青藏高原北部中新统五道梁群湖相沉积碳氧同位素变化及古气候旋回

通过对中新统五道梁群湖相沉积进行全岩心钻探和碳氧同位素测试,获得青藏高原北部中新世早期古环境变化和古气候旋回的重要地质记录.五道梁群约150 m湖相沉积以灰岩、白云质灰岩与泥灰岩为主.仅在深度50.0～51.0 m出现湖相沉积间断,上下均为连续湖相沉积.深度140～145 m湖相沉积碳氧同位素剧烈变化,碳同位素(δ13C/‰)出现2次最低峰值,氧同位素(δ18O/‰)出现2次最高峰值;深度140.7 m湖相沉积碳同位素(δ13C/‰)和氧同位素(δ18O/‰)同时出现极低值.对应于渐新世/中新世界线深海沉积记录的Mi-1全球古气候事件.深度140.7～14.2 m湖相沉积碳氧同位素记录了Mi-1期后7次1.2 Ma天文周期的古气候旋回,深度62.6～9.86 m湖相沉积碳氧同位素记录了9次周期约17.4 ka的古气候旋回.根据湖相沉积碳氧同位素记录的古气候旋回,推断青藏高原北部五道梁盆地中新世早期古大湖发育时期为(24.1±0.6)Ma～(14.5±0.5)Ma,年均气温变化范围为19～21℃,平均约20.0℃.     

咸化湖盆白云岩碳氧同位素特征及古环境意义:以柴西地区始新统下干柴沟组为例

近年来,柴西地区始新统致密碳酸盐岩中的油气勘探获得了重要进展,其中白云岩为最优质的储集岩。系统的岩石学、矿物学和地球化学特征研究表明,该套白云岩主要为泥晶结构,白云石晶间孔广泛发育,孔径多小于1μm。泥晶白云岩的δ13CPDB值变化范围为-4.5‰~0.2‰,平均值为-2.4‰,相对于典型湖相碳酸盐岩偏负,反映了周缘辫状三角洲供给富含轻碳的地表水在一定程度上改变了白云岩的碳同位素组成,且湖水中的溶解碳部分来源于有机质氧化解体产生的CO2,通过改变湖水中溶解碳的碳同位素组成从而间接影响了湖相白云岩的碳同位素组成。泥晶白云岩的δ18OPDB值变化范围为-6.1‰~0.4‰,平均值为-2.6‰,相对于典型湖相碳酸盐岩偏正,明显受咸化湖沉积环境影响。碳氧同位素相关关系投点图及其计算出的盐度、温度数据反映了泥晶白云岩是在准同生阶段、半开放-半封闭的咸化环境下、于水/沉积物界面附近由方解石交代而成的低温白云岩,这类准同生交代成因白云岩在区域上分布较广,且孔径极小的白云石晶间孔抗压实能力强,这一重要发现无论是平面展布上还是纵向深度上均开拓了柴达木盆地致密油勘探的新领域。     

Geochemistry of Miocene lacustrine sediments from the Sokolov Coal Basin (Czech Republic)

Sediments of the upper part of the Sokolov Formation of the Sokolov Brown Coal Basin consist mostly of brown lacustrine clays and claystones enriched in organic matter (2–18% TOC). The lower portion of the stratigraphic profile is formed mostly by kaolinite–illite clays and claystones, while the montmorillonite content increases in the upper portion. The change in the lithology of sediments is caused by the gradual erosion of the weathering crust in the source areas of the clastic material. Inversion of the weathering profile during erosion is manifested geochemically in a decrease in the Al₂O₃/Na₂O ratio and an increase in the SiO₂/Al₂O₃, Na₂O/K₂O and K₂O/TiO₂ ratios from the base to the top of the studied part of the Sokolov Formation. The amounts of trace elements (La, Ce, Nb, Zr, Cr, V, Sn) also decrease in the same direction. On the other hand, the amounts of Rb and Sr increase. The organic matter of the upper part of the Sokolov Formation consists primarily of Types I and II kerogen. Only near the base of the studied part of the formation, i.e., in the coal stringer, kerogen of Type III prevails. The extremely low degree of thermal maturity permits determination of the source of the organic matter and characterization of its accumulation environment. The organic material in the sediments is predominantly of algae origin. Relatively high amount of pentacyclic triterpanes of the hopane series indicates either the occurrence of cyanobacteria as primary producers or bacterial reworking during sedimentation and diagenesis. Organic matter accumulated under the conditions of a fresh-water lake or lake with slightly elevated salinity, in a dysoxic aqueous environment. The alternation of organic carbon-rich layers with layers low in organic matter is explained in terms of changes in the degree of dysoxia of the aqueous environment and a variation in the intensity of oxidation and mineralization of the organic material.     

