Melting and reaction in an incipient subarc mantle as inferred from the spatial compositional variability in the mantle section of the Oman ophiolite

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概要

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•   The Oman ophiolite is a remnant of Neo-Tethyan oceanic lithosphere that has been modified by arc-related magmatism during oceanic thrusting prior to the obduction to the Arabian continent. To understand the formation of oceanic mantle lithosphere at a spreading ridge and subsequent modification at an incipient subduction zone a km-scale mineral chemical mapping of the mantle section was conducted in the Fizh and Salahi blocks in the northern Oman ophiolite. The range of spinel Cr#[=Cr/(Cr+Al) atomic ratio] in harzburgites becomes wider from the region of paleo-ridge segment center to the margin. In the northern part, where a paleo-ridge segment margin was inferred, refractory harzburgites with spinel Cr# greater than 0.7 are abundant and are distributed as a band (3 km in width) extending from the basal thrust to the Moho. Such high refractory harzburgites are associated with thick dunite bands in which spinel Cr# is also high (greater than 0.7). Dunites in the Fizh and Salahi mantle sections have spinel with Cr# ranging from 0.45 to 0.8 and tend to have higher spinel Cr# than the harzburgites. Moreover, the dunites with high Cr# spinel (greater than 0.7) are abundant in the basal part of the mantle section. The distribution of refractory harzburgite and dunite in the northern Oman ophiolite can be modeled as follows. During oceanic thrusting the Oman ophiolite was displaced above an incipient subduction zone. The fluid released from metamorphic sole due to thermal metamorphism of altered oceanic crust infiltrated into the mantle section. Dunite channels may have acted as pathways for fluid infiltration from the base of the ophiolite, which was instrumental for flux melting of wallrock harzburgite. The presence of the high refractory harzburgites in the northern Fizh mantle section implies that the infiltration of fluid from the base of ophiolite was extensive in the ridge segment boundary region.

著者

• 高澤 栄一

  新潟大学理学部

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• Melting and reaction in an incipient subarc mantle as inferred from the spatial compositional variability in the mantle section of the Oman ophiolite
• 緒言
• オマーンオフィオライト・マントルセクションの組成マッピングからみた初期島弧におけるマントル溶融と反応過程

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