesis. possibly from a different area within the mantle. A shallower origin is indicated by the absence of kimberlite indicator minerals, mantle xenoliths and diamond.