The early exhumation history of the Western Tianshan UHP metamorphic belt, China: New constraints from titanite U-Pb geochronology and thermobarometry

Abstract

The 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 from UHP eclogites. 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 light REEs and heavy REEs, suggesting their growth during retrograde decompression. Sensitive high-resolution ion microprobe (SHRIMP) and LA-ICP-MS U-Pb dating of titanite from four samples, regardless of their textural occurrences, gave consistent ages of c. 306Ma. Phase equilibria modelling and Zr-in-titanite thermometry indicate that titanite replaced rutile at similar to 14kbar and similar to 570 degrees C, following a nearly isothermal decompression from the UHP peak conditions of 30kbar and 550-570 degrees C. SHRIMP zircon U-Pb dating yields two age groups: c. 320Ma and c.305Ma. The former is generally interpreted to be the age of UHP stage. The latter, overlapping the titanite ages, represents the timing of titanite growth during exhumation. Thus, we estimate that the near-isothermal exhumation from the UHP peak conditions to the epidote-amphibolite facies retrogression (14kbar) lasted c.15Ma, corresponding to an average exhumation rate of 3.4 +/- 2.5mm/yr (2 sigma). This exhumation rate lies within the range of oceanic LT-HP belts in the absence of continental subduction, suggesting a similar slab exhumation mechanism that operates in the UHP regime. The consistent retrograde metamorphic ages recorded by titanite from different localities, and the general agreement among P-T paths in the adjacent area, suggest that the UHP unit of Western Tianshan metamorphic belt was likely exhumed as a coherent unit.