TECTONIC JUXTAPOSITION OF CONTINENTAL AND OCEANIC SEDIMENTARY BASINS IN LATE ARCHEAN TIME, RATTLESNAKE HILLS, CENTRAL WYOMING

The Rattlesnake Hills of Central Wyoming preserve two supracrustal successions of contrasting origin and metamorphic grade that were tectonically juxtaposed in Late Archean time. Neither the Barlow Gap sequence nor the McDougal Gulch sequence is a greenstone belt in the classic sense because they are dominated by clastic sedimentary rocks rather than basalt. Although they are contemporaneous, their contrasting Nd isotopic signatures require that they formed in continental and oceanic environments, respectively.

The ensialic Barlow Gap sequence consists of quartzite, fuchsitic quartzite, pelitic schist and banded iron formation overlain by amphibolites with minor intermediate volcanic rocks. This unit is in depositional contact with the Sacawee Orthogneiss which we have dated at ~3.3 Ga and has been metamorphosed at upper amphibolite facies. A dacite within the amphibolites yields a U-Pb zircon age of ~2.82 Ga. Pelitic schists and amphibolite within the Barlow Springs give initial eNd values that range from –1.8 to –2.6.

The oceanic McDougal Gulch sequence contains dacites dated at ~2.86 and ~2.73 Ga suggesting that it was deposited synchronously with the Barlow Gap sequence. The McDougal Gulch sequence is dominated by metabasalt and graywacke that lie stratigraphically above the 2.73 Ga dacite. eNd from the graywackes at 2.73 Ga range from 3.0 to 4.0 suggesting that these graywackes are isotopically juvenile. The McDougal Gulch sequence was metamorphosed in lower amphibolite facies.

The Barlow Gap and McDougal Gulch sequences were tectonically juxtaposed at ~2.65 Ga. A mylonitic zone is exposed in McDougal Gulch where the Sacawee Orthogneiss is in tectonic contact with the lower McDougal Gulch sequence. The timing of collision is constrained by a U-Pb age of ~2.65 Ga on syndeformational sphene from a sheared mafic dike. The juxtaposition of supracrustal assemblages of contrasting provenance suggests that the juvenile McDougal Gulch sequence was laterally accreted to the southern Wyoming province in Late Archean time.