This study presents in situ strontium (Sr) isotope and Sr content data on multi-stage epidote crystals from ultrahigh-pressure (UHP) eclogites and omphacite-epidote veins therein at Ganghe (Dabie terrane, China), determined using LA-MC-ICP-MS. The Ganghe eclogites occur as lenses in mainly leucocratic UHP gneisses, and therefore, our data provide insights into the origin, composition, and transport scale of the discrete multi-stage fluids in UHP eclogites during the subduction and exhumation of a continental crust. Four textural types of epidote that record compositional and isotopic signatures of fluid at various metamorphic P-T conditions have been distinguished based on petrographic observations and compositional analyses. They are (1) fine-grained high-pressure (HP) epidote inclusions (Ep-In) in omphacite that define the earliest stage of epidote formation in the eclogite; (2) coarse-grained UHP epidote porphyroblasts (Ep-P) that contain omphacite with Ep-In inclusions in the eclogite; (3) fine-grained HP epidote in omphacite-epidote veins (Ep-V) as well as (4) the latest-stage epidote in disseminated amphibolite-facies veinlets (Ep-A), which crosscut the Ep-P or matrix minerals in the eclogite and HP vein. Both Ep-P and Ep-V crystals exhibit significant and complex chemical zonations with respect to the XFe (= Fe/(Fe + Al)) ratio and Sr content. In contrast to the varying Sr contents, Ep-In, Ep-P, and Ep-V have similar and narrow ranges of initial 87Sr/86Sr ratios (from 0.70692 to 0.70720 for Ep-In, from 0.70698 to 0.70721 for Ep-P, and from 0.70668 to 0.70723 for Ep-V), which are significantly different from those in Ep-A (from 0.70894 to 0.71172). The initial 87Sr/86Sr ratio of Ep-A is closer in value to the initial Sr isotopic composition of the gneisses (from 0.710790 to 0.712069) which enclose the UHP eclogite. These data indicate different sources of the eclogite-facies fluids and retrograde amphibolite-facies fluid in the Ganghe eclogites. The HP-UHP fluids responsible for the large amounts of hydrous minerals in the eclogites were internally derived and buffered. The omphacite-epidote veins were precipitated from the channelized solute-rich HP-UHP fluids released from the host eclogite. However, hydrated amphibolite-facies metamorphism during exhumation was mainly initiated by the low-Sr and high-87Sr/86Sr external fluid, which infiltrated into the eclogite from the surrounding gneisses. The eclogite-facies fluids in the Ganghe eclogites were locally derived, whereas the infiltration of the retrograde amphibolite-facies fluid from the gneisses required a long transport, most likely longer than 80 m. This study highlights that the in situ Sr isotopic analysis of multi-stage epidote can be employed as a powerful geochemical tracer to provide key information regarding the origin and behavior of various-stage subduction-zone metamorphic fluids. © 2014 Springer-Verlag Berlin Heidelberg.
