阿尔金山北缘蛇绿混杂岩中辉长岩锆石SHRIMPU-Pb定年及其地质意义

SHRIMP zircon dating of gabbro from the ophiolite melange in the northern Altyn Tagh and its geological implications.

阿尔金北缘阿克赛青崖子附近蛇绿混杂岩组成岩石包括蛇纹岩和辉长岩等,呈透镜状构造岩片产出。蛇纹岩以富Mg,贫Al_2O_3、CaO、TiO和∑REE为特征,与世界上典型蛇绿岩中变质橄榄岩的特征值一致。辉长岩SiO_2,含量为49.06%～50.70%,具有高的MgO(7.16%～10.01%)和Fe_2O_3(6.36%～12.85%)含量,TiO_2含量低到中等,为0.31%～1.02%,Na_2O和K_2O含量中等,平均分别为0.73%和0.75%,辉长岩在球粒陨石标准化稀土元素配分模式图上属平坦型或轻微富集型,无负Eu异常,与E-type洋中脊和OIB玄武岩类似。辉长岩的主量元素、微量元素特征表明其可能既保留了形成于MORB的背景信息,也记录了SSZ环境改造的结果,推测青崖子蛇绿混杂岩组合可能来自不同构造背景。蛇绿混杂岩中辉长岩的SHRIMP锆石U-Pb年龄测定,确定出青崖子附近蛇绿混杂岩所代表的洋壳形成于521±12Ma,为古生代早期。该年龄测定的结果还显示本区在472±10Ma有一次地质事件,可能代表了古生代的洋盆闭合的下限年龄。

The Qingyazi ophiolitic melange in the Akesai area, located in the northern margin of the Altyn Tagh, is composed of massive faulted-block rocks of serpentinite and gabbro. Analysis shows that the serpentinites are mostly characterized by high MgO contents, low Al(2)O(3), CaO, TiO(2) and Sigma REE contents, which are similar to the metamorphic peridotite of the world's typical ophiolite. The gabbrao has 49.06% similar to 50.70% SiO(2), high Fe(2)O(3) (6.36% similar to 12.85%) and MgO (7.16% similar to 10.01%) and low to moderate TiO(2) (0.31% similar to 1.02%) contents, with equal of Na(2)O and K(2)O (the average is 0.73% and 0.75%, respectively). It is similar to the E-type MORB and/(or) OIB that the gabbro has lower contents of Sigma REE with low fractionation of LREE/HREE ((La/Yb)(N) = 1.07 similar to 5.19) and no or slightly Eu anomaly. The major and trace elements characteristics of the gabrro indicated that it may not only retained the MORB origin signatures, but also records the results of the transformation of SSZ environment. All mentioned above show that some of the ophiolitic units in the melange may have originated in different tectonic settings, indicating that the Qingyazi ophiolitic melange represents a complex plate suture. The SHRIMP U-Pb age of zircons from gabbro of the Qingyazi ophiolitic melange is 521 +/- 12Ma (MSWD = 2.1), which stands for the formation time of the ocean basin. The result of the zircons SHRIMP U-Pb age shows that the gabbro has a metamorphic age of 472 +/- 10.0Ma(MSWD = 0.20), possibly representing the minimum age of the closure of Paleozoic ocean basin.