Re-interpretation of volcanic units from San Ambrosio Island and Gonzalez Islet, Southeast Pacific, Chile: Using new textural and geochemical data

Abstract

The San Felix - San Ambrosio Oceanic Volcanic Complex (SSOVOC), located in the Southeast Pacific, 940 km offshore northern continental Chile, corresponds to a case of intraplate volcanism in the Nazca Plate. The complex is comprised of San Felix, San Ambrosio and González islands, which are mainly composed of alkaline lavas and hyaloclastic sequences. San Ambrosio Island (2.86–2.93 Ma) would be made up mainly, in its subaerial section, of two eruptive shield-type events called Carpa Group and Fardelas Group, the latter event being slightly more alkaline than the Carpa Group. The González Islet (1 Ma?) is made up of a Hyaloclastic Unit, submarine and subaerial basanitic lava flows and the Punta Bari Group, the latter described previously on San Felix Island. The Hyaloclastic Unit and basanitic lava flows have textures associated with an underwater deposition environment and a subsequent transition to a subaerial environment.Geochemical data indicate an increase in alkalinity between San Ambrosio Island and González Islet and different degrees of partial melting based on Th/Nd vs Th and Rb/Nd vs Rb data and enrichment in incompatible trace elements suggest a decrease in partial melting between the generating event of San Ambrosio Island and González Islet. The isotopic mixing model of Sr and Nd indicates that this volcanic complex represents a mixing of 35%–45% between DM and EM-I members, although from the San Ambrosio Island to González-San Félix Islands there is an independent trend between both islands towards EM-I, which suggests a cortical contamination of the same volcanic edifice of which the three islands of the complex are part.