SYNTECTONIC EMPLACEMENT OF THE 1.44 GA OAK CREEK PLUTON, WET MOUNTAINS, COLORADO

Southern Colorado is host to a suite of 1.4 Ga A-type plutons. The Wet Mountains expose a number of these intrusions including the 1.44 Ga Oak Creek pluton. Controversy has centered on the origin of these 1.4 Ga A-type granites and the Wet Mountains serve as a prime locale to study this question. The Oak Creek Pluton, within the northern Wet Mtns is characterized by a strong NW-SE oriented structural grain. Lithologies include strongly foliated gneisses as well as several generations of intrusive rocks which exhibit foliations concordant with those in the country rocks. Lineations are variable with trends ranging over about 100„a. Two main populations of lineations occur, one being down dip and the other oblique and trending to the E-ESE. The southern margin of the Oak Creek Pluton dips moderately to the north-northeast, whereas the northern margin dips steeply to the south-southwest. Kinematic indicators along the margins of the pluton consistently record pluton side down normal displacements. The layered character of the intrusion and concordant country rock screens suggest the Oak Creek Pluton was constructed as a series of sills. The overall structure of the Oak Creek pluton is consistent with northeast-southwest directed extension. Sillimanite gneiss occurs within the pluton as concordant sheets within and throughout the wall rocks of the pluton. The sillimanite occurs as centimeter scale porphyroblasts, fibrolite mats and as pseudomorphs after andalusite. Sillimanite always occurs with biotite and K-feldspar and muscovite is absent, suggesting conditions above the stability field for muscovite and quartz. Metamorphic textures and mineral assemblages within the country rocks are consistent with peak metamorphic conditions between 650 and 800 C and pressures between 2 and 5 kbar. The occurrence of sillimanite after andalusite suggests an isobaric heating and cooling path, although an anticlockwise P-T path is also consistent with the textures. These relationships show that the Oak Creek pluton was emplaced during low-pressure, high-temperature metamorphism and northeast directed extensional deformation challenging the view that 1.4 Ga plutons are emplaced following metamorphism and deformation within host rocks.