中国科学院海洋研究所机构知识库

洋脊俯冲是地球圈层系统中关键的物质循环过程，具有重要的科学意义和潜在的经济价值。洋脊俯冲形成的独特“板片窗”构造，往往是促进（超）大型铜、金、钼矿床成矿的关键。长江中下游地区是我国重要的多金属成矿带，区域内中生代斑岩铜金矿、玢岩铁矿发育。前人研究表明长江中下游铜矿床的形成与早白垩世期间的洋脊俯冲密切相关。但是该区域玢岩铁矿的形成机制尚不清楚，其是否与洋脊俯冲存在联系尚不明确。本论文针对长江中下游典型玢岩铁矿——马口铁矿和陶村铁矿——进行了详尽的地球化学研究，旨在推进对于玢岩铁矿成矿机理的认知，加深对洋脊俯冲成矿效应的理解。

本研究选择与马口铁矿和陶村铁矿成矿紧密相关的围岩展开研究，主要包括来自马口铁矿的辉长岩和正长岩，以及来自陶村铁矿的闪长玢岩、矿石样品和蚀变带样品。对这些样品进行了全岩主微量元素测试、单矿物的原位主量元素测试、O同位素分析以及Sr-Nd-Ca同位素分析，探讨玢岩铁矿的成矿机理及其与洋脊俯冲的联系。

由于地质历史时期洋脊俯冲的相关构造痕迹很多已经消失，所以识别古洋脊俯冲比较困难，特殊的岩石组合是判断洋脊俯冲的重要方法。本文首先选择了庐枞盆地内与玢岩铁矿具有成因关系的马口岩体以及其附近的黄梅尖A型花岗岩作为研究对象，通过对其岩石成因的探讨来约束早白垩时期长江中下游玢岩铁矿形成时的构造背景。

马口辉长岩是庐枞盆地内目前唯一确定与玢岩铁矿形成相关的岩浆岩。锆石U-Pb定年指示其形成年龄为130 Ma。马口辉长岩具有富集的Sr-Nd-O同位素组成：锆石δ¹⁸O变化范围为5.39‰—6.87‰（n=16），初始Sr同位素组成(⁸⁷Sr/⁸⁶Sr)_i为0.70613—0.70626，ε_Nd(t)为-4.48至-5.15。其微量元素配分模式与岛弧岩浆岩类似，具有Nb-Ta-Ti负异常和Pb的正异常。以上特征表明其地幔源区有来自俯冲沉积物的贡献。马口正长岩具有与辉长岩一致的同位素组成（锆石δ¹⁸O=5.63‰—6.64‰（n=17），(⁸⁷Sr/⁸⁶Sr)_i =0.70642，ε_Nd(t)=-4.68），并且主、微量元素与马口辉长岩显示出明显的结晶分异演化趋势，说明两者同源，马口正长岩是辉长岩母岩浆经历较高程度结晶分异演化的产物。马口正长岩由于具有较高的高场强元素含量，因此在花岗岩类型判别图中落入A型花岗岩区域，但其较低的10000×Ga/Al（＜2.6）和FeO^T/MgO（＜28）等特征表明其不是典型的A型花岗岩，而是具有I-A过渡型特征。其高HFSE含量的特征继承自其母岩浆，是俯冲板片熔体在地幔源区中贡献的结果。其Nd同位素组成与盆地内A型花岗岩相似，说明两者具有相似的富集地幔源区，两者的化学成分差异可能是由岩浆后期的演化过程控制。对两者的锆石氧逸度进行对比发现，正长岩的氧逸度更高，说明高氧逸度条件有利于具有I-A过渡型特征岩石的形成，而A型花岗岩形成于低氧逸度环境。氧逸度的差异可能是源区受俯冲相关的熔/流体交代的程度不同导致的。马口岩体与A型花岗岩的地幔源区特征以及岩性变化支持扩张洋脊俯冲的存在。随着洋脊的俯冲，板片窗逐渐打开，板片边缘由于温度升高发生部分熔融，形成熔、流体交代上覆岩石圈地幔，同时上涌的软流圈地幔通过板片窗加热岩石圈地幔，导致地幔发生部分熔融：受俯冲相关的熔/流体交代程度交高的区域部分部分熔融形成马口辉长岩和正长岩；相反，受熔/流体交代程度低的区域发生部分熔融，形成A型花岗岩。玢岩铁矿在国际上又被称为铁氧化物磷灰石型矿床（iron oxide-apatite，IOA型矿床）。与国际上典型IOA矿床相比，长江中下游玢岩铁矿的形成位置距离俯冲带较远，扩张洋脊的俯冲是在深入内陆的长江中下游位置形成玢岩铁矿的重要原因。

