EARLY PROTEROZOIC ULTRAMAFIC AND MAFIC ROCKS FROM THE BADGER FLATS REGION, PARK COUNTY, CENTRAL COLORADO

Ultramafic rocks are rare in the Early Proterozoic basement of Colorado, and we report the only known occurrence in the southern Front Range. At Badger Flats south of Tarryall, a thin sill-like body of peridotite is exposed for 3 km along strike within a thick sequence of sillimanite-grade metapelites and minor interlayered amphibolites (mostly metabasalts). All the layered rocks and the sill are cut by the oldest calc-alkaline plutons of the region (ca.1.7 Ga), and all experienced regional metamorphism just prior to that. The peridotite contains primary olivine, orthopyroxene, and spinel, overprinted by two metamorphic amphiboles (anthophyllite and magnesio-hornblende, the latter forming oikocrysts > 1 cm across). It is low in silica (41-44%), high in MgO (19-25%; Mg# averages 65), and enriched in Cr (800-1600 ppm) and Ni (400-1100 ppm). Compatible and incompatible trace elements average 3-5x primordial mantle values, with greatest enrichment in U and minor depletion in Nb and Sr. REE values are nearly flat at about 10x chondrite, with Gd/Yb(N) approx.1.0 and La/Sm(N)=1.0-1.5. Geochemically the peridotite resembles a group of metavolcanic rocks of similar age termed komatiites and komatiitic basalts which Boardman (2000) studied near Mt. Ouray, 80 km to the southwest. Spatially associated metabasalts are tholeiitic with primitive island-arc affinities, typical of metabasalts throughout the southern Front Range (Folley, 1997). There is no physical or chemical evidence that the peridotite is a cumulate from the fractionation of these basalts, nor are there any possible felsic differentiates of the basalts nearby. The peridotite is thus interpreted to have formed from an ultramafic magma representing a higher degree of partial melting of a hotter Early Proterozoic mantle, perhaps within a back-arc extensional setting.