Progressive Melt Extraction from Upwelling Mantle Constrained by the Kita-Matsuura Basalts in NW Kyushu, SW Japan

Abstract

Systematic petrological, geochemical, and chronological studies of Cenozoic intra-plate volcanism in southwesternJapan, in the KitaMatsuura area in northwestern Kyushu, reveal temporal and spatial changes in mantle melting processes on a time scale of 2 5 Myr and a horizontal scale of 35 km.The most extensive basaltic activity in the area occurred between 8 5 and 6 0 Ma. Four vertical sections through the volcanic sequence from the underlying basement to the top flows were studied to encompass the distribution of the KitaMatsuura basalt. In the central and western sections (Hirado, Senryu, and Ishimori) there is a temporal variation in the chemical composition of the basalts: volcanism initiated eruption of mildly alkaline basalts (lowto medium-SiO2 group) followed by major flows of sub-alkaline basalt (mediumto high-SiO2 group). The eastern section (Kunimi section) is characterized by mildly alkaline basalts (low-SiO2 group). Each SiO2 group has a distinctive phenocryst assemblage and major element composition. The Zr/Y, Nb/Th, and Nb/Y ratios of the Kita-Matsuura basalts are positively correlated, which cannot be explained by assimilation of crustal materials but instead is linked to melting processes in the mantle. The average segregation depth of the inferred parental magmas of the three groups decreases in the order of SiO2 enrichment, indicating a temporal decrease in melt segregation depth in the central and western sections.The correlation between Zr/Yand Nb/Y can be reproduced by a series of instantaneous melts resulting from decompressional critical melting of a primitive mantle source with 2% trapped melt. In each section, both Zr/Yand Nb/Y decrease upwards, which implies an increase in the degree of melting with time. The temporal and spatial variations in basalt chemistry are most plausibly accounted for by progressive melt extraction during mantle upwelling, which started in the garnet stability field ( 3 0 GPa) and ended in the spinel field ( 1 5 GPa). Upwelling was centred beneath the Hirado section, in the westernmost area in KitaMatsuura, with a diameter inferred to be greater than 70 km.