PROVENANCE OF LATE PALAEOZOIC METASEDIMENTS OF THE SW SOUTH AMERICAN GONDWANA MARGIN: A COMBINED U-PB AND HF-ISOTOPE STUDY OF SINGLE DETRITAL ZIRCONS

Abstract

Combined U-Pb and Lu-Hf isotope measurement of single detrital zircon grains in Carboniferous metasediments from Patagonia delineate the source areas of the sediments, The detritus, represented by four metasandstone samples, was deposited prior to onset of subduction in Late Carboniferous time along the south Patagonian proto-Pacific Gondwana margin. A broad series of detrital zircon age peaks (0.35-0.7 Ga, 0.9-1.5 Ga) and a large spread (0.3-3.5 Ga) in the age spectra require numerous sources. A fifth metasediment was deposited after the onset of subduction. This syncollisional sample shows two distinct U-Pb age peaks at c. 290 Ma and 305 Ma. This points to a few sources only (Patagonia, West Antarctica). Initial Hf-isotope compositions of selected U-Pb dated zircons from the Carboniferous metasediments reveal zircon protoliths originating from both recycled crust and juvenile sources (εHf(T=0.4-3.5Ga) = -14 to +12). A comparison with crustal compositions of possible source areas indicates that the detritus mainly originated from the interior of Gondwana (Extra-Andean Patagonia, the Argentine Sierra de la Ventana, southernmost Africa, East Antarctica), as well as northern Chile and northwestern Argentina. The sediment transportation paths are consistent with an autochthonous palaeogeographical position of Patagonia with respect to Gondwana in Carboniferous time.