Three Chinese ordinary chondrites,including Jilin (H5),Boxian (LL3.8) and Lujiang (LL6),have been studied for their Re and Os abundances and Os isotopic composition in whole-chondrite samples,separated magnetic and nonmagnetic fractions,and nodules. The results indicate that the Re and Os abundances of the whole-chondrite samples are in the ranges of corresponding H-and LL-Groups,respectively. The Re and Os abundances of magnetic fraction from Boxian and Lujiang are within the range of high-Os ⅡAB and ⅢAB irons,whereas those of nonmagnetic fractions of Boxian and Lujiang are lower than the whole-chondrite values. The Re and Os abundances of nodules in Jilin are in the range of the LL-Group. 187Re/188Os and 187Os/188Os ratios of the three whole chondrites are in the range of ordinary chondrites which locate around the isochron of ⅡAB+ⅢAB irons. 187Re/188Os and 187Os/188Os ratios of the magnetic and nonmagnetic fractions from Boxian have a larger difference. The nonmag-netic fraction of Lujiang may contain a recent addition of Re,which causes deviation of the 187Re/188Os ratio from the irons isochron. The Re and Os abundances of nodules in Jilin are lower than those of the whole-chondrite,but their 187Os/188Os ratios are higher than that of the whole chondrite.
The Bangong Lake ophiolite is located in the westernmost part of the Bangong Lake-Nujiang River suture zone. It is a tectonic mélange consisting of numerous individual blocks of peridotite, pillowed and massive lavas and mafic dykes with SSZ-type ophiolitic geochemical affinity formed at the end of a Wilson circle. The SHRIMP U-Pb ages of the co-magmatic zircon domains from one gabbroic dyke (Sample 01Y-155) range from 162.5±8.6 Ma to 177.1±1.4 Ma with an average of 167.0±1.4 Ma (n = 12, MSWD = 1.2), suggesting that the subduction of the Bangong Lake Neo-Tethyan Ocean started before the Middle Jurassic. It is inferred that the tectonic transform from spreading to subduction of the Neo-Tethyan Ocean began before the Middle Jurassic in the Bangong Lake area.
SHI RenDeng1,2 1 School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
