U-PB ZIRCON GEOCHRONOLOGY OF THE SOUTH MOUNTAINS METAMORPHIC CORE COMPLEX, CENTRAL ARIZONA

The South Mountains metamorphic core complex preserves a record of Cenozoic extension and syn- to post-deformational magmatism allowing determination of time scales of magmatism and extension related to Miocene core complex development. The South Mountains footwall comprises two variably deformed plutons, termed the South Mountains granodiorite (SMg) and the Telegraph Pass granite (TPg). Field observations indicate mutually cross-cutting relationships suggesting a consanguineous emplacement. Extensional fabrics in the footwall involve crystal-plastic deformation of quartz and brittle fracturing of feldspars. The footwall was intruded by syn- to post-kinematic felsic to mafic dike swarms. Most plutonic rocks studied display evidence for extensive alteration marked by chlorite and white mica growth. The degree of alteration in general correlates with proximity to dike contacts. Greenschist-facies chloritic breccias at the detachment fault surface are cross-cut by mafic dikes suggesting magmatism continued after brittle faulting.

Here, we report 125 ²⁰⁶Pb/²³⁸U zircon LA-ICPMS dates from three SMg and TPg samples and three syn- to post-kinematic dikes to bracket the timing of mylonitic deformation and detachment faulting. Footwall samples were collected from the core complex center within 500 m from dike-pluton contacts. SMg and TPg zircons are typically small (<100 µm in length) and display core and rim textures in CL. ²⁰⁶Pb/²³⁸U core and rim dates range from ~19-23 Ma and are complex, inhibiting a simple explanation of magmatic overgrowths on xenocrystic cores. We unmix age components and consistently identify two populations in each pluton sample. Oldest populations are 21.60 ± 0.36 (SMg), 21.75 ± 0.30 (TPg₁), and 21.58 ± 0.49 Ma (TPg₂), which we interpret as crystallization ages. Youngest populations are 19.84 ± 0.35 (SMg), 20.56 ± 0.37 (TPg₁), and 20.28 ± 0.23 Ma (TPg₂), and may indicate a post-crystallization Pb-loss event.

Dikes yielded similar ages of 20.67 ± 0.42, 20.31 ± 0.26, and 19.40 ± 0.27 Ma. Sample proximity and age similarity between dikes and younger populations suggest that the dikes may have provided requisite heat to alter plutonic zircons. These new data bracket deformation between ca. 21.6 and 19.4 Ma and suggest core complex development shortly followed magmatic pluton emplacement.