Geological and geochronological implications of U–Pb dating in apatite from the Late Devonian/Early Carboniferous Sauce Guacho granite, Sierra de Ancasti, NW Argentina

Abstract

The Sierra de Ancasti is located in the southeastern part of Catamarca province in northwest Argentina. It belongs to the easternmost sector of the Eastern Sierras Pampeanas geological province. The crystalline basement of the Sierra de Ancasti consists mostly of granitoids of two different ages -Early/Middle Ordovician and Late Devonian/Early Carboniferous periods-that intrude medium to high-grade metamorphites. The Sauce Guacho (SG) granite, which corresponds to the second period mentioned above, is one of them, and it intrudes banded schists. The SG granite has similar geological and geochemical characteristics to other coetaneous granitic bodies scattered throughout the Eastern Sierras Pampeanas. To get geochronological information on SG granite andcontribute to the understanding of the Upper Paleozoic magmatism developed in the pre-Andean margin of SW Gondwana, we carried out U–Pb apatite dating on this granitic body of the Sierra de Ancasti. Several transparent, green, idiomorphic, and subidiomorphic apatite crystals were separated from the SG granite. They were used to obtain U–Pb apatite ages using LA-ICP-MS methodology. The results yielded concordia ages between 346 and 363 Ma. Due to the relatively low closure temperature of the U–Pb system in apatite (~350°-570°C), the result is considered a cooling age marking the end of the high-temperature magmatism that would have given rise to theSG granite. This revalidates a magmatic episode of the Middle Devonian-Early Carboniferous age, corresponding to the so-called ‘Early Gondwanan’ orogeny in the Sierra de Ancasti. An initial cooling rate has been estimated at ~4 °C/Ma for the SG intrusive.