ABSTRACT

This paper newly reports results of LA-ICPMS U-Pb dating for 282 zircon crystals separated from a Middle Miocene adakite in Cerro Pampa, southern Argentine Patagonia. With the exception of one spot age, 174 of the U-Pb concordia ages are markedly older (>94 Ma) than the cooling ages of the adakite magma (ca. 12 Ma). The presence of numerous exotic zircon crystals indicates that the adakitic magma carries up information related to the crustal components during its ascent through the entire crust underneath Cerro Pampa. The obtained concordia ages of exotic zircons, 94-1335 Ma, are divisible into five groups having distinctive peaks on a population diagram. The first (94-125 Ma) and second age groups (125-145 Ma) correspond to the age of plutonic activities that formed the main body of the South Patagonian Batholith. The third to fifth groups respectively correspond to activities of the El Qumado-Ibañez volcanic complex (145-170 Ma), plutonic rocks scarcely exposed in Central Patagonia (170-200 Ma), and the Eastern Andean metamorphic complex of Late Paleozoic to Early Mesozoic ages (200-380 Ma). Our data suggest that the crust underneath Cerro Pampa was formed mostly after 380 Ma, the majority forming during the Early Cretaceous to Middle Jurassic. The processes of crustal development ceased for ca. 80 m.y. until the activity of the Cerro Pampa adakite in ca. 12 Ma. In contrast to the existence of numerous Archaean-Palaeoproterozoic exotic zircons in Mesozoic plutonic rocks distributed in Andean Cordillera at around 46°S, no evidence was found for Archaean-Paleoproterozoic crust on the Cerro Pampa region at 48°S. This evidence suggests that two crusts must have aggregated along a boundary between 46°S and 48°S with the continental margin of Gondwana during Late Paleozoic times, as part of the amalgamation of Pangea.