A new P-T-t path of eclogites from Chinese southwestern Tianshan: constraints from P-T pseudosections and Sm-Nd isochron dating

The southwestern Tianshan orogenic belt hosts one of the Earth's rare occurrences of ultrahigh-pressure (UHP) metamorphic rocks derived from subducted oceanic crust. To address its poorly constrained and controversial P-T-t evolution, for this study, three representative samples, two paragonite-epidote eclogites (H505-26 and H76-10) and one glaucophane-phengite eclogite (H711-1) are selected, and Sm-Nd geochronology and detailed phase equilibria modeling are combined. Garnet porphyroblasts from the three studied eclogites have well-preserved growth zoning with rim-ward increasing pyrope and decreasing spessartine contents. Phase equilibria modeling reveals that the investigated garnets grew in the lawsonite eclogite facies; in eclogite H505-26 during heating and burial, and in eclogites H76-10 and H711-1 during heating and decompression. Temperatures of 460-590 °C derived for the three eclogites are below the closure temperature of the Sm-Nd system, and, thus, the garnet-omphacite-whole rock Sm-Nd isochron ages have the potential to reflect garnet growth, i.e., eclogite facies metamorphism. The three eclogites yield consistent Sm-Nd isochron ages of 309 ± 4.6 Ma (H505-26), 306 ± 15 Ma (H76-10) and 305 ± 11 Ma (H711-1), reflecting HP eclogite facies metamorphism. Based on the new geochronology, detailed investigation of phase equilibria and published data, a clockwise P-T-t path with post-Pmax heating - decompression, subsequent isothermal decompression and final cooling - decompression is predicted for the eclogites from Chinese southwestern Tianshan. We propose, subduction of the South Tianshan paleo-ocean began before 346 Ma, the eclogite facies prograde metamorphism of the subducting oceanic crust occurred at 346-333 Ma, peak eclogite facies metamorphism at 320-305 Ma, and buoyancy-driven exhumation of the oceanic crust at 296-226 Ma.