Abstract
The Andean continental arc is built upon the thickest crust on Earth, whose eruption products reflect varying degrees of crustal assimilation. In order to robustly model magma evolution and assimilation at subduction zones such as the Andes, the compositions of parental magmas feeding crustal magma reservoirs need to be defined. Here we present new olivine and clinopyroxene oxygen isotope data from rare mafic volcanic rocks erupted at the margins of the giant Altiplano-Puna Magma Body (APMB) of the Altiplano-Puna Volcanic Complex, Central Andes. Existing olivine and pyroxene δ18O values for the Central Andes are highly variable and potentially not representative of sub-arc parental compositions. However, new olivine (n = 6) and clinopyroxene (n = 12) δ18O values of six Central Andean volcanoes presented here display a narrow range, with averages at 6.0‰ ± 0.2 (2σ S.D.) and 6.7‰ ± 0.3 (2σ S.D.), consistent with a common history for the investigated minerals. These data allow us to estimate the δ18O values of sub-arc, parental melts to ca. 7.0‰ ± 0.2 (2σ S.D.). Parental melts feeding the APMB and associated volcanic centres are postulated to form in the felsic continental crust following assimilation of up to 28% high-δ18O basement rocks by mantle-derived magmas.
Highlights
Eruption products of frontal arc volcanoes usually exhibit heterogeneous chemical and isotopic compositions because parental magmas are compositionally modified by incorporation of continental crust either at their source via subducted sedimentary material or by crustal contamination during subsequent ascent through the crust (e.g.1)
In order to utilise oxygen isotopes to assess the magnitude of crustal material assimilated by evolved magmas along the Central Andean arc, it is necessary to know the δ18O value of the parent magma, which itself may be compositionally modified by assimilation in the deep crust
The samples selected for this study are from six individual volcanoes (La Poruña, San Pedro, Paniri, La Poruñita, Palpana and Chela), which were active at different times and are all located around the western margin of the Altiplano-Puna Magma Body (APMB) melt anomaly in the Altiplano-Puna Volcanic Complex (Fig. 1)
Summary
Eruption products of frontal arc volcanoes usually exhibit heterogeneous chemical and isotopic compositions because parental magmas are compositionally modified by incorporation of continental crust either at their source via subducted sedimentary material or by crustal contamination during subsequent ascent through the crust (e.g.1). The samples selected for this study are from six individual volcanoes (La Poruña, San Pedro, Paniri, La Poruñita, Palpana and Chela), which were active at different times and are all located around the western margin of the Altiplano-Puna Magma Body (APMB) melt anomaly in the Altiplano-Puna Volcanic Complex (Fig. 1) Based on their radiogenic isotope compositions, our samples have experienced limited degrees of crustal modification (e.g. assimilation) and are ideally suited to obtaining the parental δ18O values locked in early-formed crystals. The volcanoes included in this study comprise, in order of increasing eruption age, La Poruña, San Pedro, Paniri, La Poruñita, Palpana and Chela, all of which are situated within the Altiplano-Puna Volcanic Complex but peripheral to the proposed APMB reservoir[19] (Fig. 1) In this region, the ascending parental basaltic-andesite magma is thought to have avoided significant contamination by evolved melts from the APMB as demonstrated by the lowest 87Sr/86Sr and highest 143Nd/144Nd being towards the borders of the large felsic body[12,20]. The shape and relatively monotonous composition have been related to rapid construction of the volcanic edifices at ca. 4.1 Ma for Chela and ca. 3.8 Ma for Palpana, followed by restricted erosion and limited duration of magmatic differentiation[24]
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