Geobarometric evidence for a Latest Oligocene shallow ductile detachment system in the Santa Catalina and Tortolita Mountains, Arizona

Abstract:

The Santa Catalina and Tortolita Mountains of Southeast Arizona are footwall exposures of the mid crust, later exhumed by a ductile detachment system. Surprisingly little quantitative research has been done to constrain the evolution of this ductile detachment system, despite the reputation of this area as a prime example of a metamorphic core complex. This study presents new evidence for the formation of a shallow ductile detachment system during the latest Oligocene (~25 Ma). Hornblende thermobarometry results were obtained from electron microprobe analysis of hornblende-plagioclase pairs in 6 samples collected from the 26 Ma Catalina Granite pluton in the Northwest Catalina mountains and the Tortolita mountains. Maximum depth estimates for the emplacement of the Catalina Granite range from an average of 8.2 km in the Tortolita mountains, to 6.4 km in the Northwest Catalina mountains. Average temperature of emplacement of the pluton from the plagioclase-hornblende thermometer was calculated to be 680^oC, with some samples indicating much higher temperatures. These preliminary results suggest that the ductile detachment system formed at a shallow depth no greater than 8 km, and was either the result of, or influenced by, the thermal incursion during the emplacement of the Catalina Granite pluton. Furthermore, this study sheds some light on the development of metamorphic core complexes; while some may be very deep and long lived, others, such as the Catalina core complex, are shallow and short lived.