Timing and duration of the calc-alkaline arc of the Pampean Orogeny: Implications for the late Neoproterozoic to Cambrian evolution of western Gondwana

The Pampean Orogen in the Eastern Sierras Pampeanas contains two paired magmatic belts, an eastern calc-alkaline magmatic belt and a western peraluminous granite/high-grade metasedimentary belt. The relationship between the two belts and their relative timing are constrained through new U-Pb zircon ages on granodiorites, monzogranites, and associated volcanic rocks from Sierra Norte and the easternmost Sierras de Cordoba. These ages indicate that calc-alkaline arc magmatism was active over at least a 30-m.yr. period from 555 to 525 Ma, terminating at the same time that peraluminous magmatism and associated high-grade metamorphism began in the adjoining metasedimentary belt (525-515 Ma). These temporal relationships and the metamorphic characteristics of the two belts appear to be in conflict with previously proposed models for the Pampean Orogeny as a continental-collision event, but they are consistent with models that propose eastward-dipping subduction of oceanic crust initiated at ca. 555 Ma, followed by ridge-trench collision at ca. 525 Ma. Similar-aged belts of arc-related and peraluminous magmatism occur elsewhere along the paleo-Pacific margin of Gondwana, suggesting that similar processes of subduction and noncollisional peraluminous magmatism occurred along much of the Gondwana margin in late Neoproterozoic to Cambrian time.