Abstract
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 Cordon 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 lithological facies of increasing acidity 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 felsic rocks range from high-potassium rhyolites to shoshonites, typically depleted in Eu. This indicate partial melting of a lithospheric mantle in an average to thin crust.
Highlights
The basement of the central Andes of Argentina is largely constituted by an assemblage of rhyolites, andesite, mesosilicic igneous complexes and interbedded volcanic breccias and tuffs of Late Permian – Middle Triassic age
Due to its widespread extension, the Choiyoi has been related to the magmatism that occurred during the breakdown of the western margins of Gondwana in the Late Palaeozoic (Zeil, 1981; Ramos. 1999; Rocha-Campos et al 2011)
The base of the Choiyoi consists of mafic rocks, mostly basalts, andesite, and andesitic breccias that correlate with the Portezuelo del Cenizo Formation, from the Lower Permian
Summary
The basement of the central Andes of Argentina is largely constituted by an assemblage of rhyolites, andesite, mesosilicic igneous complexes and interbedded volcanic breccias and tuffs of Late Permian – Middle Triassic age These rocks have been included within the Choiyoi Group (Choiyoi), which covers an area of at least 200,000 km and may be up to 500,000 km in central - western Argentina and Chile (Llambias et al 2003). Most papers about the unit deal with its facies distribution and lithological features, but only a few examine the geochemical, chronological and tectonic aspects of the unit (Strazzere et al 2006) In this regard, the present work integrates petrography, geochemistry and structural data from an area where the magmatism is well represented, in order to better recognise the geologic features of the Choiyoi in central Argentina and to help constrain its tectonic evolution along the southwestern margins of Gondwana. Even though the paper provides an overview of the geology in this area, the broader aspects of the rock characteristics as well as the geodynamic processes are expected to be common to other regions and have implications for the entire formation
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