Paper No. 169-6
Presentation Time: 2:20 PM
PETROLOGICAL STUDY OF TI-CHONDRODITE- AND TI-CLINOHUMITE-BEARING SERPENTINITES FROM CHINESE SOUTHWESTERN TIANSHAN
SHEN, Tingting1, ZHANG, Lifei2, HERMANN, Jörg3, LV, Zeng1, CHEN, Jing1, XIA, Bin1, PADRÓN-NAVARTA, José Alberto4 and BADER, Thomas5, (1)Department of Geology, School of Earth and Space Sciences, Peking University, Yifu Buliding 2#, Yiheyuan Road 5, Peking University, Haidian district, Beijing, China, Beijing, 100871, China, (2)Department of Geology, School of Earth and Space Sciences, Peking University, 5 Yiheyuan Road, Beijing, 100871, China, (3)Research School of Earth Sciences, The Australian National University, Research School of Earth Sciences, The Australian National University, 0200 Canberra, Australia, Canberra, 0200, Australia, (4)Géosciences Montpellier, CNRS & Université de Montpellier 2, Géosciences Montpellier, CNRS & Université de Montpellier 2, F-34095 Montpelier Cedex 5, France, Montpellier, F-34095, France, (5)School of Earth and Space Sciences, Peking University, Beijing, 100871, China
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.
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