桂北圆石山早侏罗世A型花岗岩的岩石成因及意义  

The Early Jurassic A-Type Granites in Northern Guangxi,China:Petrogenesis and Implications

作　　者：贾小辉[1] 王晓地[1] 杨文强[1] 牛志军[1] 

机构地区：[1]中国地质调查局武汉地质矿产研究所,湖北武汉430223

出　　处：《地球科学（中国地质大学学报）》2014年第1期21-36,共16页Earth Science-Journal of China University of Geosciences

基　　金：中国地质调查局基础地质综合研究项目<南岭成矿带基础地质综合研究>(No.1212011085356)

摘　　要：桂北圆石山花岗岩中发育大量镁铁质包体.LA-ICP-MS锆石U-Pb定年结果显示,花岗岩形成于早侏罗世(179±2Ma).花岗岩的地球化学特征表现为硅含量均一,富碱更富钾、相对富铁而贫镁,具有高的104×Ga/A1比值和Zr+Nb+Ce+Y含量,属于A型花岗岩.圆石山花岗岩具有比较均一的Sr、Nd同位素组成(ISr=0.701 7~0.710 8,εNd(t)=-7.77^-4.55).镁铁质包体则显示了稍低的ISr值(0.705 0~0.707 1)和稍高的εNd(t)值(-4.87^-2.63).花岗岩的锆石原位Hf同位素组成为:(176 Hf/177 Hf)i=0.282 62~0.282 70,εHf(t)=-1.68~1.17,相应的Hf同位素两阶段模式年龄TDM2变化于1.25~1.43Ga之间.圆石山花岗岩可能是在伸展环境下由低成熟度的下地壳物质部分熔融所形成.自早侏罗世(~200Ma)以来,伸展作用是华南内陆构造背景的主体,多期次的玄武质岩浆底侵作用可能是燕山期伸展作用的直接诱因.华南内陆早侏罗世时期可能仍处于板内"后碰撞"环境.The Yuanshishan granites have large amounts of mafic enclaves.This paper presents new zircon LA-ICP-MS U-Pb age,and the age analysis shows that the granites were generated in Early Jurassic（179±2Ma）.These granites are characterized by uniform silicon contents,enriched in alkali contents and more enriched in K contents,and enriched in iron and depleted in magnesium contents,with high 10^4×Ga/A1values and Zr＋Nb＋Ce＋Y contents.The granites belong to A-type granite.The Yuanshishan granites have homogeneous Sr-Nd isotope compositions（ISr =0.701 7to 0.710 8,εNd（t）=-7.77to-4.55）.Mafic enclaves show lower Isr（0.705 0to 0.707 1）and higherεNd（t）（-4.87to-2.63）values.In situ zircons Hf isotope compositions from these granites have limited ranges（^176 Hf/^177 Hf） i =0.282 62to 0.282 70）,εHf（t）=-1.68to 1.17,and TDM2 =1.25to 1.43Ga.We suggest that the Yuanshishan granites were likely generated by partial melting of newly underplated basaltic lower crust in extension setting.Since Early Jurassic（-200Ma）,extension has been the major subject of the tectonic background in the interior of the South China.Multistage basaltic magma underplating is direct inducement to the Yan shanian extension.During the Early Jurassic,the South China interior was most likely in post-collisional setting.

关 键 词：早侏罗世 A型花岗岩 玄武质底侵 伸展作用 岩石学 地球化学 

分 类 号：P581[天文地球—岩石学]