Tonalites

Tonalite is a coarse-grained igneous rock of intermediate felsic composition, commonly classified within the tonalite–trondhjemite–granodiorite (TTG) family. It lies compositionally between diorite and granite, being richer in plagioclase feldspar and often containing quartz, while alkali feldspar is present only in minor amounts or is absent. The defining mineral assemblage is dominated by plagioclase with quartz as a secondary constituent and relatively small amounts of mafic minerals such as biotite, hornblende, or pyroxene.

Mineralogy and texture are typically phaneritic, reflecting slow cooling in plutonic settings. Tonalite rocks usually display a light to intermediate color index due to their felsic mineralogy, though the presence of mafic minerals can give a darker tone. Accessory minerals such as zircon, magnetite, and opaque minerals may occur.

Formation and tectonic context vary, but tonalites commonly form by partial melting of crustal rocks or by the differentiation and evolution of granitoid magmas in continental crust. They are frequent in continental collision zones and in large intrusive bodies such as batholiths and plutons. The tonalite family is an important component of early crustal evolution in many orogenic belts, particularly in Archean terranes where TTG rocks are widespread.

Classification and naming follow the QAPF (quartz–feldspar–plagioclase) framework: tonalite contains significant quartz and plagioclase with limited alkali feldspar, distinguishing it from granodiorite and granite. Tonalite occurs in many regions around the world, often as part of ancient continental crust or major intrusive complexes.