ALPINE PERIDOTITE IN THE ARIZONA DESERT: NEW DISCOVERY OF OROCOPIA SCHIST AND INCLUDED SERPENTINIZED PERIDOTITE IN SOUTHWEST ARIZONA

The latest Cretaceous to early Tertiary Orocopia Schist (OS) records subduction of continental-margin supracrustal rocks beneath southwest North America. Heretofore, all known exposures of OS lay along the Chocolate Mountains anticlinorium. This structure extends from the Orocopia Mountains (southern California) southeastward to Neversweat Ridge, 65 km east of the Colorado River in southwest Arizona. In 2012 we found an additional body of OS significantly farther inland—at Cemetery Ridge (CR), 90 km east of the Colorado River and 90 km west of the outskirts of greater Phoenix.

The quartzofeldspathic schist at CR possesses four features diagnostic of OS: porphyroblasts of bluish-gray to black graphitic albite, layers of Fe-Mn metachert and amphibolite schist (metabasalt), and pods of coarse-grained actinolite rock.

OS at CR is remarkable because it includes at least 17 blocks of serpentinized mantle peridotite, 200–400 m to < 30 m long. These blocks, aligned in a diffuse trend ≈ 2 km long, may be dispersed fragments of a single peridotite slab. Field relations indicate that the peridotite at CR was serpentinized but otherwise unmetamorphosed when it was emplaced into the sedimentary protolith of the OS, and was subsequently partially metamorphosed with the schist. Premetamorphic textures and fabrics are preserved in several of the blocks. Here the peridotites are serpentinized harzburgite and olivine orthopyroxenite, commonly with bastite texture and locally with probable mantle tectonite fabric; and subordinate black serpentinized dunite, typically forming dikes cutting harzburgite. We’ve found one small mass of probable chromite-serpentine rock. As two of the peridotite bodies are closely associated with metachert, the peridotite incorporated into the OS is probably oceanic mantle, possibly detached from the subducting plate, rather than continental mantle.

Thick actinolite veins in and around the peridotite reveal the origin of the enigmatic actinolite rock that is ubiquitous in the Orocopia and related schists.

OS and alpine peridotite at CR provide the farthest-inland in situ evidence for low-angle subduction beneath the Southwest. Studies of detrital-zircon and metamorphic-mineral ages will relate this outlying exposure of OS to similar schists nearer to the continental margin to the west.