TECTONIC EROSION OF THE HANGING WALL OF THE LARAMIDE LOW-ANGLE SUBDUCTION COMPLEX, SOUTHWEST U.S.A.: ACTINOLITITE, METADIORITE, AND PERIDOTITE INCLUSIONS AND BLOCKS IN THE OROCOPIA SCHIST

Schists of the Late Cretaceous Pelona-Orocopia-Rand (POR) low-angle subduction complex contain four types of foreign inclusions or blocks: actinolitite, serpentinite, metadiorite, and peridotite. Actinolitite and serpentinite are widespread; but metadiorite is known only from the Orocopia Schist of southwest Arizona and the southeast corner of California, and peridotite only from the Orocopia Schist at Cemetery Ridge, southwest AZ. Serpentinite is generally too altered to yield much information about its protolith (or has not been studied). Metadiorite has magmatic-arc-like geochemistry (in contrast to the generally MORB-like metabasalt intercalated with the POR schists). One metadiorite body has a concordant U-Pb zircon age (Mukasa et al., 1984: GSA AwP) of 163 Ma, within the age range of the Coast Range ophiolite of western CA. The peridotite at Cemetery Ridge comprises serpentinized dunite, harzburgite, and olivine orthopyroxenite. Orthopyroxene compositions and fluid-mobile-element systematics evince formation and serpentinization in a mantle-wedge (suprasubduction) environment. This peridotite is exotic to the quintessentially continental region of southern AZ, and presumably came from western CA where peridotite is common.

Actinolitite in the POR schists has high concentrations of Cr and Ni (~1000–2000 μg/g), indicating ultramafic affinity or derivation; but the sources of such actinolitite were enigmatic until discovery of the peridotite at Cemetery Ridge. There, intensive mechanical and fluid-mediated interaction between peridotite and enclosing quartzofeldspathic schist has produced voluminous tremolite- or actinolite-bearing metasomatic rocks, including abundant veins and pods of actinolitite marginal to the peridotite. We infer that somewhere along their subduction paths all the POR schists must have come into contact with peridotite, from which they acquired their characteristic actinolitite inclusions.

These data and inferences suggest a scenario in which peridotite and diorite were locally eroded or plucked from the base of a Californian Coast Range ophiolite–like terrane in the upper plate of the POR low-angle subduction zone. Interaction of schist and entrained peridotite produced actinolitite, which was then dispersed through the subducting schists.