EVOLUTION OF THE PRECAMBRIAN ROCKS OF YELLOWSTONE NATIONAL PARK (YNP): HIGH GRADE METAMORPHIC ROCKS FROM JUNCTION BUTTE TO SLOUGH CREEK

The Junction Butte to Slough Creek area of Yellowstone National Park (YNP) contains a suite of high-grade metamorphic rocks that are distinct from the Jardine Metasedimentary Sequence and Garnet Hill. The area consists of grey tonalitic gneiss cut by migmatites, which are interpreted as in situ partial melts. The deformational style is ductile, including: recumbent, isoclinal, gently plunging folds; ptygmatic folds; sheath folds; and boudins. The gneiss is medium-to-coarse grained and includes: quartz, plagioclase, biotite, some muscovite, zircon, monazite, and allanite. Minor bodies of amphibolite and metagabbro are also present. A distinct cross-cutting leucocratic granitic pluton was mapped and sampled. The leuco-granite ranges from medium-to-coarse grained and is locally pegmatitic. It is composed of 50 – 60% perthitic K-feldspar and ranges from 10 – 20% plagioclase and 30 – 40% quartz to equal parts plagioclase and quartz. Other minerals include: biotite, muscovite, amphibole, secondary chlorite, ilmenite, myrmekite, and garnets. Sub- to euhedral, 1 – 4 mm, unzoned garnets are abundant and contain quartz inclusions with muscovite and chlorite wrapped around them. The garnets indicate that the leuco-granite is a peraluminous, S-type granite. Mica-rich, psammitic xenoliths and schlieren are found in the pluton. The grey tonalitic gneiss and metasedimentary rocks in the area include discordant and concordant granitic veins and dikes that were derived from the leuco-granite pluton. Application of iterative thermobarometry to the garnet leuco-granite yields conditions of 821-863ºC and 7.0-7.3 kbar. U/Pb zircon data from the gray gneiss acquired by LA-ICP-MS indicate a crystallization age of 3.25 +/- .08 (2σ). The Junction Butte area is also distinctive relative to the Garnet Hill area as indicated by the paucity of definitive metasedimentary rocks, by the presence of migmatites developed through partial melting rather than injection and garnet-bearing leuco-granite. The gray gneisses may be compositional equivalents of similar rocks exposed in the Beartooth Mountains to the east, but this overall sequence is interpreted as being allochthonous with respect to the Garnet Hill and Jardine Metasedimentary Sequences to the west.