HYDROGEN ISOTOPE EVIDENCE FOR DETACHMENT FAULT-RELATED EXHUMATION OF THE OROCOPIA SCHIST, SOUTHEASTERN CALIFORNIA

Jacobson et al. (2002; 2007) report ⁴⁰Ar/³⁹Ar cooling ages that document a two-stage cooling and exhumation history for the Orocopia Schist at the Gavilan Hills and Orocopia Mountains in SE California. The first event began ~50 Ma and is related to exhumation and/or subduction refrigeration. The second event started 28-24 Ma and is related to syn-detachment fault chlorite alteration. We report hydrogen isotope results from the minerals that document this cooling history. Low biotite (–115 to –101‰, n = 5) and muscovite (–74‰) δD values from the upper-plate Mesozoic gneiss between the Chocolate Mountains detachment fault and the high-angle Gatuna normal fault at the Gavilan Hills record the infiltration of mid-latitude meteoric-hydrothermal water (δD ~–50‰) through these faults. Low δD values (δD_biotite = –110 to –90‰, n = 5; δD_muscovite = –83 to –64; n = 5) from the lower plate Orocopia Schist suggest these meteoric-hydrothermal fluids infiltrated into the lower plate during the Late Oligocene to Early Miocene unroofing. Our results from the Orocopia Mountains are similar with low biotite δD values (–123 to –106‰, n = 6) from leucogranite gneiss and anorthosite-syenite in the upper plate of the Orocopia Mountains Detachment Fault (OMDF). Muscovite from detachment fault-related mylonites have very low δD values (–107 to–90‰, n = 3); suggesting the involvement of meteoric-hydrothermal fluids during the first cooling event (43.9 - 49.3 Ma), but these values are also in equilibrium with metamorphic water (δD ~–70‰) at ~450°C. δD values from amphibole/biotite (–114 and –112‰) and muscovite (–90, –87, and –65‰) from chlorite zone schists just beneath the OMDF suggest a similar fluid history as the upper plate. The infiltration of variable amounts of meteoric-hydrothermal water through lower plate biotite zone schists is suggested by biotite (–115 to –95‰, n = 4) and muscovite (–105 to –78‰, n = 4) δD values. Higher biotite (–87 to –73‰, n = 3) and muscovite (–59 and –72‰) δD values from the Sierra de Salinas and the Rand Schist indicate a different fluid history. The lack of correlation between δD values and ⁴⁰Ar/³⁹Ar cooling ages indicates that meteoric fluid infiltration through the Orocopia Schist and associated detachment faults occurred during the Late Oligocene-to-Early Miocene final unroofing of the metamorphic complex.