Palaeoenvironmental reconstruction of a middle Miocene alluvial fan to cyclic shallow lacustrine depositional system in the Calatayud Basin (NE Spain)

ABSTRACT The middle Miocene sedimentary fill of the Calatayud Basin in north‐eastern Spain consists of proximal to distal alluvial fan‐floodplain and shallow lacustrine deposits. Four main facies groups characteristic of different sedimentary environments are recognized: (1) proximal and medial alluvial fan facies that comprise clast‐supported gravel and subordinate sandstone and mudstone, the latter exhibiting incipient pedogenic features; (2) distal alluvial fan facies, formed mainly of massive mudstone, carbonate‐rich palaeosols and local carbonate pond deposits; (3) lake margin facies, which show two distinct lithofacies associations depending on their distribution relative to the alluvial fan system, i.e. front (lithofacies A), comprising massive siliciclastic mudstone and tabular carbonates, or lateral (lithofacies B) showing laminated and/or massive siliciclastic mudstone alternating with tabular and/or laminated carbonate beds; and (4) mudflat–shallow lake facies showing a remarkable cyclical alternation of green‐grey and/or red siliciclastic mudstone units and white dolomitic carbonate beds. The cyclic mudflat–shallow lake succession, as exposed in the Orera composite section (OCS), is dominantly composed of small‐scale mudstone–carbonate/dolomite cycles. The mudstone intervals of the sedimentary cycles are interpreted as a result of sedimentation from suspension by distal sheet floods, the deposits evolving either under subaerial exposure or water‐saturated conditions, depending on their location on the lacustrine mudflat and on climate. The dolomite intervals accumulated during lake‐level highstands with Mg‐rich waters becoming increasingly concentrated. Lowstand to highstand lake‐level changes indicated by the mudstone/dolomite units of the small‐scale cycles reflect a climate control (from dry to wet conditions) on the sedimentation in the area. The spatial distribution of the different lithofacies implies that deposition of the small‐scale cycles took place in a low‐gradient, shallow lake basin located in an interfan zone. The development of the basin was constrained by gradual alluvial fan aggradation. Additional support for the palaeoenvironmental interpretation is derived from the isotopic compositions of carbonates from the various lithofacies that show a wide range of δ¹⁸O and δ¹³C values varying from ?7·9 to 3·0‰ PDB and from ?9·2 to ?1·7‰ PDB respectively. More negative δ¹⁸O and δ¹³C values are from carbonate‐rich palaeosols and lake‐margin carbonates, which extended in front of the alluvial fan systems, whereas more positive values correspond to dolomite beds deposited in the shallow lacustrine environment. The results show a clear trend of δ¹⁸O enrichment in the carbonates from lake margin to the centre of the shallow lake basin, thereby also demonstrating that the lake evolved under hydrologically closed conditions.     

