Subduction-related Late Cretaceous high-K volcanism in the Central Pontides orogenic belt: constraints on geodynamic implications

Mineral chemistry, major and trace elements, ⁴⁰Ar/³⁹Ar age and Sr–Nd–Pb isotopic data are presented for the Late Cretaceous Hamsilos volcanic rocks in the Central Pontides, Turkey. The Hamsilos volcanic rocks mainly consist of basalt, andesite and associated pyroclastics (volcanic breccia, vitric tuff and crystal tuff). They display shoshonitic and high-K calc-alkaline affinities. The shoshonitic rocks contain plagioclase, clinopyroxene, alkali feldspar, phlogopite, analcime, sanidine, olivine, apatite and titanomagnetite, whereas the high-K calc-alkaline rocks contain plagioclase, clinopyroxene, orthopyroxene, magnetite / titanomagnetite in microgranular porphyritic, hyalo-microlitic porphyritic and glomeroporphyritic matrix. Mineral chemistry data reveal that the pressure condition of the clinopyroxene crystallisation for the shoshonitic rocks are between 1.4 and 6.3 kbar corresponds to 6–18-km depth and the high-K calc-alkaline rocks are between 5 and 12 km. ⁴⁰Ar/³⁹Ar age data changing between 72 ± .5 Ma and 79.0 ± .3 Ma (Campanian) were determined for the Late Cretaceous Hamsilos volcanic rocks, contemporaneous with the subduction of the Neo-Tethyan Ocean beneath the Pontides. The studied volcanic rocks were enriched in the large-ion lithophile and light rare earth element contents, with pronounced depletion in the contents of high-field-strength elements. Chondrite-normalised rare earth element patterns (La_N/Lu_N = 6–17) show low to medium enrichment, indicating similar sources of the rock suite. Initial ⁸⁷Sr/⁸⁶Sr values vary between .70615 and .70796, whereas initial ¹⁴³Nd/¹⁴⁴Nd values change between .51228 and .51249. Initial ²⁰⁶Pb/²⁰⁴Pb values vary between 18.001 and 18.349, ²⁰⁷Pb/²⁰⁴Pb values between 15.611 and 15.629 and ²⁰⁸Pb/²⁰⁴Pb values between 37.839 and 38.427. The main solidification processes involved in the evolution of the volcanic rocks consist of fractional crystallisation, with minor amounts of crustal contamination ± magma mixing. According to geochemical evidence, the shoshonitic melts in the Hamsilos volcanic rocks were possibly derived from the low degree of partial melting of a subcontinental lithospheric mantle (SCLM), while the high-K calc-alkaline melts were derived from relatively high degree of partial melting of SCLM that was enriched by fluids and/or sediments from a subduction of oceanic crust.     

华北板块北缘呼兰群~(40)Ar-~(39)Ar定年及其构造意义

呼兰群发育于华北板块北缘,主要为一系列长英质火山岩和沉积岩组成的受到低绿片岩相-角闪岩相变质作用和多期变形作用改造的变质岩系,分布于吉林中部红旗岭-呼兰镇一带。大部分地区的呼兰群地层层序被严重破坏,野外观察的"层理"实际上是构造置换作用形成的构造面理,绝大多数的岩性界面为构造接触。呼兰群由一系列相互叠置的构造岩片构成,具有构造混杂岩性质。呼兰群经历了两期韧性变形作用:早期受到NNE-SSW向挤压作用,呼兰群表现为由NNE向SSW逆冲的运动学特征;晚期可能受到NE-SW向伸展作用,主要表现为由SW向NE倾向滑移的运动学特征。本次分别对采自红旗岭地区强变形带的白云石英片岩、石榴白云片岩中的白云母进行40Ar-39Ar测年,分别获得220.23±2.15Ma、221.31±2.6Ma的坪年龄,认为~220Ma年龄应代表华北板块北缘碰撞造山带造山后伸展作用发生的时间。     

