NEW PETROLOGIC ANALYSIS AND THERMOCHRONOLOGIC CONSTRAINTS ON EXHUMATION RATE USING APATITE (U-TH)/HE COOLING AGES OF THE GRANITE PEAK PLUTON, KLAMATH MOUNTAIN PROVINCE, NW CALIFORNIA
In this study, we collected eight samples for petrologic and thermochronologic analysis along a bedrock transect spanning 1.2 km of relief, and 6.4 km of horizontal distance that traverses the approximate radius of the GPP. Based on petrologic description of thin sections, the GPP is a monzogranite near the peak, while samples collected near its base have mineral percentages characteristic of granodiorite. Percent mafics change from 59% (upper three samples), to 40% (single sample), to 24% (lower four samples). If the GPP is to remain included in the Shasta Bally Belt, unit descriptions could be modified.
The GPP is located in the footwall of an unusual low-angle detachment fault, the east-west striking La Grange Fault (LGF). Previous studies constrain detachment-driven exhumation generated by the LGF to the middle Tertiary; two apatite (U-Th)/He cooling ages from the GPP range from 18-14 Ma (Piotraschke et al., 2015), yielding an exhumation estimate of 1.6 km/Ma. In this study, we present four new apatite cooling ages to supplement previous work and produce a more robust exhumation rate. Using new petrologic and thermochronologic analyses this study places the Granite Peak Pluton into a broader regional context.
This work emerged from an upper division, undergraduate elective course. Finally, we offer best practices on how undergraduate classes can incorporate students in field and laboratory intensive research that prioritizes student-led collaboration, and allows for multiple levels of student involvement.