Abstract

The Pinal terrane of southern Arizona is composed mostly of quartz-sericite schists, meta-wackes, metavolcanic rocks, and granitoid and gneissic rocks, with minor meta-chert, meta-conglomerate, and carbonate rocks. Various tectonic settings have been suggested for the origin of this terrane. The existence of tectonic mélanges within the terrane, the presence of granitic rocks in close proximity to the mélanges, the presence of both oceanic and arc-like metavolcanic rocks within the terrane, the age distributions of igneous and meta-igneous rocks within the terrane, the deformational patterns within the terrane, and the width of the terrane all support a model in which the terrane was formed as a forearc-subduction complex that experienced a ridge subduction event at ca. 1.65 Ga. The Pinal terrane was likely the forearc-subduction complex to the Mazatzal arc terrane, which lies immediately to the north. Regional variations in the mineralogy of metavolcanic and metasedimentary rocks exposed within the terrane together with structural data suggest the terrane may have experienced an arc obduction event in the Proterozoic after the ridge subduction event. If it did, most of this arc terrane was eroded off the Pinal terrane in the 300 m.y. following the obduction event and prior to deposition of the Apache Group at ca. 1.32 Ga. There is a Paleoproterozoic arc terrane exposed in northern Mexico that may represent remnants of an obducted arc terrane. High-pressure–high-temperature metamorphic rocks (e.g., blueschists) have not been identified within the Pinal terrane.

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