中国东部苏、皖、浙新生代玄武岩源区特征及深部过程研究: Li和B同位素制约

Cenozoic intra-plate basalts are widely distributed in Jiangsu, Anhui and Zhejiang Provinces, East China. The petrogenesis and deep dynamic mechanisms of these basalts have been heated debated for many years. Based on previous studies, this project intends to use the burgeoning geochemical tracers of Lithium and Boron isotopes, in combination with the bulk-rock compositions and traditional radiogenic isotopes (Sr-Nd-Pb) to carry out detailed researches on the basalts from typical areas of Jiangsu, Anhui and Zhejiang Provinces. A comprehensive discussion on the composition, nature and origin of the source rocks will be focused. Furthermore, the project plans to take the interaction between melt/fluid derived from subducted slabs and mantle rocks as sally port, to further constrain the nature and origin of the mantle metasomatic medium, and then to trace the direct causes that led to geochemical enrichment or depletion of the mantle beneath different areas. The geochemically modification mechanism on both asthenosphere and lithosphere mantle by the subducted oceanic slab which is stagnant in the mantle transition zone beneath East China will be discovered. The final objects of this project are to (1) disclose the petrogenesis of the basaltic rocks in studied areas; (2) to reveal the evolutionary process of the Cenozoic mantle beneath studied areas; (3) to propose an appropriate petrological model for these basalts, also with an aim to try to provide new constraints for further understand of the genesis of Cenozoic basalts from East-central and South China.

中国东部苏皖及浙江地区广泛出露新生代板内玄武岩，其岩石成因及深部动力学机制一直是科学家讨论的热点话题。本项目拟在以往研究的基础上，采用新兴的锂，硼同位素地球化学示踪手段对苏皖浙典型地区新生代玄武质岩石开展研究，并结合全岩成分和传统放射性同位素（Sr-Nd-Pb）地球化学示踪方法，综合讨论其源区不同物质组分的性质及来源。同时以示踪俯冲板片释放的熔/流体与地幔岩石的反应作为突破口，对地幔交代介质的性质及来源进行限定，追踪不同时空背景下地幔发生富集或亏损的原因，探讨滞留在地幔过渡带的俯冲板片对中国东部新生代以来软流圈和岩石圈地幔的化学改造机制。最终剖析研究区玄武质岩石的成因，反演新生代地幔演化过程，建立苏皖浙新生代玄武质岩浆形成的深部动力学模型，力争为进一步认识中国东部中段及华南新生代玄武岩的成因提供新的制约。

中国东部新生代玄武岩成因一直是地质学家讨论的热点话题。近年来，随着非传统稳定同位素地球化学的快速发展，将传统的放射性同位素和稳定同位素比值联合讨论，能对壳源物质的地幔循环历史有较好的约束，从而更好地揭示岩石成因，认识地球深部动力学作用机理。本项目以中国东部中段苏皖地区及东南沿海闽粤地区新生代玄武岩为研究对象，获得了全岩主微量元素组成、K-Ar年代学、Sr-Nd-Pb同位素比值以及Li-B同位素组成数据。在此基础上，深入讨论了岩石成因和深部动力学机制，再现了东南亚深部地幔晚新生代以来的演化历史。取得的主要认识有如下几点：.（1）东南沿海晚新生代火山岩从沿海到内陆，喷发时代逐渐变新，且随着玄武岩年龄减小，Sr-Nd-Pb同位素组成表现出规律性的变化趋势，暗示地幔组成的不均一性。识别出源区为由橄榄岩和榴辉岩/辉石岩组成的混合源岩，这可能与再循环的洋壳物质有关。结合地球物理深部探测资料，我们认为，储存于核幔边界的古老洋壳物质（~15亿年）被海南地幔柱携带上涌，随后，（古）太平洋板块俯冲至地幔过渡带，加入到地幔柱中继续上升，分别为沿海和内陆火山岩源区提供了富集组分。.（2）东南沿海玄武岩Li-B同位素组成呈现出较正常地幔值更大的变化范围，且二者总体表现出正相关性，暗示俯冲洋壳物质在玄武岩源区的重要作用。进一步地，δ7Li-207Pb/204Pb和 δ11B-207Pb/204Pb的负相关性表明，俯冲沉积物是玄武岩源区地球化学富集端员的主要贡献者。Li-B同位素与放射性同位素组成得出的结论一致，即不同年龄的俯冲洋壳是地幔地球化学不均一性的主要动因。.（3）华南新生代玄武岩中Pb同位素总体呈现出由北向南逐渐增加的趋势。Li-B-Sr-Nd-Pb体系显示，苏皖地区下方地幔广泛被俯冲物质的部分熔融体交代改造。本研究认为造成中国东部下覆地幔空间上的地球化学分带性主要原因是由于“海南地幔柱”的存在。