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Journal of Petrology Volume 42 Number 12 Pages 2303-2331 2001
© Oxford University Press 2001
Geochemical Evolution of Akagi Volcano, NE Japan: Implications for Interaction Between Island-arc Magma and Lower Crust, and Generation of Isotopically Various Magmas
PHEASANT MEMORIAL LABORATORY OF GEOCHEMISTRY AND COSMOCHEMISTRY, INSTITUTE FOR STUDY OF THE EARTH’S INTERIOR, OKAYAMA UNIVERSITY AT MISASA, TOTTORI, 682-0193, JAPAN
Major and trace element, and Sr, Nd and Pb isotopic compositions were determined for whole-rock samples from the ‘isotopically anomalous’ Akagi volcano in the volcanic front of the NE Japan arc. Sr and Nd isotopic compositions of phenocrysts were also analyzed together with their major and trace element compositions. Compared with the other volcanoes from the volcanic front, the whole-rock isotope compositions of Akagi show highly enriched characteristics; 87Sr/86Sr = 0·7060–0·7088, 
Nd = -0·40 to -8·6, and 208Pb/204Pb = 38·4–38·8. The rare earth element (REE) patterns are characterized by heavy REE (HREE) depletions with U-shaped patterns from middle REE (MREE) to HREE, suggesting that amphibole fractionation was induced by a reaction between clinopyroxene and H2O-rich magma in the lower crust. The integrated isotope and trace elements systematics, and tectonic structure beneath Akagi volcano, suggest that lower-crustal assimilation by the H2O-rich primary magma could have been affected by the double subduction of Philippine Sea and Pacific oceanic plates. This double subduction could have supplied larger amounts of water to the magma source region in the wedge mantle than in the case of a single subduction zone. Significant differences in isotopic compositions are observed between phenocrysts and the coexisting melts. Such isotopic disequilibrium may have resulted from magma mixing between an isotopically depleted aphyric and an enriched porphyritic magma in a shallow magma chamber. The geochemical characteristics of these end-member magmas were retained in the lower crust, despite differing extents of lower-crustal assimilation by the H2O-rich magmas.
KEY WORDS: Akagi volcano; H2O-rich magma; isotopic disequilibrium; lower-crustal assimilation; magma mixing
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