Underthrusting linked to closure of the Rocas Verdes Basin: insights from Turonian/Coniacian satellite plutons, Isla Capitán Aracena, southern Patagonia, Chile

Abstract

The Petite and Mercurio satellite plutons of the Patagonian Batholith intrude folded marine sedimentary successions and ophiolitic sequences within the Magallanes fold-and-thrust belt on Isla Capitán Aracena (~54° S), south of the Cenozoic transcurrent Magallanes-Fagnano Fault System. Their petrogenesis and tectonic significance were investigated through an integrated approach combining field observations, U-Pb zircon geochronology, whole-rock geochemistry, mineral chemistry and thermobarometry. Zircons from three samples of hornblende-bearing diorite and quartz diorite yielded Turonian/Coniacian crystallisation ages (90±1 Ma). They display calc-alkaline, metaluminous compositions, enrichment in Light Rare Earth Elements, and negative Nb-Ta-Ti anomalies, consistent with subduction-modified mantle sources in continental magmatic arcs. The compositions of amphibole, classified as magnesio-hornblende and tschermakitic hornblende, record crystallisation temperatures of ~800-850 °C and pressures of ~3-3.5 kbar (~10 km depth) in moderately oxidizing, hydrous magmas. Collectively, these data suggest that the Petite pluton was derived from water-saturated basaltic-andesite magmas whose compositions evolved through clinopyroxene and plagioclase fractionation, promoting physicochemical conditions conducive to late co-crystallisation of labradorite-andesine plagioclase and hornblende with pronounced Eu and Sr negative anomalies. The new data indicate that the oceanic remnants of the Rocas Verdes back-arc marginal basin were already buried at depths of approximately 10 km by ca. 90 Ma within a fold-and-thrust system developed during the underthrusting of continental crust.