大兴安岭中段晚三叠世四分组效应花岗岩的厘定及其地质意义

布敦化岩体位于中亚造山带东段大兴安岭地区的科尔沁右翼中旗,该岩体由中细粒二云母花岗岩和斑状中细粒白云母花岗岩组成。LA-MC-ICP-MS锆石U-Pb同位素年龄测试显示布敦化岩体形成于晚三叠世早期。地球化学分析表明,该岩体为弱过铝质-过铝质、高钾钙碱性系列,主量元素表现出高硅(73.73%~78.33%)、富碱(Na2O+K2O=7.07%~9.06%)、低钙(Ca O=0.01%~0.87%)、贫镁(Mg O=0.12%~0.52%)的特征;稀土元素具有明显的Eu负异常和M型四分组效应配分型式。岩石学和地球化学特征显示布敦化岩体为高分异I型花岗岩。Lu-Hf同位素研究显示其εHf(t)值为7.0~12.4,二阶段Hf模式年龄(tDM2)为812~473Ma。结合邻区晚三叠世地质资料可知,布敦化岩体是在古亚洲洋闭合造山后伸展的构造背景下产生的,是晚元古代-早古生代地壳物质熔融的产物。布敦化地区的四分组效应花岗岩的稀土元素总量高、岩浆侵位后经历了强烈的分异演化,暗示布敦化地区可能具有较好的稀有金属矿的找矿前景。     

华北克拉通北部迁安紫苏花岗岩LA-ICP-MS锆石U-Pb定年、地球化学及地质意义

紫苏花岗岩是太古宙高级变质地体的主要物质组成,对其开展岩石成因与年代学研究,可深入理解早期大陆生长及其机制。本文以迁安紫苏花岗岩为窗口,探讨其成因与机制,通过精确的LA-ICP-MS锆石U-Pb测年获得其成岩年龄为2544±22Ma,指示了新太古代晚期的一次岩浆活动。岩石变质年龄为2475±87Ma,证明随后的高级变质作用年龄范围为2439~2501Ma。岩石地球化学研究表明:迁安紫苏花岗岩SiO_2=59.14%~66.97%,MgO=1.62%~3.12%,富Ba(574×10-6~1572×10-6),LREE((La/Yb)N=22.3~79.0),亏损HFSE,不存在Eu负异常(δEu=1.76~3.43),其高Sr/Y值(85~179)与(La/Yb)N值指示了与TTG的相关性。结合样品地球化学特征及锆石U-Pb年龄分析,并与本区域大地构造背景相结合,迁安紫苏花岗岩可能是角闪岩相条件下偏中性含水玄武质原岩高温部分熔融的产物,应当形成于地幔柱背景。     

东昆仑祁漫塔格地区白干湖钨锡矿田白云母~(40)Ar/~(39)Ar定年及地质意义

东昆仑祁漫塔格地区白干湖钨锡矿田是认识中国西北地区钨锡矿床成矿规律的重要窗口。作者对采自含矿石英脉的2个白云母样品进行40Ar/39Ar定年,获得其坪年龄分别为(422.7±4.5)Ma和(421.8±2.7)Ma。2个样品的等时线年龄与反等时线年龄也在误差范围内一致,分别为(424±15)Ma和(418±24)Ma,表明分析数据可信。获得的白云母40Ar/39Ar坪年龄指示成矿作用发生在晚志留世,与原特提斯洋闭合事件密切相关,闭合后的陆陆碰撞使富含成矿物质的变质沉积物重熔而形成花岗岩浆;花岗岩浆侵入并析出含矿热液,导致钨锡成矿。     

Evaluation of 40Ar/39Ar Geochronology of Authigenic Illites in Determining Hydrocarbon Charge Timing: A Case Study from the Silurian Bituminous Sandstone Reservoirs,Tarim Basin,China

