ポロシリオフィオライトの生成場 : 微量成分組成からの検討  [in Japanese] Genesis of the Poroshiri ophiolite, Hokkaido, Japan : Inference from geochemical evidence  [in Japanese]

ポロシリオフィオライトの変玄武岩および変成シート状岩脈群はN-MORBに酷似しており,島弧玄武岩や背弧海盆玄武岩とは明瞭に異なった組成的特徴を示す.著しく分化した岩石が出現し組成範囲が広い特徴や,ガブロの構造などから,本オフィオライトは高速拡大海嶺で形成されたと推定される.東太平洋海膨(EPR)における不適合元素比と拡大速度の関係との比較や,地質学的特徴から,本オフィオライトは,海溝近傍の陸源砕屑物が供給されるような場にあった超高速拡大海嶺で形成されたと考えられる.後期貫入ドレライト岩脈群は,N-MORBに類似した組成的特徴を有しているが,N-MORBと比較して重希土類元素にわずかに枯渇し軽希土類元素にやや富んでいる.これらはやや肥沃なマントルソースに由来している.

Seven samples (four metabasaltic rocks and three meta sheeted-dike samples) and two samples of later metadolerite dikes of the Poroshiri ophiolite were analysed by ICP-MS. Chondrite normalized REE values of the metabasaltic rocks exhibit N-MORB pattern. Multi-element spider diagram shows a N-MORB signature except for variable concentrations in LIL elements that are ascribed to secondary modification. These metabasaltic rocks plot in N-MORB fields in discrimination diagrams using immobile elements and are distinct from such as IAT, BABB and E-MORB. Later dolerite dikes, which intruded after considerable cooling of the ophiolite, have also N-MORB signatures. However, the later dikes are slightly depleted in HREE and enriched in LREE concentrations, suggesting more fertile source than the Poroshiri ophiolite. These geochemical signatures and wide compositional range of the metabasaltic rocks with evolved rocks (e.g., 〜3 wt % in TiO_2 and 〜3.0 in FeO ^*/MgO) suggest that the Poroshiri ophiolite was generated at fast-spreading ridge. Zr/Y and Zr/TiO_2 ratios from the EPR basalts tend to decrease with increasing spreading rates. The Zr/Y and Zr/TiO_2 ratios of the metabasaltic rocks are as low as those from the ODP Hole 1256D basalts generated at the EPR with an ultra-fast spreading rate. Assuming that the mantle source of the ophiolite was similar to that of the East Pacific Rise (EPR), this fact suggests that the spreading ridge produced the Poroshiri ophiolite spread at an ultra-fast rate more than 20 cm/year.