海洋リソスフェアとオフィオライト: 類似点と相違点

Petrological constitution of the oceanic lithosphere is discussed in comparison with the origin and derivation of ophiolitic suites. Recent ODP results for Hess Deep, equatorial Pacific, indicate the petrological similarity of deep-seated oceanic rocks of a fast spreading ridge system with those of some ophiolites, such as Samail ophiolite, Oman. Mantle peridotite from Hess Deep is harzburgite with chromian spinel of Cr# from 0.5 to 0.6. Dunite is common around gabbro-troctolite intrusions. Chromian spinel is sometimes concentrated within dunite and troctolite. Interaction of high-pressure MORB with harzburgite may be more pervasive in the fast spreading ridge than in the slow spreading one. Lithospheric slice as an ophiolite complex has a much more complicated history than the present-day oceanic lithosphere. Some petrological discrepancy, therefore, is expected between ophiolite and oceanic lithosphere. Arc-related rocks in the ophiolite may have been formed at a relatively later stage of arc-like setting which is inevitable for the oceanic lithosphere to be obducted onto continental margins. Highly refractory rocks with Cr-rich spinel (Cr# > 0.7), which are commonly found in ophiolites and have not been found in the present-day ocean floors, were formed at arc-related settings or could be rarely present in the oceanic setting. It is noteworthy that all of the constituent rocks of ophiolites, and even those from ocean floor, were not always formed at the same tectonic setting