Subduction-margin Assemblages in the Proterozoic Pinal and Cochise blocks, southeastern Arizona

Abstract

Protoliths of Paleoproterozoic rocks broadly referred to as “Pinal Schist” in the Johnny Lyon Hills and the Dragoon and Little Dragoon Mountains of Cochise County, Arizona, include intrusive and subaqueous extrusive mafic rocks, turbiditic clastic rocks, and highly deformed sedimentary rocks visually similar to Phanerozoic mélanges. The combination of these lithologies and their present spatial distribution—clastic rocks to the northwest separated by a region of mélange from mafic igneous rocks to the southeast—is suggestive of an assemblage formed at a subduction-type continental margin. Protoliths of the mafic rock sequence, about 700 m thick, consist upward from its base of composite diabase dikes, pillow lavas, and pillow breccias, and pelitic sedimentary rocks; it may be an ophiolite fragment. The clastic rocks, though complexly deformed, form a largely coherent, mostly northwest-younging sequence. A fault-bounded deformed belt, present between the mafic and the clastic rocks, includes pre-metamorphic mélange and broken formation. The study area falls along or near previously suggested boundaries between lithotectonic assemblages within the Precambrian rocks of southeastern Arizona. Our results provide insight into the location and nature of the boundary and strongly suggest the presence of a suture zone juxtaposing predominantly metavolcanic rocks to the southeast against predominantly metasedimentary rocks to the northwest. Consistent with the terminology used by Eisele and Isachsen (this issue), we adopt the terms Cochise block and Pinal block for the regions to the southeast and northwest of the suture, respectively.