34 Plumes, Ophiolites and Oceanic Crust: Honoring the Scientific Contributions of John W. Shervais (Posters)

Friday, 4 April 2025
8:00 AM - 4:00 PM
Sierra Ballroom

PGE ABUNDANCE AND Re-Os ISOTOPIC SYSTEMATICS OF COAST RANGE OPHIOLITE PERIDOTITES AND CHROMITITE, AND INSIGHTS INTO FORE-ARC MAGMATIC PROCESSES

Paper number: 34-4
Program: Theme Sessions
Time: 8:00 AM – 4:00 PM

Authors

Abstract

We report platinum-group element (PGE: Os, Ir, Ru, Pt, and Pd) and Re concentrations, and Re-Os isotopic data for peridotites and podiform chromitite from the mid-Jurassic Coast Range ophiolite (CRO), California. Our aim is to provide insights into the formation and evolution of the CRO in a forearc tectonic setting. The CRO peridotites are divided into two groups: abyssal and supra-subduction zone (SSZ). They have IPGE (Os, Ir, Ru) concentrations similar to estimates for the primitive mantle, and nearly chondritic relative abundances [(Os/Ir)N ≈1.1]. Abyssal-type peridotites have slightly sub-chondritic PPGE (Pt and Pd)-Re abundances and flat chondrite-normalized patterns, whereas the SSZ-type ones are depleted overall with highly fractionated PPGE-Re patterns. The CRO peridotites have 187Os/188Os values of 0.1188 to 0.1315 (γOs = -8.3 to +1.4), and 187Re/188Os ranging from 0.022 to 0.413. The oxygen fugacity based on the V/Yb ratios of the CRO peridotites is equivalent to the FMQ buffer. The abyssal-type peridotites are residues after ≤5% melting of the primitive upper mantle, and represent a remnant of oceanic lithosphere trapped in an SSZ setting but before being re-melted or modified by subduction processes. The abyssal-type peridotites yield an aluminachron model age of ~1.5 Ga, implying that the CRO mantle had experienced episode(s) of melt extraction before the CRO crust was formed that may correspond to rifting of the Columbia supercontinent. The SSZ-type peridotites are refractory residues after ~5 to 15% melting. Extraction of fore-arc basalts generated mainly by decompression melting resulted in the SSZ-type peridotites studied. The chromitite analyzed has 187Os/188Os value of 0.1250 (γOs = -3.5), and PGE-Re patterns complementary to that of boninite, indicating a genetic link to fore-arc magmatism.

Geological Society of America Abstracts with Programs. Vol. 57, No. 4

doi: 10.1130/abs/2025CD-409424

© Copyright 2025 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.