EVOLUTION OF THE PRECAMBRIAN ROCKS OF YELLOWSTONE NATIONAL PARK (YNP): HIGH GRADE METAMORPHIC ROCKS AT GARNET HILL

High-grade Archean metamorphic rocks exposed in the Garnet Hill area in the northern part of Yellowstone National Park (YNP) exhibit distinctive features pertaining to petrography, structural style, and geothermobarometry compared with the neighboring low-grade rocks of the Jardine Metasedimentary Sequence or high-grade orthogneisses of the Junction Butte area to the east. The metasedimentary rocks of Garent Hill have been injected by a complex of muscovite-bearing granodioritic, tonalitic, and hornblende- and biotite-bearing diorite. Migmatization of the Garnet Hill assemblages is interpreted to have occurred over at least two separate injection events, the first having undergone more extensive deformation, boudinaging, and ptygmatic and isoclinal folding followed by crosscutting by a second, less deformed injection suite. The metasupracrustal rocks of the Garnet Hill area include pelitic schists and ironstones. The metapelites have a characteristic mineral assemblage of garnet, plagioclase, sillimanite, biotite, and quartz with staurolite inclusions preserved in garnet poikiloblasts. Pressure and temperature calculations using TWQ yield four independent reactions: Grt-Bt thermometer, and GBPQ , GASP, and GRAIL barometers. Using garnet rim analyses, three of the reactions (Grt-Bt, GBPQ, GASP) intersect near a common point in P-T space suggesting these rocks achieved equilibrium at metamorphic conditions of 560-585°C and 4.0-4.2 kbar. The Ti-in-biotite thermometer agrees with these results and yields 565-570°C. However, the GRAIL reaction yields pressures about 2.5 kbar higher, suggesting that this reaction is not appropriate for these rocks. The ironstones have a characteristic mineral assemblage of garnet, plagioclase, hornblende, anthophyllite, and quartz. Based on petrographic relationships, hornblende appears to replace anthophyllite and Grt-Hbl thermometry yields unrealistically low temperatures around 450°C. Based on the metamorphic conditions recorded in the metapelites of the Garnet Hill area, these rocks appear to be more closely related to the lower grade rocks of the Jardine Metasedimentary Sequence which together may represent a separate, distinctive unit that is allocthonous within the northern part of YNP.