海洋地质与环境重点实验室知识库

马努斯弧后盆地是一个活跃的弧后伸展扩张区域，受到所罗门板块和太平洋板块双重俯冲的影响，所涉及的俯冲作用过程及相应岩浆作用极为复杂。因此，通过地球化学方法，揭示马努斯岩浆活动的物质来源和演化规律，对于认识俯冲带壳-幔物质循环、弧后盆地演化具有重要意义。

本文所使用的样品来自“科学号”考察船执行的201501海上调查航次，借助水下机器人获取东马努斯弧后盆地产出的火山岩样品。样品岩性为玄武质安山岩和英安岩等，研究内容主要包括全岩主微量元素及Li、Nd同位素的测试分析；结合前人数据和研究区地质特征，重点探究锂元素、同位素在俯冲过程中的地球化学行为，为深入全面地认识俯冲带物质循环奠定基础。

根据马努斯盆地东部火山岩的常量元素组成，本文认为马努斯盆地火山岩样品主要形成于岩浆分离结晶作用，其分离结晶的矿物可能包括橄榄石、辉石、斜长石、钛铁矿及磷灰石。样品微量元素组成显示，其稀土元素分布为轻稀土富集型，且富集大离子亲石元素和流体活动性元素，亏损高场强元素。上述微量元素组成特征表明，马努斯盆地东部火山活动的岩浆源区明显受到俯冲组分的影响。

本次Li同位素组成研究显示，马努斯盆地东部火山岩Li同位素组成较为均一（0.2-6.1‰），变化幅度较小，随岩浆结晶分异演化程度没有明显变化。此外，马努斯盆地Li同位素变化明显受到俯冲组分的影响， Li同位素组成可作为盆地俯冲过程中流体和岩浆演化的优良示踪指标。

样品的¹⁴³Nd/¹⁴⁴Nd在0.512994~0.513027之间，与马努斯扩张中心MORB和新不列颠岛弧熔岩是相似的，根据样品的Li、Nd同位素相互关系可知，东马努斯弧后盆地火山岩主要来自于亏损地幔、俯冲蚀变洋壳和俯冲海底沉积物。

Manus basin is an active back-arc extensional zone, which is affected by the double subduction of the Solomon Sea plate and the Pacific Ocean plate. The subduction processes involved and corresponding magmatismare extremely complex.Therefore, revealing the material source and evolution process of Manus magmatic activity through geochemical methods, which is of great significance for understanding the crust-mantle material cycle of subduction zone and the evolution of back-arc basin.

The samples used in this paper were taken from the "Science" research ship-201501 voyage, and the volcanic rock samples from the eastern Manus back-arc basin were obtained with the help of underwater robots. The lithology of the rocks mainly consisted of basaltic andesite and dacite. The analysis testing of the main elements, trace elements, lithium isotopesand neodymium isotopes of the whole rock, combined with the previous data and the geological characteristics of the study area, to focus on the geochemical behavior of lithium elements and isotopes in the subduction process, which lays a foundation for an comprehensive understanding of the material cycle in the subduction zone.

According to the main element composition analysis of the volcanic rocks in eastern Manus basin, it shows that the volcanic rocks are mainly formed by fractional crystallization. The crystallized minerals may include olivine, pyroxene, plagioclase, ilmenite and apatite. The trace elements compositon of the samples reflect that the volcanic rocks are enriched light rare earth enrichment, large ion lithophile elements and fluid mobile elements, while are depleted high field strength elements, showing that the magma source of volcanic lavas in eastern Manusi basin are obviously influenced by the subduction components.

The Li isotope datashows that the lithium isotopic composition of volcanic rocks in Manus basin is relatively uniform and the variationsare small (0.2-6.1‰). With the evolution of magma crystallization and differentiation, the change of Li isotope ratio is not obvious. In addition, the variation of Li isotope compositions in East Manus Basinare shown to be affected by slab fluids, so Li isotope can be used as a good tracerindex of fluid and magmatic evolution in the subduction zone.

The ¹⁴³Nd/¹⁴⁴Nd of the sample are between 0.512994 and 0.513027, which is similar to the MORB of Manus Spreading Center and New Britain arc lava. According to the relationship of Li and Nd isotopes, the volcanic rocks in eastern Manus basin mainly come from depleted mantle, altered oceanic crust and sefloor sediment.

第一章 引言... 1

1.1研究目的及意义... 1

1.2拟解决科学问题... 2

1.3研究内容... 2

1.4工作量统计... 3

第二章 研究区地质背景及研究现状... 5

2.1研究区地质背景... 5

2.1.1 马努斯盆地区域地质概况... 5

2.1.2东马努斯盆地地质背景... 8

2.2国内外研究现状... 10

2.2.1马努斯盆地岩浆地球化学特征研究... 10

2.2.2马努斯盆地岩浆源区研究... 12

2.2.3马努斯盆地岩浆形成与演化研究... 13

2.2.4马努斯盆地岩浆俯冲组分来源研究... 15

第三章 样品特征与研究方法... 17

3.1样品特征... 17

3.2 研究方法... 18

第四章 岩浆岩地球化学特征... 21

4.1主量元素特征... 21

4.2微量元素特征... 23

4.3同位素特征... 28

第五章 俯冲带中锂的地球化学特征... 31

5.1 锂的地球化学特征... 31

5.2 俯冲带中锂的地球化学行为... 33

5.3俯冲组分中Li同位素组成... 38

第六章 岩浆物质来源及演化... 41

6.1俯冲组分示踪... 41

6.2岩浆端元模拟... 43

6.2.1 东马努斯火山岩Li-Nd同位素组成... 43

6.2.2 端元组分模拟... 44

6.3 岩浆演化过程分析... 44

第七章 结论... 49

参考文献... 51

致谢... 59