NEOPROTEROZOIC-EARLY CAMBRIAN CALC-ALKALINE MAGMATISM IN THE EASTERN SIERRAS PAMPEANAS, ARGENTINA: U-PB ZIRCON AND ISOTOPIC CONSTRAINTS

The Pampean orogen in the eastern Sierras Pampeanas, Argentina contains two paired magmatic belts, an eastern calc-alkaline magmatic arc and a western peraluminous granite/high-grade metasedimentary belt. Considerable uncertainty exists about the relationship between the two belts and their relative timing. New U-Pb zircon ages from a variety of granodiorites, monzogranites and a dacite from Sierra Norté and easternmost Sierras de Córdoba indicate that the calc-alkaline arc magmatism was active over a 35 myr. period from 555 to 520 Ma. There is no apparent relationship between composition and age, and only one sample showed any evidence of inheritance of older zircon components. Initial Sr and Nd isotopic compositions display evidence for significant crustal contributions to all these magmas. Initial eNd values range from -3.5 to -7.2 and Nd model ages range from 1.35 to 1.58 Ga. These isotopic results are similar to those of the peraluminous magmas, a surprising result given the marked petrological contrasts between these suites.

Results from U-Pb analyses indicate that calc-alkaline magmatism largely preceded the development of peraluminous magmatism and high grade metamorphism in the western metasedimentary belt (525-515 Ma), and that the calc-alkaline magmatism abruptly terminated shortly after the peraluminous magmatism began. 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 are consistent with models that propose eastward-facing subduction of oceanic crust initiated at ca. 555 Ma followed by ridge-trench collision at ca. 525 Ma. Similarly aged arc-related and peraluminous magmatism occurs elsewhere along the paleo-Pacific margin of Gondwana, in the Saldania (Africa), Ross (Antarctica), and Delamerian (Australia) orogenic belts. The presence of these rocks suggests the possibility that similar processes of subduction beneath a continental margin followed by ridge-trench collision occurred along much of the Gondwana margin in late Neoproterozoic to Cambrian time.