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

Abstract

The Pire Mahuida Volcanic Complex (PMVC), located in the extra-Andean region of12 southern Argentina (68º to 68º 40´W and 41º 51´ to 42º 28´ S), is a bimodal volcanic field13 that developed during the Miocene (17Ma-14Ma, Langhian-Burdigalian Stages). The14 complex partially surrounds the southern boundary of the Somún Curá basaltic plateau. The15 PMVC is mainly composed of acidic pyroclastic and lava facies (rhyolite/dacite flows, lava16 domes and coulees) and stands out as the one with the largest volume of acid rocks in the17 Somún Curà province, involving two evolutionary trends (alkaline and subalkaline).18 Subordinate in volume, basaltic flows overlie this extensive sequence of acidic rocks.19 Relationships between some units are difficult to establish because they are the result of20 different eruptive centres. However, stratigraphy, morphology and petrography allow two21 acid phases to be distinguished, one a mainly lavic phase and the other a mainly pyroclastic22 phase. U-Pb ages allow precise dating and temporal placement of the acid sequence: 1-23 lava flows and 2- lava domes and related pyroclastic phases. Two calderas and fissures24 are responsible for the emission of the felsic rocks. The basaltic facies shows a wide range25 of characteristics, which also allow two different groups to be distinguished. 1. The basalts26 of the main plateau are associated with a main fault of NW-SE pattern and 2.The younger27 basalts are associated with small volcanic edifices. The effusion of the complex was28 developed in a relatively short time with the basic episode being more prolonged than the29 acid episode.