The end of the Messinian Salinity Crisis in the western Mediterranean: Insights from the carbonate platforms of south-eastern Spain

How the Messinian Salinity Crisis (MSC) ended is still a matter of intense debate. The Terminal Carbonate Complex (TCC) is a late Messinian carbonate platform system that recorded western Mediterranean hydrological changes from the final stages of evaporite deposition till the advent of Lago-Mare fresh- to brackish water conditions at the very end of Messinian times. A multidisciplinary study has been carried out in three localities in south-eastern Spain to reconstruct the history of TCC platforms and elucidate their significance in the MSC. Overall, this study provides evidence that the TCC formed following a regional 4th order water level rise and fall concomitant with an opening-restriction trend. It can be subdivided into four 5th order depositional sequences (DS1 to DS4) recording two phases: (1) from DS1 to DS3, a tide-dominated ooidic to oobioclastic system with stenohaline faunas developed as a result of a 70 m water level rise. During this period, the TCC developed in a shallow sea with close to normal marine salinity; (2) in depositional sequence 4, a microbialite-dominated platform system developed. This is indicative of a significant environmental change and is attributed to a 30 to 40 m water level fall in the basins under study. These restricted conditions were coeval with intense evaporite deformation and brine recycling. The syn-sedimentary deformation of evaporites had a major impact on platform architecture and carbonate production, affecting the Messinian series throughout south-eastern Spain at the end of the TCC history. At that time, the TCC developed in a lake with fluctuating, brackish- to hypersaline water. These findings suggest a temporary restoration of marine conditions in the western Mediterranean marginal basins due to Atlantic water influxes prompted by a global sea level rise around 5.6 Ma. Whether marine conditions extended to the entire western Mediterranean still needs to be investigated.     

Deep-water clastic evaporites deposition in the Messinian Adriatic foredeep (northern Apennines,Italy): did the Mediterranean ever dry out?

A new genetic facies model for deep-water clastic evaporites is presented, based on work carried out on the Messinian Gessoso-solfifera Formation of the northern Apennines during the last 15 years. This model is derived from the most recent siliciclastic turbidite models and describes the downcurrent transformations of a parent flow mainly composed of gypsum clasts. The model allows clearer comprehension of processes controlling the production and deposition of clastic evaporites, representing the most common evaporite facies of the northern Apennines, and the definition of the genetic and stratigraphic relationship with primary shallow-water evaporites formed and preserved in marginal settings. Due to the severe recrystallization processes usually affecting these deposits, petrographic and geochemical analyses are needed for a more accurate interpretation of the large spectrum of recognized gravity-driven deposits ranging from debrisflow to low-density turbidites. Almost all the laminar ‘balatino’ gypsum, previously considered a deep-water primary deposit, is here reinterpreted as the fine-grained product of high to low-density gravity flows. Facies associations permit the framing of the distribution of clastic evaporites into the complex tectonically controlled depositional settings of the Apennine foredeep basin. The Messinian Salinity Crisis occurred during an intense phase of geodynamic reorganization of the Mediterranean area that also produced the fragmentation of the former Miocene Apennine foredeep basin. In this area, primary shallow-water evaporites equivalent to the Mediterranean Lower Evaporites, apparently only formed in semi-closed thrust-top basins like the Vena del Gesso Basin. The subsequent uplift and subaerial exposure of such basins ended the evaporite precipitation and promoted a widespread phase of collapse leading to the resedimentation of the evaporites into deeper basins. Vertical facies sequences of clastic evaporites can be interpreted in terms of the complex interplay between the Messinian tectonic evolution of the Apennine thrust belt and related exhumation–erosional processes. The facies model here proposed could be helpful also for better comprehension of other different depositional and geodynamic contexts; the importance of clastic evaporites deposits has been overlooked in the study of other Mediterranean areas. Based on the Apennine basins experience, it is suggested here that evaporites diffused into the deeper portions of the Mediterranean basin may consist mainly of deep-water resedimented deposits rather than shallow-water to supratidal primary evaporites indicative of a complete basin desiccation.     

