南祁连大道尔吉早古生代弧后盆地型蛇绿岩的年代学、地球化学特征及意义

The Dadaoerji ophiolite is an important part of the ophiolitic mélange situated inbetween the Central Qilian block and the South Qilian fold belt. This ophiolite mainly consists of metaperidotite, mafic-ultramafic cumulate complexes and basaltic andesite. The mafic-ultramafic cumulate complex includes three cyclic cumulate. The bottom of each cyclic cumulate is chromium spinel-bearing dunite, and overlain by mafic and ultramafic laminated complex composed of pyroxene-peridotites, diopsidite, and gabbro. The Sm-Nd isochron age of the pyroxene-peridotites in the cumulate complex is 441±58 Ma, which indicates that the ophiolite was formed in Caledonian. Geochemical data show that the metamorphic peridotites have relatively higher MgO contents of 40.41%~40.96%, Cr contents of 3590×10^-6~ 7340×10^-6, and Ni contents of 1480×10^-6~1710×10^-6, and lower Al₂O₃ contents of 0.35%~0.59%, TiO₂ contents of 0.03%~0.04%, and REE contents of 0.81×10^-6~1.84×10^-6, typical of depleted mantle rocks. The major elements of mafic-ultramafic cumulate complex show a wide range of variation, relatively enriched in HREE with positive Eu anomaly. The basaltic andesites have SiO₂ contents of 54.90%~57.76%, MgO contents of 3.50%~5.45%, TiO₂ contents of 0.72%~1.12%, Na₂O/K₂O>1; The total REE contents of the basaltic andesites are 24.9×10^-6~53.4×10^-6, La_N/Yb_N ratios are 1.0~1.7 with flat REE patterns similar to that of E-MORB. The volcanic rocks are enriched in LILE (Ba, Rb, Th) and depleted in HFSE (Nb, Ta, Zr, Ti), and are interpreted to be formed in a back-arc basin setting. Combined with the tectonic locations of the ophiolite and field observation, the authors propose that the Dadaoerji ophiolite is a relic slice of oceanic crust derived from depleted mantle, similar to the SSZ type ophiolite, and was formed in a back-arc basin setting. The North Qaidam oceanic plate was subducted northward under the Central Qilian blocks in the Early-Middle Ordovician.