马口辉长岩中的黑云母成分指示该辉长岩母岩浆的氧逸度约为ΔFMQ +0.7，与长江中下游其他地区与玢岩铁矿相关的岩浆岩氧逸度相当，指示俯冲环境。马口辉长岩单斜辉石中的磷灰石包裹体根据其Cl含量不同可以分为高Cl磷灰石和低Cl磷灰石两类，这两类磷灰石除Cl外，其他元素含量无明显差别，低Cl含量是磷灰石结晶后与低Cl熔体继续反应直至再平衡的结果。高Cl磷灰石保留了其结晶时的特征，利用其F、Cl含量，联合寄主单斜辉石结晶温度，估算出了辉长岩母岩浆的F、Cl含量，分别为0.175—1.04 wt.%，0.673—3.53 wt.%，其Cl含量甚至高于岛弧岩浆岩橄榄石中的硅酸盐熔体包裹体。长江中下游高Cl岩浆有利于高盐度岩浆热液的形成，这对于玢岩铁矿成矿流体中Fe质的搬运与富集十分重要。

前人研究曾经提出相比于岩浆作用，蒸发岩的贡献是控制玢岩铁矿形成的更重要的因素，但是目前还缺少能够有效限定蒸发岩在成矿系统中存在物质贡献的方法。本次研究创新性地将Ca同位素体系应用到岩浆热液矿床成因的探讨中，以此来判断蒸发岩地层的物质贡献。

宁芜盆地内三叠纪蒸发岩具有明显重于BSE（全硅酸盐地球）和MORB（大洋中脊玄武岩）的Ca同位素组成特征，δ^44/40Ca为1.09±0.13‰至1.35±0.04‰，因此Ca同位素具有示踪蒸发岩物质贡献的能力。通过对比陶村铁矿内新鲜闪长玢岩与MORB等端元的Ca同位素组成，发现闪长玢岩具有相对MORB略重的Ca同位素，δ^44/40Ca为0.91±0.03‰至0.97±0.06‰。前人研究认为闪长玢岩的地幔源区中有陆源沉积物和少量俯冲洋壳的贡献，但是这两种物质具有与MORB相似或更轻的Ca同位素组成，因此其加入并不会使Ca同位素重于MORB。由于样品蚀变程度很低，后期蚀变也不是导致Ca同位素变重的主要原因。闪长玢岩的母岩浆在结晶前可能经历了橄榄石、单斜辉石、斜长石、角闪石的结晶分异，其中橄榄石、单斜辉石和角闪石的结晶并不会明显改变熔体的Ca同位素组成，同时，闪长玢岩不具有明显的Eu负异常说明并没有经历高程度的斜长石结晶分异。因此，闪长玢岩略重于MORB的Ca同位素组成最可能是形成过程中混染少量盆地内蒸发岩地层的结果。此外，本文还对成矿前期钠长石化蚀变过程中Ca同位素的行为进行了约束，发现钠长石化会使Ca同位素发生分馏，导致流体的Ca同位素升高，围岩的Ca同位素下降。

综上所述，本文认为中生代时期长江中下游的岩浆作用与玢岩铁矿的成矿均为扩张洋脊俯冲的产物，扩张洋脊的俯冲是在深入内陆的长江中下游位置形成玢岩铁矿的重要原因。蒸发岩层在玢岩铁矿形成过程中有物质贡献。但是，长江中下游地区玢岩铁矿形成的主控因素是洋脊俯冲还是蒸发岩贡献，又或是两者缺一不可，有待后续的研究。

