New SHRIMP U-Pb and 40Ar/39Ar constraints on the crustal stabilization of southern South America, from the margin of the Rio de Plata (Sierra de Ventana) craton to northern Patagonia

TOHVER, E.; CAWOOD, P.; ROSSELLO, E.; LOPEZ DE LUCHI, M.G.; RAPALINI, A.E.; JOURDAN, F.

San Francisco

Congreso; AGU Fall Meet. San Francisco; 2008

American Geophysical Union

Two models exist to explain the late Paleozoic tectonic history for southern South America: an accretionary model of crustal growth through magmatism and a collisional model involving pre-existing continental elements, namely, the Rio de Plata craton and the possibly allochthonous terrane(s) of Patagonia, the Northern Patagonia Massif and the Deseado Massif. We report new U-Pb and 40Ar/39Ar results from a posited collision zone between the SW edge of the Rio de Plata craton and the northern margin of the Northern Patagonia Massif. Igneous basement samples from the Sierra de Ventana region, Buenos Aires province, were dated by ion microprobe (SHRIMP) analysis of zircon. A previously unrecognized occurrence of Paleoproterozoic basement indicates that the Rio de Plata craton extends ca.250 km farther west than considered. The majority of the basement rocks are shallow mid-Cambrian granitoids and rhyolites, including the rocks of the Cerro Colorado granite, which is intrusive into the sediments of the Curamalal Gp, signifying that these mature quartzites and conglomerates are older than early Cambrian in age, possibly correlated with the low-grade sedimentary rocks of the Tandilia Range that includes the La Tinta Fm. The 40Ar/39Ar ages from biotite, muscovite, and sericite from three different sheared basement localities demonstrates deformation in the latest Permian (265-260 Ma), some 20 Ma after the foreland deposition of the synorogenic Tunas Fm. in the upper Pilahuinco Gp, constrained by zircon ages in volcanic ashbeds. Farther south, along the northern margin of the Northern Patagonian Massif, late Ordovician cooling ages of granites intrusive into the Cambro-Ordovician Nahuel Niyeu Fm. are consistent with the presence of Ordovician magmatism along the W edge of the Rio de Plata craton. These ages alternate with late Permian cooling ages from undeformed granites and pegmatites, as well as early Jurassic cross-cutting dykes that are likely related to opening of the South Atlantic. Overall, the cooling age heterogeneity indicates that the thermal effects of igneous activity were localized, typical of higher crustal levels, whereas age homogeneization in the wake of crustal thickening and thermal relaxation should occur along the inferred collision zone. We suggest that the foreland propagation of deformation and discrete magmatism during the late Paleozoic is more consistent with accretionary tectonic models.