gems, analytical, geology, Red Sea, Saudi Arabia, olivine basalts, Eritrea, Ethiopia, Yemen

Perim Island is an eroded fragment of the southwest flank of a late Miocene (10.5 ± 1.0 Ma) volcano whose centre lay on the southwesternmost tip of Arabia. The volcano is the westernmost of the E–W line of six central vent volcanoes (the Aden Line) that extends 200 km along the south coast of Arabia from Perim to Aden. Major oxide and trace element abundances are given for 35 Perim specimens and these show that the volcano has within-plate trace element characteristics and consists of a petrographically and geochemically simple suite of alumina-poor olivine basalts, andesites, and transitional andesite–trachyandesites. Six specimens, however, are markedly enriched in Al₂O₃ and CaO, and contain abundant (20–30 mode %) highly calcic (An₇₇–₈₃) plagioclase phenocrysts. Geochemical modelling suggests that the main Perim volcanic sequence was produced by the fractional crystallization (ol + cpx + Ti-mt + plag) of a silica-saturated (SiO₂ c. 45 %) basic melt. The high Al, high Ca magmas appear to be mixing products of plagioclase-enriched basic magmas with more evolved melts. Perim is the oldest volcano of the Aden Line, which becomes increasingly younger and alkalic eastward. It is suggested that the volcanism is related to an eastwards-propagating rift produced before the most recent stage of sea-floor spreading in the Gulf of Aden (4.5 Ma–present).