Fluid-rock Interaction within the Picacho Mountains Detachment Shear Zone.

摘要

Metamorphic core complexes of southeastern Arizona extend in a southeastern trending belt that includes the Picacho, Tortolita, Santa Catalina, and Rincon Mountains. The Picacho Mountains form a solitary north-south trending belt located between Tucson and Phoenix. The Picacho Mountains are composed of a variety of Tertiary, Cretaceous, or Proterozoic granitoids, gneisses, and schists that have been subjected to low-angle normal faulting, mylonitization, and exhumation associated with Tertiary formation of a metamorphic core complex. The Picacho Mountains form the footwall of the metamorphic core complex, which is separated from the hanging wall by a gently south- to southwest-dipping detachment shear zone. The hanging wall, composed of basaltic and andesite volcanic rocks, is best exposed in the scenic Picacho Peak in Picacho Peak State Park. The detachment shear zone is divided into three plates that record a progressive gradient of deformation, from undeformed granite to (ultra)mylonites and breccia.;I present stable isotope results from mylonite samples collected across the lower plate of the detachment shear zone. Oxygen stable isotope (delta 18O) 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 stable isotope values (delta2H) of biotite and hornblende range from -99‰ to -76‰ and -93‰ to -72‰, respectively. These results suggest that, at equilibrium temperatures, the rock interacted with a fluid with delta2H values ranging from -37‰ to -56‰, consistent with a fluid of metamorphic/volcanic origin. These results are significantly different than other delta2H values reported for other 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.