Metamorphic rocks are hard to work on and even harder to describe using common language. My goal in this article is to encourage you to pick up the new map, learn more about geology and the cryptic language of metamorphic petrology/ductile structural geology, and to improve my own ability to communicate complicated ideas simply and effectively. The BQR first saw metaspeak in a 1992 article by me and Karl Karlstrom. Unfortunately it's out of print—if you don't still have yours, find a friend who does. Another extremely valuable resource would be any dictionary of geological terms. I have added a short glossary of terms at the end of this article but will try to communicate the meaning of two-bit words in the context of a sentence. Words in bold face are defined at the end of the article. For those of you who want no-holds-barred technical beta, or if you suffer from insomnia, I recommend: Ilg and others (1996) and Hawkins and others (1996)—write me for copies at Box 85, Glorieta, NM 87535. Metamorphic pressure and temperature data are from Mike Williams (U. Mass) and rock ages are from David Hawkins (MIT). Finally, thanks to Karl Karlstrom for introducing me to the "Granite Gorge Metamorphic Suite".
The illustrations are from the new Geologic Map of Grand Canyon, available from Museum of Northern Arizona or the Grand Canyon Association—the colors I refer to are those on the map.

Canadian geologist Paul Hoffman titled a 1988 article United Plates of America: Birth of a Craton. The title suggests that continents, like nation states, are dynamic bodies, growing and shrinking through time. His article describes the means by which the North American craton formed, via plate convergence, collision, and subduction, by amassing fragments of continental material (similar to Borneo, Japan, Papua-New Guinea) and island arc chains (similar to Sumatra-Java, Aleutians) at its edges. The Grand Canyon reveals a bit of this story.
Fortunately, the Colorado Plateau has behaved as a crustal “knot” since Grand Canyon Supergroup time (ca. 1300-800 Ma; Ma=million years ago), that is, it has been largely immune to crustal-scale deformation. The shortening (crunching) deformation associated with the formation of the Rocky Mountains to the north and the extensional (stretching) deformation associated with the Rio Grande Rift to the east and the Basin and Range Province to the south and west did not significantly affect the Colorado Plateau region. Thus, features observable today indicate that the rocks were initially deposited in a submarine setting about 1.6-1.8 Ga, (giga-annum, or billion years) specifically in an island-arc environment similar to the Indonesia region. The rocks were assembled onto North America during a mountain building episode called the Yavapai Orogeny 1.7 Ga ago. However, during their assembly to North America they were buried by thrusting and folding to depths of 15-20 kilometers and heated up to 550-700 degrees Celsius, fundamentally changing (meta) their form (morph). The basement rocks then remained at depth from 1.7-1.4 Ga (300 million years). At 1.4 Ga a big thermal disturbance in the mantle melted big portions of the lower crust producing large plutons, or magma bodies, like the Quartermaster pluton (river mile 260). This same event caused the buoyant rise of the basement rocks (hot rock, like hot air, rises). As they slowly rose, some 15 to 20 kilometers of overlying rock were slowly eroded away. The basement rocks were finally exposed at the surface about 1.3 Ga, just before the Grand Canyon Supergroup rocks were deposited, and have remained essentially unchanged since.
The Colorado River carved down into the Proterozoic basement of the Colorado Plateau forming the Granite Gorges of the Grand Canyon. The great clarity of rock exposure in the gorges allow an unprecedented opportunity to study the results, and understand the processes, of continents in general and of the growth of North America in particular. Although the rocks of the Granite Gorge Metamorphic Suite (GGMS) and the plutonic rocks that were injected into them make up a small part of the Colorado Plateau, similar rocks form the basement to all continents. If you could dig down through the relatively flat lying surface rocks in Kansas or the ice of Antarctica, for example, you would find essentially the same rocks as those exposed in the Granite Gorges.

Rock Types
Six Proterozoic rock types have been mapped in the new 1:62,500 scale map: the three metamorphic units of the GGMS and three distinct plutonic rock groups. Rocks of the GGMS include a new unit called the Rama Schist and Gneiss (Ilg and others, 1996; shown in blue on the map and cross section), the Brahma Schist and Amphibolite (Maxson, 1936; green on the map), and the Vishnu Schist of Walcott (1894; orange on the map). The GGMS rocks were intruded by 1.74-1.71 Ga island arc-related plutons (pink on the map) similar to those forming under Sumatra today. Later, when the island arcs were buried and heated as they crashed into North America, they partly melted and squeezed into cracks and weaknesses, forming the 1.7-1.68 Ga pegmatite dikes that lace the canyon walls (red-orange on the map). The GGMS rocks were originally submarine volcanic rocks (Rama and Brahma) and fine grained submarine sedimentary rocks (Vishnu).

