Quartzolite and quartz-rich granitoid
Quartz-rich granitoids are a group of highly evolved plutonic rocks in which quartz represents more than 60% of the felsic minerals. These rocks typically contain K-feldspar and/or plagioclase, while mafic minerals, like biotite and hornblende, are generally present as accessory minerals. On the other hand, muscovite, tourmaline, and other minerals typical of pegmatites are very common primary constituents. Quartzolite represents an extreme composition within this group of rocks, since it consists of more than 90% quartz over felsic minerals and occupies the top vertex of the QAPF diagram. Quartzolite is a synonym of silexite. However, the use of the latter term is now discouraged, since it is also a French word for chert. Quartzolites and quartz-rich granitoids occur primarily as veins, sheet intrusions (dikes and sills), apophyses, and masses within granites, sometimes in associations with pegmatites and rocks produced by hydrothermal alteration.
These quartz-rich rocks are unlikely to form by igneous processes only, since such a SiO2-rich composition would be highly viscous and unlikely to be able to fractionate from a parent, granitic melt. It is likely that they form in the pegmatite/hydrothermal late stages of crystallization of granitic magmas, as also suggested by their common association with pegmatites and leucogranites. However, there are yet not enough studies on this subject. No volcanic equivalents of these rocks exist in nature.
Quartzolite – Mindat.org