Ordovician volcanic activity in the Puna, Argentina.

COIRA, B. Y M., KOUKHARSKY

Aspects of the Ordovician System in Argentina.

INSUGEO

Lugar: Tucumán; Año: 2002; p. 267 - 280

The Ordovician volcanics in the Puna are integrated in two belts of submeridional disposition known as Faja Eruptiva de la Puna Occidental and Faja Eruptiva de la Puna Oriental. On the first one, which extends in the Northwest of the Puna and North of Chile, are found the most ancient outcroppings. Two magmatic cycles are represented there. The first cycle began with basaltic-andesitic manifestations in a shallow marine environment that were followed by dacitic-rhyolitic pyroclastic sequences. These ended in subaerial conditions. In general, they have arc like signature, with low K content denoting an evolution on an oceanic crust or a thin continental crust, though there exists basic volcanic rocks with transitional features to backarc type. This first cycle ended with the intrusion of plutonic rocks during the Upper Tremadoc.The second cycle is represented by andesistic-basaltic to andesitic volcanoes of reduced development, members of an arc to backarc setting, which evolved during the Arenig in a shallow marine environment. Episodes of an intermittent acid explosive volcanism are registered later on,meanwhile erosive stages are developed on the arc placed westwards. Towards the Late Middle to Upper Arenig, the lack of primary volcanic records, added to the existence of important turbiditic volcanoclastic sequences, mark the cease of arc volcanism and the beginning of theerosive phase.The Faja Eruptiva de la Puna Oriental developed in a basin of trasarc. It represents a submarine lavic-subvolcanic magmatism, dominantly dacitic and subordinately basaltic, which began in the Tremadoc-Arenig boundary, extending during the Middle Arenig. Its manifestations are lavas and an important swarm of synsedimentary sills emplaced on an extentional environment. The siliceous volcanic members, of peraluminous character and weak arc signature, denote a sedimentary cortical component in the source. The alkaline to subalkaline mafic volcanic rocks are plotted in the within-plate field and can be roughly divided into low-Ti and high Ti groups, being the result of different fusion degrees of a garnet bearing mantle source. The Arenig magmatic episode ended with an Upper Arenig/Llanvirn syntectonic plutonism record along the Eastern Puna. During the Ashgill a collisional plutonism in the North of Chile and an important generalized contemporaneous deformation (Ocloyic Orogenic Phase) took place, resulting on the closure of the basin.The most consistent model with the geological framework and the geochemical features for the Ordovician volcanism of the Puna considers the development of a volcanic arc during the Tremadoc with its axis in Chilean territory, which culminates with a plutonic episode.During the Arenig a new arc developed in that region, evolving in the North Eastern Puna a trasarc basin with associated bimodal volcanism, in a transpressive geodynamic regime with oblique strike-slip fault system, related to oblique arc convergence. That transtentional regime wouldhave controlled the origin and location of the magmatism. Meanwhile, under that geotectonic configuration, there would have taken place an arc magmatism in the southern Puna.