华北克拉通东南部新生代玄武岩Ba同位素地球化学研究

Basalts were widely distributed in east China during Cenozoic. They are typical OIB-like continental basalts. Though recent researches tended to argue that source of these Cenozoic basalts were mixture of depleted mantle and dehydrated subducted oceanic crust component, they did not discriminate the contributions of oceanic basalt and sediment to the geochemistry of Cenozoic basalts but generally use oceanic basalt to represent subducted oceanic crust component. Moreover, research about Cenozoic basalts from north China suggests that lower continental crustal component may also exist in the mantle source. As implicated by measured Ba isotope compositions in sedimentary BaSO4 and island arc volcanic rock, sediment may be depleted in heavy Ba isotope relative to depleted mantle; continental crust may also be depleted in heavy Ba isotope according to the measured Ba isotope composition of granodiorite. Ba is much more abundant in the sediment and continental crust components than the depleted mantle and oceanic basalt components. In this regard, Ba isotope has capability to efficiently discriminate the recycled crustal component in the mantle source. This study will measure Ba isotope besides conventional element and isotope compositions of Cenozoic basalts in the southeast part of North China Craton with the expectation of discriminating the recycled crustal component in the mantle source via combination of Ba isotope and other geochemical indices.

中国东部新生代时广泛分布有玄武岩，这些玄武岩为典型的类OIB型大陆玄武岩。近期研究虽趋向其源区应由亏损地幔与经历脱水的俯冲洋壳组分混合而成，却笼统地以洋壳玄武岩代表俯冲洋壳组分，未曾细分洋壳玄武岩和沉积物对玄武岩地球化学性质的贡献。另外对华北新生代玄武岩的研究表明其源区还可能有下地壳组分。前人对沉积成因BaSO4和岛弧火山岩样品Ba同位素测定结果暗示沉积物可能相对亏损地幔亏损重Ba同位素；对花岗闪长岩样品的测定结果暗示陆壳也可能亏损重Ba同位素。在亏损地幔、洋壳玄武岩、沉积物和陆壳四个端元组分中，后两者Ba含量远高于前两者，因此Ba同位素有潜力成为鉴别源区中再循环地壳组分的有效指标。本项研究拟对华北克拉通东南部新生代玄武岩在常规元素同位素地球化学分析外测定其Ba同位素组成，期望通过Ba同位素与其它地球化学指标的联合示踪来鉴别出源区中的再循环地壳组分。

本项目主要研究了中国东部新生代玄武岩中Ba同位素组成，也对玄武岩中锆石的REE、U-Pb年龄和Hf同位素组成进行了分析，并对中国东部新生代玄武岩的地球化学组成进行了总结。发现1. 中国东部新生代玄武岩具有相较于亏损地幔为低的δ138/134Ba。玄武岩的低δ138/134Ba应继承自低δ138/134Ba的俯冲大洋板片。这为俯冲带中Ba的再循环提供了重要约束。2. 中国东部新生代玄武岩中的锆石U-Pb年龄普遍老于玄武岩喷发年龄，表明这些锆石应为继承自地幔源区中沉积物组分的古老锆石。U-Pb年龄和Hf同位素组成表明这些锆石可能源自大别-苏鲁造山带变质岩。3. 系统总结了中国东部新生代玄武岩的全岩地球化学组成，这些玄武岩的地球化学特征可以由其地幔源区中壳源组分的性质来解释。俯冲大洋地壳部分熔融产生的熔体提供了地幔源区中的壳源组分，其中包括洋壳镁铁质火成岩、海底沉积物和大陆下地壳三种组分。因此这些玄武岩记录了古太平洋板片俯冲析出流体对地幔楔的化学交代作用。