PETROGRAPHY AND GEOCHEMISTRY OF THE PIPE-7 KIMBERLITE, ANANTAPUR DISTRICT, ANDHRA-PRADESH, INDIA

The diamond-rich Pipe-7 kimberlite found about 10km east of Lattavaram in Anantapur district, Andhra Pradesh in south India is an elongated dyke - like body, that extends over a length of 450 m and has a maximu m width of 25 m. It is porphyritic in texture and contains sub-angular to rounded xenoliths of peridotite, garnet-clinopyroxene rocks, webst erite, amphibolite, metabasic rocks and granite. Petrographic studies reveal that it is porphyritic in texture and contains euhedral to sub- hedral altered olivine, phlogopite, garnet, clinopyroxene (chronic dio pside), chromite, perovskite and ilmenite which are the unequivocal pr imary liquidus phases whereas serpentine, chlorite, phlogopite, calcit e, amphibole, magetite, monticellite, clay minerals and others represe nt secondary alteration assemblages. Chrome-diopsides and garnets repr esent an important megacryst suite. The Pipe-7 kimberlite has higher C aO and lower Mgo, Al2O3, K2O and iron contents compared to the other s ix known kimberlites in the vicinity. Fe/Mg ratio in coexisting garnet s and clinopyroxenes, indicates the equilibration temperature for(a) t he nodules to be 1040 - 1070-degrees-C and for(b) the megacrysts of ga rnet and clinopyroxene to be 1350-degrees-C respectively. Pyrope garne ts from this kimberlite show a positive correlation between CaO and Cr 2O3 and are similar in nature to those of the low calcium garnets repo rted from the diamondiferous South African kimberlites.