Complex interaction of aluminous metasedimentary xenoliths and siliceous magma; an example from Mt. Amiata (Central Italy)

Abstract

Aluminous, silica-deficient metasedimentary xenoliths in siliceous lavas of Mt. Amiata have preserved composite zoning-patterns indicative for complex processes of magma-rock interaction. Petrographic observations and small-scale mineralogical and chemical differences between up to five distinct zones (including the core and envelope of lava) provide evidence that: Quartz-poor muscovite-biotite schists are considered to be the most likely parent rocks and it is believed that release of volatiles from decomposing micas played a significant role in the high-temperature metamorphic evolution of the xenoliths and their interaction with the magma. The conditions favourable for assimilation were enhanced by injection of mafic magma into the magma chamber. Although this concurrent operation of magma-mixing precludes a quantitative estimate of contamination from the wall-rocks (which was probably of minor importance) the present example indicates that dry acid magma may potentially become more mafic by interaction with partially melted hydrous rocks.