【作者】 王建国；

【导师】 刘立； 和钟铧；

【作者基本信息】 吉林大学 ， 矿物学、岩石学、矿床学， 2014， 博士

【摘要】 本文以大兴安岭南部科右中旗地区的碱性流纹岩为研究对象，利用锆石LA-ICP-MS U-Pb同位素测年确定了碱性流纹岩的形成时代；利用碱性流纹岩地球化学分析、锆石的Hf同位素示踪作用和能谱分析、电子探针分析，讨论了碱性流纹岩的成因、岩浆作用过程和形成的构造背景；通过碱性流纹岩石英斑晶中的熔体包裹体和流体包裹体的物理化学特征研究，揭示成矿的作用过程。形成于大兴安岭南部白音高老组的碱性流纹岩与碱性花斑岩以富含钠闪石为特征，分别命名为钠闪石流纹岩和钠闪石花斑岩，两者是同源、同期次岩浆活动的产物，后者呈次火山岩相产出。钠闪石流纹岩中的组成矿物具有明显的定向特征，流动构造、球粒结构、雪球构造发育，钠闪石花斑岩呈斑状结构、交生文象结构，两者都是由石英、碱性长石、钠闪石、霓石和富稀土、稀有元素的矿石矿物所组成。碱性流纹岩的结构特征、其石英斑晶中富集流体包裹体和岩石中含有大量的钠闪石、氟碳铈矿、烧绿石等富挥发组份的矿物显示，岩浆演化过程中有卤水和气相挥发分的参与，特别是富F挥发分参与下的岩浆分异作用使碱性流纹岩的岩浆演化形成两个连续的作用阶段：岩浆阶段和残余岩浆阶段，碱性流纹岩先后经历了钠长石结晶作用过程和钠闪石结晶作用过程。钠闪石结晶作用过程使熔体充分结晶，大量的熔体分配系数很低的不相容元素在残余岩浆体系中高度富集，在最后去气成岩的过程中进入副矿物，形成赋存稀土和稀有元素的矿石矿物。钠闪石流纹岩中的岩浆锆石LA-ICP-MS U-Pb定年结果表明，其206Pb/238U年龄介于134~149之间，其加权平均年龄为141Ma，表明钠闪石流纹岩形成于早白垩世早期。锆石εHf(t)=+9.07~+12.08，二阶段Hf模式年龄介于415~616Ma之间。岩石地球化学资料表明，碱性流纹岩以富硅、碱、铁质和贫钙、镁质为特征，在地球化学上，具有典型的A型流纹岩特征。该类岩石不仅具有高的稀土元素总量(REE=306×10-6~1395×10-6)、显著的Ce正异常（Ce/Ce*=6.52~18.63）和显著的Eu负异常（Eu/Eu*=0.0067~0.0236），而且表现出高场强元素（如Zr、Hf、Nb、Ta）的显著富集、大离子亲石元素（如Ba、Sr）的强烈亏损和很高的Ga/Al比值（104×Ga/Al=4.88~6.53）。上述资料表明，钠闪石流纹岩的原始岩浆应是古俯冲蚀变洋壳部分熔融的产物，并形成于蒙古-鄂霍茨克缝合带闭合后的岩石圈伸展构造环境。碱性流纹岩具有很高的轻稀土和Nb、Y等稀有元素品位，钠闪石流纹岩岩石和钠闪石花斑岩岩体就是矿体。矿石矿物有钛铁矿、铌钛铁矿、磁铁矿、褐铁矿、锆石、钍锆石、铌钇锆石、氟碳铈矿、烧绿石、独居石、褐帘石和其它富钇铌矿物。矿石粒径微小，呈他形不规则的晶体结构，孤立状或聚晶状产出，在碱性流纹岩中呈浸染状分布，形成填隙结构、包含结构、交代结构。碱性流纹岩中La、Ce、Nd主要是以离子化合物的形式赋存在独居石、氟碳铈矿、褐帘石中，少量以杂质元素类质同象方式在锆石中；Nb在烧绿石中以离子化合物形式赋存在矿物晶格中，以杂质元素类质同象置换的形式存在铌钛铁矿和钇铌锆石中，在褐帘石中也有少量元素混入。Th、U等元素主要是以杂质元素类质同象在锆石、褐帘石等矿物中，Y元素在铌钇锆石中以杂质元素赋存。碱性流纹岩石英斑晶中原生包裹体有熔融包裹体、气液相流体包裹体、气液相含子晶矿物流体包裹体3类，并显示碱性流纹岩中的流体为中-高盐度、中-高密度流体。其中，钠闪石花斑岩中流体包裹体均一温度范围为215.9~＞500℃，钠闪石流纹岩中为168.9~336.7℃。岩石中各种类型的流体包裹体的最小捕获压力集中于7.72~32MPa，相应的最小捕获深度约为0.8~3.2km。激光拉曼光谱分析在熔体包裹体中发现有菱锰矿、方解石类的碱性矿物以及稀土类矿物拉曼谱峰，在流体包裹体中发现有碳酸根离子谱峰，这表明碱性流纹岩的岩浆成分具有显著的富碱性的物质特性，并赋存有很高的稀有稀土元素成分；碱性流纹岩的残余岩浆中的流体组分是富挥发组分的中-高盐度、中-高密度的碱性流体，并富集碱性金属元素和稀土元素离子。因此，碱性流纹岩是深源浅成的岩浆-残余岩浆作用下的稀土矿床。岩浆源岩为碱性流纹岩成矿提供了物质条件，火山岩基底岩层的背斜褶皱和多期次断裂发育的地质构造条件为稀土矿床的形成提供了重要的成矿环境。矿石特征和熔体包裹体-流体包裹体的物理化学条件表明，碱性流纹岩从岩浆结晶开始直到晚期的成岩阶段，都具有成矿作用。在岩浆阶段主要形成岩浆锆石、独居石、钛铁矿等成矿矿物；残余岩浆阶段是成矿的主要阶段，稀有稀土元素大多以阳离子形式存在，形成氟碳铈矿、褐帘石、烧绿石等。更多还原

【Abstract】 This thesis studies the formation time of the alkali-rhyolites of Keyouzhongqi in thesouthern Da Hinggan Mts., and their petrogenesis, tectonic setting and mineralization,based on LA-ICP-MS zircon U-Pb chronology, major and trace elements, electron probemicro analysis and the physical chemistry experimental data of melt inclusion and fluidinclusion in the quartz of alkali-rhyolites.The alkali-rhyolites of Baiyingaolao Formation in southern Da Hinggan Mts., wascomposed of riebeckite rhyolites and riebeckite granophyres which has the same magmasource and the same period with riebeckite rhyolites, and both of them have a similarvolcanic lithofacies. Riebeckite rhyolites shows a