Petrography and 40Ar/39Ar age of peraluminous leucogranites west of Valcheta, Northpatagonian Massif (Río Negro)

Abstract

NE trending leucogranites of the Valcheta pluton crop out west of Valcheta, between the Ríos Valcheta and Nahuel Niyeu. Petrographic and major elements data of Valcheta pluton are typical of peraluminuos leucogranites (A/CNK >1,1). It has medium grain and it is likely deformed on the border. The pluton has a leucogranite facies with muscovite-biotite-garnet and other facies with muscovite-biotite. The contact zones with the regional metamorphic rock (Nahuel Niyeu Formation) are covered and inside of the pluton there are gneisses and amphibolite xenolites. The age of the Valcheta pluton is 470 ± 1,8 My (⁴⁰Ar/³⁹Ar in muscovite). The ancient correlation between leucogranites from Valcheta with Granito Flores, a Jurassic intrusive next to Valcheta, is disregarded. The field and petrographic characteristics of Valcheta pluton are different from the intrusives of Ordovician Punta Sierra Plutonic Complex, which crops out along the coast of the Rio Negro province. Based on the new data here presented, the extension of the Ordovician magmatism of the eastern North Patagonian Massif requires a revision. The morphology, mineralogy, microstructural evidences and geochemistry from Valcheta pluton are similar to the Tapera and María Teresa plutons, from Mina Gonzalito Complex. The U-Pb zircon age of the metamorphic peak of this complex is 472 ± 5 Ma. The Valcheta pluton is compositionally similar to post-collisional peraluminous leucogranites of S-type formed in an overthickened crust.