Precambrian geology of the northern Sangre De Cristo Range, Chaffee, Fremont, and Saguache counties, Colorado

Precambrian metamorphic and igneous rocks, Paleozoic sedimentary rocks, and Tertiary igneous and sedimentary rocks crop out in the northern Sangre de Cristo Range. Miocene (?) age normal faults, Late Cretaceous or Early Tertiary thrust faults, and Paleozoic (?) reverse faults have displaced Precambrian and Paleozoic rocks. The complex structural relationships between the Sangre de Cristo, Sawatch, and Mosquito Ranges are interpreted as typical basin-and-range block faulting.

Precambrian rocks consisting of foliated and nonfoliated metaigneous and metasedimentary rocks crop out over approximately 98 square miles. Metasedimentary rocks are subdivided into mappable units of migmatitic gneiss, porphyroblastic gneiss, granitic gneiss, and quartzite.

Rb-Sr age dates record thermal events at 1.7 b. y. and between 1.3 and 1.2 b. y. ago. Field mapping and metamorphic textures suggest three regional thermal events and six periods of igneous activity during Precambrian time. The earliest recognized thermal event is recorded in statically recrystallized minerals of upper amphibolite facies. The second thermal event produced synkinematically recrystallized minerals characteristic of lower amphibolite facies. The latest thermal event is recorded by upper amphibolite facies minerals which have recrystallized statically. The earliest thermal event is not recognized in the quartzite unit.

Metasediments dip north and northeast. Correlation of metasedimentary units with similar metasedimentary rock in an adjacent area to the north corroborates the presence of an east-plunging synform.

Mineral composition, bedding character, and primary structures suggest that the original sediments were largely arkosic sands, calcareous or dolomitic shales, thin limestones, and well-sorted quartzitic sands. An unstable continental shelf environment is inferred