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

Formation of an extensive shallow water sedimentary basin before the v oluminous Matsuura basalt volcanism in the back are volcanic belt in n orthwest Kyushu, Japan, seems to indicate the lower crust erosion due to the upwelling of the mantle, which results in the decompression mel ting of mantle to form basaltic magma. The Matsuura basalts, 10 to 1 M a, range in composition from low alkali tholeiitic basalt through alka li olivine basalt to basaltic andesite. Their compositions show a wide variation in MgO from 11.6 wt.% to 2.8 wt.%. Their chemical character istics are: (1) evolution across the alkali-subalkali boundary, (2) cr escent form of the basalt distribution on a SiO2 vs. MgO diagram, (3) enrichment in normative plagioclase component and a diminution in norm ative clinopyroxene component with the magma evolution, (4) abrupt ter mination of this plagioclase-enrichment trend at MgO = 3.3 wt.%: (5) m arked enrichment in SiO2 in aphyric basalts with MgO < 5.5 wt.%, (6) a limit of magma evolution, and (7) magma mixing. Mineral crystallizati on sequence is found to be olivine, olivine + clinopyroxene, and then olivine + clinopyroxene + plagioclase. Iron oxide crystallization star ts in iron-enriched magmas a little before the joining of plagioclase in crystallization, which results in the rapid enrichment in SiO2. The chemical characteristics of the Matsuura basalts are well accounted f or by assuming open system fractional crystallization in a refilled ma gma chamber near the crust-mantle boundary. The chamber may have conta ined small amounts of water. It seems probable that the open system fr actional crystallization operative under high pressure conditions can transform low alkali tholeiitic primitive magmas through weakly nephel ine-normative basalt compositions finally to the convergent compositio n of hypersthene-normative basaltic andesire. (C) 1998 Elsevier Scienc e B.V. All rights reserved.