MECHANISM OF CRYSTAL REDISTRIBUTION IN A SHEET-LIKE MAGMA BODY: CONSTRAINTS FROM THE NOSAPPUMISAKI AND OTHER SHOSHONITE INTRUSIONS IN THE NEMURO PENINSULA, NORTHERN JAPAN

Abstract

Processes of crystal separation in a magma heavily laden with crystals without phase change are investigated from observations on frozen magma systems: Nosappumisaki and other shoshonite intrusions in the Nemuro peninsula, Japan, for which the origin of the crystals and the initial conditions are well constrained. The Nosappumisaki intrusion is 120 m in thickness and extends for more than 1.5 km. It exhibits a wide range of lithological variation, principally as a result of crystal redistribution after intrusion. Crystals in each lithology can be clearly divided into two kinds according to their composition and texture: those present before the intrusion of the magma ('phenocrysts') and those that crystallized in situ after intrusion. From the vertical change in mode and size of 'phenocrysts', it is shown that (1) augite 'phenocrysts' were rapidly deposited, with little overgrowth after intrusion, by significant coagulation or clustering on a time-scale of more than a few years, and (2) plagioclase 'phenocrysts', definitely denser than the melt but concentrated in the upper level, floated by counter flow of massive deposition of augite 'phenocrysts'. These results indicate that in a magma heavily laden with crystals of a few millimeters in size (>20 vol. %), crystal-crystal and crystal-melt interaction play an important role in the separation of crystals from the host melt. © 2006 Oxford University Press.