17 Recent Advances in Cordilleran Tectonic Evolution—2: Cenozoic

Thursday, 16 May 2019
8:00 AM - 12:00 PM
Room C124

PRESSURE AND TEMPERATURE ESTIMATES FOR SEVERAL EOCENE PLUTONS IN THE CASCADE FOOTHILLS AND EASTERN PUGET LOWLANDS OF WASHINGTON STATE: A RECORD OF EOCENE SYN-TECTONIC INTRUSION AND EOCENE TO PRESENT FAULTING

Paper number: 17-2
Program: Theme Sessions
Time: 8:20 AM – 8:40 AM

Authors

Abstract

Several Eocene intrusive bodies in western Washington State are bordered by and locally syntonically intruded fault zones such as the Pilchuck River fault (PRF) and Granite Falls fault zone (GFFZ). New pressure (P) and temperature (T) information at which these plutons formed provide insight into intrusive and tectonic history of these plutons.

The 39-43 Ma Youngs Creek intrusive complex (YCIC) consists mainly of metaluminous, medium-K, calc-alkaline granodiorite. Ca-amphiboles from the YCIC are predominantly edenite and plagioclase ranges from andesine to labradorite. The YCIC is likely a source for the volcanic rocks of Mt. Persis. The 44-45 Ma Granite Falls stock (GFS) also consists of metaluminous, medium-K, calc-alkaline granodiorite. Ca-amphiboles from the GFS are mostly edenite and plagioclase ranges from oligoclase to labradorite. The GFS is interpreted to have formed from a continental arc with a mafic to intermediate source, as supported by Sr and Nd isotopes. The GSF intruded into a transtensional segment of the GFFZ. Conversely, the ~50 Ma Bald Mountain pluton (BMP) and 49 Ma Mount Pilchuck stock (MPS), located east of the YCIC and GFS in the Cascades foothills, are interpreted to be derived from melting old sedimentary basement. Both consist of peraluminous two-mica granite with S-type compositions. Mica from the BMP and MPS are primary. Biotite has peraluminous compositions. Plagioclase is albite in the BMP, and range from albite to andesine in the MPS. K-feldspars in the BMP and MPS are Or65 to Or96.

PT estimates from amphiboles of the YCIC and GFS were estimated using Ridolfi et al. (2010). The YCIC is estimated to have crystallized at 814˚C ± 45 and 0.57 kb ± 0.08. The GFS is estimated to have crystallized at 820˚C ± 10 and 0.69 kb ± 0.07. Luhr et al. (1984) and Massone and Schreyer (1987) were utilized to estimate PT from BPM and MPS micas. The BPM is estimated to have crystallized at 629˚C ± 23 and 4.5 kb ± 0.3, while the MPS crystallized at 652.8˚C ± 17 and 5.4 kb ± 0.8. This suggests the PRF displacements have >12 km of syn- to post-Eocene displacement. This faulting may have been rapid, and may include lateral offset. Significant vertical displacement along the PRF has juxtaposed the BMP and MPS with their Eocene extrusive volcanic equivalents deposited in a transtensional basin directly south of the PRF.

Geological Society of America Abstracts with Programs. Vol. 51, No. 4

doi: 10.1130/abs/2019CD-329309

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