very characteristic by flow structure,spherulitic texture and snowball structure and all minerals in the riebeckite rhyolites wereobviously directionally arranged. Riebeckite granophyres shows porphyroid texture andgranophyric intergrowths texture. Both were composed of quartz, alkali-feldspar,riebeckite, aegirite and other minerals with rare earth elements and rare elements.Zircons from the riebeckite rhyolite are euhedral-subhedral in shape, and displaytypical oscillatory zoning on CL images, suggesting their magmatic origin. LA-ICP-MSzircon U-Pb dating results indicate that206Pb/238U ages of24analytical spots range from134Ma to149Ma, yielding a weighted mean age of141±1Ma, implying that the riebeckiterhyolite formed in the early Early Cretaceous. Their εHf(t) values range from+9.07to+12.08, and their Hf two-stage model ages(TMD2) vary from415Ma to616Ma. Thealkali-rhyolites have SiO2=74.3%~76.4%, FeO=3.96%~5.94%,(Na2O+K2O)=7.07%~8.51%, CaO=0.12%-0.84%, and MgO=0.03%~0.09%, chemically similar to typical A-type rhyolite. Additionally, the riebeckite rhyolites have high total REE contents(307×10-6~1395×10-6) and Ga/Al ratios (104×Ga/Al=4.88~6.41), strong positive Ceanomalies (Ce/Ce*=6.52~18.6), strong negative Eu anomalies (Eu/Eu*=0.007~0.009),and display enrichment in HFSEs (e.g., Zr, Hf, Nb, Ta) and strong depletion in LILEs (e.g.,Ba, Sr). Taken together, it can be concluded that the riebeckite rhyolites could be derivedfrom partial melting of a fossil altered oceanic crust during an extensional setting after theclosure of the Mongol-Okhotsk Ocean.It can be proved that there were lots of brine and volatiles involved in the magmaticevolution process of alkali-rhyolites by the present of lithophysa structure, spherulitictexture, snowball sturcure, vesicular structure, enrichment of fluid inclusions in quartzsand the characteristics of riched in riebeckites, bastnaesites, pyrochlorites in the alkali-rhyolites. Especially The magmatic differentiation processes with the full participation ofthe volatiles enriched F made alkali-rhyolite magma undergo two phases of magmatismsuccessively: magma phase and residual magma phase, and corespondingly, alkali-rhyolitehave gone through the crystalization of riebeckite after albite. Residual magma wouldenrich in incompatible elements with lower partition coefficient because the melt werewell-crystallized, and these incompatible elements entered into accessory minerals duringdegassing diagenesis and formed minerals enriched in rare earth and rare elements.The rocks of riebeckite rhyolites and riebeckite granophyres themselves are the orebody of rare rare earth deposit, and total contents of REE, Y, Nb et al. show that thesesamples