Fe and O isotopes in coesite-bearing jadeite quartzite from the Western Alps record multistage fluid-rock interactions in a continental subduction zone

Fluid is a key agent for the mass transfer between the subducting slab and the mantle wedge, which greatly affects the evo-lution 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 meta-morphism 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 whites-chist. In addition, relict domains of magmatic zircon in the three types of UHP metamorphic rocks exhibit similar U-Pb ages and delta(18)Omega values, indicating that they have the same protolith of granites. However, metamorphic zircons in the jadeite quart-zite show significantly lower delta(18)Omega values of 6.1-7.3% than the relict magmatic domains of 9.4-10.6%. Furthermore, the jadeite quartzite has whole-rock delta(18)Omega 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 delta(18)Omega values. The jadeite quartzite shows high Fe-856 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 Fe-856 but low delta(18)Omega 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. (C) 2021 Elsevier Ltd. All rights reserved.