11 Future Voices in Geoscience- Undergraduate Research Posters

This session is for two-year college and four-year college and university students presenting research posters in any subdiscipline of geosciences. Posters and presentations will be judged for awards.

Monday, 18 May 2026
8:00 AM - 5:30 PM
Alvarado D/E

Polymetamorphism of the Orocopia Schist at Cemetery Ridge, AZ

Paper number: 11-15
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Orocopia Schist is exposed at Cemetery Ridge in southwestern Arizona over an area of 5 km2, Orocopia Schist comprises metamorphosed trench sediments that were transported approximately 300 km to its present location within a subduction channel during the Late Cretaceous. U-Pb zircon dating of detrital grains show population peaks in the Proterozoic, Jurassic, Cretaceous, and Paleogene, and collectively bracket subduction metamorphism to be younger than 65 Ma. The exposure of Orocopia Schist at Cemetery Ridge is surrounded by intrusive granitic rocks that record U-Pb ages of 22-20 Ma. 

 

We collected data on four garnet-bearing samples of quartzofeldspathic schist (CR-370A, CR-235B, E and H). 17 monazite grains were analyzed using the split-stream laser ablation (LASS) system at the University of California, Santa Barbara, to simultaneously collect trace element abundances and isotopes for U-Pb dating. Pb207-corrected ages ranged from 22-26 Ma (±1) (n=63). When plotted on concordia, data from two samples showed concordance while two were discordant but yielded a lower intercept age. Ages calculated as Concordia or lower intercept methods fall within the Pb207-corrected range reported. Rare earth element (REE) patterns are similar in all samples and enriched compared to chondrite, displaying light REE enrichment and heavy REE depletion, with a pronounced negative Eu anomaly. There is a temporal increase in HREE and a decrease in LREE within the 22-26 Ma age range. Isochemical plots were generated for CR370A and CR235H. Garnet cores are high in Ca, and compositional isopleths converge at 520°C and 6 kbar, consistent with subduction zone metamorphism. Garnet in CR-235H rims preserve hotter and shallower conditions of 600°C and 5.5kbar. Discontinuities in Ca at the margins of fragments in CR235H are interpreted to have been initially sharp but are noticeably diffused, indicating temperatures >700°C were attained.  

The data are interpreted to indicate that Orocopia Schist at Cemetery Ridge has a shared history of subduction zone metamorphism with an exposure closer to the trench in the southeastern Chocolate Mountains, California, where a more complete PT path is preserved, reaching a pressure of 12.5 kb. The rocks at Cemetery Ridge underwent pre-Miocene decompression to 5.5 kb then heated to >700 °C. The second metamorphism is interpreted to be contact metamorphism due to the similarity in timing with the surrounding granitic intrusions.    

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410647

© 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.