INSTITUTO DE GEOCRONOLOGIA Y GEOLOGIA ISOTOPICA
Unidad Ejecutora - UE
congresos y reuniones científicas
Petrogenesis of the Ordovician magmatism in northeastern Patagonia, North Patagonian Massif, Argentina
MARTÍNEZ DOPICO, C.I.; LÓPEZ DE LUCHI, M.G.; RAPALINI, A.E.
San Miguel de Tucumán
Simposio; Primer Simposio de Petrología Ígnea y Metalogénesis asociada; 2011
Universidad Nacional de Tucumán
Hypothesis of a common pre-Silurian evolution for the North Patagonian Massif (NPM) and the rest of the terranes that formed West Gondwana are partially based on the continuity of the Ordovician Famatinian arc magmatism. Geochronological evidence in the Punta Sierra and Valcheta granitoids is the main support for this correlation, though recent fauna findings in the low grade metamorphic host revitalized the allochthony hypothesis and a connection of part of the NPM to Antartica (González et al., 2011). A regional integration of this magmatism focused on petrological information is presented. In the NE sector of the NPM three groups of Ordovician granitoids are represented. Group 1 (G1) comprises Amph-Bt granodiorites-tonalites with mafic enclaves emplaced in very low grade metaclastic rocks (El Jagüelito Fm). Group 2 (G2) are Bt-Ms pink monzogranites that appear as isolated plutons in the coastal areas, and Group 3 (G3) are Ms leucogranites that appear as isolated plutons intruding low to medium grade metaclastic rocks of the Nahuel Niyeu Fm. G1 and G2 are located to the east, near Arroyo Salado and Sierra Grande on the South Atlantic coast, whereas G3 is located to the west, inland (present coordinates) around Valcheta town. Major-element analysis of the source for G1 indicates a mafic component which agrees with Sm-Nd data (Pankhurst et al., 2006) and the microgranular mafic enclaves. Hybridization during a low pressure melting process involving the mixing of mafic and crustal /felsic precursors is indicated by different evolutionary paths. G2 would derive from a greywacke or igneous source that could represent the crustal/felsic component of G1 source. In any case, G1 and G2 would involve sources scarcely affected by recycling processes, whereas G3 indicates a pelite source, specifically a Ms-rich pelite as suggested by the high Rb/Sr and low Al2O3/TiO2. Depletion in Sr and CaO and a slightly negative europium anomaly suggest that a low amount of plagioclase entered into the melting reaction. Available ages of these granite groups indicate that they are mostly coeval. Sources for G1 may be related to partial melting of a mafic underplated magma which underwent crustal contamination. The predominance of a greywacke source for G2 may suggest anatexis of an immature continental platform material or derivation by fractional crystalization of G1 magmas. Westwards, the dominating process for G3 generation probably was reworking of an older crustal segment. Although the outcrops are relatively scarce, the exposed area for G2 and G3 are smaller relative to the metaluminous G1 granitoids which could indicate that significant heat input is required to generate a small amount of peraluminous magma intruding at high levels of the crust. The chemistry of these granitoids covers the compositional span of granitoids from active margins to a post-collisional settings.