ISOTOPIC PERSPECTIVES ON 150 MA OF SUBDUCTION-RELATED CRUSTAL GROWTH IN THE SOUTH PATAGONIAN BATHOLITH

The South Patagonian Batholith (47–53ºS) is a typical Cordilleran batholith formed along the active continental margin of southwestern South America. It provides a rare opportunity to examine the evolution of subduction-related magma genesis over a 150 Ma period. Previous U-Pb zircon dating has shown that the oldest intrusions were emplaced during the late Jurassic (150-157 Ma ago), and that magmatic events were not continuous, but episodic through to the Neogene. The late Jurassic to early Cretaceous (~157 to ~137 Ma) and Neogene (~25 to ~15 Ma) are notable periods of considerable magma generation. SHRIMP δ¹⁸O and LA-MC-ICP-MS ε_Hf data for previously dated zircons provide a far greater insight into the evolution of the South Patagonian batholith than was possible from U-Pb and whole-rock Sm-Nd data alone. The δ¹⁸O values in the late Jurassic are high (ranging up to +8) indicating crustally-derived magma sources. They decline rapidly during the Late Jurassic and early Cretaceous to δ¹⁸O values seen in zircon from mantle-derived sources (~+5.0 to +5.6) and even lower to values typical of hydrothermally-altered sources. This suggests that with the passage of time, the crustal sources were relatively quickly consumed by magma generation in the subduction process. Involvement of crustal melting gave way to primitive, and hydrothermally-altered sources for the generation of the middle Cretaceous and younger plutons. The ε_Hf data supports this model, but also indicates that whilst sources were primitive from middle Cretaceous time, they had already had a significant crustal residence time.