Neoproterozoic glacial and gravitational sedimentation on a continental rifted margin: The Jequitaı́–Macaúbas sequence (Minas Gerais,Brazil)

This paper is a contribution to the knowledge of the sedimentation of Neoproterozoic sequences, known as the Jequitaı́ Formation and Macaúbas Group. These sequences are present along the transitional zone between the São Francisco Craton and the Brasiliano (≌600 Ma) Araçuaı́ fold belt in Minas Gerais, Brazil. A sedimentological study of these Neoproterozoic sequences enables us to distinguish between true continental and marine glacial facies and glacial material reworked by various subaqueous gravitational processes. The cratonic Jequitaı́ Formation consists of massive and stratified diamictites up to 100 m thick. This diamictite association is tentatively interpreted as glaciomarine in origin. It continues eastward, in the Araçuaı́ fold belt, as the metasedimentary Macaúbas Group, which is composed of metadiamictites, quartzites and schists from 5–12(?)km thick. The Macaúbas Group consists of resedimented glacial material deposited by subaqueous debris flows and turbidity currents. A depositional model is proposed for the Jequitaı́–Macaúbas glacial/gravitational sequence. From west to east, a glaciomarine sequence, possibly deposited from an ice-sheet and slightly reworked by gravitational processes, was reworked along the São Francisco cratonic border and generated a slope apron system made up of diamictites associated with turbidites and rhythmites.     

Neoproterozoic glacial deposits from the Araçuaí orogen,Brazil: Age,provenance and correlations with the São Francisco craton and West Congo belt

Glacigenic diamictite successions of the Macaúbas Group are widespread in the western domain of the Araçuaí orogen, east of the São Francisco craton (Brazil). Diamictites also occur on this craton and in the African counterpart of the Araçuaí orogen, the West Congo belt. Detrital zircon grains from the matrix of diamictites and sandstones from the Macaúbas Group were dated by the U–Pb SHRIMP technique. The geochronological study sets the maximum depositional age of the glacial diamictites at 900 Ma, and indicates multiple sources for the Macaúbas basin with ages ranging from 900 to 2800 Ma. Sm–Nd T_DM model ages, determined on whole rock samples, range from 1.8 Ga to 2.5 Ga and get older up-section. Comparison of our data with those from the cratonic area suggest that these glacial deposits can be correlated to the Jequitaí and Carrancas diamictites in the São Francisco craton, and to the Lower Mixtite Formation of the West Congolian Group, exposed in Africa. The 900–1000 Ma source is most probably represented by the Zadinian–Mayumbian volcanic rocks and related granites from the West Congo belt. However, one of the most voluminous sources, with ages in the 1.1–1.3 Ga interval, has not been detected in the São Francisco-Congo craton. Possible sources for these grains could occur elsewhere in Africa, or possibly from within the Brasília Belt in western central Brazil.     

Global nature of the Paleoproterozoic Lomagundi carbon isotope excursion: A review of occurrences in Brazil,India, and Uruguay

Positive carbon isotope excursion is reported from Paleoproterozoic carbonates of the Aravalli Supergroup (northwestern India), the Minas Supergroup (Brazil), and new sections of the Paso Severino Formation (Uruguay). The 2.42 Ga Gandarela Formation, Minas Gerais, Brazil, contains red carbonate-facies BIF grading into dolostones and limestones and yielding δ¹³C values ranging from −1.6 to +0.4‰ V-PDB. The positive C-isotope excursion (up to + 11‰ V-PDB) in marine shallow-water carbonates in India and Brazil (Jhamarkotra Formation in northwestern India, and Cercadinho and Fecho do Funil formations in Minas Gerais State, Brazil) is comparable to that observed in 2.22–2.1 Ga carbonate successions worldwide that were deposited during the Lomagundi excursion. In Uruguay, δ¹³C values up to +11.6‰ V-PDB in the deep-water Paso Severino Formation of the Piedra Alta Terrane are compatible with deposition at ca. 2.15 Ga, as indicated by the 2146 ± 7 Ma U–Pb age of dacites occurring at the top of the unit. Negative δ¹³C values are also present in carbonates of the Paso Severino Formation, but an origin related to organic-matter remineralization cannot be ruled out. Thin carbonate beds in the Rio Itapicuru greenstone belt, Bahia State, Brazil, are associated, as in the Paso Severino Formation, with deep-water black shales and have carbon isotope values up to +9‰ V-PDB. High metamorphic grade carbonates of the Jacurici terrane in the Medrado-Ipueira area, Bahia, Brazil, have carbon isotope values up to +6.9‰ V-PDB, consistent with their minimum age of 2085 ± 5 Ma inferred from the intrusive contact with and the age of the Medrado norite. No evidence was found in India, Brazil, or Uruguay for Paleoproterozoic glacial events recognized in the 2.45–2.22 Ga sedimentary successions worldwide. Unconformities between the Gandarela and Cercadinho formations in Brazil and the banded gneissic Complex and the Lower Aravalli Supergroup in India might explain the absence of glacial record. Compositional and isotopic data presented here for studied Paleoproterozoic carbonate successions allow their integration into the global record of the Paleoproterozoic evolution as well as correlation with other successions of similar age. The study highlights the global nature of the Lomagundi excursion. Furthermore, it indicates that the Lomagundi excursion is recorded in both shallow-water (Aravalli and Minas supergroups) and deep-water carbonates (Paso Severino Formation and Rio Itapicuru greenstone belt) negating a significant impact of stromatolite productivity and hypersaline conditions on carbon isotope values of carbonates deposited in shallow-water, open-marine and isolated basins.     

