Metamorphic facies and relative ages of rocks on the eastern part of Brazo Huemul, Neuquén province

Abstract

The study of mineral assemblages and metamorphic facies in rocks exposed on the eastern margin of Brazo Huemul, Nahuel Huapi Lake, allowed the reconsideration of the temporal succession of units whose outcrops are isolated and difficult to locate due to the exuberant forest that covers the southestern part of Neuquén province. The rocks exposed in the study area are mainly of tonalitic composition and have been correlated with the Subcordilleran Patagonian Batholith because they are crosscut by two different types of dikes. One group of dikes shows an albite - chlorite - epidote - cummingtonite/ grunerite - hornblende mineral assemblage as a consequence of a thermal metamorphic event transitional between albite - epidote hornfels facies and hornblende hornfels conditions. This process has been attributed to the emplacement of the Cordilleran Patagonian Batholith of Cretaceous age. Because of this, the hornfels dikes were correlated with Montes de Oca Formation. The other group of dikes shows a chlorite + pumpellyite + laumontite + chlorite/esmectite + iron oxides assemblage indicating metamorphism in zeolite facies of high temperature or, sometimes shows propilitic alteration and these dikes have been correlated with the Ventana Formation. The differences between these two types of dikes allow the discussion of the relative age of the tonalitic rocks that host them. In this paper it is proposed that this tonalitic body is of Low to Middle Jurassic age.