STRUCTURAL GEOLOGY AND EMPLACEMENT HISTORY OF THE CARBON PEAK LACCOLITH, GUNNISON COUNTY, COLORADO

Carbon Peak is a mid-Tertiary pluton among about 15 similar intrusions in the vicinity of Gunnison – Crested Butte in Western Colorado which are described as the “laccolith triangle” by the Colorado Geological Survey. Whereas these laccoliths are prominent geologic and topographic features, few studies have focused on the mechanisms of emplacement of these upper-crustal intrusions. Using fundamental field geologic techniques and optical microscopy, we focused on identifying evidence of wall rock deformation, pre-full crystallization magmatic fabrics, and cross-cutting relationships to test the hypothesis that emplacement of Carbon Peak was accommodated by uplift of the Cretaceous Mesaverde and overlying Wasatch Formation. Carbon Peak pluton is very well exposed, but weathers to extensive talus fields and rock glaciers – only about 20% of the pluton-wall rock contact is exposed. The pluton consists of granodiorite – quartz monzonite porphyry with abundant large (2-6 cm) potassium feldspar phenocrysts. The pluton base is not exposed, but appears to have intruded in the lower part of the Paonia Shale member of the Cretaceous Mesaverde Formation (Gaskill and others, 1987). Excellent – but limited in extent – exposures of the western contact show a thin contact metamorphic aureole in the upper Mesaverde Formation, a thin (<10 m) chilled plutonic margin characterized by fine aphanitic to partly porphyritic quartz monzonite. A structure-contour map of the western contact shows a moderate west-dipping (~25 degree) upper contact which is conformable with the overlying west-dipping Mesaverde Formation. Potassium feldspar phenocrysts within 60 m of the contact show a magmatic fabric concordant with the upper plutonic contact. Field evidence support emplacement of Carbon Peak laccolith as an inflated sill that produced a minimum of 730 m uplift of the 660 – 1025 m thick pre-emplacement overlying sedimentary rocks.