Geochemical Evolution of Akagi Volcano, NE Japan: Implications for Interaction Between Island-arc Magma and Lower Crust, and Generation of Isotopically Various Magmas

Abstract

Major and trace element, and Sr, Nd and Pb isotopic compositions lower-crustal assimilation; magma mixing 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 INTRODUCTION whole-rock isotope compositions of Akagi show highly enriched Island-arc volcanism has been considered to result from characteristics; 87 Sr/ 86 Sr = 0·7060–0·7088, Nd =− 0·40 partial melting of the wedge mantle induced by addition to −8·6, and 208 Pb/ 204 Pb = 38·4–38·8. The rare earth of slab-derived fluid-rich materials in subduction zones. element (REE) patterns are characterized by heavy REE (HREE) This general model is supported by many geochemical depletions with U-shaped patterns from middle REE (MREE) to studies employing major and trace element compositions, HREE, suggesting that amphibole fractionation was induced by a and radiogenic isotope systematics of arc volcanic ejecta reaction between clinopyroxene and H2O-rich magma in the lower (e.g. Nakamura et al., 1985; Woodhead & Fraser, 1985; crust. The integrated isotope and trace elements systematics, and Tatsumi et al., 1986; Ishikawa & Nakamura, 1994; Miller tectonic structure beneath Akagi volcano, suggest that lower-crustal et al., 1994; Ryan et al., 1995; Shibata & Nakamura, assimilation by the H2O-rich primary magma could have been 1997). These previous works mainly focused on the source affected by the double subduction of Philippine Sea and Pacific characteristics in relation to the evolution of the mantle oceanic plates. This double subduction could have supplied larger wedge in subduction zones. The magmatic evolution of amounts of water to the magma source region in the wedge mantle individual volcanoes, after partial melting of the wedge than in the case of a single subduction zone. Significant differences mantle, and in shallow magma chambers has received in isotopic compositions are observed between phenocrysts and the less attention. Isotope and trace element geochemistry coexisting melts. Such isotopic disequilibrium may have resulted has typically been restricted to basaltic volcanic rocks, from magma mixing between an isotopically depleted aphyric and not the felsic rocks, to avoid the complexities of processes an enriched porphyritic magma in a shallow magma chamber. such as crystal fractionation, magma mixing and crustal The geochemical characteristics of these end-member magmas were assimilation. It is, however, essential to characterize the retained in the lower crust, despite differing extents of lower-crustal shallower magmatic processes along with the source characteristics. Magmas erupted at the Earth’s surface assimilation by the H2O-rich magmas.