MESOPROTEROZOIC DEFORMATION ALONG THE CHEYENNE BELT, SOUTHEASTERN WYOMING, AND IMPLICATIONS FOR CA. 1.4 GA A-TYPE GRANITES IN NORTH AMERICA

The tectonic setting for the intrusion of Mesoproterozoic A-type granites, which are widespread throughout the Proterozoic-aged crust of North America, has been the subject of recent debate. Although their geochemistry is most consistent with formation by partial melting of a mantle-derived tholeiite and has been interpreted to support an extensional setting, recent structural studies have documented syn-plutonic, reverse-sense deformation within the plutons. However, these studies have focused on pluton margins where emplacement-related effects may mask regional stresses. The Medicine Bow Mnts. of southeastern Wyoming provide an opportunity to study ca. 1.4 Ga deformation recorded in the absence of plutonism. Greenschist-grade, Mesoproterozoic fabrics are concentrated along the Cheyenne belt, a Paleoproterozoic suture between Archean and Proterozoic crust, suggesting that it acted as a zone of weakness during the continental-scale plutonic event. Only minor Mesoproterozoic plutons are exposed in the Medicine Bow Mnts., each at least 10 km from the major zones of greenschist fabrics, so reactivation of the Cheyenne belt may reflect regional tectonic stresses. Epidote-chlorite-actinolite-sphene fabrics are characterized by non-penetrative, subvertical slip surfaces and subhorizontal lineations that overprint higher-grade, accretionary-related fabrics. Syn-deformational sphene and epidote growth (established as Mesoproterozoic through U-Pb dating) are closely associated with the retrogression of hornblende and Ti-rich biotite to actinolite and chlorite. Preliminary structural analyses of slip directions using fault plane solutions are consistent with E-W to NE-SW extension. Local occurrence of ca. 1.4 Ga apatite U-Pb cooling ages in the Medicine Bow Mnts are interpreted to indicate differential unroofing along N-S-trending normal faults, which was coeval with NW-vergent thrusting along the Cheyenne belt in the Sierra Madre, 30+ km to the west. The variations in kinematics and style of deformation in these two ranges are more consistent with broad scale doming than with continent-scale shortening, and support Hoffman’s (1989) superswell theory. Broad mantle upwelling could also account for the widespread distribution of ca. 1.4 Ga plutons.