TRACE ELEMENT AND ND-HF-SR-PB GEOCHEMISTRY OF PROTEROZOIC LAMPROITES FROM THE SOUTHERN INDIAN CRATON
Chondrite normalized Rare Earth Element (REE) plots of the samples show a nearly uniform, parallel distribution pattern. All samples show light REE enrichment (La/Yb=41-88) like most global lamproites and are characterized by the absence of Eu-anomaly, high Ni (65-820ppm) and Cr (21-515ppm) contents.
The Sr isotopic composition (0.7054-0.73592) is quite variable and do not show any systematic correlation with the Nd-isotopic compositions (εNd(0)=-22.6 to 30.4). The initial Sr, Nd isotopic ratios at 1224Ma show a range of 0.7009-0.7037 and -6 to 11.5 (εNd(1224)), respectively. Some of these samples clearly show post-emplacement Rb and Nd enrichment as they fall off the generally negative Nd-Sr isotopic correlation plot.
Combined Hf-Nd data of the samples fall generally on the terrestrial array and show extremely negative Nd (εNd(0)=-22.5 to -30.5) and Hf (εHf(0)=-22 to 41) isotopic compositions.
The lamproites have variable but high U/Pb (2-37, average 17.2) and Th/U(4.6-10.7). This is reflected in the high present-day 206Pb/204Pb (16.24-20.87) and 208Pb/204Pb (37.9-49.07) ratios. The high Th/U ratios of these rocks could be the result of phlogopite melting in the mantle source.
Based on the geochemical and isotopic data, we propose a two-stage model for the generation of these rocks. The first step involves metasomatism within the mantle (causing LREE enrichment) of an ancient subducted komatiitic ultrabasic crust with high Ni, Cr. This is followed by melting of this subducted crust to generate the lamproites.