Telling the Geologic Story of Fremont County

A 523-million-year-old rock has settled in on the Geology Time Trail by Cindy Smith

A recent addition to our Geology Time Trail is a huge black and white boulder which sits comfortably at the 523-million-year time period.  This striking rock is called syenite (SIGH-en-night), and is abundant in the McClure Mountain Complex southwest of Cañon City.

The Crossroads Through Time are delighted to have this boulder for two reasons:  1) it provides part of the story of the complex uplift of the Wet Mountains about 60 million years ago, and 2) pieces of this particular boulder reside in laboratories all over the world as the standard measurement for age-dating rocks.

Paul Garrett (on the left) and Director Tony Adamic (on the right) of the Fremont County Department of Transportation delivering the syenite boulder

What is syenite?  For starters, it’s black and white.  It’s coarse grained, much like granite, but lacks the quartz content of granite (20-60% quartz in granite versus <5% in syenite).  It is primarily composed of feldspar (white), hornblende (black), and biotite (shiny black mica).  Sometimes there is a green tinge to it caused by very low degrees of partial melting, and then it’s called nepheline syenite.  Syenite is an intrusive rock, meaning it flowed as magma into older rock. and then solidified in place.

Placing the syenite boulder at the 523 million year time period on the Geology Time Trail

Interesting facts about the McClure Mountain Complex:

– The Complex is a 3.5 square-mile igneous intrusion.

– This Cambrian-aged syenite (523 million years old) intruded into much older rock, a Precambrian migmatite gneiss (1.7 billion years old).  Thus, the McClure Mountain Complex syenite is over a billion years younger than surrounding rocks.

– It has been age dated (at the time of crystallization) to 523.98 ± 0.12 million years using U-Pb (uranium-lead) geochronology (dating of rocks) on chemically abraded zircons.  U-Pb age dating is like having a birth certificate for the rock, and is the most common dating method used to date rocks older than 1 million years.

– Zircon is a gemstone mineral that can contain traces of radioactive elements, and by measuring the rate of decay (ratio of daughter elements to the parent), the age of the rock can be determined.  A half-life is the time required for half of the atoms to decay.  A few of the relevant half-life decay rates:

uranium 238 decays to lead 206 in 4.5 billion years

uranium 235 decays to lead 207 in 710 million years

thorium 232 decays to lead 208 in 14 billion years (about the age of the universe)

potassium 40 decays to argon 40 in 1.25 billion years

– Because the McClure Mountain Complex syenite is capable of consistently reproducing dates, it can be used as a standard by geologists and scientists.

– The McClure Mountain Complex syenite was first described in the early 1960’s by USGS geologists Raymond Parker and Fred Hilderbrand.

After Steering Committee members located the syenite complex near Copper Gulch Road, the Fremont County Department of Transportation generously provided transport to the Geology Time Trail. Deep thanks to Director Tony Adamic and Paul Garrett for generously donating their time and effort to bring this world-famous rock to our trail.

McClure Mountain Syenite

Dr. James Metcalf, Research Associate and Lab Manager for the Laboratory for Thermochronology, Department of Geological Sciences, at the University of Colorado, Boulder, summarizes the importance of the McClure Mountain Syenite:

The McClure Mountain Syenite is part of a Cambrian (~523 million years-old) igneous complex exposed today in the Wet Mountains of Colorado, about 13 miles west of Cañon City. The igneous complex formed when large batches of magma intruded into much older (~1700 million years old) igneous and metamorphic rocks, cooled, and crystallized deep below the surface. Later tectonic activity and mountain-building events (including the formation of the modern-day Rocky Mountains) helped bring these rocks to the surface. McClure Mountain is one of four similar Cambrian igneous complexes in the area, the others being the Gem Park, Democrat Creek, and Iron Mountain complexes.

Syenite is the name given to a specific type of igneous rock that is relatively low in silicon and consequently has very little, if any, of the mineral quartz. Instead, the mineralogy is dominated by dark-colored minerals rich in iron and magnesium, as well as feldspars rich in potassium, calcium, and sodium. The McClure Mountain syenite is primarily composed of 5 major minerals; potassium feldspar, plagioclase feldspar, hornblende, biotite and clinopyroxene.

The McClure Mountain Syenite is famous with geologists all over the world for two main reasons: First, it contains an unusually diverse assortment of accessory minerals (minerals that make up a small percent of the total volume of the rock but are very useful to geologists and geochemists), including apatite, zircon, titanite (sphene), nepheline, magnetite, ilmenite, baddeleyite, zirconolite, calcite, as well as various iron-sulfide minerals. Igneous rocks of this age are relatively rare in Colorado, so this rock and these minerals help geologists decipher an important part of the geologic history of the western United States.

The second reason the McClure Mountain Syenite, and this boulder in particular, is so well known around the world, is that it contains a mineral that is used as a standard by geochronologists. The mineral hornblende is a long and shiny black mineral that is composed of about 1.5% potassium. About 0.01% of all potassium is radioactive, some of which will decay to argon. This decay scheme forms the basis of one of the most popular and versatile geochronology (i.e geologic age-dating) methods. It turns out that the hornblende found in the McClure Mountain Syenite is incredibly uniform in composition and very well preserved, and as a consequence has been used as a dating standard in laboratories all over the world. In fact, if you visit any potassium-argon dating lab you’ll undoubtedly find a vial of hornblende labeled “MMhb-1,” which was collected from this very boulder.

Further Reading on the McClure Mountain Syenite can be found here:

Armbrustmacher, T.J., 1984, Alkaline Rock Complexes in the Wet Mountains Area, Custer and Fremont Counties, Colorado: USGS Professional Paper, v. 1269, p. 1 – 33.

Bickford, M.E., Cullers, R.L., Shuster, R.D., Premo, W.R., and Van Schmus, W.R., 1989, U-Pb zircon geochronology of Proterozoic and Cambrian plutons in the Wet Mountains and southern Front Range, Colorado, in Geological Society of America Special Papers, Geological Society of America, Geological Society of America Special Papers, v. 235, p. 49–64, doi: 10.1130/SPE235-p49.

Schoene, B., and Bowring, S.A., 2006, U–Pb systematics of the McClure Mountain syenite: thermochronological constraints on the age of the 40Ar/39Ar standard MMhb: Contributions to Mineralogy and Petrology, v. 151, p. 615–630, doi: 10.1007/s00410-006-0077-4.