Estudio geológico estructural del basamento metamórfico - ígneo en el cañadón Chacay Huarruca, Macizo Nordpatagónico Occidental, Río Negro

Abstract

The aim of this final degree project is the geological-structural analysis of the basement igneous and metamorphic rocks that outcrop in the Chacay Huarruca Creek located west of the locality of Rio Chico, in the Nordpatagonian Precordillera of the Rio Negro province (41°42’12” S - 70°29’55” W). The basement is composed of the Comallo Metamorphic Complex and Mamil Choique Plutonic Complex. The Comallo Metamorphic Complex comprises intercalations of high-grade schists, paragneisses, and migmatites (stromatites) with quartzose meta-sandstones. The regional foliation is NW-SE trending and high-grade regional metamorphism reached the amphibolite facies. The granitic leucosome of the migmatites is associated with dikes of similar composition, intruded into the metamorphic host rock which is concordant with the planar fabric. The Mamil Choique Plutonic Complex is represented by the Tunel Tonalite pluton and minor bodies of other granitoides. The pluton cuts the fabric of the rocks deformed and metamorphosed of the Comallo Metamorphic Complex. It has a quartz dioritic to tonalitic composition with biotite and hornblende-biotite bearing, which exhibits two main fabrics. One magmatic that is massive, equigranular to weakly foliated, and another is tectonic represented by mylonites that exhibit a mylonitic foliation. The geological evolution of the basement comprises deformation, metamorphism, and magmatism events associated with the Gondwanan orogenic cycle, which took place along the SW Gondwana margin during the Late Paleozoic. After the sedimentation of the metamorphic protolith during the Llandoverian (Silurian), the Comallo Metamorphic Complex was affected by three deformation episodes recognized from D1 to D3 and associated with metamorphisms M1 to M3. It was also intruded on by intrusive bodies from the Mamil Choique Plutonic Complex. D1-D2 are episodes of ductile compressive and coaxial deformation. The regional metamorphism is dynamothermal, with a prograde evolution from M1 greenschist facies to M2 amphibolite facies. The NW-SE trending penetrative fabric was formed during the tectonic-thermal climax D2-M2 in the Lower Carboniferous before the intrusion of the Tunel Tonalite that occurred in the Lower Permian. It was followed by D3-M3 which is developed in ductile shear belts that affected the tonalitic pluton and its host rock, causing the formation of mylonites and the retrogradation of the metamorphic host rocks. After the exhumation of the basement, the D4-M4 event is Andean and had a regional character due to the reactivation of the Gondwanan structures.