EVOLUTION OF THE PRECAMBRIAN ROCKS OF YELLOWSTONE NATIONAL PARK (YNP): LOW-PRESSURE METAMORPHISM OF THE JARDINE METASEDIMENTARY SEQUENCE

The Jardine Metasedimentary Sequence (JMS) is a low pressure regional metamorphic terrane composed of metapelitic, quartzitic, and minor meta-ironstone lithologies within the South Snowy Block of the Beartooth Mountains, Montana. Protoliths for these rocks are interpreted as active continental margin, turbidite deposits based on the lithologies and sedimentary structures. Fe-rich metapelites crop out along the northern boundary of YNP in the Black Canyon of Yellowstone River, from Bear Creek near Gardiner, MT to Hellroaring Creek and north to the YNP border. These biotite schists contain diagnostic metamorphic assemblages including chlorite-biotite in the western portion of the study area, increasing in grade to garnet-andalusite-chlorite-biotite and garnet-staurolite-biotite further to the east. Notable metamorphic assemblage transitions in the metapelites include the formation of garnet followed by staurolite and andalusite, and the breakdown of chlorite with increasing metamorphic grade. Staurolite occurs in a variety of textural settings, including large porphyroblasts co-existing with andalusite, abundant, small, euhedral crystals nucleating around andalusite, and euhedral grains in textural equilibrium with garnet. Muscovite is rare and typically occurs as a retrograde phase intergrown with andalusite porphyroblasts. Garnet occurs as porphyroblasts in a range of textural settings, including inclusions in andalusite, partially resorbed garnets surrounded by foliation, large, subhedral syn-kinematic porphyroblasts with rotated inclusion trails, and matrix garnet overgrowing foliation. Garnet is Fe-rich with compositions of Alm_73-82Prp_8-15Grs_3-5Sps_1-9. Slight prograde zoning in Sps is observed in some samples, but Fe/(Fe+Mg) is homogenous. Thermocalc was used to calculate average P-T for five biotite schist samples. Small variations in P-T were observed, ranging from 572-608°C and 4.1-5.8 kbar across the study area. Ti-in biotite thermometry yields consistent temperatures of 566-586°C. The JMS along with adjacent rocks in the Garnet Hill area to the E are of anomalously low metamorphic grade compared to other rocks of the northern Wyoming Province, and the JMS may represent a separate, distinctive unit that is allochthonous within this northern part of YNP.