Grain-scale Sr isotope heterogeneity in amphibolite (retrograded UHP eclogite, Dabie terrane): Implications for the origin and flow behavior of retrograde fluids during slab exhumation

Abstract To constrain the origin and flow behavior of amphibolite-facies retrograde fluids during slab exhumation, we investigate the textures, trace element contents, and in situ strontium (Sr) isotopic compositions (using LA-MC-ICP-MS) of multiple types of epidote and apatite in the UHP eclogite and amphibolites from the Hualiangting area (Dabie terrane, China). The UHP epidote porphyroblasts in the eclogite (Ep-E), which formed at ~ 28–30 kbar and 660–720 °C, contain high amounts of Sr, Pb, Th, Ba, and light rare earth elements (LREEs) and have a narrow range of initial 87 Sr/ 86 Sr ratios (0.70431 ± 0.00012 to 0.70454 ± 0.00010). Two types of amphibolite-facies epidote were recognized in the amphibolites. The first type of epidote (Ep-AI) developed in all the amphibolites and has slightly lower trace element contents than Ep-E. The Ep-AI has a same initial 87 Sr/ 86 Sr ratio range as the Ep-E and represents the primary amphibolite-facies retrograde product that is associated with an internally buffered fluid at 8.0–10.3 kbar and 646–674 °C. The other type of epidote (Ep-AII) occurs as irregular fragments, veins/veinlets, or reaction rims on the Ep-AI in certain amphibolites. Elemental X-ray maps reveal the presence of Ep-AI relics in the Ep-AII domains (appearing as a patchy texture), which indicates that Ep-AII most likely formed by the partial replacement of the Ep-AI in the presence of an infiltrating fluid. The distinctly lower trace element contents of Ep-AII are ascribed to element scavenging by a mechanism of dissolution-transport-precipitation during replacement. The Ep-AII in an individual amphibolite exhibits large intra- and inter-grain variations in the initial 87 Sr/ 86 Sr ratios (0.70493 ± 0.00030 to 0.70907 ± 0.00022), which are between those of the Ep-AI and granitic gneisses (wall rock of the amphibolites, 0.7097–0.7108). These results verify that the infiltrating fluid was externally derived from granitic gneisses. The matrix apatite in the amphibolites has the same initial 87 Sr/ 86 Sr ratio range as the Ep-AI, indicating that the amphibolite-facies fluid involved in the apatite crystallization was also internally derived. We propose that at least two separate stages of fluids were accounted for the amphibolite-facies retrogression of the Hualiangting eclogite. The fluid responsible for the growth of most of the amphibolite minerals was locally derived and behaved in a pervasive manner, whereas the influx of gneiss-derived fluid was transient, episodic, and highly channelized with a longer transport distance (> 60 m). The disparate origins and flow behavior of these fluids significantly influence the water budget and element transfer in exhumed HP–UHP slabs. This study also indicates that examining grain-scale Sr isotopic variations provides key information regarding the isotopic (dis)equilibrium, fluid origins, and fluid-flow regimes in metamorphic or metasomatic rocks that form in subduction-zone environments.