Geochemical and geochronological constraints on Quaternary volcanism in southern Mendoza, Argentina

The basaltic volcanism occurring on a continental plate behind a volcanic arc is not well studied, as this type of volcanic activity is neither common nor easily classified by conventional tectonic setting. In order to investigate the volcanism of this type, an area with abundant Quaternary monogenetic volcanism behind a volcanic arc was chosen, namely, the Payenia Basaltic Province (PBP), in central west Argentina. The PBP is subdivided into several volcanic fields; Nevado, Payún Matrú, Llancanelo and Rio Colorado. The Quaternary alkali volcanism produced in this region has been the focus of several studies since the 1960s, with the pioneering work of Llambías (1966), mainly dedicated to the polygenetic Payún Matrú caldera. More recently, several studies have dealt with the monogenetic volcanism in the PBP, classifying this setting as back-arc (Bermudez and Delpino, 1989; Bertotto et al., 2009; Gudnason et al., 2012; Jacques et al., 2013; Espanon et al., 2014b; among others), intraplate (Ramos and Folguera, 2011) or a transition between back-arc and intraplate (Søager et al., 2013; Hermando et al., 2014), highlighting the difficulty in assigning a setting. In the last few years, the understanding of this basaltic province has significantly increased; however, there are some aspects and approaches that have not been previously considered and applied. These are, for example; to integrate and compare the geochemical signatures of the volcanic fields composing the PBP, in order to obtain a general geochemical fingerprint. Furthermore, using a combined approach with geochemistry and geochronology, the evolution of the PBP can be reconstructed. The focus of this investigation is the Llancanelo and the Payún Matrú volcanic fields as they represent the latest stages of activity in the PBP and are late Quaternary in age, according to the available chronology. The aims of the investigation are i) to determine the geochemical signatures that have controlled the volcanism during the Pleistocene and the Holocene, ii) to temporally and spatially determine the volcanic evolution and to comprehend the processes that shaped these two volcanic fields, and iii) to contribute broadly to the understanding of this type of volcanism and thereby relate the findings with similar locations around the globe. In the current investigation, new geochemical data from the Llancanelo Volcanic Field (LLVF) and Payún Matrú Volcanic Field (PMVF) are presented. In addition, new numerical ages from three different dating methods are provided. These ages are from basalts of the Pampas Onduladas flow which have not been previously dated and from the Santa Maria volcano which has been regarded as young (Espanon et al., 2014a), but for which no previous reliable numerical ages were available. Of special interest are the Holocene basalts from the Santa Maria volcano, as they constitute the most recent history of the PBP

Geochronological, morphometric and geochemical constraints on the Pampas Onduladas long basaltic flow (Payún Matrú Volcanic Field, Mendoza, Argentina)

The Pampas Onduladas flow in southern Mendoza, Argentina, is one of the four longest Quaternary basaltic flows on Earth. Such flows (\u3e 100 km) are relatively rare on Earth as they require special conditions in order to travel long distances and there are no recent analogues. Favourable conditions include: a gentle topographic slope, an insulation process to preserve the melt at high temperature, and a large volume of lava with relatively low viscosity. This study investigates the rheological and geochemical characteristics of the ~ 170 km long Pampas Onduladas flow, assessing conditions that facilitated its exceptional length. The study also reports the first geochronological results for the Pampas Onduladas flow. 40Ar/39Ar step-heating analyses of groundmass reveal an eruption age of 373 ± 10 ka (2σ), making the Pampas Onduladas flow the oldest Quaternary long flow. The methods used to assess the rheological properties include the application of several GIS tools to a digital elevation model (DEM) to determine the length, width, thickness, volume and topographic slope of the flow as well as algorithms to determine its density, viscosity and temperature. The slope of the Pampas Onduladas flow determined from the initial part of the flow on the eastern side of La Carbonilla Fracture to its end point in the province of La Pampa is 0.84% (0.29°), the steepest substrate amongst long Quaternary flows. The rheological properties, such as density viscosity and temperature from the Pampas Onduladas flow are similar to values reported for other long Quaternary flows. However, the minimum volume calculated is relatively low for its length compared with other long Quaternary flows. Therefore, the extension of the Pampas Onduladas flow was probably controlled by a steep slope, combined with an insulating mechanism, which helped in providing optimal conditions for a travel length of almost 170 km

