The origin of low δ18O granites and related rocks from the Seychelles

Abstract

The 750-Ma Seychelles granites have whole-rock δ18O values that range from –1.2 to +7.5‰. Differences in δ18O values between quartz, feldspar and whole-rock for samples that have a range of δ18O values suggest that these minerals are in magmatic equilibrium, and that the whole-rock δ18O values were little affected by post-crystallization interaction with fluids. Two suites of granites (the Mahe type and the Praslin type) have previously been recognized on the basis of chemical and radiogenic isotope composition. The former have a mean whole-rock δ18O value of 5.25±0.65‰ (1σ), whereas the latter are much more variable, with a mean whole-rock δ18O value of 3.00±3.27‰. Biotite and amphibole separates from both granite types range in δD from –69 to –116‰, and show a positive correlation with whole-rock δ18O values. Dolerite dykes that intruded the granites during or shortly after their crystallization also have whole-rock δ18O values (mean 2.24±1.93‰) lower than that expected for mantle-derived basaltic rocks, but these values appear to be the result of fluid–rock interaction. We suggest that the Mahe-type granites are derived mainly from juvenile mafic to intermediate crust with 'normal' δD and δ18O values, whereas the Praslin-type granites are mixtures of this source and older crust that acquired its low δD and δ18O values by extensive interaction with meteoric water at high temperature. It is unlikely that meteoric water sufficiently depleted in D and 18O was available at 750 Ma because of the relatively low latitude of the region at that time. We suggest that alteration of the source took place significantly before production of the granite magmas. Depletion in 18O of the Seychelles granites does not necessarily require a regional extensional tectonic setting at 750 Ma, as has been proposed.