GEOCHEMISTRY OF METACHERT, SILICEOUS MARBLE, AND (NEWLY RECOGNIZED) META-UMBER IN THE PELONA-OROCOPIA-RAND SCHIST LOW-ANGLE SUBDUCTION COMPLEX, SOUTHERN CALIFORNIA AND SOUTHWEST ARIZONA

Piedmontite- or spessartine-bearing quartzite interlayered with the predominant metasandstone of the Late Cretaceous to early Paleogene Pelona-Orocopia-Rand Schist (PORS) low-angle subduction complex has been understood as ferromanganiferous metachert for at least 60 years. Subsequent recognition that the subordinate Fe-Mn component of the chert owes to submarine hydrothermal activity predicts the PORS should also contain metasedimentary rocks rich in Fe and Mn. We have now found such rocks, with Fe₂O₃* + MnO = 25–57 %, in three ranges: Sierra Pelona (CA), Orocopia Mtns (CA), and Cemetery Ridge (AZ). We interpret these rocks as meta-umber, derived from Fe-Mn-rich mudstone analogous to that of the Mediterranean region. Discovery of meta-umber, and new geochemical data, clarify the paleoenvironmental significance of metachert and associated rocks in the PORS.

Metachert, calcareous metachert, and associated siliceous marble (collectively MCSM) comprise three significant components: biogenic, detrital, and hydrothermal. The dominant siliceous to calcareous biogenic component (SiO₂ + CaCO₃ = 84–99 %; n = 35) undoubtedly derives from radiolarian and coccolith or foraminiferal oozes; Si may also have been added during diagenesis. The detrital component (typically 5–30 %) is continental, consistent with the observation that metachert grades into the enclosing continentally sourced metasandstone. The highly variable hydrothermal component reaches a maximum in Fe-Mn metachert, where Mn is enriched by factors 140–360 over the detrital background. MCSM with widely varying proportions of these three components are closely interlayered.

The MCSM present a paradox. Several characteristics suggest deposition in a continental-margin environment of nutrient upwelling and high organic productivity, above the CCD; with continental detritus available but limited. However, pronounced negative Ce anomalies (shale-normalized Ce/Ce* ≈ 0.24) in detrital-poor MCSM indicate incorporation of REE from seawater, a feature more typical of pelagic chert. And, the hydrothermal component and association with meta-umber imply proximity to submarine volcanism. We infer a tectonically and volcanically active environment, along a continental margin but open to the ocean; possibly a trench or trench-slope setting.