Back-arc shoshonitic magmatism in the Fuegian Andes, Argentina.

GONZÁLEZ GUILLOT, M.; BIEL SORIA, C.; ACEVEDO, R.; ESCAYOLA, M.; SCHALAMUK, I.

Oviedo, España

Simposio; International Applied Geochemistry Symposium (IAGS); 2007

New geochemical data have led to a different interpretation of theCretaceous magmatism in the Fuegian Andes, Argentina. In this area,several isolated plutons outcrop, ranging from pyroxenites to syenites.They were previously considered as part of the arc-related, calc-alkalinePatagonian Batholith (PB), which outcrops to the south. They have amildly alkaline shoshonitic affinity, and their location far from the arc axis,suggest a back-arc environment, allowing to be considered as a differentgeological unit. It has been proposed that these magmas generated undera similar tectonic regime than the PB, but at greater depths in the mantle(K-h model by Dickinson), thus, accounting for their high K2O content andK2O/Na2O ratio. Additionally, there is an increase in K2O for a given SiO2content in the shoshonitic rocks and some of their mafic minerals towardsthe north, accompanied by a volumetrically increase in syenites. Thesefavor a model of magma generation at gradually greater depths as theslab sinks, acquiring higher K content with distance to the trench.