南岭西段西山含铁橄榄石长英质岩石的成因、演化及其地球动力学意义

Fayalite-bearing felsic rocks are a special type of granitic rocks, and these rocks are extremely rare in nature. The special rocks provide us with a window into the processes that occur in the deep crust or deep within granitic plutons. However, the current study on deep magmatic processes of controlling their formation and evolution is weak, restricting the knowledge and understanding on genetic mechanism of fayalite-bearing felsic igneous rocks. A suite of fayalite-and ferrosilite-bearing felsic porphyroclastic lava, with abundant ultramafic enclaves, are outcropped in the Xishan granitic complex, which is situated in the western part of the Nanling region. This provides a rare opportunity for solving the foregoing scientific question. In this work, we carry out petrography, mineral microgeochemical and isotopic analysis, whole-rock chemical and isotopic analysis, as well as quantitative texture analysis comprehensively study the source, genesis and evolution of the Xishan fayalite-bearing granitic rocks. We aim to interpret the petrogenetic mechanisms of such mafic minerals as fayalite, ferrosilite and ultramafic microenclaves, and reveal the source characteristics of the fayalite-bearing granitic rocks, and further intensively discuss their deep magmatic processes. This work will shed new lights on the characteristic and evolutionary research of the Late Mesozoic deep lithosphere in inland area of South China.

含铁橄榄石长英质岩石是一种特殊的岩石类型，在自然界非常稀少。该类岩石为揭示地壳深部或花岗岩体成因过程提供了重要窗口。然而，目前该类岩石还缺乏对控制其形成与演化的深部岩浆过程的深入研究，限制了对其成因机制的理解和认识。位于南岭西段的西山花岗质杂岩体产出了含铁橄榄石、铁辉石等铁镁质矿物以及超镁铁质微型包体的长英质碎斑熔岩，为解决上述科学问题提供了难得的机遇。本项申请拟运用岩相学、矿物微区元素-同位素地球化学、岩石地球化学以及定量化结构分析等方法，对西山含铁橄榄石长英质碎斑熔岩的源区、成因及其演化进行综合研究，查明岩石中铁橄榄石、铁辉石等铁镁质矿物以及超镁铁质包体的成因机制，揭示含铁橄榄石长英质岩石的源区特征，探讨含铁橄榄石长英质岩石的深部岩浆过程，为深入研究华南内陆地区晚中生代深部岩石圈特征及其演化提供新约束。

含铁橄榄石长英质岩石是一种特殊的岩石类型，该类岩石为揭示地壳深部硅质岩浆作用以及花岗岩的成因过程提供了重要窗口。然而，究竟是怎样一种深部岩浆过程控制了该类岩石的产生仍不十分清楚。本项目选择南岭西段西山火山-侵入杂岩体中产出的含铁橄榄石长英质岩石为研究对象，开展了系统的岩石学、岩石地球化学、矿物微区元素-同位素化学以及定量化结构分析等研究。.本研究发现，西山杂岩体中除黑色碎斑熔岩以外，在灰白色碎斑熔岩和似斑状黑云母二长花岗岩中也含有铁橄榄石。通过岩相学、矿物化学以及同位素分析，揭示出岩石中的铁橄榄石、铁辉石晶体种群和结构具有多样性，且它们在源区和成因上存在一定的差异。岩相学证据、矿物化学以及斜长石Sr同位素研究表明，西山黑色碎斑熔岩中发育的微型包体与岩浆房的晶粥活化作用有关，推测它们是来自深部晶粥储库的堆晶体碎块。各类岩石具有相似的全岩Sr-Nd同位素（(87Sr/86Sr)i=0.719456～0.720918；εNd(t)= -8.14～-7.38）和锆石O同位素组成（δ18O=8.04～10.46‰）。岩石的锆石εHf(t)主体介于-7～-5之间，磷灰石εNd(t)主要介于-10～-7之间。各类同位素分析结果一致表明西山各类含铁橄榄石长英质岩石源于相同的壳源源区。此外，岩石中发育的暗色微粒包体的Sr-Nd同位素（（(87Sr/86Sr)i=0.712100～0.7125946；εNd(t)=-3.50～-3.08））及斜长石Sr同位素（(87Sr/86Sr)i=0.7075～0.7088）组成均与含铁橄榄石长英质岩石（(87Sr/86Sr)i(pl)=0.7176～0.7178）明显不同，表明在西山含铁橄榄石岩石的形成过程中有幔源岩浆的加入。综合各类研究，我们提出西山各类含铁橄榄石长英质岩石形成于晶粥体系，且该体系经历了深部和浅部等多个岩浆储库的演化以及晶体-熔体分离、晶粥再活化作用等。这一研究揭示出晚侏罗世时华南内陆地区存在复杂的同源穿地壳岩浆系统。通过综合对比研究，我们提出高温、低-中H2O含量、低氧逸度以及高分异熔体是含铁橄榄石长英质岩石产生不可或缺的条件。

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