Felsic Igneous Rocks

Abstract

Felsic igneous rocks, representing the last stages of magmatic evolution, occur across all tectono-volcanic settings. Volcanic rocks exhibit a broad range of eruptive styles, volumetrically dominated by explosive activity as a consequence of high viscosity and enrichment in volatile components. Low magmatic densities favor the emplacement of felsic magma bodies in the upper crust. The mineralogy of felsic rocks is typically more varied than other igneous rock classes, due to a wide range of compositions and saturation of silicate melt in minor phases at advanced stages of evolution. These minor phases exert a strong control on trace element behavior during the late stages of magmatic differentiation. Compositional and mineralogical zoning is common in both volcanic and plutonic rock units. During their evolution, felsic magma bodies exist as pools of crystal-poor, eruptible magma overlying piles of cognate, rheologically locked crystal mush which cannot erupt, providing a link between volcanic and plutonic rocks. Transitions between these states are accomplished by crystal settling, mafic replenishment, and mechanical disturbance, which lead to diverse outcomes for the compositional architecture of the final volcanic and plutonic rock bodies. Timescales of these processes recorded by phenocrysts in volcanic rocks range from >105years for crystal residence, to as little as 1 year for crystal growth preceding eruption. The origins of felsic rocks lie in crystallization-differentiation of mantle-derived mafic magma, often aided by melting of pre-existing country rocks. The isotopic record for continental felsic rocks indicates that crustal assimilation is especially common in the generation of rhyolites and granitoids.