AbStRAct. The Payún Matrú Volcanic Field is located in the Payenia Basaltic Province of the recent back-arc of western Argentina (35°S-38°S). This province is younger than 5 Ma, and most of its volcanic activity took place since 2 Ma. The Payún Matrú Volcanic Field contains two composite volcanoes, Payún Matrú and Payún Liso, and two basaltic fields in an E-W oriented zone, located east and west of the Payún Matrú volcano. Payún Matrú is the largest volcano of this volcanic field, and consists of a shield-shaped edifice with a circular summit caldera of 8 km in diameter. The composition of both composite volcanoes is alkaline and predominantly trachytic, having also minor intermediate lavas.The basaltic fields consist of basalts and trachybasalts, with clinopyroxene and abundant olivine as phenocrysts and also in the groundmass. Textures indicating mixing and mingling processes, such as dusty plagioclases along with clear ones, biotite replaced by anhydrous minerals and two groundmasses with a fluid-fluid relationship, are common in the early pre-caldera stage of Payún Matrú and some post-caldera lavas. The latest post-caldera lavas are trachytic, with clean sanidine phenocrysts without disequilibrium textures. A remarkable characteristic of the Payún Matrú Volcanic Field is the fact that the Payún Matrú caldera is surrounded by basaltic fields at its base, while no basalts were erupted in the caldera region. We propose that the absence of basaltic lavas in the Payún Matrú volcano is due to the presence of a magmatic chamber below it, and that the mafic magmas rising from deeper levels were unable to erupt without interaction with more evolved melts. Intermediate hybrid magmas produced as a consequence of magma mixing and mingling between basaltic and trachytic magmas, are present in the early and mid-history of Payún Matrú volcano. We present here new information about the Quaternary Payún Matrú Volcanic Field derived from field work, petrography, and major element geochemistry and 87 Sr/ 86 Sr isotopic ratios. For the first time a detailed map of the Payún Matrú Volcanic Field and its volcanic stratigraphy, primarily based on field observations, are presented. RESuMEn. Estratigrafía volcánica y evidencia de mezcla de magmas en el volcán Payún Matrú del cuaternario, en el retroarco andino de Argentina occidental. El Campo Volcánico Payún Matrú está localizado en la Provincia Basáltica Payenia, en el retroarco reciente al oeste de Argentina (35°S-38°S). Esta provincia es más joven que 5 Ma, y la mayor parte de su actividad volcánica tuvo lugar desde hace 2 Ma. El Campo Volcánico Payún Matrú contiene dos volcanes compuestos, el Payún Matrú y el Payún Liso, así como dos campos basálticos en una franja E-W, ubicados al este y oeste del Payún Matrú. El Payún Matrú es el volcán más grande de este campo volcánico, y consiste en un edificio en forma de escudo con una caldera circular en su cúspide de 8 km de diámetro. La composición de ambos volcanes compuestos es alcalina y predominantemente traquítica, con menor...

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The Payu¤ n Matru¤ Volcanic Field (Pleistocene^Holocene) is located in the Andean back-arc of the Southern Volcanic Zone, western Argentina, and is contemporaneous with the Andean volcanic arc at the same latitude. It includes two polygenetic, mostly trachytic volcanoes: Payu¤ n Matru¤ (with a summit caldera 8 km wide) and Payu¤ n Liso (a smaller stratovolcano). The volcanic field includes about 200 scoria cones and alkali basaltic and trachybasaltic lava flows, forming two basaltic fields around Payu¤ n Matru¤. New 40 Ar^3 9 Ar ages extend the activity of Payu¤ n Matru¤ up to 700 ka. The major and trace element and Sr^Nd isotopic compositions of the basaltic lavas and Payu¤ n Matru¤ rocks indicate that the trachytes of Payu¤ n Matru¤ are the result of fractional crystallization of basaltic parent magmas without significant upper crustal contamination, and that the basalts have a geochemical similarity to ocean island basalt (La/Nb ¼ 0•8^1•5, La/Ba ¼ 0•05^0•08). The Sr^Nd isotopic compositions of the basaltic to trachytic rocks range between 0•703813 and 0•703841 (87 Sr/ 86 Sr) and 0•512743 and 0•512834 (143 Nd/ 144 Nd). Mass-balance and Rayleigh fractionation models support the proposed origin of the trachytes, and an assimilation^fractional crystallization model indicates a low degree of upper crustal contamination in the youngest trachytes. Magnesium numbers (45^55) and contents of Ni (5 20^90 ppm) and Cr (30^180 ppm) in the lavas in the basaltic fields indicate that these are not primary magmas. The data also suggest that the basaltic lavas originated in the asthenospheric mantle, probably within the spinel stability field and beneath an attenuated continental lithosphere in the back-arc area. The lack of a slab-fluid signature in the Payu¤ n Matru¤ Volcanic Field rocks, along with unpublished and published geophysical results (mantle tomography and electrical conductivity anomalies) suggest that magmas were generated by decompression-induced melting of upwelling mantle.

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