Finding of corundum-bearing gabbro boulder possibly derived from the Horoman Peridotite Complex, Hokkaido, northern Japan.

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  • 幌満かんらん岩体起源のコランダムを含むガブロの発見
  • Finding of corundum-bearing gabbro boul

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Abstract

Coarse-grained corundum bearing gabbro was found as a boulder in the Ponsanushibetsu river of the Samani Town, Hokkaido, Japan. Its provenance is most probably the GBII gabbro (Shiotani and Niida, 1997) of the Horoman peridotite complex on the basis of mineral assemblage and chemical composition. Deduced from textural relationships between corundum and other minerals, corundum has not been stable in the gabbro under the latest metamorphic condition. The reaction for the corundum breakdown is possibly shown as: corundum+clinopyroxene<br>=spinel+plagioclase (i). This reaction suggests that the GBII protolith had experienced heating or decompression or both to cause the corundum breakdown. On the other hand, we can estimate two possible reactions for corundum formation. First, corundum can be formed by a reaction (i) during cooling or compression after the formation of spinel by a reaction, which is shown as: olivine+anorthite=orthopyroxene+clinopyroxene<br>+spinel (ii), in relatively aluminous protoliths (e.g., plagioclase-rich protoliths). Second, formation of corundum at much higher pressures is also possible by a reaction: Ca-tschermaks in clinopyroxene=corundum<br>+grossular (iii). This reaction means that corundum-bearing garnet-clinopyroxenite can be formed from aluminous clinopyroxenite at high-pressure. The recent interpretation of the Horoman peridotite complex as a mantle diapir ascended from the garnet stability field (Ozawa and Takahashi, 1995) may favor the latter process, the high-pressure origin of the corundum. The GBII gabbro possibly ascended as one of the member of the mantle diapir after it had been metamorphosed under high P-T conditions to form corundum as one of high-pressure minerals.

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