Spreading ridge subduction is an important cycle process on Earth. Because of its special structure, slab window, the effect of spreading ridge subduction is different from that of ordinary subduction. In particular, ridge subduction is usually associated with large ore deposits. It is therefore important to study the causal relation between ridge subduction and mineralization. The Middle and Lower of Yangtze River Belt is an important polymetallic metallogenic belt in China, where Mesozoic porphyry copper-gold deposit and porphyrite iron deposit are developed widely. Previous studies have shown that the Middle and Lower of Yangtze River Belt experienced the impact of spreading ridge subduction during the Early Cretaceous, and the process of spreading ridge subduction may be closely related to the formation of copper-gold poly-metal deposits in this region. However, the relationship between porphyrite iron ore and spreading ridge subduction in the region is still not well constrained. The study of this problem will help us to further understand the process of spreading ridge subduction and its resource effects.

Based on this, Ningwu Basin and Luzong Basin in the Middle and Lower of Yangtze River Belt (MLYRB) are selected as research areas. This research focus on the gabbro, syenite in the Makou iron deposit, and diorite porphyrite, alteration zone samples and ore samples in Taocun iron deposit. The formation of porphyrite ore mineralization is discussed based on whole rock major elements, trace elements, Sr-Nd-Ca isotope, and in situ major elements and O isotope analyses of single minerals.

Because the slab window of spreading ridge subduction in geological history have disappeared, it is difficult to identify the subduction of ancient ocean ridge. However, it can be identified and studied through special rock assemblages. Therefore, this paper first selects the Makou pluton associated with Makou deposit and Huangmeijian A-type granite in the Luzong Basin as the research object, through their petrogenesis to constrain the tectonic setting of the Middle and Lower Yangtze River Belt in the Early Cretaceous period.

Makou gabbro is the only magmatic rock in Luzong Basin that is associated with the formation of porphyrite iron ore. Zircon U-Pb dating indicates that its formation age is ~ 130 Ma. The Makou gabbro has an enrich Sr-Nd-O isotopic composition. Zircon δ¹⁸O ranges from 5.39‰ to 6.87‰ (n=16). Initial Sr isotopic composition (⁸⁷Sr/⁸⁶Sr)_i ranges from 0.70613 to 0.70626, and ε_Nd(t) ranges from -4.48 to -5.15. The trace element partitioning pattern is similar to that of island arc basalts, with Nb-Ta-Ti negative anomalies and Pb positive anomaly. These characteristics indicate that their mantle source has subducted sediments contribution. The Makou syenite has the same isotopic composition as the gabbro (zircon δ¹⁸O=5.63‰—6.64 ‰ (n=17), (⁸⁷Sr/⁸⁶Sr)_i =0.70642, ε_Nd(t)=-4.68), and the major and trace elements show obvious correlations with the Makou gabbro. These results suggest that Makou syenite is the product of high degree of magma evolution of gabbro parent magma. Because of its high content of high field strength elements, Makou syenite falls into the A-type granite region in the granite type classification figure, but its low 10000×Ga/Al (< 2.6) and FeO^T/MgO (< 28) suggest that it is not typical A-type granitoids, but has an I-A transitional characteristics. Its high HFSEs content is inherited from its parent magma and might be the result of subduction plate melt contribution in their mantle source. The Nd isotope composition is similar to that of A-type granitoids in the Luzong basin, indicating that they have similar enriched mantle source regions, and the chemical composition difference between them may be controlled by the magmatic evolution. The comparison of the oxygen fugacity of zircons shows that the oxygen fugacity of syenite is higher, indicating that the high oxygen fugacity condition is conducive to the formation of rocks with I-A transitional type, while the A-type granite is formed in a low oxygen fugacity environment. The difference of oxygen fugacity may be caused by the different contribution degree of melts/fluids in the source region. The mantle source characteristics and lithology changes of Makou pluton and A-type granite indicate the subduction of spreading oceanic ridge. During the ocean ridge subduction, the slab window gradually opened, and the slab edge melted due to the rising temperature, forming the enriched mantle domains due to metasomatisms by subduction released melts/fluids. Meanwhile, the upwelling asthenosphere mantle heated the lithospheric mantle through the slab window, resulting in partial melting of mantle: the fluid-rich domains melted easily, forming Makou gabbro and syenite, and the fluid-poor part melted, forming A-type granite. Porphyrite iron ore is also known as iron oxide-apatite (IOA) deposit in the world. Compared with the typical IOA deposits in the world, porphyrite iron deposits in the MLYRB are formed far away from the subduction zone, and the subduction of the spreading ocean ridge is an important reason for the formation of porphyrite iron deposits in this area.

