Abstract

FRACCHIA, Diego; POLO, Liza; CAFFE, Pablo J  and  COIRA, Beatriz. Stratigraphic redefinition of the Pululus Ignimbrite (Northern Puna): Volcanological and metallogenic implications. Rev. Asoc. Geol. Argent. [online]. 2010, vol.66, n.1-2, pp.271-281. ISSN 0004-4822.

The Pululus Ignimbrite -located in the north Puna of Argentina, near the international border with Bolivia- was previously defined as a relatively young (Pliocene-Pleistocene) ignimbritic shield which erupted several millions of years after the collapse of the Vilama caldera. This conclusion was mainly based on the morphology of the cerro Pululus, the location where the ignimbrite crops out. In this contribution we present stratigraphic, petrographic and geochemical arguments which refute the existence of such an eruptive centre and so give support to the conclusion that the Pululus Ignimbrite is actually the Vilama Ignimbrite, i.e. the principal stratigraphic unit erupted during the collapse of the Vilama caldera. In order to rectify the stratigraphic framework of the region, we propose to abandon the term "Pululus Ignimbrite" and to consider their outcrops as being part of the Vilama Ignimbrite. Some new stratigraphic units in Cerro Pululus were also identified; their stratigraphic position is here described, as well as lithologic and petrographic characteristics. The most important of these units is a subvolcanic intrusive -to which we propose to name Pululus Dacite- whose emplacement deformed the Vilama Ignimbrite in the shape of a dome in the highest zone of cerro Pululus. The proposed change in the stratigraphic framework also bears important consequences for volcanological and metallogenic issues in the region. They are summarised in: a) the "shield" shape of Cerro Pululus is an superimposed character due to intrusion under the Vilama Ignimbrite, then the hypothesis of a central eruption is no longer valid; b) a new topographic border of the Vilama caldera is defined; and c) the tin mineralization at cerro Pululus is probably associated to the intrusion of the Pululus Dacite, so the previous metallogenic model which linked the tin to the degassing of the ignimbrite could be discarded.

Keywords : Stratigraphy; Volcanism; Puna; Pululus Ignimbrite; Vilama Ignimbrite; Pululus Dacite.

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