New Age Constraints on the Metamorphic Evolution of the High-Pressure/Low-Temperature Belt in the Western Tianshan Mountains, NW China

The western Tianshan high-pressure/low-temperature orogenic belt in NW China contains eclogite-facies metavolcanic rocks and omphacite-bearing blueschists. Previous Sm-Nd (omphacite, garnet, glaucophane, whole rock) and Ar/Ar (crossite) dating of eclogite-facies rocks has suggested an age of ca. 345 Ma as the best approximation for the timing of peak metamorphic conditions. The samples described here are blueschist-facies rocks that formed during or after the transition from the eclogite-facies to the epidote-blueschist-facies and subsequently experienced an incipient greenschist-facies overprint. By use of white mica geochronology (K-Ar, Ar/Ar, Rb-Sr), an attempt is made to date postpeak metamorphic stages of the complex PT path. Rb-Sr and Ar/Ar ages range between 313 and 302 Ma and 323 and 312 Ma, respectively, but mostly cluster at ca. 310–311 Ma, indicating that the studied samples recrystallized at this time. However, the Ar/Ar age spectra show complex release patterns that are interpreted to be influenced by excess argon to varying degrees. This conclusion is further corroborated by K-Ar dates ranging between 385 and 309 Ma. Younger dates of ca. 302 Ma (Rb-Sr) and ca. 296 Ma (K-Ar) indicate subsequent disturbances of these isotope systems in some rocks. The new ages are significantly younger than the time of eclogite-facies metamorphism (350–345 Ma), indicating resetting of the Ar/Ar, K-Ar, and Rb-Sr systems during exhumation of the blueschist-facies rocks. Furthermore, this dataset suggests that high-pressure conditions were attained during the Carboniferous and not at Permian or Triassic time, as recently suggested by SHRIMP U-Pb zircon dating. Online enhancements: tables.