【作者】 王敏；

【导师】 莫宣学；

【作者基本信息】 中国地质大学(北京) ， 矿物学、岩石学、矿床学， 2012， 博士

【摘要】 南中国板块由扬子与华夏两大陆块拼贴而成，出露大量前寒武纪岩石，是研究前寒武纪，特别是中、晚前寒武纪地质演化的良好场所，目前已成为国内外前寒武纪地质研究的热点地区。虽然尚无明确定义，分割华夏与扬子陆块的江南造山带是研究热点。通过近一个世纪的研究，前人积累了大量研究数据，综合分析这些研究数据，本文定义江南造山带为“位于扬子地块与华夏地块之间，发生于新元古代中期，其产物组成江南古陆主体的一个晋宁期造山带”。虽然针对江南造山带的数据很多，但其构造演化模型依然如雾里看花，争议很大。特别是对于造山带西南段，问题更多，远未形成共识。造成这种状况的原因主要包括：（1）详细的地球化学、年代学分析与野外地质关系脱节，未能形成完整的数据体系；（2）关键性年代学数据缺失，未能构造区域地质时代格架；（3）岩石地球化学数据没有与岩石学特点结合，造成数据解释丧失合理性。基于上述研究现状，本论文选择黔东北梵净山地区开展详细地质解剖，以期：（1）厘清各种地质单元之间的构造关系；（2）开展详细的年代学研究，建立年代学格架；（3）挑选典型样品进行地球化学分析，探讨不同时代岩石的大地构造属性；（4）综合野外地质关系、岩石学、年代学、地球化学等资料，分析大地构造相；（5）构建江南造山西段构造演化历史，加深对江南造山带的理解。梵净山地区出露两套沉积地层，其间为角度不整合：（1）不整合面之下部为梵净山群，主要由变质砾岩（极少量，分布很局限）、砂岩、粉砂岩及泥质岩、大量火山岩组成；往上火山岩含量变少，碎屑岩粒度变细，组成了一个完整的盆地沉积系列。（2）不整合面之上为下江群碎屑岩夹火山岩组合。梵净山群中的火山岩夹层具有两种岩石类型：细碧岩（枕状玄武岩）及石英角斑岩（酸性喷出岩），以及与玄武岩渐变过渡的辉绿岩。详细的地质填图工作揭示出大量基性-超基性岩浆岩，以及少量酸性岩浆岩侵入到梵净山群浅变质地层中。火山碎屑岩锆石原位U/Pb同位素测年结果显示，梵净山群地层沉积时代在850-815Ma之间，而不是Zhou et al. （2009）认为的870-800Ma，也不是Wanget al. （2010）认为的800Ma之后。详细的年代学研究结果揭示出梵净山地区具有两期基性-超基性岩浆活动，形成两个基性-超基性岩浆岩系列：（1）早期岩浆活动形成梵净山群中的基性火山岩，以及顺层产出的层状辉绿-辉长岩-橄榄辉石岩-辉石橄榄岩。原位锆石U/Pb测年结果表明，该系列基性岩-超基性岩形成时代在856-820Ma之间，与地层沉积同时。详细的地球化学分析结果显示，基性岩具有与富集型洋中脊玄武岩类似的稀土元素配分曲线，但初始地幔标准化微量元素曲线具有明显的Nb、Ta、Sr、P、Ti负异常，表明岩浆受到了地壳物质的强烈混染，以及明显的结晶分异作用。（2）晚期基性岩浆活动时代为815-745Ma，形成辉绿-辉长岩及少量超基性岩，侵入梵净山群地层中。晚期基性岩具有与早期基性岩相同的地球化学特点：球粒陨石标准化稀土元素曲线显示富集型洋中脊玄武岩样式；初始地幔标准化微量元素曲线具有Nb、Ta、Sr、P、Ti负异常。表明两次基性岩浆活动具有相同的成岩源区、相似的岩浆演化过程。但两个岩浆系列间的细微差别表明，早期岩浆可能源自较高程度的地幔局部熔融；而晚期岩浆活动过程中矿物结晶分离程度可能较高。在梵净山群地层中发育的石英角斑岩，与基性岩浆岩一起组成双峰式火山岩组合。石英角斑岩SiO₂含量达72%，轻、重稀土均强烈分异，高场强元素也强烈分异，与裂谷型酸性岩浆岩十分相似；具有明显的Ti负异常、中等程度的Nb、Ta、Sr、P负异常，指示遭受明显地壳物质的混染。伴随早期岩浆活动，梵净山地区发育高硅、强过铝花岗岩—白云母花岗岩。其SiO₂含量超过72%，A/CNK值大于1.1。稀土元素标准化曲线显示典型的海鸥型，具有明显四分量效应。这种岩石的存在指示上涌软流圈地幔提供了额外热量造成地壳富泥质岩石高温局部熔融。综合沉积学、岩石学、地球化学、同位素年代学资料，江南造山带西南段新元古代地质演化大致可分为两个阶段：晋宁I期—860-815Ma，晋宁II期—815-750Ma，其间被梵净山群与下江群之间的角度不整合面分割。两阶段演化均表现为火山裂谷盆地的发育。结合区域地质资料，晋宁I期可能与扬子-华夏陆块之间洋盆的形成有关，梵净山地区的梵净山群代表该洋盆西部末端（现代地理位置）坳拉槽；而晋宁II期显然与大型地幔柱活动有关，是Rodinia超大陆裂解在华南地区的响应。更多还原

