PRELIMINARY EVIDENCE FOR SYNTECTONIC EMPLACEMENT OF MESOPROTEROZOIC PLUTONS IN THE SOUTHERN WET MOUNTAINS, COLORADO OWEN CALLAHAN Hampshire College Sponsor: Steve Roof, Hampshire College INTRODUCTION As outlined by Siddoway (this volume), a growing body of work in the southwestern US supports syntectonic emplacement and dynamic metamorphism of Mesoproterozoic granite. Alternatively, the geochemical signatures of these plutons have long been attributed to anorogenic events (Cullers, et al, 1992). The area around Bear Creek and Williams Creek in the southwestern Wet Mountains, Colorado, is situated in a key geologic setting, capable of adding considerably to the “anorogenic” debate. This area is composed of Paleoproterozoic gneiss, in close proximity to two Mesoproterozoic plutons—the Bear Creek Granite, isotopic age of 1486+/-36 Ma (Bickeford, et al, 1989), and the San Isabel Granite, 1360 Ma (Cullers, et al, 1992). Additionally, two lines of evidence from the southern Wet Mountains suggest retrogression through amphibolite grade conditions circa 1365 Ma. Siddoway et al (2000) support regional overprinting of 500°C between 1369+/-4 Ma and 1342+/-6 Ma using 40Ar/39Ar Hbl closure from amphibolite north of the San Isabel pluton. Titanite in granite from the Bear Creek field area likewise records temperatures between 450-500°C at 1364 Ma (unpublished U-Pb titanite age, Jones and Connelly, UT Austin 2002). The evidence suggests that the southern Wet Mountains were heavily affected by Mesoproterozoic events. The month of July 2001 was spent mapping structural relationships and sampling in this rugged yet promising area with George Perkins of Colorado College. The common mineral textures and fabrics shared by metamorphic and plutonic units, coupled with the temperature and temporal constraints established by earlier workers, led to the hypothesis that dynamic metamorphism and subsequent retrogression through amphibolite conditions at 1365 Ma was synchronous with Mesoproterozoic plutonism. Work is currently underway to test this hypothesis through in-situ monazite geochronology and petrologic analysis. FIELD METHODS and DESCRIPTIVE ANALYSIS The Bear Creek field area is located on the southwestern flank of the Wet Mountains, just north of the junction of Bear and Williams creeks. The area is composed of Proterozoic units up-thrown against Phanerozoic sedimentary and volcanic rocks along the Wet Mountain Fault, a SE-NW striking Laramide feature. The fault scarp is heavily dissected, with local relief on the order of 500m, providing good access to the Proterozoic units. Detailed structural mapping and sampling of this 2 km by 3 km region was completed during July 2001, and added to a regional