Metamorphic P-T constraints for the low-temperature assemblages overimposed on metamorphic and igneous rocks nearby Ñorquinco Lake, Aluminé, North-Patagonian Andes

MARTÍNEZ DOPICO, C.I.

Niza, Francia

Simposio; 7th International Symposium on Andean Geodynamics; 2008

The area of the Ñorquinco-Pulmarí valley, 50 km west Aluminé city, North-Patagonian Cordillera (Fig. 1) 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 igneous-metamorphic Complex (Dalla Salda et al., 1991; Varela et al., 2005). This basament is intruded by the Paso de Icalma Granodiorite (Cucchi et al., 2005; Latorre et al., 2001), a local Jurassic to Upper Cretacic igneous episode of a mayor Jurassic to Miocene event known as North-Patagonian Batolith. These rocks are covered by a Tertiary to Quaternary Andean thick andesitic to basaltic pile (Auca Pan Formation, Rancahue Basalt, Hueyeltué Basalt and Lanín Basalt). Vattuone et al. (2005), among other authors, have characterized a low to very low grade metamorphism in the Eastern Andean volcanic pile. Through the fieldwork, the petrographical study, EDAX on amphibols, pumpellyite and zeolites crystals (Martínez Dopico, 2007; Gallegos, 2007), and the use of an internally consistent thermodynamic data as the one proposed by Berman (1988, 2007), the mineral assemblages and metamorphic facies are diagnosed to characterize and establish a sequence of paragenesis from the medium to very low grade metamorphism overimposed on the andean southern basement and volcanic cover. Figure 1: a) Geodynamic framework for Patagonia and location of the concern area within the North Patagonian Cordillera b) Studied localities in the Ñorquinco-Pulmary valley. Metamorphic mineralogy and P-T constraints   Four assemblages are found for the amphibolites of the Colohuincul Complex. 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 and 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 of 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.   The tonalites compiled under the Granodiorita Paso de Icalma have a slight secondary overprint evidenced by chl/ smectites + act + prh in quartz veins. These features indicate non-coaxial deformation operating under prenhite-actinolite facies conditions of metamorphism, probably at temperatures between 300 and 200°C. This T interval is compatible with the observed mineral assemblages in the other protoliths.   The metamorphic assemblage in metadacites of the paleocene Auca Pan Formation consist of ab+ ep + chl + phl, formed under zeolite facies PT conditions (underneath 250°C), evidenced by the presence of phillipsite. In the Upper Miocene Rancahue Basalt the secondary assemblage observed in amygdales consists of thomsonite-Ca, faujasite-Ca and smectites, in the matrix phillipsite, scolecite and epidote were found (Gallegos, 2007). These associations reach their equilibrium under 250°C. Slightly secondaries processes are represented in minor veins and fractures by cb + act in the Middle Pleistocene Hueyeltué olivinic basalt. Discussion   These data allow us to relate these metamorphic events to three historical pulses barothermically different. The higher grade metamorphism (>550°C) in amphibolite- greenschists facies and its local reversions, are assigned to an Upper-Paleozoic to Jurassic pulse, associated with the emplacement at different crustal levels of 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 and, in the concern area, by the intrusion of the Paso de Icalma Granodiorite The lower grade event in prehnite-pumpellyite facies (300-350°C). could play as an overimposed metamorphism linked to the early upper Cretaceous metamorphic ages associated with an extensional regime, crustal attenuation and subsidence developed within the Andes as proposed by Aguirre et al.(1999). The very low grade event (<250°C), in zeolite facies is able to be subdivided in two stages, Paleogene- Miocene represented in the volcanic rocks of the Auca Pan Formation and Rancahue Basalt and an upper Miocene to Pleistocene stage associated with the secondary formation of actinolite in the Hueyeltué Basalt and Granodiorite Paso de Icalma. This last event in prehnite- actinolite facies was associated with the proximal Andean Miocene granitoids and dated by Ar40/Ar39 in actinolite crystals in 8 Ma of the Rio Damas metabasites in the western Andean margin by Oliveros et al., (2008). Regarding the tectonic enviroment of the Neuquén Andes it is considered that these changes in the steady mineralogy are consistent with a polymetamorphic evolution according to the variation in the angle of steepening of the Wadati- Benioff zone that generates the progressive stages of compression and extension at this latitude of the Andes as discussed by Folguera et al. (2007).