稀有金属花岗伟晶岩锆石、锡石与铌钽铁矿U-Pb和白云母~(40)Ar/~(39)Ar测年对比研究——以阿尔金中段吐格曼北锂铍矿床为例

稀有金属花岗伟晶岩年代学是花岗伟晶岩型稀有金属矿床研究的重要内容。目前测年方法较多,但不同方法测试结果的对比研究亟待开展。我们选择吐格曼北花岗伟晶岩型锂铍矿床3条含矿伟晶岩开展锆石、锡石、铌钽铁矿及白云母四种矿物不同方法的测年对比研究。结果显示:1)ρ31白云母-锡石伟晶岩中锡石238U/206Pb-207Pb/206Pb谐和年龄为468±8.7Ma (MSWD=1.1,N=39)、白云母-钠长石-锂辉石伟晶岩中锆石206Pb/238U谐和年龄为458.7±2.3Ma (MSWD=7.2,N=16); 2)ρ38白云母-钠长石-锂辉石伟晶岩中锆石206Pb/238U谐和年龄为454.7±4.0Ma (MSWD=8.0,N=10)、白云母40Ar/39Ar坪年龄为350.2±1.6Ma (MSWD=4.7); 3)ρ87含铌钽铁矿-白云母-石英伟晶岩中铌钽铁矿206Pb/238U谐和年龄为464.1±2.7Ma (MSWD=5.2,N=39)。可以看出,铌钽铁矿与锡石的U-Pb年龄在误差范围内一致,可能代表花岗伟晶岩岩浆结晶的年龄; 2件样品的蜕晶化锆石U-Pb年龄也可以对比,可能代表岩浆锆石蜕晶化后经流体交代作用及重结晶作用导致U-Pb同位素系统重置的时间。白云母Ar-Ar年龄明显晚于铌钽铁矿、锡石和锆石的U-Pb年龄,鉴于ρ38白云母-钠长石-锂辉石伟晶岩脉中白云母与锂辉石发生了强烈变形与蚀变,认为变形的白云母记录的是叠加变形与热液蚀变的时间。由此推断吐格曼北锂铍花岗伟晶岩形成于468～454Ma,这意味着阿尔金山地区中-晚奥陶世可能存在持续时间较长的稀有金属成矿事件。基于花岗伟晶岩矿物成因与吐格曼北锂铍花岗伟晶岩不同方法测年对比结果,可以得出以下结论:铌钽铁矿与锡石的U-Pb年龄可代表伟晶岩岩浆结晶的年龄,蜕晶化的锆石U-Pb年龄记录的是岩浆锆石蜕晶化后经流体交代作用及重结晶作用导致U-Pb同位素系统重置的时间,含钾矿物的40Ar/39Ar年龄能够约束伟晶岩的变形与蚀变年龄;多种定年方法的联合约束可以更好地限定稀有金属花岗伟晶岩的各个阶段成矿事件时间。     

湘中矿集区杨家山石英脉白钨矿床的白云母40Ar-39Ar和LA-ICP-MS锡石U-Pb年龄及其地质意义

杨家山中型石英脉型白钨矿床位于华南湘中低温锑金钨矿集区.白钨矿石英脉产于晚泥盆世黑云母二长花岗岩和新元古界板岩中,是全球为数不多的与花岗岩有关的石英脉白钨矿床,其成矿机制还不清楚.本文开展了与白钨矿共生的锡石的LA-ICP-MS微区原位U-Pb同位素测年,获得206Pb/238U加权平均年龄值为410.4±5.7Ma(...     

Comparison of the surface structure of the tetrahedral sheets of muscovite and phlogopite by AFM

 The surface structure of the tetrahedral sheet of dioctahedral mica muscovite was compared to that of the tetrahedral sheet of trioctahedral mica phlogopite using atomic force microscopy (AFM). AFM revealed distinct structural differences between the tetrahedral sheet surfaces of the two micas. The hexagonal ring in the AFM image of muscovite elongates in the [3 1 0] direction, and the groove runs perpendicular to the [3 1 0] direction. On the phlogopite surface, the hexagonal ring contracts slightly in the a axis direction, but the groove is not apparent. These results were consistent with the bulk structure data of the two micas determined by X-ray diffraction (XRD). The degree of surface relaxation was much larger in muscovite than in phlogopite. In muscovite, the interlayer K reduces the amount of tetrahedral rotation that actually occurs, since the interlayer K is too large for its hexagonal hole after full tetrahedral rotation. Thus, it is naturally expected that muscovite will show more tetrahedral rotation after removal of the interlayer K. It is also expected that muscovite will show more tilting of SiO₄ tetrahedra after cleaving, since an attractive force between the hydrogen in the OH group and the lower basal oxygen should be in operation, due to the decreased distance between them following interlayer K removal. Received: 14 March 2000 / Accepted: 29 July 2000     

