The Choiyoi Group in the Cordón del Plata range, western Argentina: structure, petrography and geochemistry.

The Choiyoi Group from the Permo-Triassic, is one of the most conspicuous volcano-sedimentary suites of southern South America, considered critical to understand the geological evolution of the western margins of Gondwana. In this regard, petrography, geochemistry, and structural data were examined to better elucidate the physical character and emplacement conditions of the unit in the Cordón del Plata range, within the Frontal Cordillera of Mendoza, Argentina. The site is representative of the magmatism and deformation through different Andean cycles. Results of the study indicate three facies of increasing felsic composition upwards. Mafic units consist of basalts, andesite and andesitic breccias at the base of the sequence. Felsic rocks such as rhyodacites, granites and welded tuffs are predominant above. The fault zone of La Polcura -- La Manga is the most prominent structural feature in the region, which presumably controlled the emplacement of breccias and ignimbrites within the middle and upper members. These compositional variations suggest a magma evolution from subduction to a rifting environment after the San Rafael orogeny in the Late Palaeozoic. In this line, the Lower Choiyoi was observed to overlie the San Rafael structures indicating thus, that compression ceased before the volcanic extrusion. Geochemistry data indicate that mafic rocks are mostly high-potassium, calc-alkaline volcanics derived from the mantle wedge above the subduction zone. In contrast, the Upper Choiyoi corresponds to high-potassium rhyolites to shoshonites, typically depleted in Eu. This indicates partial melting of a lithospheric mantle in an average to thin crust. [ABSTRACT FROM AUTHOR]