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A geochemical investigation of the Sara Sara volcano (Peru): a Pleistocene edifice of the northern termination of the Andean Central Volcanic Zone

Abstract:

In order to understand the contribution of the continental crust on arc magmas, we proceeded with a geochemical study of the Pleistocene eruptive products emitted by Sara Sara volcano in Southern Peru. This quaternary stratovolcano, and the surrounding monogenetic cones, are located at the northern arc termination of the Central Volcanic Zone (CVZ) of the Andes, an arc segment constructed on a thick continental crust. Based on major, trace elements and Sr-Nd-Pb isotopic ratios, we demonstrate that the observed geochemical variations are related to an assimilation-fractional crystallisation (AFC) process dominated by plagioclase and amphibole fractionation coupled with assimilation of the upper continental crust. To decipher the magmatic processes located in the lower continental crust, we extrapolated these geochemical trends towards more basic terms using the compositions of surrounding monogenic volcanoes. We achieve this by using trace element ratios vs. SiO₂ diagrams to estimate the hypothetical composition of Sara Sara's parental magmas before their differentiation by fractional crystallisation. These hypothetical compositions are similar to those of the cones in the Andahua-Huambo and Yura monogenic volcanic fields, representing the most primitive lavas in the Peruvian CVZ. By doing so, we demonstrate that the parental andesitic magmas of Sara Sara volcano exhibit a garnet-rutile signature, characterised by HREE depletion, and elevated Dy/Yb, Sr/Y and Nb/Ta ratios. We discuss the origin of this signature and conclude that it is acquired by partial melting of mafic lithologies leaving residues with garnet and rutile at the deep continental crust. These processes formed enriched basaltic melts and rhyodacitic melts which were mixed with mantle-derived magma, thereby generating hybrid andesitic parental magmas. In our scenario, arc andesites are generated in two steps including lower continental crust partial melting and subsequent hybridation with mantle-derived magmas, followed by an assimilation-fractional process in the upper crust. One sentence summary In the context of the termination of a continental volcanic arc, the high-pressure melting process at the base of the continental crust allows the andesites to acquire a garnet-rutile signature, which is progressively modified by assimilation-crystallisation processes during differentiation at lower pressures.