The Ryozen Formation, which crops out on the trench side of the NE Japan arc, contains middle Miocene rhyodacite with adakite-like trace element geochemical characteristics (Ryozen adakitic rhyodacite) and spatially and temporally related basalt (Ryozen basalt) and andesite (Ryozen andesite). K-Ar age data for the basalt and a zircon U-Pb age for the adakitic rhyodacite, combined with the stratigraphy, suggest that all of these volcanic rocks were erupted at about 16-14 Ma. The primitive nature of the Ryozen basalt is shown by its high MgO (maximum 14·1wt %), Ni (392 ppm) and Cr (1193 ppm) contents. Using the olivine maximum fractionation model, the segregation depth of the parental primary magma to this basalt is estimated at c. 50 km (about 1·5 GPa).The Ryozen andesite has slightly higher 87Sr/86Srinitial (SrI) and lower 143Nd/144Nd initial (NdI) ratios than the Ryozen basalt.This, and the characteristics of the variation trends defined by basalt and andesite samples in SiO2 versus major and trace element variation diagrams, suggests that the andesite may have resulted from fractional crystallization of basaltic magma with minor assimilation of pre-Cretaceous sedimentary rocks (i.e. an AFC process). The Ryozen rhyodacite has phenocrysts of plagioclase, amphibole, garnet and titanomagnetite, and is characterized by low Sr/Y ratios (o30), low Yconcentrations (510 ppm), high chondrite-normalized La/Yb [(La/Yb)cn] (425) and low chondrite-normalized Yb [(Yb)cn] values (55 ppm). These geochemical characteristics are similar to those of a new adakite subgroup (rhyodacite lavas in eastern Jamaica; Jamaican-type adakite). Thus, we define the Ryozen rhyodacite as the Ryozen low Sr/Yadakitic rhyodacite. A potential mechanism for the generation of this rhyodacite is crystal fractionation of plagioclase, orthopyroxene, clinopyroxene, amphibole, garnet, titanomagnetite and minor apatite from an andesitic parent magma.This mechanism is consistent with mass-balance modeling, which matches the observed major and trace element chemistry, as well as SrI and NdI, for the Ryozen andesite and low Sr/Yadakitic rhyodacite.The most likely tectono-magmatic model for the production of the volcanic rocks of the Ryozen Formation involves the upwelling of depleted hot asthenosphere, which modified the thermal structure of the mantle wedge beneath the trench side of the arc during the middle Miocene.This resulted in partial melting of both mantle wedge peridotite and the relatively cool subducting Pacific plate, leading to the simultaneous production of primitive basalt, normal andesite, high-magnesium andesite and low and high Sr/Y adakitic rhyodacites.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology