STRUCTURE AND METAMORPHISM OF THE EOLUS GRANITE, NEEDLE MOUNTAINS, COLORADO: IMPLICATIONS FOR REGIONAL METAMORPHISM AND PROTEROZOIC TECTONICS IN SOUTHERN COLORADO

The 1.44 Ga Eolus Granite makes up the core of the Needle Mountains in southwestern Colorado and is composed of two separate batholithic plutons. Our study area is within the metamorphic country rocks that sit in between these two batholiths in the Vallecito River Valley. Along the eastern contact of the Eolus Batholith within the Vallecito River Valley, a well developed contact aureole occurs within the Irving Formation. Far from the pluton mafic schists commonly contain actinolite+ biotite + chlorite + plagioclase + quartz whereas near the pluton actinolite is rimmed by hornblende, and is locally completely eliminated from the mineral assemblage. Additionally, in felsic gneiss sillimanite and K-feldspar occur together along the margins of the pluton. These relationships suggest that regional metamorphism reached the upper greenschist facies, and that amphibolite facies metamorphism is restricted to the margins of the pluton. Three foliations are present within the country rocks. The S0-1 foliation is an east-west trending bedding-parallel cleavage associated with isoclinal folds and is only locally preserved. The S2 foliation is the dominant foliation in the study area. It strikes north-south and transposes S0-1 throughout much of the study area. It is associated with sheath folds attesting to intense ductile shearing during west side up contraction. The S3 foliation is an east-west striking subvertical foliation found only near the margins of the Eolus Granite. This foliation is defined by sillimanite and hornblende, indicating it developed during contact metamorphism. L3 mineral lineations plunge down dip. Kinematic indicators within S3, record north side up shear. Since the S3 foliation transects the contact of the pluton at a high angle, it cannot have developed due to ballooning of the pluton. Early deformation during the development of S1 is cryptic. S2 developed during east-west contraction and greenschist facies metamorphism. S3 developed during amphibolite facies contact metamorphism and north-south shortening. These relationships suggest much of the amphibolite facies metamorphism within the Needle Mountains may be Mesoproterozoic, not Paleoproterozoic as previously thought.