【作者】 谭俊；

【导师】 魏俊浩；

【作者基本信息】 中国地质大学 ， 矿物学、岩石学、矿床学， 2009， 博士

【摘要】 深源脉岩是区域性岩石圈引张环境下的产物，对大陆动力学研究具有重要的指示意义。其与金矿床密切伴生的时空关系也一直是地学界研究的重要课题。论文选取胶东金矿集中区中郯庐断裂带的次级郭城深大断裂带内脉岩为研究对象，在充分野外地质调查和岩相学研究基础上，进行了详细的锆石U-Pb年代学、岩石地球化学、同位素地球化学、斑晶矿物微区地球化学研究，同时对脉岩斑晶矿物中硫化物熔融包裹体进行了细致的形态学及微区成分测试，探讨了脉岩的岩石成因、源区性质、成岩过程，以及其所揭示的壳幔岩浆混合信息对华北克拉通东部岩石圈演化及金成矿深部过程的制约。主要成果和结论如下：1．郭城断裂带内脉岩控岩构造以NE向张性断裂为主，受控于郯庐断裂带早白垩世左行走滑引张体系。95％以上脉岩成矿后侵位，井下穿脉巷道工程（垂直脉岩走向）揭露的局部地段脉岩累加厚度远远超过围岩厚度，有相当数量脉岩宽度超过5 m，是华北克拉通内罕见的脉岩高密度侵位地区。这些高密度产出的脉岩，其岩浆演化过程对华北克拉通中生代晚期岩石圈演化具有很好的指示意义。岩相学研究将断裂带脉岩划分为闪长玢岩、煌斑岩及二长斑岩三类，以闪长玢岩最为发育，煌斑岩产出有限（1％～2％）。2．锆石LA-ICP-MS U-Pb测年结果表明二长斑岩和闪长玢岩分别形成于114±2和116±1 Ma。结合最新关于金成矿的同位素年代学资料（黄铁矿亚样品Rb-Sr等时线及绢云母和白云母Ar-Ar坪年龄，123～119 Ma），进一步确认了郭城断裂带脉岩的侵位，代表了胶东地区成矿后一次重要的岩浆活动事件。3．主量元素组成显示断裂带不同类型脉岩SiO₂含量介于44.75％～71.38％，属高钾钙碱性—钾玄质岩系列。具明显高MgO（Mg^#=30～70）、Cr(5.4×10^-6～571.4×10^-6)和Ni(3.3×10^-6～235.5×10^-6)含量，且变化范围较大。岩石富集大离子亲石元素（如Sr、Rb、Ba、U等）和轻稀土元素（（La／Yb）_N=18～66），相对亏损高场强元素（如Nb、Ta、Zr、Hf、Ti等）和重稀土。Sr-Nd-Pb同位素组成不均一，具高I_Sr（t）（0.70775～0.71164）、低ε_Nd（t）（-20.8～-13.5）和²⁰⁶Pb／²⁰⁴Pb（17.128～17.557）特点，且线性关系明显。岩石成因与幔源岩浆地壳物质混染、富集地幔部分熔融以及拆沉下地壳部分熔融熔体与地幔橄榄岩反应模式相区别。地球化学组成继承了岩浆源区性质并与幔源铁镁质岩浆和壳源长英质岩浆的混合作用有关，表现为主成分共分母协变图及微量元素相关图和同分母比值图中样品分析点的直线性相关特点，以及同时具有高的Mg、Cr、Ni含量和富集Sr、K、Rb、Ba、Pb等元素特征，与同位素组成所揭示的壳幔岩浆混合信息吻合。Sr-Nd同位素二元混合模拟显示，软流圈岩浆与地壳重熔型花岗质岩浆混合能够满足脉岩的同位素及元素组成要求，煌斑岩和闪长玢岩地幔物质参与成岩的比例（60％～95％）大于二长斑岩（＜40％）。定量模型计算表明，脉岩的不相容元素和稀土元素组成也受到了辉石、角闪石、钾长石、斜长石、黑云母、磷灰石、锆石等矿物分离结晶作用的影响。MFC（岩浆混合—分离结晶）是控制岩浆演化过程的重要机制，进一步确定了华北克拉通东部中生代晚期岩石圈深部壳幔作用存在的地质事件。4．电子探针微区分析结果显示，断裂带脉岩中辉石和角闪石斑晶幔部相对核部均出现了MgO（Mg^#）、Cr₂O₃含量急剧升高的组成特点，斜长石斑晶具有反环带，斑晶成分变异揭示脉岩为壳幔岩浆混合作用的产物，与岩相学观察见角闪石斑晶包裹黑云母及出现大量长针状磷灰石现象吻合，丰富了胶东及部分报道的华北克拉通东部同时期脉岩的富集地幔来源认识。壳幔作用形成机制与岩石圈引张密切相关，张性活动导致了软流圈上涌，并可诱发地壳物质局部熔融形成长英质岩浆房。当深部高温软流圈岩浆注入壳源岩浆房后，由于温度、粘度、化学成分等不同，二者之间发生物质—能量交换，形成了不均一混合岩浆源区。温压计算显示脉岩源区处于中—下地壳范围，压力最大为0.79～1.33 GPa（26～44km），温度约924～1278℃。斑晶矿物微区地球化学研究的成果，有力确认了华北克拉通东部早白垩世软流圈上涌底侵的深部动力学过程。5．郭城断裂带高密度壳幔岩浆混合成因（软流圈岩浆注入至壳源长英质岩浆房）脉岩的侵位，为华北克拉通东部中生代晚期岩石圈的底侵减薄过程提供了直接证据。表明该阶段软流圈上涌，以及其对岩石圈地幔及下地壳的侵蚀过程应是岩石圈减薄的主导机制。动力学背景与早白垩世郯庐断裂带受伊泽奈崎板块高速斜向俯冲所引起的大规模第二次左行走滑引张活动有关。结合华北克拉通中生代构造演化，阶段性活动岩浆岩研究资料，及中生代早期拆沉模式研究基础（如徐淮地区高镁闪长岩中榴辉岩捕虏体、辽西兴隆沟高镁火山岩等），论文明确提出了华北克拉通东部岩石圈减薄的拆沉—底侵双阶段演化模式，并第一次明确指出了郯庐断裂带在岩石圈减薄过程中具有非常重要的作用—应力释放效应。6．岩相学研究在断裂带闪长玢岩斑晶矿物中发现了硫化物（磁黄铁矿）熔融包裹体与硅酸盐熔融包裹体共生现象。硫化物球粒状及“卡脖子”状形态学特征，以及电子探针微区成分测试结果表明，壳幔岩浆混合过程中能在岩浆房局部出熔形成富金硫化物熔体。脉岩斑晶矿物中岩浆硫化物残余与Fe-氧化物共生现象，指示岩浆演化后期挥发份的析出能够导致硫化物熔体破坏分解，进而释放金属元素进入成矿流体。硫化物包裹体与矿石硫化物一致的Au／Cu（～2.5）和Ag／Au（～2.0）比值，以及脉岩与矿石硫化物接近的Pb同位素组成，表明成矿流体继承了源区硫化物熔体的元素和同位素组成特点。该发现在华北克拉通中生代脉岩中尚属首次，为脉岩与金成矿的密切关系的研究提供了重要证据。更多还原

