CONICET | Buscador de Institutos y Recursos Humanos

The area of the Ñorquinco lake (39° 8´ S 71° 18´O), 50 km west Aluminé city, is distinguished by the presence of isolated outcrops of medium to high metamorphic grade rocks accredited by amphibolites and gneises. These rocks compone the Upper Paleozoic Colohuincul igneousmetamorphic Complex. Through the fieldwork, the petrographical study and the use of a barothermic tool, the mineral assemblages and metamorphic facies are diagnosed to characterize the low P / high T metamorphism and establish a sequence of paragenesis from the medium- high to low grade. Four paragenesis are found for the amphibolites. The higher grade ones in amphibolite- greenschists facies are developed by the associations: opx + cpx + pl+ tsc (equilibrated at 550°C and 4,7 kbar) and ed + act + ab + chl + ep. Subsequently, there are two overimposed re-equilibrated facies, the first under greenschist P-T conditions set up as act + ep + chl ± ab (450°C y 2,3 kbar) and the latter, under a prehnite-pumpellyite facies conditions defined by prh + pmp + chl ± ab + ep (300°C and 2,4 kbar). The proposed protolith for these amphibolites has a mafic to ultramafic affinity. Two paragenesis are recognised in the paragneises, the higher grade one in sillimanite facies is composed by kfs + sil + and+ crd + bt, its equilibrium point was found at 1,8 kbar and 630°C, the other association, steady at temperatures below 250 °C, in the biotite zone is detected from chl + ms ± bt ± ab ± cb ± kln. Both lithologies show a progressive decrease in the temperature of the determined associations. This fact allows us to relate these events to three historical pulses barothermically different. The higher grade metamorphism (>600°C) in amphibolite- greenschists facies and its local reversions, are assigned to an Upper-Paleozoic to Cretacic pulse, associated to the emplacement at different crustal levels of different plutonic episodes. South of the studied area, in San Martín de los Andes, this igneous activity could be represented by the igneous fraction of the Colohuincul Complex (previously referred as Huechulafquen Formation) and, in the concern area, by the intrusion of the Paso de Icalma Granodiorite, episode included in a regional event known as Patagonic Cordilleran Batholith. The lower grade event could play as an overimposed metamorphism linked to the Andean arc activity. This last event is able to be subdivided in two stages, Paleocene-Miocene (250-350°C) represented by the volcanic rocks of the Auca Pan Formation and Rancahue Basalt and a Quaternary stage (<250°C) associated to the Hueyeltué and Lanín basalts. This last event is associated to low temperature metamorphic processes with hydrothermal affinities. Regarding the tectonic enviroment of the Neuquen Andes it is considered that these changes in the steady mineralogy are consistent with a poliphasic metamorphism.