THE ARCHEAN WYOMING PROVINCE: NUCLEUS OF NORTH AMERICA

The Wyoming province contains some of the oldest rocks in North America; Nd isotopic evidence suggests that the crustal province was extensive by the Mesoarchean. It has been divided into three sub-provinces: the Montana metasedimentary province, the Bighorn-Beartooth magmatic zone (BBMZ), and the Late Archean southern accreted terranes (SAT). Preserved between the BBMZ and SAT is a ~70 km long, east-west striking block of Paleo- to Mesoarchean crust, named the Sacawee block, which preserves 3.8 Ga of geologic history.

Paleoarchean rocks of the Sacaweee block include tonalitic gneisses and peraluminous high-K granitic gneisses. In depositional contact with these gneisses is a cover sequence consisting of quartz arenite, pelitic schist, iron-formation, metabasalt, and graywacke. All are intruded by Neoarchean unfoliated biotite granites and Precambrian mafic dikes.

The oldest gneisses range in age from 3.45 Ma to 3.30 Ma. Two samples contain inherited 3.8 Ga zircons. Some gneisses are calcic with low K₂O/Na₂O similar to the average TTG. Others are calc-alkalic, peraluminous and with higher K₂O/Na₂O. REE patterns vary from those with HREE-depletion typical of TTG to those with negative Eu anomalies and flatter HREE similar to modern arc magmatism. Sr/Y ranges from high values of TTG to lower values of modern calc-alkalic arc magmas.

Hf and O isotopic compositions of 50 zircons were obtained on the same spots that were analyzed for U-Pb geochronology, including 9 analyses of 3.8 Ga zircon. Initial ε_Hf are all well below contemporary depleted mantle compositions. The initial ε_Hf values correspond to Hadean Hf depleted mantle model ages, suggesting derivation from a crustal reservoir that had been separated from depleted mantle for several hundred million years. 46 analyses are within error of mantle δ¹⁸O values; 4 are lower and may indicate derivation from high-temperature altered materials. No analyses have the elevated δ¹⁸O that would suggest interaction with high δ¹⁸O supracrustal material. The paucity of elevated δ¹⁸O in zircon may be consistent with crustal growth by mantle plumes; there is no evidence of modern-style plate tectonics in the Paleoarchean rocks of the Sacawee block of the Wyoming province despite the calc-alkalic, potassic character of some of the gneisses.