藏北羌塘红脊山地区香桃湖变质堆晶辉长岩地球化学特征及其地质意义

香桃湖变质堆晶辉长岩是由原"猫耳山岩组"解体出来的早古生代基性岩,其对于研究区域上龙木错—双湖—澜沧江缝合带的构造演化具有重要意义。岩相学研究表明,该堆晶辉长岩经历了高级角闪岩相变质,并遭受了绿片岩相退变质作用的改造;地球化学研究显示,其Si O2含量为44.7%~46.2%,Mg O为7.05%~8.48%,Mg#值为51.02~53.90,相容元素Cr含量为168×10-6~288×10-6、Ni为60×10-6~103×10-6,远低于原生玄武质岩浆,反映香桃湖变质堆晶辉长岩经历了一定程度镁铁质矿物的分离结晶作用;部分样品微量和稀土元素显示出OIB的特征,另一部分显示E-MORB的特征。因此,该变质堆晶辉长岩的原岩应形成于受洋中脊中轴型地幔热柱叠加的洋中脊,其可能为红脊山地区早古生代的大洋残片。

Geochemical characteristics of Xiangtaohu metamorphic cumulate gabbro in the Hongjishan area of Qiangtang, north Tibet and their geological significance

Xiangtaohu metamorphic cumulate gabbro is the Early Paleozoic basic rock resulting from disintegration of "Maoershan Formation", and it is important to comprehend the tectonic evolution of the Lungmu Co-Shuanghu-Lancang River suture zone. Petrographic studies indicate that the cumulate gabbro suffered from high amphibolite facies and subsequent greenschist facies metamorphism, and geochemical studies yielded the following data: SiO2=44.7%~46.2%, MgO=7.05%~8.48%, Mg#=51.02~53.90, compatible element Cr=168×10-6~288×10-6, Ni=60×10-6~103×10-6, by far lower than the corresponding values of the primary basaltic magma, which suggests that Xiangtaohu metamorphic cumulate gabbro has undergone a certain degree of fractional crystallization of mafic minerals. The REE and trace elements of some samples are similar to those of OIB, and the other elements are similar to those in E-MORB. It is thus concluded that the cumulate gabbro was formed in a mid-ocean range, with the superposition of mid-ocean ridge axis type mantle plume, thus probably belonging to the fragments of the Eopaleozoic Ocean.