新疆阿拉套山西段察哈乌苏二长花岗岩成因及构造意义

察哈乌苏岩体位于新疆西天山阿拉套山南缘的西段，大地构造位置属阿拉套山南缘泥盆纪—石炭纪弧后盆地，岩体主要由灰白色中粒二长花岗岩、灰白色中-细粒二云二长花岗岩组成。岩石地球化学数据显示，岩石中SiO2、Al2O3、碱含量较高，A/CNK=1.08～1.14，属过铝质高钾钙碱性花岗岩系列；轻稀土元素相对富集，重稀土元素相对亏损，稀土元素配分模式呈轻稀土富集的缓右倾型，具有明显的Eu负异常；强烈富集大离子亲石元素（LILE） Rb、Th、U、K、Hf，亏损高场强元素（HFSE） Ba、Nb、Ta、Sr、P、Ti；岩石具有较低的ISr值（0.701 5～0.702 6）。综合研究，表明察哈乌苏二长花岗岩为低Sr高分异S型花岗岩，兼具A型花岗岩的特征，是长石分离结晶的产物。二长花岗岩具有较高的正εNd （t）值（+0.37～+1.18）；高的Th、Ta含量及Th/Ta值（6.12～8.54）指示岩浆源区为亏损地幔，但受到古老地壳物质同化混染的影响。构造判别图解中所有样品落入火山弧花岗岩和同碰撞花岗岩的交界部位，更靠近同碰撞花岗岩区域。察哈乌苏岩体的侵位时代为313 Ma，结合区域地质背景，笔者认为伊犁微板块在310 Ma前已与准噶尔微板块碰合，北天山洋盆闭合时限早于晚石炭世末，310 Ma以后，阿拉套山地区逐渐进入后碰撞伸展阶段。因此，察哈乌苏二长花岗岩应为后碰撞花岗岩，岩体形成于同碰撞挤压向后碰撞伸展转换过渡阶段的张性环境，是幔源玄武质岩浆底侵和发生ACF作用的结果，其构造背景可能为活动大陆边缘。

Genesis and Tectonic Significance of the Chahawusu Monzonitic Granite in the Western Section of the Alataw Mountain,Xinjiang

The Chahawusu pluton, located in the Alataw mountains of west Tianshan, belongs to the Devonian Carboniferous back-arc basin tectonically.The pluton is mainly composed of gray white medium-grained monzonitic granite and gray white medium-fine grained two-mica monzonitic granite. The geochemical data shows that the rock has a high content of SiO2, Al2O3, and alkali, A/CNK=1.08~1.14, belonging to the peraluminum high potassium calcium-alkaline granite series; the light rare earth elements are relatively enriched and the heavy rare earth elements are relatively depleted.The rare earth distribution mode is a right-leaning type enriched with light rare earth elements, with an obvious negative Eu anomalies;large ion stonephilic elements (LILE) Rb, Th, U, K, Hf are strongly enriched and high field strength elements (HFSE) Ba, Nb, Ta, Sr, P, Ti are depleted. The low ISr value (0.701 5~0.702 6) shows that the Chahawusu monzonitic granite is a low-strontium high-differentiation S-type granite. It has the characteristics of the A-type granite, and it is the product of feldspar separation and crystallization. The high positive εNd(t) value (+0.37~+1.18), high content of Th and Ta, and Th/Ta ratio (6.12~8.54) in Monzonitic granite indicate that the magma source area is a long-term depleted mantle, affected by the assimilation and contamination of ancient crustal materials.All samples of the structural discriminant diagrams are in the junction of the volcanic arc granite or the syn-collision granite, even more closer to the syn-collision granite area. Chahawusu intrusion is in 313 Ma. Combined with the regional geological background, the authors believe that the Yili microplate has already met with the Junggar microplate before 310 Ma, and the closure of North Tianshan Ocean Basin was earlier than the end of the Late Carboniferous. After 310 Ma, the Alataw mountains area gradually entered the post-collision extension stage.Therefore,the Chahawusu monzonitic formed in a transitional environment from co-collision compression to post-collision extensional, being the post-collision granite. It is the result of mantle-derived basaltic magma underplating and the ACF process, the tectonic background being the active continental margin.