Petrologic and geochemical characteristics of “barkinite” from the Dahe mine, Guizhou Province, China

Although “barkinite” has long been studied by many geologists, its geochemical characteristics and environment of deposition are still not known in detail. In order to study the petrography and geochemical characteristics of “barkinite”, coal samples from two Permian coal seams were taken from the Dahe mine, Guizhou Province. The samples were separated into maceral fractions, and then analyzed by microscopical, isotopic, Rock-Eval, and geochemical methods. The microscopical results indicate that “barkinite” occurs as four main types. According to their relationship to other maceral groups, “barkinite” is ostensibly formed under variably dry–wet or oxidizing–reducing conditions. The extract yield, isotope data and Rock-Eval values of “barkinite” are different from other macerals. Microscopical and geochemical results indicate that “barkinite” forms part of the liptinite group.     

华南二叠纪生物成礁模式探讨

二叠纪生物礁广泛发育于我国南方碳酸盐岩地区。鉴于许多研究者从成礁的外部环境因素来探讨生物礁的发育模式,笔者从我国南方二叠纪生物礁的内部成礁因素分析,即从主要造礁生物——钙质海绵和钙质藻类等的生物学和生态学特征、埋藏和保存特点方面进行分析,继而提出了华南二叠纪海绵礁的成礁模式。此模式不同于其它地质历史时期生物礁的成礁模式。     

华南地区晚二叠世煤的稀土元素特征

在华南地区贵州六盘水、江西乐平和湖北黄石矿区以及重庆钟梁山和磨心坡煤矿晚二叠世煤的稀土元素电感耦合等离子质谱分析数据的基础上,系统研究了华南地区煤的稀土元素的质量分数分布特征、赋存特征、配分模式及其地质控制因素.煤的稀土元素质量分数与灰分质量分数呈正相关关系,部分煤样品由于有富稀土元素的重矿物组合的存在而出现稀土元素的异常高值.同一矿区晚二叠世煤的稀土元素配分模式具有一定的相似性,而不同地区晚二叠世煤的稀土元素配分模式则差异较大.煤的沉积环境、煤中无机矿物的组成以及海水的影响是控制煤的稀土元素质量分数及其配分模式的主要地质因素.     

华南晚二叠世层序—古地理及聚煤规律*

晚二叠世是地质历史中重要的成煤时期,在中国南方多个省区都形成了可采煤层。受海平面变化的影响,华南地区晚二叠世发育一套完整的陆相—海陆过渡相—海相含煤岩系。通过对野外露头及钻孔剖面资料分析,识别出了区域不整合面、下切谷底部冲刷面、河道间古土壤层和沉积相转换面等关键层序地层界面,以及依据海相石灰岩标志层横向分布规律反映的区域最大海泛面,同时结合前人关于华南地区地层对比及煤层对比方面的成果,将华南上二叠统划分为3个三级层序及9个体系域。利用单因素分析多因素综合作图法恢复了各层序中岩相古地理及煤层分布规律,结果表明:层序Ⅰ中煤层主要形成于潮控下三角洲平原以及潟湖潮坪环境,厚煤层的展布受这些沉积相带控制;层序Ⅱ中三角洲平原环境中煤层最厚,开阔台地煤层最薄,平面上煤层厚度表现为西部厚、中部次之、东部无煤的阶梯式递变;层序Ⅲ中煤层主要形成于三角洲平原环境,此时聚煤作用主要发生于华南西部的康滇古陆东侧,在华南中部及东部大部地区发育的浅海台地和深水盆地均不利于聚煤作用发生。     

Distinctive features of Late Palaeozoic massive sulphide deposits in South China

More than 20 sediment-hosted massive sulphide deposits occur in Late Palaeozoic basins in South China. These deposits are accompanied by a certain amount of volcanic rocks in the host sequence and are economically important for their Cu, Pb, Zn, Au and Ag reserves. The deposits and their host strata were commonly intruded by Mesozoic granitoids. Remobilisation of sedimentary ores and magmatic hydrothermal overprinting processes resulted in the coexistence of massive sulphides with vein-, skarn- and porphyry-type orebodies in the same region or within a single deposit. The ore-containing basins occur in different tectonic settings. The Lower Yangtze basin occurs on a passive continental margin, where the deposits are high in Cu and Au with minor Pb and Zn and recoverable Ag, Co and Mo. The ores have a lower concentration of radiogenic lead, and δ³⁴S values close to zero. Fluid inclusions are highly saline and Na-rich. Fluids and metals of the Lower Yangtze Region are interpreted to have been derived essentially from deep sources including the Precambrian basement. By contrast, basins of the Nanling Region formed in an intracontinental setting developed on a folded Caledonian basement. These deposits are higher in Pb, Zn, Sn and W, as well as Cu, with recoverable Ag, Sb, Hg, U, Bi, Tl and Mo. The ores are characterised by a higher concentration of radiogenic lead and a wide variation of δ³⁴S composition. Fluid inclusions have lower salinities and higher K⁺/Na⁺ ratios. Fluids are considered to have been sourced substantially from seawater by convection. Metals for the Nanling deposits were essentially derived from the Caledonian basement by leaching. The contrast in ore composition between these two regions appears to have been controlled by differences in basement composition of the ore-forming basins.     

