浙江建德新生代玄武岩源区特征及其成因  

Genesis and Characteristics of Mantle Sources of Cenozoic Jiande Basalts from Zhejiang Province

作　　者：何叶 陈立辉[1] 曾罡[1] 王小均[1] HE Ye;CHEN Lihui;ZENG Gang;WANG Xiaojun(State Key Laboratory for Mineral Deposits Research,School of Earth Science and Engineering,Nanjing University,Nanjing 210023,China)

机构地区：[1]南京大学地球科学与工程学院,内生金属矿床成矿机制研究国家重点实验室,南京210023

出　　处：《高校地质学报》2020年第3期241-254,共14页Geological Journal of China Universities

基　　金：国家自然科学基金项目(41672049,41688103)资助。

摘　　要：中国东南部浙江境内分布有大量新生代板内玄武岩,这些玄武岩的分布受到三条北东—南西向的断裂控制,将浙江玄武岩分为西部、中部、东部三个区域。其中,出露于浙江西部江山-绍兴断裂带的建德玄武岩是浙江境内最古老的新生代玄武岩(约40 Ma)。为进一步认识浙江境内新生代岩浆作用的本质,测定了建德玄武岩的元素组成和Sr、Nd、Hf、Pb同位素组成,并在与浙江境内其它新生代玄武岩对比的基础上,探讨它们之间的成因联系。建德玄武岩为碧玄岩,与浙江西部其它新生代玄武岩一样,碱性程度明显高于浙江中部和东部的新生代玄武岩(以弱碱性的碱性橄榄玄武岩和拉斑玄武岩为主)。这些玄武岩具有较低的SiO2(41.3~42.3 wt%)和Al2O3(9.70~12.6 wt%)含量,较高的MgO(8.90~15.6 wt%)、CaO(8.92~12.1 wt%)、TiO2(2.78~3.18 wt%)和Fe2O3T(14.1~16.2 wt%)含量以及较高的Ca/Al(1.02~1.16)比值。不相容微量元素组成上与火成碳酸岩具有亲缘性,即蛛网图上表现为明显的K、Zr、Hf、Ti的负异常(Hf/Hf*=0.74~0.77,Ti/Ti*=0.70~0.74),同时具有高的Zr/Hf比值(48.5~50.1),表明其地幔源区含有碳酸盐组分。建德玄武岩具有亏损的Sr-Nd-Hf同位素组成(87Sr/86Sr=0.7032~0.7034,εNd=5.85~5.95,εHf=7.78~8.56)和较高的206Pb/204Pb(18.491~18.554)、207Pb/204Pb(15.488~15.518)和208Pb/204Pb(38.387~38.523)比值。相比浙江中部和东部玄武岩,浙江西部玄武岩及建德玄武岩具有更高的碱含量、不相容微量元素含量、La/Yb比值和更明显的K、Zr、Hf、Ti负异常,表明浙江西部玄武岩是碳酸盐化地幔低程度熔融的产物。浙江新生代玄武岩的Ti/Ti*与Ba/Th、K/La比值之间较好的正相关性表明其源区存在两端元混合的特征。其中,以浙江西部玄武岩(包括建德玄武岩)为代表的地幔端元是由含碳酸盐的再循环洋壳熔融产生的碳酸盐熔体与亏损地幔反应形成的碳酸盐化地幔,以较低的Ba/Th、The distribution of Cenozoic continental basalts from Zhejiang Province,southeast China are controlled by three northeast-southwest trending faults.According to their outcropping locations and eruption ages,these basalts are divided into three groups:western part,central part and eastern part basalts in Zhejiang province.Here,we carried out a geochemical study on Cenozoic Jiande basalts,the oldest basalts(~40 Ma)cropping out at Jiangshan-Shaoxing fault in the west of Zhejiang,to further explore the nature of mantle source beneath Zhejiang province.These volcanic rocks are mainly alkaline(sodic)basanites with low contents of SiO2(41.3-42.3 wt%)and Al2O3(9.70-12.6 wt%),high contents of MgO(8.90-15.6 wt%),CaO(8.92-12.1 wt%),TiO2(2.78-3.18 wt%)and Fe2O3T(14.1-16.2 wt%).They are characterized by negative K,Zr,Hf,and Ti anomalies(Hf/Hf*=0.74-0.77,Ti/Ti*=0.70-0.74),and high Zr/Hf(48.5-50.1),Ca/Al(1.02-1.16)ratios,which suggest the contribution of recycled carbonated materials in the mantle source.They show depleted Sr,Nd,Hf isotopic compositions(87Sr/86Sr=0.7032-0.7034,εNd=5.85-5.95,εHf=7.78-8.56)and have 206Pb/204Pb ratios of 18.491-18.554,207Pb/204Pb ratios of 15.488-15.518 and 208Pb/204Pb ratios of 38.387-38.523.Compared to weakly alkaline basalts in central and eastern part of Zhejiang,the strongly alkaline basalts in western part of Zhejiang show higher contents of trace elements and more pronounced negative Zr,Hf,and Ti anomalies,which suggest that the strongly alkaline basalts in western part of Zhejiang are generated by low-degree partial melting of a carbonated mantle source.The good correlations of Ti/Ti*with Ba/Th and K/La of Zhejiang Cenozoic basalts suggest a mixing process of two components.Carbonated melts derived from partial melting of carbonate-bearing oceanic crust reacted with the depleted asthenosphere mantle,generating the carbonated mantle source with low Ba/Th,K/La,and Ti/Ti*ratios(reflected in basalts in western part of Zhejiang).Another enriched component is the melting residue of recycled oc

关 键 词：碳酸盐组分 强碱性玄武岩 中国东南部 低程度部分熔融 地幔岩性 

分 类 号：P588.145[天文地球—岩石学]