Metamorphic Rocks

Intro

As the name indicates, metamorphic (meta = change, morph = form) rocks are pre-existing igneous, sedimentary, or metamorphic rocks that have been altered, or metamorphosed, deep within Earth’s crust. The rocks changed form in response to intense fluctuations in temperature, pressure, shearing, stress, or chemical environment. During Colorado’s mountain-building events, the intrusion of igneous bodies increased the crustal temperature to result in contact and regional metamorphism. The dominant metamorphic rock types in Colorado are gneiss, schist, amphibolite, and quartzite.

Details

Contact metamorphism occurs when hot magma intrudes into cooler rock. The intrusion heats the surrounding rock, making the low-temperature minerals unstable. These minerals change to minerals that are stable at the new, higher temperatures. Contact metamorphism of the Leadville limestone created the Yule Marble.

Regional metamorphism occurs because pressure and temperature change over a broad area. Different pressure and temperature combinations create a variety of minerals. In the course of a drive through Big Thompson Canyon from Loveland to Estes Park, you traverse all of the mineral zones of regional metamorphism. The presence of biotite near the mouth of the canyon signals an area of low-grade metamorphic rocks. Farther up the canyon are the garnet and staurolite zones indicative of formations subjected to higher temperatures. Within five miles, you reach the community of Drake in the highest (sillimanite) temperature and pressure zone of regional metamorphism. Exposed in the hills above Drake are coarse-grained pegmatites that are vein-like offshoots of the batholith. The high-grade rocks and pegmatites are indicators that you are approaching the 600-square-mile granitic batholith that surrounds Estes Park.

Around Estes Park and Rocky Mountain National Park, the metamorphic rocks were raised to temperatures and pressures at or near their melting point. This gave rise to migmatites, an intimate mixture of igneous and metamorphic rocks. Migmatites are found throughout much of the Front Range.

Archean Metamorphic

The Archaean rocks (of the Owiyukuts Complex) are the oldest in Colorado. They were metamorphosed about 2.7 billion years before present. That means that the original rocks were even older. Unfortunately, there aren’t very many of them, only about 40 acres outcrop in extreme northwest Moffat County, Colorado along Beaver Creek.

Metamorphic folding

Rocks are subjected to intense pressures and temperatures during metamorphism. This makes them quite ductile and allows them to fold into weird and wonderful shapes. Sometimes they are folded multiple times such that early formed folds will be re-folded. The metamorphic rocks in Colorado endured three, and perhaps four, periods of ductile deformation.

Refolded Folds

The oldest folds in Colorado are in Precambrian metamorphic rocks. The layers were folded during different regional metamorphic events. When previously folded rocks were again subjected to heat and pressure, they were refolded and became refolded folds.

Early geologists studying the Precambrian structures of the Front Range found many clues indicating two periods of folding, but were unable to find a place where they could see both sets of folds in the same outcrop. Finally, they found an outcrop in Clear Creek Canyon east of Blackhawk that has both sets of folds.

It is fairly easy to pick out large-scale (many sq km) anticline and synclines. However, it requires careful searching to discern the smaller-scale (cm-sized or less) tight folds that were folded once during an earlier period of folding, then folded again and tightened by a second period of folding.

Preserved Original Features

Commonly the original features occurring in rocks are completely altered or destroyed by the recrystallization and/or deformation of the metamorphic process. However, there are places in Colorado where the original features can be discerned, even in high-grade metamorphic rocks.