74 How are Plutons Made? Physical and Chemical Records of Pluton Construction and Evolution (Posters)

This session integrates structural and petrologic views of plutonic systems, exploring the crustal response to pluton formation, and processes that control the physical and chemical characteristics of pluton-host rock systems from initial emplacement to solidification.

Sunday, 19 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

The Freeman Creek Pluton as a Window into Pre-eruptive Magma Storage and Eruptibility in the Elevenmile Canyon Caldera Complex, Nevada

Paper number: 74-3
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

During the Middle Cenozoic, the Great Basin experienced an extensive period of explosive silicic volcanism, which produced massive ash-flow tuff deposits created by ~250 eruptions. Subsequent tilting of crustal blocks due to Basin and Range extension revealed cross sections of these deposits, occasionally exposing plutonic levels. The Freeman Creek Pluton (FCP; 25.16 Ma) in the southern Stillwater Range is thought to be part of the subsurface magma storage region for the Tuff of Elevenmile Canyon (TEC; ≤5000km3, 25.12 Ma) due to similarities in age, zircon geochemistry, and whole rock compositions (Colgan et al., 2018). The exposure of the intrusive FCP linked to the TEC provides a unique opportunity to understand the subsurface conditions of a magma storage system associated with a major caldera-forming eruption. By comparing the erupted TEC to the unerupted FCP, we seek to understand why portions of the magma erupted explosively, while others did not erupt. By understanding this relationship in an ancient system, this work advances our knowledge of active magmatic systems and improves our ability to anticipate future eruptions worldwide.

We will present results of geologic mapping, whole rock and mineral geochemistry from the FCP. Field mapping and whole rock geochemistry indicate that the pluton represents 7-8 distinct granite to granodiorite intrusions. The pluton is primarily composed of a coarse-grained biotite-rich (~10%) granite (70.36-71.88 wt% SiO2) with variable amounts of orthoclase (~5%-45%), and a granodiorite intrusion (69.73 wt% SiO2). Several younger intrusions of finer grained leucogranites (76.03-77.36 wt% SiO2) crosscut other intrusions within the pluton.

We collected feldspar compositions from 8 representative FCP samples via EPMA. Plagioclase anorthite contents range from An13-73 in the biotite granite, An17-60 in granodiorite, and An9-65 in leucogranite, with the lowest values generally in the crystal cores or rims. Most plagioclase inner cores were heavily altered, while rims, separated by sharp boundaries were surprisingly fresh (3-8 wt.% CaO). This may indicate a two-stage crystallization and alteration history, where rims represent later magmatic overgrowths during final stages of pluton cooling. Perthitic textures were abundant in orthoclase grains, consistent with slow cooling. By establishing the internal complexity of the unerupted FCP, this work provides a baseline for future comparisons with the TEC and broader efforts to evaluate controls on eruptibility.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414088

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