26-7 Constraining Aplite Melt Extraction Pressures with rhyolite-MELTS

Session: Subduction Zones and Their Volcanic Arcs: Initiation and Evolution, Structure, Metamorphism, Magmatism

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Abstract:

The Tuolumne Intrusive Complex (TIC) is the youngest (90–82 Ma) intrusive suite in Yosemite National Park and provides a natural laboratory for examining late-stage magmatic processes in large Cordilleran plutonic systems. The TIC consists primarily of medium- to coarse-grained granodiorite and granite intruded by aplite dikes—fine-grained, silica-rich rocks commonly interpreted as late-stage residual melts extracted from crystal-rich magma reservoirs. Aplite dikes throughout the TIC exhibit varying field relationships with their host granodiorites, ranging from sharp to gradational contacts, as well as a range of related textures. Aplite textures range from fine-grained “sugary” aplites, coarser-grained variants, and aplites with locally developed feldspar megacrysts near aplite-host contacts. However, despite their abundance and recorded variability, the pressure–temperature conditions under which aplite-forming melts are extracted and stored prior to emplacement remain poorly constrained.

This study integrates original field observations, whole-rock geochemistry, and thermodynamic modeling to constrain the extraction pressures of aplite-forming residual liquids at the TIC. Fifteen aplite samples and five host granodiorite samples were collected in October 2024, with whole-rock major element compositions determined by XRF at Hamilton College throughout 2025. Sixteen rhyolite-MELTS simulations of the aplites yielded preliminary results indicating that aplite-forming melts equilibrated at pressures corresponding to depths of ~8–12 km.

Rhyolite-MELTS simulations constrain aplite melt extraction to ~200 MPa, with an estimated uncertainty of ±20–30 MPa. Rhyolite-MELTS pressures also broadly align with independent depth estimates previously proposed for the TIC based on field and petrologic constraints. Moreover, the relatively narrow pressure range obtained from rhyolite-MELTS modeling represents an improvement relative to traditional mineral-based geobarometers, which commonly yield larger uncertainties in felsic systems like the TIC. Overall, these results support a model in which the TIC’s aplites represent extracted residual melts derived from shallow crustal magma reservoirs during the late stages of pluton assembly, and demonstrate the utility of rhyolite-MELTS as a complementary geobarometer for constraining magma storage and/or melt extraction pressures in silicic plutonic systems where mineral-specific barometers are limited or ambiguous.

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