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Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 17-2
Presentation Time: 8:20 AM

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


MACDONALD Jr., James H.1, DRAGOVICH, Joe D.2, THOMPSON, Glenn T.3, TEPPER, Jeffrey H.4, WISE, Kevin J.1, DUFRANE, S. Andrew5 and ANDERSON, Megan6, (1)Marine & Ecological Sciences, Florida Gulf Coast University, 10501 FGCU Blvd South, Ft. Myers, FL 33965, (2)Associated Earth Sciences, Inc., 1552 Commerce Street, Suite 102, Tacoma, WA 98402, (3)Department of Earth Sciences, Syracuse University, Heroy Geology Laboratory, Room 011 141 Crouse Drive, Syracuse, NY 13244, (4)Geology Department, University of Puget Sound, 1500 N. Warner #1048, Tacoma, WA 98416, (5)Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Science Building, Edmonton, AB T6G 2E3, Canada, (6)Washington Department of Natural Resources, Washington Geological Survey, 1111 Washington St SE, Olympia, WA 98504-7007

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.

Handouts
  • MacDonald et al pt talk.pdf (6.0 MB)

    • Session No. 17

    T5. Recent Advances in Cordilleran Tectonic Evolution—2: Cenozoic
    Thursday, 16 May 2019: 8:00 AM-12:00 PM
    Room C124 (Oregon Convention Center)
    Geological Society of America Abstracts with Programs. Vol. 51, No. 4
    doi: 10.1130/abs/2019CD-329309
    © Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.
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