1. Evolution of the Precambrian Rocks of Yellowstone National Park (YNP): High Grade Metamorphic Rocks at Garnet Hill Benjamin Parks 1, Cameron Davidson 1, Darrell Henry 2, David Mogk 3, Paul Mueller 4, and David Foster 4 1 Carleton College, 2 Louisiana State Univ., 3 Montana State Univ., 4 Univ. of Florida Introduction and Scientific Rationale Archean metasedimentary rocks exposed in the Garnet Hill area in northern YNP exhibit distinctive field relations, structural style, and metamorphic conditions compared with the low-grade Jardine Metasedimentary Sequence (JMS) to the west and high-grade orthogneisses of the Junction Butte area to the east. The Garnet Hill area has been injected by numerous generations of felsic dikes and sills, which cut metasedimentary and lenticular dioritic units. In this study, we calculate the metamorphic conditions of the Garnet Hill metasedimentary rocks, and have obtained radiometric ages of some of the felsic to dioritic dikes and sills. The Garnet Hill area contains key evidence about the metamorphic, structural and magmatic processes that have contributed to Archean crustal evolution in this area. Petrography Pelitic schist contains: garnet, plagioclase, sillimanite, biotite, and quartz with staurolite inclusions preserved in garnet poikiloblasts. Foliation defined by bt, mm-scale quartzofeldspathic layers present. Ironstones contain: garnet, plagioclase, hornblende, grunerite, and quartz. Anhedral crystal texture. Hbl & grun display ingrown texture, indicating disequilibrium. Geochronology Ages were obtained for hornblende diorite plutons from Garnet Hill to determine its relation to other nearby igneous units. Ages were determined by measuring U-Pb systematics in zircons using Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS). Ages below are based on 207 Pb/ 206 Pb ratios from data that is <10% discordant. Age determinations from two hornblende quartz diorite bodies on Garnet Hill are: 2798 ± 4 Ma (2σ), 10-DM-7-3-03 2815 ± 9 Ma (2σ),10-PM-7-14-01 These ages are similar to the Crevice and Hellroaring Plutons. Geothermobarometry Pressure and temperature calculations using TWQ yield four independent reactions in both metasedimentary units: the Grt-Bt thermometer, and Mg-model GBPQ, GASP, and GRAIL barometers in the metapelites, and Grt-Bt, Mg- and Fe-model GBPQ, and GRIPS in the ironstone. Grt-Bt, GASP, and GBPQ reactions indicate metamorphic conditions of 560-615°C and 4.0-5.5 kbar. However, GRAIL and GRIPS reactions yield pressures about 1.5- 2.5 kbar higher than other barometers, suggesting that these reactions are not appropriate for these rocks. The Ti-in-biotite thermometer applied to same data agrees with the metapelite results in TWQ and yields 565-570°C. For diorite intrusions, iterative Al-in-hbl and hbl-plag igneous thermobarometry yield 685-727°C and 5.8-6.5 kbar, interpreted as crystallization pressure and temperature. Field Relations Injection of the Garnet Hill assemblages is interpreted to have occurred over at least two separate events, the older injection having undergone more extensive deformation, boudinaging, and ptygmatic and isoclinal folding followed by crosscutting by a second, less deformed set of dikes. The development of ductile structures and injection of igneous sills and dikes are contemporaneous with high grade metamorphism at Garnet Hill. Summary P-T values recorded for metasedimentary units in the Garnet Hill and JMS areas are similar (560-615°C, 4.0-5.5 kbar); however these are quite different compared to orthogneisses at Junction Butte to the east (~800 o C, 7-8 kbar). Field observations and P-T results from the migmatites at Garnet Hill indicate that the felsic dikes and sills were emplaced by injection and not in-situ melting of the metasedimentary rocks. Their age and composition are similar to the adjacent Crevice and Hellroaring plutons. Garnet Hill is the easternmost extent of the metasedimentary sequence that includes the lower grade JMS rocks. A major structural discontinuity is interpreted between the Garnet Hill rocks and the higher grade and older (~3.2 Ga) tonalitic orthogneisses to the east at Junction Butte. P-T plot of ironstone sample 10-EHG-705-01, using the Grt-bt thermometer, the Mg- and Fe-model GBPQ barometer, and the GRIPS barometer. The aluminosilicate triple point is plotted in gray. P-T plot comparing metamorphism undergone by JMS (beige box), Junction Butte (JB) orthogneiss, and Garnet Hill (GH) metapelites and ironstones. Metapelite sample 10-CO-713-02 displaying quartzofeldspathic layering (A) aligned with bt foliation (B). X-ray map (C) of garnet crystal seen in (B) displays Ca zoning pattern. Ironstone sample 10-AL-719-01 displaying anhedral crystal shape and ingrown texture between hornblende and grunerite crystals. A B C Meter-scale granitic dike cutting pelitic schist. Hand lens for scale. Two dike generations seen cutting hornblende diorite. This project was supported through the NSF REU program, Division of Earth Science grants EAR 0852025, 0851752, and 0851934. 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. Map of the northern border of Yellowstone National Park mapped by Casella et al.,1982. The Garnet Hill area is defined by the reference box. Tight to isoclinal disharmonic folding in pelitic schists with leucosome veins. Acknowledgements