西昆仑岩碧山超单元花岗岩特征及构造环境

Petrological characteristics and tectonic setting of Yanbimountain super-unit granite in West Kunlun

岩碧山超单元花岗岩出露于西昆仑东段,居华北板块与华南板块的结合部位,锆石U-Pb模式年龄(239±30.3)Ma,由三个单元组成。从早次单元到晚次单元,岩性分别为英云闪长岩→花岗闪长岩→二长花岗岩,具有较明显的从中酸性向酸性演化的趋势。SiO262.30%～69.45%,Na2O>K2O,σ值1.27～1.99,A/CNK>1,属钙碱性铝过饱和钠质花岗岩类型。大离子亲石元素及相容元素Cr等较富集。ΣREE(84.85～157.49)×10-6,LREE/HREE10～20,早次单元Eu略有亏损,其他单元δEu值均大于1。研究表明,岩浆主要来源于下地壳物质的部分熔融,并有部分上地幔物质及中上地壳物质加入。各单元为同源岩浆分异演化的产物。岩体形成于火山弧构造环境,为被动侵位中的岩墙扩张兼岩浆顶蚀作用的结果。

Located in the east part of West Kunlun Mountains and in the combinative part of North China and South China plates,the Yanbi mountain super-unit granite has a zircon U-Pb age of (239±30.3)Ma and consists of three units:tonalite,granodiorite and admellite,showing a evolution trend with emplacement ages from intermediate-acidic to acidic.The petrological and trace element geochemical features of the grantoids are as follows:(1)contents of SiO_2 range from 62.30 to 69.45%,Na_2O>K_2O;(2)σ values range from 1.27 to 1.99,A/CNK>1 and rich in LILE and Cr;(3)ΣREE ranges from 84.85×10^(-6) to 157.49×10^(-6) with the LREE/HREE ratios changing from 10 to 20,all the granitoids show obvious positive Eu anomalies except for the early unit.The features mentioned above indicate that the grantoids belong to calc-alkalic and aluminum-oversaturated agpaitic granite and the magma might be derived from partial melting of the Lower Crust with adding of some upper mantle and middle-upper crust components.Research shows that the three units were resulted from evolution of the same mother magma through passive emplacement in a volcanic island environment.