Early Proterozoic metasediments from north-central Colorado: metamorphism, provenance, and tectonic setting ( USA).

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. 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 S Wyoming in which unroofed Proterozoic granitic rocks and quartzites of the Snowy Pass Supergroup served as the principal sources.-from Authors