Crustal anatexis and evolution of granitoid magma in Permian intra-oceanic island arc, the Asago body of the Yakuno ophiolite, Southwest Japan

Abstract

A late Paleozoic crustal section of an intra-oceanic island arc occurs in the Middle Unit (M-Unit) of the Asago body from the Yakuno ophiolite, SW Japan. It consists of mafic metamorphic rocks and felsic to mafic later intrusions. The M-Unit is composed of three lithologic zones: the Lower migmatite zone (L-MZ), the Middle metagabbro zone (M-MZ), and the Upper amphibolite zone (U-AZ). The metamorphic grade of the lowest horizon of the M-Unit reaches granulite-facies conditions (P = 3.5-5.5 kbar at T = 850-860°C) and an assemblage of plagioclase + clinopyroxene + orthopyroxene (pseudomorph) + hornblende (tschermakite or magnesiohastingsite) appears in the mafic metamorphic rocks. Structural relationships of the migmatite and related rocks suggest that a small-scale leucosome (i.e. melt) was generated at the bottom of L-MZ and its upward melt segregation led to the formation of medium- to large-scale plutons and dykes of later intrusions into the M-MZ and U-AZ. The bulk rock chemistry of these rocks suggests that the later intrusions were derived from the partial melting of mafic metamorphic rocks, leaving melanosome as residuum. Major and trace element compositional variation of the later intrusions indicates that the leucosome (SiO₂ 48-71 wt%) may be a source of the plutons and dykes of medium-silica content (SiO₂ 52-67 wt%), whereas chemical compositions of the plutons and dykes of high-silica content (SiO₂ 72-76 wt%) and those of the leucosomes do not overlap. The fact indicate that the plutons and dykes of high-silica content may have been derived from either compositionally more differentiated magma or a completely exotic one.