Compositional classification of “kimberlitic” and “non-kimberlitic” ilmenite

Ilmenite is one of the common kimberlitic indicator minerals recovered during diamond exploration, and its distinction from non-kimberlitic rock types is important. This is particularly true for regions where these minerals are present in relatively low abundance, and they are the dominant kimberlitic indicator mineral recovered. Difficulty in visually differentiating kimberlitic from non-kimberlitic ilmenite in exploration concentrates is also an issue, and distinguishing kimberlitic ilmenite from those derive from other similar rocks, such as ultramafic lamprophyres, is practically impossible. Ilmenite is also the indicator mineral whose compositional variety has the most potential to resolve provenance issues related to mineral dispersions with contributions from multiple kimberlite sources.

Various published data sets from selected kimberlitic (including kimberlites, lamproites, and various ultramafic lamprophyres) and non-kimberlitic rock types have been compiled and evaluated in terms of their major element compositions. Compositional fields and bounding reference lines for ilmenites derived from kimberlites (sensu stricto), ultramafic lamprophyres, and other non-kimberlitic rock types have been defined primarily on MgO–TiO₂ graphs as well as MgO–Cr₂O₃ relationships.     

中国南方二叠纪层序地层时空格架及充填特征

据华南二叠纪不同沉积盆地层序地层区域分布、纵向演化及其充填特征分析研究,初步建立了上扬子克拉通-右江盆地-钦防盆地、中扬子克拉通及全区的层序地层时空格架模型;提出层序充填可分为两个阶段:早二叠世碳酸盐缓坡-台地层序充填阶段,晚二叠世陆源碎屑为主与火山碎屑相混层序充填阶段;认为全区二叠纪四个Ⅰ型层序受控于构造隆升事件,晚二叠世层序TST普遍发育火山碎屑沉积,古陆边缘HST或少量SMT/LST大多由三角洲沉积体系进积叠加而成,上扬子克拉通及右江地区的生物礁以海侵和高位型为特色。     

华南地区二叠纪栖霞组碳酸盐岩成因研究及其地质意义

二叠系栖霞组是华南地区一个特殊的碳酸盐岩地层,富含有机质和燧石结核,具广泛的空间分布,但是岩相和厚度变化极小,同时发育天青石结核和海泡石。概述前人对这些沉积特殊性的研究成果及存在问题。对其特殊性的深入成因分析表明,栖霞组其他沉积特殊性均与栖霞期的缺氧沉积环境有关,而缺氧沉积环境的成因可能与栖霞期较高的生物产率有关。根据栖霞期全球古气候、古海洋背景研究的最新进展,结合栖霞组的沉积学特征,认为栖霞组特殊性的成因可能与当时较高生物产率有关,进一步研究、确定栖霞期较高生物产率的成因及其变化将具有重要的地质意义。     

中国南方二叠纪层序地层划分与对比

对中国南方二叠纪时期不同沉积盆地类型的沉积层序进行了深入研究,综合各种盆地类型的沉积层序,该期可识别11个层序单元(S₁～S₁₁),其中,栖霞、茅口、吴家坪三阶分别含三个层序,长兴阶两个层序,S₁、S₆、S₇、S₁₁为Ⅰ型,其余为Ⅱ型。层序对比显示,上、中、下扬子的层序数量相近,界面特征各不一致,靠近古陆处在P/C、P₁/P₂界线间缺少层序1～2、3～5个不等;华南太平洋构造域影响区层序体系域具HST>TST结构,特提斯域控制区则具HST     