A review of the stratigraphy of Marwar Supergroup of west-central Rajasthan

The lithostratigraphy, depositional environment and age of the Marwar Supergroup have been reviewed in the light of report of δ¹³C depletion recorded in the carbonates of the Bilara Group (middle part of Marwar Supergroup) and discovery of trilobite-like trace fossils from the ·Red bedsŽ of Nagaur Group (upper part of Marwar Supergroup). The δ¹³C depletion observed in Bilara carbonates is not a result of glaciation rather due to rapid burial and poor water circulation in the low energy water of the protected basin. Secondly, the trace fossils are, in fact, traces of notostracan crustaceans found in shallow fluvial and shallow lacustrine environment. The present paper also records a spiral, burrowing trace-fossil, possibly Gyrolithes, from a cross-bedded sandstone of the Jodhpur Group.     

Provenance shift through time in superposed basins: From Early Cryogenian glaciomarine to Late Ediacaran orogenic sedimentations (Araçuaí Orogen,SE Brazil)

Records of Precambrian glaciation have been reported from southeastern Brazil for over a century. We present sedimentological, stratigraphic and isotopic (U-Pb and Lu-Hf on detrital zircons, C and O on carbonates) studies on diamictite-rich to diamictite-free successions of the Araçuaí Orogen, the Brazilian counterpart of the Araçuaí – West Congo Orogenic System (AWCO). From base to top, the Chapada Acauã Formation (Macaúbas Group) includes a diamictite-rich unit, with lenses of graded sandstone and clast-supported conglomerate, that gradually passes to graded sandstone, pelite with sparse oversized clasts and rare carbonate lenses on top, representing mass flow and turbidity current deposits of submarine fan, followed by finer-grained turbiditic sedimentation with iceberg discharges along fan fringes, from glaciomarine to post-glacial scenarios. The Salinas Formation comprises a deep-sea sand-mud sequence composed of pelites and pelitic wackes. The Chapada Acauã Formation was deposited in-between 750 and 667 Ma. It shows wide spectra of zircon ages (typical of continental rift to passive margin settings) that start around 3.2 Ga and display main age peaks indicating sediment provenance from the Rhyacian-Orosirian basement (εHf_(t) = +14.6 to −18.5) and Early Tonian anorogenic rocks (950–880 Ma; εHf_(t) = −3.2 to −23.2) for the diamictite-rich lower unit and, for the upper unit, also in Early Cryogenian anorogenic rocks (ca. 715 Ma, εHf_(t) = −9.6). In contrast, the Salinas Formation shows most ages in-between 676 and 620 Ma and a maximum sedimentation age around 551 Ma, with εHf_(t) from +6.9 to −18.2, unravelling an important shifting of sediment provenance to sources located in magmatic arcs and collisional granites of the Araçuaí and Ribeira orogens. The Early Cryogenian age and negative Hf signature for the glaciomarine Chapada Acauã Formation point to sediment sources in the 720–670 Ma anorogenic igneous rocks found in AWCO and adjacent Congo – São Francisco Craton, and suggest correlation with the Sturtian global glaciation.     

