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引用本文:余吉远,计波,王国强. 中天山阿拉塔格环状杂岩体中闪长质包体地球化学与岩浆混合作用[J]. 中国地质, 2018, 45(4): 767-782.
YU Jiyuan,JI Bo,WANG Guoqiang. Geochemistry of dioritic enclaves related to magmatic mixing in the concentrically zoned Alatage igneous complex, central Tianshan Mountains[J]. Geology in China, 2018, 45(4): 767-782(in Chinese with English abstract).
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中天山阿拉塔格环状杂岩体中闪长质包体地球化学与岩浆混合作用
余吉远1,2, 计波1,2, 王国强1,2
1.中国地质调查局造山带地质研究中心, 中国地质调查局西安地质调查中心, 陕西 西安 710054;2.国土资源部岩浆作用成矿与找矿重点实验室, 中国地质调查局西安地质调查中心, 陕西 西安 710054
摘要:
环状杂岩体的岩相分带有多种成因模式。为了查明新疆阿拉塔格环状岩体的成因,本文特就其中暗色包体的成因及其岩浆演化进行探讨。包体分布极不均匀,在酸性岩单元的东南角集中发育,大部分呈浑圆状或次圆状,微细粒结构,部分包体中含有寄主岩石中的长石斑晶。通过对暗色包体主量、微量元素、Sr-Nd同位素及单颗粒锆石U-Pb年龄测试,认为该环状岩体中的暗色包体主要为闪长质-花岗闪长质岩浆包体,包体SiO2(56.72%~61.80%)低,K2O+Na2O(8.12%~10.55%)高,具有高钾富碱的特征,属于高钾钙碱性岩石系列;里特曼指数(σ)为4.59~4.85,稀土元素富集。包体及寄主岩的主量和微量元素协变图呈不同程度的线性关系,而且两者稀土、微量元素曲线形态相近,显示出包体和寄主岩在地球化学特征上既有相似性,又有不同的特点。这种特征表明,环状花岗岩岩浆的形成至少与两种岩桨的混合有关。包体的(87Sr/86Sr)t较低(0.705204~0.705914)、εNdt)=1.65~2.57,全部为正值,揭示包体的原始岩浆为幔源玄武质岩浆。包体和寄主岩石的关系显示岩浆的混合方式为基性岩浆注入到已经开始结晶的酸性岩浆。本研究为环状杂岩体的多种成因过程提供了重要依据,认为其中的环状花岗岩单元为壳源酸性岩浆与幔源基性岩浆混合作用的产物。包体和寄主岩石的特征均反映在晚古生代中天山造山带发生过一定程度的后碰撞地壳垂向生长。
关键词:  环状岩体  岩浆包体  地球化学  岩浆混合作用  中天山
DOI:10.12029/gc20180409
分类号:P588.11+5
基金项目:国家自然科学基金(41202045)和中国地质调查局项目“祁连成矿带肃南—大柴旦地区地质矿产调查”(12120113046400)联合资助。
Geochemistry of dioritic enclaves related to magmatic mixing in the concentrically zoned Alatage igneous complex, central Tianshan Mountains
YU Jiyuan1,2, JI Bo1,2, WANG Guoqiang1,2
1.Research Center for Orogenic Geology, Xi'an Center of Geological Survey, Geological Survey of China, Xi'an 710054, Shaanxi, China;2.Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, Ministry of Land and Resources, Xi'an Center of Geological Survey, Geological Survey of China, Xi'an 710054, Shaanxi, China
Abstract:
There are various genetic models about the concentrically zoned igneous complexes. In order to investigate the genesis of the concentrically zoned Alatage igneous complex in central Tianshan Mountains, the authors studied the origin of dark coloured enclaves within the complex and their magmatic evolution in this paper. The enclaves are extremely unevenly distributed, and are concentrated in the southeastern part of the more acidic granitoids within the complex. Most of them are rounded or sub-rounded, with a microlithic to fine-grained texture. Some enclaves contain feldspar phenocrysts that were derived from the host rocks. Based on an analysis of major and trace elements, Sr-Nd isotope, and U-Pb geochronology of individual zircons of the dark coloured enclaves, it is suggested that they are mainly dioritic and granodioritic enclaves and belong to high-K calc-alkaline or K-rich basalt series, with low SiO2 (56.72%-61.80%) and high K2O+Na2O (8.12%-10.55%) content. Their Rittman index is 4.59-4.85, with enriched rare earth elements. The covariance diagrams of the major and trace elements of the enclaves and host rocks show varying degrees of linear relationships, and the curve patterns of REE and trace elements of the enclaves and host rocks are similar, which indicates that there are both similarities and differences in the geochemistry of the enclaves and host rocks. These characteristics indicate that the genesis of the circular granitoid complex is related to magma mixing of at least two types of magma. The enclaves have relatively low (87Sr/86Sr)t ratios (0.705204-0.705914), and positive εNd(t) values (1.65-2.57), which indicates that the primary magma of the enclaves is a mantle-derived basaltic magma. The relationship between the enclaves and host rocks suggests that mafic magma was injected into acidic magma to result in magma mixing. The results obtained by the authors provide important evidence for the multiple origins of the concentrically zoned Alatage igneous complex that was generated by magma mixing between crust-sourced acidic magma and mantle-derived mafic magma.
Key words:  concentrically zoned igneous complex  magmatic enclaves  geochemistry  magma mixing  central Tianshan Mountains