Differentiation of a Hydrous Arc Magma Recorded in Melt Inclusions in Deep Crustal Cumulate Xenoliths from Ichinomegata Maar, NE Japan. (15th March 2018)

• Record Type: Journal Article
• Authors: Yanagida, Y.; Nakamura, M.; Yasuda, A.; Kuritani, T.; Nakagawa, M.; Yoshida, T.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 19:Number 3(2018)
• Page Start: 838
• Page End: 864
• Publication Date: 2018-03-15
• Subjects: hornblende fractionation -- lower crust -- cumulate xenoliths -- calc‐alkaline -- alumina saturation index; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1002/2017GC007301
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: The Ichinomegata maar, located in the back‐arc side of the northeastern Japan arc, erupted calc‐alkaline andesitic magma with abundant deep‐crustal and mantle xenoliths at about 60–80 ka. We investigated the relationship between fractionated solids and differentiated silicic melts at middle to lower crust conditions through petrologic analyses of hornblende‐bearing cumulate xenoliths and melt inclusions. The Sr and Nd isotope compositions are similar to those of the host magmas, suggesting their cognate origin. 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 five types of xenolith studied, the leuco‐hornblende gabbro preserved interstitial glass and melt inclusions with a 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, which corresponds to a depth of 15–24 km. 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 inclusions, starting from primitive basalt via successive fractionation of a calculated wherlite and cumulate xenoliths. In middle to late stage differentiation, hornblende gabbros are the dominantly fractionated assemblages. Our results are consistent with published differentiation experiments of hydrous arc magmas and the lithology of the deep crust beneath the back arc of the NE Japan arc as estimated from seismic tomography. Key Points: Crystallization order were determined for a series of mid to hornblende bearing cumulate xenoliths from Ichinomegata maar (NE Japan arc) Corundum‐normative calc‐alkaline suite was reproduced by fractionation of the xenoliths, supporting experiments at lower crustal pressure Melt inclusions in the cumulate contain < 8.1 wt % H2 O, supporting the presence of super hydrous melt inferred from magnetotelluric survey