新疆西准噶尔达尔布特蛇绿岩套柳树沟镁铁质杂岩SHRIMP锆石U-Pb年龄及地球化学特征

对新疆西准噶尔达尔布特蛇绿岩套柳树沟镁铁质杂岩中橄榄辉长岩、蛇纹石化橄榄辉长岩、辉长岩和蚀变辉长岩地球化学分析表明:辉长岩和蚀变辉长岩具有钙碱性和拉斑玄武岩的双重特征,橄榄辉长岩和蛇纹石化橄榄辉长岩属镁铁质堆积岩,为蛇绿岩组成单元;稀土总量较高,具微弱正Eu异常,稀土元素配分模式为略左倾平坦型,与SSZ型镁铁质堆晶岩稀土配分模式相同;微量元素蛛网图上,富集大离子亲石元素Cs、K、Th、U,相对亏损高场强元素Nb、Ta,可能代表俯冲板片的流体交代上覆地幔楔使地幔岩石发生部分熔融。地球化学构造环境判别柳树沟镁铁质岩石岩浆源区为亏损型地幔向富集型地幔过渡的适度富集型地幔,其形成的最佳模型是在成熟岛弧基础上裂谷化形成的一个不成熟的类似边缘海性质弧后盆地,不具成熟大洋或盆地那样的洋壳-上地幔结构,在盆地扩张初期岩浆具岛弧特征,随着盆地被进一步打开,镁铁质岩石具N-MORB特征。对辉长岩采用SHRIMP锆石U-Pb同位素年龄测试,获得特柳树沟镁铁质岩石的结晶时间为314.9±1.7 Ma。

SHRIMP Zircon U-Pb age and geochemical characteristics of mafic rock from Liushugou-Darbut in western Junggar,Xinjiang, China

Geochemical characteristics of olive gabbro, serpentinizated olive gabbro, gabbro and altered gabbro in the marie intrusive complex located in Liushugou-Darbut, western Junggar, Xinjiang are reported in this paper. The gabbro and altered gabbro have a dual characteristic of Ca-Alkali and tholeiite. Olive gabbro and serpentinizated olive gabbro are marie accumulated rocks which are the units of ophiolite. The amount of rare element is high which is faint-abnormal in positive-Eu, and distributed mode of rare element is left-leaning and smooth which is the same as SSZ-type mafic cumulate. When normalized to primitive mantle, they are rich in large-ion lithophile element Cs, K, Th and U, and relatively deficient in high field strongth element Nb, Ta, which probably indicate that originate from mantle wedge metasomatized by the liquid of subducted panel make pyrolite partly melt. The geochemical tectonic environment distinguish that mafic rocks in Liushugou Channel is moderately en- riched mantle which transits from depleted mantle into enriched mantle. The best model of its formation is that： a mature island arc rifting formed on the basis of an immature nature similar to the edge of the sea-arc basin, with- out upper mantle structure of mature oceanic or oceanic basins, and then the early expansion of the magma basin with arc characteristics, as the basin is further opened, mafic rocks with N-MORB features. SHRIMP Zircon U-Pb dating of gabbro in Liushugou-Darbut mafic complex is yielded ages of 314.9 ±1.7 Ma.