244 Subduction Zone Processes: Insights from Geology, Geochemistry, and Petrochronology

We invite contributions that explore the geologic processes of subduction zones from new field observations, analytical methodologies, and numerical modeling of metamorphic rocks along the subduction interface or try to integrate or reconcile the rocks record with geophysical observations.

Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 217C

Pressure-temperature path from the Laramide shallow-angle subduction channel determined from Orocopia Schist, Chocolate Mountains, southeastern California, and implications for evolution of the subduction system

Paper number: 244-10
Program: Technical Sessions
Time: 10:35 AM – 10:50 AM

Authors

Abstract

Orocopia Schist is hypothesized to be trench sediments subducted and transported within a Late Cretaceous to Paleogene low-angle subduction system along the southwest margin of North America.  Exposures in southeastern California and western Arizona document at least 300 km of transport from the trench.  The conditions recorded provide constraints on the evolution of the low-angle subduction system. We analyzed minerals in garnet-bearing quartzofeldspathic schist from the southeastern Chocolate Mountains to determine to a pressure-temperature (P-T) path.  Analyzed samples possess the assemblage plagioclase + biotite + quartz + muscovite +  clinozoisite + garnet.  Variably present accessory phases include rutile, titanite, allanite, zircon, and ilmenite. Garnet in three analyzed samples is bimodally zoned, with high-Ca rims (Xgr =0.35–0.38) and intermediate-Ca (Xgr=0.22–0.25) cores.  The interface between the zones is an idioblastic surface, suggesting continuous growth without resorption. Compositional isopleths of garnet cores and rims display convergence on isochemical plots calculated using Perple_X. The interface is interpreted to have developed when the trajectory of the P-T path paralleled a garnet mode isopleth, thus not shrinking nor growing as conditions changed.  The core and rim are interpreted to have grown when the trajectory of the path paralleled an Xgr isopleth. This yielded a zigzag 3-segment path starting at 550°C and 0.72 GPa and ending at 585°C at 1.2 GPa. Preliminary quartz-in-garnet elastic barometry indicates garnet core growth at 0.4–0.7 GPa and temperatures of 400–620°C determined by Ti-in-quartz and Zr-in-rutile thermometry applied to inclusions in garnet.  Applying the temperature determined from the garnet core isopleth intersections (550°C) to quartz-in-garnet isomekes from garnet cores yields pressures of 0.6–0.9 GPa, broadly consistent with conditions determined from isochemical plots. The timing of metamorphism is constrained from published data to be younger than 70 Ma (youngest detrital zircon age) and older than 58 Ma (oldest hornblende Ar-Ar cooling age). Numerical time-dependent modeling suggests these conditions were attained early in the low-angle subduction history, there was a significant heat contribution from frictional and/or viscous heating, and the geometry likely resembled that of the modern Guerrero flat-slab system along the Pacific margin of southwest Mexico, where the near-trench portion dips moderately then flattens.  We infer that the rocks were metamorphosed within the moderately-dipping near-trench portion of the system.

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

doi: 10.1130/abs/2025AM-413867

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