The biotite composition in the Makou gabbro suggest that the oxygen fugacity of the gabbros parent magma is about ΔFMQ +0.7, which is similar to that of magmatic rocks associated with porphyrite iron ore in other areas of MLYRB, indicating subduction environment. The apatite inclusions in Makou gabbro clinopyroxene can be divided into high Cl apatite and low Cl apatite according to their different Cl content. There is no obvious difference in the content of other elements between the two kinds of apatite except Cl, and the low Cl content is the result of the reaction between the apatite and the residue low Cl melt after crystallization. Based on the F and Cl content in the apatite and the crystallization temperature of the host clinopyroxene, the content of F and Cl in magma could be obtained. Uisng the high Cl apatite, the content of F and Cl in the gabbro parent magma is 0.175—1.04 wt.% and 0.673—3.53 wt.%, respectively, which is even higher than that in the silicate melt inclusions in olivine from the island arc magma. The high Cl magma in the MLYRB is conducive to the formation of high salinity magmatic hydrothermal fluid, which is very important for the Fe transport and enrichment in porphyrite iron ore-formation process.

Previous studies have suggested that the contribution of evaporites is a more important factor controlling the formation of porphyrite iron ore than magmatism, but there is still a lack of effective methods to constrain the contribution of evaporites in the metallogenic system. In this study, Ca isotope system is innovatively applied to the genesis of magmatic hydrothermal deposits to determine the contribution of evaporite strata.

The Ca isotopic characteristics of the Triassic evaporites in the Ningwu Basin are significantly heavier than those of BSE (full silicate Earth) and MORB (mid-ocean ridge basalt), with δ^44/40Ca ranging from 1.09±0.13‰ to 1.35±0.04‰, so Ca isotope has the ability to trace the contribution of evaporites. By comparing the Ca isotopic composition of fresh diorite porphyrite in Taocun iron deposits with MORB and other end members, it is found that the diorite porphyrite has a slightly heavier Ca isotopic composition than MORB, with δ^44/40Ca ranging from 0.91±0.03‰ to 0.97±0.06‰. The mantle source region of diorite porphyrite has the contribution of terrigenous sediments and a small amount of subducted oceanic crust, but these two materials have similar or lighter Ca isotope composition than MORB, so their addition does not make Ca isotope heavier than MORB. Since the degree of sample alteration is very low, the late alteration is not the main reason for the heavy Ca isotope. The parent magma of diorite porphyrite may have undergone the crystallization of olivine, clinopyroxene, plagioclase and hornblende, but olivine, clinopyroxene, and hornblende crystallization does not significantly change the Ca isotope composition of the melt. And the lack of obvious negative Eu anomaly of diorite porphyrite indicates that it did not experience a high degree of plagioclase crystallization. Therefore, the heavier Ca isotopic composition of diorite porphyrite is most likely the result of mixing a small amount of evaporite strata in the basin during the formation process. In addition, the behavior of Ca isotope during the albitization in the early stage of mineralization is constrained. It is found that this process will cause the fractionation of calcium isotope, resulting in the increase of Ca isotope in fluid and the decrease of Ca isotope in the surrounding rock.

In conclusion, the magmatism and porphyrite ore mineralization in the MLYRB in the Mesozoic period are the products of subduction of the spreading ocean ridge, which is an important reason for the formation of porphyrite iron ore in this region. Evaporative rock has contribution to the formation of porphyrite iron ore. However, the main controlling factor of porphyrite iron ore formation is subduction of ocean ridge or contribution of evaporite, or both, which needs further research.

