STABLE ISOTOPE EVIDENCE FOR A TWO-STAGE FLUID HISTORY THE OROCOPIA SCHIST AT THE OROCOPIA MOUNTAINS AND GAVILAN HILLS, SOUTHEASTERN CALIFORNIA

A two-stage fluid history of the Orocopia Schist at the Gavilan Hills and Orocopia Mountains in SE California is documented with mineral oxygen and hydrogen isotope data and ⁴⁰Ar/³⁹Ar geochronology (Jacobson et al., 2002; 2007). The first event is related to prograde metamorphism followed by subduction refrigeration at ~50 Ma. This event is evidenced by quartz, muscovite, amphibole, and biotite δ¹⁸O values from Orocopia Schist in the lower plate of the Orocopia/Chocolate Mountains detachment fault(s) and Gatuna normal fault in apparent equilibrium at 450-550°C with metamorphic water (δ¹⁸O ~ +8.5). A few high D/H muscovites and biotites from the Orocopia Schist suggest these metamorphic fluids had δD ~ –40. The second event is associated with final exhumation, syn-detachment fault chlorite alteration and the influx of meteoric-hydrothermal fluids at 28-24 Ma. This event is evidenced by low biotite (–90 to –122‰), amphibole (–100 to –119‰), and muscovite (–75 to –107‰) dD values from almost all structural levels at the Orocopia Mountains and Gavilan Hills that indicate D/H exchange involving meteoric fluids with δD values between –60 and –80‰. Muscovite from detachment fault-related mylonites have the lowest δD values, suggesting that these faults were an enhanced zone of meteoric fluid circulation. More variable δD values in lower plate Orocopia Schist from both the chlorite and biotite zones indicate a lesser degree of interaction with meteoric-hydrothermal fluids. The preservation of high-temperature oxygen isotope fractionations (T ~ 500°C) in upper plate rocks indicates low water-rock ratios for the second event. Low biotite δ¹⁸O values (+3.4 to +4.5) in the upper plate at the Orocopia Mountains are out of isotopic equilibrium with coexisting quartz (δ¹⁸O ~ +10‰). This indicates a more vigorous meteoric-hydrothermal system at the Orocopia Mountains Fault relative to that related to the Chocolate Mountains/Gatuna fault system at Gavilan Hills.