felsischhell

Felsischhell is a descriptive term used in German geology to designate light-colored rocks with a felsic composition. It sits at the light end of the spectrum of silica- and alkali-rich rocks and is contrasted with more dark-colored, mafic rocks.

Chemical and mineralogical characteristics include a relatively high silica content and a predominance of light-colored minerals such as quartz and feldspars (orthoclase and plagioclase). Accessory minerals may include muscovite, biotite, zircon, and apatite. The feldspar-rich assemblage gives the rocks their pale appearance, while mafic minerals are typically scarce.

Common rock types classified as felsischhell encompass granitoid rocks like granite and granodiorite, as well as felsic volcanic rocks such as rhyolite and rhyodacite (including related pumice or obsidian in some cases). Textures range from coarse-grained in plutonic rocks to fine-grained or glassy in volcanic rocks; many felsischhell rocks are porphyritic, with visibly larger phenocrysts of quartz or feldspar.

Formation and occurrence: felsischhell rocks crystallize from silica- and alkali-rich magmas, typically in continental crust settings. They are commonly associated with crustal melting processes, subduction-related magmatism, or extensional tectonics that generate felsic magmas.

Geological mapping and classification: the term is used descriptively to group rocks by color and composition on field maps. It is not a strict chemical boundary, and in precise petrographic classifications there is a continuum with intermediate felsic rocks. The term highlights the light hue and light-toned mineralogy typical of these rocks.