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 variabil-ities 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.

Crystal -melt separation has been invoked as a mechanism that generates compositional variabil-ities 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.