Sm-Nd systematics and petrology of postorogenic granitoids in the northern Baltic Shield

In the northern part of the Baltic Shield, quartz diorites, diorites, and monzodiorites compose massifs of postorogenic granites, in which younger granite phases are restricted to their central parts, and dike rocks (aplites, pegmatites, and granite porphyries) occur in the apical parts. The rocks of the Litsa-Araguba Complex (which is located in the northwestern part of the Kola Peninsula and was examined most thoroughly) compose seven intrusions 850 km² in total area, which were formed in mesoabyssal and hypabyssal depth facies. The massifs consist of quartz diorites and monzodiorites dated at 1774 ± 9 Ma, diorites, diorite porphyries, and lamprophyres, which are distinguished as phase 1. The porphyritic and equigranular granites, granodiorites, quartz monzonites, granites, alaskites and related vein leucogranites, pegmatites, and granite porphyries of phases 2 (main), 3, and 4 have an age of 1772–1762 Ma. Data obtained on the Sm-Nd systematics of the rocks indicate that their ?_Nd(1765) values are close to those for rocks of phases 1, 2, and 3 (from ?6.8 to ?8.8) and vary from ?5.0 to ?11.9 for the leucocratic granites of phase 4. The model age values are, respectively, 2.37–2.62 and 2.58–3.23 Ga. These data suggest that the parental melts were of anatectic genesis and were produced by the melting of mostly metasomatically altered garnet granulites from the lower crust. The leucogranites and alaskites of phase 4, which occur as relatively thin bodies in the rocks of the Archean Complex penetrated by the Kola Superdeep Borehole, were derived from a Neoarchean sialic source or produced by the contamination of the parental melts with the material of the Late Archean upper crust. The SHRIMP-II zircon age of the lower crustal migmatized garnet granulites lies within the range of 1831 ± 23 to 1392 ± 21 Ma in the concordia plot. All dates of the rocks are characterized by a unimodal distribution with most values lying within the range of 1650–1800 Ma and approximated by a discordia with T₁ = 1750 ± 30 Ma, MSWD = 3.1. This age value can be interpreted as an averaged age of the lower crustal granitization and corresponds, within the errors, to the age of postorogenic granite intrusions in the upper crust.     

Sm-Nd systematics of a tonalitic augen gneiss and its constituent minerals from northern Michigan

The Sm-Nd isotopic system of a tonalitic augen gneiss and its constituent minerals from northern Michigan was disturbed during metamorphism. Sm-Nd zircon ages are lower than the wholerock Sm-Nd model age. However, closely associated pairs of minerals (for example, sphene and biotite or apatite and plagioclase) retain their apparent metamorphic ages. The Sm-Nd model age for the tonalitic augen gneiss of 3919 ± 30myr, appears to reflect open system behavior during metamorphism. A mineralogically different gneiss from the same location has a Sm-Nd model age of 3520 ± 70 myr. The two whole rocks differ in their Sm-Nd and Rb-Sr systematics and in their chondrite-normalized rare earth element (REE) patterns. The whole-rock-normalized mineral REE patterns show the contribution of the major and trace minerals to the REE content of the whole rock. The trace minerals contain a significant amount of the total REE.     

Current crescents from the Herreria Formation (Lower Cambrian) of northern Spain

Current crescents are well preserved on the upper surfaces of quartzite beds in the Lower Cambrian Herreria Formation of the Cantabrian Mountains in northern Spain. Within a sequence of nearshore deposits, the current crescents are interpreted as having formed by flow separation of wave backwash around obstacles on a beach. The obstacles are not preserved and are inferred to have been perishable organic material, such as stranded algae or jellyfish, which subsequently decayed.     

Lower boundary of the Cambrian

A comprehensive evaluation of the Precambrian — Lower Cambrian stratigraphy of the Siberian Platform. Because of the lack Of trilobites in the basal Lower Cambrian, all available fossil groups were studied (archaeocyathids, stromatolites, problematica, sponges, brachiopods, al, gae, and trilobites). The boundary was placed at the lowest occurrence of skeleton bearing fauna (archaeocyathids, sponges, brachiopods). Below this level the fauna consists principally of stromatolites, problematica, and algae. Comparison of the stratigraphy and fauna of the Siberian Platform is made with the Altay-Sayan folded zone, Western Europe, North Africa, and Australia.—D.W. Trexler     

