Petrogenesis of Neoproterozoic granitoids and related rocks from the Seychelles: The case for an Andean-type arc origin

The Seychelles islands consist of undeformed and unmetamorphosed, metaluminous monzogranites and granodiorites of Neoproterozoic age (similar to750 Ma). Subsolvus, and lesser hypersolvus granitoids are crosscut by coeval dolerite dykes, dominantly of olivine tholeiite composition. Field relations suggest that mixing between granitoid and doleritic magmas generated a variety of minor intermediate rocks that occur as irregular masses and enclaves; their compositions plot as linear arrays between those of dolerites and granitoids. Two groups of granitoids can be distinguished based on colour, chemistry and isotopic signature. Mahe Group granitoids are grey, with relatively low incompatible element concentrations and primitive isotopic signatures that cluster at epsilon(Nd 750) = +2.85 +/- 0.17 and I-Sr 750 = 0.7031 +/- 0.0008 (some samples with impossibly low I-Sr <0.700 were affected by open-system processes). Praslin Group granitoids are characteristically red to pink, with Rb >180 ppm, U >4.2 ppm, Th >20 ppm, Pb > 30 ppm, and correspondingly evolved and variable isotopic signatures (epsilon(Nd, T) = +0.80 to -3.71 and I-Sr,I- T = 0.7032-0.7263). Both groups of granitoids appear to have been derived from a mixed source dominated by a juvenile, mantle-derived component, with variable amounts of an ancient, possibly Archaean, silicic source constituent or contaminant. Potential source materials are unexposed in the Seychelles, but the juvenile component may resemble similar to750 Ma intermediate to mafic metavolcanic rocks of NW Madagascar or NE India; candidates for the ancient silicic component, whose signature is slightly enhanced in granitoids of the Praslin Group, may be represented by 2.5-3.3 Ga tonalitic gneisses of the Banded Gneiss Complex in Rajasthan (NW India), or by similar materials present in central-northern Madagascar. Seychelles dolerites show variable Nd and Sr isotopic compositions (epsilon(Nd 750) = +5.46 to -0.87; I-Sr 750 = 0.7021-0.7061) that can be modelled as basaltic magmas derived from depleted mantle, variably contaminated (0-15%) by Archaean silicic crust. All petrologic, petrographic, geochemical, isotopic and chronologic data for Neoproterozoic magmatic rocks of the Seychelles, coupled with palaeomagnetic data indicating its position at the margin of the Rodinia supercontinent at similar to750 Ma, are at least consistent with, if not suggestive of, a continental or Andean-type arc setting. We argue, therefore, that the conventionally accepted notion of an extensional (i.e. rift or plume) setting for Seychelles magmatism is vulnerable.