EXHUMATION OF THE PINALEÑO-JACKSON MOUNTAIN METAMORPHIC CORE COMPLEX, SOUTHEASTERN ARIZONA: CONSTRAINTS FROM MULTI-SYSTEM THERMOCHRONOLOGIC ANALYSIS

Metamorphic core complexes are found throughout the North American Cordillera and are responsible for accommodating extension following the Sevier and Laramide orogenies. Southern Arizona, in the southern U.S. Cordillera contains a series of SE–NW trending core complexes where regional low-temperature thermochronologic studies have constrained footwall exhumation from ca. 30–15 Ma. The Pinaleño Mountains comprise part of the footwall of the Pinaleño-Jackson Mountain core complex and represents the easternmost core complex in North America. This study investigates the thermal history of the footwall of the Pinaleño-Jackson Mountain core complex using multiple techniques including zircon U-Pb geochronology, biotite and muscovite ⁴⁰Ar/³⁹Ar thermochronology, zircon and apatite (U-Th)/He thermochronology, and apatite fission-track thermochronology. We present thermochronologic data from five igneous rocks ranging from mafic to felsic (46–73 wt. % SiO₂) in composition with zircon U-Pb crystallization ages ranging from 1.4 Ga to 55 Ma. One muscovite incremental heating experiment from these rocks yields an ⁴⁰Ar/³⁹Ar inverse isochron age of 24.8 ± 1.1 Ma, whereas two biotite experiments yield an ⁴⁰Ar/³⁹Ar inverse isochron age of 22.4 ± 0.5 Ma and a diffusion spectrum from 22.5 to 19 Ma. Zircons display a range of eU values from 67–11,500 ppm with positive (U-Th)/He date-eU correlations and yield (U-Th)/He dates of 20.6 ± 1.3 Ma to 13.3 ± 0.7 Ma. Apatites display a range of eU values from 3–57 ppm with positive (U-Th)/He date-eU correlations and yield (U-Th)/He dates of 22.5 ± 3.8 Ma to 6.4 ± 2.6 Ma. Fission-track analyses of apatite yield dates of 25.1 ± 2.9 Ma to 18.1 ± 1.5 Ma. These data are consistent with footwall exhumation from ca. 29–18 Ma and cooling rates of ~50 °C/Myr during this interval. These data, in combination with previous studies from the region show that core complex exhumation and associated extensional deformation was diachronous across southern Arizona, generally younging toward the west.