Felsic magmatism and hydrothermal activity of the Central Igneous Complex of Boa Vista (Cape Verde islands): Insights from zircon, titanite and apatite geochronology

Abstract

Studies focused on the U-Pb geochronology and oxygen isotope composition of magmatic zircon formed in intraplate oceanic islands are very scarce. The Boa Vista Island (Cape Verde archipelago) displays one of the few felsic igneous complexes (alkaline volcanic and plutonic rocks) in an oceanic setting and provides the opportunity to date and characterise magmatic-hydrothermal events based on the isotopic records of zircon. Due to the scarcity of this mineral in these igneous rocks, we have separated detrital zircon from sediments of dry riverbeds of the Central Igneous Complex of the island of Boa Vista, which is dominantly felsic in composition. Zircon has been analysed to determine the U–Pb–O isotopic composition and trace element contents. All zircon grains display complex CL textures indicative of a strong transformation due to interaction with a hydrothermal agent. This reactive event led to a strong disturbance of the U-Pb system, enrichment of trace elements (Ti, Nb, Ta, Th, U, Pb, Y and REE) and decrease of δ18O values (down to −6 ‰). These features suggest that the magmatic U–Th-rich zircon crystallised from highly evolved melts and experienced partial metamictization prior to its transformation. High-T hydrothermal fluids were likely mixed with groundwater (imprinting such low oxygen isotope composition) and reacted with and transformed zircon crystals from the main felsic igneous formations. The geochronological results obtained by SHRIMP on zircon have been complemented with U-Pb ages obtained by laser ablation SF–ICP–MS on titanite and apatite of syenitic intrusions. Overall, the geochronological results fall within a restricted range of ages from 13.3 to 15 Ma. These results, together with literature data, imply that most of the felsic volcanic and plutonic rocks from Boa Vista were formed in the range ∼ 12–15 Ma (episodes 1 and 2 of the second stage of magmatism). The hydrothermal process observed in the studied detrital zircon likely occurred during the last felsic magmatic events (∼10–12 Ma).