Two syenitic phases in the Early Paleogene Silhouette Island volcano-plutonic complex, Seychelles

The Silhouette alkaline volcano-plutonic complex is the largest exposed Early Paleogene (62–64 Ma) igneous complex of the Seychelles microcontinent. The rocks of this study were collected from Baie Cipailles, Pte Vareur and Pte Ramasse Tout and include syenite (SiO₂ = 60–63 wt%), microgranite (SiO₂ = 70–74 wt%), tuffaceous trachyte (SiO₂ = 64–65 wt%) and a basaltic xenolith (SiO₂ = 44 wt%; MgO = 6.4 wt%; Mg# = 47.6). The silicic rocks of this study are ferroan, metaluminous to weakly peralkaline and classify as A1-type granitoids. The whole-rock Sr–Nd (⁸⁷Sr/⁸⁶Sr_i = 0.703894–0.706534; $ε$_Nd(t) = +0.5–+1.8) isotopes of the silicic rocks are similar to those of the basalt xenolith (⁸⁷Sr/⁸⁶Sr_i = 0.703576; $ε$_Nd(t) = +1.9). There is limited to no geochemical evidence for crustal contamination in any of the rocks. The syenitic rocks were probably derived by fractional crystallization of an alkaline basaltic parental magma in the upper crust and under reducing conditions (⁠$Δ$FMQ = −1). The Sr–Nd isotopes and zircon Hf (⁠$ε$_Hf(t) = +3.3–+9.1) isotopes of the silicic rocks are indicative of a moderately depleted source. We found distinct textural, mineralogical and compositional differences between the syenitic rocks, suggesting that the Silhouette complex is composed of at least two syenitic magma pulses.