Bolivian hornblendite cumulates: Insights into the depths of Central Andean arc magmatic systems

Abstract

The eruptive products of continental arc volcanoes provide wide-ranging insights into the processes governing storage, differentiation, and final eruption (or emplacement) of magmas from trans-crustal magmatic systems. The important role and fractionation of amphibole in continental arc magmatic systems is well documented even if amphibole is generally absent as a prevalent phenocrystic phase in erupted, differentiated lavas. This study presents a comprehensive textural and geochemical investigation of rare plagioclase-bearing hornblendite cumulates entrained in c. 1.4 Ma (trachy-)andesites at the Quillacas monogenetic center on the Bolivian Altiplano, Central Andes. MicroXRF mapping and imaging via SEM-EDS clearly reveals their mesocumulate, idiomorphic nature characterized by cumulus hornblende (≥90%) with intercumulus sodic plagioclase (~8%), minor Fe-Ti oxides and accessory apatites. The trace element compositions of cumulus amphiboles are consistent with their crystallization from a silicate melt with (La/Yb)N < 5 and low (Pb/Ce)N at <0.6. Amphibole TiO2 contents imply crystallization between 700 and 764 °C consistent with depths in the arc crust of 50–55 km as constrained by the Central Andean geotherm. Bulk hornblendite isotopic signatures (Sr-Nd-Pb) correspond to compositions associated with the Central Andean mid-lower arc crust consistent with the presence of hornblendite cumulate piles at this depth. Intercumulus (titano)magnetite exhibits oxy-exsolution ilmenite lamellae, the formation of which is associated with oxygen fugacity (ƒO2) increase during magmatic cooling and differentiation. The ubiquitous presence of gabbroic dehydration (reaction) rims (20–40 μm in width) on the amphiboles is interpreted to be the result of a decrease in ƒO2 during ascent. The hornblendite mesocumulates from this study validate the extensive “cryptic” fractionation of amphibole at arcs and its important role on the petrogenesis of (continental) arc magmas.