Early Proterozoic metasediments from the Big Thompson Canyon area in north-central Colorado increase in metamorphic grade from the biotite zone to the sillimanite zone, representing a low-pressure facies series that probably developed in response to a steep geotherm over a thermal high. High-grade rocks are enriched in K2O, Al2O3, Fe2O3, MgO, TiO2, Cr, Sc, Cs, Rb, Ba, and light REE and are depleted in SiO2, CaO, and Zr relative to low-grade rocks. These differences chiefly reflect differences in the original sandstone/shale ratio.
Detrital grains in medium and coarse metasediments are quartz, quartzite, and feldspar, and they suggest a recycled orogen provenance in which granitic rocks and quartzites dominate in the source. The abundance of shale in the original succession also attests to the importance of granite in the source. Major- and trace-element contents of the rocks constrain the input of mafic to dacitic detritus to less than a few percent. REE patterns in all rocks are similar to Phanerozoic shales and cratonic sandstones. The Big Thompson Canyon metasediments may have been derived from an uplifted collisional orogen in southern Wyoming in which unroofed Proterozoic granitic rocks and quartzites of the Snowy Pass Supergroup served as the principal sources.