WAS THE CHEYENNE BELT ONCE A TRANSFORM MARGIN?: IMPLICATIONS OF DETRITAL ZIRCON AGES FROM THE GREEN MOUNTAIN ARC AND BARBER LAKE BLOCK, SOUTHERN WYOMING

The Barber Lake block is a narrow (~3 km), dominantly metasedimentary terrane lying within the composite Cheyenne belt in southern Wyoming. It is separated from the Archean Wyoming province to the north and the Green Mountain arc to the south by the Central and Southern mylonite zones, respectively. The Green Mountain arc is a ~1780 Ma oceanic arc terrane that was accreted to the southern margin of the Wyoming province at ~1750 Ma. Previous workers interpreted the Barber Lake block as a possible arc-marginal or intra-arc basin developed far from a continent. The current proximity of the Barber Lake block to the Green Mountain arc suggested that they may have formed together in a single Green Mountain arc system. Detrital zircons were analyzed from metasedimentary rocks of the Green Mountain arc and the Barber Lake block using LA-ICP-MS. Probable igneous detrital ages from the Green Mountain arc form a single peak at 1805 Ma with no Archean grains present. Conversely, ages from the Barber Lake block show a dominant peak at 1872 Ma with scattered older ages up to 2983 Ma. Statistical modeling limits any contribution from the Green Mountain arc in the Barber Lake block to a maximum of 2%, suggesting no contribution at all. Therefore, the Barber Lake block was probably formed far from both the Wyoming craton and the Green Mountain arc. Possible source regions for the ~1872 Ma zircons in the Barber Lake block include the Trans-Hudson and Penokean orogens. Either of these regions could have been along strike of the Cheyenne belt between 1872 and 1750 Ma depending on the tectonic model chosen for the suturing of the Wyoming and Superior cratons. In either case, the Barber Lake block could represent a tectonic sliver emplaced by significant margin-parallel transport. Implications for the Cheyenne belt include: (1) a possible transform history sometime between 1872 and 1750 Ma, and (2) the possibility that the southern margin of the Wyoming province was truncated during transform faulting prior to Green Mountain arc accretion. If continuing work can identify a specific Trans-Hudson and Penokean affinity for the Barber Lake block, it would provide an additional test of competing suturing hypotheses for the Wyoming and Superior cratons.