GEOLOGIC MAP OF PERIDOTITE AND ASSOCIATED METASOMATIC ROCKS IN THE OROCOPIA SCHIST LOW-ANGLE SUBDUCTION CHANNEL AT CEMETERY RIDGE, SOUTHWEST ARIZONA

The Late Cretaceous Pelona-Orocopia-Rand Schist low-angle subduction complex of southern California and southwest Arizona contains sparse serpentinite bodies, and common inclusions of Cr- and Ni-rich, peridotite-derived actinolitite. However, well-preserved peridotite is seen only at Cemetery Ridge, southwest AZ (120 km west of Phoenix), where it forms tectonic blocks within the Orocopia Schist. We have prepared a detailed (1:2000) geologic map of four of the largest (~50–300 m) and most informative peridotite bodies, exposed in a pediment area on the southwest flank of northern Cemetery Ridge. The map shows three major units: (1) Garnet-biotite quartzofeldspathic schist of the Orocopia Schist. (2) Peridotite and subordinate pyroxenite (collectively, peridotite), comprising variably serpentinized harzburgite and olivine orthopyroxenite, and serpentinized dunite. Delicate isotropic serpentinization textures are widely preserved, indicating the peridotite is internally undeformed. (3) Metasomatic rocks that envelop the peridotite, and were produced by intensive mechanical and fluid-mediated interaction of peridotite and enclosing quartzofeldspathic schist.

The principal metasomatic rock is porphyroblastic albite-actinolite gneiss, called spotted gneiss (SG). SG is derived from and grades out of schist. Our map depicts three intergradational zones, inward from schist toward peridotite: schist with minor SG or incipient alteration toward SG; mixed schist and spotted gneiss, in subequal proportions; SG with subordinate relict schist. SG includes abundant veins and pods of coarse-grained actinolitite, concentrated along the margins of peridotite. Ductile, disharmonic folds are common.

Foliation in schist and SG away from peridotite is consistent in orientation, but foliation in SG and schist near peridotite is highly variable, with pronounced deviation from outcrop to outcrop. We infer that this pattern records a change from uniform to nonuniform flow as the schist passed around the obstacle presented by the peridotite. The SG evidently formed as a boundary layer that accommodated differential flow between subducting schist and entrained but undeformed bodies of peridotite. Margins of peridotite bodies were mechanically and chemically eroded, to make actinolitite and SG.