Pikes Peak granite
The Pikes Peak granite is a 1.08 billion year old widespread geologic formation found in the central part of the Front Range of Colorado. It is a coarse-grained pink to light red syenogranite with minor gray monzogranite, and it has a distinctive brick-red appearance where it outcrops. The granite gets its name from the 14,115-foot (4,302 m) Pikes Peak mountain, which is made up almost entirely of this rock.
Origin
The Pikes Peak granite was emplaced in three major intrusive events in the southern Front Range, now exposed in Colorado's Lost Creek Wilderness, Buffalo Peaks Wilderness, and Pikes Peak. It is the geochemically potassic series of plutons compromising most of the Pikes Peak batholith, a batholith formed of two major types of plutons, the potassic Pikes Peak granite and later plutons, plus late stage sodic syenite and granite plutons. Both the batholith and the Pikes Peak granite are A-type, meaning granites that originate in anorogenic, or non mountain building, tectonic settings, with an alkaline geochemistry and arising from more anhydrous magmas.[1]
Erosion
Over the next billion years, the now cooled granite was gradually exposed through erosion of overlying rocks. About 60 million years ago, parts of the Western U.S. were subjected to a series of uplifts, known as the Laramide orogeny, that eventually formed the modern Rocky Mountains and raised Pikes Peak to its current height. Pikes Peak, like other portions of Colorado Rockies is still being uplifted today as a part of larger tectonic processes effecting the Western United States.
Today, the Pikes Peak Batholith and Granite is exposed over a large part of the central Front Range of Colorado. It is found as far north as the southern slopes of Mount Evans west of Denver, west to South Park, and as far south as Cañon City. The batholith is about 80 miles (130 km) long in the north-south direction and about 25 miles (40 km) wide east to west. Even more of it remains hidden underground. Geologists have found the granite at the bottom of deep wells on the plains and magnetic sensors have detected it as much as 80 miles (130 km) to the east.
Mineralogy
The granite ranges in color from light pink to almost red. The pink color is due to large amounts of microcline feldspar and various iron minerals that permeate the rock. The long cooling time and the chemical composition of the original magma allowed large crystals to precipitate out of the magma. As a result, in many places the granite is very coarse grained, made up almost entirely of large crystals of feldspar, typically about a centimeter across. This makes the granite easily weathered and very crumbly. Almost every hill and slope in the Pikes Peak region is covered with thick blankets of loose gravel (scree) made up of marble-sized grains of feldspar.
In some places, the cooling process lasted long enough to form pegmatites that contain large, pure crystals of various minerals. The chemistry of the cooling magma produced a complex and unique mineralogy that attracts collectors from around the world and the Pikes Peak region is famous for its spectacular mineral specimens.
Smoky quartz crystals and topaz are found in many places in the Pikes Peak granite. Probably the most famous mineral from the area is amazonite, a bluish form of microcline feldspar that is relatively rare in other parts of the world. Many museum collections have stunning specimens of deep blue amazonite crystals studded with jet-black smoky quartz crystals.
References
- ↑ Loiselle, MC; Wones, DR (1979). "Characteristics and origin of anorogenic granites. Geological Society of America Abstracts with Program" 11: 468.
1. Granite Tectonics Of Pikes Peak Composite Batholith Colorado Pegmatite Symposium - 1986 R.M. Hutchinson - Colorado School of Mines
2. Colorado Gem Trails and Mineral Guide Richard M. Pearl 3rd rev. ed. 1993
3. A Brief Summary of the Mineral Deposits of the Pikes Peak Batholith, Colorado. Rocks & Minerals Article September 1, 2001