MINERALOGY AND GEOCHEMISTRY OF CHIGICHERLA KIMBERLITE AND ITS XENOLITHS, ANANTAPUR DISTRICT, SOUTH-INDIA

Petrography and chemistry of newly discovered Chigicherla Kimberlite p ipe of Andhra Pradesh, South India are presented. The textural charact eristics of the rock suggest it to be more close to the hypabyssal typ e of kimberlite. Presence of melilite classifies these rocks as extrem e variety of kimberlite. Mineralogy of the spinel harzburgite xenolith s that occur within these Kimberlite reveals two types of xenoliths: H arzburgite xenoliths having spinel which is Cr rich (61.6%) and Al (10 .29%) poor. Harzburgite xenolith having spinet which is Al rich (43.58 %) and Cr (25.90%) poor. Temperature estimates reveal that the Cr-rich type xenolith has equilibrated at 972-degrees-C while the Cr- poor on es were equilibrated at 839-degrees-C +/- 31-degrees-C. The maximum pr essure estimated for these xenoliths is 25 kb. Presence of chrome spin el and absence of magnesium aluminium spinel in the xenoliths, indicat e that the uppermost 45 or 50 km of the mantle section has highly ''de pleted'' peridotite composition. The study supports the contention of the steady state of thermal condition in the mantle lithosphere beneat h the continental shield and has not changed appreciably through the l ast one billion years.