俯冲带火山岩硼及其同位素特征：华南中生代玄武岩初步研究  

Boron and Its Isotopes in the Subduction Zone： A Preliminary Study of Mesozoic Basalts in Southeast China

作　　者：戴宝章[1] 蒋少涌[1] 赵葵东[1] 高剑锋[1] 

机构地区：[1]南京大学地球科学系成矿作用国家重点实验室,江苏南京210093

出　　处：《地球学报》2005年第B09期105-108,共4页Acta Geoscientica Sinica

基　　金：本项目受日本C0E-21合作研究基金资助.

摘　　要：硼及其同位素能够有效地反映由于俯冲板片脱水引起的湿地幔楔部分熔融过程在火山岩中存留的地球化学信息。本次研究中使用同位素稀释法对华南中生代玄武岩进行了硼含量的测定，结合稀土与微量元素分析结果对玄武岩成因及构造背景进行探讨。玄武岩的硼含量主要集中在（1-5）×10^-6，只有几个样品具有较高的含量（最高可达19×10^-6），按时代和空间位置划分出的3组样品在硼含量上没有明显差别。在B／Ce，Nb／Be的协变图解上，中侏罗世玄武岩表现出OIB特征。最临近太平洋板块的白垩世东区玄武岩富集LILE，并出现Nb-Ta亏损，具有岛弧火山岩特征，但硼没有富集。进行系统的B-Li-O同位素分析能有效地揭示华南中生代玄武岩的形成过程及该区的构造演化。Although not the dominant facies, basalts are quite widely distributed in southeast China, and thus have special significance in studying the tectonic evolution of this region. The dominant eruption ages of basalts are 175 ± 5 Ma and 80-110 Ma. In this study, all basalt samples （31 samples） were divided into three groups： Group Ⅰ （13 samples） of Middle Jurassic, Group Ⅱa （4 sampies） of Cretaceous basalts from the western part of southeast China, and Group Ⅱb（ 14 samples） of Cretaceous basalts from the eastern part of southeast China. Samples of Group Ⅱb are rich in LILE and show evident depletion of Nb and Ta in the spider diagram of primary mantle normalized incompatible dements, which is generally thought to be related to hydrated mantle wedge by fluids from the subduction slab. In this study, the authors aim to employ boron geochemistry to trace the source of boron in basalts from southeast China and to assess the contribution of the subduction slab to the basalt generation, in particular the generation of Group Ⅱb basalts. The boron contents of basalt samples were analyzed using the isotope-dilution （ID） ICP-MS method at the State Key Laboratory for Mineral Deposits Research, Nanjing University. The results show that, compared with recent basalts, all basalts under study are relatively poor in boron, being mainly in the range of （1 -5） × 10^-6, with only a few samples having higher abundances （up to 19.0 × 10^-6）. The boron versus other element plots （e. g., B/Ce versus Ce） for Group I hasalts indicate a clear OIB affinity for Middle Jurassic basalts, which possibly erupted in an intra-continental rift setting without any contributions from subducted slab related to Pacific Plate. Further analyses of B-O-Li isotopes for these basalts may provide a better clue to their origin and tectonic settings.

关 键 词：华南中生代玄武岩 俯冲板片脱水 B含量 同位素特征 岛弧火山岩 玄武岩 硼含量 中生代 华南 俯冲带 

分 类 号：P618.510.4[天文地球—矿床学] P588.14[天文地球—地质学]