川北地区下寒武统龙王庙组沉积相 及与储层的关系

以四川盆地北部下寒武统龙王庙组为研究对象,利用野外露头、测井及岩心等资料,探讨了川北地区下寒武统龙王庙组的沉积相类型及特征,并分析了其对储层的影响。研究认为:川北地区在早寒武世龙王庙期的沉积环境为碳酸盐台地,发育潮坪、局限台地和开阔台地等3种相带,这些相带又进一步细分为混积潮坪、台内点滩、潟湖和滩间海等4种亚相。其中,混积潮坪亚相主要发育砂质白云岩坪、白云岩坪等,部分地区可见潮道;台内点滩亚相以砂屑白云岩最为常见,沉积厚度不大;潟湖亚相以白云质潟湖沉积和白云质灰质潟湖沉积为主,分布面积大;滩间海亚相沉积物以深灰色块状石灰岩为主,局部夹白云质豹斑灰岩。纵向上,大部分地区龙王庙组沉积由2次海退旋回构成,潟湖主要发育在单个沉积旋回的中下部;台内点滩主要发育在沉积旋回的中、上部,单个滩体厚度较薄;混积潮坪发育在旋回顶部,即旋回末期。平面上,由于受古陆影响,研究区自西向东具有物源供应减少、沉积水体逐渐变深,以及沉积相从潮坪—局限台地—开阔台地过渡的特征,台内点滩主要围绕局限潟湖分布。结合不同沉积相带储层物性的分析,认为台内点滩的颗粒白云岩储集性能最好,晶粒白云岩次之,膏盐岩最差。龙王庙组储层发育的最有利沉积环境为台内点滩。     

Hopanes and hopenes of the Lower Cambrian Sinyaya Formation (northern slope of the Aldan Anteclise)

Hopane hydrocarbons contained in organic matter from the Lower Cambrian Sinyaya Formation, southeast of the Siberian platform, were analyzed by gas chromatography/mass spectrometry. This study first identified C₂₈ 28, 30-bisnorhopane and its homologues present in high concentrations in autochthonous organic matter from East Siberian sedimentary basins. These biomarkers can be used in distinguishing between the sources of naphthides on the northern slope of the Aldan anteclise. Two saturated naphthenic hydrocarbons, C₃₁ homohopene and C₃₁ 28-norhomohopene, were also detected. This study provides some details on hopane distribution, organic matter accumulation conditions, and the degree of OM catagenesis.     

渝东南-黔北地区下寒武统牛蹄塘组页岩裂缝有效性研究

基于对研究区下寒武统牛蹄塘组页岩岩心裂缝观察描述,利用测井、区域地质资料定量研究裂缝有效性,对其主控因素进行定性分析,结合页岩岩心现场解吸实验,探讨页岩含气量与裂缝有效性关系。结果表明:牛蹄塘组页岩岩心裂缝发育,当裂缝开度大、角度陡倾、延伸较远时,深、浅侧向电阻率值差异明显,裂缝有效性高;若裂缝角度平缓、纵向上延伸较浅、开度小,深、浅侧向电阻率值无明显差异,裂缝有效性差,且多为矿物所封堵。页岩裂缝有效性主要受充填程度、溶蚀作用、异常流体高压、构造活动等因素控制。其中矿物封堵对裂缝有效性造成破坏较大;溶蚀作用和页岩中的异常流体高压对改善页岩孔渗、提高裂缝有效性有积极效果,区域构造活动对不同构造部位裂缝的有效性产生不同影响。有效裂缝有助于吸附态页岩气的解吸,增加游离气含量,成为天然气运移、开采的通道,特别对改善页岩储层渗透率作用更大。然而过大规模尺寸的裂缝会破坏页岩密封性和超压,使天然气散失,不利于页岩气保存。     