华南二叠纪层序地层与盆地演化

新兴的层序地层学,是盆地分析的重要手段。华南二叠纪是华南板块海西-印支阶段盆地演化的重要时期,沉积盆地类型划分为克拉通盆地、板内及板缘伸展盆地,以及东吴运动后出现的前陆盆地和弧后盆地。南方二叠系由11个层序和1个半超层序组成,不同类型沉积盆地中层序地层具有不同的层序特征和充填形式。二叠纪是海西-印支期盆地演化由成熟到消亡的转折阶段,它们发育于扩张环境下,总体上是由两个克拉通盆地夹持一个伸展盆地。两个克拉通盆地的发展不均衡,中央伸展盆地沿着加里东的拼合带发育。这一古地理古构造格局与加里东期十分相似,由此认为,华南晚古生代沉积的构造背景,并不是稳定的克拉通环境。     

Holocene dryness and human occupation in Brazil during the “Archaic Gap”

An overview of the archaeological data produced over the last decades for Brazil, coupled with a background of recent paleoenvironmental studies, suggests that during the mid-Holocene vast areas of Central Brazil ceased to be occupied by human groups. Independent data from dated human skeletons, rock-shelter stratigraphy, and chronology of open-air sites converge to support the inference that these areas were depopulated or altogether abandoned. Paleoenvironmental data suggest that dryness events constitute the major cause behind the observed trends. This phenomenon expands the already perceived notion that climatic stresses had a major role in the shaping of human settlement patterns in marginal environments, such as deserts and high-altitude settings.     

Mineralogy and geochemistry of different morphological pyrite in Late Permian coals,South China

The mineralogical and morphological characteristics, concentration of major and trace elements, and sulfur isotopic composition of three pyrite and two coal samples in the Upper Permian high-sulfur coals from Xingren, Zhijin, and Hefeng mining area, South China, were investigated, by using optical microscopy, field emission-scanning electron microscopy in conjunction with an energy-dispersive X-ray spectrometer (FE SEM-EDS), X-ray powder diffraction (XRD), X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), cold vapor atomic absorption spectrometry (CVAAS), and isotopic ratio mass spectrometer. The pyrite in Xingren and Zhijin coals mainly occurs as nodular, lens-shaped, thin-layer, and massive forms, and it occurs mainly as fine vein fillings in the Hefeng coals. Microscopically, pyrite in the coals from Xingren, Zhijin, and Hefeng mainly occurs as framboidal, cell-filling, and vein-filling forms, respectively. There is a distinct difference in X-ray powder diffractogram and diffraction data of the three pyrite samples. There is a maximum diffraction peak (2.709 Å) in pyrite in the coals from Xingren and (2.707 Å) in pyrite in the coals from Zhijin; however, the maximum diffraction peak is 3.343 Å in pyrite in the coals from Hefeng. The average unit cell length (a ₀) is 5.4169 Å for the Xingren pyrite, 5.4159 Å for the Zhijin pyrite, and 5.4170 Å for the Hefeng pyrite. The ratio of S/Fe is 2.16 for the Xingren pyrite, 2.09 for the Zhijin pyrite, and 2.01 for the Hefeng pyrite. Copper (701 μg/g), Ni (369 μg/g), and Co (29.6 μg/g) concentrated in the Hefeng pyrite. The concentration of As is 126, 19.6, and 19.1 μg/g in the Hefeng, Zhijin, and Xingren pyrite, respectively. Mercury is 11.7 μg/g in the Xingren pyrite, 2.79 μg/g in the Zhijin pyrite, and 0.512 μg/g in the Hefeng pyrite. There is a clear tendency that elements Cu, Ni, Co, Cr, Se, Mo, and As are significantly enriched in the Hefeng pyrite. Mercury is greatly enriched in the Xingren pyrite, and Zn is enriched in Zhijin pyrite. Rare earth elements and yttrium (REY) are not abundant (8.276 μg/g) in the Hefeng pyrite and are characterized by maximum positive anomaly of Eu (Eu/Eu* = 6.54). The δ³⁴S value is ?26.9 ‰ in the Xingren pyrite, +3.8 ‰ in the Zhijin pyrite, and +3.7 ‰ in the Hefeng pyrite. The trace elements in the Hefeng pyrite and coal are As (126 and 6.1 μg/g), Hg (0.512 and 0.158 μg/g), Zn (276 and 56.7 μg/g), Se (16.5 and 1.07 μg/g), Mo (45.5 and 9.93 μg/g), Cu (701 and 37.8 μg/g), Ni (369 and 16.9 μg/g), Co (29.6 and 8.63 μg/g), Sb (2.64 and 0.742 μg/g), Cd (3.49 and 0.366 μg/g), and Pb (62.8 and 33.5 μg/g), demonstrating that these potentially toxic trace elements were mainly concentrated in pyrites. The strongly positive Eu anomaly (Eu/Eu* = 6.54) in the netted pyrite vein filled in the cleats of the Hefeng coal may be the product of epigenetic hydrothermal fluid.     