The Silurian bituminous sandstones(SBS) in the Tarim Basin, China are important basinwide reservoirs with an estimated area of approximately 249000 km2. We investigated the ages of authigenic illites in the SBS reservoirs and constrained their formation timing by using the ~(40)Ar/~(39)Ar step wise heating method. The age spectra, ~(39)Ar recoil loss and their controlling factors were investigated systematically. The ~(40)Ar/~(39)Ar ages were compared with the conventional K/Ar ages of identical clay fractions. The clay in the SBS reservoirs is dominated by orderly mixed-layer illite/smectite(I/S) with 5%–30% smectite layers. The I/S minerals morphology comprises primarily honeycomb, short filamentous and curved-lath particles, characteristic of authigenic illites. The unencapsulated ~(40)Ar/~(39)Ar total gas ages(UTGA) of the authigenic illites range from 188.56 ± 6.20 Ma to 491.86 ± 27.68 Ma, which are 7% to 103% older than the corresponding K/Ar ages of 124.87 ± 1.11 Ma to 383.45 ± 2.80 Ma, respectively. The K-Ar ages indicate multistage accumulations with distinct distribution patterns in the Tarim Basin: older(late Caledonian-early Hercynian) around the basin margin, younger(late Hercynian) in the basin centre, and the youngest(middle to late Yanshanian) in the Ha-6 well-block, central area of the North Uplift. The age difference is believed to have been caused by the ~(39)Ar recoil loss during the irradiation process. Compared with the K/Ar ages, the estimated ~(39)Ar recoil losses in this study are in the range from 7% to 51%. The ~(39)Ar recoil loss appears to increase not only with the decreasing particle sizes of the I/S, but also with increasing percentage of smectite layers(IR) of the I/S, and smectite layer content(SLC) of the samples. We conclude that due to significant ~(39)Ar recoil losses, UTGA may not offer any meaningful geological ages of the authigenic illite formation in the SBS and thus can not be used to represent the hydrocarbon charge timing. ~(39)Ar recoil losses during ~(40)Ar/~(39)Ar dating can not be neglected when dating fine authigenic illite, especially when the ordered mixed-layer I/S containing small amount of smectite layers(IR30%) in the reservoir formations. Compared with the unencapsulated Ar-Ar method, the conventional K-Ar method is less complicated, more accurate and reliable in dating authigenic illites in petroleum reservoirs.     

The Cretaceous Ofuku Pluton and Its Relation to Mineralization in the Western Akiyoshi Plateau,Yamaguchi Prefecture,Japan

The relationship between the magmatism of the Cretaceous Ofuku pluton and mineralization in and around the Akiyoshi Plateau, Yamaguchi Prefecture, Japan was investigated using a combination of field observation, petrographic and geochemical analyses, K–Ar geochronology, and fluid inclusion data. The Ofuku pluton has a surface area of 1.5 × 1.0 km, and was intruded into the Paleozoic accretionary complexes of the Akiyoshi Limestone, Ota Group and Tsunemori Formation in the western part of the Akiyoshi Plateau. The pluton belongs to the ilmenite‐series and is zoned, consisting mainly of early tonalite and granodiorite that share a gradational contact, and later granite and aplite that intruded the tonalite and granodiorite. Harker diagrams show that the Ofuku pluton has intermediate to silicic compositions ranging from 60.4 to 77.9 wt.% SiO₂, but a compositional gap exists between 70.5 to 73.4 wt.% SiO₂ (anhydrous basis). Modal and chemical variations indicate that the assumed parental magma is tonalitic. Quantitative models of fractional crystallization based on mass balance calculations and the Rayleigh fractionation model using major and trace element data for all crystalline phases indicate that magmatic fractionation was controlled mainly by crystal fractionation of plagioclase, hornblende, clinopyroxene and orthopyroxene at the early stage, and quartz, plagioclase, biotite, hornblende, apatite, ilmenite and zircon at the later stage. The residual melt extracted from the granodiorite mush was subsequently intruded into the northern and western parts of the Ofuku pluton as melt lens to form the granite and aplite. The age of the pluton was estimated at 99–97 Ma and 101–98 Ma based on K–Ar dating of hornblende and biotite, respectively. Both ages are consistent within analytical error, indicating that the Ofuku pluton and the associated Yamato mine belong to the Tungsten Province of the San‐yo Belt, which is genetically related to the ilmenite‐series granitoids of the Kanmon to Shunan stages. The aplite contains Cl‐rich apatite and REE‐rich monazite‐(Ce), allanite‐(Ce), xenotime and bastnäsite‐(Ce), indicating that the residual melt was rich in halogens and REEs. The tonalite–granodiorite of the Ofuku pluton contains many three‐phase fluid inclusions, along with daughter minerals such as NaCl and KCl, and vapor/liquid (V/L) volume ratios range from 0.2 to 0.9, suggesting that the fluid was boiling. In contrast, the granite and aplite contain low salinity two‐phase inclusions with low V/L ratios. The granodiorite occupies a large part of the pluton, and the inclusions with various V/L ratios with chloride daughter minerals suggest the boiling fluids might be related to the mineralization. This fluid could have carried base metals such as Cu and Zn, forming Cu ore deposits in and around the Ofuku pluton. The occurrence and composition of fluid inclusions in the igneous rocks from the Akiyoshi Plateau are directly linked to Cu mineralization in the area, demonstrating that fluid inclusions are useful indicators of mineralization.     