Messinian palaeoclimate and palaeo-environment in the western Mediterranean realm: insights from the geochemistry of continental deposits of NW Sardinia (Italy)

To evaluate the palaeo-environmental parameters of a portion of the Sardinia–Corsica microplate during the Messinian drop in sea level, we examined the chemistry and mineralogy of upper Tortonian–lower Messinian (late Miocene) clayey continental deposits from NW Sardinia. Differences exist between the uppermost part of the succession, which is devoid of carbonate phases, and the lower part, reflecting changes in provenance and climate. The carbonate-free samples were probably derived from quartzite of the metamorphic basement and were deposited under a climate characterized by alternating dry and relatively wet periods. The other samples were derived from basement phyllite and were deposited under a warm, dry climate that promoted the capillary rise of Ca²⁺ and bicarbonate from a shallow water table, and therefore, the precipitation of carbonate. This part of the succession contains both calcite and dolomite. The presence of barite indicates an important concentration of SO₄ ^2? in the solution from which the CaMg(CO₃)₂ precipitated. The formation of dolomite under hypersaline conditions may be explained by bacterial degradation of organic matter, which produced CO₂ and ammonia, thereby increasing the solution alkalinity. The succession formed in an oxic environment, except for a calcite-rich level that formed under relatively reducing conditions. For this level, the large amount of calcite and the lack of dolomite indicate an alkaline environment and a very low Mg²⁺/Ca²⁺ ratio in the soil solution. These observations, coupled with the reducing conditions, indicate the availability of large amounts of degraded organic matter, probably related to a period typified by a wetter climate.     

南羌塘坳陷扎仁地区中侏罗统布曲组晶粒白云岩成因分析

以青藏高原南羌塘坳陷扎仁地区中侏罗统布曲组晶粒白云岩为对象对其进行成因的研究。通过显微镜观察、流体包裹体数据以及碳氧同位素分析,认为研究区白云岩可分为细粉晶白云岩、中晶白云岩以及粗晶白云岩,在裂隙附近还广泛发育晶粒较粗大的鞍形白云石。白云石中流体包裹体均一温度在150.2～216.0℃,盐度均值达到了24.5%NaCl,远高于方解石包裹体均一温度与盐度,表明白云石的形成经历了高温高盐度的过程。白云石碳氧同位素分析显示其δ~(13)C_(PDB)值为-0.01‰～3.43‰,δ~(18)O_(PDB)值为-11.17‰～-7.68‰,通过白云石-水氧同位素分馏方程得到白云化流体的δ~(18)O_(SMOW)值为4.82‰～12.85‰,δ~(13)C_(PDB)对比认为白云石受寄主灰岩环境的影响。通过碳氧同位素数据对比以及前人的研究结果,认为研究区白云岩为相对封闭环境下受岩浆活动加热的高盐度流体对寄主灰岩交代的产物,高盐度流体由于镁离子的消耗导致流体对方解石过饱和,继而沉淀了高温的方解石。因此,热液活动对研究区中侏罗统布曲组白云岩的发育具有重要意义,值得加强对这一方向的探索研究。     

青藏高原北部新生代湖相碳酸盐岩碳氧同位素特征及古环境意义

对青藏高原北部地区新生代陆相地层中湖相碳酸盐岩进行了系统采样分析，以碳氧同位素作为古环境和古气候变化的替代指标,试图从古湖泊演化的角度，阐明高原新生代早期环境演变历史和过程。研究结果表明，该区所有分析样品中生物碎屑灰岩类样品的δ¹⁸O和δ¹³C显示最低值，反映它们沉积在一个水体滞留时间短的开放性淡水湖泊系统中。泥晶灰岩类样品的δ¹⁸O和δ¹³C之间具有正相关关系，表明它们发育在蒸发作用明显的封闭性咸水湖泊体系中，而叠层石灰岩类明显富集13C可能与微生物活动有关。从始新世到渐新世至中新世，湖相碳酸盐岩δ¹⁸O值具有逐渐增大的趋势，δ¹³C则由古近纪的负值变为新近纪的正值，反映该区古湖泊系统经历了一个由开放到封闭的过程，预示古近纪到新近纪之交高原古气候格局发生了重大变革。     

