NEW U-PB DATES OF SYN-DEFORMATIONAL MINERALS THAT DIRECTLY DATE MULTIPLE TECTONIC EVENTS AT 1.75 AND 1.62 GA ALONG THE CHEYENNE BELT SUTURE ZONE, SOUTHEASTERN WYOMING

The Cheyenne belt, a network of mylonite zones exposed in the Medicine Bow Mountains, Richeau Hills, and Sierra Madre of southeastern Wyoming, has long been proposed as the primary suture between the Wyoming Archean craton and the Paleoproterozoic island arc terranes and back arc basins of the Colorado Province. The penetrative fabrics of the mylonite zones are northeast-trending, steeply southeast-dipping, and contain steeply plunging lineations that record ductile deformation under amphibolite-grade conditions. However, the timing of the collision, known as the Medicine Bow Orogeny, has previously only been constrained to ca. 1.78 Ga – 1.74 Ga based on U-Pb zircon ages of pre- and post-deformational granites. New results from the Medicine Bow Mountains directly date the timing of the Medicine Bow Orogeny: U-Pb zircon analysis from the syn-deformational Albany granite (1753 ± 3 Ma) and U-Pb dates of syn-deformational sphene from the French Creek Shear Zone (1748 ± 6 Ma) and the Barber Lake Block (1748 ± 10 Ma) are interpreted to indicate that the amphibolite-grade fabrics of the mylonite zones developed at 1.75 Ga.

In the Medicine Bow Mountains, the mylonite zones of the Cheyenne belt were reactivated by a greenschist-grade fabric that is defined by sub-vertical epidote-chlorite-tremolite ± sphene veins that crosscut the mylonitic fabrics at high angles and contain sub-horizontal lineations. These fabrics are non-penetrative and unevenly spaced, and range in style from brittle offset across veins to ductile transposition of the pre-existing amphibolite-grade fabrics. Kinematic analysis of the greenschist-grade fabrics indicates that northwest/southeast-directed, subhorizontal shortening was accommodated by complex strike-slip movement along the veins. U-Pb analysis of syn-deformational epidote (1611 ± 36 Ma) and sphene (1634 ± 9.5 Ma) are interpreted to indicate that this fabric developed at 1.62 Ga when southeastern Wyoming lay far inboard of an active plate margin, and likely represents a previously unrecognized tectonic event.

This study highlights the use of U-Pb dates of syn-deformational minerals to directly date the timing of the development of metamorphic fabrics. This technique can yield more precise ages of tectonic events than those from crosscutting relationships.