HISTORY AND DEVELOPMENT OF CONCEPTS IN THE STUDY OF METAMORPHISM IN THE CONTEXT OF THE PRECAMBRIAN OF THE COLORADO FRONT RANGE: CONTRIBUTIONS TO THE FIELD BY MEMBERS OF THE COLORADO SCIENTIFIC SOCIETY

In the early days of geology, metamorphic rocks were an enigma. With the emergence of modern geology and the ideas of Hutton, Lyell, and Darwin came real progress in understanding the origin of metamorphic rocks. Nevertheless, it was common for metamorphic and Precambrian rocks to be mapped, labeled, or otherwise simply referred to as “basement”. This still occurs in many modern sedimentology or stratigraphy-related studies.

Precambrian metamorphic rocks of the Colorado Front Range attracted attention in 1858 when gold was found on Chicago Creek at Idaho Springs. Because the Tertiary mineralized veins are hosted in PC rock, it was important to understand the host metamorphics. Notable geologists and engineers of the era, including founders and early members of the CSS, produced detailed studies of mineralization and host rocks of the Colorado Mineral Belt in the Front Range. The area was further investigated by the 1874 Hayden Survey. In these early studies metamorphic rocks were not merely lumped as “basement”, but were broken into units that even today convey meaning to those who work the area, despite now outdated interpretations. Workers disagreed on interpretations of units as originally sedimentary, igneous, or volcanic, with even less agreement on age and contact relationships.

Studies ca. 1900-50 further refined unit descriptions and interpretations. Geologic quadrangle mapping in the 1950-70s by U.S.G.S. geologists - involving past and present CSS members, provided excellent field maps, unit descriptions, and a good base for further interpretive study. In the 1980s, a new direction of study was initiated by geologists, including CCS members, to interpret the regional Precambrian geology of Colorado in terms of modern plate tectonics theory, protolith determination, and geochemical associations. Interpretations of accretionary origin are being refined and questions of tectonic and stratigraphic associations, also provenance, remain to be addressed.