UHP METAMORPHISM DOCUMENTED IN TI-CHONDRODITE- AND TI-CLINOHUMITE-BEARING SERPENTINITES FROM SOUTHWESTERN TIANSHAN, CHINA
The peak metamorphic mineral assemblage of a serpentinized wehrlite comprises Ti-chondrodite + olivine + antigorite + chlorite + magnetite + brucite. A computed pseudosection shows that the Al-content in antigorite is mostly sensititive to temperature but can also be used to constrain pressure. The average Al-content of antigorite included in Ti-chondrodite and in the matrix result in a well constrained peak metamorphic temperature of 510-530º C. Peak pressures are less precisely constrained at 37 ± 7 kbar. The Tianshan serpentinites thus record UHP metamorphic conditions and represent the deepest subducted serpentinites discovered so far.
To refine the metamorphic path of the ultramafic rocks, we have investigated the stability fields of Ti-chondrodite and Ti-clinohumite using piston-cylinder experiments. A total of 11 experiments were conducted at 25-55 kbar and 600-750º C in a F-free natural system. Combined with previous experiments and information from natural rocks we constructed a petrogenetic grid for the stability of Ti-chondrodite and Ti-clinohumites in F-free peridotite compositions.
To constraint the UHP metamorphism of serpentinite, we did in-situ secondary ion mass spectrometry analyses of perovskite from the blackwalls and obtained a well-defined U-Pb age of 309 ± 9 Ma, which represents the uplift time of UHP serpentinite in Chinese southwestern Tianshan.