EVOLUTION OF ARCHEAN ROCKS OF THE SOUTH SNOWY BLOCK, YELLOWSTONE NATIONAL PARK: RESULTS OF AN REU SITE PROJECT

Archean rocks of the South Snowy block provide important insights into the petrogenesis, architecture, and evolution of the northern Wyoming Province. The South Snowy block assemblage includes biotite schists, quartzites and minor iron formation of the Jardine Metasedimentary Sequence (JMS). These immature sediments preserve primary sedimentary structures and are interpreted as mid- to distal fan turbidites deposed along a continental margin. The sediments were metamorphosed to the chlorite zone in the west, and up to the andalusite-staurolite zone in the east closer to a tectonic boundary with older (3.2 Ga) high-grade orthogneisses. Thermobarometry results indicate metamorphic pressures of 3-4 kbar. The JMS was intruded by two bulbous quartz monzonite plutons emplaced at depths of about 12 km, a zone of injection migmatite, and a suite of quartz diorite sheets. U-Pb zircon data indicate crystallization ages of ca. 2.8 Ga for the quartz monzonite and dioritic intrusions. Geochemical data and inherited zircons suggest that the felsic intrusions are related to the dioritic rocks and were contaminated by the metasediments. The dioritic intrusions are geochemically similar to deeper-level (~20 km) orthogneisses in the Long Lake magmatic complex of the Beartooth Mountains. Detrital zircons from the turbidites constrain the age of deposition between 2.8 and 2.9 Ga and indicate that the detritus could not have been derived from the Long Lake complex. The iron formations are gold-bearing and dissimilar to the Cherry Creek sequences to the west, which contain marbles. The JMS turbidite package must have been emplaced onto this part of the Wyoming craton prior to plutonism at ca. 2.8 Ga. Polyphase deformation in the South Snowy block includes rare isoclinal folds and two generations of kink folds that pre-date the plutons. Mylonitic shear zones cut the plutons and metasedimentary rocks. The JMS assemblage represents the upper crust equivalent to the mid- to lower crustal exposures in the Beartooth Mountains. This 2.8 Ga tilted crustal section is cut by at least one major mylontic shear zone at the boundary between the JMS and the Angel Lake orthogneiss. This shear zone is tentatively interpreted as a major normal-sense detachment that juxtaposes lower and higher-grade rocks.