Geochemical variations in the Central Southern Volcanic Zone, Chile (38–43°S): The role of fluids in generating arc magmas

Abstract

We present new Sr–Nd–Pb–Hf–O isotope data from the volcanic arc (VA, volcanic front and rear arc) in Chile and the backarc (BA) in Argentina of the Central Southern Volcanic Zone in Chile (CSVZ; 38–43°S). Compared to the Transitional (T) SVZ (34.5–38°S; Jacques et al., 2013), the CSVZ VA has erupted greater volumes over shorter time intervals (Völker et al., 2011) and produced more tholeiitic melts. Although the CSVZ VA monogenetic cones are similar to the TSVZ VA samples, the CSVZ VA stratovolcanoes have higher ratios of highly fluid-mobile to less fluid-mobile trace elements (e.g. U/Th, Pb/Ce, Ba/Nb) and lower more- to less-incompatible fluid-immobile element ratios (e.g. La/Yb, La/Sm, Th/Yb, Nb/Yb), consistent with an overall higher fluid flux and greater degree of flux melting beneath the CSVZ stratovolcanoes compared to the CSVZ monogenetic centers and the TSVZ VA. The CSVZ monogenetic centers overlap the TSVZ in Sr and Nd isotopes, but the stratovolcanoes are shifted to higher Sr and/or Nd isotope ratios. The Pb isotopic composition of the CSVZ overlaps the TSVZ, which is clearly dominated by the composition of the trench sediments, but the CSVZ monogenetic samples extend to less radiogenic Pb isotope ratios. δ18Omelt from the CSVZ stratovolcano samples are below the MORB range, whereas the CSVZ monogenetic and the TSVZ samples fall within and slightly above the MORB range. The Nd and Hf isotopic ratios of the CSVZ VA extend to more radiogenic compositions than found in the TSVZ VA, indicating a greater contribution from a more depleted source. These correlations are interpreted to reflect derivation of fluids from hydrothermally altered oceanic crust and/or serpentinized upper mantle of the subducting plate. CSVZ BA basalts largely overlap TSVZ BA basalts, displaying less or no subduction influence compared to the VA, but some CSVZ BA basalts tap more enriched mantle, possibly subcontinental lithosphere, with distinctively lower Nd and Hf and elevated 207Pb/204Pb and 208Pb/204Pb isotope ratios.