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

(1)利用斜辉石斑晶估算岩浆微量元素组成阿曼蛇绿岩的火山岩多受热液交替作用，并没有保持其原始成分。本研究通过对斜辉石残斑晶的分析，估算了蚀变岩中岩浆的原始微量元素组成。根据流体可移动的微量元素组成，识别出A、B、C、D、E和F 6种不同类型的岩浆。(2)利用变质底液估算流体微量元素谱在阿曼蛇绿岩基底观测到角闪岩相变质岩（变质底液）的光谱。这些岩石被认为代表了古代大洋俯冲岩石圈的最上层。根据变质底的成分随变质温度的变化，估算了角闪岩相板释放流体的微量元素谱。(3)稀土元素与Sr、Nd、Pb同位素分析方法的发展建立了高效离子色谱法分析稀土元素的方法。采用热电离质谱法和多收集器电感耦合等离子体质谱法对Sr、Nd和Pb同位素进行了分析。(4)讨论本研究结果表明，在扩张轴（A型）岩浆活动之后，发生了与俯冲有关的B型和C型（拉斑岩）岩浆活动，最后发生了D型、E型和F型（拉斑岩和波斑岩）岩浆活动。b型、d型和c型岩浆的微量元素谱与变质底的流体作用一致。这表明这些岩浆来自于俯冲初期角闪岩相板块释放的流体对地幔楔的熔融交代作用。

(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.