Pb, Sr and Nd Isotopic Systematics of the Carbonatites of Sung Valley, Meghalaya, Northeast India: Implications for Contemporary Plume-Related Mantle Source Characteristics

Abstract

A Pb/Pb–Pb/Pb age of 134± 20 Ma with a model INTRODUCTION l1 of 8·19 ± 0·02 has been obtained for the carbonatites from Carbonatites are carbonate-rich igneous rocks of mantle Sung Valley, Meghalaya, placing their time of emplacement very origin. They are found mainly in continental settings and close to the time assigned for the break-up of Gondwana. Initial their emplacement ages range from Late Archaean to eSr (+5·3 to+7·8), eNd (+1·7 to+2·3), Pb/Pb (19·02), Quaternary (Woolley, 1989). Their high abundances of Pb/Pb (15·67) and Pb/Pb (39·0) indicate a mantle Sr (average 7000 ppm) and Nd (average 250 ppm) source region with a somewhat higher Rb/Sr ratio than Bulk Earth, preclude significant changes in Sr and Nd isotopic ratios minor light rare earth element (LREE) depletion, and a timecaused by crustal contamination and provide a unique integrated enhancement of U/Pb. The isotopic characteristics of the means of characterizing the evolution of the mantle from Sung Valley carbonatites, however, differ significantly from those of Archaean to Recent times (Bell et al., 1982; Bell & basalts from Rajmahal and the Ninetyeast Ridge, which have a Blenkinsop, 1987). In the Indian Peninsula, carbonatites relatively more depleted upper-mantle source component, indicating have been reported from various geological settings with involvement of different mantle sources. The mantle source region of ages ranging from Early Proterozoic to Tertiary (Krishnathe carbonatites is characterized by an EM2–HIMU signature, murthy, 1988; Natarajan et al., 1994). Petrological, minsimilar to that observed for Ambadongar, western India, but different eralogical and chemical data are available for most of from the carbonatites of East Africa, which have an EM1–HIMU the Indian carbonatite complexes (Krishnamurthy, 1988). signature. It is speculated that the Sung Valley carbonatites were derived from a parental magma generated by partial melting of the However, isotopic data remain scarce and are confined sub-continental lithosphere, which was previously metasomatized to Sr/Sr analyses from Sevattur (Deans & Powell, by fluids derived from an EM2–HIMU type mantle plume. The 1968; Kumar & Gopalan, 1991), an Rb–Sr study on data suggest that the pre-130 Ma sub-continental mantle exhibited carbonatites of Hogenekal (Natarajan et al., 1994), an preferentially an EM2 type signature. isotopic study of Sr, Nd and Pb from the Ambadongar carbonatite complex (Veena et al., 1993; Simonetti et al., 1995) and a recent Pb/Pb age determination on the Newania and Sevattur carbonatites of India (Schleicher et al., 1997). This study attempts to define the combined Sr, Nd and Pb isotopic systematics of the Sung Valley