Late Archean metamorphic history of the Wind River Range, Wyoming

The Wind River Range of western Wyoming provides a unique exposure of Archean metasediments and metavolcanics and is an ideal location in which to study early crustal formation and the evolution of the thermal structure of the crustal. The metamorphic history of the WRR has been revised by new thermobarometric techniques, field mapping, and radiogenic dating of metamorphic minerals.; Mineral assemblages included in garnet from unusual high V and Cr metapelites of the Paradise Basin area indicate peak metamorphic conditions in the granulite facies (>9.5 kbar, 780°C). Matrix assemblages preserve lower grade conditions and reveal the retrograde history of the region. Many of the silicate and oxide minerals contain high levels of V and Cr, and K_D values have been calculated for V and Cr between garnet, sillimanite and hercynite. Electron microprobe U-Th-total Pb dating of included monazites from this area yield peak metamorphic ages of 2.62 Ga. Matrix monazite grains yield ages ∼200 Ma younger, which is likely the result of a pervasive late fluid.; Piston cylinder experiments have been undertaken in order to determine the P-T dependence of Zr in garnet as a thermobarometer. Results indicate that the garnet-zircon-quartz (GZQ) thermobarometer functions as a reliable indicator of P-T conditions, even in rocks which have been strongly retrogressed. The GZQ thermobarometer has been applied to samples from the Paradise Basin, indicating two separate periods of garnet growth and providing a new P-T path for these rocks.; Detailed field mapping in the Atlantic City region, southern Wind River Range, indicates a far more complex structure in these Archean supracrustal sediments than previously recognized. The 40Ar/39Ar ages of hornblendes from mafic schists within the mapping area yield ages of 2.65–2.62 Ga. These ages agree well with cooling ages for the adjacent Louis Lake Batholith (LLB), which has been determined to be a syn-metamorphic rather than a post-metamorphic intrusion. Unmetamorphosed mafic dikes which cross-cut the LLB have been dated at 1575–1435 Ma.