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Cenozoic-Present |
1 Department of Earth Sciences. The University of Hong Kong, Pokfulam Road, Hong Kong, China probins{at}hkucc.hku.hk
2 Laboratory of Continental Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037, China
3 Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada
4 Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5
5 Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093-0225, USA
Numerous ultra-high-pressure minerals have been recovered from podiform chromities in the Luobusa ophiolite, Tibet. Recovered minerals include diamond, moissanite, Fe-silicides, wüstite, Ni-Fe-Cr-C alloys, PGE alloys and octahedral Mg-Fe silicates. These are accompanied by a variety of native elements, including Si, Fe, Ni, Cr and graphite. All of the minerals were hand-picked from heavy-mineral separates of the chromitites and care was taken to prevent natural or anthropogenic contamination of the samples. Many of the minerals and alloys are either enclosed in, or attached to, chromite grains, leaving no doubt as to their provenance. The ophiolite formed originally at a mid-ocean ridge (MOR) spreading centre at 177±33 Ma, and was later modified by suprasubduction zone magmatism at about 126 Ma. The chromitites were formed in the suprasubduction zone environment from boninitic melts reacting with the host peridotites. The UHP minerals are believed to have been transported from the lower mantle by a plume and incorporated in the ophiolite during seafloor spreading at 176 Ma. Blocks of the mantle containing the UHP minerals were presumably picked up by the later boninitic melts, transported to shallow depth and incorporated in the chromitites during crystallization.
