Across-Arc Mineralogical and Geochemical Variations in Kirishima Volcano Group and its Implication in the Genesis of the Volcanism of the Ryukyu Arc (I)

Kirishima Volcano Group (KVG) lies at the northern end of the Ryukyu arc that extends discontinuously in a NNE direction from northern Taiwan to Kyushu Island of Japan. Within the KVG, three active stages are recognized on the basis of field geology and tephrochronology. The third stage volcanoes, the subject of this paper, comprises thirteen youngest volcanoes( < 18,000 years ) and were erupted from a series of vents aligned NW-SE direction. The arrangement direction is oblique to the volcanic front of the arc and is parallel to the proceeding direction of the Philippine Sea plate.
In basalts or basaltic inclusions, olivine crystallization is followed either by plagioclase(the plagioclase basalt suites of Morrice and Gill(1986) : PBS ) or augite( the augite basalt suites : ABS ). PBS occur at the volcanic front side of the KVG and evolve to quartz-bearing pyroxene andesite. ABS occur behind the front side and evolve to hornblende-bearing pyroxene andesite, and have higher AlIV contents in calcic pyroxene. All tholeiitic lavas and ejecta are confined to the low K2O volcanic products near the volcanic front, while calc-alkaline materials are erupted more randomly. Regardless of their rocks series, the eruptive products near the volcanic front have low incompatible elements concentrations( average K60=1.7 ) and low K2O/Na2O ratios. In contrast to this, the lavas behind the volcanic front have high incompatible elements concentrations( av. K60=2.25 ), high K2O/Na2O ratio and higher modal clinopyroxene /plagioclase ratios. These results have clarified the lateral variations of magma compositions of the KVG, in which silica activity decreases, and incompatible elements and water contents increase away from the front. These variations are well correlated with the depth of Wa-dati-Benioff zone beneath the KVG, the depth of which abruptly changes from 110 to 170 km away from the front. Based on these facts, it is concluded that the lateral variations of the KVG in geochemistry represent the across-arc geochemical variations of the Ryukyu volcanic arc. Moreover, comparison of K2O/Na2O ratios in lavas from the KVG and N-type MORBs indicates that the above lateral variations are probably caused by the lateral heterogeneity of the mantle wedge beneath the KVG.