Petrology of P-5 and P-13 “Kimberlites” from Lattavaram Kimberlite Cluster, Wajrakarur Kimberlite Field, Andhra Pradesh, India: Reclassification as Lamproites

Abstract

To date, all the potassic-ultramafic pipes from the Wajrakarur Kimberlite Field (WKF), Eastern Dharwar Craton (EDC), India, have been classified as kimberlites. Recent reclassification of a kimberlite pipe P-2 from the WKF as a lamproite necessitated a re-examination at the other kimberlite bodies in the region with a view to placing them in their appropriate slots in the mineralogic-genetic classification scheme of ultrapotassic rocks. The Pipes P-5 and P-13 of Lattavaram cluster in WKF were chosen for this present work. Both pipes exhibit macrocrystal textures and have two generations of olivine. The prominent groundmass minerals constitute spinel, perovskite, phlogopite, apatite, pyroxene, serpentine, chlorite and calcite/magnesite. The phlogopites from both the pipes are low in aluminium and barium, are high in total iron in comparison to archetypal kimberlites and follow lamproite trends. The pipes are characterized by the groundmass spinels of four varieties, for example, ulvospinel, Mg-ulvospinel, Mg-Cr-spinel and magnetites. Both the pipes have atoll and necklace-textured spinels besides the euhedral groundmass spinels. These spinels exhibit compositional trends identical to those found in kimberlite magmatic trend T2. Perovskites with bimodal size distribution occur as discrete grains throughout the rock. The two pipes represent polyphase intrusions that are in close proximity within Lattavaram region. The mineral chemistry data of phlogopites, spinels, perovskites and pyroxenes from these pipes are more akin to those in lamproite magmas rather than that in archetypal (Group-1) kimberlites. Thus, a careful reassessment of the petrological characteristics of all the 45 kimberlitic pipes so far discovered in the EDC is necessary keeping in view the economic and academic importance of these pipes due to their diamond and mantle xenolith potential.