山西南部中条山古元古代花岗质片麻岩 白云母40Ar/39Ar测年及其地质意义

中条山前寒武纪岩石是洞悉华北克拉通前寒武纪基底构造演化的重要窗口之一,该区的前寒武纪岩系主要由涑水杂岩、绛县群、中条群、担山石群,以及西阳河群和汝阳群组成.本文以中条山地区涑水杂岩中古元古代花岗质片麻岩为研究对象,挑选其中的变形变质白云母进行激光40Ar/39Ar和常规40Ar/39Ar测年分析.激光40Ar/39Ar法获得的白云母等时年龄1830Ma±20Ma为白云母氩封闭温度年龄的最小估计,常规40Ar/39Ar法给出的白云母坪年龄1852Ma±11Ma为白云母氩封闭温度年龄的最佳估计.白云母1852Ma± 11Ma与先前获得的独居石电子探针U-Th-Pb主峰值年龄相近,并且与华北克拉通中部带的变质年龄一致,表明中条山地区涑水杂岩中古元古代花岗质片麻岩记录了古元古代晚期的一次变质作用事件.这一事件与华北克拉通中部怀安-吕梁-恒山-五台-赞皇等地的变质变形作用同时发生,揭示华北克拉通东、西部陆块沿中部带的碰撞拼合应发生在古元古代晚期,而非新太古代.     

湖南龙山锑金矿床白云母~(40)Ar-~(39)Ar年代学及其意义初探

龙山锑金矿是湘中Sb-Au矿集区内规模最大的脉状锑金矿床,但其精确的成矿时代尚未厘定。本次工作发现该矿床第Ⅰ成矿阶段的石英硫化物脉中发育少量的热液白云母。通过40Ar-39Ar年代学测定,确定了白云母的~(40)Ar-~(39)Ar同位素坪年龄为162. 5±1. 8Ma,相应的等时线年龄为161. 1±1. 2Ma(MSWD=1. 0),反等时线年龄为161. 1±1. 2Ma(MSWD=1. 0)。根据矿物共生组合特征,认为白云母的~(40)Ar-~(39)Ar坪年龄能代表龙山锑金矿床的成矿年龄。结合湘中地区其他锑(金)矿床的年代学研究成果,本文认为,该地区在155~162Ma之间有一次热液成矿事件,与南岭地区165~150Ma与花岗岩有关的钨锡多金属矿床的成矿时代相一致。湘中地区已有的S同位素地球化学和地球物理学资料表明,该地区锑金矿床的形成可能与岩浆作用有关,类似于南岭地区晚侏罗世钨锡多金属矿床的构造背景。     

Muscovite surface structure imaged by fluid contact mode AFM

The surface structure of muscovite was imaged using an atomic force microscope (AFM) in contact mode in water. The following three types of AFM images were observed: (1) those showing clearly the arrangement of hexagonal rings of SiO₄ tetrahedra; (2) those showing a hexagonal array of bright spots separated by a distance of about 5.3 Å; and (3) those changing gradually from image (2) to image (1). Image (1) successfully provides information on the tetrahedral tilt and basal surface corrugation that are particularly characteristic of dioctahedral micas. The mean unit cell dimensions for the muscovite surface measured from Image (1) were slightly longer than those of the bulk structure, due to the rehydration of the tetrahedral sheet and/or surface relaxation. Image (2) was made by varying the scan angles, even on the same surface in which Image (1) was obtained. Image (3) has information on a single plane rather than on two or more planes involving steps, kinks and so on. Therefore, what is depicted in Images (2) and (3) is not the arrangement of interlayer K ions but the basal plane of the tetrahedral sheet. Some structural relaxation of the tetrahedral sheet surface was also observed. Gradual expansion and contraction of hexagonal rings were randomly found on the muscovite surface. The surface relaxation results from a tetrahedral rotation and/or tilting after cleaving, since significant variations of both distances and bond angles between adjacent SiO₄ tetrahedra on the surface were found.     

Clayey materials in river basin enhancing microbial contamination of river water

Mineral constituents of clay materials may promote interaction, adsorption and attachment of microorganisms, often resulting in biofilms’ formation. In this study investigation is made to determine how littoral clayey materials on the shores of a river promote accumulation of bacteria and increase contamination of river water. Clayey samples were collected at various points along the shore of a river around Mondeor in Johannesburg and the mineralogical composition was determined using XRD and XRF. Microorganisms in clay-biofilm and river water were identified by DNA sequencing and plate count. Results showed that total coliforms, Escherichia coli, Pseudomonas sp. and presumptive indigenous microorganisms attached to littoral clayey materials containing the mineral muscovite (characterising argillaceous soils). Bacteria number on clayey materials was significantly higher than on overlying water especially before rainy season. However a decrease of the number of bacteria in clayey materials concurrent with an increase in the number of suspended bacteria after rain events, was the result of the action of high and fast flows in the basin, eroding the biofilms. Attachment of microorganisms in clayey material as observed in this study could be ascribed to the glue-like aspect of soil (due to muscovite) that facilitates adhesion. It therefore demonstrates the potential of clayey materials to encourage biofilm formation and enhance microbial contamination of river water as shown here.     

