PERMO-TRIASSIC MAGMATISM OF THE CHOIYOI GROUP IN THE CORDILLERA FRONTAL OF MENDOZA, ARGENTINA: GEOLOGICAL VARIATIONS ASSOCIATED TO CHANGES IN PALEO-BENIOFF ZONE

The petrological and geochemical characteristics of the Late Paleozoic to Lower Triassic igneous rocks from Cordillera Frontal of Argentina encompassed in the Choiyoi Group show changes through time. The purpose of the present work was to propose a tectonic model which characterizes the genesis of the Choiyoi Group. In order to achieve this, petrographycal, geochemical, and structural studies were done. The two different areas were selected to achieve the goal of the present study, both located in the Cordillera Frontal of Mendoza. The igneous sequence is composed of breccias and andesitic lava flows, crystalline, glassy, and litihic ignimbritic pyroclastic flows, rhyolitic and plutonic granitic rocks. The San Rafael compressional orogenic phase took place during the Early Permian and is interpreted as linked to a shallowing episode of the subducted slab. Associated with this process, basic rocks of the lower section of the Choiyoi Group were emplaced at the base of the continental crust and contributed to produce an important crustal melt that led to the formation of the acid volcanism observed in the middle section. The melts originated at the base of the hydrated and metasomatized mantle lithosphere is in agreement with high values of light Rare Earth Elements. This magmatic event is thought to be related to a change of the paleo-Benioff zone which caused gravitacional collapse of the orogenic belt. This extensional regime gave place to an appropriate environment for the development of the rift basins. Late granitic bodies with the upper section were emplaced after the San Rafael orogeny in the upper levels of the continental crust. They show evidence that point out to an alkaline, anhydrous and post-orogenic origin, probably related to water depleted magmas produced as a consequence of the cease of the subduction linked to the quasiestatic period of the Gondwana supercontinent.