黔北下寒武统牛蹄塘组烃源岩的生物标志物 特征和沉积环境

为研究黔北地区下寒武统牛蹄塘组烃源岩母质来源和沉积环境,开展了生物标志化合物分析,结果表明：①正构烷烃OEP接近1.0,无明显奇偶优势,受轻微生物降解影响,正构烷烃多表现为双峰型(C18/C25),(nC21+nC22)/(nC28+nC29)值为0.54~1.01,显示重烃组分占绝对优势;Pr/Ph值范围在0.51~0.82之间,具有植烷优势。②烃源岩样品组成相对丰度五环三萜烷>三环萜烷>四环萜烷,三环萜烷中C21、C23、C24呈倒V字形分布,Ts/(Tm+Ts)为0.45~0.50。③C27规则甾烷/C29规则甾烷略小于1或者接近1,芳烃化合物DBT/P为0.05~0.09。根据这些特征并结合干酪根同位素及正构烷烃同位素组成和变化规律,认为该区域下寒武统牛蹄塘组烃源岩形成于浅海还原环境,且有机质主要来源于细菌、藻类等低等水生生物,有机质热演化程度较高。根据常量、微量元素指标并结合有机地化指标刻划出从灯影组到牛蹄塘组和明心寺组沉积时期的水体环境,将牛蹄塘组烃源岩的形成分为3个阶段,第一阶段为早寒武世初始快速海侵阶段,第二阶段为深水还原高有机质产力阶段,第三阶段为海水缓慢变浅高有机质产力阶段,而到了明心寺组沉积时期为浅水氧化非烃源岩阶段。     

黔北下寒武统牛蹄塘组烃源岩的生物标志物特征和沉积环境

为研究黔北地区下寒武统牛蹄塘组烃源岩母质来源和沉积环境,开展了生物标志化合物分析,结果表明：④正构烷烃OEP接近1．0,无明显奇偶优势,受轻微生物降解影响,正构烷烃多表现为双峰型（c18／c25）,（nC2＋nC22）／（nC28＋nC29）值为0．54～1．01,显示重烃组分占绝对优势;Pr／Ph值范围在0．51-0．82之间,具有植烷优势。②烃源岩样品组成相对丰度五环三萜烷〉5-g,萜烷〉四环萜烷,三环萜烷中C21、C23、C24呈倒V字形分布,Ts／（Tm＋Ts）为0．45-0．50。⑧C27规则甾烷／c∞规则甾烷略小于1或者接近1．芳烃化合物DBT／P为0．05-0．09。根据这些特征并结合干酪根同位素及正构烷烃同位素组成和变化规律,认为该区域下寒武统牛蹄塘组烃源岩形成于浅海还原环境,且有机质主要来源于细菌、藻类等低等水生生物,有机质热演化程度较高。根据常量、微量元素指标并结合有机地化指标刻划出从灯影组到牛蹄塘组和明心寺组沉积时期的水体环境,将牛蹄塘组烃源岩的形成分为3个阶段。第一阶段为早寒武世初始快速海侵阶段,第二阶段为深水还原高有机质产力阶段,第三阶段为海水缓慢变浅高有机质产力阶段,而到了明心寺组沉积时期为浅水氧化非烃源岩阶段。     

山东济南下寒武统朱砂洞组软沉积变形中燧石的Sm-Nd年龄测定及其意义

在济南南部下寒武统朱砂洞组丁家庄段硅质白云岩中,发育地震引起的硅质触变脉、硅质触变沉陷构造和触变楔等软沉积变形构造。从同一地震触变沉陷构造中,采得一组灰黑色燧石样品。使用英国VG354同位素质谱仪和同位素质谱分析方法,测得每个样品的 ¹⁴⁷Sm/¹⁴⁴Nd 和 ¹⁴³Nd/¹⁴⁴Nd 同位素值。采用Sm-Nd 等时线法,对燧石进行了测年。获得燧石的成岩年龄为530.8±6.1 Ma,由于软燧石(硅胶体)成岩至少经历了0.1 Ma的时间,所以,地震沉积事件发生在530.7±6.1 Ma,属早寒武世早期。由于测年结果与沧浪铺阶的下界年龄接近,山东朱砂洞组丁家庄段无化石且河南省标准地层剖面中的整个朱砂洞组属沧浪铺阶,这暗示济南地区朱砂洞组丁家庄段属沧浪铺阶。本次测年得到了鲁西地区第1个下寒武统的同位素年龄值,这对确定该地区早寒武世地震沉积事件发生的时间具有参考价值和意义,也为深入研究鲁西地区寒武纪地层提供了新资料。     

