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

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.