Journal of Petrology Volume 43 Number 1 Pages 45-83 2002
© Oxford University Press 2002
Petrogenesis of Neoproterozoic Granitoids and Related Rocks from the Seychelles: the Case for an Andean-type Arc Origin
1DEPARTMENT OF GEOLOGY, RAND AFRIKAANS UNIVERSITY, PO BOX 524, AUCKLAND PARK 2006, SOUTH AFRICA
2DÉPARTEMENT DES SCIENCES DE LA TERRE ET L’ENVIRONNEMENT, UNIVERSITÉ LIBRE DE BRUXELLES, CP 160/02, AV. F. D. ROOSEVELT 50, B-1050 BRUSSELS, BELGIUM
3GEOLOGICAL SURVEY OF NORWAY, PO BOX 3006 LADE, N-7002, TRONDHEIM, NORWAY
The Seychelles islands consist of undeformed and unmetamorphosed, metaluminous monzogranites and granodiorites of Neoproterozoic age (
750 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. Mahé Group granitoids are grey, with relatively low incompatible element concentrations and primitive isotopic signatures that cluster at 
Nd 750 = +2·85 ± 0·17 and ISr 750 = 0·7031 ± 0·0008 (some samples with impossibly low ISr <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 (Nd, T = +0·80 to -3·71 and ISr, 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 750 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 (Nd 750 = +5·46 to -0·87; ISr 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 750 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.
KEY WORDS: Seychelles; Neoproterozoic; granitoid; Andean-type arc; petrogenesis
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