Invertebrate traces in pseudo–coprolites from the upper Cretaceous Marília Formation (Bauru Group), Minas Gerais State,Brazil

Pseudo–coprolites are inorganic structures often confused with fossil faeces. The absence of some diagnostic features, such as inclusions, coprofabrics, grain adhesion, and defined shape, suffices to disregard these structures as coprolites. Herein we revise the so–called “coprolites” from the Serra da Galga Member of the Marília Formation (Maastrichtian of Bauru Group, Paraná Basin), at “Ponto 1 do Price” locality near the town of Peirópolis (Uberaba municipality, Minas Gerais State, Brazil) and conclude that they are, in fact, pseudo–coprolites related to calcretes. These data also agree with the geological setting of “Ponto 1 do Price”, composed mainly of coarse sandstones and conglomerates, in which these pseudo–coprolites were found. In addition, some of these specimens exhibit superficial traces, here described as a new boring ichnospecies, Asthenopodichnium fallax isp. nov., produced by invertebrates in Late Cretaceous fresh–water settings of Brazil.     

Pleistocene chronology: Long or short?

European geologists, in general, have tended to favor a “short” chronology for the glacial Pleistocene, with four major glacial cycles in the past 500,000 or 600,000 yr. Interpretation of ocean floor sediments by Emiliani and others has accorded with this view, in contrast to the “long” chronology of Ericson and Wollin and their followers, who spread the four North American glacial episodes over a 2-m.y. period. An examination of the available radiometric dates and age estimates from paleomagnetic polarity zones serves to confirm Richmond's view that the four major European glacials do not equate with the four North American glacials in a simple one-to-one manner, but that the Illinoian matches the Elster (Mindel) rather than the Saale (Riss). The Alpine Günz is then equated broadly with the Kansan and overlaps in time with the Jaramillo normal polarity event at about 900,000 y.a. The Nebraskan is older than 1.2 m.y. and is thus coeval with the European Upper Villafranchian, within which the Donau and Biber glacial events may be traced. Montane glaciation certainly extended back into the Tertiary but cold pulses of sufficient duration to produce continental glaciation were more marked through the past 1.5 m.y. More critical studies of the terrestrial record are needed before firm correlations can be made.     

Trilobite Biostratigraphy of the lower Paleozoic (Cambrian–Ordovician) Joseon Supergroup, Taebaeksan Basin, Korea

In Korea,trilobites are among the most intensively studied fossil groups in the past century and provide invaluable information about lower Paleozoic stratigraphy,paleogeography,and tectonics of the Korean Peninsula. Trilobites occur in the lower Paleozoic Joseon Supergroup of the Taebaeksan Basin which was part of the Sino-Korean Craton in the Paleozoic. The Joseon Supergroup is divided into the Taebaek,Yeongwol,and Mungyeong groups. The Taebaek and Yeongwol groups are richly fossiliferous,while the Mungyeong Group is poorly fossiliferous. Contrasting trilobite faunal contents of the Taebaek and Yeongwol groups resulted in two separate biostratigraphic schemes for the Cambrian–Ordovician of the Taebaeksan Basin. A total of 22 biozones or fossiliferous horizons were recognized in the Taebaek Group; 19 zones were established in the Yeongwol Group; and four biozones were known from the Mungyeong Group. These trilobite biozones of the Taebaeksan Basin indicate the Joseon Supergroup ranges in age from the Cambrian Series 2 to Middle Ordovician and can be correlated well with the formations of North China,South China,and Australia.     

The Weichselian glaciation in Svalbard before 15,000 B.P.*

Th/U dating and radiocarbon dating of 'old' shells are discussed, and amino acid ratios from shells are used as a method of relative-age dating. The Svalbard area has been completely covered by an extensive ice sheet at leats once. New data from Sjuøyane indicate that such glaciation took place in the Early Weichselian. The Middle Weichselian was a period of interstadial conditions. Series of beaches of assumed Middle Weichselian age occur in several places in western Spitsbergen while no such beaches are known in the eastern part of the archipelago. The maximum glaciation in the Late Weichselian is assumed to have taken place about 18,000 B.P. In the western part of Spitsbergen, the Late Weichselian glaciation was limited and local, while the eastern part of the archipelago was covered by an ice sheet. Kongsøya has a pattern of Holocene shoreline displacement which indicates that the centre of this ice sheet was east of kong karts Land.     