第1章 绪论... 1

1.1选题背景... 1

1.2研究现状... 2

1.2.1长江中下游中生代成岩成矿作用与洋脊俯冲... 2

1.2.2长江中下游玢岩铁矿... 4

1.2.3高温地质过程Ca同位素... 6

1.3拟解决关键科学问题和研究方法... 7

1.3.1长江中下游中生代马口岩体形成的动力学背景... 7

1.3.2长江中下游早白垩世中基性岩浆F、Cl组成... 7

1.3.3玢岩铁矿的Ca同位素研究... 8

1.4论文工作量统计... 9

1.5论文创新点... 10

1.6论文结构和布局... 10

第2章 研究区地质背景及样品介绍... 12

2.1研究区地质背景... 12

2.1.1长江中下游地区... 12

2.1.2庐枞盆地... 13

2.1.3宁芜盆地... 14

2.1.4陶村铁矿矿床地质特征... 17

2.2样品介绍... 18

第3章 样品制备和分析方法... 20

3.1样品制备... 20

3.2分析方法... 20

3.2.1岩相学观察... 20

3.2.2全岩主量元素分析... 20

3.2.3全岩微量元素分析... 21

3.2.4全岩Sr-Nd同位素分析... 21

3.2.5全岩Ca同位素分析... 22

3.2.6锆石U-Pb定年及O同位素分析... 22

3.2.7磷灰石、单斜辉石、黑云母原位主量元素分析... 23

第4章 长江中下游马口岩体辉长岩、I-A过渡型花岗岩的形成与扩张洋脊俯冲    24

4.1引言... 24

4.2样品岩相学与地球化学特征... 25

4.2.1岩相学特征... 25

4.2.2地球化学特征... 26

4.3讨论... 32

4.3.1马口岩体年代学... 32

4.3.2马口正长岩属于A型花岗岩类吗？... 32

4.3.3马口辉长岩、正长岩成因... 35

4.3.4庐枞盆地I-A过渡型正长岩与A型花岗岩的成因联系... 37

4.3.5庐枞盆地马口岩体和A型花岗岩形成的地球动力学背景... 40

4.3.6玢岩铁矿形成的动力学背景... 41

4.4小结... 41

第5章 长江中下游早白垩世中基性岩浆F、Cl组成及对玢岩铁矿成因的启示    43

5.1引言... 43

5.2样品岩相学及地球化学特征... 43

5.2.1单矿物岩相学特征... 43

5.2.2单矿物地球化学特征... 45

5.3讨论... 51

5.3.1磷灰石结晶的物理化学条件... 51

5.3.2马口辉长岩母岩浆的F、Cl含量... 53

5.3.3对玢岩铁矿成因的启示... 55

5.4小结... 56

第6章 长江中下游玢岩铁矿Ca同位素研究... 57

6.1引言... 57

6.2样品岩相学及地球化学特征... 58

6.2.1样品岩相学特征... 58

6.2.2样品地球化学特征... 59

6.3讨论... 63

6.3.1三叠纪蒸发岩Ca同位素组成... 63

6.3.2闪长玢岩Ca同位素指示膏盐层的物质贡献... 63

6.3.3 Sr-Nd同位素对成矿热液流体来源的限制... 65

6.3.4 Ca同位素在钠长石化蚀变过程中的行为... 65

6.4小结... 66

第7章 结论与展望... 67

参考文献... 69

附录一 马口岩体锆石LA-ICP-MS U-Pb年龄数据表... 86

附录二 马口岩体锆石原位REE及Ce⁴⁺/Ce³⁺数据表... 88

附录三 马口岩体锆石原位氧同位素数据表... 90

附录四 马口岩体与黄梅尖岩体主微量元素数据表... 91

附录五 马口岩体Sr-Nd同位素数据表... 94

附录六 马口岩体锆石Ti温度计计算结果数据表... 95

致谢... 96

作者简历及攻读学位期间发表的学术论文与其他相关学术成果 98