TRACE ELEMENT AND ND-HF-SR-PB GEOCHEMISTRY OF PROTEROZOIC LAMPROITES FROM THE SOUTHERN INDIAN CRATON

We report trace element and Nd, Sr, Hf, Pb isotopic data for 10 lamproite samples from the Krishna river region in south India. These Proterozoic lamproites (reported ages are 1224 ± 14 Ma, 1384 ± 18 Ma.) were emplaced along the eastern margin of the Precambrian Dharwar craton at the contact between the proterozoic Cuddapah basin and Archean basement rocks.

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 (ε_Nd(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 (ε_Nd(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 (ε_Nd(0)=-22.5 to -30.5) and Hf (ε_Hf(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.