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

Abstract

AbstractThe Chinese Western Tianshan terrane is a well‐known ultrahigh‐pressure (UHP) metamorphic belt that mainly consists of subducted oceanic crustal materials. While two uplift stages have been recognized, the early exhumation history has not been fully constrained. We conducted U–Th–Pb and trace element microanalysis of titanite as well as zircon fromUHPeclogites. Apart from several inclusions in garnet, three types of retrograde titanite have been identified based on their petrographic occurrences: (a) in matrix enclosing relict rutile; (b) as coarse‐grained coronas around garnet; and (c) in veins coexisting with epidote and/or apatite. The three types of titanite are homogeneous in chemical composition, low in Al, enriched in middle rare earth elements (REEs), and depleted in lightREEs and heavyREEs, suggesting their growth during retrograde decompression. Sensitive high‐resolution ion microprobe (SHRIMP) andLA‐ICP‐MSU–Pb dating of titanite from four samples, regardless of their textural occurrences, gave consistent ages ofc. 306 Ma. Phase equilibria modelling and Zr‐in‐titanite thermometry indicate that titanite replaced rutile at ~14 kbar and ~570°C, following a nearly isothermal decompression from theUHPpeak conditions of 30 kbar and 550–570°C.SHRIMPzircon U–Pb dating yields two age groups:c. 320 Ma andc. 305 Ma. The former is generally interpreted to be the age ofUHPstage. The latter, overlapping the titanite ages, represents the timing of titanite growth during exhumation. Thus, we estimate that the near‐isothermal exhumation from theUHPpeak conditions to the epidote–amphibolite facies retrogression (14 kbar) lastedc. 15 Ma, corresponding to an average exhumation rate of 3.4 ± 2.5 mm/yr (2σ). This exhumation rate lies within the range of oceanicLT‐HPbelts in the absence of continental subduction, suggesting a similar slab exhumation mechanism that operates in theUHPregime. The consistent retrograde metamorphic ages recorded by titanite from different localities, and the general agreement amongP–Tpaths in the adjacent area, suggest that theUHPunit of Western Tianshan metamorphic belt was likely exhumed as a coherent unit.