MUSINGS ON THE PRECAMBRIAN ROCKS OF THE COLORADO FRONT RANGE

In 1874 A.R. Marvine described the “Archean" of the Front Range and remarked: ”…when sufficient time is expended on their examination I conceive that some exceeding interesting and clear results will follow”. Subsequent studies have amply born out his speculation, although we are still a lot longer on interesting results than on clear ones! Marvine produced the first real geologic map of parts of the Front Range basement rocks; since then mapping has progressed from studies of mining districts to detailed mapping of major parts of the range, to modern compilations of most of the basement geology.

Radiometric dating the 1960s had an enormous impact on understanding of the Front Range basement rocks---it allowed recognition of the various ages of plutonic rocks and to an understanding of the timing of metamorphism and deformation in the enclosing rocks. It also precipitated the first efforts in the U.S to develop a time scale for the Precambrian as a result of questions from P.B. King on how to depict throcks of the Front Range and adjacent ranges on his 1976 Geologic map of the United States.

Until development of plate tectonic theory, no regional tectonic interpretation of basement rocks of Colorado and adjacent states had been suggested. However, in 1979 it was proposed that most of the Proterozoic rocks of the southwestern states were the products of accretion of island arc systems to the southern edge of Archean North America. This interpretation has been widely accepted and refined, and has guided much of the research on the Front Range basement rocks for more than two decades. However several workers have pointed out some of the difficulties with this hypothesis and are currently searching for alternative explanations.

While further mapping may contribute to the solution of this dilemma, it is probable that the application of new techniques that are beginning to be applied may be more fruitful. These include seismic profiling, studies of P-T metamorphic history, application of new structural techniques, and more widespread applications of detailed geophysics. Maybe these will help produce some of the clear results that Marvine anticipated so long ago.