Tonalite

Tonalite is a coarse-grained intrusive igneous rock dominated by quartz and plagioclase feldspar, with little potassium feldspar. It is classified within the granitoid family.

Plagioclase: A feldspar mineral series containing sodium-rich and calcium-rich end members. Plagioclase is one of tonalite’s dominant minerals.
Igneous differentiation: Changes in magma composition caused by processes such as crystal removal and magma mixing. Differentiation can produce the silica-rich compositions that crystallize as tonalite.
Granodiorite: A coarse-grained granitoid containing quartz and plagioclase, with more potassium feldspar than tonalite. The higher potassium-feldspar proportion is the main distinction from tonalite.
Tonalite–trondhjemite–granodiorite suite: A suite of Archean granitoid rocks dominated by tonalite, trondhjemite, and granodiorite. Tonalite is a principal rock type in this widespread ancient crustal association.
Quartz: A crystalline mineral composed of silicon dioxide. Quartz is abundant in tonalite and helps define its classification.
Fractional crystallization: Magma evolution in which early-formed crystals separate from the remaining melt. Removing early minerals can shift residual magma toward tonalitic composition.
Trondhjemite: A pale, sodic tonalite rich in plagioclase and quartz and poor in dark minerals. Trondhjemite is a distinctive tonalite variety with especially low potassium content.
Archean craton: A stable part of continental crust formed or consolidated during the Archean Eon. Tonalite–trondhjemite–granodiorite rocks make up much of many Archean cratons.
Potassium feldspar: A group of feldspar minerals rich in potassium, including orthoclase and microcline. Its scarcity separates tonalite from potassium-feldspar-rich granitoids.
Partial melting: The incomplete melting of rock, which creates melt with a composition unlike its source. Partial melting of crustal rocks can generate tonalitic magmas.

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