Geochronology and Nd and Sr Isotope Characteristics of Gabbro Dikes from the Shemalagou Area in the Middle Sector of the Bangongco—Nujiang Ophiolite Belt,North Tibet

This paper reports a combined internal and whole-rock Sm-Nd isochron age, and Nd-Sr isotopic data for gabbro of the Shemalagou ophiolite in north Tibet. The Sm-Nd isochron data for two whole rocks and garnet and hornblende separates from the rocks yield an early Jurassic age of 191 ±22 Ma, which agrees with the geological facts that there are Jurassic Radiolarian fossils found in the abysmal cherts, which are nipped between the pillow lava and volcanic clastic sediments. This suggests that the Sm-Nd isochron age of 191 ± 22 Ma is reliable and proves the formation of the Shemalagou ophiolite during the early Jurassic. The e_(Nd)(t) values range from +7. 0 to +5. 4, indicating that gabbro was formed in an oceanic island-arc setting. Compared with the Yalung Zangbo ophiolite located in South Tibet, the Shemalagou ophiolite samples have lower e_(Nd)(t)values, which reflect that they occur in different geological settings. The initial ratios of strontium (~(87)Sr/~(86)Sr)_i of three whole rock samples from the Shemalagou ophiolite are 0. 7059, 0. 7061 and 0. 7045, respectively, with an average of 0. 7055. They are higher than modern abysmal lavas (0. 7020～0. 7035). The isotope characteristics of Nd and Sr suggest that the Shemalagou ophiolite was formed on the basis of spreading of the back-arc basin that was developed during the early Jurassic. The magma source may have been contaminated by subducted ocean crust or ocean sediments.