THERMAL HISTORIES OF THE WILDERNESS SUITE GRANITOIDS FROM THE CATALINA METAMORPHIC CORE COMPLEX REVEALED BY K-FELDSPAR 40AR/39AR AGES AND MDD MODELS

The Wilderness leucocratic suite of granitoids intrudes the lower plate of the Catalina metamorphic core complex (MCC) of southern AZ. These variably deformed S-type granites were emplaced as a series of seven sills that have inflated the crust by ~4 km. Five K-feldspars from the Wilderness suite, including granites from the Wilderness and Catnip Canyon sills and a pegmatite that intrudes the Seven Falls sill, were analyzed by the ⁴⁰Ar/³⁹Ar step heat method in order to assess their thermal history. The Wilderness sill samples include a deformed granite, an undeformed granite and a pegmatite. The Catnip Canyon granite and the Seven Falls pegmatite are undeformed. All ⁴⁰Ar/³⁹Ar age spectra show variable gradients with apparent ages ranging from 24-30 Ma. The age gradients common to all the spectra can be interpreted to result from slow cooling or partial resetting due to thermal events. ⁴⁰Ar/³⁹Ar apparent ages for the pegmatite K-feldspars are similar at low temperature steps and range from 24-25 Ma, but the apparent ages differ at higher temperature steps. Similarly, the deformed and undeformed granites yield ⁴⁰Ar/³⁹Ar apparent ages, which range from 25-26 Ma for low temperature steps, however the deformed Wilderness granite yields apparent ages up to 30 Ma whereas the undeformed granites yield apparent ages from 27-29 Ma for the highest temperature steps. Lovera's multi-diffusion domain (MDD) models were used to derive T-t paths with models starting at 50 Ma, the suggested crystallization age of the sills based on previous U-Pb and K-Ar studies (Keith et al., 1980). All monotonic histories suggest the onset of rapid cooling at 30-27 Ma from 350-250^oC. An exception is the deformed Wilderness granite that has cooled to 75^oC by 30 Ma. This suggests the deformed granite was exhumed to shallow crustal levels prior to the other samples. The reheating models, with the exception of the deformed Wilderness granite, are similar to the monotonic histories; they indicate cooling at 30-27 Ma from 300-250^oC. The reheating model for the deformed granite indicates a possible thermal pulse at 30-28 Ma. A potential heat source for a reheating event is emplacement of the Catalina granite in the Oligocene. Ongoing thermochronologic studies are aimed at further constraining the mid-Tertiary history of magmatism and origin of the Catalina MCC.