Pb, Sr and Nd isotopic systematics of the carbonatites of Sung Valley, Meghalaya, Northeast India: Implications for contemporary plume-related mantlesource characteristics

A Pb-206/Pb-204-Pb-207/Pb-204 age of 134 +/- 20 Ma with a model mu(1) of 8. 19 +/- 0.02 has been obtained for the carbonatites from Swing Valley, Megha laya, placing their time of emplacement very close to the time assigned for the break-up of Gondwana. Initial epsilon(Sr) (+5.3 to +7.8), epsilon(Nd) (+1.7 to +2.3), Pb-206/Pb-204 (19.02), Pb-207/Pb-204 (15.67) and Pb-208/Pb- 204 (39.0) indicate a mantle source region with a somewhat higher Rb/Sr rat io than Bulk Earth, minor light rare earth element (LREE) depletion, and a time-integrated enhancement of U/Pb. The isotopic characteristics of the Su ng Valley carbonatites, however, differ significantly from those of basalts from Rajmahal and the Ninetyeast Ridge, which have a relatively more deple ted upper-mantle source component, indicating involvement of different mant le sources. The mantle source region of the carbonatites is characterized b y an EM2-HIMU signature, similar to that observed for Ambadongar, western I ndia, but different from the carbonatites of East Africa, which have an EM1 -HIMU signature. It is speculated that the Sung Valley carbonatites were de rived from a parental magma generated by partial melting of the sub-contine ntal lithosphere, which was previously metasomatized by fluids derived from an EM2-HIMU type mantle plume. The data suggest that the pre-130 Ma sub-co ntinental mantle exhibited preferentially an EM2 type signature.