A TWO-STAGE FLUID HISTORY FOR THE OROCOPIA SCHIST AND ASSOCIATED ROCKS RELATED TO FLAT SUBDUCTION AND EXHUMATION, SOUTHEASTERN CALIFORNIA: A COLD AND DRY METAMORPHIC CORE COMPLEX
Mineral 18O/16O disequilibrium recorded in hydrothermally altered mylonites combined with lower plate schists with δD values as low as –120‰ in the Orocopia Mountains provide evidence for a second stage of fluid–rock interaction related to schist exhumation during the late Oligocene–early Miocene. A moderate temperature contrast across the Orocopia Mountains Detachment Fault drove circulation of meteoric-hydrothermal fluids along the fault at this time. The general absence of large negative shifts in the δ18O values of easily exchanged minerals (e.g. plagioclase) indicates that low water/rock ratios characterize this hydrothermal system. As such, the scale of fluid circulation associated with tectonic exhumation of the Orocopia Schist was much smaller than has been previously described for detachment faulting in large core complexes in the continental interior of the western United States and Canada (e.g., Shuswap Complex), places with hotter lower plates during extension. In conclusion, exposures of Orocopia Schist in southeastern California represent a cold and dry metamorphic core complex, one for which metamorphism predated exhumation along detachment faults by ~25 my.