Reversed Metasomatism at the Slab-Mantle Interface in a Continental Subduction Channel: Geochemical Evidence from the Ultrahigh-Pressure Metamorphic Whiteschist in the Western Alps
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摘要: 前人对深俯冲板片释放熔/流体交代地幔楔形成弧岩浆源区的过程和机制已得到充分认识，然而对地幔楔岩石能否脱水交代深俯冲地壳并不清楚.在对欧洲西阿尔卑斯造山带Dora-Maira地体白片岩的地球化学研究中，首次发现地幔楔交代岩能够脱水反向交代深俯冲地壳岩石，从而极大影响俯冲地壳的地球化学组成.结合白片岩和围岩的全岩地球化学特征以及锆石学结果，查明了白片岩的原岩为S型花岗岩，澄清了关于Dora-Maira白片岩原岩属性的长期争议.在此基础上，发现白片岩中变质锆石相对残留岩浆锆石δ18O值显著降低，指示原岩形成后受到低δ18O变质流体的交代作用.白片岩具有高温岩石中最高的δ26Mg值达0.75‰，显著高于围岩变花岗岩，指示交代流体具有重Mg同位素组成.基于地球主要岩石储库的Mg同位素组成，推测交代流体来自俯冲隧道中富滑石地幔楔蛇纹岩在弧下深度的脱水分解，而这些地幔楔蛇纹岩是新特提斯洋壳在弧前深度变质脱水产生的流体与地幔楔浅部橄榄岩反应形成.这些结果不仅提供了利用Mg-O同位素示踪俯冲隧道中流体来源的新思路，也提供了地幔楔蛇纹岩来源流体反向交代深俯冲地壳岩石的首个典型实例.这种反向交代不仅极大改变了深俯冲地壳的地球化学组成，而且对弧岩浆岩重Mg同位素成因具有重要意义.Abstract: It is well known that the mantle wedge metasomatized by fluids derived from the subducting slab serves as the source of arc magmas. However, it is uncertain whether the subducting crust would be metasomatized by fluids released from mantle wedge metasomatites. Now this is firstly illustrated by a detailed study of petrology and geochemistry for whiteschist from the Dora-Maira Massif in the Western Alps.Based on the whole-rock geochemistry and zirconology for the whiteschist and its country rock, it is concluded that the protolith of the whiteschist is a kind of S-type granites, providing a resolution to the long-standing controversy on the protolith nature of the whiteschist in this region. The δ18O values of metamorphic zircon in the whiteschist are significantly lower than those of magmatic zircon, suggesting that the protolith was metasomatized by low δ18O fluids before the peak UHP metamorphism. The whiteschist shows the highest δ26Mg values up to 0.75‰ among high-T silicate rocks, which are much higher than those of the metagranite, suggesting that the metasomatic fluids have heavy Mg isotope compositions.Based on the Mg isotope systematics of major terrestrial silicate reservoirs, it is proposed that such fluids would probably originate from talc-rich serpentinites that were generated at forearc depths by hydration of the mantle wedge peridotite during prograde subduction of the Neotethyan oceanic slab. The mantle wedge serpentinites were then metastably carried by the subducting continental crust to subarc depths, where they underwent dehydration for reversed metasomatism of the deeply subducting continental crust at the slab-mantle interface in the continental subduction channel. The results provide not only the new idea for tracing fluid sources in the continental subduction channel, but also the first example that the deeply subducting continental crust underwent the reversed metasomatism by the fluids derived from dehydration of the mantle wedge metasomatite. This reversed metasomatism would greatly modify the geochemical composition of the deeply subducted continental crust, which has bearing on the origin of arc magmas rich in heavy Mg isotopes.
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Key words:
- Mg isotopes /
- O isotopes /
- subduction channel /
- reversed metasomatism /
- western Alps /
- geochemistry
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图 1 西阿尔卑斯造山带Dora-Maira地体白片岩和围岩变花岗岩中残留岩浆锆石U-Pb年龄和Hf-O同位素组成对比
Fig. 1. The comparison of δ18O values, U-Pb ages and εHf(t1) values for the relict magmatic zircons of the metagranite and whiteschist from the Dora-Maira Massif, western Alps
图 2 西阿尔卑斯Dora-Maira地体白片岩和变花岗岩的Mg同位素组成以及地球主要硅酸盐储库的Mg同位素组成
Fig. 2. The Mg isotope composition of whiteschist and metagranite in the Dora-Maira Massif and the major silicate reservoirs of the Earth
图 3 全岩δ26Mg值与δ18O值、Na2O+CaO、MgO以及CIA的关系
数据来自Chen et al.(2016)；CIA为化学风化指数，指Al2O3/(Al2O3+Na2O+K2O+CaO)的摩尔数比值
Fig. 3. Correlations between whole-rock δ26Mg values and δ18O values, Na2O+CaO, MgO and CIA
图 4 西阿尔卑斯造山带Dora-Maira (DM)地体白片岩形成过程的构造示意
修改自Chen et al.(2016)；图中Europe指欧洲陆块，BR指Brianç onnais地体
Fig. 4. The tectonic model for the formation of Dora-Maira whiteschist in the western Alps
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