Fluid is a key agent for the mass transfer between the subducting slab and the mantle wedge, which greatly affects the evolution of the crust-mantle system at convergent plate boundaries. A geochemical study was carried out for coesite-bearing jadeite quartzite and its country rock granitic gneiss from the Dora Maira Massif in the Western Alps. The results provide new insights into the composition of metamorphic fluids and fluid-rock interaction processes in the continental subduction zone. Coesite inclusions are found for the first time in metamorphic zircons of the granitic gneiss. The ultrahigh-pressure (UHP) metamorphism was dated to occur at 34.7 ± 0.3 Ma, confirming that the country rock experienced the UHP metamorphism synchronously with the whiteschist and jadeite quartzite. The jadeite quartzite occurs as layers and boudins within the coesite-bearing whiteschist, and shows similar whole-rock REE distribution patterns to both granitic gneiss and whiteschist. In addition, relict domains of magmatic zircon in the three types of UHP metamorphic rocks exhibit similar U-Pb ages and δ¹⁸O values, indicating that they have the same protolith of granites. However, metamorphic zircons in the jadeite quartzite show significantly lower δ¹⁸O values of 6.1–7.3‰ than the relict magmatic domains of 9.4–10.6‰. Furthermore, the jadeite quartzite has whole-rock δ¹⁸O values of 7.9–8.7‰, significantly lower than those of 9.5–10.9‰ for the granitic gneiss but higher than 6.5–7.9‰ for the whiteschist. These differences suggest that the jadeite quartzite was probably formed through metasomatism of metagranite by external fluids with relatively low δ¹⁸O values. The jadeite quartzite shows high δ⁵⁶Fe values of 0.69–0.87‰, considerably higher than those of 0.18–0.38‰ for the granitic gneiss, but falling within the range of 0.32–1.03‰ for the whiteschist from the same outcrop. The Fe isotope modeling results suggest that the metasomatic fluids responsible for the formation of jadeite quartzite were possibly derived from the host whiteschist under UHP metamorphic conditions. Such fluids are enriched in Si, Al and Mg, and have high δ⁵⁶Fe but low δ¹⁸O values. Therefore, the jadeite quartzite records the action of metamorphic supercritical fluids at the subarc depth. If the metasomatized rocks in the continental subduction zone would dehydrate to generate fluids with variable geochemical compositions due to multistage fluid-rock interactions, the fluids could further infiltrate the surrounding rock and possibly the mantle wedge to result in further geochemical transfer from the subducting slab to the mantle wedge.

流体是俯冲板片与地幔楔之间传质的关键因素，对会聚板块边界的壳幔系统演化有很大影响。对来自西阿尔卑斯山多拉迈拉地块的含柯石英的硬玉石英岩及其围岩花岗片麻岩进行了地球化学研究。结果为大陆俯冲带变质流体的组成和流体-岩石相互作用过程提供了新的见解。柯石英包裹体首次在花岗片麻岩的变质锆石中被发现。超高压 (UHP) 变质作用发生在 34.7 ± 0.3 Ma，证实了该围岩与白片岩和硬玉石英岩同步经历了超高压变质作用。硬玉石英岩在含柯石英的白片岩中以层和布丁形式出现，并显示出与花岗片麻岩和白片岩相似的全岩 REE 分布模式。此外，三种超高压变质岩中岩浆锆石的残余域表现出相似的U-Pb年龄和δ¹⁸ O 值，表明它们具有相同的花岗岩原岩。然而，硬玉石英岩中的变质锆石的δ ¹⁸ O 值显着低于6.1-7.3‰ 的残存岩浆域 9.4-10.6‰。此外，硬玉石英岩的全岩δ ¹⁸ O 值为7.9-8.7‰，明显低于花岗片麻岩的9.5-10.9‰，但高于白片岩的6.5-7.9‰。这些差异表明硬玉石英岩很可能是由具有较低δ ¹⁸ O 值的外部流体对变花岗岩的交代作用形成的。硬玉石英岩显示高δ ⁵⁶Fe值为0.69-0.87‰，明显高于花岗片麻岩的0.18-0.38‰，但同一露头白片岩的Fe值在0.32-1.03‰范围内。Fe同位素模拟结果表明，导致硬玉石英岩形成的交代流体可能来自超高压变质条件下的寄主白片岩。此类流体富含Si、Al和Mg，具有高δ ⁵⁶ Fe但低δ ¹⁸O 值。因此，硬玉石英岩记录了亚弧深变质超临界流体的作用。如果大陆俯冲带交代的岩石由于多级流体-岩石相互作用而脱水产生地球化学成分可变的流体，流体将进一步渗入围岩和可能的地幔楔，导致进一步的地球化学从俯冲板片转移到地幔楔。