中国科学院海洋研究所机构知识库

南海是我国最大的边缘海，几乎经历了完整的地质演化旋回，理解其形成演化过程对完善边缘海盆形成理论具有重要意义。南海处于欧亚板块、太平洋板块、印度-澳大利亚板块相互作用的地带，同时南海北缘发育海南地幔柱，因此南海的形成演化过程受板块俯冲、地幔柱等多种因素影响，导致其具有复杂的地幔源区。由于南海海盆内部被巨厚沉积物覆盖，难以获得洋壳岩石样品，从而缺乏从岩石地球化学的角度限制南海地幔源区特征以及对南海形成演化过程的理解。本博士论文利用国际大洋发现计划（IODP）在南海实施的第349航次获取的洋壳玄武岩，通过对这些岩石的全岩主微量元素、Sr-Nd同位素、以及新兴的Mg-Fe-Zn金属稳定同位素和橄榄石化学组分分析，深入探究南海地幔源区特征，并进一步揭示板块俯冲作用对南海形成演化的影响。

本文研究的南海玄武岩形成于扩张晚期，属于拉斑质玄武岩。其中，东次海盆（U1431）玄武岩主要为N-MORB，个别层位出现E-MORB；西南次海盆（U1433和U1434）玄武岩主要为E-MORB。主微量元素特征显示这些玄武岩经历了不同程度的橄榄石、辉石、斜长石等矿物分离结晶作用；微量元素蛛网图显示富集流体活动性元素，如Rb、Ba、U等，以及Pb正异常；同时具有不同于典型地幔的Ce/Pb（7.56 ~ 31.18）、Nb/U（21.53 ~ 55.45）比值；Sr-Nd同位素结果显示具有比N-MORB略微高的⁸⁷Sr/⁸⁶Sr值（0.703113 ~ 0.704190），但是与N-MORB相似的¹⁴³Nd/¹⁴⁴Nd值（0.512963 ~ 0.503091）；与全球MORB中橄榄石相比，南海玄武岩中的橄榄石具有高Ni含量和低CaO含量，指示南海橄榄岩地幔源区中存在辉石岩组分。

镁同位素结果（d²⁶Mg）显示，南海玄武岩的d²⁶Mg值从-0.24‰变化至0.00‰，系统地高于典型MORB值（d²⁶Mg = -0.25‰）。由于d²⁶Mg值与指示样品蚀变、部分熔融和岩浆演化的地球化学指标之间不存在相关性，表明南海玄武岩的Mg同位素变化不受蚀变和岩浆过程的影响，而是反映了其岩浆源区特征。结合微量元素特征，本文认为南海玄武岩比MORB重的Mg同位素特征反映了其岩浆源区受到了俯冲板片中滑石/蛇纹石来源的高d²⁶Mg流体的影响。通过对比分析南海周缘地区具有轻Mg同位素特征的新生代玄武岩，本文提出这两类具有不同Mg同位素组成的玄武岩皆是板块俯冲的产物，两者之间的Mg同位素差异受控于俯冲过程中具有不同Mg含量及Mg同位素组成的矿物在不同俯冲深度分解释放流体的影响。在俯冲浅部，滑石/蛇纹石脱水发生分解，产生具有重Mg同位素组成的流体，而白云石可以俯冲至深部地幔，产生具有轻Mg同位素组成的流体/熔体。因此，南海玄武岩的Mg同位素结果支持板块俯冲作用对南海形成演化的影响。

南海玄武岩的Fe同位素组成（d⁵⁶Fe）为0.08‰ ~ 0.16‰。通过校正分离结晶对样品Fe同位素组成的影响，获得了初始熔体的d⁵⁶Fe值（0.02‰ ~ 0.10‰），与典型MORB铁同位素值（d⁵⁶Fe = 0.067‰）类似。基于混合地幔端元（橄榄岩+辉石岩）的微量元素模拟计算表明，南海橄榄岩地幔源区中辉石岩组分比例约10~40%，由此混合源区产生的南海玄武岩理论上应具有比典型MORB重的Fe同位素组成。但是，由于受俯冲过程中蛇纹岩流体（具有低d⁵⁶Fe值）交代作用的影响，降低了南海玄武岩的d⁵⁶Fe值，使得其与典型MORB 的d⁵⁶Fe值相似。进一步地，本人认为辉石岩熔体是在南海扩张过程中以洋脊-地幔柱相互作用的形式参与了南海玄武岩的形成；结合橄榄石高Ni特征，认为辉石岩组分属于由俯冲洋壳形成的榴辉岩/辉石岩熔体交代地幔橄榄岩，形成的二次辉石岩。

进一步利用本次研究获得的洋壳岩石样品，探究了板块俯冲过程中Zn同位素行为。结果显示南海玄武岩Zn同位素组成（d⁶⁶Zn = 0.22‰ ~ 0.31‰）与典型MORB（d⁶⁶Zn = 0.27‰）类似，高于上地幔平均值（d⁵⁶Fe = 0.18‰）。由于d⁶⁶Zn值与指示样品蚀变、岩浆演化和俯冲流体交代的地球化学指标之间不存在相关性，表明南海玄武岩的Zn同位素组成不受蚀变、分离结晶和板块俯冲作用的影响。南海玄武岩与典型MORB相似的Zn同位素特征主要受地幔熔融过程控制。通过收集、整理形成于俯冲背景下具有不同d⁶⁶Zn值的火山岩数据，本次研究指出板块俯冲过程中，Zn同位素行为主要受白云石、菱镁矿等富Zn矿物控制。