中国南方二叠纪海平面变化及升降事件

在对中国南方二叠纪层序地层、充填特征及沉积演化认识基础上,通过研究海平面升降变化及事件后认为:华南二叠纪相对海平面升降旋回与其层序数量一致,升降型式可有两种且互成影像;二级海平面升降包括“二分式”和“单分式”,前者以扬子区为代表,后者典型见于华夏区;海平面升降事件发生在栖霞早期、茅口早期、茅口末期、吴家坪早期及长兴中后期;二级、三级海平面变化在升降样式、数量、相位上与欧美各地有所不同,其成因与所处构造域有关。     

Revisiting platinum group elements of Late Permian coals from western Guizhou Province, SW China

Twenty five coal samples from the Late Permian coal-bearing strata in Weining, Nayong, and Zhijin, western Guizhou Province, SW, China, were analyzed for platinum group elements (PGEs). The coal ashes were digested by the Carius tube technique and accurately measured by isotope dilution-inductively coupled plasma mass spectrometry (ID-ICP-MS) for all PGEs. The results are much lower than the previous reported values. Our study suggested that the previously reported PGE values are incorrect and may due to the polyatomic interferences in ICP-MS measurements. In our study, samples from the Weining coalfield have the lowest PGE contents (from 0.019 Ir to 0.42 ng/g Pd), which represent the PGE background value in coal in western Guizhou province. Some of the coals have Pt and Pd contents about 20-times higher than the background value, indicating PGEs are concentrated. We also reported new and reliable PGE data and background value of coal in western Guizhou province, SW, China, and suggested to rework the PGE background values of Chinese coals.     

Late Cretaceous Foraminiferal Faunas from the Saiqu “mélange” in Southern Tibet

As one of the mélanges in the southern side of the Yarlung-Zangbo suture zone, the Saiqu mélange in southern Tibet is important for understanding the evolution of the Neo-Tethys ocean. The age of the Saiqu mélange, however, has been debated due to the lack of reliable fossil evidence in matrix strata. Based on lithological similarities with platform strata in southern Tibet and limited fossils from exotic blocks, previous studies variously ascribed the Saiqu mélange to be Triassic in general, Late Triassic, or Late Cretaceous. Here we reported planktonic foraminiferal faunas from the matrix strata of the Saiqu mélange. The new fossils yield a Late Cretaceous age, which is so far the best age constraint for the mélange. Regional stratigraphic correlation indicates that the Cretaceous Oceanic Red Beds (CORBs) in Saiqu may be time equivalent to the CORBs of the Zongzhuo Formation in neighboring regions. Thus the Saiqu mélange should be correlated to the Upper Cretaceous Zongzhuo Formation rather than the Triassic Xiukang Group, as previously suggested.     

Thermal models of arcs and ridges — A “Source-span-sink” model

A theory of two-dimensional geothermic problems is elaborated by the active temperature function at the vertical contact of two horizontally layered media. The approach offered before for oceanic ridges is extended to the case of continental margins and the upper part of a descending slab, i.e. “sink”, in island-arc areas. It is assumed that the plate motion in the oceanic area exists; in a descending area it is directed downward but remains zero on a continental side. Mathematically it symbolizes a “source—span—sink” thermal model. Numerical parameters are given for a theoretical thermal model of the heat-flow profile across the Kuril island arc, from the trench through Iturup Island, Sakhalin Island and the Tatarian Trough.     