Petrographic and geochemical evidence for the formation of primary, bacterially induced lacustrine dolomite: La Roda 'white earth' (Pliocene, central Spain)

Upper Pliocene dolomites (‘white earth’) from La Roda, Spain, offer a good opportunity to evaluate the process of dolomite formation in lakes. The relatively young nature of the deposits could allow a link between dolomites precipitated in modern lake systems and those present in older lacustrine formations. The La Roda Mg‐carbonates (dolomite unit) occur as a 3·5‐ to 4‐m‐thick package of poorly indurated, white, massive dolomite beds with interbedded thin deposits of porous carbonate displaying root and desiccation traces as well as local lenticular gypsum moulds. The massive dolomite beds consist mainly of loosely packed 1‐ to 2‐μm‐sized aggregates of dolomite crystals exhibiting poorly developed faces, which usually results in a subrounded morphology of the crystals. Minute rhombs of dolomite are sparse within the aggregates. Both knobbly textures and clumps of spherical bodies covering the crystal surfaces indicate that bacteria were involved in the formation of the dolomites. In addition, aggregates of euhedral dolomite crystals are usually present in some more clayey (sepiolite) interbeds. The thin porous carbonate (mostly dolomite) beds exhibit both euhedral and subrounded, bacterially induced dolomite crystals. The carbonate is mainly Ca‐dolomite (51–54 mol% CaCO₃), showing a low degree of ordering (degree of ordering ranges from 0·27 to 0·48). Calcite is present as a subordinate mineral in some samples. Sr, Mn and Fe contents show very low correlation coefficients with Mg/Ca ratios, whereas SiO₂ and K contents are highly correlated. δ¹⁸O‐ and δ¹³C‐values in dolomites range from ?3·07‰ to 5·40‰ PDB (mean=0·06, σ=1·75) and from ?6·34‰ to ?0·39‰ PDB (mean=?3·55, σ=1·33) respectively. Samples containing significant amounts of both dolomite and calcite do not in general show significant enrichment or depletion in ¹⁸O and ¹³C between the two minerals. The correlation coefficient between δ¹⁸O and δ¹³C for dolomite is extremely low and negative (r=?0·05), whereas it is higher and positive (r=0·47) for calcite. The lacustrine dolomite deposit from La Roda is interpreted mainly as a result of primary precipitation of dolomite in a shallow, hydrologically closed perennial lake. The lake was supplied by highly saturated HCO₃^?/CO₃^2? groundwater that leached dolomitic Mesozoic formations. Precipitation of dolomite from alkaline lake waters took place under a semi‐arid to arid climate. However, according to our isotopic data, strong evaporative conditions were not required for the formation of the La Roda dolomite. A significant contribution by bacteria to the formation of the dolomites is assumed in view of both petrographic and geochemical evidence.     

鄂尔多斯盆地中上三叠统延长组长7段碳酸盐结核成因讨论

鄂尔多斯盆地南部铜川地区露头区中三叠统延长组长7段细砂岩、粉砂岩、泥质粉砂岩、粉砂质泥岩层序中发育有大量圆球状、椭球状、扁球状碳酸盐岩结核,对于其成因存在争议.对铜川地区多个剖面的结核开展宏观描述,在微观薄片观察基础上开展岩石矿物组成、稳定同位素、簇同位素以及微区同位素等分析,特别是选取一个结核开展了详细的解剖.结果显示结核成分以方解石为主,少量白云石;内部结构有差异,在球体横切面上明显分为两个圈层;碳同位素值正偏,由内部圈层向外部圈层降低;氧同位素与碳同位素变化趋势一致,计算结核形成古温度变化区间为28.6～76.5℃;簇同位素揭示结核中部形成温度为42.9℃.认为该类结核为早期发酵带内细菌参与甲烷生成作用形成,甲烷菌活动引起有机质碳同位素分馏,碳酸盐结核中方解石和白云石圆球粒可能是蓝细菌细胞方解石化或白云石化的结果.根据埋藏史,认为该类结核形成于成岩作用早期,后期在成岩过程中叠加生长,同心环层结构明显.     

