File:Pegmatitic granite dike in Black Canyon Schist (Proterozoic; No Thoroughfare Canyon, Colorado National Monument, Colorado, USA) 1 (23856419271).jpg

Pegmatitic granite dike intruding schist in the Precambrian of Colorado, USA.

Mottled pinkish-colored rock = pegmatitic granite Black rock = schist

Colorado National Monument is a colorful and scenic park just southwest of the town of Grand Junction in western Colorado, USA. It's located in the northeastern part of the Colorado Plateau Physiographic Province, a large section of continental crust that was significant uplifted during the Cenozoic. The rocks in the park are gently folded into a large monocline that has been erosionally dissected into a series of dramatic canyons.

The above photo shows a large loose block of the oldest rock unit in the park. The black-colored material is Precambrian basement rock, consisting of schist. Schist is a foliated, coarsely-crystalline, intermediate- to high-grade metamorphic rock. It usually forms by intense alteration of phyllite (which was formerly slate, which was formerly shale). The mottled pinkish-colored rock is an igneous dike that intruded well after the schist formed. Igneous dikes are planar intrusions that cut across country rocks. They can be thin or very wide. This dike is composed of pegmatitic granite, a felsic, crystalline, intrusive igneous rock dominated by the minerals quartz and potassium feldspar. Pegmatitic granite has all, or almost all of its crystals larger than 1 centimeter. Large to huge crystals in a granite is the result of cooling of a water-rich magma. The large pink crystals are potassium feldspar (KAlSi3O8 - potassium aluminosilicate). The pegmatitic granite is not metamorphosed, so it postdates regional metamorphism in this area. The pegmatite dates to about 1.4 billion years.

Geologic unit & age: Black Canyon Schist (late Paleoproterozoic, 1.759 Ga) with a ~1.4 Ga pegmatitic granite dike (Mesoproterozoic)