THE MESOPROTEROZOIC RECORD IN THE WET MOUNTAINS, COLORADO

The Wet Mountains of Colorado present a clear record of Mesoproterozoic plutonism, metamorphism, and tectonism. Three plutons, ranging from 1.47 Ga age in the north to 1.36 Ga in the south, intrude paragneisses, Kfeldspar-biotite migmatites, amphibolites, and granitoid gneisses. HT-LP metamorphism was coeval, based on regional 40Ar/39Ar hornblende closure ages and on ~1.425 Ga growth of monazite included within cordierite poikiloblasts, determined from in situ U-Th-Pb analysis. Monazite overgrows crenulation cleavage preserved within cordierite, and cordierite gneisses are in turn deformed within kms-scale, E-plunging folds. The folds are sharply truncated by the Five Points Gulch shear zone, a steep ~NNW-SSE deformation zone marked by moderately N-plunging sillimanite lineation. Along the margin of the 1.44 Ga Oak Creek pluton, NE- to E-directed extensional fabrics overprint the regional foliation. Thus, the Wet Mountains record regional ~N-S contraction and ~E-W extension, accompanied by transcurrent faulting.

New mapping reveals widespread, penetratively deformed Kfs-Bt granitoids in the Wet Mountains. Two large intrusions are the Williams Creek orthogneiss in the southern Wet Mountains and the Newlin Creek orthogneiss in the north. Their fabrics show SE-to SW-verging reverse kinematics, and they are cut by fine-grained granite dikes with concordant foliation. Thus the fine-grained dikes are syntectonic, and together with the orthogneiss they bracket the time of deformation. New U-Pb zircon ages for both the Williams Creek orthogneiss and the crosscutting granite are ca. 1.43 Ga, constraining ~N-S contraction to be approximately this age.

Further geochronology is needed to elaborate the timing and extent of Mesoproterozoic plutonism in the Wet Mountains. However, it is clear that sustained HT metamorphism continued until ~ 1.36 Ga and emplacement of the San Isabel pluton, based on U-Pb ages on metamorphic titanite in the orthogneiss and 40/39Ar geochronometry. A strong positive magnetic anomaly over the Wet Mountains suggests extensive Fe enriched plutonic rocks, such as a Mesoproterozoic A-type batholith. If so, the Wet Mountains may reflect the presence of a shallower Mesoproterozoic heat source, or expose a deeper crustal level than other Precambrian exposures in Colorado and New Mexico.