IS THE CHEYENNE BELT REALLY A PALEOSUBDUCTION ZONE?: TECTONIC IMPLICATIONS OF NEW GEOLOGIC MAPPING AND U-PB AGES FROM THE BIG CREEK GNEISS

The Cheyenne belt is a major tectonic boundary in southeastern Wyoming, juxtaposing juvenile Paleoproterozoic crust with Archean crust to the north. This tectonic zone has been interpreted as a paleosubduction zone along which the Green Mountain arc was accreted during an arc-continent collision at ~1777 Ma (e.g., Tyson et al., 2002). The southward dip of the inferred subduction zone is based on the dip of the mylonite zones as well as the lack of arc-aged rocks north of the Cheyenne belt.

Geologic relationships and new U-Pb zircon ages from the Big Creek gneiss (BCG), a high-grade composite unit in the southeastern Sierra Madre, southeastern Wyoming, indicate: (1) intrusion of a bimodal arc-related gabbro-tonalite suite into paragneiss at ~1780 Ma; (2) intrusion of a bimodal Fe-rich gabbronorite-granite suite (with strong affinities to bimodal A-type suites worldwide) into the arc at ~1763 Ma; and (3) deformation, burial, and metamorphism at ~1750 Ma, apparently coeval with Cheyenne belt deformation.

In addition, new detrital zircon ages from the Barber Lake block (a fault-bounded paragneiss unit within the Cheyenne belt) and BCG show distinctly different populations. Grains from the Barber Lake block yield Archean and Trans-Hudson ages, but no arc ages. Conversely, the BCG samples yield Trans-Hudson and arc ages, but no Archean ages. These results suggest that juxtaposition of the blocks in the Cheyenne belt involved significant tectonic transport.

The intrusion of the mantle-involved A-type suite, possibly in an extensional regime, prior to Cheyenne belt deformation suggests that the accretion event is not simply an arc-continent collision. Furthermore, if the arc and A-type suites are allochthonous, as suggested by the detrital zircon data, the lack of Paleoproterozoic intrusions in the Archean basement does not constrain the subduction dip direction. We therefore suggest two competing tectonic models: (1) the arc accreted above a south-dipping subduction zone, followed by slab breakoff (and A-type magmatism); or (2) the arc formed over a north-dipping subduction zone as a fringing arc and was later extended (A-type suite). In either model, the arc was then accreted to the Wyoming province along the Cheyenne belt, possibly as a result of back-arc shortening in a subduction system to the south.