Resolving the origin of the Seychelles microcontinent: Insight from zircon geochronology and Hf isotopes

The Seychelles microcontinent is located in the western Indian Ocean and is exposed as a series of granitic islands. The Main Islands of the Seychelles consist of the Mahé and Praslin granite groups. They are characterized as ferroan, metaluminous to peraluminous, alkali-calcic to calc-alkalic, and resemble post-collisional A-type granite but have distinct Sr-Nd-Pb-O isotopic compositions. Previous studies showed that the granites and syn-plutonic dolerites were emplacement at 760–750 Ma, although rocks from Île aux Récifs yielded an older age (808.8 ± 1.9 Ma). New in situ zircon U-Pb geochronology of granites from northern Mahé (751.9 ± 4.6 Ma, 755.5 ± 5.8 Ma, 756.2 ± 7.7 Ma) and western Praslin (757 ± 11 Ma, 753.0 ± 7.5 Ma, 751.3 ± 5.8 Ma) yielded weighted-mean ²⁰⁶Pb/²³⁸U ages similar to previous studies but a significant amount of inherited zircon was identified with ages ranging from ~880 Ma to ~770 Ma. The weighted-mean ²⁰⁶Pb/²³⁸U ages of the principal inherited zircon populations from the Mahé (777.5 ± 5.7 Ma, 807.7 ± 7.7 Ma) granites are within uncertainty of those identified from the Praslin (777.5 ± 6.5 Ma, 814 ± 13 Ma) granites indicating there was an older period of magmatism within the Seychelles microcontinent. The Hf isotopic data of the magmatic and inherited zircons from the Mahé granite are generally more depleted (ε_Hf(t) = +6.7 to +13.2) and have younger model ages (T_DM2 = 902 Ma to 1219 Ma) than those from the Praslin granite (ε_Hf(t) = +2.2 to +10.9, T_DM2 = 986 to 1524 Ma). The differences between the zircon Hf isotopes from the Mahé and Praslin granites is consistent with other isotopic systems and indicates that the isotopic dichotomy is an intrinsic feature of the Seychelles microcontinent that extends back to at least ~840 Ma and possibly to the Mesoproterozoic (1000 Ma to 1400 Ma). It is likely that the Mahé and Praslin granites were emplaced during a period of post-collisional extension/crustal relaxation and that the older inherited zircons are derived from rocks that record an earlier period of magmatism that was likely subduction-related.