Triassic collision of western Tianshan orogenic belt, China: Evidence from SHRIMP U–Pb dating of zircon from HP/UHP eclogitic rocks

A newly recognized ultrahigh-pressure (UHP) terrane in the Chinese Western Tianshan orogenic belt contains blueschists, eclogites and metapelites. This belt extends westward to the “South Tianshan” in Tajikistan, Kyrgyzstan, Kazakhstan and Uzbekistan for more than 2500 km long in central Asia. New ion microprobe (SHRIMP) U–Pb dating of zircon from HP-UHP eclogites and metapelites indicates Triassic ages for the collision in western Tianshan. Zircon from four eclogites yields magmatic ages of 310–413 Ma in the cores and one metapelite contained detrital zircon cores as old as 1886 ± 20 Ma. Zircon rims reveal the HP-UHP metamorphic ages of 233 ± 4–226 ± 4.6 Ma. The geochronologial data suggest that a South Tianshan paleo-ocean was developed between the Tarim continent and the Yili-central Tianshan Craton before the Carboniferous (> 310 Ma). During the Permian–Triassic subduction and continent collision, oceanic basalts underwent HP/UHP metamorphism. A new tectonic model for HP-UHP metamorphic rocks of the Chinese Western Tianshan orogenic belt represented by HP-UHP metamorphic eclogitic rocks is proposed in the light of recent paleomagnetic, paleontologic, sedimentary and stratigraphic studies.