【Abstract】 Deep-source mafic dikes, controlled by regional lithosphere extension, play an important role inunderstanding the continental dynamics. Their temporal-spatial relationship to gold deposits hasbeen a significant research field for many geologists. In this study, we focus on the EarlyCretaceous dikes intruded intensively in the Guocheng fault zone secondary to the Tan-Lu faultsystem （deep to upper mantle）, Jiaodong Peninsula within the southeast margin of the North Chinacraton （NCC）. After extensive field work and petrographic study, the results of zircon U-Pbchronology, whole-rock geochemistry, isotopic geochemistry and microscopic geochemistry of theGuocheng dikes, and the morphology and microscopic analyses of sulfide melt inclusions inphenocrysts from these rocks are presented. These results are used in an attempt to investigate thesource and petrogenesis of these dikes, constrain the Mesozoic lithosphere thinning of the easternNCC, and understand the gold enrichment process in the magmatic chamber and the transfer of oremetals from the magma to the hydrothermal fluid during gold ore formation. The main results andconclusions are summarized as the follows:（1） These dikes in the Guocheng fault zone dominantly intruded in NE direction extensionalfaults, resulted from the extensional strike-slip movement of the Tan-Lu fault system in the EarlyCretaceous. 95 % volume of the dikes emplaced immediately after gold ore formation, and thecumulative thickness is locally far more than their wall rocks revealed by laneways crosscut theserocks vertically, which is perhaps the best example and accordingly an ideal place to test the dikepetrogenesis and their implications for the evolution of the eastern NCC. Petrographic study showsthat these dikes are dominated by diorite porphyry with lesser amounts of lamprophyre andmonzonite porphyry.（2） Zircon cathodoluminescence （CL） images show well-developed magmatic oscillatory zonesand the acquired LA-ICP-MS zircon U-Pb weighted mean ²⁰⁶pb/²³⁸U ages are 114±2 Ma （MSDW=1.5） for monzonite porphyry and 116±1 Ma （MSDW=0.8） for diorite porphyry,respectively. Earlier magmatic events in the northwest Jiaodong Peninsula represented by someinherited or captured zircons also occur in these dikes. Our results, combined with preciouslypublished data on gold deposits （Rb-Sr dating of auriferous pyrite and Ar-Ar dating of muscoviteand sericite）, further confirm a significant magmatic action after gold ore formation in theJiaodong Peninsula.（3） Bulk-rock geochemical data show that the Guocheng dikes range in composition from quartzmonzonite to monzogabbro with an SiO₂ content varying from 44.75% to 71.38%. These dikes arecomposed of high-K calc-alkaline to shoshonitic or ultrapotassic rocks, which are characterized byhigh and wide MgO （Mg^#= 30-70）, Cr （5.4-571.4 ppm） and Ni （3.3-235.5 ppm） contents. Theyare enriched in large ion lithophile elements （LILE, e.g., Sr, Rb, Ba） and light rare earth elements（LREE; （La/Yb）_N=18-66）, depleted in high field