QUANTITATIVE DETERMINATION OF PEAK METAMORPHIC TEMPERATURES IN A PALEOSUBDUCTION ZONE FROM RAMAN SPECTROSCOPY OF CARBONACEOUS MATERIAL

The Pelona-Orocopia-Rand (POR) schists and the schist of Sierra de Salinas are correlated units in southern California and southwest Arizona thought to represent accretionary wedge material underplated during Laramide-age subduction. Petrologic constraints suggest that metamorphic grade in these schist units decrease structurally downward. This inverted metamorphic gradient is interpreted to either be the result of shear heating generated along the subduction interface, or conductive heating from the subduction zone hangingwall during progressive underplating of successive slabs. Quantitative temperature determinations from a range of structural depths would help to distinguish between these two models.

Raman spectroscopy of carbonaceous material (RSCM) allows the precise determination of peak metamorphic temperatures of rocks containing primary carbon. When carbonaceous material in sedimentary rocks is metamorphosed, it experiences graphitization, and becomes progressively more ordered. The degree of the organization is correlated to temperature. As this process is irreversible, it records the peak metamorphic temperature, regardless of the final state. The pelitic schists of the POR and Sierra de Salinas have compositions ideal for RSCM. Preliminary samples from structurally high schists of the Orocopia Mountains yield a temperature of 508 +/- 2 °C using this method, consistent with thermobarometric estimates for the schist of Sierra de Salinas and Sierra Pelona. This close correlation suggests that the RSCM method is a robust technique constraining temperatures in areas where metamorphic history and mineral assemblages are less suitable for standard thermobarometric methods.

Continued sampling in the Orocopia Mountains may confirm the presence and determine the magnitude of an inverted metamorphic gradient, and contribute significantly to our understanding of subduction processes along the Late Mesozoic western margin of North America.