Characterization and origin of spongillite-hosting sediment from João Pinheiro,Minas Gerais,Brazil

Spongillite from João Pinheiro, Minas Gerais, Brazil is mainly known for its use in brick production and in the refractory industry. Very few studies have focused on its geological context. Spongillite-rich deposits occur in shallow ponds on a karstic planation surface developed on rocks of the Neoproterozoic São Francisco Supergroup. Cenozoic siliciclastic sediments are related to this surface. A field study of these deposits and analysis of multispectral images showed a SE–NW preferential drainage system at SE, suggesting that Mesozoic Areado Group sandstones were the source area of the spongillite-hosting sediments. Mineralogical and textural characterization by optical microscopic analysis, X-ray diffraction (XRD), differential and gravimetric thermal analysis (DTA-GTA), infrared spectroscopy (IR) and scanning electron microscopy (SEM) of seven open-pit spongillite-rich deposits (Avião, Carvoeiro, Vânio, Preguiça, Divisa, Severino, Feijão) showed a sedimentological similarity between the deposits. They are lens-shaped and are characterized at the bottom by sand facies, in the middle by spicules-rich muddy-sand facies and at the top by organic matter-rich muddy-sand facies.Petrographically, the spongillite-hosting sediments and the siliclastic sediments of the Areado Group show detrital phases with similar mineralogical and textural features, such as the presence of well-sorted quartz grains and surface features of abrasion typical of aeolian reworking that occurred in the depositional environment in which the sandstones of the Areado Group were formed. Detrital heavy minerals, such as staurolite, zircon, tourmaline, and clay minerals, such as kaolinite, low amounts of illite, scarce chlorite and mixed-layer chlorite/smectite and illite/smectite occur in the spongillite-hosting sediments and in sandstones from the Areado Group. In both formations, staurolite has similar chemical composition. These mineralogical and textural features show that the sediments of the Areado Group constitute the main source of the pond sediments that host spongillite.     

五台山西南麓“龙泉关群”地层、构造特征及有关问题的讨论

根据造山带地层学地层划分原则,“龙泉关群”可划分为两个性质截然不同的地层单元:其一是“龙泉关构造岩层”,为构造地层单元;其二是“跑泉厂变沉积岩组”,属经典地层单元。前者经历了至少两期变形、变质作用,保留着走向近SN和近EW两组矿物拉伸线理以及角闪岩相和角闪岩相→绿片岩相退变质特征;后者仅发育有一组走向近SN矿物拉伸线理和一期绿片岩相变质作用。这表明:(1)“龙泉关群”实际上包含着两个世代不同的地层单元,应予解体。其中,“龙泉关构造岩层”形成于晚太古代末期,属于“阜平古陆块”刚性基底一部分;“跑泉厂变沉积岩组”的地层层位相当于“五台群”,是“阜平古陆块”西北陆缘带沉积,时代归属早元古代早、中期。(2)“龙泉关群”不是由一个统一的应力场同时形成的一套构造岩,因而不应将其作为一个大型韧性剪切带。(3)“龙泉关群”构成五台碰撞造山带前陆地区的活化基底和活化盖层两个大地构造相。“龙泉关群”的解体及其地层时代的重新厘定支持“板峪口组应归属滹沱群一部分”和“铁堡运动即为五台运动”的看法。     

The Archean BIF-hosted Cuiabá Gold deposit,Quadrilátero Ferrífero,Minas Gerais,Brazil

