Evolution of the Precambrian Rocks of Yellowstone National Park (YNP): Late Archean Felsic Plutons Kate Philbrick 1, Bryant Ware 2, Darrell Henry 3, David Mogk 4, Paul Mueller 5, and David Foster 5 1 Wellesley College, 2 Boise State Univ., 3 Louisiana State Univ., 4 Montana State Univ., 5 Univ. of Florida Conclusions The Crevice and Hellroaring Plutons are distinct in their emplacement style (bulbous magmatic bodies intruded at 4 Kb, ~10-15 km depth) compared with deeper seated (~20-25 km) magmatic bodies emplaced in series of sill-like sheets in the adjacent Beartooth Mountains. The plutons formed from the partial melting of continental crust, perhaps from a pelitic, metasedimentary source. The emplacement of these granitic bodies is significant because they place a minimum age of ~2.8 Ga on sedimentation as well as the regional metamorphism and deformation of the Jardine Metasedimentary Sequence. References Casella, C.J., Levay, J., Eble, E., Hirst, B., Huffman, K., Lahti, V., and Metzger, R., 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, p Montgomery, C.W., 1982, Preliminary Zircon U-Pb Dating of Biotite Granodiorite from the South Snowy Block, Beartooth Mountains: in Precambrian Geology of the Beartooth Mountains, Montana and Wyoming, MBMG Sp. Publ., 84, p Geochronology For the Crevice Pluton, a U/Pb zircon age of / σ Ga has been determined. The inherited zircon in the Crevice Pluton is consistent with zircon populations in the adjacent metasedimentary rocks. The Hellroaring Pluton has a provisional U/Pb zircon age of / σ Ga Age of the plutons 2.8 Ga Age of deposition of the metasedimentary sequence Ga } Time (in Ga) Figure 1: View of the Hellroaring Pluton looking to the north Figure 2: View of the Crevice Pluton looking to the east, featuring Rattlesnake Butte Introduction The Precambrian metasedimentary rocks of northern Yellowstone National Park have been intruded by two plutonic granitic bodies, the Crevice and Hellroaring Plutons. Montgomery (1982) determined an age of 2.73 to 2.79 Ga for one of the plutons in the northern portion of Yellowstone National Park, 0.3 km east of Tower Junction. Casella, et al. (1982) hypothesized that the plutons have a similar geologic history to the well-studied plutons in the Beartooth Mountains, 40 miles to the east. The purpose of this study is to determine the timing of the intrusion of the felsic plutons in relation to the sedimentation and metamorphism of the surrounding metasedimentary sequence and the plutons in the Beartooth Mountains. Figure 3: Map of the northern border of Yellowstone National Park showing the location of the two major felsic plutons mapped by Casella et al.,1982. The Crevice Pluton is located in the northwest of the map while the Hellroaring Pluton can be viewed near the central area of the map. Sample locations and type are shown by the different colored dots; biotite quartz monzonites are magenta, and muscovite-bearing quartz monzonites are pink. Over 170 samples were collected throughout the month of field work. Thin sections were made for petrographic analysis. The photomicrographs of samples 10-DM and 10-KP show anhedral biotite against subhedral muscovite. This relationship is consistent throughout the pluton, including samples from near the contact and in the interior of both the Crevice and Hellroaring Plutons. Photomicrographs of samples 10- WF and 10-BW show a good representation of the mineralogy of the Crevice and Hellroaring Plutons that include sub-equal modal abundances of quartz-plagioclase and microcline (perthitic). Petrography Figure 8: Photomicrographs of the Hellroaring and Crevice Plutons. A) Texture and mineralogy of sample 10-WF : Biotite Quartz Monzonite, Hellroaring Pluton. B) Texture and mineralogy of sample 10-BW : Muscovite Granite, Crevice Pluton. C) : Biotite-muscovite intergrowth in interior of Hellroaring Pluton (sample 10- KP ). Note that biotite is anhedral against subhedral muscovite. Crossed-polarized light. D) : Biotite-muscovite intergrowth in interior of Crevice (10-DM ). Note that biotite is anhedral against subhedral muscovite. Crossed-polarized light. CD AB Acknowledgements 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 David Hawkins for his expertise and guidance. Thanks to Dr. Mark Schmitz for his ever extending guidance, knowledge and patience. Figure 10: Relative Al 2 O 3 concentrations in samples from the Crevice and Hellroaring Plutons Figure 13: Phase diagram showing the stability of magmatic muscovite, indicating a minimum crystallization pressure of 0.38 GPa; from Winter (2010, Figure 18.5a) Figure 11: Spider diagram showing HFSE depletion characteristic of formation in an arc environment. Figure 12: REE diagram showing pronounced negative Eu anomaly indicting a crustal source for these magmas. Figure 14: Tectonic variation diagrams indicating a volcanic arc or collisional setting for this magmatism. In the northern YNP the Jardine Metasedimentary Sequence has been intruded by two main igneous bodies: the Crevice Pluton and the Hellroaring Pluton. A dynamic mix of different contact relations were observed adjacent to the Crevice and the Hellroaring Plutons. Locally, there are areas of sharp cross-cutting contacts between the granite plutons and the metasedimentary rocks, areas where xenoliths are present indicating forceful emplacement (Figure 4), and some thin Field Relations Figure 4: Metasedimentary xenoliths in the SE margin of Crevice Pluton. Figure 5: Biotite-rich xenoliths near the margin of Crevice Pluton. Figure 6: Slickenlines from the Crevice Pluton. Figure 7: View of the Crevice Pluton Metasedimentary relations from Bear Creek across the Yellowstone River injection migmatites. The plutons have not developed a strong deformation fabric near the contacts and clearly cross cut large folds of the metasedimentary rocks (most notably in the Bear Creak area) indicating post kinematic intrusion. Peraluminous Metaluminou s Geochemistry Figure 9: Total alkali v. SiO 2 diagram showing restricted rhyolitic compositions compared to the compositional range of the Long Lake magmatic complex of the Beartooth Mountains. Crevice Pluton = dots, Hellroaring Pluton = triangles