日高変成帯における地殻溶融と酸性マグマの形成  [in Japanese] Crustal anatexis and related acidic magma genesis in the Hidaka metamorphic belt, Hokkaido, northern Japan  [in Japanese]

日高変成帯主帯最下部(深度23〜24 km)には, ザクロ石-菫青石-黒雲母片麻岩(GCB片麻岩), ザクロ石-斜方輝石-菫青石あるいは斜方輝石-斜長石片麻岩(GOC片麻岩)が分布し, 変成条件は870℃, 720MPaを示す。GCB片麻岩では黒雲母の分解溶融反応(黒雲母+Anに乏しい斜長石+石英±ザクロ石=斜方輝石±菫青石+Anに富む斜長石+メルト)の結果, 斜方輝石-斜長石-石英あるいは斜方輝石-菫青石-斜長石-石英の鉱物組み合わせを持つ優白質部が形成されたと考えられる。GCB片麻岩2試料を用いた850℃・700MPa, 含水条件での溶融実験の結果, 天然の含斜方輝石優白質部は, 溶融量は33〜38%程度の初期アナテクシスによって形成されたことが示された。GOC片麻岩はメルトが抜けた後のレスタイトと考えられる。一方, 大規模に分布するS-タイプトーナル岩は, 現在の主帯では見られないさらに深部(深度27〜30 km)での大規模な地殻溶融(50〜60%程度)によってもたらされたものと考えられる。

The Main Zone of the Hidaka metamorphic belt in Hokkaido island is an imbricate stack of an island arc-type crust in which a sequence of units with creasing metamorphic grade from high to low structural levels is exposed. The basal part (23-24 km in depth) of the metamorphic sequence consists mainly of garnet-cordierite-biotite gneiss (GCB gneiss), garnet-orthopyroxene-cordierite and orthopyroxene-plagioclase gneisses (GOC gneiss), orthopyroxene-bearing amphibolite, and orthopyroxene-bearing S-type tonalite. These metamorphic rocks underwent granulite-facies metamorphism with peak P-T conditions of 720 MPa, 870℃. In this zone pelitic granulite (GCB gneiss) includes leucosomes (orthopyroxene-plagioclase-quartz or orthopyroxene-cordierite-plagioclase-quartz) which are explained by biotite-breakdown anatectic reactions such as biotite+An-poor plagioclase+quartz±garnet+vapor=orthopyroxene±cordierite+An-rich plagioclase+melt during the peak metamorphic event. To test the proposition that the leucosome was derived by partial melting of the surrounding metapelite, melting experiments of the two samples of GCB gneiss were carried out for water-saturated and dry systems at 700 MPa and 850℃. The chemical composition of the leucosome from natural rocks shows a peraluminous S-type tonalitic affinity and islocated very close to the tie-line between the melts produced in water-saturated systems and the composition of the equilibrate minerals (orthopyroxene+plagioclase or orthopyroxene+cordierite+plagioclase). This suggests, therefore, that the leucosome was formed by in-situ incipient anatexis of GCB gneiss at close to the highest P-T condition of the Main Zone. It is also suggested that GOC gneiss represents restite after anatexis. On the other hand the S-type tonalite magmas that formed large intrusive masses in the Main Zone were probably generated by large-degree crustal melting in deeper parts (27-30 km) which is not exposed in the Hidaka metamorphic belt.