The indigenous origin of Witwatersrand “carbon”

In the Witwatersrand approximately 40% of the gold is intimately associated with so-called “carbon” in “carbon seam reefs”, which occur in over a dozen paleoplacers, many of them concentrated at two stratigraphic levels in the 7000-m-thick succession of Archean siliciclastic sedimentary rocks. This is reduced carbon, present as kerogen admixed in various proportions with derivative (now solid) bitumen(s). Oil generation and migration were active geological processes during Early Earth history. Numerous possible source rocks for oil generation, including the carbon seams themselves, occur within the Witwatersrand basin. In the Witwatersrand ore, oil-bearing fluid inclusions are also present, derived like the bitumen, by thermal maturation of the kerogen. The presence of kerogen and bitumen in the Witwatersrand sedimentary rocks, together with a wealth of observations on the spatial distribution of the carbon seams confirm that the carbon originated in situ from living organisms in microbial mat cover, as opposed to flowing in from elsewhere as liquid hydrocarbons as some researchers have suggested. Paleochannels, which truncated auriferous carbon seams early in the depositional history, are of widespread occurrence, and micro-synsedimentary faults offset carbon seams. The carbon seams are thus indigenous biogenic markers that grew contemporaneously with placer development. The various features highlighting the nature and spatial distribution of Witwatersrand carbon seams provide a classic case where field evidence trumps laboratory data in the reconstruction of geological processes.     

关于“海陆对比”研究的若干实践和思考

通过剖析和总结两个重要的研究经历和学术成果:(1)应用东北印度洋的深海钻心研究喜马拉雅山—青藏高原隆升,(2)应用南海周边陆域的岩浆-沉积记录研究“古南海”的消亡和南海的早期开裂,作者阐释了对于这一特殊的“海陆对比”研究的理解、策划与心得体会。在前一项凭海观山的研究中,首先明确在东北印度洋区存在两类组分、成因各不相同的深海沉积序列,它们分别坐落于孟加拉海底扇和东经90°海岭,在响应山脉隆升的方式上各具优势。经过对两个序列的替代性指标的严格筛选与对比,确定3.6~3.2 Ma和1.0~0.6 Ma是晚中新世以来山脉与高原隆升影响最为深刻的关键时段。在第二项由陆识海的研究中,须要处理的则是较第一项更为复杂的包括岩浆、构造、沉积甚至陆上钻井在内的地质记录,而且其保存条件远逊深海沉积系列。此类研究的优势在于,可以避免单一钻孔记录的局限性,助力研究者在更广阔的区域内综合各种适用的基础材料,构建反映海陆一体化的区域构造演化框架。在华南大陆边缘,存在中生代晚期活动陆缘向新生代被动陆缘的重大构造转换。为建立一个完整的陆缘弧体系,作者于研究区布设了“十字形”考察路线,在东西向追索最初发现于海南的陆缘弧的展布特征,在南北向查明陆缘弧的结构样式,发现在白垩纪中期(110~80 Ma)发生强烈的因板块汇聚而产生的区域隆升,且由南向北隆升强度减弱。经过与同期浙闽陆缘岩浆-沉积记录的综合对比,认为中生代向北俯冲的“古南海”很可能属于业已消亡的特提斯域。中生代末华南陆缘进入全新的发展阶段,三水盆地因展现白垩纪—始新世规模最大且保存最好的岩浆-沉积过程被选为被动陆缘破裂研究的中心地区。古新世晚期(~57 Ma)以碱性玄武岩-粗面岩-钠闪碱流岩为代表的碱性系列双峰式火山喷发活动在研究区兴起,并一直持续到盆地停止发育(42~38 Ma)。实验数据显示:(1)岩浆源区位于软流圈地幔,即使喷发规模最大的粗面岩和碱流岩,也是来自幔源玄武质岩浆的分阶段结晶分异,(2)计算得到的地幔热异常并不明显。作者综合所得结果判断,研究区不存在主导区域构造运动的深源地幔柱,三水盆地发达的火山岩系产出的真实背景在于,中生代晚期的俯冲-碰撞使得岩石圈缩短加厚,于中新生代之交发生拆沉作用和软流圈上涌。这一区域构造环境不仅导致新生代早期的华南裂谷作用,很可能对其后的南海扩张也产生重要影响。现代地球科学将海和陆这两个最大的地理单元紧密地联系在一起,从海洋采集相关的地质信号研究大陆构造,抑或反之,都给我们提供了审视和解决科学问题的新的有效视窗。     

华南晚二叠世碳酸盐岩碳同位素旋回对海平面变化的响应

华南晚二叠世的碳同位素变化曲线是逐渐降低的,与全球海平面变化趋势相一致。华南晚二叠世碳酸盐岩碳同位素旋回可分为两个三级旋回,与两个三级海平面变化响应,其中吴家坪晚期和长兴期组成了一个三级旋回。华南长兴期识别出了三个四级碳同位素地层旋回可能与四级海平面变化旋回相对应。华南晚二叠世碳同位素旋回记录了全球海平面变化的信息:高值区和高海平面相对应,而低值区和低海平面相对应。