HR: 0800h
AN: V41C-0717    [Abstracts]
TI: Multi-stage Development From Ultrahigh-pressure Environment to Low-pressure Magmatic Processes: New Insight From Podiform Chromitites in the Luobusa Ophiolite, Southern Tibet
AU: * Yamamoto, S
EM: syamamot@geo.titech.ac.jp
AF: Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, 152-8551, Japan
AU: Komiya, T
EM: tkomiya@geo.titech.ac.jp
AF: Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, 152-8551, Japan
AU: Maruyama, S
EM: smaruyam@geo.titech.ac.jp
AF: Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, 152-8551, Japan
AB: Unusual silicate lamellae were discovered within chromites of podiform chromitites in the Luobusa ophiolite, southern Tibet, especially from massive- and nodular-type chromites. Using analytical transmission electron microscopy (ATEM), we found coesite, clinopyroxene and MgSiO3 phase as exsolution lamellae from the host chromites. There is no evidence that the Luobusa ophiolite itself formed at great depth, and the presence of coesite lamellae in a chromite directly indicate that podiform chromitites originate from deep mantle environment. On the other hand, magmatic structures are still observed in the podiform chromitite ore-body, such as banding chromites and the sharp contact between host peridotite and dunite envelopes. The characteristic features of the disseminated- and banded-type chromites, such as their interstitial distribution, euhedral to subhedral morphology and absence of exsolution lamellae, suggest they formed under low-pressure magmatic conditions. According to petrographic investigations, nodular-type chromites with numerous lamellae seem to be changing into disseminated-type chromite with no lamellae. Therefore, we conclude that the podiform chromitites at Luobusa retain evidence of their multi-stage development from ultrahigh-pressure environment to low-pressure magmatic processes under a ridge. On the basis of our results, we propose that a significant component of the podiform chromitite at Luobusa originate from the ultra-deep environment, and that chromitites were transported with mantle upwelling from a deep mantle to a shallow level under a mid-ocean ridge. Subsequently, the mantle peridotite with its podiform chromitite underwent partial melting, and the chromites without exsolution lamellae were largely recrystallized or newly precipitated by a shallow-level magmatic process under the mid-ocean ridge. As chromite is a highly refractory mineral, the petrological ultrahigh-pressure evidence can be preserved in spite of its long evolved history. Moreover, our new approach of nano-scale measurement with ATEM for refractory chromites may become a new tool to trace ultra-high pressure signature not only for ophiolitic massifs but also for ultra-high pressure massifs or mantle xenoliths.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3621 Mantle processes (1038)
DE: 3625 Petrography, microstructures, and textures
DE: 3654 Ultra-high pressure metamorphism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting