2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 224-8
Presentation Time: 12:45 PM

NEW P-T PATHS FOR METAMORPHISM OF AMPHIBOLITE FROM THE SALMON RIVER SUTURE ZONE, IDAHO


BOLLEN, Elizabeth M., Geological Sciences, University of Alabama, Tuscaloosa, AL 35487, STOWELL, Harold H., Department of Geological Sciences, University of Alabama, Tuscaloosa, AL 35487, SCHWARTZ, Joshua J., Department of Geological Sciences, California State University Northridge, 18111 Nordhoff Street, Northridge, CA 91330 and MCKAY, Matthew P., Department of Geology and Geography, West Virginia University, 98 Beechurst Ave, 330 Brooks Hall, Morgantown, WV 26506

The Salmon River Suture Zone (SRSZ) in west-central Idaho marks the earliest stages of terrane accretion along the Mesozoic margin of cratonic North America. East-dipping, N-S trending thrust faults parallel the suture. The thrust faults separate rocks of different metamorphic grades, with higher grade rocks in the hanging wall. Of these faults, the Pollock Mountain fault, juxtaposes two structural thrust sheets, the upper greenschist-grade Rapid River and amphibolite-grade Pollock Mountain plates.

Amphibolite from the Pollock Mountain plate containing hornblende, biotite, garnet, quartz, plagioclase, ilmenite, rutile, and ± clinozoisite was modeled using THERIAK-DOMINO to construct a detailed P-T path. Garnet is texturally and compositionally zoned with cores containing abundant rotated helicitic inclusions and inclusion free rims. Overall, asymmetrically from garnet core to rim, Mn and Ca decrease, while Fe and Mg increase. The boundary between inclusion density corresponds with a fluctuation in Fe, Mn, and Mg. Garnet core compositions coupled with observed inclusions indicate that garnet growth initiated at ca. 6 kbar and 700°C. Peak metamorphism for this rock occurred at 7.5 kbar and 700°C, based on the observed mineral assemblage and garnet rim compositional isopleths. This isothermal loading path is compatible with rapid thrust loading after heating.

Previous geochronologic and petrologic studies interpreted the sharp textural and compositional discontinuity between core and rim as multi-stage growth from distinct accretion events at 144 and 128 Ma (Getty & Selverstone, 1993) or protracted growth during a single, long-duration (>10 Ma) metamorphic event caused by thrust loading (McKay, 2011). Comparisons of phases included in garnet with those in the matrix, coupled with exchange thermobarometry, are being used to evaluate the single event hypothesis. Preliminary results are being used in conjunction with garnet Sm-Nd ages and a P-T-t path for the Rapid River plate (McKay, 2011) which shows similar near isothermal loading.

Session No. 224

Petrology: Metamorphic/Igneous (Posters)
Tuesday, 21 October 2014: 9:00 AM-6:30 PM
Exhibition Hall C (Vancouver Convention Centre-West)
Geological Society of America Abstracts with Programs. Vol. 46, No. 6, p.554
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