ISOTOPIC EVIDENCE FOR AN EARLY ARCHEAN CORE OF THE WYOMING PROVINCE: NORTHEASTERN GRANITE MOUNTAINS, CENTRAL WYOMING

Some of the oldest rocks of the Archean Wyoming province are exposed in the northeastern Granite Mountains of central Wyoming. Gneisses and supracrustal rocks in the Black Rock Mountain region (BRMR) and Barlow Gap area (BGA) of the Granite Mountains offer a window into the Early Archean history of the Wyoming province. Supracrustal rocks in this region contain varied amounts of quartzite, banded iron formation, dacite, rhyolite, amphibolite, basalt, and pelite.

Preliminary U-Pb zircon analyses from exposed gneisses yield ages ranging from 3.3 Ga to 3.6 Ga. Based on field mapping, we interpret this older suite of rocks as the basement onto which a younger succession of supracrustal rocks was deposited. U-Pb zircon data from a metarhyolite collected in the BRMR are interpreted to constrain deposition to ca. 2.86 Ga.

Preliminary isotopic (Rb-Sr, Sm-Nd) investigations of lithologic units exposed in the BRMR and BGA indicates the presence of pre-existing isotopically evolved crust as old as 3.8 Ga during the time of their formation. e_Nd values from basement gneisses and supracrustal rocks collected in the BRMR, calculated at 2860 Ma, range from –0.823 to –8.674, with depleted mantle Nd model ages ranging from 3.3-3.8 Ga. Interpretation of the isotopic data from the supracrustal sequence suggests these units were deposited in ensialic basins and are not related to the accretion of juvenile arc rocks. The analyses from basement gneisses in the study area have the most negative e_Nd values and oldest Nd model ages (3.7-3.8 Ga) of the central Wyoming province.

We interpret a prolonged geologic history in the Black Rock Mountain region and Barlow Gap area that extends back to at least 3.8 Ga. Furthermore, we suggest that the northeast Granite Mountains of Wyoming represent part of an old exposed core of the central Wyoming province.