川西北平武地区南一里花岗岩体地球化学特征及其构造环境

Geochemical Features and Tectonic Setting of the Nanyili Granite in the Pingwu Area,Northwestern Sichuan

对出露于碧口地块西南部的南一里岩体进行了系统的地球化学研究,重点讨论了南一里岩体的岩石成因、成岩物质来源及其构造环境。该岩体的SiO2和Al2O3的含量均很高,SiO2含量变化为71.29%～73.05%,A/CNK在1.07～1.11之间;FeO/(FeO+MgO)比值较低,为硅和铝过饱和类型,属典型的强过铝质花岗岩。稀土元素总量(∑REE)为56.80×10-6～89.12×10-6,稀土元素配分曲线呈右倾型,具有弱的负铕异常。Ta、Nb、P、Ti等高场强元素具有明显的负异常,而Rb和Sr等大离子亲石元素具有明显的正异常。岩浆源区可能是以杂砂岩成分为主的沉积岩部分熔融形成的花岗质岩浆上升侵位形成的,是一种典型的壳源成因类型。南一里岩体具有后碰撞岩浆活动的特征,是印支期华北地块和扬子地块碰撞导致地壳加厚环境下下地壳部分熔融的产物,形成于同碰撞(挤压环境)向碰撞后(伸展环境)转化阶段,为后造山花岗岩类。

The late Indosinian Nanyili granite, exposing in the southern margin of the Bikou area, was analyzed and discussed so as to better understand its genesis, ore-forming material source and structural setting. The results show that the Nanyili granite is high in SiO2 （71.29%-73.05%） and Al2O3 contents, slightly low ratio of FeO/（FeO＋MgO）, has A/CNK ratios ranging 1.07-1.11. All these characteristics suggest sthat it belongs to typical peraluminous granite. The ∑ REE ranges from 56.80 × 10^-6 to 89. 12 × 10^-6 , REE diagram shows rightward incline and obvious negative Eu abnormity. High field strength elements such as Ta,Nb,P and Ti show distinct nagetive anomly while lithophile elements such as Rb and Sr exhibit obvious positive anomalies. Magma source is probably rising and emplacement of partially melt granitic magma dominated by greywacke, which is typical of crustal genesis. With the features of the postcollisional magmatic activity, the Nanyili granite is the product of partially melt crust in such a geologic condition of crustal thickening resulted from the collision between the North China massif and Yangtze massif during the Indosinian. It formed during the transition period from syncollision （compressional） to post collision （extensional environment）.