U-PB AND LU-HF ISOTOPIC CONSTRAINTS ON THE PROVENANCE OF PERMIAN DETRITUS IN TURBIDITES FROM PATAGONIA AND LIVINGSTON ISLAND, ANTARCTICA; IMPLICATIONS FOR PLATE RECONSTRUCTIONS

Provenance of sedimentary rocks has been revolutionised through application of isotopic methods, in particular U-Pb detrital zircon dating. Such studies reveal not only the age spectrum for the source of zircon, but invariably a young age component that places significant constraints on, and sometimes redefines, time of deposition. In many cases this young age component is derived from a magmatic source that is coeval with deposition. Laser ablation, multi-collector ICP MS Lu-Hf analyses of previously U-Pb dated zircon can further constrain isotopic sources. The Hf isotope signature of an individual zircon age component will provide a characteristic signature of the protolith and information as to crustal residence time, enabling more refined correlations and reconstructions. Within the Late Palaeozoic accretionary wedge of the western margin of Patagonia and Antarctic Peninsula there are packages of metasedimentary rocks that lie outboard of the southern Patagonian and Antarctic Peninsula batholiths. Detrital zircons in turbiditic sandstones from both areas indicate a similar source characterized by an abundance of Permian-age detrital zircons. Previously it has been suggested that these have come from a Permian arc, possibly the Choiyoi province of South America. Hf isotopic data for Permian-age zircons from turbidites in a N to S transect from 50°S to 60°S yield dominant initial epsilon Hf values between -5 and -10. This indicates that the source for these Permian zircons had a significant crustal residence time and that they were not derived from a juvenile magmatic arc. Other grains record even more enriched Hf isotope signatures indicating derivation from yet older crustal sources. The Hf isotopes reinforce communality of the detrital zircon source and indicates that juvenile crust was not dominant, but that there may have been mixing between older sources of, say, Pan African (Brasiliano) and Grenville ages, together with a minor but significant subduction-related magmatic input. This supports the proposal that recently recognised but widespread Permian magmatism in Patagonia represents crustal melting in response to subducted slab break-off; the inferred Permian arc is not juvenile. The data is consistent with the premise that the Antarctic Peninsula was side by side with Patagonia during the Jurassic, receiving detritus from the same source.