NEOARCHEAN POLYMETAMORPHISM DURING CRUSTAL ASSEMBLY OF THE SOUTHERN WYOMING PROVINCE

The Wyoming Province is dominated by Mesoarchean to Neoarchean granitoids that have Pb and Nd isotopic signatures indicating derivation from older Archean to Hadean crust. The southern margin of the Wyoming province is composed of a series of accreted terranes (the Southern Accreted Terranes) that have isotopic compositions indicating younger sources than compose the rocks of the rest of the Wyoming Province. The Sacawee block of south-central Wyoming, which forms the southernmost extent of Paleoarchean crust in the Wyoming province, preserves key evidence about this Neoarchean accretion. Black Rock Mountain lies on the eastern end of the Sacawee block and exposes a complex series of deformation zones. The main portion of the mountain consists of metagabbro and metabasalt that is interlayered with pelitic gneiss. A NE-SW trending, steeply-dipping shear zone on the west end of the mountain marks the contact between these rocks and the 3.33-3.30 Ga tonalitic to granitic Sacawee orthogneiss. Another NE-SW trending shear zone on the east marks the contact with the ~3.4 Ga tonalitic UC Ranch orthogneiss. Locally the amphibolite on Black Rock Mountain is garnet-bearing, and records equilibration at 550° and 5.5 kilobars. NE-SW trending shear zones cutting Black Rock Mountain commonly contain pelitic rocks with coarse-grained, radiating porphyroblastic andalusite. The assemblage garnet-andalusite-sillimanite-biotite-quartz from these pelitic rocks yields temperatures and pressures of 550° and 3.5 kilobars. The deformation zones on Black Rock Mountain contain no lineation and indicate that the deformation in the eastern Sacawee block occurred by pure shear. The shearing was accompanied by extensive fluid flux, as indicated by the coarse-grained minerals in the deformation zones. The NE-SW trending foliations and steeply dipping deformation zones at Black Rock Mountain have the same trend as the shear zones farther west in the Sacawee block that have been dated at 2635 Ma. We interpret the polymetamorphism and NE-SW fabrics to record collision of the Sacawee block with the Southern Accreted Terranes shortly before the intrusion of the 2625 Ma Wyoming batholith, which stitches them together and completes the assembly of the Wyoming Province.