Maryland medicine : MM ; a publ. of MedChi, the Maryland State Medical Society

Southern Mendoza, Argentina, is characterised by abundant Pleistocene to Holocene volcanism associated with back-arc magmatism, influenced by the subducting Nazca plate. Age determinations in this volcanic area have been improved during the last 5 years

Journal of the Institute of Nuclear Materials Management : JNMM

The Payenia Basaltic Province (PBP) is located 450. km east of the Chile-Peru trench in central west Argentina, behind the Andean arc front, constituting the back-arc. In order to evaluate the influence of the subducting slab as well as the magmatic source of this region, two volcanic fields located at comparable distance to the trench, having abundant basaltic products and similar eruptive timeframes were chosen. The Llancanelo (LLVF) and the Payún Matrú (PMVF) volcanic fields are part of the PBP and exhibit abundant basaltic activity during the Pleistocene. The geochemical data suggest that the LLVF has some arc signatures which have been described as weak as they are not as pronounced as in the Andean arc. The weak arc signature is not derived from slab dehydration as high Th enrichment relative to U cannot be explained by this process. We relate the Th enrichment as well as the lack of large residual garnet signatures, to slab sediments in the source. In the case of the PMVF, no arc signature has been inferred despite being only 30. km south of the LLVF. However the PMVF has a composition similar to that of the local intraplate end member, represented by the Rio Colorado volcanic field. The two volcanic fields, LLVF and PMVF, show indications of lower crustal assimilation as they trend towards the lower continental crust end member in Nb/U vs Ce/Pb and Nb/Yb vs Th/Yb diagrams.The geochemical differences between the LLVF and the PMVF as well as between several volcanic fields are illustrated using spatial distribution maps of geochemical ratios. Using this new approach, the decrease in arc signature can be traced in the back-arc and the higher enrichment in high field strength elements (HFSE) relative to large ion lithophile elements (LILE) in the PMVF compared to the LLVF is explicitly shown. These geospatial maps provide a graphical manner to illustrate the presence of two distinct types of volcanism (OIB-like and arc-like) occurring in the same Quaternary basaltic province

Localised magmatic constraints on continental back-arc volcanism in southern Mendoza, Argentina : the Santa Maria Volcano

The Payún Matrú Volcanic Field constitutes part of the continental back-arc in Argentina. This volcanic field has been the focus of several regional investigations; however, geochemical analysis of recent volcanoes (<8 ka) at the scale of an individual volcano has not been conducted. We present a morphological description for the Santa Maria Volcano in addition to results from major and trace element analysis and ²³⁸U-²³⁰Th-²²⁶Ra disequilibria. The trace element evidence suggests that the Santa Maria magmatic source has a composition similar to that of the local intraplate end member (resembling an ocean island basalt-like source), with a slight contribution from subduction-related material. The U-series analyses suggest a high ²²⁶Ra excess over ²³⁰Th for this volcano, which is not derived from a shallow process such as hydrothermal alteration or upper crustal contamination. Furthermore, intermediate-depth processes such as fractional crystallisation have been inferred for the Santa Maria Volcano, but they are not capable of producing the ²²⁶Ra excess measured. The ²²⁶Ra excess is explained by deep processes like partial melting of mantle lithologies with some influence from subducted Chilean trench sediments. Due to the short half-life of ²²⁶Ra (1600 years), we infer that fast magma ascent rates are required to preserve the high ²²⁶Ra excess.13 page(s