T171 Subduction Zone HP-UHP Metamorphism and Its Relation to the Coeval Magmatism I

Monday, 20 October 2014
1:00 PM - 5:00 PM
122

PETROLOGICAL STUDY OF TI-CHONDRODITE- AND TI-CLINOHUMITE-BEARING SERPENTINITES FROM CHINESE SOUTHWESTERN TIANSHAN

Paper number: T171-6
Program: Technical Sessions
Time: 2:20 PM – 2:35 PM

Authors

  • Tingting Shen Presenting Author
    School of Earth and Space Sciences, Peking University, Beijing, N/A
  • Lifei Zhang
    School of Earth and Space Sciences, Peking University, Beijing, N/A
  • Jörg Hermann
    The Australian National University, Canberra, N/A
  • Zeng Lv
    School of Earth and Space Sciences, Peking University, Beijing, N/A
  • Jing Chen
    School of Earth and Space Sciences, Peking University, Beijing, N/A
  • Bin Xia
    School of Earth and Space Sciences, Peking University, Beijing, N/A
  • José Alberto Padrón-Navarta
    CNRS & Université de Montpellier 2, Montpellier, N/A
  • Thomas Bader
    Peking University, Beijing

Abstract

The southwestern Tianshan (China) metamorphic belt records HP-UHP conditions corresponding to a cold oceanic subduction zone setting, where coesite is commonly found in eclogitic rocks. Serpentinites enclosing retrograded eclogite and rodingite occur in the UHP metamorphic belt. There are three kinds of serpentinites based on the protolith: dunite-serpentinite, harzburgite-serpentinite, wehrlite-serpentinite. Ti-clinohumite and Ti-chondrodite have been reported in these serpentinites. The host rock of both minerals is described petrographically, and, with the aid of a P-T pseudosection and high-pressure experiments, the conditions permitting the formation of these rare minerals are investigated. Based on petrologic investigations, we infer that the peak metamorphic mineral assemblage of serpentinized wehrlite comprises Ti-chondrodite + olivine + antigorite + chlorite + magnetite. The lack of geikielite/ilmenite in the sample indicates that the reaction:

Ti-clinohumite <=> olivine + geikielite/ilmenite

marking the maximum temperature stability limit of Ti-clinohumite was not reached. However, reaction textures suggest the occurrence of:

Ti-chondrodite + olivine <=> Ti-clinohumite

and

clinopyroxene + olivine + antigorite <=> Ti-clinohumite + diopside + chlorite + H2O.

We have investigated the stability field of Ti-chondrodite and Ti-clinohumite in the TFMASH system using piston-cylinder apparatuses. A total of 11 experiments were conducted at 25-55 kbar, 600-750 ºC for 168 hours. Combined with the temperature constrained by isopleth of the Al content of antigorite in a pseudosection calculated for the bulk composition of the serpentinized wehrlite, we estimate P-T conditions of >36 kbar, 520-550 ºC reflecting subduction along a very cold geothermal gradient. Because of the low temperature in the paleo-subduction zone, antigorite and Ti-clinohumite have not been completely decomposed to olivine + orthopyroxene or olivine + geikielite/ilmenite, respectively, during uplift and heating. This finding implies that serpentinites can be subducted to depths of at least 120 km without being complete dehydrated and the minerals antigorite, Ti-chondrodite and Ti-clinohumite still stable at these depths are able to transport considerable amounts of water deep into the mantle.