DECIPHERING TRANSLITHOSPHERIC MAGMATIC PROCESSES THROUGH A DETAILED TEXTURAL AND MINERALOGICAL STUDY OF A MONOGENETIC VOLCANO CRYSTAL CARGO

For decades, the fundamental processes that influence melt generation in Earth’s mantle and subsequent transport through the overriding crustal lid have been studied intensely. In continental arc environments, identifying translithospheric phenomena can be complicated due to the multitude of processes which occur: fractional crystallization, magma mixing/mingling, and crustal contamination. This is particularly true for the Central Andes of South America where continental crust can reach up to 70-75km in thickness. Lava from the monogenetic volcanic center of Quillacas in the back-arc region of the Bolivian Andes form the basis of this study. Here, the andesitic erupted lavas not only contain products of crystallization from primitive melts in the form of Cr spinel-bearing olivine mineral clusters but also abundant evidence of open system magmatic processes in the form of antecrysts, hornblendite cumulates, and crustal xenoliths. These lavas therefore offer an opportunity to interrogate and identify the inputs into translithopheric magmatic systems in continental arc environments.

Based on petrography and geochemical analysis of olivine grains (Fo_85-87) and their peritectic orthopyroxene rims we suggest the presence of a mid-crustal mush zone where olivine resided for a prolonged time and re-equilibrated recording temperatures of ~1060⁰C as determined by Al in olivine thermometry.

Recent work on these lavas has also proposed the presence of a mid-crustal mush zone at ~16-24km. Grains of quartz and biotite in the lavas are interpreted as xenocrystic. Plagioclase macro- and microphenocrysts show a variety of sizes and disequilibrium textures including oscillatory, normal, reverse, and patchy zoning. These data are interpreted as representing multiple crystal populations.

Combined, the mineralogical, textural, and geochemical signatures of the Quillacas mineral populations are consistent with open magmatic processes and the presence of a cumulate-bearing mid-crustal storage zone. This is consistent with multiple geophysical studies in the Central Andean region that have identified low seismic velocity zones.

The mineral assemblage of the Quillacas lava thus captures the complexity of translithopheric magmatic systems beneath monogenetic volcanic systems in continental arc environments.