Petrogenesis of Neoproterozoic Granitoids and Related Rocks from the Seychelles: the Case for an Andean-type Arc Origin

Abstract

compositions (Nd 750 = +5•46 to −0•87; I Sr 750 = 0•7021-The Seychelles islands consist of undeformed and unmetamorphosed, 0•7061) that can be modelled as basaltic magmas derived from metaluminous monzogranites and granodiorites of Neoproterozoic depleted mantle, variably contaminated (0-15%) by Archaean age (>750 Ma). Subsolvus, and lesser hypersolvus granitoids are silicic crust. All petrologic, petrographic, geochemical, isotopic and crosscut by coeval dolerite dykes, dominantly of olivine tholeiite chronologic data for Neoproterozoic magmatic rocks of the Seychelles, composition. Field relations suggest that mixing between granitoid coupled with palaeomagnetic data indicating its position at the and doleritic magmas generated a variety of minor intermediate margin of the Rodinia supercontinent at >750 Ma, are at least rocks that occur as irregular masses and enclaves; their compositions consistent with, if not suggestive of, a continental or Andean-type plot as linear arrays between those of dolerites and granitoids. Two arc setting. We argue, therefore, that the conventionally accepted groups of granitoids can be distinguished based on colour, chemistry notion of an extensional (i.e. rift or plume) setting for Seychelles and isotopic signature. Mahé Group granitoids are grey, with magmatism is vulnerable. relatively low incompatible element concentrations and primitive isotopic signatures that cluster at 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, KEY WORDS: Seychelles; Neoproterozoic; granitoid; Andean-type arc; U >4•2 ppm, Th >20 ppm, Pb >30 ppm, and correspondingly petrogenesis evolved and variable isotopic signatures (Nd, T = +0•80 to −3•71 and I Sr, 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. INTRODUCTION Potential source materials are unexposed in the Seychelles, but the The Seychelles (Figs 1 and 2), correctly interpreted by juvenile component may resemble >750 Ma intermediate to mafic both Wegener (1924) and Du Toit (1937) as a continental metavolcanic rocks of NW Madagascar or NE India; candidates fragment left behind after Gondwana break-up, have for the ancient silicic component, whose signature is slightly enhanced long been considered as a geological curiosity, repin granitoids of the Praslin Group, may be represented by 2•5-3•3 resenting small, but conspicuous granitic islands in the Ga tonalitic gneisses of the Banded Gneiss Complex in Rajasthan Indian Ocean, at least 1000 km from the prominent (NW India), or by similar materials present in central-northern continental land masses of Madagascar or Africa. About 25 of the >100 islands in the Seychelles archipelago Madagascar. Seychelles dolerites show variable Nd and Sr isotopic

Biodiversity hotspots and Ocbil theory

“…Soils at the Seychelles archipelago are derived from weathering of Neoproterozoic granites (Ashwal et al 2002). Lodoicea maldivica is a remarkable monotypic palm endemic to the most impoverished-soils (especially in P) in two islands.…”

Provenance and tectonic setting of Neoproterozoic sedimentary sequences in the South China Block: evidence from detrital zircon ages and Hf–Nd isotopes

“…Another relationship between the Yangtze Block and India is the occurrence of Neoproterozoic mafic volcanic and low-18 O magmatic rocks in both cratons (Yan et al 2002;Zheng et al 2004). In addition, the continental arc setting for the sediments in the present study requires a subduction environment along the Yangtze Block, which probably extended into the Neoproterozoic continental arcs along the western India, Seychelles and Madagascar (Torsvik et al 2001a;Tucker et al 2001;Ashwal et al 2002).…”

Proterozoic Tectonostratigraphy and Paleogeography of Central Madagascar Derived from Detrital Zircon U‐Pb Age Populations

“…Somewhat younger provinces in the Seychelles (∼750 Ma; Ashwal et al 2002) and the Bemarivo block of northern Madagascar (∼715-750 Ma; Tucker et al 1999a;Buchwaldt and Tucker 2001) may represent a companion arc on the opposite side of the Mozambique ocean.…”