粤北坪田地区三叠纪A型花岗岩年代学、地球化学及其构造意义  

Geochronology,Geochemistry and Tectonic Implications of Triassic A-Type Granites in Pingtian Area,Northern Guangdong

作　　者：席振 刘清泉[2] 吴德华[1] 陈肇华 Xi Zhen;Liu Qingquan;Wu Dehua;Chen Zhaohua(School of Municipal and Geomatics Engineering,Hunan City University,Yiyang 413000,China;Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring,Ministry of Education,Central South University,Changsha,410083,China;Hunan Key Laboratory of Remote Sensing Monitoring of Ecological Environment in Dongting Lake Area,Hunan Natural Resources Affairs Center,Changsha 410004,China)

机构地区：[1]湖南城市学院市政与测绘工程学院,湖南益阳413000 [2]中南大学有色金属成矿预测与地质环境监测教育部重点实验室,湖南长沙410083 [3]湖南省自然资源事务中心洞庭湖区生态环境遥感监测湖南省重点实验室,湖南长沙410004

出　　处：《地球科学》2024年第7期2508-2525,共18页Earth Science

基　　金：国家自然科学基金项目(No.42101005);有色金属成矿预测与地质环境监测教育部重点实验室(中南大学)开放基金项目(No.2022YSJS13);洞庭湖区生态环境遥感监测湖南省重点实验室开放课题(DTH Key Lab.2022-06)。

摘　　要：坪田岩体位于华南南岭中部,是认识华南三叠纪岩浆作用及其地球动力背景的理想窗口.以坪田花岗岩类为研究对象,对其开展了系统的全岩地球化学、锆石U-Pb定年和锆石原位Hf同位素研究.结果表明,坪田岩体由粗粒似斑状黑云母花岗岩、粗粒似斑状钾长花岗岩和中粒二长岩组成,成岩年龄为238~239 Ma,形成于中三叠世.地球化学特征显示,岩石轻稀土元素富集,有明显铕负异常(δEu平均为0.42).富集Zr、Hf、Y、Ce,明显亏损Sr、P和Ti,属于准铝质到弱过铝质碱性花岗岩类,为A型花岗岩.锆石ε_(Hf)(t)值为-37.7~-5.0,tDM2二阶段模式年龄为1578~3597 Ma之间,结合全岩地球化学特征,揭示其原始岩浆来源于地壳中长英质物质在低温高压环境下部分熔融,可能混入古老地壳物质,并经历了一定的结晶分异作用,形成于后碰撞伸展背景.综合华南A型花岗岩和碱性正长岩的地球化学特征和空间分布,认为华南内部三叠纪区域构造演化主要受华南地块与印支地块碰撞带和华南地块与华北地块碰撞带共同控制,华夏地块在中三叠世(238 Ma左右)发生构造环境的转变,从早三叠世的碰撞挤压环境,到中晚三叠世过渡到后碰撞伸展环境.Pingtian pluton is located in the central part of South China,providing an ideal window for understanding the Triassic tectonic-magmatic activities and the geodynamic setting in South China.Systematic whole-rock geochemistry,zircon U-Pb dating and zircon in situ Hf isotope studies were carried out,taking the Pingtian granite group as the object of study.The results show that the Pingtian pluton consists of coarse-grained porphyritic biotite granite,coarse-grained porphyritic potassium feldspar granite and medium-grained monzonite,with diagenetic ages ranging from 238 to 239 Ma,and it was formed in the Middle Triassic.Geochemical characteristics show that the rocks are enriched in light rare earth elements with obvious europium negative anomalies(δEu average=0.42).It is enriched in Zr,Hf,Y and Ce,and significantly depleted in Sr,P and Ti.It belongs to the metaluminous to weakly peraluminous alkaline granite type,and is A-type granite.The zirconε_(Hf)(t)values range from-37.7 to-5.0,and the tDM2 two-stage model ages range from 1578 to 3597 Ma.Combined with the whole-rock geochemical characteristics,it reveals that the magmas were derived mainly from the partial melting of felsic material in the crust under low-temperature and high-pressure environment,probably mixed with old crustal material,and experienced crystalline differentiation,and was formed in the post-collisional extensional background.Integrating the geochemical characteristics and spatial distribution of A-type granites and alkaline syenite in South China,we propose that the tectonic evolution of the Triassic was mainly controlled by the collisional interaction of the South China block with the Indochina block and the North China block,and that the tectonic setting changed at around 238 Ma,from a collisional extrusion environment in the Early Triassic to a transition to a post-collisional extensional environment in the Middle to Late Triassic.

关 键 词：A型花岗岩 中三叠世 后碰撞伸展环境 坪田 华南 地球化学 

分 类 号：P588.12[天文地球—岩石学]