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1 Institute of Geosciences, Shizuoka University, 836 Oya, Shizuoka 422-8529, Japan
The discovery of boninite, a typical high-MgO andesite, in the Oman ophiolite is reported. The boninites in the Oman ophiolite occur as lavas and dikes of the Alley volcanic sequence that overlie or crosscut the spreading-ridge–derived lavas (Geotimes volcanic sequence) and sheeted dikes. The phenocryst mineral assemblage and the major and trace element compositions observed for these boninites resemble those of the Izu-Mariana forearc boninites, indicating that the Alley boninites represent primitive melt generated by partial melting of hydrous peridotite. The occurrence of boninite provides strong thermal and chemical constraints on the formation of the Oman ophiolite that require hot, hydrous shallow mantle (>1250 °C at <30 km depth) to have underlain the proto–Oman ophiolite at the time of boninite generation. The initiation of subduction of the young, hot oceanic lithosphere (and obduction of the future Oman ophiolite) near the spreading ridge and the resultant melting of the highly depleted, shallow-mantle wedge metasomatized by slab-derived fluid represent the most favorable mechanism for the genesis of the Alley boninites.
Key Words: boninites • Oman ophiolite • mineralogy • geochemistry • subduction zones • thermal structure
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