KURILE ISLAND-ARC VOLCANISM - STRUCTURAL AND PETROLOGICAL ASPECTS

This study discusses the structural and spatial characteristics of all known Kurile island-arc Quaternary volcanoes, which include 105 that are subaerial and 96 submarine. Volcanic activity in the island arc is distributed in two zones parallel to the Kurile-Kamchatka deep-sea trench. The depth to the seismo-focal zone in the frontal zone is 110-140 km and in the rear zone 160-190 km. These frontal and rear volcanic zones show distinct variations in geochemical, petrologic, and mineralogic characteristics. Concentrations of K, Rb, Ba, Sr, F, Be, La, Ce, Nb, Zr, U, Th, Ni, and Cr in similar volcanic rocks of the rear zone are typically more than twice as high as those in rocks of the frontal zone, whereas concentrations of Fe and V are half as high in the rear-zone lavas as in those of the frontal zone. Variations in concentration along the strike of the arc are less pronounced. The average Sr-87/Sr-86 ratios for the frontal and rear zones are: Northern Kuriles 0.70322 and 0.70304, Middle Kuriles 0.70302 and 0.70295, and Southern Kuriles 0.70347 and 0.70306, respectively. Nd-143/Nd-144 values in lavas from the frontal and rear zones are 9-10 and 7-8. respectively. Be-10 abundances in the Kurile island arc lavas range from 2.2 . 10(6) at/g to 7.9 . 10(6) at/g. No differences in Be-10 abundances corresponding to the frontal and rear-arc zones have been found. Acid and intermediate lavas from the volcanic front are characterized by a two-pyroxene phenocryst association, while similar lavas from the rear arc have amphibole- and biotite-bearing associations. Mineral compositions (plagioclase, pyroxene, amphibole, olivine) in the two zones also show distinct differences. Plagioclase in the rear-arc lavas is less calcic, with higher K, Sr, Ba and lower Fe than in the frontal lavas. Ferromagnesian minerals in the rear-arc lavas contain less Fe and more Mg than in frontal lavas. These data are used to develop a model for magma generation related to two levels of volatile separation from the subducting lithospheric plate.