9 Tectonic and Magmatic Thickening of Mesozoic North American Cordillera

Thursday, 13 May 2021
8:30 AM - 12:30 PM
UNR Dept of Geological Sciences & Engineering Room

CRITERIA FOR RECOGNITION OF SUBDUCTED (OROCOPIA) SCHIST IN WESTERN ARIZONA

Paper number: 9-12
Program: Theme Sessions
Time: 11:15 AM – 11:20 AM

Authors

Abstract

The Orocopia Schist (OS) is a latest Cretaceous low-angle subduction channel. Its principal locus, the Chocolate Mountains anticlinorium extending from the Orocopia Mountains (SE California) east to Neversweat Ridge (SW Arizona), has been known since the mid 1970s. Recently, two more exposures of the oceanic OS have been found at isolated localities farther inland in SW Arizona: Cemetery Ridge (2012) and northern Plomosa Mountains (2016). These discoveries raise the possibility that additional inboard areas of OS have yet to be found. Hence a review of criteria for recognition of OS is warranted.

Positive criteria: P1—Homogeneous quartzofeldspathic schist (metasandstone) with planar schistosity parallel to transposed bedding. P2—Porphyroblasts of bluish-gray to black graphitic plagioclase. P3—Metamorphic biotite ± accessory garnet. P4—Sparse layers of morb-like metabasalt. P5—Sparse layers of Fe-Mn metachert and siliceous marble, with negative Ce anomalies inherited from seawater. P6—Pods of coarsely bladed, high-Cr and -Ni actinolitite. P7—Pods or blocks of serpentinite, calc-serpentinite, or peridotite (rare). P8—Detrital-zircon age spectra with Paleoproterozoic, Mesoproterozoic, Jurassic, and Cretaceous peaks.

Negative criteria: N1—Sedimentary or volcanic features visible in hand specimen: sand grains, pebbles, quartz or feldspar phenocrysts, lithic fragments, pumice lapilli, or eutaxitic foliation. N2—Pelitic rocks. N3—Quartzite derived from quartz arenite. N4—Banded iron formation or associated metachert. N5—Mesozoic igneous intrusions.

Regional or local units that fail several or most of these criteria include: (1) Proterozoic metasedimentary rocks in the Harcuvar-Buckskin core complex, Bouse Hills, and hills west of Tonopah. (2) Metamorphosed Paleozoic and Triassic strata. (3) Metamorphosed Jurassic and Cretaceous sedimentary, volcanic, volcaniclastic, and hypabyssal rocks widespread in southwest Arizona. (4) Jurassic metarhyolite in a Miocene rock-avalanche breccia southeast of the McCracken Mountains (Elliott and Corones 2019). This so-called “Alamo schist” fails all of P1 to P8. Equally important, it decisively fails N1.

We hope these criteria can further the search for more exposures of subducted schist in western Arizona by averting more false positives.

Geological Society of America Abstracts with Programs. Vol. 53, No. 4

doi: 10.1130/abs/2021CD-362443

© Copyright 2021 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.