塔里木板块东北部坡北基性–超基性岩体岩浆序列研究

Study on Magmatic Sequence of the Pobei Basic-Ultrabasic Complex in the Northeast Tarim Plate

坡北岩体位于新疆塔里木板块东北部,可划分为五个岩浆单元,对应五个岩相,分别为辉长岩相、辉长苏长岩相、橄榄辉长岩相、橄榄岩相和辉长岩岩枝。各单元岩石之间的矿物学特征存在差异,说明坡北岩体是不同期次岩浆侵入的产物。模拟计算获得橄榄岩相、橄榄辉长岩相和辉长苏长岩相岩石中橄榄石的平均结晶温度分别为:1303℃、1280℃和1204℃;橄榄岩相的原生岩浆的MgO含量为18%,属苦橄质岩浆;辉长岩相岩石原生岩浆MgO含量为8.61%,辉长苏长岩相岩石原生岩浆MgO含量为8.74%,均为玄武质岩浆。微量元素图解与Sr、Nd同位素指示岩浆演化过程中存在同化混染作用,辉长岩相和辉长苏长岩相受到的混染作用明显高于橄榄岩相和橄榄辉长岩相;辉长岩相、辉长苏长岩相和橄榄岩相的岩浆源区均为亏损岩石圈地幔,较低的部分熔融程度形成辉长岩相和辉长苏长岩相,较高的部分熔融程度形成橄榄岩相。

The Pobei complex, located in the northeastern margin of the Tarim plate, can be divided into five magma units, corresponding to five lithofacies: gabbro facies, gabbronorite facies, olivine gabbro facies, peridotite facies and gabbro apophysis. The mineralogical characteristics of rocks from each unit are different, indicating that the Pobei complex is a product of different magma pulses. The average crystallization temperatures of olivine in peridotite facies, olivine gabbro facies and gabbronorite facies are 1303?C, 1280?C and 1204?C, respectively. The MgO content of the primary magma of peridotite facies is 18%, suggesting a picritic magma. The MgO contents of the primary magma of gabbro facies and gabbronorite facies are 8.61% and 8.74%, respectively, all of which are basaltic magma. Trace element diagrams and Sr-Nd isotope indicate that contamination existed during the magmatic evolution process, and the degree of contamination in gabbro facies and gabbronorite facies is significantly higher than olivine gabbro facies and peridotite facies. The magma sources of gabbro facies, gabbronorites facies and peridotite facies are deplete lithosphere mantle, and the gabbro facies and gabbronorite facies are formed by lower degree of partial melting, while the peridotite facies is higher than them.