| The Dongqiao ophiolite lies in Dongqiao, Amdo County, northern Tibet. It is oneof the most important ophiolites from the northern part of the Bangong-Nujiang suturezone, with complete rock assemblages. Furthermore, the Dongqiao ophiolite is one ofthe earliest chromite deposits exploited in China. Therefore, it plays a key significanenot only in discussing the evolution of the Bangong-Nujiang suture zone, bu also inthe exploration of chromite.According to field words, the Dongqiao ophiolite is composed by harzburgite(including magnesium olivine, enstatite and chrome spinel), cumulate gabbro(diopside/salite and labradorite), mafic dikes (isotropic gabbro), pillow/massive lavasand the overlying red chert in the west of Dongqiao, and pillow lavas in Luobuzhong.In this study, we estimate the temperature and pressure conditions of the rocks fromthe Dongqiao ophiolite using the geothermobarometers of Roeder and Emslie (1970)and Mercier (1976). The results indicate that these rocks were formed in the conditionof978-1077oC and27.2-38.8kb, corresponding to the depth of99~141km, that is thedepth of the upper mantle. Hence, the magma of the Dongqiao ophiolite was derivedfrom a environment of the garnet lherzolite field. Major, trace and rare earth elementsanalyses show that basalts in the west of Dongqiao have different geochemicalcharacters with pillow lavas in Luobuzhong, Sr-Nd isotopic compositions indicatethat the rocks were derived from a depleted mantle source above the supra-subductionzone. SIMS zircon U-Pb dating on the cumulate gabbro sample yielded a late EarlyJurassic206Pb/238U ages ranging from185.3Ma to191.3Ma, with a weighted meanage of188.4±1.2Ma (MSWD=0.3). This age is the crystallized timing of thecumulate gabbro, suggesting that the Dongqiao ophiolite was formed in the late EarlyJurassic. According to these data, we suggest that the Dongqiao ophiolite formed inan initial ocean basin in the suprasubduction zone. |
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