Evolution of the Precambrian Rocks of Yellowstone National Park (YNP): Low-Pressure Metamorphism of the Jardine Metasedimentary Sequence Carly Osborne 1, Julia Baldwin 1, Darrell Henry 2, David Mogk 3, Paul Mueller 4, and David Foster 4 1 University of Montana, 2 Louisiana State University, 3 Montana State University, 4 University of Florida Introduction Field Relations Petrography & Mineral Chemistry Thermobarometry Conclusions Garnet-bearing mica schist from the JMSGarnet bearing rock from Garnet Hill to the east (higher grade) X-ray elemental maps of garnet. A: Ca, B: Fe, C: Mg, D: Mn References Casella, C.J., Levay, J., Eble, E., and others, 1982, Precambrian geology of the southwestern Beartooth Mountains, Yellowstone National Park, Montana and Wyoming, in Precambrian Geology of the Beartooth Mountains, Montana and Wyoming, MBMG Sp. Publ., 84, Mueller, P.A., and Frost, C.D., 2006, The Wyoming Province: a distinctive Archean craton in Laurentian North America, Canadian Journal of Earth Sciences, 43, Powell R., Holland, T., and Worley, B., 1998, Calculating phase diagrams involving solid solutions via non-linear equations, with examples using THERMOCALC, Journal of Metamorphic Geology, 16, Thurston, P., 1986, Geochemistry and Provenance of Archean Metasedimentary Rocks in the Southwestern Beartooth Mountains [MS thesis]: Bozeman, MSU. The JMS metasedimentary rocks along with adjacent rocks in the Garnet Hill area to the E are of anomalously low metamorphic grade compared to other rocks of the northern Wyoming Province The JMS may represent a separate, distinctive unit that is allochthonous within this northern part of YNP This project was supported through the NSF REU program, Division of Earth Science grants EAR , , and Special thanks to YNP staff, Christie Hendrix, Stacey Gunther, Carrie Guiles, Bridgette Guild and Hank Heasler for their support and interest. Thanks to Donna Whitney and Ellery Frahm, Univ. of Minnesota for assistance with the microprobe analyses. Acknowledgements Fig. 1. General geologic map of the Beartooth-Snowy Block region. From Mueller & Frost (2006). Archean metasedimentary rocks in the Jardine Metasedimentary Sequence (JMS) within northern YNP (Fig. 1) are unique in their metamorphic grade and preservation of sedimentary structures. The JMS contains lower grade rocks compared to rocks further to the east in the Garnet Hill or Junction Butte areas (Fig. 2) The primary goal of this study was to use quantitative thermobarometry to determine the metamorphic conditions of pelitic rocks in the JMS and to determine whether metamorphic P-T gradients are preserved in the JMS from west to east where higher grade rocks are exposed at Garnet Hill (Parks et al., 2011). The JMS contains unique garnet-andalusite-staurolite assemblages that provide important new data for interpreting the Archean crustal evolution of the South Snowy Block region Study area is located east of Gardiner, Montana (Fig. 1) and extends east to Coyote Creek (Fig. 2) and has been termed the Jardine Metasedimentary Sequence Outcropping rocks consist primarily of fine-grained, sandy biotite schists and minor intercalated meta-ironstones These rocks typically preserve sedimentary structures such as graded bedding, crossbeds, and sole marks and are interpreted as low velocity turbidite deposits (Goldstein et al., 2011). Porphyroblasts include garnet, andalusite, and staurolite. Typical outcrop of biotite schist Exposed outcrop of sandy biotite schist near Bear Creek, western JMS Fig. 2. Geologic map of northern YNP. Boxes outline the Jardine Metasedimentary Sequence (this poster) and Bear Creek and Garnet Hill study areas. Sample locations for P-T determination are labeled on the map. Modified from Casella et al. (1982). Thin section scan of sample AL showing andalusite and staurolite porphyroblasts. Small staurolite grains also occur in the top left corner of the slide. Metapelites contain diagnostic metamorphic assemblages including chlorite-biotite in the western study area near Bear Creek (Figs. 2 & 3) (DM ), increasing in grade to andalusite-staurolite-biotite (AL72) and garnet-staurolite-biotite (JH715) further east (Fig. 3) Eastern assemblages (e.g. JH713) are dominated by garnet-biotite- chlorite with pseudomorphs of white mica after andalusite (Fig. 3) Notable metamorphic assemblage transitions in the metapelites include the formation of garnet followed by staurolite and andalusite, and the breakdown of chlorite with increasing metamorphic grade Muscovite is rare and typically occurs as a retrograde phase intergrown with andalusite KS7-16 AL7-2 JH7-13 BW7-12 JH7-15DM Fig. 3. Photomicrographs (PPL) of the biotite schist from Bear Creek to the west (DM ) to east (JH7-13) across the JMS study area. Staurolite is most abundant in samples to the west, whereas garnet increases in size and abundance to the east.Sample JH7-13 contains pseudomorphs of white mica after andalusite. Garnet occurs in a range of textures, including inclusions in And, partially resorbed grts surrounded by foliation, large, subhedral syn-kinematic porphyroblasts with rotated inclusion trails, and matrix grtovergrowing foliation. Grt is Fe-rich with compositions ranging from Alm Prp 8-15 Grs 3-5 Sps 1-9. Slight prograde zoning in Sps is observed in some samples, but Fe/(Fe+Mg) is homogenous. Thermocalc (Powell et al., 1998) was used to calculate average P-T for five biotite schist samples. Slight variations in P-T were observed, ranging from °C and kbar across the study area. Ti-in biotite thermometry yields consistent temperatures of °C. Pressure-Temperature results AL7-2: 586 ± 67°C, 4.1 ± 0.9 kbar BW7-12: 572 ± 21°C, 3.4 ± 0.9 kbar JH7-15: 576 ± 15°C, 5.6 ± 1.3 kbar KS7-16: 595 ± 15°C, 5.9 ± 1.3 kbar JH7-13: 609 ± 20°C, 4.7 ± 1.5 kbar JH7-13BW7-12JH7-15 KS7-16 A B C D BtBt Chl Qtz