Origin of Low-K Intermediate Lavas at Nekoma Volcano, NE Honshu Arc, Japan: Geochemical Constraints for Lower-Crustal Melts

Extremely low-K basaltic andesite to andesite lavas at Nekoma volcano, situated in the frontal volcanic zone of the NE Honshu arc, were produced from melts that originated in the lower crust. Multiple incompatible trace element model calculations suggest that extremely low-K basalt found in the same arc is a natural analog for the source composition. However, fractional crystallization, magma mixing, and crustal contamination models of primary low-K basalt cannot reproduce the Nekoma chemical composition. Derivation of melts from an extremely low-K amphibolitic lower-crustal rock with the residual mineral phases hornblende, olivine, pyroxenes, plagioclase, and magnetite is plausible. Major element compositions of Nekoma lavas are very similar to those of experimental melts of amphibolite dehydration melting, which further support the proposal. Light rare earth elements are slightly enriched, but total rare earth element abundances are relatively low, suggesting a high degree of partial melting of the source. Ba/Th ratios are low for frontal arc lavas, reflecting modification of the ratio during partial melting. Zr/Hf and Nb/Ta ratios are significantly greater than is usual for arc lavas, suggesting an anomalous source composition. Markedly low K, Rb, Cs contents in the extremely low-K lavas are attributed to an extremely low-K source. Underplating of an extremely low-K basalt originating from a hydrous depleted mantle wedge could form such an amphibolite. In contrast, Nd and Sr isotope ratios fall close to Bulk Earth values, indicating an isotopically enriched source. Hornblende-bearing rocks may predominate in the lower crust of the NE Honshu arc, based on the observation of crustal xenoliths. The presence of large low-Vp regions at lower-crustal depths beneath the frontal arc is suggested by geophysical observations. These observations further support lower-crustal melting beneath Nekoma as the origin of the intermediate low-K lavas.