caa a22 4500 386402345 CHVBK 20180307112204.0 cr unu---uuuuu 161130e198903 xx s 000 0 eng 10.1017/S0016756800006300 doi S0016756800006300 pii (NATIONALLICENCE)cambridge-10.1017/S0016756800006300 de Silva S. L. Department of Earth Sciences, Open University, Milton Keynes, Bucks. MK7 6AA, U.K. The origin and significance of crystal rich inclusions in pumices from two Chilean ignimbrites [Elektronische Daten] [S. L. de Silva] Crystal rich (∼ 70-98% phenocrysts) magmatic inclusions in pumices for compositionally heterogeneous ignimbrites from the Central Andes of northern Chile are interpreted as the products of crystal accretion at the sidewalls of the magma chambers. The inclusions are typically andesitic in composition and are found as ‘peppery textured' lenses and bands, or as discrete ovoid ‘blobs' within dacitic pumices from the early erupted portions of the ignimbrites. The inclusions have a bimodal gain size with large phenocrysts (> 1 mm), typical of those of the host pumice, set in a dominant finegrained framework (< 0.5 mm) of plagioclase, with lesser amounts of hornblende and biotite in equal proportions, and ubiquitous titanomagnetite in a matrix of vesiculated high-Si rhyolite glass. An igneous microgranular texture is defined by this framework. The mineralogy of the inclusions, as well as the compositions of the phenocrysts and glass, are very similar to those of the host pumices. These characteristics, in addition to available major, trace and REE data, are best reconciled if the inclusions represented samples of fractionated crystals and glass from the same magma as the host pumice. The restricted occurrence of these inclusions in the early erupted portions of the ignimbrites suggests that these crystal accumulations occurred in the upper portions of the magma chambers, at the sidewall; the dominantly fine grain size and crystal rich nature of the inclusions are considered to be the result of the higher thermal gradient in the boundary layer. These inclusions may be an important link between the experimental and geochemical models for the origin of compositional layering in magma chambers by sidewall crystallization. The presence of similar inclusions in other ignimbrites and volcanics, as well as plutonics, suggest that they may be a common feature of silicic magmas. Copyright © Cambridge University Press 1989 Geological Magazine Cambridge University Press 126/2(1989-03), 159-175 0016-7568 126:2<159 1989 126 GEO https://doi.org/10.1017/S0016756800006300 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1017/S0016756800006300 text/html Onlinezugriff via DOI NATIONALLICENCE 100 1- de Silva S. L. Department of Earth Sciences, Open University, Milton Keynes, Bucks. MK7 6AA, U.K NATIONALLICENCE 773 0- Geological Magazine Cambridge University Press 126/2(1989-03), 159-175 0016-7568 126:2<159 1989 126 GEO CC0 http://creativecommons.org/publicdomain/zero/1.0 nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-cambridge