strength elements （HFSE, e.g., Nb, Ta, Zr, Hf, Ti）and heavy rare earth elements （HREE）, and possess uniform isotopic compositions having highinitial ⁸⁷Sr/⁸⁶Sr （0.70775-0.71164） and lowε_Nd （t） （-20.8-13.5） and ²⁰⁶pb/²⁰⁴pb （17.128-17.557）.Since crustal contamination during the magma ascent is insignificant, the Guocheng dikes couldreflect the nature of their magma source. Their geochemical features can not been interpreted bypartial melting of an enriched mantle or melts derived from foundered lower crust, withsubsequent interaction with mantle peridotite. Therefore, we propose that these rocks originatedfrom the mixing between mantle-derived mafic and crust-derived felsic magmas, resulting in highcontents of Mg, Cr and Ni and enrichment of Sr, K, Rb, Ba and Pb simultaneously, which isconsistent with linear relationship of their isotopic compositions. A simple modeling of Sr-Ndisotope show that mixing the depleted mantle with granitic melt remelted from crust can accountfor the present Sr-Nd isotope features and element compositions, in which the proportion of crustalmaterials participated in monzonite porphyry （＞60%） is higher than those in diorite porphyry andlamprophyre （5%-40%）. Quantitative model indicates the behaviors of incompatible elements andREE are also influenced by fractional crystallization of pyroxene, hornblende, plagioclase,K-feldspar, biotite, apatite and zircon. MFC （mixing and fractional crystallization） dominantlycontrol the magma evolution of the Guocheng dikes, confirming the existence of crust-mantleinteraction in the eastern NCC during the Late Mesozoic.（4） Electron probe microanalyses （EPMA） show that the amphibole and clinopyroxenephenocryst’s mantle in diorite porphyry and lamprophyre respectively has sharply higher MgO（Mg^#） and Cr₂O₃ contents in contrast to their cores. The plagioclase phenocryst in monzoniteporphyry has reverse zoning. These compositional variations of phenocrysts indicate that the dikesoriginated from a mixing magma source, which coincides with the petrography exhibiting biotiteenclaves in amphibole phenocrysts and the presence of acicular apatites. This crust-mantleinteraction is related to the lithosphere extension, which results in asthenosphere upwelling leadingto decompressional melting of crustal materials to form felsic magma reservoir. Along with hightemperature asthenosphere magma injecting into the crust-derived magma chamber, material andenergy exchange occurred, as different temperature, viscosity and chemical compositions of themixing members, to form the inhomogeneous mixing magma source of these dikes. Mineral chemistry show that the pressure and temperature of the magma source is about 0.79-1.33 GPa（26-44 km; middle-lower crust level） and 924-1278℃respectively. Our findings make sure adeep dynamical process of asthenosphere upwelling in the eastern NCC during the Late Mesozoic.