Geology and petrology of the Parkdale Gneiss, Fremont County, Colorado

The Farkdale Gneiss in Fremont County, Colorado, Is a massive homogeneous rock unit 9 miles long and 4 1/2 miles wide. The massiveness, homogenity, lack of relic structures, and porphyroblastic texture suggest an original igneous intrusive body. The long axis is in a north-south direction which is characteristic of the regional structural trends in the Colorado Front Range and Wet Mountains. The north-south strike of the enclosing metasedimentary rocks suggests the bedding served as a guide to the emplacement of the Farkdale unit.

The Farkdale Gneiss changes character from a strongly foliated, dark-gray gneiss in the southern part of the unit and a weakly foliated, pinkish-gray gneiss in the northern area. Mode analysis and zircon study indicate the two rock types to be members of the same intrusive body. Petrographic work shows that a gradation exists between the two rock types. The prophyroplastic character and pinkish color of the rock in the northern area are accompanied by a gradational increase in potassium-feldspar and corresponding decrease in biotite. It is postulated that the breakdown of biotite during subsequent metamorphism produced the potassium ion needed for the porphyr9blasts of potassium-feldspar.