Granite Gorge Metamorphic Suite
Rama rocks are metamorphosed rhyolite to andesite flows and ash deposits similar to those that erupted from Mount Saint Helens in 1980. The best and most easily accessible example of Rama rocks is just above 127-Mile Rapid on river right in the Middle Granite Gorge (reddish color, not the black Brahma rocks)
Brahma rocks are metamorphosed basalts similar to the lava flows in Hawaii and Iceland. The best examples of Brahma pillow basalts are about 3 miles up Shinumo Creek and in “Pillow Basalt” canyon just below Travertine Grotto on the right. Other good Brahma rock examples are at Schist Camp (upper end of the beach), the upper beach at Blacktail, or up Specter Chasm.
Vishnu rocks are metamorphosed volcanic arc basin sediments similar to those being shed from the islands in the Indonesia archipelago. Most of the gray rocks of the Upper Gorge are Vishnu rocks. Relict bedding is preserved in Vishnu Canyon across from Grapevine Camp, between Lower Bass Camp and 110-Mile Camp and especially up Waltenberg and Hakatai Canyons.
Plutons
GGMS rocks were intruded by arc plutons including the Zoroaster, Pipe Creek, Horn Creek, 96-Mile, Crystal, Trinity, and Ruby plutons. These plutons intruded as large, thick sheets and, as the cross section shows, some were incorporated into large folds during the “big crunch” at 1.7 Ga. The best example of a folded arc pluton is the Zoroaster pluton. The big amphitheater wall between Zoroaster Rapid and Cremation Canyon is a cross-sectional view of Zoroaster antiform (Figure 1).

Pegmatite Dikes
As the volcanic arcs were “wrapping” onto the Wyoming Archean core (Karl Karlstrom makes an analogy of a series of boats successfully wrapping in Bedrock Rapid), they were shortened by as much as 500% resulting in crustal thickening and many folds (e.g. Sockdolager, Zoroaster, Trinity folds; Figures 1 and 2) and shear zones (Figure 3). Rocks deposited at the surface found themselves as deep as 20 kilometers. Deeper rocks melted and moved up through the crust as magma, bringing, or “advecting” their heat with them. The pegmatites we now see in the walls of the canyon (reddish orange on the map) may only represent a small fraction of the total magma that moved through the crust. The advected heat of the pegmatites combined with the mantle heat conducted from below, melted the GGMS rocks in several areas. The migmatites (small-scale mixtures of Vishnu rocks and pegmatites) from Hance to Grapevine, and those from Cremation to 96-Mile Canyon record peak temperatures of up to 750° Celsius and they show the effects of melting at 1.7 Ga. If you look closely at the centimeter-scale pegmatite blobs, you will see dark rims around them. These rims contain the harder-to-melt minerals like biotite. The quartz and feldspar melted and segregated to form the small pegmatite blobs.

Shear Zones
The Upper Gorge is segmented into several blocks by shear zones. Shear zones are zones of very high strain and are simply the deeper equivalent of brittle faults. As you move from the brittle (i.e. breaking) upper crust to the plastic (flowing) middle and lower crust, high strain is more diffuse and occurs in zones rather than along discrete fault planes. As you float downstream you cross shear zones at Vishnu, Bright Angel, 96-Mile, Crystal (Figure 3), and Lower Bass Camp. The shear zones separate blocks which record different pressures and temperatures. One of the most dramatic breaks occurs at 96-Mile Canyon. Rocks upstream were heated to 750° C and were buried to about 20 kilometers. From Schist Camp to Crystal, rocks were “only” heated to 550 degrees C and buried to about 12 kilometers. We think the Boucher area is the best area to work out the earliest history of the crunch because the rocks there didn't get hot enough to destroy many of the early deformation fabrics.

The oldest rocks in Grand Canyon
One of the most interesting finds of our work is that the Elves Chasm pluton is 1.84 Ga. This is about a 100 million years older than any other rock in the southwestern US. Characteristics such as: the old age of the Elves pluton, chemical evidence, garnets that record 3 or more growth stages (compared to 1 or 2 growth stages east of Crystal), and strange rocks that may indicate a metamorphosed soil layer just below Waltenberg, in 115-Mile Canyon, in Blacktail Canyon, and in the Middle Gorge suggest that the rocks west of Crystal Creek might be part of a volcanic arc 100 million years older than the rest of the basement to the SW.
If we are correct in our thinking, the Crystal shear zone would be the first “suture” or relict subduction zone recognized in the Proterozoic rocks of the west. Next time you run Crystal, you might be crossing from one volcanic arc to another, much older arc. Try not to get subducted.

Brad Ilg

Glossary

Andesite—An extrusive (erupted) igneous rock that is rich in hornblende, quartz, and feldspar. The San Francisco Peaks and Mount St. Helens are examples of andesitic volcanoes.
Basalt—An extrusive (erupted ) igneous rock that is rich in pyroxene, olivine, and plagioclase. Sunset Crater is formed of basalt.
Craton—A part of the earth's crust that has been stable and undeformed for a long time.
Crust—The outer most layer of the earth.
Mantle—The layer of the earth below the crust and above the core.
Orogeny—Mountain building episode. The orogeny that deformed the GGMS rocks was probably more like the Andean orogeny than the Himalayan orogeny. The Andean orogeny is characterized by an ocean crust subducting under continental crust, the Himalayan case by continent-continent collision.
Pegmatite—A vein with big crystals. Big crystals mean the magma had a long time to cool.
Pluton—A molten mass of rock that cools and crystallizes beneath the surface of the earth.
Proterozoic—At a coarse time scale the earth can be divided into the Archean (4.5-2.5 Ga) and Proterozoic (2.5 Ga- 540 Ma) Eras which make up about 90% of earth history and the Phanerozoic Eon (540 Ma-present). The Archean makes up nearly half of earth history yet there are no rocks of this age in the Grand Canyon.
Rhyolite—An extrusive (erupted) igneous rock that is rich in quartz and feldspar. It is usually pink in color. Rhyolite is more viscous than other lavas and therefore tends to form very explosive eruptions (Yellowstone caldera for example).