Chronostratigraphy and compositional characteristics of miocene volcanism in the Sierra de Pire Mahuida, extra-Andean Patagonia, Argentina

Abstract

The Pire Mahuida Volcanic Complex (PMVC), located in the extra-Andean region of southern Argentina (68° to 68° 40′W and 41° 51′ to 42° 28′ S), is a bimodal volcanic field that developed during the Miocene (17Ma-14Ma, Langhian-Burdigalian Stages). The complex partially surrounds the southern boundary of the Somún Curá basaltic plateau. The PMVC is mainly composed of acidic pyroclastic and lava facies (rhyolite/dacite flows, lava domes and coulees) and stands out as the one with the largest volume of acid rocks in the Somún Curà province, involving two evolutionary trends (alkaline and subalkaline). Subordinate in volume, basaltic flows overlie this extensive sequence of acidic rocks. Relationships between some units are difficult to establish because they are the result of different eruptive centres. However, stratigraphy, morphology and petrography allow two acid phases to be distinguished, one a mainly lavic phase and the other a mainly pyroclastic phase. U-Pb ages allow precise dating and temporal placement of the acid sequence: 1- lava flows and 2- lava domes and related pyroclastic phases. Two calderas and fissures are responsible for the emission of the felsic rocks. The basaltic facies shows a wide range of characteristics, which also allow two different groups to be distinguished. 1. The basalts of the main plateau are associated with a main fault of NW-SE pattern and 2.The younger basalts are associated with small volcanic edifices. The effusion of the complex was developed in a relatively short time with the basic episode being more prolonged than the acid episode.