The Cuiabá Gold Deposit is located in the northern part of the Quadrilátero Ferrífero, Minas Gerais State, Brazil. The region constitutes an Archean granite–greenstone terrane composed of a basement complex (ca. 3.2 Ga), the Rio das Velhas Supergroup greenstone sequence, and related granitoids (3.0–2.7 Ga), which are overlain by the Proterozoic supracrustal sequences of the Minas (< 2.6–2.1  Ga) and Espinhaço (1.7 Ga) supergroups.The stratigraphy of the Cuiabá area is part of the Nova Lima Group, which forms the lower part of the Rio das Velhas Supergroup. The lithological succession of the mine area comprises, from bottom to top, lower mafic metavolcanics intercalated with carbonaceous metasedimentary rocks, the gold-bearing Cuiabá-Banded Iron Formation (BIF), upper mafic metavolcanics and volcanoclastics and metasedimentary rocks. The metamorphism reached the greenschist facies. Tectonic structures of the deposit area are genetically related to deformation phases D₁, D₂, D₃, which took place under crustal compression representing one progressive deformational event (E_n).The bulk of the economic-grade gold mineralization is related to six main ore shoots, contained within the Cuiabá BIF horizon, which range in thickness between 1 and 6 m. The BIF-hosted gold orebodies (> 4 ppm Au) represent sulfide-rich segments of the Cuiabá BIF, which grade laterally into non-economic mineralized or barren iron formation. Transitions from sulfide-rich to sulfide-poor BIF are indicated by decreasing gold grades from over 60 ppm to values below the fire assay detection limit in sulfide-poor portions. The deposit is “gold-only”, and shows a characteristic association of Au with Ag, As, Sb and low base-metal contents. The gold is fine grained (up to 60 μm), and is generally associated with sulfide layers, occurring as inclusions, in fractures or along grain boundaries of pyrite, the predominant sulfide mineral (> 90 vol.%). Gold is characterized by an average fineness of 0.840 and a large range of fineness (0.759 to 0.941).The country rocks to the mineralized BIF show strong sericite, carbonate and chlorite alteration, typical of greenschist facies metamorphic conditions. Textures observed on microscopic to mine scales indicate that the mineralized Cuiabá BIF is the result of sulfidation involving pervasive replacement of Fe-carbonates (siderite–ankerite) by Fe-sulfides. Gold mineralization at Cuiabá shows various features reported for Archean gold–lode deposits including the: (1) association of gold mineralization with Fe-rich host rocks; (2) strong structural control of the gold orebodies, showing remarkable down-plunge continuity (> 3 km) relative to strike length and width (up to 20 m); (3) epigenetic nature of the mineralization, with sulfidation as the major wall–rock alteration and directly associated with gold deposition; (4) geochemical signature, with mineralization showing consistent metal associations (Au–Ag–As–Sb and low base metal), which is compatible with metamorphic fluids.     

The succession of the Lower Old Red Sandstone (Siluro-Devonian) along the Ross-Tewkesbury Spur Motorway (M.50), Hereford and Worcester

The motorway cuts approximately 1900 m of strata, of which about 56 per cent are continuously exposed in numerous long sections, a new standard being afforded for the Lower Old Red Sandstone in the southern Welsh Borders. The succession recognized is: LOWER OLD RED SANDSTONE Brownstones (L. Dev.) at least 795 m St. Maughan's Group (L. Dev.) 630 m Raglan Marl Group (Siluro-Dev.) 385 m Clifford's Mesne Sandstone (Sil.) at least 9 m LUDLOW SERIES (Silurian) at least 5 m The uppermost Ludlow Series and the Clifford's Mesne Sandstone are exposed in the core of the May Hill-Woolhope upfold. They are overlain in the complimentary syncline to the west by the Raglan Marl Group, St. Maughan's Group and Brownstones, in ascending order. Above the Clifford's Mesne Sandstone, there is a progressive upward increase in the number and thickness of sandstone relative to mudstone beds. Pedogenic limestones are largely concentrated in two short stratigraphic intervals. The lowest, mainly in the uppermost Raglan Marl Group but reaching into the St. Maughan's Group, includes the “Psammosteus” Limestone recognised elsewhere in the area. The second concentration occurs in the uppermost St. Maughan's Group. Vertebrates in the Raglan Marl Group and the St. Maughan's Group allow the Downtonian and Dittonian “stages” to be recognized.     

辽河群时代探讨

笔者通过参加编写辽宁早前寒武纪地层典,对近年来争论的辽河群时代问题有一点粗浅的看法。研究和解决辽河群时代问题,对探讨辽东前寒武纪构造演化具有一定的实际意义。因此,本文拟就这一问题作一初步讨论。一、辽河群的由来和发展辽东前寒武纪地层最初被划分为太古界片麻岩结晶片岩和前寒武纪层,或者是太古界片麻结晶岩片岩和前寒武纪下部层与前寒武纪上部层。嗣后,斋藤林次创建辽河系,指出辽河系相当于前寒武纪下部层。辽河群是由辽河系演变来的,其发生和发展的主要过程见表1。     

