西昆仑岔路口西花岗岩地球化学特征及构造意义

Geochemistry and Tectonic Significance of Granite from the West of Chalukou in Western Kunlun,NW Chian

岔路口西花岗岩侵位于塔什库尔干-甜水海微地块下一中二叠统神仙湾组。通过对岩体进行锆石U—Pb测年，获得两组年龄为（99．1±1．7）Ma（MSWD=14）和（93．2±1．1）Ma（MSWD=4．4），属晚白垩世早期。岩石地球化学特征显示，岩体SiO2含量较高（平均74．32％），全碱含量7．82％～7．97％，低CaO（0．38％～0．5％）、MgO（0．16％～0．27％），FeOt／MgO比值较高（6～9．7）；微量元素富集Rb，Th，Nh，zr，Hf，Y，LILE元素Sr，Ba极亏损；REE元素具“燕式”分布特征，极亏损Eu（6Eu=0．21～0．41），属A1型花岗岩。结合地球化学特征及区域构造演化认为，自晚白垩世新特提斯洋北支洋已转入陆内伸展机制，该区进入伸展减压环境，使得下地壳发生减压部分熔融形成A1型花岗岩。

The west of Chalukou granite intrudes the Early-Middle Permian Shenxianwan Formation in the Tashikuer- gan-Tianshuihai Terrane.LA-ICP-MS zircon U-Pb geochronological study on the granitic intrusion produced two groups of ages of （99.1 ± 1.7） Ma（MSWD=1 4） and （93.2± 1.1） Ma（MSWD=4.4）,suggesting the emplacement age of the intrusion belongs to the early stage of Late Cretaceous.The bulk geochemistry show that the granite is characterized by relatively high contents of SiO2（average of 74.3%） and total alkalis（7.82%-7.97%）, low contents of CaO （0.38%-0.5%） and MgO （0.16%-0.27%） as well as high FeOt/MgO ratios （6-9.7）. Furthermore,the granitic samples display enrichment of Rb,Th, Nb,Zr, Hf, Y and strong depletions of Sr, Ba.The rocks show prominent wing-shaped REE patterns and strong Eu negative anomalies （SEu = 0.21-0.41）,resembling the A1 granites.In combination with the whole-rock geochemistry and regional tectonic evolution,it is proposed that the northern branch ocean of the Neo-Tethys has been transformed into intra-continental extensional setting by the Late Cretaceous,and in this tectonic framework,the decompressional partial melting of the upper crust leads to the formation of the A1 granite in the study area.