【作者】 李超；

【导师】 闫峻；

【作者基本信息】 合肥工业大学 ， 地质学， 2021， 博士

【摘要】 岩浆岩作为探视地球深部物质组成的探针以及成矿物质的载体,对其形成时代、岩石类型、地球化学特征、源区位置、岩浆性质进行探索可以为区域成岩成矿机制、深部地球动力学背景以及区域构造演化提供重要的信息。本文选择华北克拉通东南缘蚌埠隆起区内的荆山、老山、蚂蚁山、东芦山、西芦山、曹山、锥山、淮光岩体以及大巩山和江山金矿区中与金成矿相关的岩浆岩作为研究对象,开展详细的全岩地球化学和全岩Sr-Nd-Pb同位素、锆石U-Pb年代学、锆石Lu-Hf同位素、以及锆石、磷灰石、角闪石、黄铁矿化学成分研究,旨在(1)确定各阶段岩浆岩的类型、形成时代、源区位置、岩石成因、部分熔融条件、岩浆过程以及构造背景;(2)探索白垩纪不同类型岩浆岩与同期金成矿作用之间的联系,识别出与成矿相关岩浆岩;(3)确定与成矿相关岩浆岩在金成矿过程所扮演的角色;(4)与胶北隆起同期岩浆岩进行对比研究,探讨华北东部中生代构造演化与成岩成矿之间的关系。蚌埠隆起晚侏罗世淡色花岗岩常与淡色细晶岩以及伟晶岩相伴生,具有高硅、高碱、高铝、低铁、镁、钛氧化物含量,高的分异指数、低的Zr/Hf和Nb/Ta比值、低的石英氧同位素组成、并且稀土配分模式显示四分组效应,表明其属于高分异I型花岗岩。此外,结合新元古代-晚三叠世的继承锆石的存在、较低的岩浆锆石钛饱和结晶温度、低的石英氧同位素和放射性Pb同位素比值以及富集的Sr-Nd-Hf同位素组成,表明蚌埠淡色花岗岩是由三叠纪俯冲至蚌埠地区之下的北大别片麻岩,发生水致部分熔融后经历了高程度的结晶分异作用之后形成的。蚌埠早白垩世早期岩浆岩形成于115～130Ma,主要由埃达克质花岗岩和少量基性脉岩组成。此阶段岩浆岩与金矿体具有密切的时空关系。其中埃达克质花岗岩富集轻稀土、亏损重稀土、未显示Eu异常,并且其具有高Sr、低Y和Yb含量、低的Mg#值、具有变化范围较大的Sr–Nd–Pb–Hf同位素组成,表明蚌埠早白垩世早期花岗岩起源于华南或/和华北板块加厚下地壳。此外,根据锆石和磷灰石矿物化学成分反映,这些埃达克质花岗岩浆具有相对较高的挥发分含量和氧化性,这种特点有利于金的运输与富集。蚌埠早白垩世早期的基性岩具有低硅高镁的特点,并且富集大离子亲石元素、亏损高场强元素,以及富集轻稀土元素、亏损重稀土元素,具有低的Sr-Nd同位素组成以及变化的放射性Pb同位素比值,暗示这些基性岩起源于被俯冲华南板块地壳物质交代的岩石圈地幔。此外,磷灰石和角闪石矿物化学成分反映基性岩浆具有高氧逸度和高含水量,这种性质为成矿物质的源区活化和运移提供了良好的条件。蚌埠地区早白垩世晚期岩浆岩形成于109～114 Ma,分布广泛,以普通钙碱性花岗岩为主。在野外未见其与金矿体有紧密的时空关系。与前两阶段花岗岩相比,本阶段花岗岩具有低Sr、高Y和Yb含量、低的Sr/Y、(La/Yb)_N和(Dy/Yb)_N比值以及具有Eu负异常。此外,此阶段岩石中没有发现新元古代继承锆石,结合它们负的Nb-Ta异常、高的Nb/La比值以及均一的Sr-Nd-Pb-Hf同位素组成、较为古老的两阶段Hf-Nd模式年龄,表明它们起源于华北克拉通的中下地壳,而不是加厚地壳。通过对比蚌埠隆起与胶北隆起中生代同期成岩成矿作用,基于区域中生代构造演化格架,本文认为蚌埠地区中生代成岩成矿作用与华南板块和古西太平洋板块(伊泽奈崎板块)的俯冲均有重要联系。中三叠世时期,华南板块呈低角度长距离向北俯冲至华北板块下方,致使华南上地壳含金岩系被携带至华北板块深部。中侏罗世时期,伊泽奈崎板块向西俯冲至华北克拉通东部,俯冲板片释放的流体交代上覆岩石圈地幔,降低了其稳定性。随后,在晚侏罗世,伊泽奈崎板块俯冲方向缓慢向西北偏转,触发了晚侏罗世岩浆活动。直至早白垩世早期,伊泽奈崎板块俯冲方向继续沿顺时针方向偏转,但俯冲速度明显加快,随着俯冲角度逐渐变陡,导致正在俯冲的板块发生回撤,造成沿郯庐断裂带发生了大规模的区域拉张。强烈的拉张运动不仅触发了岩石圈地幔和加厚下地壳发生部分熔融,分别产生了基性和埃达克质熔体,而且还造成郯庐断裂带发生了走滑运动形成了大量NNE走向的次级断裂。华南板块上地壳物质属性的岩浆将部分成矿物质携带至地壳深度,流体将其运送至走向平缓的次级断裂中富集,从而形成金矿床。随后,岩石圈进一步失稳发生减薄,区域热异常触发了减薄后的华北板块下地壳物质发生了部分熔融,产生了普通的钙碱性花岗质岩浆。更多还原