黔北下寒武统黑色岩系的沉积环境与地球化学响应

下寒武统黑色岩系在我国华北、华南及塔里木盆地等广泛发育,但富有机质泥页岩的分布特征及其控制因素未有定论。本文对黔北地区下寒武统牛蹄塘组(及同时期地层)不同沉积相带典型剖面中的富有机质泥页岩及其上下层位进行了地球化学分析,识别出寒武纪早期不同沉积相带氧化还原环境的明显差异。台内凹陷相主要岩性为黑色页岩,底部为不等厚的薄层磷块岩和硅质岩或硅磷质结核,沉积的黑色页岩厚度大,有机质丰度高,V、Ni、Mo、U等微量元素显著富集,反映了贫氧-缺氧环境,以及短暂动态的硫化环境,Mo/TOC反映了其为局限盆地特征;上斜坡相为贫氧到氧化的沉积环境,但更多地是表现为氧化的沉积环境,其中牛蹄塘期存在局部的硫化环境,沉积的黑色页岩厚度小,但有机质丰度最高,V、Ni、Mo、U等微量元素的富集程度较低;盆地相为黑色硅质岩与富有机质的黑色硅质页岩间互沉积,同期沉积厚度较小,有机质丰度相对较低,V和U的富集程度大于台内凹陷相,而Ni和Mo的富集程度则低于台内凹陷相,指示了缺氧环境,期间伴随有贫氧和短暂的氧化环境存在,且可能存在局部的硫化环境,Mo/TOC反映了中等的局限程度。总的来说,黔北下寒武统黑色岩系的有机质丰度...     

川北青川地区下寒武统沥青质砾石的发现及其意义

沥青质砾石被发现于四川省青川县白家镇一带的下寒武统碎屑岩中。黑色的沥青质砾石多呈次圆状或次棱角状,表面光滑但无光泽,与胶结物之间有明显的界线,直径最大可达5cm。推测这些砾石可能来自于被剥蚀的震旦系灯影组。沥青、碳酸盐岩和页岩以砾石的方式沉积于下寒武统．表明在早寒武世筇竹寺期之前川北青川地区已存在有古油藏,前寒武系中可能还有生烃的烃源岩。     

雪峰山隆起北缘热历史及其对牛蹄塘组生烃作用的影响

对于复杂构造带的古老- 深层页岩，烃类的生成过程可能是多阶段的。准确恢复页岩的成熟演化过程是研究页岩气富集机理的先决条件。本文依托雪峰山隆起北缘新钻探的地质调查井，恢复雪峰山北缘古生代以来的热历史，从热演化的角度讨论牛蹄塘组页岩的生烃潜力。元素分析显示，热液活动对牛蹄塘组黑色页岩有机质的富集产生了积极影响，特别是下部页岩热液指示指标异常高、显著的Ce负异常、Eu正异常和Y正异常，都证明了牛蹄塘组早期受热液作用的影响。古温标联合反演结果显示，雪峰山北缘自古生代以来先后经历了三次升温—降温过程。三次热演化高峰依次出现在晚奥陶世末期、中三叠世末期和早白垩世末期，所达到的最高温度依次降低。三次升温过程分别受到早古生代拉张作用和岩浆活动、晚古生代—早中生代快速沉降作用和早白垩世岩浆活动的控制。受沉积埋藏作用和早期热事件的影响，牛蹄塘组页岩在寒武纪—早志留世快速经历了生油高峰、原油裂解高峰等生烃关键时期，在晚奥陶世达到过成熟阶段。随后第一次的抬升剥蚀作用，破坏了页岩及上覆盖层的封闭性，形成有利于气体扩散的裂缝或断层通道，使得早期形成的烃类散失。     

REE chemistry and Sm-Nd systematics of late Archean weathering profiles in the Fortescue Group, Western Australia

