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 Al2O3contents of 0.35%~0.59%, TiO2contents of 0.03%~0.04%, and REEcontents 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 SiO2contents of 54.90%~57.76%, MgO contents of 3.50%~5.45%, TiO2contents 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, LaN/YbN 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.%大道尔吉蛇绿岩是中祁连地块和南祁连褶皱带之间蛇绿混杂岩带的重要组成部分。出露的岩石单元包括地幔橄榄岩、镁铁–超镁铁质堆晶杂岩和玄武安山岩。其中镁铁–超镁铁质堆晶杂岩包括三个堆晶旋回,单个堆晶旋回底部为含铬尖晶石纯橄岩,向上逐渐变为透辉石岩(辉石橄榄岩)→辉长岩等。堆晶杂岩中辉石橄榄岩 Sm-Nd 同位素等时线年龄为441±58 Ma,为加里东中期岩浆活动的产物。地球化学特征显示：地幔橄榄岩MgO(40.41%~40.96%)、Cr(3590×10–6~7340×10–6)、Ni(1480×10–6~1710×10–6)含量高,而Al2O3(0.35%~0.59%)、TiO2(0.03%~0.04%)和∑REE(0.81×10–6~1.84×10–6)含量低,具亏损地幔岩的特征；镁铁–超镁铁质堆晶杂岩的主量元素变化大,重稀土元素富集,正Eu异常(δEu=1.22~2.15)；玄武安山岩SiO2为54.90%~57.76%, MgO为3.50%~5.45%, TiO2为0.72%~1.12%, Na2O/K2O>1,∑REE为24.9×10–6~53.4×10–6, LaN/YbN=1.0~1.7,具平坦稀土配分模式,与E-MORB相似,同时富集Ba、Rb、Th等大离子亲石元素,亏损Nb、Ta、Zr、Ti 等高场强元素,具弧后盆地火山岩地球化学特征。综合区域地质、岩石组合和地球化学特征,认为大道尔吉蛇绿岩属构造肢解的蛇绿岩残片,具有俯冲带(SSZ)型蛇绿岩的特征,形成于弧后盆地环境,为奥陶纪柴北缘洋向中祁连地块俯冲,引起弧后扩张形成。
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