Lower crustal metasomatism inferred from mafic xenoliths from Ichinomegata crater, Northeast Japan arc

Abstract

Gabbroic xenoliths from Ichinomegata crater, Northeast Japan arc, contain a large amount of amphiboles. Some of them are obviously of secondary origin in hornblende-pyroxene gabbros; clinopyroxene in the pyroxenespinel symplectite, which is subsolidus reaction products of olivine and plagioclase by cooling, was replaced in part with amphibole. Minerals (amphibole, clinopyroxene and spinel) in the symplectite contain appreciable TiO 2 (up to 1.6 wt%), although olivine and plagioclase are free from TiO 2 . The amphibole in the symplectite is significantly enriched in Na, K, Rb and Ba relative to the other symplectite minerals that contain very low amounts of these elements. The metasomatic formation of amphibole in the symplectite was accompanied with addition of, at least, Ti, Na, K, Rb and Ba. The metasomatism is characterized by enrichment of incompatible elements, especially LILE but not HFSE (Nb, Ta, Zr and Hf ). This was caused by infiltration of fluid possibly released from hydrous arc magmas. Metasomatic processes observed in the lower crustal gabbros are also recorded in peridotite and websterite xenoliths of upper mantle origin from Ichinomegata. This means widespread modification of mineral chemistry and mineral assemblage from the upper mantle to the lower crust beneath the arc.

Differentiation of a Hydrous Arc Magma Recorded in Melt Inclusions in Deep Crustal Cumulate Xenoliths from Ichinomegata Maar, NE Japan

“…Their Sr 87 /Sr 86 ratio ranges from 0.703316 to 0.703582, which is slightly higher (by 0.0005) than the basalts from the Ichinomegata and Sannomegata maars (Yamamoto et al, 2013). This isotopic similarity with the host magma, as well as absence of any distinct features of secondary reactions (e.g., hornblende-bearing symplectite) as described by Takeuchi and Arai (2014), suggests the cognate origin of the cumulate xenoliths and formation of the silicic end-member magma via fractionation in the lower crust.…”

“…A hornblende-lithology of the deep crust beneath the back arc of the NE Japan arc is estimated from seismic tomography and experimental measurements of seismic wave velocities at high pressure and temperature conditions (Nishimoto et al, 2005(Nishimoto et al, , 2008. The lithology may be form by fractionation of hydrous magmas as investigated in this study, or by hydration of a preexisting dry gabbroic assemblage (olivine, pyroxene and plagioclase) by aqueous fluids exsolved from late stage of hydrous arc magma differentiation (Takeuchi & Arai, 2014).…”

“…The Sr and Nd isotope compositions of the six representative cumulate xenoliths are similar to those of the host magma, suggesting the cognate origin of the xenoliths and formation of the silicic end‐member magma via fractional crystallization in the lower crust (Figure ). A hornblende‐lithology of the deep crust beneath the back arc of the NE Japan arc is estimated from seismic tomography and experimental measurements of seismic wave velocities at high pressure and temperature conditions (Nishimoto et al, ). The lithology may be form by fractionation of hydrous magmas as investigated in this study, or by hydration of a preexisting dry gabbroic assemblage (olivine, pyroxene and plagioclase) by aqueous fluids exsolved from late stage of hydrous arc magma differentiation (Takeuchi & Arai, ).…”

“…Takahashi () summarized modal proportions of the xenoliths from the Ichinomegata maar and reported that the hornblende gabbroic xenoliths were more abundant than pyroxene‐hornblende gabbro and amphibolite xenoliths. Takeuchi and Arai () reported the occurrence of symplectic hornblende in hornblende‐pyroxene gabbros and concluded that it was formed by the reaction between preexisting anhydrous minerals with fluid released from hydrous arc magmas at the lower crust. In this study, we selected 16 samples of the hornblendite‐hornblende gabbro series with clear cumulus characters, i.e., relatively course and euhedral grains without remarkable foliation and the symplectitic hornblende (Takeuchi & Arai, ), determined their mineral assemblage, modal proportions, and mineral compositions, and then classified into five types (Figure ) as described in detail later.…”

“…The geophysical observation that the maximum V p is 7.0 km/s may be explained by an assemblage mainly composed of hornblende and plagioclase. Such a lithology may be formed by the fractionation of hydrous magmas as investigated in this study, or the hydration of a pre-existing dry gabbroic assemblage (olivine, pyroxene and plagioclase) by aqueous fluids exsolved from late stage hydrous arc magmas (Costa et al, 2002;Takeuchi & Arai, 2014).…”