Lithology and age of the Cushamen Formation. Devonian magmatism in the western North Patagonian Massif. Argentina

LOPEZ DE LUCHI, M.G.; CERREDO, M.E.; MARTINEZ DOPICO

Bolletino di Geofisica teorica ed applicata,

Istituto Nazionale di Oceanografia e di Geofisica Sperimentale

Año: 2010 vol. 51 p. 71 - 74

0006-6729

A minimum age for the metamorphism ca 370 Ma could be inferred based on the isochron for the granitoids the area Río-Chico-Cushamen which underwent the D1-D2 deformation. In the area of Laguna del Toro the 372 Ma Cáceres orthogneiss (Pankhurst et al., 2006) is emplaced in a metamorphic pile which exhibit similar lithologies than the CF. (Fig. 1). Recalculation of the U-Pb data for metamorphic rims of zircons of El Maitén paragneiss (Pankhurst et al., 2006) indicate a peak between 360-380 Ma and subordinate peak at 330-340 Ma. Results extracted from the analysis of the relativity probability vs. age plot of the zircons of the Pto.Miranda area (Hervé et al., 2005) indicate that only three data have 206Pb/238U ages younger than 350 Ma. In this connection, we want to highlight that our new isochron enlarges the group of Late Devonian intrusions occurring along the western border of the NPM. From San Martin de los Andes in the north to the area of Colonia Cushamen in the south, several ~400-360 Ma (Varela et al., 2005; Pankhurst et al., 2006;) deformed tonalitic to granitic intrusions show isotopic signatures pointing to crustal sources (Sr/Sri > 0,710, eNd < -4). Such a widespread episode of crustal melting might result either within a thickened chemically closed crust or within a crust of normal thickness with advective heat contributions in the form of mantle derived melts. Current scenarios for the Devonian in the western NPM propose a magmatic arc setting (Pankhurst et al. 2006) whereas the Devonian tectonomagmatic evolution has been also related to the Chanic movements of Precordillera (Varela et al 2005). The lack of complete chemical data is a severe restriction to properly constrain the meaning of this magmatism. Our younger Rb-Sr WR isochron age ca 320 Ma is close to the reported U-Pb SHRIMP 329 Ma age for intermediate rocks of the Tonalite El Platero and Cordón El Serrucho (Pankhurst et al. 2006). Tonalite El Platero intrudes CF south of Pto Miranda. Giacosa et al. (2004) mentioned that the NW planar fabric of the tonalite is subvertical whereas the remaining igneous and clastic rocks of CF exhibit a shallower dipping parallel NW fabric. Rocks at Cordón el Serrucho belong to a belt of foliated calc-alkaline rocks which were interpreted as syn to late tectonic regarding the deformation of its host metamorphic rocks. On the contrary Pankhurst et al. (2006) proposed that the ca. 330 Ma represent a subduction event developed along the southwestern margin of the NPM. Furthermore in the area of Chacay Huarruca, immediately north of Cañadón La Angostura (Fig. 1) Varela et al. (2005) obtained a 206Pb/238U lower intercept at 302±39 Ma for a leucogranite which intrudes the CF. Pankhurst et al. (2006) reported U-Pb SHRIMP ages for S-type peraluminous Grt-bearing leucogranites, 314±2Ma from Paso del Sapo and 318± 2Ma from Sierra de Pichiñanes south of the studied areas. Both were interpreted as crustal granitoids emplaced in a syn-collisional setting after the ca. 330 Ma subduction. Therefore we propose that at least a part of CF underwent a Devonian metamorphic event and was intruded by a series of granitoids with different chemical signatures probably emplaced in different tectonic settings. Accurate interpretation of the timing and meaning of penetrative fabric in these granitoids needs further study.