Mafic granulite xenoliths from the East Indian Shield: Evidence for recycled continental crust in the Archean mantle

Abstract

Massif-type charnockites of the Eastern Ghats granulite terrain, India, abound in mafic enclaves, which are generally hornblende mafic granulites with relatively minor occurrences of pyroxenite enclaves in the marginal segments only. The mafic granulite enclaves may be interpreted as earlier mafic melts within plutonic charnockite, where prograde heating in the hornblende in these mafic granulite enclaves was probably due to the host charnockite crystallization. Pyroxenite enclaves, on the other hand, are likely to be cumulates from an episode of mafic magmatism. The trace-element characteristics of hornblende–mafic granulite xenoliths are akin to arc-derived basalt, indicating a tectonic setting of subduction and slab melting. Further, low values of primitive mantle–normalized Nb/U ratios and enriched radiogenic isotopic compositions in the mafic xenoliths clearly indicate recycled continental crust in the mantle source region. While most of the internal segments of the Eastern Ghats mobile belt are Paleoproterozoic domains, the marginal (cratonic) segments in the north and west are Archean domains. The average Nd model age of ca. 2.5 Ga for the protoliths of hornblende–mafic granulite xenoliths for the Paleoproterozoic domains may be interpreted as the age of arc magmatism. For the marginal segments, the average Nd model age of ca. 3.3 Ga probably represents earlier Archean arc magmatism. Initial 87 Sr/ 86 Sr ratios calculated at these periods of mafic magmatism are high, which, together with negative e Nd values calculated for 2.5 and 3.3 Ga mafic magmatism, indicate recycled continental crust in their mantle source region. However, juvenile crustal addition seems to have been significant at 2.5 Ga, as is evident from the positive e Nd values for the majority of samples representing 2.5 Ga magmatism.