DETRITAL-ZIRCON GEOCHRONOLOGY OF THE FUEGIAN ANDES

Patagonia and Tierra del Fuego form the southern terminus of the modern Andes, a 6500 km long mountain belt that spans more than 60° of latitude over which dramatic variations in subduction style, convergence, elevation and climate have provided an exceptional laboratory to evaluate the relationships between these phenomena and mountain-building. The southernmost Andes are an important component of this orogen, as convergence in this region has been augmented by development of the Patagonian orocline that may have added hundreds of kilometers of shortening to that experienced by the Andes north of 50º S. In order to better understand this region's kinematics, this study reports U-Pb detrital-zircon age data collected by LA-MC-ICP-MS from metasedimentary basement, marginal basin and foreland basin strata of the southernmost Andes and its associated Rocas Verdes and Magallanes basins. The zircon composition of a metasedimentary sample collected from the Cordillera Darwin complex is dominated by late Paleozoic (250-350 Ma) grains, and contains subsidiary 450-650 Ma and 700-1100 Ma age peaks, as well as scattered individual Mesoproterozoic, Paleoproterozoic and Archean grains. The zircon composition of Jurassic-Cretaceous Rocas Verdes samples are dominated by 110-130 Ma grains, with a subordinate 70-100 Ma age peak. Zircons collected from Eocene-Miocene foreland basin strata are largely post-Triassic in age, with dominant middle Jurassic, late Cretaceous and Eocene-Oligocene age peaks, and subordinate early Cretaceous, middle Cretaceous, and early Paleogene peaks. Pre-Jurassic zircon age peaks identified in foreland basin strata are centered around the late Paleozoic, the Neoproterozoic-Eocambrian and the late Mesoproterozoic. Collectively, these data suggest a protracted history of Meso-Cenozoic orogenesis that contributed sediment from metamorphic basement, extensional volcanic, and batholith terranes, and encourage reconsideration of depositional ages suggested by other means.