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

The 750-Ma Seychelles granites have whole-rock delta(18)O values that range from -1.2 to + 7.5parts per thousand. Differences in delta(18)O values between quartz, feldspar and whole-rock for samples that have a range of 6180 values suggest that these minerals are in magmatic equilibrium, and that the whole-rock delta(18)O values were little affected by post-crystallization interaction,kith 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 delta(18)O value of 5.25 +/- 0.65parts per thousand (1sigma), whereas the latter are much more variable, with a mean whole-rock PO value of 3.00 +/- 3.27parts per thousand. Biotite and amphibole separates from both granite types range in deltaD from -69 to -116parts per thousand, and show a positive correlation with whole-rock delta(18)O values. Dolerite dykes that intruded the granites during or shortly after their crystallization also have whole-rock PO values (mean 2.24 +/- 1.93parts per thousand) 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' deltaD and delta(18)O values, whereas the Praslin-type granites are mixtures of this source and older crust that acquired its low deltaD and delta(18)O values by extensive interaction with meteoric water at high temperature. It is unlikely that meteoric water sufficiently depleted in D and O-18 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 O-18 of the Seychelles granites does not necessarily require a regional extensional tectonic setting at 750 Ma, as has been proposed.