Small‐scale disequilibrium in a magmatic inclusion and its more silicic host

Abstract

Basaltic andesite inclusions and their host dacite from the Purico‐Chascon complex in northern Chile are isotopically distinct. Textural characteristics of the inclusions are typical of those resulting from magma mingling. Serial sectioning across the interface of an inclusion and its host dacite, complemented by microdrill sampling and detailed microprobe work, has enabled us to examine the scale of mixing and chemical (isotopic and trace element) disequilibrium. The results of this work show that (1) the composition of the inclusion is relatively homogeneous; (2) the dacite host is generally higher in 87Sr/86Sr and lower in 143Nd/144Nd than the enclave, but it is heterogeneous on a small scale, and probably a “hybrid”; (3) the isotopic composition in the marginal zone, apparently on both host and inclusion sides of the weakly chilled interface, actually shows the highest 87Sr/86Sr and lowest 143Nd/144Nd; (4) large plagioclase crystals in the inclusions and host are xenocrystic, with higher 87Sr/86Sr than any of the other samples. These observations are reconciled with a model of magma evolution in a crustal magma chamber. In such a scenario the mafic magma is overlain by a cap of rhyolite—a partial melt of the dacitic ignimbrites which now underlie the Purico‐Chascon complex. Overturn of such a magma system gives rise to a hybrid dacite containing discrete mafic inclusions.