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 40Ar/39Ar ( 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, 40Ar/39Ar, Rb-Sr), an attempt is made to date postpeak metamorphic stages of the complex PT path. Rb-Sr and 40Ar/39Ar 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 40Ar/39Ar 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 40Ar/39Ar, 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.