STRATIGRAPHY AND EVOLUTION OF THE CHOIYOI GROUP AT PRECORDILLERA AND CORDILLERA FRONTAL MENDOZA, ARGENTINA

Permian basic to acidic volcanic rocks cropping out in western Argentina were assigned to the Choiyoi Group. Based on detailed mapping and profiling allow us to identified three stages of magmatic evolution. The first stage is represented by more than 800 m of basic to intermediate breccias and lava flows, resting unconformably on lower Permian marine sequences. Their geochemical signature indicates a relatively primitive calc alkaline trend with strong negative Ta and Nb anomalies.

The second stage includes both intermediate to acidic volcanic rocks and continental sequences, reaching 1500 m thick. The first ones are represented by low and high-welded ignimbrites, surge deposits and lag and ground breccias. This association was produced during collapse of plinian plumes associated to large calderas. Dacites and rhyolites (> 60% < 75 %), display small negative Ta and Nb anomalies. The sedimentary rocks, conglomerates, tuffaceous sandstones, limestones and lahar deposits, are considered as moat deposits and were generated during caldera collapse.

The third stage is characterized by 700 m thick of low and high-welded tuff, lava flows and subvolcanic bodies. They are associated with caldera collapse and dome resurgence. Moat deposits are restricted to the margins of the caldera, reaching less than 100 meters thick. High silica (> 75%) and strong negative Eu anomalies characterize this intraplate volcanism. U/Pb radiometric dating indicates a lower Permian age.

The tectonic evolution of this volcanic event start with a magmatic arc developed on continental crust on the Gondwana active margin. This stage passes transitionally to a mature volcanic arc with incipient extensional periods represented by intraplate magmatism. Finally, extensional conditions produce a widespread intraplate volcanism, associated with basin and rift development in the Gondwana margin.

Key words: Choiyoi Group, Gondwana magmatism, tectonic evolution, Precordillera, Cordillera Frontal Mendoza, Argentina.