DIRECT DATING OF DEFORMATION - U-PB AGE OF SYNDEFORMATIONAL SPHENE GROWTH IN THE PROTEROZOIC LARAMIE PEAK SHEAR ZONE

In this paper, we show that deformation can be dated by combining meso scopic and microscopic structural observations with an understanding o f metamorphic mineral reactions and U-Pb ages of newly grown sphene (t itanite). This approach can be used on a variety of rock types that ha ve been deformed at a wide range of metamorphic conditions. In an exam ple from the Proterozoic Laramie Peak shear zone of southeastern Wyomi ng, a single period of syntectonic sphene growth in sheared mafic dike s is documented both by a strong spatial relationship between deformat ion and metamorphism and by sphene microtextures. U-Pb analyses of sph ene separates give overlapping concordant or nearly concordant ages wi th a weighted mean of the Pb-207/Pb-206 ages of 1763 +/- 7 Ma. We inte rpret this direct age of deformation as evidence that the Laramie Peak shear zone records the cratonic response to the 1.78-1.74 Ga Cheyenne belt collisional event.