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

We welcome diverse submissions on the petrology, structure, geochemistry, and geophysics of subduction zones from all approaches. Example topics include initiation, evolution, volatile cycling, ophiolites, arc tempos, high-P metamorphism, mélanges, seismicity, and accretionary wedges.

Thursday, 23 April 2026
1:30 PM - 5:30 PM
LMH, Deli

Constraining Aplite Melt Extraction Pressures with rhyolite-MELTS

Paper number: 26-8
Program: Technical Sessions
Time: 3:50 PM – 4:10 PM

Authors

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.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3

doi: 10.1130/abs/2026CD-410858

© Copyright 2026 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.