Abstract
The granitic and associated gabbroic rocks of the Tanzawa plutonic complex of Miocene age occurring in the northern part of the Izu-Bonin arc are characterized by low abundances of K (229–6790 ppm) and Rb (0.414–12.1 ppm), low K 2 O/Na 2 O ratios (0.037–0.21), moderately high K/Rb ratios (541–630), low Rb/Sr ratios (0.00137–0.0579) and low initial 87 Sr/ 86 Sr ratios (0.70332–0.70372). This indicates that acid to intermediate plutonic rocks with these geochemical characteristics also occur in island arc environments besides mid-oceanic ridge environments. They represent, together with associated gabbroic rocks, a low-potash island arc plutonic complex and are expected to occur beneath young island arcs, although now unexposed. The Tanzawa plutonic complex may have been formed by differentiation of low-K calc-alkaline magma.
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