T52 Bridging Two Continents: Advances in Crustal Subduction and Ultrahigh-Pressure Metamorphism

Sunday, 1 November 2015
1:30 PM - 5:30 PM
Room 349/350

UHP METAMORPHISM DOCUMENTED IN TI-CHONDRODITE- AND TI-CLINOHUMITE-BEARING SERPENTINITES FROM SOUTHWESTERN TIANSHAN, CHINA

Paper number: T52-3
Program: Technical Sessions
Time: 2:10 PM – 2:25 PM

Authors

Abstract

The southwestern Tianshan (China) metamorphic belt records HP-UHP conditions corresponding to a cold oceanic subduction-zone setting. Serpentinites enclosing retrogressed eclogite, and two kinds of rodingite and “blackwall” occur as lenses within metapelites, which also hosts coesite-bearing eclogites. Based on petrology and petrography of these serpentinites and rodingites, five events are recognized: (1) formation of a wehrlite-harzburgite-dunite association in the mantle; (2) retrograde metamorphism and partial hydration during exhumation of the mantle rocks close to the seafloor; (3) oceanic metamorphism leading to the first serpentinization and rodingitization; (4) UHP metamorphism during subduction; (5) retrograde metamorphism during exhumation together with a second serpentinization and rodingitization. Moreover, perovskite is commonly found in the associated blackwalls.

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.