CONTACT METAMORPHISM AROUND THE 1.4 GA EOLUS GRANITE, NEEDLE MOUNTAINS, SOUTHWEST COLORADO

We are investigating contact metamorphism around the NE margin of the Eolus granite in the Vallecito River valley, Needle Mountains Colorado. In the Vallecito Valley, the Eolus granite intrudes a diverse package of metavolcanic and metasedimentary rocks. A well developed contact aureole is present and records an andalusite-sillimanite type field gradient.

Amphibolites in the southern part of the study area contain chlorite, biotite, and actinolite at distances greater than 1 km form the pluton margin. Near the pluton margin, both chlorite and actinolite have been replaced by hornblende, indicating amphibolite to upper amphibolite facies metamorphic conditions.

In the northern part of the study area the granite intrudes quartzites and schists. At distances greater than 1 km from the pluton andalusite is present with chlorite, biotite and muscovite. Closer to the pluton sillimanite appears with muscovite, and at the pluton margin muscovite is eliminated from the assemblage.

Taken together, the mineral assemblages suggest that contact metamorphism occurred at pressures between 2 and 4 kbar, with peak temperatures near 650 C. We are working to refine these pressure and temperature estimates using quantitative thermobarometry. These results suggest that like many 1.4 Ga plutons, the Eolus granite was emplaced at relatively shallow levels within the crust.