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Processes of crystal separation in a magma heavily laden withcrystals without phase change are investigated from observationson frozen magma systems: Nosappumisaki and other shoshoniteintrusions in the Nemuro peninsula, Japan, for which the originof the crystals and the initial conditions are well constrained.The Nosappumisaki intrusion is 120 m in thickness and extendsfor more than 1·5 km. It exhibits a wide range of lithologicalvariation, principally as a result of crystal redistributionafter intrusion. Crystals in each lithology can be clearly dividedinto two kinds according to their composition and texture: thosepresent before the intrusion of the magma (phenocrysts)and those that crystallized in situ after intrusion. From thevertical change in mode and size of phenocrysts,it is shown that (1) augite phenocrysts were rapidlydeposited, with little overgrowth after intrusion, by significantcoagulation or clustering on a time-scale of more than a fewyears, and (2) plagioclase phenocrysts, definitelydenser than the melt but concentrated in the upper level, floatedby counter flow of massive deposition of augite phenocrysts.These results indicate that in a magma heavily laden with crystalsof a few millimeters in size (>20 vol. %), crystalcrystaland crystalmelt interaction play an important role inthe separation of crystals from the host melt. KEY WORDS: magma chamber; sill; crystal settling; plagioclase flotation; Nosappumisaki 相似文献
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