Evolution of the Precambrian Rocks of Yellowstone National Park (YNP): Regional Overview David Mogk 1, Darrell Henry 2,Paul Mueller 3, and David Foster 3 1 Montana State Univ., 2 Louisiana State Univ., 3 Univ. of Florida Scientific Rationale Setting in the Wyoming Craton Preliminary Results The Precambrian rocks of northern Yellowstone National Park provide important insights into the petrogenesis, architecture, and evolution of the northern Wyoming Province (WP). These rocks are distinguished by: A suite of metasedimentary rocks. The Jardine Metasedimentary Sequence(JMS), composed primarily of biotite schists, quartzites and minor iron formation have preserved primary sedimentary structures (graded bedding, cross-beds, channels). Original sediments are relatively immature and this suite is interpreted as a turbidite sequence. In contrast, metasedimentary rocks throughout the northern WP are more typically stable platform sediments (marble, quartzite, pelite). Metamorphism is at an anomalously low-grade ranging from chlorite zone to andalusite-staurolite zone in most of the metasedimentary rocks, compared with upper amphibolite to granulite facies rocks in most of the northern WP. The structural style is dominated by kink-folding and late mylonitic shear zones, whereas adjacent areas (Yankee Jim Canyon) have developed pervasive ductile shear zones. Magmatism in this area is dominated by two epizonal quartz monzonitic plutons (emplacement depth ~12 Km), whereas magmatism in the Long Lake magmatic complex in the adjacent Beartooth Mountains is at much deeper levels (~20 Km), and with a range of compositions from gabbroic to granitic. Setting of the South Snowy Block in the Wyoming Province Key Research Questions Geologic map of the crystalline rocks of northern YNP; Casella et al., (1982) Relationship of the South Snowy Block to the Beartooth Mountains Hellroaring Pluton, YNP Detrital zircon age spectra from quartzites; J-2 (JMS) could not have been derived from adjacent Beartooths. Long Lake Magmatic Complex, Beartooth Mtns. 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. The metasedimentary rocks (JMS) are interpreted as a turbidite sequence deposited in a mid- to distal-fan setting based on the type and distribution of preserved primary sedimentary structures. These rocks were most likely deposited along an active continental margin. The source area(s) represent a bi-modal association of mafic and tonalitic rocks. The age of deposition is constrained between Ga based on detrital and magmatic zircon ages. These rocks could not have been derived from the contemporaneous, adjacent Long Lake Magmatic Complex (interpreted as a continental arc). Two quartz monzonitic plutons have been emplaced at high crystal levels (~12 km), at 2.8 Ga, are peraluminous, and have distinct compositions compared with the LLMC. Although their emplacement is contemporaneous with the LLMC, they appear to have had a different source. Mafic to intermediate plutons intruding the JMS are also 2.8 Ga, but are geochemically identical to rocks of the LLMC. Metamorphism is relatively low-grade ranging from chlorite zone in the west to staurolite-andalusite zone in the east; the low pressure, high temperature metamorphism is of a “Buchan" style. An injection migmatite complex is located at Garnet Hill, and a migmatitic orthogneiss (3.2 Ga) and garnet leucogranite occur in the Vantage Butte areas. Polyphase deformation includes rare isoclinal folds and two generations of kink folds that pre-date the plutons. Mylonitic shear zones cut both the plutons and the metasedimentary rocks. Details of these results can be seen in the adjacent posters. Sketch map showing the locations of the Jardine (J) area in the South Snowy Block, Yankee Jim Canyon (YJC), and locations in the main Beartooth massif. Because of these differences in lithology, metamorphism, structural style, and magmatism, earlier work (Mogk, 1984) interpreted these rocks and adjacent rocks in the South Snowy Block as an allochthonous unit, tectonically emplaced in the latest Archean. The purpose of this study is to test that hypothesis. Metasedimentary packages at Bear Creek Some of the key research questions addressed in this project include: 1) What is the relationship of this suite of rocks to the voluminous 2.8 Ga magmatic rocks of the main Beartooth massif to the east and the high-grade, migmatitic and mylonitic gneisses of Yankee Jim Canyon to the west? 2) What are the ages, provenance(s) and depositional environments of the metasedimentary rocks? 3) What is the igneous, metamorphic, and structural history of this area, and what does this tell us about crustal genesis and evolution? 4) Is this suite of rocks allochthonous in their current setting (Mogk, 1984), or do they represent unusual preservation of upper crustal rocks in the midst of mesozonal plutons and higher grade metamorphic sequence exposed across the northern Wyoming Province?