HR: 0830h
AN: V21D-0559    [PDF]
TI: Interesting Inclusions From Podiform Chromitites in Luobusa Ophiolite, Tibet
AU: * Yamamoto, S
EM: syamamot@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences Tokyo Institute of Technology, 2-12-1, Ookayama Tokyo, 152-8551 Japan
AU: Komiya, T
EM: tkomiya@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences Tokyo Institute of Technology, 2-12-1, Ookayama Tokyo, 152-8551 Japan
AU: Hirose, K
EM: kei@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences Tokyo Institute of Technology, 2-12-1, Ookayama Tokyo, 152-8551 Japan
AU: Maruyama, S
EM: smaruyama@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences Tokyo Institute of Technology, 2-12-1, Ookayama Tokyo, 152-8551 Japan
AB: For the past decade, diamonds and unusual mineral asemblages were reported in podiform chromitites of the Luobusa ophiolite, southern Tibet, China (Bai 1993, Bai 2000, Yan 2001). These minerals were found from heavy mineral separation of chromitites. These minerals include (1) native elements, (2) alloys, (3) carbide (SiC, CrC), (4) platinium group elements (PGE) and arsenides, (5) silicates (Ol, Opx, Cpx, Amp, Srp, Chl, Uv, Prp, Alm, Wo, Zrn, Ap, Bt, Spn, Rt, Pl, Kfs, Phl, Sil, Qz and octahedral serpentine (possible pseduomorph after ringwoodite?), (5) oxide (corundum and chromite), (6) carbonates. Despite many questions as to these minerals above still remain open, these mineral inclusions would provide us the important infomation on the formation of the podiform chromitites. In this study, octahedral serpentine was discovered both on a thin section and from the heavy mineral separation. These octahedral inclusions exist within chromites, forming a line. These minerals are approximately 5-15$\mu$m in diameter and have well octahedral morphology. EPMA, laser raman spectrometer and transmission electron microscopy (TEM) were used to determine the structure and chemical composition of this crystal. For the present, there are several interpretations of this octahedral silicate. One possibility is that if the octahedral structuer is euhedral so this octahedral serpentine may be pseudomorph after ringwoodite because of its chemical composition and octahedral crystal shape. Another is that ocahedral minerals are melt inclusions. Linear occurrence of octahedral minerals is similar to that of fuluid inclusions. If the octahedral structuer is negative crystal shape reflecting octahedral crystal of cromian spinel, then octahedral inclusions may be melt inclusions judging from linear occurrence. At the same time, zircons were obtained from the mineral separation from chromitites. U-Pb dating of these zircons by LA-ICP-MS yielded two different ages. One group has relatively younger age 107-534 Ma, which nearly plots on a concordia line. Another group has older age 1460-1822 Ma, which plots off the concordia line. Cathode luminescence images of these zircons indicate that some zircons have clear oscillatory zoning whereas other zircons show apparent homogeneous overgrowth. But any correlation between CL image and the U-Pb age was not identified in particular. Luobusa ophiolite has been recognized as fragment of Tethys oceanic crust formed in Cretaceous at 100-120 Ma (Allegre et al. 1984). The minimum age 107 Ma corresponds to the age of the formation of Luobusa ophiolite and all other age of zircons in chromitites is much older than that of ophiolite. In addition, the inclusions in the zircons were analyzed by EPMA and laser raman spectrometer. Several zircons contain some inclusions, which are quartz, feldsper, mica, apatite, titanite and others. These inclusions are the minerals composed of crustal material, which means that these zircons were crystalized in the low pressuer crustal condition. On the other hand, Yu et al. (2001) reported that zircons from chromitites in Luobusa ophiolite have shorter inter-atomic distances for Zr-O and Si-O bonds. They concluded that Tibetan-zircons were derived from the high-pressure mantle environment. Judging from the line of evidence mentioned avobe, it is highly possible that these zircons captured by chromitites were originated from recycled crustal materials convecting through upper mantle.
DE: 1213 Earth's interior--dynamics (8115, 8120)
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3640 Igneous petrology
DE: 3665 Mineral occurrences and deposits
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting