The Oligo-Miocene Anamizu Formation in the Ushitsu-Matsunami area consists of a lower volcano sedimentary member and an upper volcanic member. The upper member is essentially composed of three lava series; tholeiite (basalt and basaltic andesite), calc-alkaline (basaltic andesite) and high-magnesian andesite (bronzite andesite), in ascending order. The tholeiitic basalt is further divided into K-poor and K-rich (K-feldespar bearing) types. The tholeiitic basaltic andesite has higher FeO*/MgO (∼3) than the others (1-1.5). The calc-alkaline basaltic andesite has higher Cr and Ni than tholeiitic basaltic andesite. Bronzite andesite contains higher MgO, Cr and Ni than common calc-alkaline andesite of the same SiO2 content (∼60 wt%), but lower than those of boninite and sanukite. All the rock series are depleted in HFSE (Nb and Ti) in comparison with N-MORB and OIB, suggesting typical subduction-related arc magmas. High Zr/Y ratios of the tholeiitic basalt resemble those of active continental margin magmas rather than island-arc magmas. The HFSE, Ni and Cr compositions indicate a progressive depletion or increasing degree of partial melting of the mantle wedge source in the order tholeiite basalt (basaltic andesite) → calc-alkaline basaltic andesite → bronzite andesite. Spinel in the three series shows different trends: Crpoor (Cr # 0.49 to 0.54) in the tholeiitic basalt, fairly Cr-rich (Cr # 0.61) in the calc-alkaline basaltic andesite, and Cr-rich (Cr # 0.73) in the high-magnesian andesite. These trends indicate different mantle restites; Iherzolite for the tholeiitic basalt and harzburgite for the calc-alkaline basaltic andesite and highmagnesian andesite, and increasing degree of depletion in the order as above. On the contrary, LILE and LREE exhibit a gradual enrichment of the source in the same order. Corresponding decrease of TiO2/K2O suggests that the enrichment has been due to addition of fluids derived from the descending slab. These data indicate that the K-poor tholeiitic basalt magma has been formed by partial melting of the Iherzolitic upper mantle wedge under almost anhydrous conditions; whereas the K-rich tholeiitic basalt magma may have been produced by partial melting of the metasomatized Iherzolitic mantle source under slightly hydrous conditions. The calc-alkaline and bronzite andesite magmas have been produced by partial melting of the hydrous, metasomatized, harzburgitic mantle wedge. The stratigraphy of the lava succession in the studied area (tholeiite basalt-basaltic andesite-> calc-alkaline basaltic andesite → bronzite andesite) indicates that depletion and hydration (metasomatism) of the mantle source have progressed simultaneously.
|ジャーナル||Journal of Mineralogical and Petrological Sciences|
|出版ステータス||Published - 2002|
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