西秦岭北缘天水地区新元古代花岗质片麻岩地球化学特征及其形成环境  

Geochemical features and tectonic setting of the Neoproterozoic granitic gneiss in Tianshui area on the northern margin of West Qinling

作　　者：刘成军[1] 裴先治[1,2] 李佐臣[1,2] 李瑞保[1] 裴磊[3] 高景民[1] 魏方辉[1] 王银川[1] 吴树宽[1] 陈有炘[1] 

机构地区：[1]长安大学地球科学与资源学院,陕西西安710054 [2]西部矿产资源与地质工程教育部重点实验室,陕西西安710054 [3]中国地质大学(北京)地球科学与资源学院,北京100083

出　　处：《地质通报》2012年第10期1588-1601,共14页Geological Bulletin of China

基　　金：国家自然科学基金项目<西秦岭北缘变形构造解析及其祁连-秦岭构造交接关系研究>(批准号:40972136);中央高校基本科研业务费专项资金项目<中央造山系(中西段)关键构造部位复合造山作用研究>(编号:CHD2011TD020);中国地质调查局地质调查项目<天水市幅1:25万区域地质调查(修测)>(编号:200013000154)

摘　　要：西秦岭北缘新元古代花岗质片麻岩位于天水地区分隔北秦岭造山带和北祁连构造带的新阳-元龙韧性剪切带中。花岗质片麻岩具高Si、高Al的特征,属高钾钙碱性系列,A/CNK在1.104～1.389之间,为硅、铝过饱和类型,属典型的强过铝质S型花岗质岩石。轻、重稀土元素分馏较强,具中等负Eu异常。岩石富集大离子亲石元素(Cs、Rb、Ba、Th、U、K、La等),强烈亏损高场强元素(Nb、Ta、Ti、Hf等),明显的Ba、P、Sr负异常,相对富集Zr。该花岗质岩石是一种典型的壳源成因类型,主要为上地壳中以成熟度较低、含泥质成分较高的杂砂岩、岩屑杂砂岩为原岩的古元古界秦岭岩群中含水矿物相脱水部分熔融形成的,可能存在少量的分离结晶作用。该花岗质片麻岩具有同碰撞型花岗岩的特征,可能是北秦岭微地块与相邻地块在新元古代早期发生汇聚的产物,是Rodinia超大陆在西秦岭地区汇聚的响应。The Neoproterozoic granitic gneisses in Tianshui area are located in the Xinyang -Yuanlong ductile shear belt which is the boundary between the North Qinling orogenic belt and the North Qilian tectonic zone. Geochemistry, petrogenesis and tectonic set- ting of the granitic gneisses are discussed in this paper. The granitic gneisses are rich in SiO2 and Al2O3, with A/CNK being 1.104- 1.389, showing the supersaturation of A1 and Si and suggesting the features of typical strongly peraluminous S-type granite. The ∑REE range from 128.27×10^-6 to 242.09×10^-6. Chondrite-normalized REE patterns show a rightward inclination with medium negativeEu anomalies. The trace element geochemistry is characterized by positive anomalies of Cs, Rb, Ba, Th, U, K, La, strongly-depleted negative anomalies of Nb, Ta, Ti, Hf, and evident negative anomalies of Ba, P, St. The granitic gneisses have features of crust-detived and typical continent-continent syn-collisional granites. The Paleoproterozoic Qinling Group whose protoliths were immature psam- mites containing higher pelitic compositions probably served as the source rocks. The generation of granitic gneisses resulted from de- hydration melting due to the breakdown of micaceous minerals, with the possible existence of some fractional crystallization. The granitic gneisses suggest a collision between the North Qinling micro-block and its adjacent block in the Early Neoproterozoic period. It is a response of the assembly of Rodinia in West Qinling.

关 键 词：西秦岭北缘 强过铝质S型花岗质片麻岩 新元古代 同碰撞 RODINIA 

分 类 号：P534.3[天文地球—古生物学与地层学] P595[天文地球—地质学]