Andesitelike

Andesitelike is a descriptive term used in igneous petrology to characterize rocks whose mineralogical and chemical properties closely resemble andesite but are not formally named as andesite in lithologic descriptions. It signals an intermediate silica content and a plagioclase-dominated mineralogy, often with ferromagnesian minerals such as amphibole or pyroxene.

In terms of composition, andesitelike rocks typically lie in the mid-silica range, roughly 57–63 wt% SiO2. They commonly contain plagioclase as the principal feldspar and one or more ferromagnesian minerals (hornblende, amphibole, or pyroxene). Quartz is usually scarce or absent, though it can occur in some textures. On geochemical plots, these rocks frequently fall into the calc-alkaline field, reflecting arc-related differentiation processes. They may share similar trace-element signatures with true andesite.

Formation and occurrence: andesitelike rocks are often associated with subduction-zone magmatism and volcanic arcs, where fractional crystallization, crustal assimilation, and fluid-rich melting yield intermediate compositions between basalt and rhyolite. They can describe both extrusive lavas and plutonic equivalents that resemble andesite in mineralogy and chemistry, even when precise modal proportions or field relationships prevent a formal rock name.

Usage and caveats: “andesitelike” is not a formal lithologic name but a descriptive qualifier. It is used when a rock resembles andesite in composition and mineralogy but lacks the exact proportions, texture, or tectonic context needed to assign a strict andesite designation. Petrographic analysis and geochemical data are typically employed to confirm the resemblance and interpret petrogenesis.

See also: Andesite; Calc-alkaline series; Subduction-zone volcanism.