U-PB ZIRCON GEOCHRONOLOGY OF PROTEROZOIC GRANITES OF THE TAYLOR RIVER AREA, SW SAWATCH RANGE, GUNNISON COUNTY, CENTRAL COLORADO

Polydeformed, supracrustal metasedimentary and mafic metavolcanic rocks of the Black Canyon Succession, exposed in the SW Sawatch Range of central Colorado, experienced at least two phases of granitic magmatism during the Proterozoic. These phases are manifest as three distinct granitic plutons, two that are coarse-grained and contain a well-developed foliation and a third that is fine-grained and relatively undeformed. These intrusive rocks were mapped by earlier workers (Tweto, 1972; Montrose, CO 1¢ª x 2¢ª) and interpreted to be Paleoproterozoic (Xg) and Paleoroterozoic or Mesoproterozoic (YXg) respectively based on texture and presence or absence of deformation. LA-ICP-MS analysis of zircons from these plutons constrains the timing of magmatism and provides new temporal constraints for fabrics that are crosscut by or related to these Proterozoic intrusive phases.

The oldest intrusive unit, the Taylor River granite, is a coarse-grained to megacrysticgranite, that was emplaced ca. 1724 Ma and contains a localized steeply dipping, to nearly vertical, NNW striking foliation, which is consistent with the dominant fabric in neighboring country rocks. In some areas, the granite contains a well developed foliation defined by large, tabular K-feldspar megacrysts that has an average orientation of 220/56 WNW and is interpreted to be magmatic in origin. Subsequent deformation within the Taylor River granite is observed to be limited to localized shear zones, parallel to dominant fabric in the area. This granite is cut to the SW by a much finer-grained granite that is relatively undeformed. Most zircons from this granite give Paleoproterozoic ages, but a few small, euhedral grains yield Mesoproterozoic ages. We interpret the older ages to reflect inheritance and suggest instead that this fine-grained granite crystallized ca. 1450 Ma. The third granitic pluton is exposed 5-10 km NW of Taylor Park Reservoir, and it is texturally and compositionally similar to the Taylor River granite. Zircon analysis indicates that this pluton crystallized ca. 1470 Ma, and its relationship to wall rock fabrics and structures is unknown.