东地中海黎凡特盆地白垩纪阿尔布期白云岩成因研究

“白云岩问题”一直是碳酸盐岩研究的热点问题。镁作为白云石的主量元素之一,其同位素组成包含了白云岩化过 程和白云岩沉积环境的信息。本文以以色列黎凡特盆地早白垩世末阿尔布期（~100 Ma）Givat Ye’arim组和Soreq组白云岩 为研究对象,在岩石学、微量元素和C-O-Sr同位素地球化学研究基础之上,开展了高密度镁同位素分析。结果显示白云岩 主要由半自形-自形的泥微晶白云石组成,岩性致密,后期成岩改造作用较弱,因此元素及同位素组成基本可以反映原始 白云岩化流体的地球化学特征;C-Sr同位素组成与Albian期海水保持一致,表明同期咸化海水是白云岩化流体并为白云石 形成提供镁离子;白云岩δ26Mg值稳定在-2.0‰左右,没有明显的垂向差异,表明白云岩化过程中咸化海水供应充足,流体 以渗流方式而不是以扩散方式在松散沉积物中运移,孔隙水化学组成相对均一,白云岩镁同位素组成得以与同期海水相平 衡。白云岩中陆源风化指示元素Rb和ΣREE以及海水盐度指标Na元素含量表现出多期旋回性变化,反映半封闭体系对周期 性气候和环境变化的响应,然而垂向上稳定的白云岩δ26Mg值表明区域性的气候变化不会显著改变沉积盆地体系中的镁同位 素组成,因此对于地质历史时期在半局限海洋环境中沉积的巨厚白云岩而言,其镁同位素组成可以用来示踪全球海水镁同 位素波动。     

南海北部中新世以来粘土矿物特征及东亚古季风记录

应用X射线衍射(XRD)、扫描电子显微镜(SEM) 和X射线能谱(EDS) 分析技术对南海北部ODP1146站中新世以来(~20 Ma) 粘土矿物的组成、结晶学特征、微形貌和化学成分进行了研究, 分析了粘土矿物的物质来源及其记录的东亚季风演化历史.1146站粘土矿物组合的总体特点是以伊利石和蒙脱石为主, 高岭石和绿泥石含量较低.物源分析表明, 1146站蒙脱石主要来自于吕宋岛, 伊利石和绿泥石来自于珠江和台湾(长江), 而高岭石则主要来自于珠江.1146站的粘土矿物不仅被南海周围物源的同时代气候所控制, 而且为相互消长的不同传输作用(表层洋流) 的强度所影响.1146站(伊利石+绿泥石) /蒙脱石比值可以用来作为东亚季风演化的矿物学标志.指标变化显示出东亚冬季风强度和冬季风相对夏季风的强度在15 Ma、8 Ma和3 Ma左右发生了3次显著加强, 结果可以与黄土、北太平洋风尘沉积、南海微体古生物记录等很好对比.青藏高原的阶段性隆升可能促进了东亚季风的这3次加强.      

