西南日本,後期新生代玄武岩類の起源マントルの特徴  [in Japanese] Chemical and isotopic characteristics of the source mantle for late Cenozoic basalts in southwest Japan  [in Japanese]

西南日本に分布する後期新生代の玄武岩類は北部九州地域, 中国山地地域, 阿武地域の3地域に区分される。これら3地域の玄武岩類について, その全岩化学組成とSr-Nd同位体組成を検討した。北部九州地域にはソレアイト〜アルカリ玄武岩が分布し, いずれもWPB-typeの化学的特徴を有する。一方, 中国山地と阿武地域にはWPB-typeとIAB-typeの両方の特徴を有するアルカリ玄武岩が認められる。このうちWPB-type を示す玄武岩について起源マントル中のインコンパチブル元素組成を推定し, Sr-Nd同位体組成との相関関係を検討した結果, 北部九州地域のものはN-MORBとenrichした起源マントルとの間に成因的な相関が認められるが, 中国山地地域では相関関係を示さないことが明らかになった。このことは, 北部九州地域の玄武岩の起源マントルはN-MORB起源マントルとEMII-typeのenrichマントルとの成因的関与があることを示し, 一方, 中国山地地域の玄武岩の起源マントルは, EMI-typeの起源マントルそのものの不均質性を示していると考えられる。

Late Cenozoic basalts (ca. <12m.y.) are widely distributed along the southwest Japan arc. These basalts are divided into the three groups: North-Kyushu district basalt (NKD), Abu district basalt (ABD) and Chugoku district basalt (CHD) on the basis of distribution, age, and chemistry. In the north-Kyushu district, voluminous eruption of weakly differentiated basalts formed lava plateaus during the Miocene to Pliocene. Basalts in the Otsu area of northwest Yamaguchi Prefecture are included with NKD because of their similar chemistry. These basaltic rocks have in compatible-element patterns typical of ocean-island basalt (OIB) or within-plate basalt (WPB), such as positive Nb and Ba anomalies. They occupy the within-plate alkali basalt (WPA) and within-plate tholeiite (WPT) fields on trace element discrimination diagrams. In contrast, basalts from Aso volcano and Kirishima volcano located along the active volcanic front plot in the volcanic arc basalt (IAB) field. In the Chugoku district, Miocene to Pleistocene alkali basalt are sporadically distributed as monogenetic volcanoes. These basalts are primitive and contain ultramafic xenoliths. Two types of basalts, MF-type and SA-type, are recognized. MF-type basalt has within-plate basalt signature and SA-type basalt has island-arc basalt signature. In the Abu district, Quaternary monogenetic volcanic groups (consisting of both alkaline basalt and calc-alkaline andesite) are dominant. They have trace element characteristics of WPB and IAB. WPB-type basalts of NKD and ABD have similar chemical and isotopic characteristics. This suggests that the source mantle compositions of ABD was similar to that of NKD. Calculated-mantle/N-MORB source mantle normalized diagrams show the source mantle region of NKD was enriched in LIL-elements, with negative Nb and positive K anomalies. N-MORB source mantle normalized patterns of the CHD source mantle are enriched in LIL-elements. Late Cenozoic basalts with WPB-type signature from CHD have Sr and Nd isotopic ratios similar to those of oceanic island basalts, which plot along the mantle array. NKD and ABD basalts have relatively high ^<87>Sr/^<86>Sr ratios and therefore plot on more flat-lying trends to the right of the mantle array. Based on the chemical and isotopic features of the source mantle and the tectonic setting, a genesis of the following model for late Cenozoic basalts in the southwest Japan is proposed. In the Chugoku district, an enriched mantle plume with an EMI signature ascended from deep in the mantle. Mantle diapirs rose rapidly and produced liquids formed small degrees of partial melting. The liquid would segregated at deeper levels in the upper mantle. On the other hand, mantle diapirs characterized by an EMII signature rose slowly and were entrained an overlying N-MORB like depleted mantlebe low the north Kyushu district. The enriched plumes were progressively melted, with these enriched melts mixed with depleted melts produced by partial melting of the shallow level depleted N-MORB source mantle. Thus, compositions were changed from alkalic to tholeiitic. The liquids were segregated at shallow level 1n the upper mantle, then stored in lower and/or middle crust as reservoirs where fractional crystallization and/or magma mixing took place.