23 Subduction Zones and Their Volcanic Arcs: Initiation and Evolution, Structure, Metamorphism, Magmatism (Posters)

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
8:00 AM - 5:30 PM
LMH, 5th Floor Chapel

From Ash to Insight: Pressure, Temperature, and the Secrets of Santa Catalina’s Garnet Quartzites

Paper number: 23-8
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Santa Catalina Island has long been recognized as a valuable natural laboratory for subduction processes. Within the complex’s amphibolite-facies mélange there are mafic garnet hornblendite blocks, as well as felsic garnet quartzite blocks, the latter being relatively less studied. Due to elevated oxygen isotope ratios, these quartzites are categorized as metacherts (Page, et. al. 2019). This study examines three of these garnet quartzite blocks in order to make P-T estimates. The samples are mainly composed of quartz, with varying amounts of garnet, chlorite, white mica, biotite, anthophyllite, hornblende, rutile, sphene, and apatite. Accessory minerals include monazite, allanite, zircon, and clinozoisite. Texturally, the garnets are organized as discrete bands through the quartzite matrix, but some samples appear to be more heterogeneous, sometimes even interlayered with the amphibolite. 

This study used optical and scanning electron microscopy (SEM) to characterize and obtain bulk chemical compositions of these garnet quartzite blocks, with which Perple_X thermodynamic modeling was then carried out. Through this modeling we were able to obtain Equilibrium Assemblage Diagrams (EADs) that illustrated new pressure and temperature constraints. This, combined with past Zr-in-Rt measurements, allowed us to constrain upper and lower temperature limits. The EADs yielded a 525 - 670 C temperature range for all samples, while the Zr-in-Rut temperatures obtained by Hartley et al. (2016) establish a slightly higher range of 600 - 710 C at 15kbar. However, the pressure is more difficult to constrain due to the high variance assemblages. The EADs yielded a wide range of pressures, from 7-20 kbar, with some blocks showing better pressure constraints than others. These temperatures and pressures are consistent with thermobarometry of mafic blocks in the same mélange. 

Major element trends among samples suggest a protolith that is a mixture between metachert SiO2 and a single other mafic to intermediate volcanic source. Relict igneous zircon cores reveal a narrow age range for this protolith, 121-124 Ma (Page, et. al. 2019). The Santiago Peak Volcanic Group at the NW margin of the Peninsula Range Batholith contains a variety of volcanic rocks of the exact same age (Herzig and Kimbrough, 2014). Given Catalina’s position immediately adjacent to these units at the time of eruption, they make a tantalizing prospect for an ash source.

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

doi: 10.1130/abs/2026CD-411061

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