Geochemical variations in the Quaternary Andean back-arc volcanism, southern Mendoza, Argentina

Abstract

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.

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

“…The Llancanelo volcanic field has weak subduction signature while the same type of signature was not recognised in the Payún Matrú Volcanic Field (PMVF; Espanon et al, 2014). The basalts from the Payún Matrú volcanic field have an intraplate geochemical signature similar to that of the Rio Colorado volcanic field (Espanon et al, 2014). The Pampas Onduladas flow is part of the PMVF and is located on the eastern side of this volcanic field where the oldest basaltic flows are found ( Figure 1).…”

“…Available geochemical data from the Pampas Onduladas flow are summarised in Table 1 ( Pasquarè et al, 2008;Espanon et al, 2014). The extent of the Pampas Onduladas flow was determined using existing maps (Pasquarè et al, 2008) as well as a digital elevation model and surface maps.…”

“…In this sense, the Nevado volcanic field has a defined subduction signature while the Rio Colorado volcanic field has a typical intraplate signature resembling that of ocean island basalts (Kay et al, 2013;Søager et al, 2013). The Llancanelo volcanic field has weak subduction signature while the same type of signature was not recognised in the Payún Matrú Volcanic Field (PMVF; Espanon et al, 2014). The basalts from the Payún Matrú volcanic field have an intraplate geochemical signature similar to that of the Rio Colorado volcanic field (Espanon et al, 2014).…”

“…The magmatic source region for this extensive flow has been inferred to be affected by metasomatism associated with the subduction of the Nazca plate (Pasquarè et al, 2008), although recent studies suggest that the Payún Matrú Volcanic Field (PMVF, Figure 1) shows minimal (Jacques et al, 2013;Søager et al, 2013) to negligible (Espanon et al, 2014) evidence for subduction signatures. The basalts in the PMVF have geochemical characteristics similar to the local ocean island basalt (OIB) source (Espanon et al, 2014), taken as the Rio Colorado Volcanic Field previously described by Søager et al (2013) as OIB-type. In addition, lower crustal assimilation has been suggested (Espanon et al, 2014) for the Pampas Onduladas flow.…”

“…The basalts in the PMVF have geochemical characteristics similar to the local ocean island basalt (OIB) source (Espanon et al, 2014), taken as the Rio Colorado Volcanic Field previously described by Søager et al (2013) as OIB-type. In addition, lower crustal assimilation has been suggested (Espanon et al, 2014) for the Pampas Onduladas flow.…”