EVOLUTION OF THE PRECAMBRIAN ROCKS OF YELLOWSTONE NATIONAL PARK (YNP): METASEDIMENTARY ROCKS

The Jardine Metasedimentary Sequence (JMS) is a suite of low-grade metamorphosed sandstones and mudstones in the South Snowy Block of the Beartooth Mountains. Samples from a ~25 km transect within Yellowstone National Park (YNP) between Gardiner, MT and Tower Junction, north to the YNP boundary near Hellroaring Creek consist of intercalated sub-arkosic (plagioclase>Kspar) quartz-biotite schist and biotite schist that were intruded by two major granitic plutons (~2.8 Ga) and several mafic stocks and dikes. Preserved sedimentary structures include graded bedding, crossbeds, sole marks, and lenticular, coarse-grained bodies interpreted as channels. Measured stratigraphic sections reveal an overall coarsening of the sedimentary rocks from west to east. Metaconglomerates, rip-up clasts, and well-defined erosional surfaces are notably absent. The sediments are interpreted as low velocity turbidite deposits based on the observed lithologies and sedimentary structures. Whole rock and trace element geochemistry data will be used to compare the JMS to other metasedimentary terranes within the region in order to determine possible correlations and to aid in tectonic reconstructions. The lithologic sequence, sedimentary structures, and preliminary whole-rock geochemical data (Thurston, 1986) indicate deposition occurred along an active continental margin or back-arc basin. Scanning electron microscopy and CL imaging have identified detrital minerals that include zircon, monazite, apatite, ilmenite, and sulfides. Cross-cutting relations of the granitic plutons (2.8 Ga) and the ages of the youngest detrital zircons form two samples (2.95 Ga) constrain the age of deposition and metamorphism to this 150 Ma period, which makes these rocks younger than the high grade quartzites of the central Beartooth Mountains that contain older grains (4.0 vs 3.6 Ga in JMS) and a much higher concentration of grains in the 3.2-3.4 Ga range. These attributes make the JMS distinct in the northern Wyoming Province in terms of the lithologic package, preserved sedimentary structures, and low-grade metamorphism. We interpret it as an allochtonous sequence in the otherwise higher grade rocks of the northern Wyoming Province, or a uniquely preserved upper crustal section.