Magnatism in the initial stage of subduction : inference from the Oman ophiolite

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(1)Estimation of the trace element composition of magmas using clinopyroxene phenocrystsMost of the volcanic rocks of the Oman ophiolite suffer from hydrothermal alternation, and does not preserve their original compositions. In this study, primordial trace element composition of the magma was estimated for the altered rocks by analyzing the relict clinopyroxene phenocrysts. Six distinct types of magma (A,B,C,D,E and F) were recognized in terms of the fluid-mobile trace element composition.(2)Estimation of the trace element spectrum of the fluids using metamorphic soleSlivers of the amphibolite-facies metamorphic rocks (metamorphic sole) are observed at the base of the Oman ophiolite. These rocks are considered to represent uppermost part of the ancient subducted oceanic lithosphere. Trace element spectrums of the fluids liberated from the amphibolite-facies slab were estimated based on the compositional change of the metamorphic sole as a function of the metamorphic temperature.(3)Development of the analytical methods of rare-earth elements and Sr,Nd, and Pb isotopesAn analytical method of rare-earth elements using a high-performance ion chromatography was developed. Analytical techniques of Sr,Nd and Pb isotopes were also developed using thermal ionization mass spectrometry and multi-collector inductively coupled plasma mass spectrometry.(4)DiscussionThe results of this study shows that magmatism at the spreading axis (type A) was followed by subduction-related magmatism of types B and C (tholeiitic), and then magmatism of types D,E and F (both tholeiitic and boninitic) finally took place. Trace element spectrums of the type-B, type-D and type-C magmas are consistent with the involvement of the fluids similar to those estimated from the metamorphic sole. This suggests that these magmas were derived from melting of the mantle wedge metasomatized by fluids liberated from the amphibolite-facies slab in the initial stage of subduction.