DUSTY AND HONEYCOMB PLAGIOCLASE - INDICATORS OF PROCESSES IN THE UCHINO STRATIFIED MAGMA CHAMBER, IZU PENINSULA, JAPAN

Two types of melt-inclusion-bearing plagioclase coexist in the Uchino basalt, Izu peninsula, Japan: (1) dusty plagioclase with an Ab-rich core mantled by a dusty zone consisting of more An-rich plagioclase and fine melt inclusions; and (2) honeycomb plagioclase with a central core containing larger melt inclusions. The dusty plagioclase probably forms through partial dissolution of Ab-rich plagioclase, as dusty zones truncate the concentric zonal structure of the Ab-rich inner core. The honeycomb plagioclase is formed by skeletal growth under supercooling conditions. This is indicated by the euhedral geometry of the boundary between the inner inclusion-free and the outer inclusion-bearing zones of the honeycomb plagioclase which has a concentric inner core. Bimodal distribution of the chemical compositions of phenocryst cores and assemblages in crystal clots suggest that hybridization of basaltic and andesitic magmas took place before crystallization of phenocryst rims. The core-rim boundaries of phenocrysts derived from the andesitic magma and the basaltic magma have a rounded and a euhedral geometry, respectively. This suggests the andesitic magma was heated and the basaltic magma was cooled prior to hybridization. These features are consistent with the development of a thermal diffusive interface between overlying andesitic magma and underlying basaltic magma in a stratified magma chamber.