Provenance of Late Paleozoic sediments in the southern Patagonian Andes: age estimates, sources, and depootional setting

In the southern Patagonian Andes of Chile and Argentina the Late Palaeozoic basement of the Andes is mainly composed of metasediments. SHRIMP U-Pb ages of detrital zircons from turbiditic rocks of the Cochrane unit, which belongs to the Eastern Andean Metamorphic Complex in Chile, constrain the maximum depositional age to Devonian to Early Carboniferous for this unit. SHRIMP U-Pb ages further restrict deposition of the Bahía de la Lancha Formation in Argentina to late Early Carboniferous times. The main sediment sources of the Cochrane unit and the Bahía de la Lancha Formation were generally dominated by felsic rocks, as revealed by the whole-rock chemistry (Th/Sc = 0.6-1.9). The sedimentary detritus is dominated by recycled metasedimentary and metamorphic material, as revealed by cathodoluminescence of quartz, chemistry of tourmaline and zoning of zircon. The source material was of continental origin and had been recycled within the crust before becoming part of the studied turbidite deposits as indicated by whole-rock Sr and Nd isotope signatures and U-Pb ages of detrital zircons (87Sr/86Sr(T = 250 Ma) = 0.709-0.718; e Nd (T = 320-350 Ma) = -8 to -2; Nd TDM* = 1170-1540 Ma; ca. 90 % of the zircons have U-Pb ages of < 1500 Ma, with a dominance in the interval 350-700 Ma). As indicated by the U-Pb ages and Hf isotope signatures of single zircons, the original main source areas for the detrital grains encompassed present extra-Andean Patagonia, as well as the Argentinean Sierra de la Ventana slightly north of Patagonia. Furthermore, southern Africa, East Antarctica and the Falkland Plateau, which formed an area in the interior of Gondwana, were probably important original source areas for the sediments. The part of the Eastern Andean Metamorphic Complex that crops out on the shoreline of Fiordo Peel (ca. 50°45'S) in the southern Chilean archipelago has a Late Carboniferous - Early Permian maximum depositional age, as deduced from the SHRIMP Upb dating. Magmatic source material dominates the sediments, as revealed by the abundance of zircons with magmatic zoning. The primary sources were mainly felsic and had a continental setting (Th/Sc = 0.8-1.1; 87Sr/86Sr(T = 250 Ma) = 0.705-0.710; e Nd (T=280) = -5 to -4; Nd TDM* = 1270-1320 Ma; > 40 % of the zircons have U-Pb ages between 290 and 330 Ma). The sediments most likely had short transportation paths. With the data above, this study supports an onset of subduction at the Late Palaeozoic Pacific margin of Gondwana (present coordinates) in Late Carboniferous times.