| 四川会理中元古代竹箐基性岩体群及Fe-Ti-V氧化物矿床成因 |
| 范宏鹏
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| 导师 | 朱维光
; 钟宏
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| 2013
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| 学位授予单位 | 中国科学院研究生院
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| 学位授予地点 | 北京
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| 学位名称 | 博士
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| 学位专业 | 地球化学
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| 关键词 | 中元古代基性岩体
Fe-ti-v氧化物矿床
扬子陆块
Columbia超大陆
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| 摘要 | 竹箐岩体群位于扬子地块西缘，主要由辉长岩组成，这些岩体多呈岩墙或小岩体侵入到中元古界会理群黑山组板岩以及白云岩中。这些岩体可分为三个旋回五个岩相带，Fe–Ti–V氧化物矿床位于上部和中部旋回的底部。本文在总结前人地质工作的基础上，通过对野外地质观察、年代学、矿物学、主量和微量元素，以及铂族元素（PGE）地球化学等进行深入研究，目的是探讨竹箐岩体群的形成时代、岩浆源区特征、构造背景，母岩浆特征和演化过程及Fe-Ti-V氧化物矿床的形成机制。 通过SIMS 锆石U-Pb同位素分析，锆石的U-Pb年龄为1494 ± 6 Ma，斜锆石的U-Pb年龄为1486 ± 3 Ma和1490 ± 4 Ma。这些锆石和斜锆石U–Pb年龄在误差范围内一致，表明竹箐岩体群形成于~1.5 Ga。而且，这个年龄与下昆阳群鹅头厂组（与竹箐岩体群所侵入的地层层位相当）凝灰岩中的锆石U-Pb年龄（1503 ± 17 Ma）一致。 竹箐辉长岩的PGE明显亏损。岩石中的Ir、Ru和Rh含量主要受磁铁矿控制，且磁铁矿相对富集Ir、Ru和Rh而亏损Pt和Pd。通过计算得出各个PGE在竹箐辉长岩中相对相容性从低到高排序是Pd < Pt < Rh < Ir < Ru。Ir-Ru-Rh和Pt-Pd之间发生的分异主要是由铬铁矿在深部以及磁铁矿在浅部岩浆房分离结晶导致，Ru的正负异常是由PGE之间相容性相对差别及岩石中磁铁矿所占比例的差别导致。竹箐岩石的原始岩浆形成时已经达到硫饱和状态且亏损PGE，这可能是较低程度部分熔融导致富集PGE的硫化物残留在源区。 在微量元素蛛网图上，所有竹箐岩体群的竹箐辉长岩样品都呈现"驼峰状"的配分曲线，其特点是不同程度地富集不相容元素，且没有明显的Nb–Ta亏损，与典型的来自板内的大陆溢流玄武岩（CFB）省以及洋岛玄武岩（OIB）省中的碱性玄武质岩石的微量元素特征极其类似。所有的岩石样品都具有高Fe-Ti和低Mg#等特征，指示竹箐岩体群的母岩浆是富Fe–Ti且强烈演化的基性岩浆。 竹箐辉长岩表现出负的εNd(T)值、亏损的PGE以及轻稀土元素相对重稀土元素富集的特征，说明其原始岩浆是略微富集的软流圈地幔较低程度部分熔融的产物。竹箐岩体群各个旋回内的岩石成分主要是受Fe-Ti氧化物的分离结晶控制，而各旋回界面上成分的变化可能与多期岩浆补给有关。此外，竹箐辉长岩在形成过程中没有发生明显的地壳混染。 形成于约1.5 Ga的竹箐岩体群指示扬子地块西缘的岩浆活动可以和前寒武纪全球大部分克拉通内部广泛分布的1.6-1.2 Ga造山后和非造山岩浆活动进行对比，而这些岩浆活动被认为与Columbia超大陆裂解有关。本文提出竹箐岩体群的形成很可能与导致Columbia超大陆裂解的地幔柱活动产生的全球性岩浆事件有联系。因此，扬子地块就很可能是古元古代-中元古代的Columbia超大陆的组成部分。 |
| 其他摘要 | The Zhuqing intrusions in western Yangtze Block are dominantly composed of gabbroic rocks. They occur as dykes or small irregular intrusions that intruded the dolomite or shales of the Mesoproterozoic Heishan Formation of the Huili Group. The Zhuqing intrusions can be divided into three cycle unites and five zones. The Fe-Ti-V ore-bearing deposits are hosted in the base of the Upper and Middle Zones. In this study, geochronology, mineralogy, elemental geochemistry and platinum-group element (PGE) are used to determine the precise crystallization age and to discuss the origin and nature of the parental magma, petrogenesis and Fe–Ti–V mineralization for these intrusions. The mafic intrusions are dated at 1494 ± 6 Ma (zircon U–Pb), 1486 ± 3 Ma (baddeleyite U-Pb) and 1490 ± 4 Ma (baddeleyite U–Pb). These ages are consistent with each other within errors, which is also consistent with the 1503 ± 17 Ma age of a tuff sample from the E’touchang Formation, suggesting