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

Abstract

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 paths 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 meters) 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 ℃ around 318.0 ± 2.3 Ma to HP peak metamorphism ~ 2.45 ± 0.2 GPa, 540 ± 20 ℃ at 316.8 ± 0.8 Ma, then, with eclogite-facies deformation ~ 2.0 ± 0.15 GPa, 525 ± 25 ℃ at 312 ± 2.5 Ma, exhumed to near surface within ca. 303 to ca. 280 Ma. Our P-T-time-isotopic results combined to 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. Highlights  Constant δ 18 O 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.

High‐ and ultrahigh‐pressure rocks—keys to lithosphere dynamics

“…The authors note that their P – T – t path is similar to those already published for various lithologies in the same UHP belt (e.g. Li, Klemd, Gao, Jiang, & Song, ; Tan et al., and references therein). Therefore, the authors conclude that the whole UHP terrane of the Western Tianshan metamorphic belt has been exhumed as a coherent unit.…”

The early exhumation history of the Western TianshanUHPmetamorphic belt, China: New constraints from titanite U–Pb geochronology and thermobarometry

“…Coesite (e.g. Lü & Zhang, ; Lü, Zhang, & Chen, ; Lü, Zhang, Du, & Buche, , ; Tan et al., ; Yang, Zhang, Tian, & Bader, ) and its pseudomorphs (e.g. Zhang, Ellis, & Jiang, ; Zhang, Song, Liou, Ai, & Li, ) have been discovered in the belt.…”

“…The P–T paths of the UHP eclogites within this metamorphic belt have been well established in previous studies. The peak pressure exceeds 30 kbar at ~550°C, and the subsequent exhumation to pressures of, or lower than, 15 kbar took place through near‐isothermal decompression (Tan et al., ; Tian & Wei, , ; Zhang, Zhang, Lü, Bader, & Chen, ). Eclogites are commonly pristine but may show variable degrees of retrogression.…”

“…Previous insights into the metamorphic evolution history of the Western Tianshan UHP belt were obtained through zircon and rutile U–Pb, white mica and amphibole 40 Ar– 39 Ar, mica Rb–Sr, garnet Lu–Hf and garnet‐omphacite Sm–Nd geochronology (Figure b, also see Tan et al., , for review). In this study, we employ titanite (CaTiSiO 5 ), a typical retrograde phase, to investigate the timing of exhumation.…”

“…(); #21: coesite‐bearing eclogite, zircon SIMS U–Pb age, Tan et al. (); #22: coesite‐bearing eclogite, Grt–Omp– WR Sm–Nd isochron age, Tan et al. ()…”