Retroarc Neogene volcanism at Payenia: A review

The Payenia basaltic province is a typical retroarc association developed along the foothills of the Andes between 33°40 ́S and 38°00'S. It records two main events, an older, mostly Miocene one (26 to 8 Ma) and a younger Pliocene to Holocene one (younger than 5 Ma). It covers an N-S lowland belt named here the central depression. To the North of 36°20'S, the region was described as Los Huarpes depression, a partially deformed sedimentary basin characterized by a 1000 m-thick, undifferentiated Cenozoic sequence. To the south, Upper Cretaceous to Lower Paleogene layers, instead of the undifferentiated Cenozoic deposits, are covered by basalts. The volcanism additionally covers the western side of the San Rafael Block as well as the fold and thrust belt of the Andean foothills. The Payenia province consists of more than 800 monogenetic basaltic cones, and scarce polygenetic volcanos fed by shallow magmatic chambers. Among the latter, the following examples can be mentioned: Miocene Chachahuén volcano (7 to 5 Ma), composed of andesites, rhyodacites and basalts with high-K and amphibole; Pliocene El Nevado volcano, with calc-alkaline, basaltic trachyandesites, trachytes, dacites and rhyolites; and Upper Pleistocene to Holocene Payún Matrú volcano, composed of trachytes, trachyandesites and trachybasalts. The southernmost area of the central depression is covered by the Pleistocene, Auca Mahuida basaltic shield (1.7 to 0.88 Ma), consisting of trachybasalts, basaltic trachyandesites and trachyandesites. This shield is aligned with Tromen and Domuyo volcanos, defining an NW-SE volcanic belt, oblique to the Andes, whose southern tip corresponds to the Cortaderas lineament. Some of the monogenetic cones are of hydromagmatic origin, whereas a few others exhibit small mantle inclusions. The Payenia retroarc province develops to the south of the Pampean flat slab segment, where the heights of the Andes are smaller. This height difference might have been caused by a higher thermal gradient that softened the crust in the steeper subduction segment, while the colder lithosphere in the flat slab segment allowed greater crustal thickening and height. From the tectonic point of view the height difference has been attributed to a smaller orogenic contraction in the southern segment. Holocene volcanism appears exclusively at Payun Matru volcano. The associated thermal anomaly may explain the segmentation of the central depression, separating the Cenozoic Los Huarpes basin in the north and the section without coeval sedimentation in the south. The only surface evidence of this segmentation is the E-W, La Carbonilla fault, running both sides of the Payun Matru volcano. Other authors have explained this thermal anomaly as a process of crustal thinning and stretching associated with hot injection from the asthenosphere.Fil:Llambías, E.J. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Risso, C. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

The Mencué Batholith: Permian episodic arc-related magmatism in the western North Patagonian Massif, Argentina

The Mencué Batholith, western North Patagonian Massif, includes three major bodies. The Mencué Granodiorite, Cura Lauquén Granite and La Blancura Syenogranite. There are dikes of granitic and basaltic rocks cutting the above-cited rocks. The Mencué Batholith represents several episodes of magmatism, with ages varying between 294 and 239 Ma. The Mencué Granodiorite and the Cura Lauquén Granite are solid-state deformed and are S-type. They have high-K and normal calc-alkaline affinities. These rocks contain significant quantities of subduction-zone chemical components that decrease towards younger lithofacies. La Blancura Syenogranite lack subduction zone chemical components and represent A-type granite, typical of within-plate magmatism. The partial melting of metapelites could be the process of formation of these bodies. The older lithofacies of the Mencué Batholith, found westernmost, display a stronger deformation, but there is a progressive eastward change to younger and mildly deformed bodies and even non-deformed bodies. We conclude that the evolution of the Mencué Batholith start in the Sakmarian-Roadian (Early Permian) period. At this time, a subduction zone was active to the west and its thermal influence affected sedimentary or metamorphic rocks producing S-type granites During the Wordian and Capitanian, (Middle Permian) periods, the Mencué Batholith was mildly deformed, possibly in the process of the vanishing of the deformation and has a minor subduction chemical signature. Between the Wuchiapingian and Olenekia periods, the alkaline facies of the Mencué Batholith show an absence of deformation and the characteristics of within-plate magmatism. The Early Permian magmatic events in the western North Patagonian Massif are represented by the older bodies of the Mencué Batholith and were produced by subduction in the western margin of the Gondwanan continent. The Late Permian-Early Triassic magmatic events show a noticeable decreasing influence of subduction and an increasing influence of within-plate chemical components.Fil: Gregori, Daniel Alfredo. Universidad Nacional del Sur. Departamento de Geología. Cátedra de Geología Argentina; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Geológico del Sur. Universidad Nacional del Sur. Departamento de Geología. Instituto Geológico del Sur; ArgentinaFil: Strazzere, Leonardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Geológico del Sur. Universidad Nacional del Sur. Departamento de Geología. Instituto Geológico del Sur; ArgentinaFil: Barros, Mercedes Virginia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Geológico del Sur. Universidad Nacional del Sur. Departamento de Geología. Instituto Geológico del Sur; ArgentinaFil: Benedini, Leonardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Geológico del Sur. Universidad Nacional del Sur. Departamento de Geología. Instituto Geológico del Sur; ArgentinaFil: Marcos, Paulo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Geológico del Sur. Universidad Nacional del Sur. Departamento de Geología. Instituto Geológico del Sur; ArgentinaFil: Kostadinoff, Jose. Universidad Nacional del Sur. Departamento de Geología. Cátedra de Geología Argentina; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Geológico del Sur. Universidad Nacional del Sur. Departamento de Geología. Instituto Geológico del Sur; Argentin