1. Education


Pictures of Igneous Rock Types


Tonalite is a widespread but uncommon plutonic rock, a granitoid without alkali feldspar that may also be called plagiogranite and trondjhemite. (more below)
Much quartzier than diorite
Photo (c) 2010 Andrew Alden, licensed to About.com (fair use policy)

The granitoids all center around granite, a fairly equal mixture of quartz, alkali feldspar and plagioclase feldspar. As you remove alkali feldspar from proper granite, it becomes granodiorite and then tonalite (dominantly plagioclase with less than 10 percent K-feldspar). Recognizing tonalite takes a close look with a magnifier to be sure that alkali feldspar is truly absent and quartz is abundant. Most tonalite also has abundant dark minerals, but this example is almost white (leucocratic), making it a plagiogranite. Trondhjemite is a plagiogranite whose dark mineral is biotite. This specimen's dark mineral is pyroxene, so it's plain old tonalite. See another example, with thin sections, at Ron Schott's Geology Home Companion blog.

An extrusive rock (lava) with the composition of tonalite is classified as dacite. Tonalite gets its name from the Tonales Pass in the Italian Alps, near Monte Adamello where it was first described along with quartz monzonite (once known as adamellite).

  1. About.com
  2. Education
  3. Geology

©2014 About.com. All rights reserved.