Late Miocene fish otoliths from the Colombacci Formation (Northern Apennines,Italy): implications for the Messinian ‘Lago‐mare’ event

A fish otolith assemblage from the Messinian ‘Lago‐mare’ deposits of the Colombacci Formation cropping out in the Montecalvo in Foglia Syncline, Marche, central Italy, is described. The assemblage displays a low diversity and consists of seven taxa belonging to three families: the Gobiidae, Myctophidae and Sciaenidae. Sciaenid otoliths are the most abundant elements representing 88% of the entire assemblage. The palaeoecological analysis reveals a coastal shallow marine environment strongly influenced by continental outflow. The low diversity and high abundance of the euryecious sciaenids are indicative of a very simplified food web, which probably represented an ecological response to the fluctuating environmental parameters and available food resources. The fish remains documented here provide an unambiguous evidence that normal marine conditions were present in the Mediterranean, at least in the upper part of the ‘Lago‐mare’ event, and unquestionably demonstrate that the marine refilling preceded the Mio‐Pliocene boundary. These findings clearly demonstrate that fishes, because of their mobility and migratory behaviour, represent a useful tool for the large‐scale interpretation of the environmental conditions of the Messinian Mediterranean water body. The necessity of a new scenario of palaeoenvironmental evolution for the post‐evaporitic Messinian of the Mediterranean is also discussed. Copyright © 2006 John Wiley & Sons, Ltd.     

Characteristics of Clay Minerals in the Northern South China Sea and Its Implications for Evolution of East Asian Monsoon since Miocene

Clay mineral assemblages, crystallinity, chemistry, and micromorphology of clay particles in sediments from ODP Site 1146 in the northern South China Sea (SCS) were analyzed, and used to trace sediment sources and obtain proxy records of the past changes in the East Asian monsoon climate since the Miocene, based on a multi-approach, including X-ray diffraction (XRD) and scanning electron microscopy combined with energy dispersive X-ray spectrometry (SEM-EDS). Clay minerals consist mainly of illite and smectite, with associated chlorite and kaolinite. The illite at ODP Site 1146 has very well-to-well crystallinity, and smectite has moderate-to-poor crystallinity. In SEM the smectite particles at ODP Site 1146 often appear cauliflower-like, a typical micromorphology of volcanic smecites. The smectite at ODP Site 1146 is relatively rich in Si element, but poor in Fe, very similar to the smectite from the West Philippine Sea. In contrast, the chemical composition of illite at ODP Site 1146 has no obvious differences from those of the Loess plateau, Yellow River, Yangtze River, and Pearl River. A further study on sediment source indicates that smectite originates mainly from Luzon, kaolinite from the Pearl River, and illite and chlorite from the Pearl River, Taiwan and/or the Yangtze River. The clay mineral assemblages at ODP Site 1146 were not only controlled by continental eathering regimes surrounding the SCS, but also by the changing strength of the transport processes. The ratios of (illite+chlorite)/smectite at ODP Site 1146 were adopted as proxies for the East Asian monsoon evolution. Relatively higher ratios reflect strongly intensified winter monsoon relative to summer monsoon, in contrast, lower ratios indicate a strengthened summer monsoon relative to winter monsoon. The consistent variation of this clay proxy from those of Loess plateau, eolian deposition in the North Pacific, planktonic, benthic foraminifera, and black carbon in the SCS since 20 Ma shows that three profound shifts of the East Asian winter monsoon intensity, and aridity in the Asian inland and the intensity of winter monsoon relative to summer monsoon, occurred at about 15 Ma, 8 Ma, and the younger at about 3 Ma. The phased uplift of the Himalaya-Tibetan plateau may have played a significant role in strengthening the Asian monsoon at 15 Ma, 8 Ma, and 3 Ma.     

辽西下白垩统义县组湖相碳酸盐岩碳氧稳定同位素组成及其沉积环境

通过辽西义县盆地上白垩统义县组湖相碳酸盐岩碳氧稳定同位素组成,分析了赋存热河生物群的义县盆地湖湘沉积环境.该地层序列中碳酸盐岩的δ13C值在-4.825‰～6.870‰之间;δ18O值在-16.190‰～-7.143‰之间,均呈负值,这表明当时水文特征趋于封闭,气候炎热干燥,湖水温度和盐度均较高,火山活动可能是影响义县...     

