Quartz monzonite

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.[1] The fine grained volcanic rock equivalent of quartz monzonite is quartz latite.[1]

Quartz monzonite
Igneous rock
Quartz monzonite bedrock from a USGS drill core at Western Cape Cod, Massachusetts
Quarry for the Salt Lake Temple with boulders and detached masses being worked by stone cutters

Quartz monzonite porphyry is often associated with copper mineralization in the porphyry copper ore deposits.[2]

Geographic distribution

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 Church of Jesus Christ of Latter-day Saints' Salt Lake Temple, Church Administration Building, and Conference Center, as well as the Utah State Capitol.

The large boulders of Joshua Tree National Park in southern California are quartz monzonite.

In Queensland, Castle Hill, Mount Stuart and Mount Louisa around Townsville represent a large quartz monzonite province.

A large pluton in the Atlanta lobe of the Idaho Batholith, near McCall, Idaho, is made of quartz monzonite.[3]

The Guilford Quartz Monzonite and Woodstock Quartz Monzonite, probably comagmatic, are located in central Maryland.

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References
  1. Classification of Igneous Rocks Archived September 30, 2011, at the Wayback Machine
  2. Titley, Spencer R. and Carol L. Hicks, Geology of the Porphyry Copper Deposits, University of Arizona Press, 1966, p. 35
  3. Idaho Batholith
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