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

A two-stage fluid evolution of Orocopia Schist is documented at the Orocopia Mountains and Gavilan Hills with stable isotopes and ⁴⁰Ar/³⁹Ar thermchronology (Holk et al., International Geology Review, v. 59, p. 639-663). Mineral ¹⁸O/¹⁶O fractionations from the Gavilan Hills record fluid conditions at the peak of metamorphism (490–580°C) during early Cenozoic accretion to the base of the continental crust in a nearly flat subduction system. Prograde subduction-related fluids most likely had δD between –30‰ to –50‰ and δ¹⁸O between +7‰ and +11‰. We found no compelling evidence that fluids related to prograde metamorphism significantly infiltrated the overlying crust, circulated through deep shear zones, or drove anatexis at the inboard position of the shallow subduction system represented by the Orocopia Schist at the Orocopia Mountains and Gavilan Hills. These prograde fluids most likely migrated into the upper plate trenchward of the current position of the Orocopia Schist. No δD evidence exists for downward infiltration of seawater along extensional faults that is predicted to accompany early Cenozoic detachment faulting.

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.