Record of multiple stage channelized fluid and melt activities in deeply subducted slab from zircon U–Pb age and Hf–O isotope compositions

Abstract

Geochemical features of island arc rocks require two slab-derived components of a melt and a fluid added to their sources. These two liquids may unite to an intermediate aqueous melt at the base of volcanic arc. Direct evidence of this process has been scarce yet. Here we present an integrated study of in situ U–Pb age, trace element, and O–Hf isotope for zircons from a suite of a quartz vein, a felsic vein, their host eclogite, and surrounding gneiss at the Xitieshan UHP terrane of the North Qaidam metamorphic belt (NQMB). Oscillatory zoned zircons from the quartz vein are enriched in U, Li, Nb, and REE, with a formation age of 442±6Ma, consistent with the timing of the peak UHP metamorphism. This demonstrates there is no temporal gap between the UHP metamorphism and fluid flow in the Xitieshan terrane. In particular, the oscillatory zoned zircons of the quartz vein have similar Hf isotope compositions to zircons from the surrounding gneiss, but variable O isotope compositions between the host eclogite and surrounding gneiss. The fluid from surrounded gneiss was thus an aqueous melt with high transport ability for Lu and Hf and O, while the fluid from the host eclogite was an aqueous fluid only with the ability to transport O. According to the similar behaviors of granitic rock and sediment, it is inferred that a channelized hydrous melt can be formed in the downgoing sediment layer, where an aqueous fluid can be formed from underlying basalt and/or peridotite layers at the sub-arc depth. The two types of fluid are fed to the overlying mantle wedge from subducting slab to form island arc rock. By contrast, the zircons in the felsic vein yield a weighted mean 206Pb/238U age of 420±4Ma, which can be regard as the timing of partial melting during the exhumed stage for the Xitieshan UHP rocks. The zircon O and Hf isotopic features of the felsic vein are between the country gneiss and host eclogite, compatible with their emanating from hydrous melt of both the eclogite and gneiss during the exhumation stage of the UHP rocks. This indicates that hydrous melt would be released from both the sediment and basalt or peridotite layers of the subducting slabs at relatively shallow depth.