郯庐断裂带南段张八岭群变质岩的原岩时代及其构造意义

大别造山带东缘郯庐断裂带上分布着绿片岩相变质的张八岭群。对于它们的原岩时代长期没有同位素年代学数据,而其变形与变质原因也一直没有明确的认识。本次工作中选择了该带上8处张八岭群变火山岩进行了锆石LA-ICP-MS U-Pb定年。结果表明,它们的原岩时代为748~750 Ma,属于新元古代中期的南华纪,为扬子板块下部盖层而非前人认为的变质基底。结合张八岭群的变形与变质特征及前人白云母40Ar/39Ar定年结果,并与大别造山带进行对比,本文认为大别造山带东南缘张八岭群的变形与变质是造山带内俯冲与折返的结果,而其东缘郯庐断裂带内张八岭群的变形与变质是碰撞造山期该断裂带左行走滑活动所致。这些认识再次为郯庐断裂带起源于华北与扬子板块的碰撞过程中提供了重要的证据,也支持其造山期起源于陆内转换断层或斜向汇聚边界。     

Iron-formation and glaciogenic rocks of the Rapitan Group,Northwest Territories,Canada

In northwestern Canada, iron-formation occurs as part of the Rapitan Group, a dominantly sedimentary succession of probable Late Precambrian age. The Rapitan Group contains abundant evidence of glaciogenic deposition. It includes massive mixtites which contain numerous faceted and striated clasts. Finely bedded and laminated sedimentary rocks of the Lower Rapitan contain many large isolated (ice-rafted?) intra- and extra-basinal clasts. The Lower and Middle Rapitan are interpreted as products of a glacial marine regime. The iron-formation is interbedded with thin mixtite beds and contains large exotic clasts which are probably indicative of the existence of floating ice at the time of deposition of at least part of the iron-formation. If the apparently low paleolatitudes are confirmed, then glacial marine interpretation of the Rapitan, and the probably correlative Toby Conglomerate of southern British Columbia, support the postulate of a very extensive Late Precambrian ice sheet in North America.Similar iron-formations of similar age are present in South America (Jacadigo Series), in South-West Africa (Damara Supergroup) and in South Australia (Yudnamutana Sub-Group). All of these iron-formations are associated with glaciogenic rocks. In addition to the iron-formations, dolostones, limestones and evaporites (?) are intimately associated with Late Precambrian mixtites, considered by many to be glaciogenic.Huronian (Early Proterozoic) and correlative sequences of North America, and rocks of similar age in South Africa also contain closely juxtaposed undoubted glaciogenic rocks, iron-formations, dolostones and aluminous quartzites. The dolostones and aluminous sedimentary rocks have been interpreted as having formed under warm climatic conditions, but might also be explained by invoking higher P_CO2 levels in the Early Proterozoic atmosphere. By analogy with the Huronian succession, preservation of “warm climate” indicators in mixtite-bearing Late Precambrian sequences does not preclude a glacial origin for the mixtites.     

The Statherian itabirite-bearing sequence from the Morro Escuro Ridge,Santa Maria de Itabira,Minas Gerais,Brazil

The itabirite-bearing metasedimentary sequence from Morro Escuro Ridge comprises the basal units of the Espinhaço Supergroup and makes up a small tectonic inlier developed during one of the Brasiliano orogenic events (800-500 Ma), amongst horses of the Archean TTG gneisses, including sheared granites of the anorogenic Borrachudos Suite (∼1700 Ma). The metasedimentary rocks are comprised of low-to intermediate-amphibolite facies schists, quartzites, conglomerates and banded iron formation (itabirite) correlatable with the sequences of the Serro Group, which underlies the metasedimentary rocks of the Espinhaço Supergroup in the Serra da Serpentina Ridge. A maximum Statherian deposition age (1668 Ma) was established using SHRIMP U–Pb isotopic constraints on zircon grains from conglomerate and quartzite units overlying the itabirite. The itabirite is predominantly hematitic and its geochemical characteristics are typical of a Lake Superior-type BIF deposited in a platformal, suboxic to anoxic environment distant from Fe-bearing hydrothermal vents. Close to the contact zone with amphibolites of the Early Neoproterozoic Pedro Lessa mafic suite, an increase of the magnetite content and crystallization of metasomatic Mg-hornblende and Ce-allanite can be observed. These mineralogical changes developed preferentially along the igneous contact zone but are probably co-genetic with the formation of alteration haloes in zircon grains during the Neoproterozoic Brasiliano orogeny (506 ± 6 Ma).     