Exploratory Spatiotemporal Analysis in Risk Communication during the MERS Outbreak in South Korea

The 2015 Middle East respiratory syndrome (MERS) outbreak in South Korea gave rise to chaos caused by psychological anxiety, and it has been assumed that people shared rumors about hospital lists through social media. Sharing rumors is a common form of public perception and risk communication among individuals during an outbreak. Social media analysis offers an important window into the spatiotemporal patterns of public perception and risk communication about disease outbreaks. Such processes of socially mediated risk communication are a process of meme diffusion. This article aims to investigate the role of social media meme diffusion and its spatiotemporal patterns in public perception and risk communication. To do so, we applied analytical methods including the daily number of tweets for metropolitan cities and geovisualization with the weighted mean centers. The spatiotemporal patterns shown by Twitter users' interests in specific places, triggered by real space events, demonstrate the spatial interactions among places in public perception and risk communication. Public perception and risk communication about places are relevant to both social networks and spatial proximity to where Twitter users live and are interpreted in reference to both Zipf's law and Tobler's law.     

Nitrogen, Oxygen and Argon Dissolved in the Northern North Pacific in Early Summer

Major gases dissolved in seawater were accurately determined with a shipboard gas chromatographic method. The standard deviations were 0.28, 0.34 and 0.36% for N₂, O₂ and Ar, respectively. The method was applied to water from the northwestern North Pacific Ocean collected in May to June 2000. We got 127 duplicate seawater samples from the surface 200 m layer at 11 stations. The O₂ concentrations obtained by this method agreed with those given by the Winkler method. All the seawater samples from the surface 200 m, especially those from the upper 30 m, were supersaturated with respect to atmospheric N₂ and Ar concentrations. In the topmost 30 m layer, the degrees of supersaturation in the inventory were 2.7–4.3% for N₂ (ΔN₂) and 1.7–2.6% for Ar (ΔAr), and their ratios, ΔN₂/ΔAr, ranged from 1.53 to 1.81. This supersaturation seems to be chiefly due to air bubbles injected into the water and dissolved due to the water pressure, because the N₂/Ar ratio of the air is around 2. The amounts of air bubbles dissolved in the upper 30 m water were relatively large, with mean value of 0.41 ml/kg or 18.4 μmol/kg. The ΔN₂, ΔAr and ΔN₂/ΔAr values were all positively well correlated with the wind velocities averaged for the last 24 hours prior to sampling, allowing the conclusion to be drawn that the weaker the wind velocity, the dissolved gas composition approaches in equilibrium with the air; while the stronger the wind velocity, it approaches in the air composition. This revised version was published online in July 2006 with corrections to the Cover Date.     

云贵高原红土性风化壳定年对象--锰结核的基础研究

以野外剖面调查为基础，结合物理学、地球化学、矿物学和同位素年代学手段和方法，对云贵高原红土性风化壳中可资定年的对象——锰结核的产出状态、内部构造、地球化学特征、矿物组成及锰结核全岩的^40K／^40Ar定年进行了探索性研究，为解决我国风化壳的发育年龄奠定基础。研究结果表明：锰结核的形成晚于其赋存介质(风化壳)的生成；在风化壳中，锰结核的内部构造以(准)同心圆状构造为主；结核形成过程中发生了MnO和Fe2O3的富集，其中以MnO的富集较为显著；并且Fe、Mn分布具有较明显的负相关关系；锰结核中K2O含量明显高于CaO、MgO及Na2O，但K和Mn不存在明确的相关关系；结核中次生锰氧化物的主要存在形式为锰钡矿；锰结核的全岩^40K／^40Ar定年结果是一混合年龄，其不具有明确的地质意义；造成这一结果的主要原因在于锰结核是由不同时期、多种矿物构成的复杂集合体。