Two weathering profiles, each consisting of an upper, sericite-rich zone and a lower, chlorite-rich zone, are preserved between flows of the Mt. Roe Basalt in the Fortescue Group, Hamersley Basin, Western Australia. REE concentrations in samples from these two profiles, which originally developed ca 2,760 Ma, show large variations depending on stratigraphic position. LREE abundances and (La/Yb)N are greatest at depths of 3-6 m below the paleosurface of the Mt. Roe #1 profile and are somewhat lower in samples above this level. The LREEs reach concentrations 6-9 times greater than in the underlying basalt, and thus appear to have been mobilized downward in the paleosol and concentrated in its middle part. LREE concentrations in the #2 profile show a similar distribution but with a sharp increase in all REE concentrations within 50 cm of the paleosurface. The distinction between the REE profiles in the two paleosols may be related to the difference in the overlying material. The #1 paleosol is overlain by a few meters of sediments and then by basalt, whereas the #2 paleosol is directly overlain by basalt. The LREEs appear to have been mobilized both during chemical weathering of the parental basalt and during later lower-greenschist-facies metamorphism and metasomatism of the paleosols. Remobilization of the REEs during the regional metamorphism of the Fortescue Group is confirmed by a whole-rock Sm-Nd reference isochron of Mt. Roe #1 samples with an age of 2,151 +/- 360 Ma. Variable initial 143Nd/144Nd values of unweathered basalt samples which may represent the paleosol protolith prevents a confident determination of the magnitude of LREE mobility. Both the initial mobilization of the REEs during weathering and the metasomatic remobilization appear to have taken place under redox conditions where Ce was present dominantly as Ce3+, because Ce anomalies are not developed within the sericite zone samples regardless of concentration. Europium anomalies in the paleoweathering profile are somewhat variable and were probably modified by mobilization of Eu2+ at metamorphic conditions. In all samples, the HREEs appear to have been relatively immobile and correlate with Al, Ti, Cr, V, Zr, and Nb. Sm-Nd systematics and REE patterns of four unweathered basalt samples indicate derivation of the Mt. Roe Basalts from a heterogeneous and enriched source having epsilon Nd between -4.0 and -7.4. Initial 143Nd/144Nd values of these basalts are even lower than those reported by NELSON et al. (1992) for Fortescue Group basalts and indicate a substantial crustal component in the generation of Mt. Roe Basalts.     

To the problem of stage subdivision of the Lower Cambrian

It was a serious mistake when subdivision stratotypes had been disposed of in practice of constructing the General stratigraphic scale, because they cannot be substituted by the boundary stratotypes. A subdivision stratotype and respective GSSP are complementary parts characterizing any stratigraphic unit. The best candidates for stage stratotypes of the Lower Cambrian are certainly sections of the Lena-Aldan region in the Siberian platform, which have been studied in detail during many years, are well exposed, and contain abundant and diverse fossils, being insignificantly disturbed by tectonic dislocations. Nomenclature, stage and boundary stratotypes of the Siberian standard can be successfully used therefore for stratigraphic subdivision of the Lower Cambrian.     

The age of the Stillwater complex—a comparison of U-Pb zircon and Sm-Nd isochron systematics

A zircon U-Pb age of 2713 ± 3 Myr confirms the less precise age of about 2710 Myr (corrected for new decay constants of Jaffeyet al., 1971) obtained from the same sample of zircon from the Stillwater complex chill zone (Nunes and Tilton, 1971). This age compares with Sm-Nd mineral and whole-rock isochron ages of 2701 ± 8 Myr (Depaolo and Wasserburg, 1979); or 2706 ± 8 Myr if the Lugmairet al. (1975) normalization procedure is used. The agreement of these ages to within about 0.4% indicates that the λ₁₄₇ = 0.00654 × 10^?9yr^?1 value is less than 0.8% too large relative to the U decay constants determined by Jaffeyet al. (1971) and enables more correct geological syntheses to be made when working out detailed absolute age stratigraphies using precise data from both systems.     

First occurrences of Lapworthella in Lower Cambrian of the Siberian Platform

Two new species of Lapworthella are described from abundant silicified specimens from the Lower Cambrian Tyuser Shaya strata along the Lena river in northern Kharaulakh. Based on shape and size of shell, shell structure and composition (calcium phosphate), and character and arrangement of sculptural elements, Stenothecopsis schodackensis Lochman (1956) and several species of Stenothecopsis described by Poulsen (1942) are placed in synonymy with Lapworthella. The family Lapworthellidae Missarzlaevskiy, fain. nov. is proposed to include Lapworthella and Stenothecopsis. The two genera are removed from Crustacea (Cobbold, 1921, 1935) and placed in the unassigned order Hyolithelminthes Fisher, 1962.—M. E. Taylor.     

湘中地区下寒武统烃源岩沉积环境及其特征

本文通过对湘中地区早寒武世岩相古地理的研究,认为早寒武世期间,湘中地区为发育在陆架之上的缺氧滞留盆地沉积环境,沉积了一套以黑色炭质板状页岩及含硅质炭质泥岩等为主的黑色沉积。该套黑色沉积沿着湘中地区以及扬子东南缘呈带状稳定分布,反映出当时湘中地区是一"富烃沉积区",具有生烃潜力;该套寒武纪海相地层是油气勘探的重要层位。