【Abstract】 Because the magmatic rock is not only the probe of the earth’s deep materials but also the carrier of ore-forming materials,the study of its formation age,type,geochemical characteristics,magma sources,and magma properties can provide many important information for regional rock-and ore-forming mechanism,geodynamics background,and tectonic evolution.In this paper,magmatic rocks collected from Jingshan,Laoshan,Mayishan,Donglushan,Xilushan,Caoshan,Zhuishan,and Huaiguang plutons,and Jiangshan and Dagongshan deposits in the Bengbu uplift are selected as the research objects.This paper has carried out detailed whole-rock geochemistry and whole-rock Sr-Nd-Pb isotope,zircon U-Pb geochronology,zircon Lu-Hf isotope and mineral chemistry of these magmatic rocks,aiming to（1）Determine their types,formation ages,sources,petrogenesis,partial melting conditions,magmatic processes,and tectonic background;（2）Explore the relationship between different magmatism and gold mineralization in the Cretaceous,and identify ore-causative rocks;（3）Determine the role of ore-causative rocks for gold mineralization;（4）Conduct a comparative study with the Jiaobei magmatic rocks,and explore the relationship between the Mesozoic tectonic evolution and rock-and ore-forming mechanism in the eastern North China Craton.The Late Jurassic leucogranites are characterized by high Si O₂,alkali,and Al₂O₃contents.In addition to two samples exhibiting the rare-earth element tetrad effect,all rocks have high differentiation index values,extremely low iron–magnesium–titanium oxide contents,and low O isotopic compositions,low Zr/Hf and Nb/Taratios,suggesting a highly fractionated granite.Furthermore,the samples contain Neoproterozoic and Triassic zircons,have low Ti-in-zircon temperatures,low radiogenic Pb isotopic compositions,and highly enriched Sr–Nd–Hf isotopic compositions.Taken together,the geochronological and geochemical results suggest that the BLG was formed by high-degree fractional crystallization after hydrous partial melting of the North Dabie granitic gneisses,which have been subducted beneath the Bengbu area by Triassic continental subduction.The early Early Cretaceous magmatic rocks in the Bengbu area were formed at115–130 Ma and are mainly composed of adakitic granites and a few mafic dikes.These rocks have a close spatiotemporal relationship with the gold orebodies.The adakitic granites samples are characterized by pronounced enrichment in light rare earth elements（REE）,depletion in heavy REE,an absence of Eu anomalies,and they have relatively low Mg#values and variable Sr–Nd–Pb–Hf isotopic compositions.These features indicate that the parental magmas of the adakites were derived from partial melting of thickened lower crust of the South China Plate and/or the North China Craton.In addition,based on mineral chemical composition of apatite and zircon,the adakitic magmas were highly oxidized and hydrated,which would favor transpotation for ore-forming materials.The mafic dikes have low Si O₂ and high Mg O contents;high Ba/Rb ratios（>18）,low Sr-Nd values;and a wide range of radiogenic Pb isotopic ratios,indicating an enriched mantle source metasomatized by the upper crust of the subducted South China Block.Their mineral chemical composition of apatite and amphibole suggest that the mafic magmas were more highly oxidized and hydrated,which are favorable conditions for activation of ore-forming source and transportation.The late Early Cretaceous magmatic rocks in the Bengbu area were formed at 109–114 Ma and are mainly composed of calc-alkaline granites.There is no close temporal and spatial relationship between them and gold ore bodies in the field.Compared with granites of the first two stages,the granites have low Sr,high Y and Yb content,small negative Nb–Ta anomalies,high Nb/La,low and homogeneous Nd–Pb–Hf isotopic compositions,indicating they were derived from the middle to lower crust of the North China Craton.By comparing the Mesozoic magmatism and mineralization of the Bengbu Uplift and the Jiaobei Uplift,based on the regional tectonic evolution,this paper suggests that the Mesozoic magmatism and mineralization in the Bengbu area are importantly related to the subduction of the South China Block and the Izanagi Plate.During the Middle Triassic,the South China Block subducted northward under the eastern North China Craton.The continental subduction caused Neoproterozoic pyritic sedimentary rocks with Au and S anomalies in the South China upper crust to be transported to the deep part of the North China Craton.In the Middle Jurassic,the Izanagi plate subducted westward to the North China Craton,and the fluids released by the subducted slab hydrated the overlying lithospheric mantle,reducing its stability.During the Late Jurassic,the subduction direction of the Izanagi Plate slowly changed to the NWN direction,leading to the emplacement of the Late Jurassic intrusions.Subsequently,the subduction of the Izanagi Plate accelerated,and the subduction direction continued to deflect to the north.As the subduction angle steepened,the subducting Izanagi Plate rolled back during the middle Early Cretaceous,causing the large-scale tectonic extension along the Tanlu fault zone.The strong extensional movement induced partial melting of the lithospheric mantle and the lower crust,generating gold-related mafic and granitic melts,respectively.In addition,the pre-existing NE-NNE striking faults adjacent to and inside of the Tanlu fault zone reactivated as normal faults in this stage.These gentler dipping faults provided structural sites for precipitation of the ore-forming minerals.At the same time or slightly later,lithospheric thinning was completed after 115 Ma.Regional thermal anomalies triggered partial melting of the thinned lower crust,generating calc-alkaline granitic magma.更多还原