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Geological Society, London, Special Publications; 2003; v. 218; p. 279-298;
DOI: 10.1144/GSL.SP.2003.218.01.15
© 2003 Geological Society of London

Magmatic, Metamorphic and Tectonic Processes in Ophiolite Genesis

Multi-stage evolution of the Tertiary Mineoka ophiolite, Japan: new geochemical and age constraints

N. Hirano1, Y. Ogawa2, K. Saito3, T. Yoshida4, H. Sato5 & H. Taniguchi6

1 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan nhirano{at}geo.titech.ac.jp
2 Institute of Geoscience, University of Tsukuba, Tsukuba 305-8571, Japan
3 Department of Earth and Environmental Sciences, Faculty of Science, Yamagata University, Yamagata 990-8560, Japan
4 Institute of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku University, Sendai, 980-8578, Japan
5 Ocean Research Institute, University of Tokyo, Tokyo 164-8639, Japan
6 Komazawa University Senior High School, Tokyo 158-0098, Japan

The Mineoka ophiolite in the southern Boso Peninsula is situated in a unique tectonic setting in the collisional zone between the Izu and Honshu arcs in Japan. The ophiolitic rocks are composed mainly of tholeiitic pillow basalts and dolerites, alkali-basaltic sheet flows, and calc-alkaline dioritic to gabbroic rocks. The tholeiitic basalts show variable trace element compositions ranging from mid-ocean ridge basalt to island-arc basalt, whereas the alkali-basalts have a within-plate affinity. High-Fe and -Ti tholeiitic basalt and within-plate alkali-basalt have Ar/Ar ages of 49 ± 13 Ma and 19.62 ± 0.90 Ma, respectively. Three plutonic rocks have K-Ar ages of c. 25, 35 and 40 Ma. These ages are inconsistent with the known ages from the Pacific or Philippine Sea Plate. We infer that the Mineoka ophiolitic assemblage was part of another Tertiary oceanic plate, the ‘Mineoka Plate’, which underwent island-arc volcanism in the Miocene as a result of subduction initiation at a fracture zone or a transform fault system due to a change in the position of the Euler rotation pole of the Pacific Plate at c. 43 Ma. Eruption of within-plate type alkali basalts on the Mineoka Plate took place near the palaeo-Japan continental arc just before the emplacement of the Mineoka ophiolite into the Japanese continental margin.





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[Abstract] [PDF]