Geochemistry and C-O and Nd-Sr isotope characteristics of the 2.4 Ga Hogenakkal carbonatites from the South Indian Granulite Terrane: evidence for an end-Archaean depleted component and mantle heterogeneity

摘要

The South Indian Granulite Terrane (SGT) is a collage of Archaean to Neoproterozoic age granulite facies blocks that are sutured by an anastomosing network of large-scale shear systems. Besides several Neoproterozoic carbonatite complexes emplaced within the Archaean granulites, there are also smaller Paleoproterozoic (2.4 Ga, Hogenakkal) carbonatite intrusions within two NE-trending pyroxenite dikes. The Hogenakkal carbonatites, further discriminated into sovite and silicate sovite, have high Sr and Ba contents and extreme light rare earth element (LREE) enrichment with steep slopes typical of carbonatites. The C-and O-isotopic ratios [delta C-13(VPDB) = -6.7 to -5.8% and delta O-18(VSMOW) = 7.5-8.7% except a single O-18-enriched sample (delta O-18 = 20.0%)] represent unmodified mantle compositions. The epsilon Nd values indicate two groupings for the Hogenakkal carbonatites; most samples show positive epsilon Nd values, close to CHUR (epsilon Nd = -0.35 to 2.94) and named high-epsilon Nd group while the low-epsilon Nd group samples show negative values (-5.69 to -8.86), corresponding to depleted and enriched source components, respectively. The Sr-87/(86)Sri ratios of the two groups also can be distinguished: the high-epsilon Nd ones have low Sr-87/Sr-86(i) ratios (0.70161-0.70244) while the low-epsilon Nd group shows higher ratios (0.70247-0.70319). We consider the Nd-Sr ratios as primary and infer derivation from a heterogeneous mantle source. The emplacement of the Hogenakkal carbonatites may be related to Paleoproterozoic plume induced large-scale rifting and fracturing related to initiation of break-up of the Neoarchean supercontinent Kenorland.