rhyolitegranite

Rhyolitegranite is not a formally defined rock name in standard igneous nomenclature. In geological literature it is used informally to describe granitoid rocks that have rhyolite-like chemistry or textures, or to indicate rocks that show both granitic (coarse-grained) and rhyolitic (high-silica) characteristics within a single intrusion or lithologic unit. The term is typically hyphenated as rhyolite-granite.

Composition and texture: Rhyolitegranite is typically felsic, with high silica and light-colored minerals. It commonly contains quartz and alkali feldspar as major phases, with plagioclase present in varying amounts, and accessory minerals such as biotite, hornblende, zircon, and apatite. The rock may have a granite-like coarse texture but show rhyolite-like microstructures or intergrowths that reflect the rhyolitic chemistry. Some specimens are porphyritic, with larger quartz or feldspar phenocrysts in a finer groundmass.

Classification and use: In formal QAPF-based schemes, rhyolite and granite plot in different fields, one volcanic and the other plutonic. A rock labeled rhyolitegranite does not represent a standard category, and its classification depends on mineral modal percentages rather than the name. The term is therefore best viewed as a descriptive placeholder for rhyolite-like chemistry within a granitoid texture.

Formation and occurrence: Such rocks are associated with continental crustal magmatism and may form by late-stage differentiation of granitic magmas or by crystallization of rhyolitic magmas within crustal magma systems. They occur in regions with long-lived crustal processing and in volcanic-plutonic complexes of subduction and post-collision settings.

See also: Granite, Rhyolite, Granitoid, QAPF classification.