Differentiation of an upper crustal magma reservoir via crystal-melt separation recorded in the San Gabriel pluton, central Chile

Abstract

Crystal-melt separation has been invoked as a mechanism that generates compositional variabilities in magma reservoirs hosted within the Earth’s crust. However, the way phase separation occurs within such reservoirs is still debated. The San Gabriel pluton of central Chile is a composite pluton (12.82 ± 0.19 Ma) with wide textural/compositional variation (52–67 wt% SiO2) and presents a great natural laboratory for studying processes that occur in upper crustal magma reservoirs. Geochemical and geochronological data supported by numerical models reveals that shallow magma differentiation via crystal-melt separation occurred in magma with intermediate composition and generated high-silica magmas and cumulate residues that were redistributed within the reservoir. The pluton is composed of three units: (1) quartz-monzonites representing the main hosting unit, (2) a porphyritic monzogranite located at the lowest exposed levels, and (3) coarse-grained quartz-monzodiorites with cumulate textures at the middle level of the intrusive. Calculations of mass balance and thermodynamic modeling of major and trace elements indicate that <40 vol% of haplogranitic residual melt was extracted from the parental magma to generate quartz-monzonites, and 50–80 vol% was extracted to generate quartz-monzodiorites, which implies that both units represent crystal-rich residues. By contrast, the monzogranites are interpreted as a concentration of remobilized residual melts that followed 30–70 vol% fractionation from a mush with 0.4–0.55 of crystal fraction. The monzogranites represent the upper level of a pulse that stopped under a crystal-rich mush zone, probably leaving a mafic cumulate zone beneath the exposed pluton. This case study illustrates the role of the redistribution of residual silicic melts within shallow magma reservoirs.