P-T-time-isotopic evolution of coesite-bearing eclogites: Implications for exhumation processes in SW Tianshan

- Constant δ18O of multistage zircon domains suggest a nearly closed metamorphic system.
- Multistage zircon ages are tied to metamorphism by test REE partitioning with garnet.
- Detailed P-T-t-isotopic history of SW-Tianshan coesite-bearing eclogite is obtained.
- Zircon chronology and Sm-Nd isochron constrain peak burial at ca. 315 ± 5 Ma.
- Exhaustively compiled regional data outline a short-lived detachment and exhumation.

The Chinese Southwestern Tianshan high- to ultra-high pressure low temperature (HP-UHP/LT) metamorphic belt exhibits well-preserved mafic layers, tectonic blocks/slices and boudins of different sizes and lithology embedded within dominant meta-volcanosedimentary rocks. Despite a wealth of previous studies on UHP relicts, P-T path estimates and age constraints for metamorphism, controversies still exist on P-T-t assessments and regional exhumation patterns (i.e., tectonic mélange versus internally coherent “sub-belt” model). This study focuses on a group of coesite-bearing eclogite samples from a thick (~ 5 m) layered metabasalt outcrop in order to unravel its detailed tectono-metamorphic evolution through space and time (both prograde, peak and exhumation). Using SIMS zircon U-Pb and oxygen isotope analyses, TIMS Sm-Nd multi-point isochron dating, in situ laser-ICP-MS trace-element analyses, classical thermobarometry and thermodynamic modeling, we link the multistage zircon growth to garnet growth and reconstruct a detailed P-T-time-isotopic evolution history for this UHP tectonic slice: from UHP peak burial ~ 2.95 ± 0.2 GPa, 510 ± 20 °C around 318.0 ± 2.3 Ma to HP peak metamorphism ~ 2.45 ± 0.2 GPa, 540 ± 20 °C at 316.8 ± 0.8 Ma, then, with eclogite-facies deformation ~ 2.0 ± 0.15 GPa, 525 ± 25 °C at 312 ± 2.5 Ma, exhumed to near surface within ca. 303 to ca. 280 Ma. Our P-T-time-isotopic results combined with the compilation of regional radiometric data and P-T estimates notably point to the existence of a short-lived period of rock detachment and exhumation (< 10 Ma, i.e. at ca. 315 ± 5 Ma) with respect to subduction duration.