【Abstract】 Voluminous Precambrian rocks expose along the margin of, or within the SouthChina Block （SCB）, which have become a hot study-object to reveal the Precambrianhistory of the Earth. The Jiangnan orogenic belt that separates the Yangtze andCathaysian blocks, is one of the hottest topics concerning the SCB geology, regardlessthe fact that this belt has not been well defined. A growing body of geological andgeochronological data led to below definition for the Jiangnan orogenic belt:“a largebelt of middle Neoproterozoic rocks between the Yangtze and Cathaysian blocks, thatcompose the major portion of the previously defined Jiangnan ancient land.”In spiteof numerous previously-published high quality data, no consensus appears on thetectonic evolution of the Jiangnan orogenic belt, especially concerning the westernsegment of this belt. Reasons may include: （1） Field relationship has not beenpositively taken into account in interpreting most geochemical and geochronologicaldata; （2） Some key geochronological data are still absent, which obscured making areasonable tectonic model; and （3） Detail petrological information is commonlyunavailable for most geochemical studies.Accordingly, this paper tries to report a detailed case study of the Fanjingshan Mt.region, northeast Guizhou Province, South China, in order to: （1） reveal the fieldrelationship between the different geologic units; （2） make a reasonablegeochronological scheme on the basis of much more detail sampling; （3） select typicalrock samples for geochemistry study; （4） integrate all available data from field works,petrological observations, geochronological studies, and geochemical analyses tosuggest possible tectonic facies, and/then, （5） synthesis a tectonic evolution model forthe western segment of the Jiangnan orogenic belt.Two sedimentary sequences in the Fanjingshan region have been identified,which are separated by an angular unconformity: （1） below the unconformity is theFanjingshan Group, consisting mainly of meta-volcanics, meta-volcaniclastics andmeta-fine clastics. The content of volcanics and the grain-size of clastic decreaseupward, defining a complete basin sedimentary circle. （2） Resting upon the unconformity is another volcaniclastic sequence, the Xiajiang Group.The volcanics in the Fanjingshan Group include: spilite-keratophyre sequence（mainly meta-basalt with pillow）, quartz spilite （silicic extrusion）, and diabase dykesthat has a gradual boundary with the basalt. Besides, geological mapping has revealednumerous mafic-ultramafic and minor granitic plutons intruded the FanjingshanGroup. In-site LA-ICP-MS zircon U/Pb analyses demonstrate that the volcanism andsimultaneous sedimentation of the Fanjingshan Group took place during 860 to 815Ma, rather than previously-thought 870-800 Ma （Zhou et al., 2009） or after 800 Ma（Wang et al., 2010）.On the basis of detail geochronological studies, two stages mafic-ultramaficmagmatism were revealed that gave rise two suites of mafic to ultramafic igneousrock in Fanjingshan region: （1） The earlier stage magmatism has resulted in basalthorizons and bed-paralleled diabase or gabbroic dykes composing the FanjingshanGroup. In-site LA-ICP-MS zircon U/Pb results indicate that the mafic magmatism andsedimentation were simultaneous during 856-820 Ma. Geochemistry revealed anE-MORB-like Chondrite-normalized REE pattern for the earlier stage mafic rocks,while their primitive-mantle normalized trace element spider diagrams are generallyE-MORB like but with obvious Nb, Ta, Sr, P, and Ti negative anomalies. Suchgeochemical feature suggests an intensive crustal contamination and magmadifferentiation during the mafic magmatism. （2） The later stage mafic magmatismoccurred during 815 to 745 Ma according to new in-site LA-ICP-MS zircon U/Pbresults, which gave rise voluminous diabase, gabbro, and minor ultramafic plutonsintruding the Fanjingshan Group. The later stage mafic rocks have similargeochemical characteristic as those of the earlier stage rocks: both have anE-MORB-like Chondrite-normalized REE pattern and E-MORB like primitive-mantlenormalized trace element spider diagrams with obvious Nb, Ta, Sr, P, and Ti negativeanomalies, suggesting they have a similar petrogenesis process. However, subtledifference in geochemistry suggests a higher degree mantle partial melting for theearlier mafic magma and a higher degree magma differentiation for the later stagemagmatism. The quartz spilite and basalt of the Fanjingshan Group define bi-modelvolcanism. The quartz spilite has high SiO₂-content up to 72 wt. %, and is highlycomparable with the silicic volcanics in a continental rift featured by strongdifferentiation of light and heavy rare earth elements （LREE and HREE）, and traceelements including high-field-strength elements. The silicic volcanics have obviousnegative Ti-anomaly and medium Nb, Ta, Sr, and P negative anomalies, suggesting,again, an intensive crustal contamination.During the earlier stage mafic magmatism, some high-SiO₂, stronglyperaluminous leucogranites developed in Fanjingshan region. Geochemical studiesdemonstrate that the leucogranite is strongly peraluminous with very highSiO₂-content （up to 72 wt. %） and high A/CNK value > 1.1. The chondrite-normalizedREE patterns are eagle-like, exhibiting strong negative Eu-anomaly and obvioustetrahedral effect, suggesting they likely have resulted from a high temperature（>875oC）partial melting of clay-enriched psammitic rocks.Synthesizing available sedimentary petrologic, igneous petrologic, geochemical,and geochronological data, the Neoproterozoic tectonic evolution of the westernsegment of the Jiangnan orogenic belt may be divided into two stages: 860-815 Ma isthe Jinning-I stage, and 815-750 Ma is the Jinning-II stage, between them is theangular unconformity separating the Fanjingshan and Xiajiang Groups. Both theJinning-Ⅰand -Ⅱmovements are displayed as continental-rifting-related volcanismand sedimentation. According to geologic data from other areas, the JinningⅠstagerifting may have resulted in an aulacogen, likely representing the western tip of anocean between the Yangtze and Cathaysian blocks, while the JinningⅡstage rifting iscommonly thought as the represent of the mantle plume activity that has led tobreakup of the Rodinia super-continent.更多还原