EVIDENCE FROM A SHEARED TONALITE PLUTON IN THE EASTERN PENINSULAR RANGES MYLONITE ZONE (EPRMZ), CALIFORNIA FOR INFILTRATION OF HIGH-18O, HIGH D/H WATERS, PROBABLY DERIVED AT DEPTHS >20KM BY DEHYDRATION OF ALTERED OCEANIC CRUST

Holk et al. (1991) demonstrated infiltration of metamorphic water at depths of at least 15-20 km during early Tertiary thrust faulting of a 15X15 km tonalite pluton west of Borrego Springs, CA. New d18O and dD data confirm and amplify this conclusion. Whole-rock d18O values from undeformed tonalite are +11.2±0.4 (n=7), whereas d18O values of mylonitized tonalite are +12.3±0.3 (n=22). Thus, open-system water-rock exchange has increased the d18O of the sheared tonalite by about 1‰. Quartz, feldspar, and biotite d18O values in undeformed tonalite are +12.8±0.4 (n=7), +11.0±0.7 (n=7), and +7.7±0.7 (n=7), compared with +13.4±0.2 (n=9), +11.7±0.3 (n=10), and +9.2±0.3 (n=11) in mylonitized tonalite. The extremely uniform quartz-feldspar and quartz-biotite 18O-fractionations suggest equilibrium at 450-550°C for the mylonitized tonalite. Biotite and hornblende dD values from undeformed tonalite are –73±4 (n=13), compared with –52 to –68 (n=19) in mylonitic tonalite. Constant biotite and hornblende Fe/Mg (≈ 1.25; Anderson, 1983) from all the tonalities requires that the EPRMZ was infiltrated by large amounts of high-D/H water (dD ≈ –10 to –30 at 450-550°C) that was most likely derived from the dehydration of seawater-altered subducted oceanic crust. Quartz and feldspar d18O values from syndeformational tonalitic pegmatites are +13.5±0.2 (n=4) and +12.2±0.3 (n=6), while biotite and muscovite dD values are –64±1 (n=2) and –42 (n=1). The similarity of d18O and dD values between pegmatite and mylonitic tonalite suggest a genetic relationship between the pegmatitic magmas and these deep-seated fluids. Higher whole-rock d18O values (+13.0 to +16.1, n=3) and much lower dD values (–98 to –114, n=3) occur where the EPRMZ cuts amphibolite-facies metasediments. The much lower dD values of these finer-grained upper plate rocks strongly suggest D/H exchange with meteoric groundwaters during Pliocene detachment faulting related to the opening of the Gulf of California.