Geobarometric and Thermochronologic Evidence for a Latest Oligocene Shallow Ductile Detachment System in the Santa Catalina and Tortolita Mountains, Arizona

摘要

The Santa Catalina and Tortolita mountains of Southeast Arizona are footwall exposures of the mid crust, 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). Chemical compositions of hornblende-plagioclase pairs were obtained by electron microprobe analysis of 6 samples collected from the Catalina Granite pluton in the northwest Catalina mountains and the Tortolita mountains, and pressure of emplacement is calculated using the aluminum-in-hornblende thermo-barometer. In addition to the pressure data, zircon U-Pb ages were calculated for 4 of the samples. Maximum depth estimates for the emplacement of the Catalina Granite range from an average of 11.3 km in the Tortolita mountains, to 6.6 km in the northwest Catalina mountains. Average temperature of emplacement for the pluton is calculated to be 663°C in the northwest Catalina mountains and 693°C in the Tortoliata mountains. U-Pb age results indicate mean ages ranging from 25.04 to 24.79 Ma (MSWD = 1.7 to 3.9) for each sample, and a combined mean age of 24.91 Ma (MSWD = 2.6). A positive trend between pressure and age is observed for some of the samples. Scatter in the U-Pb ages could be a result of continuous magmatism between 26 and 24 Ma, and the pressure-age trend could be the result of melt migration within the shallow crust. These results suggest that the Catalina ductile detachment system formed at a depth no greater than 8 km due to the thermal incursion produced as the Catalina Granite pluton rose through the shallow crust.