Study On The Origin And Tectonic Environment Of The Dongco Ophiolite Of Tibet

The Dongco ophiolite lies in Dongco Lake, Gaze Country, and northern Tibet. It’s one of the most important ophiolite which located the middle and western part of the Bangong Lake-Nujiang suture zone, with complete rock assemblages. Under the observation of detailed field work, including metamorphic peridotite（Hazarburgite、iherolite,serpentine dunite）,mafic-ultramafic cumulate mafic dike, pillow basalt and sediment rocks.The geochemical analyses show that the Hazarburgite of Dongco Ophiolite is metamorphic peridotite of ACM diagram. Combined with previous research data, the REE element and Hake diagram show that its geochemical characteristic similar to SSZ-type ophiolite; The geochemical of gabbros indicate that it belongs to tholeiitic series of sub-alkaline basalts which similar to the form of REE and N-MORB trace digram,these gabbros were derived from³0% partial melting of a spinel iherzolite mantle, which was enriched by interaction with slab-derived fluids and melt, the cumulate gabbros and isotropic gabbros have island arc and back arc basin affinities, suggesting that they were originated in an oceanic back arc basin. The geochemical of basalts indicate that it belongs to alkaline basalt series, the rare earth element abundances are higher than that of chondrite and all samples show the enrichment distribution patterns of light rare earth element which like to the OIB-type,and the spider diagram indicates that the normative value of incompatible element to primitive mantle similar to OIB-type, the Dongco OIB-type basalt probably represents the product from the hotspots and undergoing relatively low-degree partial melting, suggesting that they were originated within ocean plateLA-ICP-MS zircon U-Pb dating on the isotropic gabbros sample yielded the ²⁰⁶Pb/²³⁸ U ages ranging from 215 Ma to 239 Ma,with a weighted mean age of 225.3±2.4Ma（MSWD=2.3）. This age is the crystallized timing of the gabbro, suggesting that the Dongco ophiolite was formed in the late Triassic. The two other samples（cpx –basalt） did not yield an ideal zircon U-Pb age, the single zircon ²⁰⁶Pb/²³⁸ U ages ranging from 117 Ma to 163 Ma, suggesting that the crystallized timing of the basalt is between Late Jurassic and Early Cretaceous, it represent that the ocean of Dongco is still exist.According to the member of ophiolite, the metamorphic peridotite and gabbros allow us to recognize that the Dongco ophiolite is of SSZ affinity and should be the relics of an immaturity back arc basin. The geochemical of basalt is of OIB –type affinity and should be represent the effects of Mantle Plume（or Hot spot）. The members of the Dongco ophiolite have different geochemical characteristics were emplaced in Bangong Lake-Nujiang suture zone by the structural.