The role of magma mixing and mafic recharge in the evolution of a back-arc quaternary caldera: The case of Payún Matrú, Western Argentina

Abstract

The Quaternary Payún Matrú volcano is a long-lived edifice that developed a summit caldera 8 km wide, with abundant pre- and post-caldera volcanic activity. It is the main volcano of the Payún Matrú Volcanic Field, which is located in the back-arc Payenia Basaltic Province, at mid-western Argentina. The composition ofPayún Matrú is mainly trachytic, with lesser amounts of trachy andesitic and basaltic trachy andesitic lavas. The Payún Matrú Volcanic Field includes also the Payún Liso stratovolcano and more than 200 monogenetic basaltic cones and associated lava flows that are located east and west of the caldera. The aim of this work is to show the Payún Matrú evolution based on the mineralogical and textural characterization and to make inferences on the trigger mechanism for the explosive eruption which leads to the caldera formation. Some intermediate lavasand trachytes include phenocrysts with contrasting textures and composition, such as inverse zoning in plagio clase and clinopyroxenes, sieved plagio clases with amore calcic rim and calcic plagio clases with a more sodic rim,indicating that they could not have formed together and suggesting magma mixing processes. In addition, a few lavas show mafic enclaves or texturally 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. The trachytic pyroclastic deposits related to the caldera collapse do not exhibit evidence of hybridization,although they present a minor proportion of dissolved feldspars and biotite replaced by anhydrous minerals, suggesting that a restricted zone of the chamber was reheated. This may be explained by a mafic recharge event where part of the host trachytic magma was heated, but without a complete homogenization between thehost and recharging magmas. These features, in addition to the abundant basaltic volcanism east and west of the caldera, suggest that the trigger mechanism for the explosive eruption was the injection of basaltic magma into the trachytic chamber of Payún Matrú.