通过本论文的研究，我们从岩石地球化学角度提供了南海地幔源区受板块俯冲影响的直接证据，并定量模拟了南海玄武质岩浆源区不均一性成分的比例；此外，我们限定了板块俯冲过程中影响锌同位素行为的主要因素。

Sr-Nd isotope results show that they have a slightly high ⁸⁷Sr/ ⁸⁶Sr (0.703113 ~ 0.704190), but ¹⁴³Nd/¹⁴⁴Nd (0.512963 ~ 0.503091) similar to N-MORB; Comparing with olivine chemistry in global MORB, the chemical compositions of olivine in studied basalts are with high Ni contents and low CaO contents, indicate that there are pyroxenitic components in peridotitic mantle source of the South China Sea.

The Mg isotopic compositions show that d²⁶Mg of the South China Sea basalts are from -0.24‰ to 0.00‰, which are higher than that of typical MORB (d²⁶Mg = -0.25‰). Due to the lack of correlation between d²⁶Mg and indicators of alteration and magmatic processes, the heavy Mg isotopes of the studied basalts are inherited from their magma source, which are most likely from subducted talc- and/or serpentine-derived fluids with high d²⁶Mg. Through the comparative analysis of the light Mg isotopes characteristics of basalts in the surroundings of the South China Sea, we proposed that the two types basalts with different Mg isotopic compositions are all the products of plate subduction, and the difference in Mg isotopic compositions between them was controlled by minerals with different Mg contents and Mg isotopes during subduction. Specifically, the shallow subduction was mainly affected by heavy Mg isotopic fluids released from talc and/or serpentine, while the deep mantle was mainly affected by light Mg isotopes fluids/melts released from dolomite.

The Fe isotopic compositions (d⁵⁶Fe) of the South China Sea basalts are from 0.08‰ to 0.16‰. After evaluating the effects of alteration and fractional crystallization, we got the Fe isotopic compositions of primitive melts (d⁵⁶Fe = 0.02‰ ~ 0.10‰), which are similar to that of typical MORB (d⁵⁶Fe = 0.067‰). The simulation calculation of mixed mantle shows that about 10~40% pyroxenitic melts was involved into the peridotite mantle source in the South China Sea, which theoretically should produce basalts with heavier Fe isotopes than typical N-MORB. However, due to the effect of fluid-metasomatism released from serpentinite with low d⁵⁶Fe, the Fe isotopic compositions of the magma source were further reduced, and similar to that of typical MORB. Pyroxenitic components was involved into the formation of the South China Sea basalts by ridge - plume interaction during the expansion of the South China Sea. Combined with the high Ni contents of olivine, the pyroxenitic components in the peridotitic mantle was the secondary pyroxenite formed by reaction of mantle peridotite and the eclogite or pyroxenite melts originated from subducted oceanic crust.

In this study, the influence of plate subduction on the mantle source of basalt in the South China Sea was further illustrated by Zn isotopes. The results show that Zn isotopic compositions (d⁶⁶Zn) of the South China Sea basalts are from 0.22‰ to 0.31‰, which are similar to that of typical MORB (d⁶⁶Zn = 0.27‰), but higher than that of average upper mantle (d⁶⁶Zn = 0.18‰). Since there is no correaltion between d⁶⁶Zn and indicators of alteration, fractional crystallization and subducted fluid-metasomatism, it indicates that alteration, fractional crystallization and plate subduction have a limted effect on the variation of Fe isotopes. The MORB-like Zn isotopes of the South China Sea basalts mainly reflect the mantle melting process. By collecting and sorting out the data of volcanic rocks with different d⁶⁶Zn formed in the background of subduction, this study proposed that the behavior of Zn isotopes during subduction was mainly affected by dolomite and other Zn-rich minerals.

This study provides direct evidence that the mantle source in the South China Sea was affected by plate subduction from the perspective of petro-geochemistry, quantitatively simulates the proportion of heterogeneous materials in the basaltic magma source of the South China Sea, and limits the main factors that affect the behavior of zinc isotopes in plate subduction.

      4.2.2 全岩Sr-Nd同位素特征

      4.2.3 橄榄石矿物学特征

第5章 南海玄武岩地幔源区的俯冲组分：镁同位素证据.

5.1 引言

5.2 结果

5.3 讨论

       5.3.1 南海玄武岩高镁同位素的成因

       5.3.2 板块俯冲过程中镁同位素行为及其对南海扩张的指示意义

5.4 小结.

第6章 辉石岩熔体对南海玄武岩地幔源区的贡献：铁同位素证据

6.1 引言

6.2 结果

6.3 讨论

       6.3.1 南海玄武岩MORB-like铁同位素的成因

       6.3.2 南海及周边地区地幔中辉石岩成因

6.4 小结.

第7章 南海玄武岩锌同位素特征及其对板块俯冲过程中锌同位素行为的指示

7.1 引言

7.2 结果

7.3 讨论

       7.3.1 南海玄武岩锌同位素特征

       7.3.2 板块俯冲过程中锌同位素行为

7.4 小结

第8章 结论与展望

参考文献