赣南中侏罗世玄武岩的Pb-Nd-Sr同位素地球化学研究：中生代地幔源区特征及构造意义

赣南中侏罗世玄武岩的Pb-Nd-Sr同位素地球化学研究：中生代地幔源区特征及构造意义

章邦桐, 陈培荣, 凌洪飞, 孔兴功

南京大学内生金属矿床成矿机制研究国家重点实验室，南京 210093

收稿日期:2004-06-20 修回日期:2004-06-20 出版日期:2004-06-20 发布日期:2004-06-20

Pb-Nd-Sr Isotopic Study of the Middle Jurassic Basalts in Southern Jiangxi Province：Characteristics of Mantle Source and Tectonic Implication

ZHANG Bang-tong, CHEN Pei-rong, LING Hong-fei, KONG Xin-gong

State Key Laboratory for Mineral Deposits Research and Department of Earth Sciences, Nanjing University, Nanjing 210093, China

Received:2004-06-20 Revised:2004-06-20 Online:2004-06-20 Published:2004-06-20

摘要/Abstract

摘要： 赣南龙南—寻邬地区余田群菖蒲组底部的玄武岩形成于中侏罗世(172．6～175．6Ma)，其Pb，Nd，Sr同位素组成的特征为：富放射性成因铅((^206Ph／^206Ph)i=17．872～18．653，(^207Pb／^204Pb)i=15．434～16．131，(^208pb／^204pb)i=37．837～39．194)，中等的εNd(t)(-0．4～+1．1，平均值为0．1)及较高的ISr(0．70835～0．71115)。玄武岩在Ph-Ph图解上均投影于NHRL上方，A7／4Pb值为0．19—61．7(平均值为22．1)，AS／4Pb值为59．2～101．5(平均值为71．03)，△Sr值为80．0～111．5(平均值为91．5)，表明存在典型的Dupal同位素异常。根据Sr-Nd，Sr-Ph，Nd-Pb和Pb-Pb相关特征，判明赣南玄武岩是由亏损地幔端元(DMM)和EMⅡ型富集地幔端元在源区混合形成的。按Sr-Nd双变量二元混合模型计算得出源区物质中亏损地幔端元和EMⅡ富集地幔端元所占份额分别为58％～64％和42％～36％。赣南地区呈东西向展布的中侏罗世双峰式火山岩带反映了华南板块内部在燕山早期发生的一起重要伸展构造事件。根据Ph-Nd-Sr同位素地球化学及构造特征，推测赣南中生代地幔源区中的EMⅡ富集地幔端元组分可能源自冈瓦纳古陆。

Abstract: The basalts of the Changpu Formation of the Yutian Group in the Longnan-Xunwu area of Southern Jianxi were formed in the Middle Jurassic Epoch with a Rb-Sr isochron age of 173-176 Ma，and are the basic endmenber of the bimodal volcanic rock assosiation．The Pb-Nd-Sr isotope compositions of basalts are as follows：enriched in radiogenic lead，(206Pb／204Pb)i= 17．872～18．653，(207Pb／204Pb)i=15.434 ～16.131，(208Pb／204Pb)i=37．837 ～39．194；intermediate initial Nd isotopic ratios，εNd(t)= -0.4+1.1；and distinctive high initial Sr isotopic ratios，ISr=0．70835～0．71115．On the Pb-Pb diagrams．the basalts are plotted above NHRL line and have obvious Dural anomaly：Δ7／4Pb=0．19～61．7(22．1 in average)；△8／4Pb=59．2～101．5(71．03 in averag)；ΔSr=80～111．5(91．5 in average)．The Sr-Nd，Sr-Pb，Nd-Pb，Pb-Pb relationships of basalts strongly suggest a mixing source of the depleted mid．ocean ridge basalt mantle endmenber(DMM)and with the EM II enriched mantle endmenber．According to double-variation and two-component(Sr-Nd)mixing equation，calculation shows that the depleted mantle endmember and the EMII enriched mantle endmember for rock-forming materials of southern Jiangxi basalts are 58％一64％ and 42％一36％．respectively．The E-W trending Middle Jurassic bimodal volcanic rock belt in East Nanling area implies a major extensional tectonic event during the Early Yianshanian period．On the basis of Sr-Nd-Pb isotope geochemical and tectonic features mentioned above，it is assumed that the endmember component of EM II of Mesozoic mantle souses in southern Jiangxi was derived from the Gondwanaland．

章邦桐, 陈培荣, 凌洪飞, 孔兴功. 赣南中侏罗世玄武岩的Pb-Nd-Sr同位素地球化学研究：中生代地幔源区特征及构造意义[J]. J4.

ZHANG Bang-tong, CHEN Pei-rong, LING Hong-fei, KONG Xin-gong. Pb-Nd-Sr Isotopic Study of the Middle Jurassic Basalts in Southern Jiangxi Province：Characteristics of Mantle Source and Tectonic Implication[J]. J4.