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

The Klamath Mountain Province (KMP) occupies the southernmost extent of the Cascadia Subduction Zone, in northern California and southern Oregon. The KMP predominantly consists of a sequence of Paleozoic accreted terranes that have been intruded by Jurassic-Cretaceous plutons associated with the Nevadan Orogeny. Detailed mapping and petrologic studies place plutons of the KMP in suites of similar emplacement ages, composition, and melt source. A few small plutons remain undescribed, including the Granite Peak Pluton (GPP). The GPP is mapped intruding into the Trinity subterrane of the Eastern Klamath Terrane. Irwin (1994) tentatively mapped the GPP within the Shasta Bally Belt, which is primarily quartz diorite, trondhjemite, and granodiorite, although its affiliation and emplacement age are unknown.

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.