FLUID-ROCK INTERACTION IN THE PICACHO PEAK DETACHMENT SHEAR ZONE (AZ)

The Picacho Mountains form the western part of an extensive Miocene metamorphic core complex, including the Tortolita, Catalina, and Rincon Mountains in southern Arizona. The study area of Picacho Peak is located on the south end of a north-south running mountain range, 40 miles NW of Tucson. The Picacho Peak detachment shear zone is divided into three levels: (1) a lower plate, consisting of undeformed to mylonitized quartz-feldspatic Oracle granite, (2) a middle plate is made of altered and fractured Oracle granite, (3) an upper plate entails Miocene allochtonous volcanic and sedimentary rocks. The three plates are separated with detachment zones associated with chloritic breccia. The rocks in this area have undergone middle Tertiary mylonitic deformation.

We present preliminary stable isotope (oxygen and hydrogen) results from mylonite samples collected across the lower plate of the detachment shear zone. Oxygen stable isotope analyses of biotite, hornblende and quartz have values that range from 4.4 ‰ to 5.7 ‰, 4.6 ‰ to 7.0 ‰, and 10.2 ‰ to 11.9 ‰, respectively. Geothermometry based on quartz – biotite and quartz – hornblende pairs yield temperatures for equilibrium ranging from 415 to 632°C, similar to previously published data. Hydrogen isotope values of biotite and hornblende from -99‰ to -76‰ and -93‰ to -72‰, respectively. These results suggest that, at temperature of equilibrium, the rock interacted with a fluid with δ²H values ranging from -35‰ to -51‰, consistent with a fluid of metamorphic/volcanic origin. These results are significantly different than other δ²H values reported for other detachment shear zones associated with North American metamorphic core complexes, suggesting that the Picacho shear zone had a lesser meteoric water component, or that the detachment shear zone was deeper than others.