SIGNIFICANCE OF NEWLY DISCOVERED SUBDUCTION COMPLEX, INCLUDING PARTIALLY SERPENTINIZED MANTLE PERIDOTITE, SOUTHWEST ARIZONA

The Late Cretaceous to early Paleogene Pelona, Orocopia, and Rand Schists (PORS) of southern California and southwest Arizona are anomalous because they exhibit many characteristics of a subduction complex, yet occupy positions far inland from the continental margin. Prolonged debate has led to consensus that the PORS are tectonic correlatives of the Franciscan Complex, carried beneath southwest North America during low-angle, northeastward subduction. However, several important aspects of their origin remain unclear.

Among the three units, Orocopia Schist is exposed the farthest inland, in southwest Arizona as much as a few hundred kilometers from the present or former continental margin. In 2012 we found an additional, unexpected exposure of Orocopia Schist even farther inland—90 km west of the outskirts of greater Phoenix. This body of Orocopia Schist (at a place called Cemetery Ridge), is extraordinary because it includes numerous blocks of serpentinized mantle peridotite. These peridotite blocks, as much as 200–400 m long, are in large part converted to metasomatic rocks dominated by tremolite-actinolite. Locally, however, partially serpentinized, bastitic orthopyroxene-rich harzburgite and olivine orthopyroxenite are well preserved, with abundant relict orthopyroxene and some relict olivine.

The newly discovered Orocopia Schist at Cemetery Ridge is important for two independent reasons. (1) The peridotite blocks will provide insight into mechanisms of incorporation and transport in a sub-continental low-angle subduction zone. Only two observations presently constrain the origin and provenance of the CR peridotite—it includes rocks unusually rich in orthopyroxene, and is associated with metachert. The former observation suggests subarc or suprasubduction mantle; the latter suggests some period of exposure on the sea floor. We infer, tentatively, that the peridotite most likely came from the leading corner of the mantle wedge, along the early Paleogene continental margin. However, we are also considering at least three other possible sources. (2) Detrital-zircon U-Pb ages (in progress, to be reported at the meeting) and Ar-Ar cooling ages for the Orocopia Schist at Cemetery Ridge will enable us to resolve several ambiguities associated with age patterns already established for the PORS.