TECTONO-STRATIGRAPHY OF THE MESOARCHEAN SACAWEE BLOCK, CENTRAL WYOMING PROVINCE

Mapping and geochronological studies demonstrate that the Sacawee block of central Wyoming extends well beyond the locations where it was originally described: Mesoarchean basement gneisses and two overlying supracrustal cover sequences of the Sacawee block are now known to extend from the type localities at Barlow Gap and Beulah Belle Lake areas of the central Granite Mountains across the western Granite Mountains to South Pass at the southern end of the Wind River Range.

Basement gneisses include the medium- to coarse-grained leucocratic 3.30 Ga Sacawee Orthogneiss, which contains distinctive, 1-3 cm long, alkali feldspar porphyroclasts. The Sacawee Orthogneiss has been identified at West Sage Hen Rocks, Stampede Meadows, and Tin Cup Mountain in addition to Barlow Gap and Beulah Belle Lake areas. Two newly-dated samples yield precise U-Pb ages and no evidence of older, inherited zircon.

The overlying Barlow Gap group consists of a rift-drift cover sequence of quartzite, pelitic schist, iron-formation, and metabasalt. One Barlow Springs group quartzite contains a single population of detrital zircon grains yielding a date of 2853 ± 6 Ma, similar to previous age determinations for rhyolite and dacite in the Barlow Springs group of 2.86-2.83 Ga. Another quartzite from Stampede Meadows contains two populations of detrital zircon of ~2.9 and ~3.2 Ga; other quartzite samples lack the 2.8-2.9 Ga population and contain only > 3.2-3.3 Ga detrital zircons.

The ~2.7 Ga Rattlesnake Hills group is composed of metabasalt, local felsic metavolcanic rocks, and abundant biotite paragneiss interpreted as metagraywacke. The biotite paragneiss, which is characterized by radiogenic initial e_Nd values, is relatively zircon poor. Most zircons are 2.66-2.72 Ga with sparse older grains. The biotite paragneiss in Stampede Meadows is partially melted; zircon from leucosome yielded an age of 2636 ± 2 Ma. This partial melting is interpreted as associated with accretion of juvenile terranes to the southern margin of the Wyoming province between 2.65 and 2.62 Ga; the 2.62 Ga Granite Mountains batholith was intruded shortly after accretion.

We note the similarities in ages and stratigraphy of the Sacawee block and the Slave province and suggest that both the Wyoming and Slave provinces could have been part of a single Archean supercontinent.