EVOLUTION OF THE PRECAMBRIAN ROCKS OF YELLOWSTONE NATIONAL PARK (YNP): LATE ARCHEAN FELSIC PLUTONS

The Precambrian metasedimentary rocks of northern YNP have been intruded by two plutonic granitic bodies, the Crevice and Hellroaring Plutons. Both plutons are post-kinematic, with only minor fabric development at contacts, and contacts that clearly cross cut the foliation and mesoscopic folds in the surrounding metasedimentary rocks. The contact zones between plutons and country rock show a dynamic mix of thin injection migmatitic zones and xenoliths with varying degrees of assimilation. Both plutons are primarily composed of homogeneous granite with hypidiomorphic texture. The mineralogy includes sub-equal modal abundances of quartz-plagioclase and microcline (perthitic). Biotite content varies from ~5-10% and muscovite occurs in discrete zones within the plutons. Petrographic observations indicate that the majority of the muscovite in the plutons grew prior to or simultaneously with the biotite. The presence of magmatic muscovite is of particular importance because it constrains a minimum pressure of crystallization of ~ 4 Kb, consistent with pressure estimates for metamorphism in the adjacent metasediments. This also requires, at least locally, the occurrence of a peraluminous granite (S-type) that may be the result of melting of a sedimentary source at depth or local assimilation. These possibilities will be tested using whole-rock geochemistry and zircon xenocrysts. For the Crevice Pluton, a U/Pb zircon age 2.808 +/- 0.007 2σ Ga has been determined. The inherited zircon in the Crevice Pluton is consistent with zircon populations in the adjacent metasedimentary country rocks. The Hellroaring Pluton has a provisional U/Pb zircon age of 2.805 +/- .010 Ga. The granites are also locally cut by muscovite-bearing mylonites, and ⁴⁰Ar-³⁹Ar thermochronologic data will be used to determine the age of this deformation. The Crevice and Hellroaring plutons are distinct in their emplacement style (bulbous magmatic bodies intruded at ~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 emplacement of these granitic bodies is significant because they place a minimum age on sedimentation as well as the regional metamorphism and deformation of the gold-bearing Jardine Metasedimentary Sequence.