CONICET | Buscador de Institutos y Recursos Humanos

The Quaternary Payún Matrú volcano is a long-lived edifice that developed a summit caldera 8 kmwide, with abundant pre- and post-caldera volcanic activity. It is the main volcano of the PayúnMatrú Volcanic Field, which is located in the back-arc Payenia Basaltic Province, at mid-westernArgentina. The composition of Payún Matrú is mainly trachytic, with lesser amounts oftrachyandesitic and basaltic trachyandesitic lavas. The Payún Matrú Volcanic Field includes also thePayún Liso stratovolcano and more than 200 monogenetic basaltic cones and associated lava flowsthat are located east and west of the caldera. The aim of this work is to show the Payún Matrúevolution based on the mineralogical and textural characterization and to make inferences on thetrigger mechanism for the explosive eruption which lead to the caldera formation. Someintermediate lavas and trachytes include phenocrysts with contrasting textures and composition,such as inverse zoning in plagioclase and clinopyroxenes, sieved plagioclases with a more calcicrim and calcic plagioclases with a more sodic rim, indicating that they could not have formedtogether and suggesting magma mixing processes. In addition, a few lavas show mafic enclaves ortexturally different groundmasses with fluidal contacts, indicating mingling between two magmas.These lavas are found in the pre-caldera stage and the early post-caldera stage of Payún Matrú. Thetrachytic pyroclastic deposits related to the caldera collapse do not exhibit evidence ofhybridization, although they present a minor proportion of dissolved feldspars and biotite replacedby anhydrous minerals, suggesting that a restricted zone of the chamber was reheated. This may beexplained by a mafic recharge event where part of the host trachytic magma was heated, but withouta complete homogenization between the host and recharging magmas. These features, in addition tothe abundant basaltic volcanism east and west of the caldera, suggest that the trigger mechanism forthe explosive eruption was the injection of basaltic magma into the trachytic chamber of PayúnMatrú.