Formation of layered structure in the Murotomisaki gabbroic complex, especially picrite gabbro, Kochi Prefecture, Japan : Quantitative evaluation of crystal accumulation

• Published in: Chishitsugaku zasshi Vol. 105; no. 11; pp. 771 - 788
• Main Authors: Akatsuka, Takashi, Obata, Masaaki, Yokose, Hisayoshi
• Format: Journal Article
• Language: Japanese; English
• Published: The Geological Society of Japan 1999
• Subjects: Kochi Prefecture
• ISSN: 0016-7630; 1349-9963
• DOI: 10.5575/geosoc.105.771

The Murotomisaki gabbroic complex is a sill-like, sheeted layered intrusion, located in Cape Muroto, Kochi Prefecture, and is composed of gabbro, olivine gabbro and dolerite. In order to investigate the origin of the layered structure, whole rock chemical analyses and mineral chemical analyses were carried out for a series of constituent rock types. On the basis of mass balance calculations using the analytical data, a quantitative assessment was made for the crystallization and crystal settling processes. In particular, "picrite gabbros" which are situated in the lower and the upper parts of the intrusion, just above and below the chilled margins respectively, were examined in detail. The composition of the melt phase in the initial magma was estimated from the whole rock and mineral chemical analyses of the chilled margins. It was shown that olivine crystals (Fo83) accumulated (20 vol%) to form the lower picrite gabbro with an interstitial melt up to 74 vol%. Observed REE abundance data are consistent with this model. Whereas, accumulation of more Fe-rich olivine and plagioclase are necessary to account for the the upper picrite gabbro, where the volume fraction of the melt was estimated to be 68 vol%.