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

Abstract

The physical process of rhyolite segregation from crystal mushes remains elusive as microstructural evidence of conventional segregation mechanisms is not available. This study provides direct fabric evidence for deformation‐assisted segregation of eruptible rhyolite in the Chilean Andean arc. The shallow (<7 km), 6.4–6.2 Ma Huemul pluton comprises domains of quartz monzonite, granite, and high‐silica granite. Compositional modeling shows that rhyolitic melt (high‐silica granite) was extracted from a granitic parent, leaving behind silicic cumulates (quartz monzonite). To understand mechanisms of rhyolite segregation, we investigate magmatic fabrics in the pluton. Anisotropy of Magnetic Susceptibility analyses reveal oblate magnetic fabrics and NNW‐striking, subvertical magnetic foliations throughout Huemul. Within the high‐silica granite, magnetic lineations are subvertical and parallel to elongate miarolitic cavities. Magnetic lineations in the quartz monzonite plunge moderately to the NNW, away from the high‐silica granite. In the quartz monzonite, the Shape‐Preferred Orientation of early feldspars is parallel to the magnetic lineation and developed while suspended in melt. Estimations of early feldspar clustering and crystallinity yield ~38% of interstitial volume loss in the quartz monzonite and no volume loss in the granite. These fabric data suggest ENE tectonic shortening coeval with rhyolite extraction. We explain these observations with a model of tectonic filter pressing in which shortening is accommodated by interstitial melt flow at slow (10−5 km3/yr) rates, segregating moderate volumes of rhyolite in Myr time scales. These interactions link plutonism, tectonic deformation, and upward mobility of eruptible rhyolite in tectonically active margins.

Transient rhyolite melt extraction to produce a shallow granitic pluton

“…This translates to melt fluxes between 0.00017 and 0.0015 km 3 /yr. These magma accumulation rates are minimum estimates, as they do not account for potential volume losses associated with dike propagation or eruption of the high-silica granite ( 35 ).…”

“…We note that, in this context, the average velocity, is solely implemented to displace the crust and intrusions to accommodate successive intrusions ( 84 ). There is some uncertainty in the accommodation mechanism, and for the purposes of these calculations, we assume that the intrusions are accommodated by crustal shortening resulting in vertical translation ( 35 ).…”

Schlieren-Bound Magmatic Structures Record Crystal Flow-Sorting in Dynamic Upper-Crustal Magma-Mush Chambers

“…Tectonism has been proposed to cause magma mush instability on the timescales of seconds to minutes (e.g., earthquakes; Davis et al, 2007), and at the m.y. time-scale can form fabrics and extract melts (e.g, Paterson et al, 1998;Garibaldi et al, 2018;Bachmann and Huber, 2019). Regional tectonism during emplacement of the TIC was dextral transpressive, with metavolcanic and metasedimentary host-rock fabric orientations consistent with the regional NW-SE magmatic foliation found in TIC domains (e.g., Cao et al, 2015;Hartman et al, 2018).…”

“…This suggests that internal magmatic processes dominate in the formation and orientation of schlieren-bound structures. Garibaldi et al (2018) proposed that magmatic fabrics and magmatic structures (e.g., miarolitic cavities) in the Huemul pluton record evidence for a crystalrich mush mobilizing interstitial rhyolitic melts by tectonic filter pressing. Other studies have also considered tectonism as a mechanism to mobilize crystal-mushes and re-orient earlier-formed magmatic structures, including fabrics (e.g., Žák and Paterson, 2010;Alasino et al, 2019).…”

Multiple, Coeval Silicic Magma Storage Domains Beneath the Laguna Del Maule Volcanic Field Inferred From Gravity Investigations

“…The LdMVF (Figure 1), located on the crest of the Andes (36°S) within the Southern Volcanic Zone, comprises the region's largest concentration of crystal poor rhyolite and has hosted at least 50 post-glacial rhyolitic eruptions comprising >40 km 3 since 25 ka (Andersen et al, 2017b;Hildreth et al, 2010). The LdMVF lies 20-30 km south east of the 7.2 to 6.2 Ma Risco Bayo and Huemul plutonic complexes (Garibaldi et al, 2018;Schaen et al, 2018Schaen et al, , 2017 to 75 wt% SiO 2 and strong depletion of Ba and Eu. This composition suggests these granites are frozen remnants of rhyolitic melt extracted from a mush that solidified at 4-6 km depth analogous to those that likely are growing beneath the present day LdMVF (Andersen et al, 2017b(Andersen et al, , 2018Le Mevel et al, 2016;Schaen et al, 2017;Wespestad et al, 2019).…”