Combined U-Pb SHRIMP and Hf isotope study of the Late Paleozoic Yaminue Complex,Rio Negro Province,Argentina:Implications for the origin and evolution of the Patagonia composite terrane

We have carried out zircon U-Pb SHRIMP dating and Hf isotope determinations on a biotite paraschist and on a tonalitic orthogneiss of the Yaminué Complex, and re-evaluate this complex in the broader context of the tectonic evolution of the Patagonia composite terrane. In the metasedimentary unit （msuYC）, the youngest detrital zircon dated at 318 ± 5 Ma （Mississippian/Pennsylvanian boundary） indicates a Pennsylvanian （or younger） depositional age. The three main age populations peak at 474, 454 and 374 Ma. Preliminary Hf isotope data for two detrital zircons （447 and 655 Ma） yielded ε（Hf） values of -0.32 and 0.48, indicating that their primary sources contained small amounts of recycled crustal components （of Calymmian age; TDM 1.56 Ga）. Zircons from the orthogneiss （miuYC; intrusive into msuYC） show a crystallization age of 261.3 - 2.7 Ma （Capitanian; late middle Permian） which is broadly coeval with deformation, and Neoarchean-Paleoproterozoic inheritance. Meaningful core-rim relationship between Neoarchean zircon cores and late Permian rims is well defined, indicating the occurrence of Archean crust in this sector of Patagonia. Hf TDM of Permian zircons is mainly Meso-Paleoarchean （2.97-3.35 Ga）, with highly negative e（Hf） values （ca, -33）. Hf TDM of inherited Neoarchean zircon cores is also Meso-Paleoarchean （3.14-3.45 Ga） but more juvenile （ε（Hf） = -0.3）. Hf isotopes reinforce the presence of unexposed ancient crust in this area. Combining geological and isotope data, as well as geophysical models, we identify the Yaminué Complex within the La Esperanza-Yaminué crustal block flanked by two other, distinct crustal blocks： the Eastern block which forms part of the Patagonia terrane sensu stricto, located in the eastern Patagonian region, and the Western block forming part of the Southern Patagonia terrane. Their origins and timing of amalgamation to form the Patagonia composite terrane are also discussed.

The Rhyacian El Cortijo suture zone: Aeromagnetic signature and insights for the geodynamic evolution of the southwestern Rio de la Plata craton, Argentina

The amalgamation of the southern Rio de la Plata craton involves two possibly coeval Rhyacian sutures associated with the Transamazonian orogeny, rather than a single one as previously envisaged, i.e. the El Cortijo suture zone and the Salado suture. We circumscribe the Tandilia terrane to the region between these two sutures. The E1 Cortijo suture zone runs along a roughly WNW oriented magnetic low aligned along the southern boundary of the Tandilia terrane, i.e. boundary between the Tandilia and Balcarce terranes. This extensive magnetic low, ca. 300 km long, and ca. 90 km wide, would be caused by demagnetization associated with shearing. At a more local scale, the trend of the El Cortijo suture zone often turns toward the E-W. At this scale, WNW trending tholeiitic dykes of Statherian age are seen to cut the Rhyacian E1 Cortijo suture zone. Spatially associated with the El Corti]o suture zone, there are small magnetic highs interpreted to be related to unexposed basic bodies of ophiolitic nature related to those forming part of the El Cortijo Formation. We envisage the pre-Neoproterozoic evolution of the Tandilia belt to have been initiated by the extension of Neoarchean （ - 2650 Ma） crust occurred during Siderian times （2500-2300 Ma）, causing the separation between the Balcarce, Tandilia and Buenos Aires terranes, and the development of narrow oceans at both north and south sides of the Tandilia terrane, accompanied by -2300-2200 Ma sedi- mentation over transitional -continental to oceanic- crust, and arc magmatism developed in the Tan- dilia terrane. The island arc represented by the El Cortijo Formation was also developed at this time. At late Rhyacian times, it occurred in both the closure of the narrow oceans developed previously, the entrapment of the El Cortijo island arc, as well as anatectic magmatism in the Balcarce terrane.