Keywords:hornblende fractionation, lower crust, cumulate xenoliths
The Ichinomegata maar, located in the back-arc side of the northeastern (NE) Japan arc, erupted calc-alkaline andesitic magma with abundant deep-crustal and mantle xenoliths ca. 60–80 ka. In this study, we show the relationship between the fractionated solids (i.e. cumulate xenoliths) and silicic melts differentiated at middle to lower crust conditions. We selected 16 hornblende-bearing cumulate xenoliths characterized by clear cumulus texture and divided them into 5 types (olivine-hornblende clinopyroxenite, clinopyroxene hornblendite, mela-hornblende gabbro, hornblende gabbro and leuco-hornblende gabbro) based on mineral assemblages. The crystallization sequence is determined to be olivine, spinel → clinopyroxene → hornblende, magnetite → plagioclase → apatite, based on the observation of texture and solid solution compositions. Of the 5 types of xenolith studied, the leuco-hornblende gabbro preserved interstitial glass and melt inclusions with silica content (SiO2) of 63.9–74.0 wt% and high water content (up to 8.1 wt%). Hornblende geobarometry indicates an equilibrium pressure of 0.39–0.64 GPa. The delay of plagioclase crystallization due to high water content characterizes differentiation of the hydrous arc magma. We successfully constructed an internally consistent differentiation model of the corundum-normative calc-alkaline trend to the silicic melt inclusion compositions, starting from an assumed primitive basalt via successive fractionation of calculated wherlite and cumulate xenoliths. In the middle to late stage differentiation, the dominant fractionation of hornblende gabbros efficiently increases SiO2 in the melt. The crystallization sequence and melt composition trend show very good agreement with the results of recent differentiation experiments at lower crustal conditions (Nandedkar et al., 2014, Contrib. Mineral. Petrol.). As for major elements, the silicic end-member composition of the Ichinomegata host magma overlapped with the melt inclusion compositions and thus was reproduced by the differentiation model. The Sr and Nd isotope compositions are also similar to those of the host magmas, suggesting the cognate origin of cumulate xenoliths. This petrogenetic relationship provides a rare opportunity to elucidate pre-eruptive magmatic activity in the deep crust of NE Japan arc.