Evolution of the Precambrian Rocks of Yellowstone National Park (YNP): Metasedimentary Rocks Emily Hernandez Goldstein 1, Kirsten Sauer 2, Jeremy Harwood 3, David Mogk 3, Darrell Henry 4, Paul Mueller 5, and David Foster 5 1 Columbia Univ., 2 Cornell Univ., 3 Montana State Univ., 4 Louisiana State Univ., 5 Univ. of Florida Introduction Field Relations Geochemistry Geochronology Conclusions Outcrops of metasediments contain preserved sedimentary structures which help delineate the paleo-depositional environment. Preserved sedimentary structures include: graded bedding, crossbeds, sole marks, and lenticular, coarse-grained bodies interpreted as channels (Fig. 1). Measured stratigraphic sections reveal an overall coarsening of the clastic sequences from west to east (Fig. 2). Metaconglomerates, rip-up clasts, and well-defined erosional surfaces are absent. Based on the observed lithologies and sedimentary structures, the original sediments are interpreted as low velocity turbidite deposits. The preserved sedimentary structures, geochemistry and geochronology indicate these clastic metasedimentary rocks: resemble clastic turbidite deposits of greenstone belts. were deposited on a mid-to-distal submarine fan on an active continental margin. were deposited between 2.9 and 2.8 Ga, based on the youngest detrital zircon (2.9 Ga) and cross-cutting granitic plutons (2.8 Ga). are not consistent with derivation from the Archean rocks of the Beartooth Mountains based on the detrital zircon age spectrum. a cb c b a Figure 1. Relict sedimentary features from the JMS (bottom photos highlight interpreted structure). a)Bottom of sedimentary layer with a preserved sole mark. b)Interbedded metamorphosed sandstone and shale. Metasandstone layers highlighted by gray lamination and metashale layers highlighted by garnet growth from clay minerals during metamorphism. c) Inferred relict cross bedding in metamorphosed sandstone indicating flow during deposition. Isotopic analytical results and field relationships constrain the possible depositional age range. Detrital zircons were analyzed by LA-ICP-MS ( 207 Pb/ 206 Pb of <10% discordant) yield the ages of source detritus from 2.9 Ga to 3.66 Ga. Cross-cutting relationship of 2.8 Ga granite plutons constrain the age of deposition of the JMS between 2.9 Ga and 2.8 Ga. Figure 3.Tectonic setting discrimination ternary plots with JMS samples and representative Archean compositions plotted (KOM = komatiite, BAS= basalt, TTG, FVO = felsic volcanics, GR = granite). OIA = Ocean Island Arc, CIA = Continental Island Arc, ACM = Active Continental Margin, PM = Passive Margin (after Bhatia 1983, 1986). Whole-rock geochemical analysis provides insight into the nature of the provenance and tectonic setting. The JMS is characterized by both felsic and mafic tendencies Felsic: Elevated Zr/Y (4-14), enriched in incompatible elements Mafic: High Cr, Ni, and FeO T Range of REE patterns- HREE depletion and LREE enrichment, Eu/Eu* from 0.4 to 0.9. Provenance The geochemistry indicates that the source was bimodal. Sources likely include felsic and mafic volcanics along with tonalitic- trodjhemitic-granitoids. Tectonic Setting Tectonic discrimination diagrams after Bhatia (1983, 1986) indicate that the JMS has a geochemical signature most similar to a Continental Island Arc setting. This is consistent with the bimodal nature of the provenance. Figure 4. Histogram of detrital zircon grain populations. Field areas west to east: Bear Creek, Crevice Lake, Blacktail Deer Creek East, Cottonwood Creek. Metasedimentary rocks of the Wyoming Province are typically high grade metamorphic rocks with substantial amounts of deformation. The nature of the protolithic sedimentary rocks is typically uncertain due to this deformation. In contrast to this stereotype, the relatively low-grade clastic metasediments of the Jardine Metasedimentary Sequence (JMS) of YNP provide an opportunity to: investigate sedimentary features as they relate to sedimentary environments consider the possible sources of the detrital material and its relation to tectonic reconstruction constrain the age of deposition Acknowledgements References This project was supported through the NSF REU program, Division of Earth Science grants EAR , , and Special thanks to YNP staff, Christie Hendrix, Stacey Gunther, Carrie Guiles, Bridgette Guild and Hank Heasler for their support and interest. Figure 2. Stratigraphic sections of metasediments along an west to east (left to right) transect. Bhatia, M.R. (1983) Plate-Tectonics and Geochemical Composition of Sandstones. Journal of Geology, 91(6), Bhatia, M.R. (1985) Plate-Tectonics and Geochemical Composition of Sandstones - a Reply. Journal of Geology, 93(1),