Quartz monzonite or adamellite is an intrusive, felsic, igneous rock that has an approximately equal proportion of orthoclase and plagioclase feldspars. It is typically a light colored phaneritic (coarse-grained) to porphyritic granitic rock. The plagioclase is typically intermediate to sodic in composition, andesine to oligoclase. Quartz is present in significant amounts. Biotite and/or hornblende constitute the dark minerals. Because of its coloring, it is often confused with granite, but whereas granite contains more than 20% quartz, quartz monzonite is only 5–20% quartz. Rock with less than five percent quartz is classified as monzonite. A rock with more alkali feldspar is a syenite whereas one with more plagioclase is a quartz diorite. The fine grained volcanic rock equivalent of quartz monzonite is quartz latite.
Quartz monzonite porphyry is often associated with copper mineralization in the porphyry copper ore deposits.
A massive outcrop of this igneous rock can be seen on the bald summit of Croydon Mountain near Cornish, New Hampshire. Stone Mountain in Georgia is a large quartz monzonite monadnock.
Quartz monzonite extracted from a quarry in Little Cottonwood Canyon was used to build several buildings in Salt Lake City, Utah, including the LDS Church’s Salt Lake Temple, the Utah State Capitol, the LDS Church Administration Building, and the facade of the nearby LDS Conference Center.
The large boulders of Joshua Tree National Park in southern California are quartz monzonite.
A large pluton in the Atlanta lobe of the Idaho Batholith, near McCall, Idaho, is made of quartz monzonite.
The Guilford Quartz Monzonite and Woodstock Quartz Monzonite, probably comagmatic, are located in central Maryland.
Igneous rock (derived from the Latin word ignis meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. The magma can be derived from partial melts of existing rocks in either a planet’s mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses.
Igneous and metamorphic rocks make up 90–95% of the top 16 km of the Earth’s crust by volume.Igneous rocks form about 15% of the Earth’s current land surface.Most of the Earth’s oceanic crust is made of igneous rock.
Igneous rocks are also geologically important because:
- their minerals and global chemistry give information about the composition of the mantle, from which some igneous rocks are extracted, and the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted;
- their absolute ages can be obtained from various forms of radiometric dating and thus can be compared to adjacent geological strata, allowing a time sequence of events;
- their features are usually characteristic of a specific tectonic environment, allowing tectonic reconstitutions (see plate tectonics);
- in some special circumstances they host important mineral deposits (ores): for example, tungsten, tin, and uranium are commonly associated with granites and diorites, whereas ores of chromium and platinum are commonly associated with gabbros.
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