EVOLUTION OF THE SOUTH-CENTRAL ANDEAN ARC DETERMINED BY DETRITAL ZIRCON U-PB AND HF ANALYSIS OF RETROARC BASIN STRATA, CENTRAL ARGENTINA (32°S-40°S)

The Andes Mountains provide a natural laboratory to analyze the relationship between the tectonic evolution of a subduction margin, basin morphology, and volcanic activity. Magmatism in Cordilleran-style orogenic systems varies through time both in magnitude and in chemistry which are controlled by the rate of subduction, angle of subduction, absolute convergence velocity and the amount of shortening in and composition of the overriding plate. The Andean magmatic arc of Chile and Argentina has been active since at least 190 million years ago with variable volcanic output caused by changing tectonic regimes including extension from the Late Triassic to Late Cretaceous, contraction from the Late Cretaceous to Eocene, and variable extension and contraction along-strike in the late Paleogene to Neogene. Strata from the Andean retroarc in central Argentina ranging in age from Middle Jurassic to Miocene were sampled and analyzed to determine their detrital zircon U-Pb age spectra and Hf isotopic composition. When all detrital zircon data are combined, significant pulses in magmatic activity occur from 190-145 Ma, 130 Ma, 110 Ma, 70 Ma, 16 Ma, and 7 Ma. There are two significant hiatuses in magmatic activity, between 90 and 70 Ma and 40 and 20 Ma. Epsilon Hf values were relatively evolved (between 0 and -5) when the arc initiated at 190 Ma. Epsilon Hf values increased by 15-20 epsilon units between 160 and 150 Ma. The high magmatic output and large influence of juvenile material at 165 Ma coincide with extensional tectonism in the volcanic arc, which may have also been influenced by a reduction in convergence velocity. There is a relatively gradual transition to more evolved isotopic values between 100 Ma and 70 Ma which is conterminous with initial shortening at these latitudes. There was a large shift towards more juvenile Hf isotopic values at 20 Ma that is consistent with renewed extension in the back-arc during the Oligocene-early Miocene. Hf values become gradually more evolved through the Miocene, coincident with renewed shortening in the fold-and-thrust belt at that time. Overall, between 250 Ma and present there are three times when Hf values move towards more juvenile compositions, and all three are associated with extension and there are three episodes when values become more evolved which are associated with compression.