have a high ore grade. Ore minerals are composed of ilmenite, niobium ilmenite,magnetite, limonite, zircon, thorium zircon, niobium and yttrium zircon, bastnaesite,pyrochlore, monazite, allanite and other minerals enriched in niobium and yttrium, and areformed by microcrystal xenomorphic granular in the shape of isolated form or synneusischaracterized by interstitial structure, poikilitic texture and metasomatic texturedisseminated in alkali-rhyolites.Most La, Ce, Nd in monazite, bastnaesite and allanite exist in the form of ioniccompounds, while few in zircon are in the form of somorphism. Nb occurrence inpyrochlore and niobium ilmenite, niobium and yttrium zircon respective by ioniccompounds and somorphism, and the little sneak into allanite. Th, U occurrence in zircon and allanite by somorphism, and Y occurrence in niobium and yttrium zircon bysomorphism.Abundance inclusions in quartzs of alkali-rhyolites can be classified into three types:melt inclusions, gas-liquid fluid inclusions and daughter minerals of salt crystal bearinggas-liquid fluid inclusions, indicating the fluid inclusions was middle-high salinity andmiddle-high density. The fluid inclusion homogenization temperature ranges from168.9to336.7℃in riebeckite rhyolites and215.9to great than500℃in riebeckite granophyres,and the trapping pressures and trapping depths of all inclusions respectively concentrateon7.72to32MPa and0.8to3.2km. The magma of alkali-rhyolites enriched in alkalis andrare-earth and rare elements based on electron probe micro-analysis. And its componentsin residual magma was volatile component with middle-high salinity and middle-highdensity fluid, enriched in ions of alkali-metal elements and rare earth elements.Mineralization of alkali-rhyolites is related with gas-liquid activity, its’ genetic typebelongs to the polygenetic superposition of rare earth element and rare element ore depositwith the action of magmatism-residual magmatism that originated in deep-sourced andformed hypergene zone. The magma souce provided ore-forming materials condition foralkali-rhyolites, and the anticlinal fold and the complicated faulting system in basementstrata provided a ore-forming environment.The ore character and physical and chemical characteristics of fluid melt inclusionsreveal that ore minerals are crystallized throughout all the time of magma stage andmagmatic-hydrothermal transitional processes. These ore minerals such as magmaticzircons, monazites and ilmenites were crystallized in magma stage. Residual magmaticstage is the main crystallization period, and in this period, rare and rare earth elementsmainly exist in bastnaesites, allanites，pyrochlores et al. in the form of cation.更多还原