The Colorado orogen, formerly called the Colorado
province, is a >500-km-wide belt of
oceanic
arc rock (1.78–1.65 Ga) that extends southward into
New Mexico and composes a major part of the
Proterozoic provinces of
southwestern United States. This transcontinental collisional event occurred during the
Paleoproterozoic (
Statherian Period). The
Wyoming sector of the Colorado orogeny was formerly called the
Medicine Bow orogeny. The eastern sector extends into the
High Plains and is called the
Central Plains orogeny. The boundary between the Colorado orogeny and the
Wyoming craton is the
Cheyenne belt, a 5-km-wide
mylonitic shear zone that
verges northward. The Cheyenne belt transects and cuts off the south edge of the older
Trans-Hudson orogeny. The Paleoproterozoic
volcanic and
sedimentary rocks that resulted from the Colorado orogeny underwent
metamorphism followed by plastic
folding under moderate pressure and temperature (PT) conditions (temperature about 500 °C and pressures in excess of 1.2
GPa). The metamorphism was accompanied by intrusion of intermediate
calc-alkalic rocks, such as the
granodiorites of the
Boulder Creek batholith. The accompanying
amphibolite facies metamorphism is characterized by
sillimanite and, locally,
garnet,
andalusite, and
cordierite. Contemporaneity of emplacement of the granodioritic rocks with folding is indicated by concordant
plutonic boundaries and by conformity of the internal structure (of
solid-state recrystallization) in the batholith with that in the
supracrustal wall rocks. Comparable mineral facies in the country rocks and batholiths indicate that emplacement took place at moderate depths. ==References==