九重-別府鮮新世沈降盆地の北西縁に分布する高マグネシア安山岩  [in Japanese] High-magnesian andesites from the northwestern margin of the Pliocene Kuju-Beppu Tectonic Basin, northern Kyushu, Japan  [in Japanese]

九州中北部の九重-別府鮮新世沈降盆地北西端に未分化な組成を有する高マグネシア安山岩(深倉-土師山HMA)を見いだした。これらの岩石は高いMgO, Cr-Ni含有量と低いFeO^*/MgO比を示す。含まれる鉱物の組成もMg#に富んでおり, 単斜輝石のMg#は最高90に達する。このことから深倉-土師山HMAは初生HMAの条件を満たすcpx-HMAである。液相濃集元素の特徴は瀬戸内のHMAに類似している。Sr-Nd同位体組成と液相濃集元素の特徴から, 深倉-土師山HMAの起源マントルは, 耶馬渓ソレアイト玄武岩のソースマントルに, スラブ由来の流体と沈み込む堆積物に伴ってLIL元素が付加されたと考えると, 岩石学的特徴をうまく説明できる。ただしマントルに付加されたLIL元素量によって, LIL元素に富んだグループ(HKG)と, LIL元素にあまり富まないグループ(LKG)の二つが存在していたと考えられる。

Voluminous andesitic lavas and pyroclastics with calc-alkaline affinities are distributed in the northern part of central Kyushu. Basaltic andesites (Fukakura-Doshiyama IIMA) erupted in the northwest margin of the Pliocene Kuju-Beppu Tectonic Basin have been studied petrographically and analyzed for mineral compositions and whole-rock major element, trace element, and Sr-Nd isotopic compositions. The Fukakura-Doshiyama HMAs are characterized by high MgO, Ni, Cr contents and low FeO^*/MgO ratios, and have in compatible element contents similar to those of the west Setouchi-HMA. Compared with the Yabakei tholeiitic basalt, the Fukakura-Doshiyama HMAs show a marked enrichment in K, Rb and Ba relative to Sr, whereas the concentrations of Sr and HFS-elements are similar. The Fukakura-Doshiyama HMAs can be divided into a LILE enriched group (HKG), and a group less-enriched in LILE (LKG). Both the HKG and the LKG were derived by partial melting of the mantle, which had similar compositions to the source mantle of Yabakei basalt. However, the source mantle of Fukakura-Doshiyama HMA was metasomatized by a slab-derived aqueous fluid enriched in LILE; the HKG originated from a mantle source with a higher H_2O content than that of the LKG, assuming the same degree of melting. On the projection from diopside to the plane ol-pl-Q, the HKG and the LKG rocks originated at lower pressure than the Yabakei basalt. The existence of H_2O in the mantle source of the Fukakura-Doshiyama HMAs shifted the multiple saturation boundary to the low pressure side. This is consistent with the incompatible element characteristics observed in the two groups of Fukakura-Doshiyama HMA.