Genesis of calc-alkali andesite magma in a hydrous mantle-crust boundary: Petrology of lherzolite xenoliths from the Ichinomegata crater, Oga peninsula, northeast Japan, part II

Abstract

The Ichinomegata volcano, northwestern Honshu, Japan, consisting of three explosion craters, is characterized by the presence of contemporaneous basalt (high-alkali tholeiite) and calc-alkali andesite and a variety of mafic and ultramafic xenoliths of deep-seated origin. The population of the rock types decreases exponentially as a function of increasing depth of their origin. Based on the Ichinomegata xenolith mineralogy, it is inferred that the lower crust and uppermost mantle beneath this area is partially hydrated, consisting dominantly of hornblende gabbro and hornblende-bearing spinel lherzolite, respectively. Chemical analysis on spinel-pyroxene symplectite (so called garnet pseudomorph) in some Ichinomegata lherzolites suggests a calcic-plagioclase primary chemistry rather than garnet. In lherzolite xenoliths which have undergone a preheating event, primary partial melting textures are observed. The composition of the glass formed along the grain boundaries of the partially melted lherzolites are similar to those produced in hydrous melting experiments on natural peridotite at about 10 kbar between 1000 and 1100°C. The high-alkali tholeiite and calc-alkali andesite of the Ichinomegata volcano are considered to have been formed by the following two-stage melting processes; (1) derivation of the basalt magma from partial melting of a peridotite diapir in the upper mantle at 40–50 km depth; (2) derivation of the calc-alkali andesite magma at 25–30 km depth by wet partial melting of the rocks at the mantle/crust boundary caused by emplacement of hot basaltic magma body. It is proposed that similar wet partial melting takes place more extensively beneath major island-arc volcanoes in the world, because the lower crust and the upper mantle beneath them may be hydrated due to continuous water supply from the subducting plate, and the amount of heat energy liberated at the mantle/crust boundary would be much larger in major stratovolcanoes than in the comparatively small Ichinomegata volcano. From consideration of energy balances, it is expected that the calc-alkali magma formed by this mechanism will be similar in volume to the hot basaltic magma solidified in the deep crust, and the base of the crust is the most likely site where substantial amounts of calc-alkali magma can be produced. It is emphasized that the model accords well with various constraints on the petrogenesis of calc-alkali rocks, among which most important are: (1) within the history of an island-arc volcano, calc-alkali rocks (andesite and dacite) occur in association with tholeiitic or alkalic rocks; (2) the ratio of calc-alkali rocks to other rocks (basalts and their derivatives) increases systematically with average crustal thickness beneath the volcanic belt; (3) calc-alkali rocks frequently show evidence of magma mixing.