A HUGE UHP METAMORPHIC BELT OF OCEANIC-TYPE IN SW TIANSHAN, CHINA: ZIRCON U-PB DATING AND PETROTECTONIC EVOLUTION

The UHP metamorphic rocks have been documented in more than twenty orogenic belts in the world so far; most of them occur in the Eurasian continental collision belts and possess similar supercrustal protoliths, whereas UHP belt of oceanic-type is extremely rare. Recently, a huge UHP metamorphic belt of oceanic-type with more than 80 km long has been recognized in SW Tianshan, China. Petrological studies show that the UHP eclogitic rocks record three discrete stages of recrystallization: a peak UHP eclogite stage at 560-600ºC and 4.9-5.0 GPa, a main UHP eclogite stage at 598-496℃ and 2.6 -2.7 GPa and retrograde overprinting under epidote blueschist-facies conditions. The new SIMS zircon dating from two eclogite samples shows that one has 308.1±4.7 Ma (average N=4) rim ages, another shows 243.3±2.9 Ma (average N=6) rim ages. The SIMS zircon dating from three Gt mica schist samples produces 308.3±4.7 Ma (average N=5), 308.9±2.0 Ma (average N=21) and 311.8±2.8 Ma (average N=10) rim ages respectively. The Lu/Hf isochron of three eclogite samples shows the metamorphic ages of 263±10 Ma, 306±10 Ma and 326.8±5.2 Ma. The Sm/Nd isochron of three eclogite samples yields the three similar metamorphic ages of 305±15 Ma, 305±26 Ma and 309.8±5.6 Ma respectively. It is obvious that two metamorphic stages can be recognized, i.e., 308.9±2.0 to 326.8±5.2 Ma for the peak stage and 243.3± 2.9 to 263±10 Ma for the retrograde stage in western Tianshan eclogitic rocks.

A tectonic model of the paired metamorphic belt for UHP belt in western Tianshan, China, was proposed as following: the ancient Tianshan ocean began to close before 420 Ma, and the Tarim plate was subducted northwards underneath the Yili-central Tianshan plate which resultes in the coeval arc magmatism and low P/T metamorphism occurred on the active continental margin, subsequently, the oceanic crust and accretionary wedge sediments were further subducted to the mantle depth at 308.9±2.0 to 326.8±5.2 Ma and exhumed with the low P/T metamorphic belt to the surface at 243.3± 2.9 to 263±10 Ma.