ALBERT

Description: Crustal assimilation (e.g. Hildreth and Moorbath, 1988) and/or subduction erosion (e.g. Stern, 1991; Kay et al., 2005) are believed to control the geochemical variations along the northern portion of the Chilean Southern Volcanic Zone. In order to evaluate these hypotheses, we present a comprehensive geochemical data set (major and trace elements and O-Sr-Nd-Hf-Pb isotopes) from Holocene primarily olivine-bearing volcanic rocks across the arc between 34.5-38.0°S, including volcanic front centers from Tinguiririca to Callaqui, the rear arc centers of Infernillo Volcanic Field, Laguna del Maule and Copahue, and extending 300 km into the backarc. We also present an equivalent data set for Chile Trench sediments outboard of this profile. The volcanic arc (including volcanic front and rear arc) samples primarily range from basalt to andesite/trachyandesite, whereas the backarc rocks are low-silica alkali basalts and trachybasalts. All samples show some characteristic subduction zone trace element enrichments and depletions, but the backarc samples show the least. Backarc basalts have higher Ce/Pb, Nb/U, Nb/Zr, and Ta/Hf, and lower Ba/Nb and Ba/La, consistent with less of a slab-derived component in the backarc and, consequently, lower degrees of mantle melting. The mantle-like δ18O in olivine and plagioclase phenocrysts (volcanic arc = 4.9-5.6 and backarc = 5.0-5.4 per mil) and lack of correlation between δ18O and indices of differentiation and other isotope ratios, argue against significant crustal assimilation. Volcanic arc and backarc samples almost completely overlap in Sr and Nd isotopic composition. High precision (double-spike) Pb isotope ratios are tightly correlated, precluding significant assimilation of older sialic crust but indicating mixing between a South Atlantic Mid Ocean-Ridge Basalt (MORB) source and a slab component derived from subducted sediments and altered oceanic crust. Hf-Nd isotope ratios define separate linear arrays for the volcanic arc and backarc, neither of which trend toward subducting sediment, possibly reflecting a primarily asthenospheric mantle array for the volcanic arc and involvement of enriched Proterozoic lithospheric mantle in the backarc. We propose a quantitative mixing model between a mixed-source, slab-derived melt and a heterogeneous mantle beneath the volcanic arc. The model is consistent with local geodynamic parameters, assuming water-saturated conditions within the slab.

Description: 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.

Description: Highlights • New trace element and Sr-Nd-Pb isotope data for SVZ crustal and trench sediment samples. • New whole-rock Hf–O isotope data for crustal and trench sediment samples. • Nd model ages confirm a Mesoproterozoic crustal residence age for the Guarguaráz complex. • NSVZ trench sediments reflect the source composition of eroded mafic material. Abstract This paper provides new trace element and Sr-Nd-Hf-Pb-O isotope data on Neoproterozoic to Phanerozoic mid to upper crustal lithologies of the Andean basement in central Chile and western Argentina (33°-35°S; 69°-72°W). It also provides additional data on trench sediments being subducted offshore the northern segment of the Southern Volcanic Zone in Chile. Neoproterozoic metamorphic and igneous rocks from the Guarguaráz complex (Argentina; 33.6°S, 69.5°W), when back corrected to 350 Ma, display a narrow range in 87Sr/86Sri (0.713–0.718), 143Nd/144Ndi (0.5118–0.5121), εNdi (−8.1 to −1.1), εHfi (−11.4 to +1.2) and δ18O (9–13‰). Nd model ages (TDM = 1.08–1.65 Ga) for the Guarguaráz complex points to a Mesoproterozoic crustal residence age for these rocks. Metasedimentary rocks from the Carboniferous accretionary prism in central Chile (~34°S) overlap with these ranges, but differ by having lower initial 87Sr/86Sri (0.7052–0.7093) and higher δ18O (14–17‰). The Guarguaráz metamorphic and igneous rocks, when back corrected to 350 Ma, have similar Pb isotope ratios than the Chilean Carboniferous metasedimentary rocks (206Pb/204Pbi = 17.58–18.52 vs. 18.33–18.46; 207Pb/204Pbi = 15.50–15.64 vs. ~15.64; 208Pb/204Pbi = 37.70–38.36 vs. 37.98–38.18). Two Guarguaráz samples are shifted towards less radiogenic Pb isotope ratios, similar to samples representative of the Cuyania basement. This suggests that Chilenia hosts at least two geochemical components: (1) a component with unradiogenic Pb isotopes, similar to the Proterozoic Cuyania basement, and (2) a component with more radiogenic Pb isotopes, similar to Chilean Phanerozoic metasedimentary and igneous rocks. The ranges in Pb isotope ratios for the Chilean Mesozoic (206Pb/204Pbi = 18.44–19.86; 207Pb/204Pbi = 15.59–15.69; 208Pb/204Pbi = 38.30–40.30) and Miocene (206Pb/204Pbi = 18.43–18.57; 207Pb/204Pbi = 15.58–15.60; 208Pb/204Pbi = 38.33–38.46) igneous rocks are similar to those of the accretionary prism. The Mesozoic and Miocene intrusive rocks are characterized by low 87Sr/86Sri (0.704–0.708 and ~0.704, respectively) and high εNdi (−6.2 to +4.0 and + 3.9 to +5.9, respectively) and εHfi (+7.0 to +12.7 and + 8.5 to +10.8, respectively). They can be divided into two groups. Group (1), consisting exclusively of Mesozoic samples, has negative εNdi, 87Sr/86Sri 〉 0.706, elevated e.g., Ba/Th, Nb/Yb, Zr/Y and lower Nb/La, reflecting derivation from enriched (most likely overriding crust or mantle) material. Group (2), consisting of Mesozoic and Miocene rocks, has positive εNdi, εHfi, and lower initial 87Sr/86Sri than group (1) reflecting depleted mantle melts addition to the crust. Finally, Sr-Nd-O isotopic compositions of the trench sediments at latitude 33°-33.3°S are almost identical to those at latitude 35°-40°S, indicating a relative homogeneous material input along the SVZ, although there are subtle differences in REE and Pb isotopic compositions. Based on Nd–Hf isotopes, trench sediments offshore Chile (εNd〉 +1; εHf 〉 +2) and offshore Peru (εNd 〈 −2; εHf 〈 +1) have distinct compositions, reflecting the differences in input material. The positive εNd and εHf values suggest derivation from eroded depleted mantle-derived mafic material.