（5） Crust-mantle magma mixing genesis （with asthenosphere magma injecting into crust-derivedmagma reservoir） of the Guocheng dikes provides direct evidence for asthenospheric underplatingthinning process of the eastern NCC during the Late Mesozoic. It indicates that asthenosphereupwelling and its chemical erosion to lithospheric mantle and lower crust may be the dominantthinning mechanism in the Cretaceous. Dynamical background is probably due to the secondextensional sinistral strike-slip movement of the Tan-Lu fault system triggered by the obliquesubduction of the Izanagi plate. Combined with tectonic evolution of the NCC and stagedmagmatisms during the Mesozoic, and the existence of the Early Mesozoic foundering lower crust（e.g., researches on eclogite xenoliths entrained by adakitic intrusions from the Xuzhou-Huaibeiregion, Xinglonggou high Mg volcanic rocks in western Liaoning）, we propose that the NCC hadexperienced staged thinning process, i.e., delamination thinning in the Early Mesozoic andunderplating thinning in the Late Mesozoic respectively, and firstly point out the Tan-Lu faultzone may play a significant role in the lithosphere thinning, i.e., stress release.（6） Based on petrographic study in detail, we discovered primary melt inclusions （PMIs） inequilibrium with silicate melt inclusions in phenocrysts （including hornblende, plagioclase andbiotite） from diorite porphyry. The globular shape and necking of the sulfides and differentsulfide/silicate melt ratios in single inclusions indicate that these sulfides were trapped in amolten stage, documenting the fact that the magma had exsolved sulfide melt upon crust-mantlemagma mixing in some parts of the magma chamber. EPMA microscopic analyses show that Auwould be preferentially enriched in the sulfide liquid. Relicts of the magmatic sulfides coexistingwith Fe-oxides in phenocrysts from the diorite porphyry indicate that the sulfide melt can beinstantaneously destabilized during later stage of the magma evolution and release Au to anore-forming hydrothermal fluid. The mean Au/Cu and Ag/Au ratios of sulfide melt inclusions,within an average factor of about 2.5 and 2.0 respectively, is identical to those of ore sulfides,and the dikes have Pb isotopic compositions similar to ore sulfides, suggest that the ore-forminghydrothermal fluid inherited the element and isotope features of the sulfide melt in the magmasource. This discovery in the Mesozoic dikes in the NCC is the first, providing a critical linkbetween mafic dikes and gold deposits and showing new insights into the formation of othermagmatic hydrothermal deposits.更多还原