Formation Age and Evolution Time Span of the Koktokay No. 3 Pegmatite,Altai, NW China: Evidence from U–Pb Zircon and 40Ar–39Ar Muscovite Ages

The Koktokay No. 3 pegmatite is the largest Li–Be–Nb–Ta–Cs pegmatitic rare‐metal deposit of the Chinese Altai orogenic belt, and is famous for its concentric ring zonation pattern (nine internal zones). However, the formation age and evolution time span have been controversial. Here, we present the results of LA‐ICP–MS zircon U–Pb dating and muscovite ⁴⁰Ar–³⁹Ar dating. Four groups of zircon U–Pb ages (～210 Ma, ～193–198 Ma, ～186–187 Ma and ～172 Ma) for Zones II, V, VI, VII, and VIII, and a weighed mean ²⁰⁶Pb/²³⁸U age of 965 ± 11 Ma for Zone IV are identified. Also, Zones II, IV, and VI have muscovite ⁴⁰Ar–³⁹Ar plateau ages of 179.7 ± 1.1 Ma, 182.1 ± 1.0 Ma, and 181.8 ± 1.1 Ma, respectively. Considering previous U–Pb age studies (Zones I, V, and VII), the ages of emplacement, Li mineralization peak, hydrothermal stage of the No. 3 pegmatite are in ranges of 193–198 Ma, 184–187 Ma and 172–175 Ma, with weighted mean ²⁰⁶Pb–²³⁸U ages of 194.8 ± 2.3 Ma, 186.6 ± 1.3 Ma and 173.1 ± 3.9 Ma, respectively. The No. 3 pegmatite formed in the early Jurassic. The results of xenocrysts suggest that there is another pegmatite forming event of around 210 Ma in the mining district and the old zircon U–Pb ages imply that Neoproterozoic crustal rocks pertain to sources of the No. 3 pegmatite. Including the previous muscovite ⁴⁰Ar–³⁹Ar age studies (Zones I and V), a cooling age range of 177–182 Ma is considered as the time of hydrothermal stage and end of formation. The evolution process of the No. 3 pegmatite lasted 16 Ma. Therein, the magmatic stage continued for 9–11 Myr and the magmatic–hydrothermal transition and hydrothermal stages were sustained at 5–7 Ma. These time spans are long because of huge scale, cupola shape, large formation depth, and complex internal zoning patterns and formation processes. Considering some pegmatite dikes in the Chinese Altai, there is an early Jurassic pegmatite forming event.     

扬子地块西南缘大红山群石榴白云母-长石石英片岩的白云母40Ar-39Ar定年及其地质意义

大红山群是扬子地块西南缘出露的古元古代结晶基底,主要经历了绿片岩相-低角闪岩相变质作用.本研究对大红山群老厂河组变质中酸性岩和变质沉积岩——石榴白云母-长石石英片岩中的白云母进行了40Ar-39Ar测年,得到三个样品的坪年龄和40Ar/39Ar等时线年龄结果较统一,坪年龄代表的变质年龄分别为837.7±4.2Ma、839.6±4.2Ma和844.2±4.2Ma.变质沉积岩和变质中酸性岩的变质时代类似,均介于837～845Ma.大红山群变质基性岩中变质锆石的U-Pb定年年龄为849±12Ma(杨红等,2012),40Ar-39Ar测年数据与锆石定年数据相结合,说明大红山群古元古代结晶基底中的火山岩和沉积岩均在新元古代经历了同期变质作用,其主期低角闪岩相变质作用发生于新元古代837～850Ma.结合前人发表的扬子西缘～750Ma的变质年龄,扬子西缘从北向南的区域变质作用时限可扩展到750 ～850Ma.此外,扬子西缘存在750～850Ma的岩浆事件,本文研究结果说明,扬子地块西缘在新元古代不仅发生了大规模岩浆作用,也发生了750～850Ma的区域变质作用,扬子西缘存在新元古代的岩浆-变质事件.岩浆事件与变质事件之间可能存在相关性,即新元古代岩浆作用引起了扬子西缘的区域动力热流变质作用.     

Fabrication of synthetic calcite–muscovite rocks with variable texture — An analogue to cataclasite fabrics?

A series of large diameter calcite–muscovite aggregates has been prepared from calcite and muscovite powders, in order to gain a better understanding of how texture develops in impure carbonate rocks. The development of the microstructure and the crystallographic preferred orientation (CPO, texture) during the preparation process is described. The synthetic rocks have been fabricated from powders of calcite and muscovite by uniaxial cold-pressing at loads up to 400 MPa and subsequent hot isostatic pressing (HIPping) at pressures of 150 to 170 MPa and a temperature of 670 °C. The resulting textures and microstructures are homogeneous throughout the samples. The calcite CPO is generated by rigid body rotation and twinning during cold-pressing and is not significantly altered by recrystallization during HIPping. Grain growth during HIPping is observed in pure calcite samples, but is inhibited through high porosity and the presence of muscovite in the mixed aggregates. The preferred orientation of the calcite c-axes is found to increase with increasing uniaxial cold pressure, and to be independent of the muscovite content. The magnetic bulk susceptibility of the starting material has been changed by the formation of ferromagnetic impurities during fabrication. Comparison of the samples to natural calcite fabrics from fault zones show the potential of the experiments and fabric analyses presented to analyze and to better understand the deformation mechanisms of fault zones.