Composition and genesis of Precambrian metalliferous conglomerates

Precambrian metalliferous conglomerates are the most important source of gold, uranium, and other metals. They concentrate no less than 30% of world gold reserves and provide 30–50% of world gold production. The metalliferous conglomerates are known at various chronological levels of the Early Precambrian: the Neoarchean (Witwatersrand Supergroup, South Africa), the Neoarchean-Paleoproterozoic (Huronian Supergroup, Canada), and the Paleoproterozoic (Tarkwaian Group, West Africa; Roraima Group, the Guiana Shield; Jacobina and Sierra de Carrego groups, the Brazil Shield; Mount Bruce Group, West Australian Shield). They are related to different stages of the tectonic evolution: preorogenic stage (Huronian Supergroup), orogenic stage (Tarkwaian Group), and postorogenic or protoplatformal stage (Witwatersrand). Long-term stabilization of the Earth’s crust and deposition of thick sedimentary sequences were the most favorable conditions for the formation of metalliferous conglomerates.     

Depositional and tectonic setting of the Paleoproterozoic Lower Aillik Group,Makkovik Province,Canada: evolution of a passive margin-foredeep sequence based on petrochemistry and U–Pb (TIMS and LAM-ICP-MS) geochronology

The Paleoproterozoic Lower Aillik Group is a deformed metasedimentary–metavolcanic succession located in the Makkovik Province of Labrador, eastern Canada. The group is situated near the boundary between reworked Archaean gneiss of the Nain (North Atlantic) craton and juvenile Paleoproterozoic crust that was both tectonically accreted and formed on or adjacent to this craton during the ca. 1.9–1.78 Ma Makkovikian orogeny. The Lower Aillik Group is structurally underlain by Archaean gneiss and structurally overlain by ca. 1860–1807 Ma bimodal, dominantly felsic volcanic and volcaniclastic rocks of the Upper Aillik Group. We present geochemical data from metavolcanic rocks and U–Pb geochronological data from several units of the Lower Aillik Group in order to address the depositional and tectonic history of this group. U–Pb data were obtained using both thermal ionization mass spectrometry (TIMS) and laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS). Two quartzite units near the structural base of the Lower Aillik Group contain detrital zircons only of Archaean age, and are interpreted to have been deposited on the Nain craton during post-2235 Ma rifting and initiation of a passive continental margin. Overlying mafic metavolcanic rocks contain thin horizons of intermediate tuff, one of which is dated at 2178±4 Ma. This relatively old age, and an inferred stratigraphic relationship with underlying sedimentary units, suggest that the volcanic rocks represent transitional oceanic crust, consistent with their geochemical similarity to tholeiitic rifted margin sequences of Mesozoic age in eastern North America. A package of interlayered psammitic and semipelitic metasedimentary rocks that appears to stratigraphically overlie the mafic volcanic unit is dominated by Paleoproterozoic detrital zircons but also contains Archaean grains. This package was deposited after 2013 Ma, the age of the youngest concordant zircon. The U–Pb data imply a minimum 165 m.y. time gap between mafic volcanism and sedimentation, and are consistent with deposition of the psammite–semipelite unit in an evolving foredeep that heralded the approach of a Paleoproterozoic arc terrane. Accretion of this terrane to the Nain cratonic margin at ca. 1.9 Ga initiated the Makkovikian orogeny. Although the Lower Aillik Group is highly deformed and may contain internal tectonic boundaries or be incomplete, the U–Pb and geochemical data allow quantitative assessment of a prolonged rift-drift-basin closure cycle that characterized the Early Paleoproterozoic evolution of the southern Nain cratonic margin.     

Complex and imitation Tectonites from the Kosciusko Pluton,snowy mountains,New South Wales

Rocks of Upper Precambrian age near Adelaide show evidence of two or more phases of deformation. The first phase has resulted in concentric and similar folds with an associated slaty cleavage. Structures of this phase are overprinted by folds with associated crenulation cleavage. Minor occurrences of later kink folds are also observed. The hypothesis that the first phase folds overprint very large folds not observable in the field is examined. The observed variation in the attitude of first phase folds could also have resulted from large scale inhomogeneities of strain.

1 “Torrens Group” is used in place of the “Torrensian Series” of Mawson and Sprigg (1950) at the suggestion of Daily (1963) since the Torrensian Series has an unwarranted time significance.

2 The scale of folds follows that of Weiss (1957). Macroscopic‐folds larger than a single outcrop. Mesoscopic‐folds on the scale of a hand specimen or single outcrop. Microscopic‐folds on the scale of a thin section.