Multi-stage Evolution of Late Neogene Mantle-derived Magmas from the Central Andes Back-arc in the Southern Puna Plateau of Argentina

Abstract

The late Miocene and younger mafic back-arc lavas in the southern Puna of the central Andean plateau have been attributed to the aftermath of crustal and mantle lithospheric delamination or foundering. In this paper, we analyze in more detail the nature of the back-arc mafic suite magmas, including the conditions of magma generation in the mantle and of magma evolution during ascent and ponding in the crust, using extensive compositional data for phenocryst minerals and olivine-hosted melt inclusions in combination with published and new whole-rock chemical and isotopic data. We estimate that the primary melts last equilibrated with an enriched mantle source at temperatures near 1375ºC and pressures near 2 GPa, which is near the base of the seismically determined 60 km thick crust. A mantle source geochemically enriched by continental material introduced through delamination and subducted erosion processes is required to explain the coincidence of the high 87Sr/86Sr ratios (>0·705) and high Sr concentrations (>700 ppm) of the most primitive lavas (e.g. 9-10 wt % MgO, olivine Fo88). The crystallization conditions inferred from mineral-melt equilibria indicate that olivine (T=1320-12208C) was followed by clinopyroxene (T =1230-1140ºC). Clinopyroxene-melt equilibration pressures of 0·7 to near 1GPa in the most mafic samples indicate that the magmas crystallized at mid-crustal depths of 20-35 km, within a region of inferred partial melt accumulation based on the presence of low seismic velocity zones. Olivine-hosted melt inclusions indicate relatively dry melts (maximum 0·5 wt % H2O) with unusual high-Al basaltic compositions, which are attributed to the high-pressure suppression of plagioclase crystallization. A first stage of crustal contamination before mid-crustal accumulation and crystallization of the mafic magmas is suggested by high O-isotope ratios in olivine phenocrysts and negative Eu anomalies in clinopyroxene from the plagioclase-free mafic lavas. Mixing models based on trace elements and radiogenic isotopes suggest assimilation of silicic melt in the lower crust, similar to contemporaneous glassy dacites with steep REE patterns and negative Eu anomalies. A second stage of crustal assimilation at shallower depths is indicated by the mismatch of incompatible elements in clinopyroxene relative to bulk-rock compositions, by strong positive correlations of radiogenic isotopes with wt % SiO2, and by petrographic observation of partly resorbed and reacted quartz xenocrysts. Mixing calculations require the erupted magmas to have assimilated in total some 15-25% crust.