TIMING OF MID-CRUSTAL DEFORMATION WITH RESPECT TO MAGMATISM AND DETACHMENT FAULTING IN THE SOUTH MOUNTAINS METAMORPHIC CORE COMPLEX, ARIZONA, USA

The South Mountains metamorphic core complex (SMCC) contains a record of Cenozoic extension and gravitational collapse that is fundamental to the post-orogenic crustal evolution of western North America. The SMCC comprises two variably deformed and mutually intrusive Miocene plutons, termed the South Mountains granodiorite (SMg) and the Telegraph Pass granite (TPg), both of which are intruded by syn- to post- kinematic felsic to mafic dike swarms. Extensional fabrics in the footwall are defined by crystal-plastically deformed quartz and brittlely fractured feldspars. The SMCC is unique because fabric development in the Tertiary footwall rocks cannot be attributed to older (e.g., Cretaceous) deformation, thus presenting an opportunity to precisely determine timescales of mid-crustal deformation and mylonitic fabric development related to Miocene core complex faulting.

Two samples were collected from the SMg, and one from the TPg. One SMg sample (SMg₁) was collected from the center of the core complex, whereas the second sample (SMg₂) was collected ~1 km west of the first sample, away from any observed or mapped mafic dikes. The TPg sample was collected at the boundary between the SMg and TPg units, and is the least deformed footwall pluton sampled. The three samples yielded statistically indistinguishable ²⁰⁷Pb_c-corrected ²⁰⁶Pb/²³⁸U zircon SHRIMP-RG ages of 20.6 ± 0.3 Ma (SMg₁), 20.8 ± 0.2 Ma (SMg₂) and 20.1 ± 0.5 Ma (TPg). Ti-in-zircon thermometry gave emplacement temperatures of 720 ± 25°C and 710 ± 25°C for SMg₁ and SMg₂, and 740 ± 25°C for the TPg sample. Integrating our new data with a previous ²⁰⁶Pb/²³⁸U zircon LA-ICPMS age of 19.4 ± 0.3 Ma from an undeformed dike allows us to bracket the timing of mylonitic deformation between ca. 20.1 and 19.4 Ma. These new data suggest that both the SMg and TPg were emplaced contemporaneously, and crystal-plastic deformation occurred within <1 Ma of igneous crystallization. Calculated cooling rates from magmatic emplacement at 700°C to ~120°C yielded a time-integrated value of ~220°C/Ma. These data highlight the rapid tempo of mylonitic deformation and cooling related to Miocene core complex development.