crystallization age of the Zhuqing intrusions are ~1.5 Ga. All the Zhuqing gabbros are highly depleted in PGE concentrations. The Ir, Ru and Rh distributions are mainly controlled by magnetite. Moreover, magnetites have relatively concentrated Ir, Ru and Rh in preference to Pt and Pd in the Zhuqing gabbros. The calculation indicates the order of relative incompatibility of PGE in the Zhuqing gabbros appears to be Pd < Pt < Rh < Ir < Ru. The fractionation between Ir-Ru-Rh and Pt-Pd in the Zhuqing gabbros is mainly attributed to fractional crystallization of chromite at depth and magnetite in the upper chamber, whereas Ru anomalies are mainly due to variable degrees of compatibility of PGE in magnetite. The very low PGE contents, Cu/Zr ratios, and high Cu/Pd ratios for the Zhuqing gabbros suggest the magma parental to these rocks was initially S-saturated and highly depleted in PGE were mainly due to low degrees of partial melting that left sul?des concentrating PGE behind in the mantle. All the studied rocks are high-Ti and alkaline in composition, and exhibit light rare earth element enrichment and “humped” incompatible trace-element patterns with no obvious Nb–Ta depletion, similar to intraplate alkali basaltic rocks in continental flood basalt (CFB) and ocean island basalt (OIB) provinces. In addition, the Zhuqing gabbros have high Fe, Ti contents and Mg# values, suggesting the Zhuqing intrusions were likely crystallized from a relatively high Fe–Ti and highly evolved mafic magma.Negative εNd(T) values, depleted PGE concentrations and fractionation of the HREE of these rocks indicate that the Zhuqing gabbros were generated by low degrees of partial melting of a slightly enriched asthenospheric mantle source with minor crustal contamination. Fractional crystallization and cumulate processes are responsible for the formation of the Zhuqing gabbros and magnetite layer in a single cycle, however, multiple magma recharges may be responsible for the gabbros and Fe-Ti oxide layers. The ~1.5 Ga mafic magmatism was likely a part of the global 1.6-1.2 Ga anorogenic magmatism related to the break-up of the supercontinent Columbia due to upwelling of mantle plume, suggesting that the Yangtze Block may have been a component of the supercontinent. |
| 学科领域 | 矿床地球化学
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| 语种 | 中文
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| 文献类型 | 学位论文
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| 条目标识符 | http://ir.gyig.ac.cn/handle/352002/5813
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| 专题 | 研究生_研究生_学位论文
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推荐引用方式
GB/T 7714 |
范宏鹏. 四川会理中元古代竹箐基性岩体群及Fe-Ti-V氧化物矿床成因[D]. 北京. 中国科学院研究生院,2013.
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