Back-arc shoshonitic magmatism in the Fuegian Andes, Argentina

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

Oviedo, España.

Simposio; 23th International Applied Geochemistry Symposium; 2007

IAGS

New geochemical data have led to a different interpretation of the Cretaceous 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-alkaline Patagonian Batholith (PB), which outcrops to the south. They have a mildly alkaline shoshonitic affinity, and their location far from the arc axis, suggest a back-arc environment, allowing to be considered as a different geological unit. It has been proposed that these magmas generated under a 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 and K2O/Na2O ratio. Additionally, there is an increase in K2O for a given SiO2 content in the shoshonitic rocks and some of their mafic minerals towards the north, accompanied by a volumetrically increase in syenites. These favor a model of magma generation at gradually greater depths as the slab sinks, acquiring higher K content with distance to the trench.