THE OLDEST FRAGMENT OF THE CENTRAL WYOMING PROVINCE: THE SACAWEE BLOCK, NORTHERN GRANITE MOUNTAINS, CENTRAL WYOMING

The Sacawee block is a narrow belt (~10 km wide) of Archean rocks that extends for 70 km along the northern part of the Granite Mountains of central Wyoming. The block is comprised of a series of Middle Archean tonalitic and calc-alkalic gneisses, dominated by the ~3.3 Ga Sacawee orthogneiss, and two supracrustal sequences. The ~2.86 Ga Barlow Gap Group, composed of iron formation, quartzites, metapelites, and metavolcanic rocks, is the best preserved supracrustal unit of the Sacawee Block. However, west of Beulah Belle Lake, we recognize an earlier Middle Archean (>~3.1 Ga) supracrustal unit dominated by quartzites. The Sacawee block is distinguished from other parts of the Wyoming Province by very negative εNd values of gneisses and some metasediments at 2.6 Ga, which range between -10 to -13.7. Crustal residence model ages are between 3.6 and 3.9 Ga, and are the oldest of the central Wyoming Province. Less negative εNd values, between -1.1 and -8.5, characterize the Middle Archean granites of Beulah Belle Lake, as well as some Late to Middle Archean metasediments, suggesting that different sources contributed to this group of rocks. Two deformation events are recognized within the Sacawee block. A localized Middle Archean event is recorded in the portion of the block exposed west of Beulah Belle Lake. Here, Sacawee and related basement gneisses, quartzites, and ultramafic-mafic intrusive rocks have been metamorphosed and deformed to produce a foliation that has been folded into shallowly plunging isoclinal folds, with east-west trending axes. U-Pb zircon dating of a weakly foliated granite dike that lacks the primary deformational fabrics provides a minimum deformation age of ~3074 Ma. At West Sage Hen Rocks, a sheared amphibolite within the Sacawee block yields a U-Pb zircon age of ~2.65 Ga. This age corresponds to a Late Archean deformation that extends through the northern Granite Mountains. The Sacawee block is either an exposure of the ancient basement that underlies all of the Wyoming Province, or an exotic block that was accreted to the core of the Wyoming Province, perhaps at ~3.1 Ga. If the latter, lateral accretion of old crustal fragments was an important mechanism of crustal growth for the central Wyoming Province during the Middle Archean.