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Abstract
Orocopia Schist in the northern Plomosa Mountains, west-central Arizona: A Laramide subduction complex exhumed in a Miocene metamorphic core complex
Highlights
Age of Intrusion and Mylonitization Here we report U-Pb zircon isotopic ages for the Northern Plomosa intrusive complex (NPIC) determined at the Laser Ablation ICPMS Laboratory at the University of Texas at Austin (USA) (Fig. 15; Table 2; refer to the Data Repository for raw ages and detailed methods)
Based on field observations and geochemical evidence, we conclude that the 10 km2 exposure of quartzofeldspathic schist in the northern Plomosa Mountains is the Orocopia Schist
This study presents the first documentation of Orocopia Schist exhumed within a Miocene metamorphic core complex, with Miocene mylonitization through the entire structural thickness of the schist
Summary
The Pelona-Orocopia-Rand Schists (PORS) of southern California and southwestern Arizona (USA) (Fig. 1) are interpreted as Late Cretaceous to early Paleocene metamorphosed trench sediments and other minor rock types (e.g., Haxel and Dillon, 1978; Jacobson et al, 1988, 2000) subducted during the Laramide orogeny and accreted beneath the lower continental crust during slab flattening of a segment of the Farallon plate (Grove et al, 2003; Saleeby, 2003; Jacobson et al, 2011). Field and Petrographic Description The Orocopia Schist comprises ~10 km of the exposed footwall of the Plomosa detachment fault and most commonly crops out as gray flaggy layers with well-developed centimeter-scale layering, or is homogenous with a purple-red color, and commonly has a well-developed S-C′ mylonitic fabric It is composed dominantly of quartz (26%–50% modal abundance, used throughout) and feldspar (24%–50%), with biotite (8%– 34%, variably chloritized) and locally muscovite (2%–20%, generally ~10%), minor opaque minerals (≤4%), and accessory apatite (≤1%), rutile, zircon, and local garnet (≤0.5% where present) (Supplementary Table 1 in the GSA Data Repository). Garnets have been resorbed (Fig. 4E) and altered to plagioclase + biotite + epidote + hornblende(?) This unit is similar to amphibolite layers observed in Orocopia Schist of other localities, which are generally interpreted as metamorphosed basalt, consistent with geochemical analysis of three samples from the northern Plomosa Mountains (see section Comparative Geochemistry of Orocopia Metabasalt, Actinolitite, and Metachert). Previous and ongoing studies reveal that PORS meta chert (and associated siliceous marble, here not considered separately) comprises three principal components: dominant biogenic and subordinate
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