中国东部寒武系与奥陶系界线地层的碳氧同位素研究

对我国东部不同地层区一些代表性的寒武系-奥陶系界线层位开展了碳?氧同位素研究?结果表明,由于沉积环境差异和成岩蚀变影响,界线层位的δ18O值变化较大,规律性不明显,δ13C值变化虽小,但在界线处均发生不同程度漂移?漂移的层位与寒武纪-奥陶纪主要生物群面貌变化的界线相一致?综合各方面资料分析,笔者认为,碳同位素组成的漂移很可能由晚寒武世到早奥陶世时期内海平面的升降所引起.     

Formation of wollastonite-bearing marbles during late regional metamorphic channelled fluid flow in the Upper Calcsilicate Unit of the Reynolds Range Group,central Australia*

Abstract Granulite facies marbles from the Upper Calcsilicate Unit of the Reynolds Range, central Australia, contain metre-scale wollastonite-bearing layers formed by infiltration of water-rich (X_CO2= 0.1–0.3) fluids close to the peak of regional metamorphism at c. 700° C. Within the wollastonite marbles, zones that contain <10% wollastonite alternate on a millimetre scale with zones containing up to 66% wollastonite. Adjacent wollastonite-free marbles contain up to 11% quartz that is uniformly distributed. This suggests that, although some wollastonite formed by the reaction calcite + quartz = wollastonite + CO₂, the wollastonite-rich zones also underwent silica metasomatism. Time-integrated fluid fluxes required to cause silica metasomatism are one to two orders of magnitude higher than those required to hydrate the rocks, implying that time-integrated fluid fluxes varied markedly on a millimetre scale. Interlayered millimetre -to centimetre-thick marls within the wollastonite marbles contain calcite + quartz without wollastonite. These marls were probably not infiltrated by significant volumes of water-rich fluids, providing further evidence of local fluid channelling. Zones dominated by grandite garnet at the margins of the marl layers and marbles in the wollastonite-bearing rocks probably formed by Fe metasomatism, and may record even higher fluid fluxes. The fluid flow also reset stable isotope ratios. The wollastonite marbles have average calcite (Cc) δ¹⁸O values of 15.4 ± 1.6% that are lower than the average δ¹⁸O(Cc) value of wollastonite-free marbles (c. 17.2 ± 1.2%). δ¹³C(Cc) values for the wollastonite marbles vary from 0.4% to as low as -5.3%, and correlations between δ¹⁸O(Cc) and δ¹³C(Cc) values probably result from the combination of fluid infiltration and devolatilization. Fluids were probably derived from aluminous pegmatites, and the pattern of mineralogical and stable isotope resetting implies that fluid flow was largely parallel to strike.     

塔里木盆地北部下奥陶统碳酸盐岩孔洞充填物特征及其成岩环境分析

为了正确分析和评价碳酸盐岩溶蚀孔洞在埋藏成岩环境中的发育和保存条件,对塔里木盆地北部露头剖面中下奥陶统白云岩及其溶蚀孔洞的充填物进行了详细的矿物学和地球化学研究,试图对这类孔洞充填物沉淀时的成岩环境进行恢复。大型洞穴充填物的矿物学特征和微孔洞充填物扫描电镜的分析表明,此类孔洞的环带状充填物具有明显的世代性,由基岩向孔洞中心,至少可识别出4期充填物,即第1期的乳白色白云石,第2期的铁质浸染的杂色高镁方解石,第3期的无色透明白云石和第4期的自形石英。碳、氧同位素分析结果表明,孔洞充填物形成于高温条件,结合微量元素、稀土元素所示的成岩信息可进一步推断其形成于高温埋藏环境,而非现代表生暴露期的充填物;第1期和第3期白云石充填物形成在相对还原且高盐度的流体性质条件下,而第2期高镁方解石充填物沉淀时相对偏氧化,盐度也偏低。据下奥陶统此类4期充填物的特征分析可知,第2期高镁方解石充填物的发育可能与构造抬升导致成岩环境的改变有关。