浙东南晚中生代玄武岩的地球化学特征及其对源区物质的指示意义  

Geochemical Characteristics of the Late Mesozoic Basalts in Southeastern Zhejiang Province and Constraints on Magma Source Materials

作　　者：崔玉荣[1,2] 谢智[1] 王斌[1] 陈江峰[1] 俞云文 贺剑锋[1] 

机构地区：[1]中国科学技术大学地球和空间科学学院,中科院壳慢物质与环境重点实验室,合肥230026 [2]天津地质矿产研究所,天津300170 [3]浙江省地调院,萧山311200

出　　处：《高校地质学报》2011年第4期492-512,共21页Geological Journal of China Universities

基　　金：中科院知识创新项目(KZCX2-EW-QN508,KZCX2-YW-Q08-3-3);国家自然科学基金项目(40873002,40673008);新世纪优秀人才支持计划(NCET-07-0795)共同资助

摘　　要：浙江东南部晚中生代上、下火山岩系(以下简称上、下岩系)中均有玄武岩产出,本文对这些玄武岩分别进行了元素地球化学和Sr-Nd-Pb同位素研究。不同岩系玄武岩的主量元素均表现出富碱、富Al等特征。但微量元素存在差异,下岩系天台和青田样品具有轻稀土富集以及Ba,Pb和Sr富集,Eu负异常,Nb,Ta,Zr和Hf亏损的特征。上岩系玄武岩的元素特征也有差别,永嘉花坦、宁波玄坛地、新昌镜岭和永嘉镜架山等地样品的元素特征表现出的性质与下岩系样品相似,武义玄武岩样品没有Ta,Nb亏损特征,金衢盆地玄武岩的元素特征则介于两者之间。对应的,这些玄武岩样品的同位素组成也有明显差异,下岩系玄武岩的初始同位素组成范围为ISr=0.70850～0.70897,εNd(t)=-5.6～-4.1,(206Pb/204Pb)i=18.21～18.38,(207Pb/204Pb)i=15.55～15.58,(208Pb/204Pb)i=38.26～38.49,接近下岩系中酸性岩浆岩的范围,反映了下地壳物质对其岩浆源区的显著影响。上岩系玄武岩有明显差异,表现出与元素特征对应的分组现象。其中永嘉花坦、宁波玄坛地、新昌镜岭和永嘉镜架山样品ISr=0.70734～0.70936,εNd(t)=-7.1～-2.1,(206Pb/204Pb)i=18.01～18.40,(207Pb/204Pb)i=15.54～15.62,(208Pb/204Pb)i=37.99～38.62,具有富集特征,可能来自活动大陆边缘;而武义和金衢盆地样品的ISr=0.70533～0.70589,εNd(t)=0.4～3.3,(206Pb/204Pb)i=17.23～18.11,(207Pb/204Pb)i=15.46～15.53,(208Pb/204Pb)i=36.91～38.43,具有类似OIB特征,趋向亏损地幔端元。上下岩系玄武岩的元素和同位素组成的研究表明,玄武岩的物质来源有较明显的差别,且表现出随时间变化的特征。其中下岩系玄武岩源区中可能有古老岩石圈地幔、下地壳物质和俯冲蚀变洋壳物质的贡献,而上岩系中玄武岩源区有可能是类似下岩系玄武岩性质的岩石圈、软流圈地幔和下地壳物质等的贡献。浙东南晚中生代岩石圈演化的动力学过程可�The late Mesozoic basalts occur in the southeastern Zhejiang Province together with acidic volcanic rocks,which can be divided into upper and lower rock series（URS and LRS） . Major,trace elements and Sr-Nd-Pb isotopic features of thesebasalts are studied in this paper to provide insights into the nature of their magma sources and evolution processes. Both the URS and LRS basalts are enriched in alkaline and Al elements. However,their characteristics of trace elements are different. The samples from Tiantai and Qingtian of LRS are enriched in LREEs,Ba,Pb and Sr,and depleted in Nb,Ta,Zr and Hf with Eu negative anomalies. The basalts from Yongjiahuatan,Ningboxuantandi,Xinchangjingling and Yongjiajingjiashan basalts of URS show similar trace elements and REEs natures to those of the samples from LRS,but basalts from Wuyi of URS show no negative Ta,Nb anomalies,while the trace element features of the basalts from Jinqu Basin are on the average. Correspondingly,isotopic compositions of these basalts are also variable apparently. The initial isotopic ratios of basalts from LRS range are as follows：I Sr = 0.70850～ 0.70897,ε Nd（t） =-5.6～-4.1,（206 Pb/204 Pb） i =18.21～18.38,（207 Pb/204 Pb） i =15.55～15.58,（208 Pb/204 Pb） i =38.26～38.49,which are approximate to those of the intermediate-acid magmatic rocks in LRS,indicating that their magmatic sources were contaminated significantly by the lower crustal materials. Basalts from URS demonstrate a marked difference from those of the LRS in isotopic features by showing similar groups as the characteristics of trace elements. The samples from Yongjiahuatan,Ningboxuantandi,Xinchangjingling and Yongjiajingjiashan of URS have enriched Sr-Nd isotopic compositions with I Sr =0.70734～0.70936,ε Nd（t） =-7.1～-2.1,（206 Pb/204 Pb） i =18.01～18.40,（207 Pb/204 Pb） i =15.54～15.62,（208 Pb/204 Pb） i = 37.99～38.62,suggesting that these basalts are most likely generated from active continental margin. The samples from Wuyi and Jinqu Basins

关 键 词：晚中生代 玄武岩 地球化学特征 源区物质 浙东南 

分 类 号：P595[天文地球—地球化学] P591.1[天文地球—地质学]