Magma evolution observed in the Matsuura basalts in northwest Kyushu, Japan: An example of high-pressure open system fractional crystallization in a refilled magma chamber near the crust–mantle boundary

Abstract

Formation of an extensive shallow water sedimentary basin before the voluminous Matsuura basalt volcanism in the back arc volcanic belt in northwest Kyushu, Japan, seems to indicate the lower crust erosion due to the upwelling of the mantle, which results in the decompression melting of mantle to form basaltic magma. The Matsuura basalts, 10 to 1 Ma, range in composition from low alkali tholeiitic basalt through alkali olivine basalt to basaltic andesite. Their compositions show a wide variation in MgO from 11.6 wt.% to 2.8 wt.%. Their chemical characteristics are: (1) evolution across the alkali–subalkali boundary, (2) crescent form of the basalt distribution on a SiO 2 vs. MgO diagram, (3) enrichment in normative plagioclase component and a diminution in normative clinopyroxene component with the magma evolution, (4) abrupt termination of this plagioclase-enrichment trend at MgO=3.3 wt.%, (5) marked enrichment in SiO 2 in aphyric basalts with MgO<5.5 wt.%, (6) a limit of magma evolution, and (7) magma mixing. Mineral crystallization sequence is found to be olivine, olivine+clinopyroxene, and then olivine+clinopyroxene+plagioclase. Iron oxide crystallization starts in iron-enriched magmas a little before the joining of plagioclase in crystallization, which results in the rapid enrichment in SiO 2. The chemical characteristics of the Matsuura basalts are well accounted for by assuming open system fractional crystallization in a refilled magma chamber near the crust–mantle boundary. The chamber may have contained small amounts of water. It seems probable that the open system fractional crystallization operative under high pressure conditions can transform low alkali tholeiitic primitive magmas through weakly nepheline-normative basalt compositions finally to the convergent composition of hypersthene-normative basaltic andesite.