Geología y metamorfismo del basamento Ígneo-Metamórfico del arroyo Cochicó Grande en el paraje Kilka, Precordillera Neuquina Sur

Abstract

The aim of this contribution is the study of the stratigraphy, internal structure and petrography of the igneous-metamorphic basement in arroyo Cochicó Grande, Kilka area and surroundings (38°51’00”S-70°48’00”O, Villa Pehuenia, Neuquén). As a results, up to three complexes of basement rock and their volcano-sedimentary cover have been identified. The basement consists of low-grade and high-grade metamorphic rocks of the Kilka Igneous-Metamorphic Complex and the Pehuenia Metamorphic Complex respectively, which are not in contact with each other. The intrusive igneous bodies were grouped into the Cochicó Grande Plutonic Complex, which is post-orogenic as it cuts the internal structure of metamorphic complexes and produces thermal contact metamorphism. The Kilka Igneous-Metamorphic Complex is composed of meta-volcanic rocks, phyllites, and hornfels. The axial plane of schistosity S1 has NW-SE trending and dips between 67° and 80° to the NE. The metamorphic association of the phyllites is quartz-white mica-chlorite, and that of the meta-volcanic rocks is quartz-albite-epidote-chlorite ± tremolite-actinolite, both indicative that the regional metamorphism reached the greenschist facies. The Pehuenia Metamorphic Complex consists of paragneises with quartz-plagioclase-biotite-muscovite ± cordierite, migmatites and amphibolite. Its penetrative metamorphic foliation S2 is NW-SE tranding and dips to the SW at high angles. Regional metamorphism reached the amphibolite facies. The volcano-sedimentary cover is composed of pyroclastic and volcanic rocks that unconformably cover the three basement complexes. It is assigned to the Choiyoi Group of the Lower Permian-Triassic, so the geological-structural evolution of the basement is pre-Permian. Since the rocks of the sedimentary cover are affected by very-low to low-grade regional metamorphism and by compressive deformation, they can also be compared with those of the Pre-Cuyano cycle (Upper Triassic-Lower Jurassic) of the Neuquén Basin, which also exhibit these same geological attributes.