Constraints on the origin and evolution of magmas in the Payún Matrú Volcanic Field, Quaternary andean back-arc of western Argentina

HERNANDO, IRENE RAQUEL; ARAGÓN, EUGENIO; FREI, ROBERT; GONZÁLEZ, PABLO DIEGO; SPAKMAN, WIM

JOURNAL OF PETROLOGY

OXFORD UNIV PRESS

Lugar: Oxford; Año: 2014 vol. 55 p. 209 - 309

0022-3530

The Payún Matrú Volcanic Field (Pleistocene ? Holocene) is located in the Andean back-arc of the Southern Volcanic Zone, western Argentina, and it is contemporaneous with the Andean volcanic arc at the same latitudes. The Payún Matrú Volcanic Field presents two polygenetic volcanoes, mostly trachytic: Payún Matrú (with a summit caldera 8 km wide) and Payún Liso (a smaller stratovolcano). This field presents about 200 scoria cones and alkaline basaltic and trachybasaltic lava flows, which constitute two basaltic fields around Payún Matrú. New 40Ar-39Ar ages extend the life of Payún Matrú up to 700 ka. Major and trace elements and Sr ?Nd isotopic composition of basaltic lavas and Payún Matrú rocks indicate that the trachytes of Payún Matrú are the result of fractional crystallization of basaltic magmas without significant upper crustal contamination, and that basalts have a geochemical similarity with OIB basalts (La/Nb = 0.8 ? 1.5, La/Ba = 0.05 ? 0.08). Isotopic ratios of basaltic to trachytic rocks range between 0.703813 and 0.703841 (87Sr/86Sr), and between 0.512743 and 0.512834 (143Nd/144Nd). Mass balance and Rayleigh fractionation models support the proposed origin of the trachytes, while an assimilation-fractional crystallization model indicates a low degree of upper crustal contamination in the youngest trachytes. Magnesium number (45 ? 55) and contents of Ni (<20 ? 90 ppm) and Cr (30 ? 180 ppm) of lavas in the basaltic fields indicatethat these are not primary magmas. The obtained data also suggest that the basaltic lavas were 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 The lack of slab fluids influence inthe Payún Matrú Volcanic Field rocks, along with unpublished and published geophysical results (mantle tomographies and electrical conductivity anomalies) suggest that magmas were generated by mantle upwelling and partial fusion by decompression.