ポロシリオフィオライト中にトラップされた日高変成帯起源の異地性岩塊

Abstract

The boundary between the Hidaka metamorphic belt (HMB) and the Poroshiri ophiolite is a major thrust fault ; the Hidaka Main Thrust. The westernmost part of the HMB consists of an intense mylonite zone (western-margin mylonite zone) along the Hidaka Main Thrust. The ophiolite is cut by several brittle shear zones which strike NW-SE and dip NE. Exotic slices consisting of tonalitic mylonite (including hornblende-biotite gneiss and garnet bearing biotite gneiss) and brown hornblende amphibolite occur along the shear zones. The brown hornblende amphibolite is exposed as blocks in the tonalitic mylonite or intercalated with the mylonite. The bulk compositions of the hornblende-biotite gneiss and garnet bearing biotite gneiss are comparable with the lower or basal tonalite of the HMB. Major and minor element chemistries of the brown hornblende amphibolite show MORB features. K2O contents of these exotic rocks are different from those of amphibolites in the Poroshiri ophiolite, but are very similar to those of brown hornblende amphibolite of the HMB. Garnets in the tonalitic mylonite have the composition similar to those of basal tonalites of the HMB, and exhibit a reverse zoning at the margin. Hornblendes of the hornblende-biotite gneiss and the brown hornblende amphibolite have a brownish core enclosed by a greenish rim. Ti contents of the hornblende cores are similar with those of brown hornblende amphibolite in the HMB, and decrease in the green rims. Such compositional changes suggest that they underwent retrograde metamorphism. On the other hand, surrounding ophiolite rocks suffered from prograde metamorphism. The above facts indicate that these exotic slices are not originated in the ophiolite, but derived from the HMB. Studies of deformed structures in these slices and the surrounding rocks lead to the following conclusions: these rocks are ‘far-travelled horse’ accreted to the footwall (the Poroshiri ophiolite) derived from the hangingwall (the lower sequence of the HMB) of the HMT. The lower sequence of the HMB thrust to the west over the ophiolite and formed a nappe. Then, a part of the lower sequence of the HMB was tectonically intermingled with the ophiolite along the NW-SE trending shear zones.