Mississippian volcanism in the south-central Andes: New U-Pb SHRIMP zircon geochronology and whole-rock geochemistry

摘要

In the northern extension of the Famatina and the southern Puna (NW Argentina) prominent rhyolitic volcanic rocks traditionally referred to as Ordovician are exposed, resting on metamorphic basement and covered by thick Late Paleozoic siliciclastic successions. We report new U-Pb SHRIMP ages from these rhyolites that show them to be of Mississippian (348-342Ma) age, thus identifying a previously unknown volcanic event in this portion of western Gondwana. Whole-rock geochemistry and Sr-Nd isotopic analyses suggest a crustal source for these rocks but with a juvenile input (ε_(Nd)(t) between -2.91 and -0.3, and T_(DM) values between 1.09 and 1.1Ga). This is different from the Early Paleozoic magmatism of western Argentina where crustal recycling took place without any involvement of mantle material. The Carboniferous magmatism is compatible with an extensional environment developed along the Terra Australis accretionary orogen as a result of tectonic switching processes. These rhyolites may be related to the coeval Mississippian A-type granites exposed to the east, in the Sierras Pampeanas, confirming the regional character of this magmatism.