Complementary crystal accumulation and rhyolite melt segregation in a late Miocene Andean pluton

Abstract

High-silica granites are hypothesized to form via fractionation in the shallow crust, yet the predicted residues are rarely identified and can be difficult to distinguish within plutons whose rocks otherwise plot along liquid lines of descent. Bulk-rock compositional mass balance in the late Miocene Risco Bayo-Huemul plutonic complex (Chile) suggests that lithological differences within the Huemul pluton reflect residual crystal concentration in response to melt extraction. A compositional gap from 70 to 75 wt% SiO 2 and strong depletion in Ba and Eu suggest that Huemul alkali feldspar (Afs) granites are frozen remnants of highly evolved rhyolitic melt extracted from a mush. Quartz monzonites enriched in Zr and Ba with Eu/ Eu* near unity are interpreted to represent the complementary residual silicic cumulates of this fractionation process. Compositional variations in Afs granite zircon (Eu/Eu*, Dy/ Yb) further support extraction of this melt from a zircon-saturated mush. U-Pb zircon dates indicate that Huemul rocks evolved ~800 k.y. after initial crystallization of more mafic Risco Bayo rocks, likely precluding their evolution via fractionation from mafic forerunners. This pluton records a means to produce rhyolite in the upper crust, which has propelled large silicic eruptions during the Quaternary within the Andean subduction zone.

Interpreting Granitic Fabrics in Terms of Rhyolitic Melt Segregation, Accumulation, and Escape Via Tectonic Filter Pressing in the Huemul Pluton, Chile

“…Crustal shortening is currently evidenced by geodetic ENE interseismic crustal velocities (Ruegg et al, ). The crust beneath the SVZ experienced ENE shortening prior to, during, and after emplacement of the Huemul pluton 6.4–6.2 Ma (Schaen et al, ).…”

“…The Risco Bayo‐Huemul plutonic complex comprises two temporally and compositionally distinct plutons: the mafic‐to‐intermediate Risco Bayo (7.2 to 6.4 Ma) and the silicic Huemul (6.4 to 6.2 Ma; Schaen et al, , ). The plutonic complex is exposed below the Quaternary Tatara‐San Pedro volcanic system (Figure ), in NNW‐trending valleys to the northeast and southeast of the edifice (Nelson et al, ; Singer et al, ).…”

“…Liquid line of descent (LLD) produced by differentiation of modeled mafic to intermediate parent suggests that quartz monzonite rocks are non‐liquid compositions. Schaen et al () show that rhyolite (i.e., high‐silica granite ) melt extraction from the granite domain is complemented by crystal accumulation in the quartz monzonite . Schematic diagrams show evolution of compositional domains (b) before and (c) after rhyolite segregation.…”

“…Schaen et al () provide valuable insight on the temporal and compositional relationship of the three Huemul domains and their relation through rhyolite segregation. The goal of the present study is to constrain how the flow of rhyolite within the system leads to segregation and how this flow is related to external deformation (Figures b and c).…”

“…This setting increases the likelihood of the reservoir experiencing subsolidus (solid-state) deformation, which acts to overprint and often obliterate of the fabric record of magmatic processes. Schaen et al (2017Schaen et al ( , 2018 document extraction of rhyolitic melt from the shallow (<7 km) Huemul pluton in the Chilean Andes. The Huemul pluton was syntectonically emplaced during contractional deformation of the arc and preserves magmatic fabrics developed while in a partially molten ("mushy") state.…”