Mid-Proterozoic plume-related thermal event in Eastern Indian craton: evidence from trace elements, REE geochemistry and Sr-Nd isotope systematics of basic-ultrabasic intrusives from Dalma volcanic belt

Abstract

Trace, REE, Sr and Nd isotopic studies have been carried out on gabbro-pyroxenite intrusives (Rb-Sr isochron age ~1619±38 Ma; Sr_i ~0.70552±0.00002) of the Dalma volcanic belt from eastern Indian craton. Primitive mantle-normalised trace element patterns show a general depletion of high field strength elements and LREE but more or less flat pattern in most compatible elements. Chondrite-normalised REE plots show depleted LREE-flat HREE patterns [(SLREE/SHREE)_N < 1, (Ce/Yb)_N < 1] strikingly similar to the komatiitic and tholeiitic lavas from this belt. Nd isotopic data with mean f_Sm/Nd ~+0.2704 and high e_Nd (mean +7.8) values indicate that the source of these rocks was depleted in LREE for considerably long time. When plotted on the global e_Nd evolution path for the upper mantle the Dalma intrusives fall exactly around the depleted MORB-type mantle at 1.6 Ga. Enrichment in some LILE like Rb, Ba, Th is found both in the tholeiitic lavas and the residues indicating them to be source characteristics. Positive DNb values of most of the mafic-ultramafic units (including komatiitic lavas) of this belt indicate that they originated from a mantle plume with thick envelope of hot upper mantle producing MORB-like depleted komatiites, tholeiites and intrusives. The mid-Proterozoic plume eventually rifted the continent above, forming a rapidly subsiding basin which was subsequently collapsed and compressed. The plume also caused widespread thermal events recorded in charnockitisation, migmatisation and granitisation around 1.6 Ga. This was possibly part of a global ~1.6 Ga thermal anomaly which affected the pre-existing large landmass comprising atleast Antarctica, Australia and India (Mawson continent?).