Subduction channel fluid-rock interaction: Indications from rutile-quartz
veins within eclogite from the Yuka terrane, North Qaidam orogen
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Abstract
High-pressure (HP) or ultrahigh-pressure (UHP) rutile-quartz veins that form at mantle depths due to fluid-rock
interaction can be used to trace the properties and behavior of natural fluids in subduction zones. To explore the
fluid flow and the associated element mobility during deep subduction and exhumation of the continental crust,
we investigated the major and trace elements of Ti-rich minerals. Additionally, U–Pb dating, trace element
contents, and Lu–Hf isotopic composition of zircon grains in the UHP eclogite and associated rutile-quartz veins
were examined in the North Qaidam UHP metamorphic belt, Yuka terrane. The zircon grains in the rutile-quartz
veins have unzoned or weak oscillatory zonings, and show low Th/U ratios, steep chondrite-normalized patterns
of heavy rare earth elements (HREEs), and insignificant negative Eu anomalies, indicating their growth in
metamorphic fluids. These zircon grains formed in 431 3 Ma, which is consistent with the 432 2 Ma age of
the host eclogite. As for the zircons in the rutile-quartz veins, they showed steep HREE patterns on one hand, and
were different from the zircons present in the host eclogite on the other. This demonstrates that their formation
might have been related to the breakdown of the early stage of garnet, which corresponds to the abundance of
fluids during the early exhumation stage. The core-rim profile analyses of rutile recorded a two-stage rutile
growth across a large rutile grain; the rutile core has higher Nb, Ta, W, and Zr contents and lower Nb/Ta ratios
than the rim, indicating that the rutile domains grew in different metamorphic fluids from the core towards the
rim. The significant enrichment of high field strength elements (HFSEs) in the rutile core suggests that the peak
fluids have high solubility and transportation capacity of these HFSEs. Furthermore, variations in the Nb vs. Cr
trends in rutile indicate a connection of rutile to mafic protolith. The zircon grains from both the rutile-quartz
veins and the host eclogite have similar Hf isotopic compositions, indicating that the vein-forming fluids are
internally derived from the host eclogite. These fluids accumulated in the subduction channel and were triggered
by local dehydration of the deeply subducted eclogite during the early exhumation conditions.
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