Magmatic Evolution of the Southernmost Andes and its Relation with Subduction Processes

Abstract

The magmatic arc in the SouthernmostAndes is represented by the Fuegian Batholith and satellite plutons, plus minorvolcanic and volca-niclastic sequences. Magmatism was active from the LateJurassic to the Miocene in response to subduction of the Pacific plates beneathSouth America. The batholith was constructed episodically and includes acalc-alkaline trench-side arc and shoshonitic rear-arc suites. The oldestcomponents are minor S-like granitoids; recent volcanism is very scarce. Thevolcanic front shifted, and the composition of magmas changed several times dueto a variable subduction configuration. Major tectonic events included theopening and closure of the Rocas Verdes marginal basin in theJurassic-Cretaceous, and oroclinal bending in the latest Cretaceous-Paleogene.A well established slab shallowing event initiated in the mid-Cretaceous,causing a cratonward migration of the volcanic front, the widening of the arc,and inception of potassic magmatism in the rear-arc. This was associated withmarginal basin inversion, regional peak metamorphism and orogenic deformation.Different timing and position of rear-arc suites with respect to the volcanicfront in the Fuegian and Southern Patagonian Andes may have been influenced bydifferential development of the marginal basin, which was wider to the south.The absence of magmatism in the Miocene may have responded to highly obliqueconvergence plus buoyant subduction.