Precambrian Sedimentary Rocks Research Articles

Succession of Laramide Magmatic and Magmatic-Hydrothermal Events in the Patagonia Mountains, Santa Cruz County, Arizona

This investigation of the space-time progression of magmatism and hydrothermal activity in the Patagonia Mountains of southern Arizona is based on field and paragenetic relationships, and on U-Pb and 40 Ar/ 39 Ar geochronology of igneous and hydrothermal minerals. The Patagonia Mountains consist of Precambrian, Paleozoic, and Mesozoic sedimentary, granitic, and volcanic rocks, Laramide volcanic rocks, and a core of Laramide intrusions that comprise the Patagonia Mountains batholith. Laramide igneous rocks and adjacent Paleozoic and Mesozoic rocks contain significant porphyry Cu-Mo deposits, Mo-Cu breccia pipes, Ag replacement deposits, and numerous other Cu-Pb-Zn-Ag replacement and vein deposits. Ages of igneous and hydrothermal minerals from 20 U-Pb and 52 40 Ar/ 39 Ar determinations define four magmatic and magmatic-hydrothermal events that formed the batholith and altered parts of it and adjacent rocks; cumulatively the events span at least 16 m.y., from ~74 to 58 Ma. The oldest event of this succession includes the 74 Ma Washington Camp stock and spatially associated Cu-Pb-Zn-Ag replacement deposits in Paleozoic carbonate rocks of the Washington Camp-Duquesne district. Eruption of 73 to 68 Ma volcanic rocks in the northern part of the range was the next youngest event, which coincides temporally with replacement and vein deposits in Paleozoic carbonate rocks at the Flux mine (~71 Ma). An event at 65 to 62 Ma is marked by emplacement of small-volume quartz monzonite, granodiorite, and diorite intrusions, formation of the Ventura breccia deposit in Jurassic granite at 65 to 64 Ma, and formation of other Pb-Zn-Ag-Cu replacement and vein deposits (~62 Ma; Blue Nose and Morning Glory). The Red Mountain porphyry Cu-Mo system is hosted by ~62 Ma granodiorite and Laramide volcanic rocks (73–68 Ma) at the northern end of the batholith. It includes a deep, chalcopyrite-bornite resource (~60 Ma) that is associated with potassic and sericitic alteration and a near-surface chalcocite-enargite resource (60 Ma) that is associated with advanced, supergene-enriched argillic alteration. The youngest event includes the Sunnyside porphyry Cu-Mo system and a Cu-Mo breccia deposit at Red Hill (Four Metals mine), both of which formed in large-volume quartz monzonite, granodiorite, quartz monzonite porphyry, and quartz feldspar porphyry (~61–59 Ma). Similar to the Red Mountain system, the Sunnyside system consists of a deep chalcopyrite resource that occurs in ~60 to 59 Ma quartz feldspar porphyry, and a near-surface, slightly younger (~59–58 Ma) enargite-chalcocite-tennantite resource that occurs in quartz feldspar porphyry, quartz monzonite porphyry, and Mesozoic rocks. The Red Hill Cu-Mo breccia deposit is hosted by large-volume quartz monzonite, granodiorite, and quartz monzonite porphyry (~61–59 Ma). Discrepancies between field and paragenetic relationships and some analytic ages at Sunnyside and Red Hill preclude precise dating of mineralization stages, and may reflect disturbance of isotope systems by multiple, co-spatial to juxtaposed intrusive and hydrothermal events, and/or by unrecognized intrusions. Numerous vein and replacement deposits at the northern end of the batholith, including the Hardshell Ag resource and the Three R supergene chalcocite resource, are distal deposits of the Sunnyside and Red Mountain systems. Small, ~61 to 59 Ma Cu-Mo deposits in large-volume intrusions in the southern part of the batholith consist of hydrothermal quartz, biotite, K-feldspar, muscovite, chalcopyrite, and molybdenite. The age span of magmatic and magmatic-hydrothermal events in the Patagonia Mountains, minimally 16 m.y., is comparable to that of certain other magmatic-hydrothermal successions that contain porphyry Cu-Mo systems. Magmatic-hydrothermal events of the Wasatch-Oquirrh igneous trend, Utah, and the Boulder batholith, Montana, both span ~17 m.y. and include the Bingham and Butte porphyry Cu-Mo, vein and replacement deposits, respectively. Plutons and mineral deposits in the Pima district, Arizona, which includes the porphyry Cu-Mo deposits at Sierrita-Esperanza, Mission-Pima-San Xavier North, and Twin Buttes, formed over an interval of ~14 m.y. The diversity of igneous and hydrothermal products likely reflects evolutionary processes occurring at multiple sites in the lithosphere and at different time scales from >10 m.y. to less than the geochronologic precision currently achievable.

Late Precambrian terrigenous rocks of the Anamakit–Muya zone of the Baikal–Muya belt: geochemistry and results of LA-ICP-MS dating of detrital zircons

A comparative geochemical characteristics of Late Precambrian sedimentary rocks (Ust’-Kelyana and Tuluya rock units) in the Anamakit–Muya zone of the Baikal–Muya belt is given, and the conditions of their sedimentation are considered. The first results of U–Pb (LA-ICP-MS) dating of detrital zircons and Sm–Nd isotope data on the Tuluya unit deposits are presented. Petrogeochemical study showed that the studied sediments are first-cycle rocks similar in composition to terrigenous island-arc sediments. The low contents of Th, Rb, Zr, Hf, and LREE and high contents of Co, Ni, Sc, V, Cr, and Fe2O3* in the sandstones of the Ust’-Kelyana unit evidence that these rocks are similar to oceanic-arc deposits. In contrast, the enrichment of the Tuluya unit rocks in Zr, LREE, Th, Rb, and Nb indicates their similarity to deposits of continental island arcs or active continental margin. Isotope-geochronological studies of the Tuluya rock unit showed the mixing of detrital material resulted from the erosion of Neoproterozoic island-arc igneous rock associations (625–700 Ma), like those in the Karalon–Mamakan zone (Yakor’ and Karalon Formations), and more ancient associations, like the Kelyana (812–824 Ma) and/or Dzhaltuk Groups. Judging from the minimum age of detrital zircon, the lower time bound of sedimentation corresponds to 0.6 Ga.

Ewolucja i tektonika staropaleozoicznego kompleksu strukturalnego południowego Bałtyku

Przedstawiono charakterystyke tektoniczno-strukturalną utworow starszego paleozoiku poludniowego Baltyku i przyleglej strefy lądowej, wyrozniając 21 jednostek. Na podstawie analizy paleostrukturalnej zrekonstruowanej na 6 mapach odtworzono zmiennośc rozwoju subsydencji basenu staropaleozoicznego od wendu - kambru poźnego po dewon gorny. Określono wiek powstania syneklizy baltyckiej i czas jej uformowania udowadniając, ze jest ona, tak jak strefa T -T, strukturą heterochroniczną. Evolution and tectonics of the Lower Palaeozoic structural complex in the southern Baltic Structural foundations of the Baltic Syneclise should be connected with a pre-platform stage of evolution of the Southern slope of the Baltic Shield. Fragments of the oldest cover, dated as Jotnian (about 1.5 MA) are known among others from Gotska Sandon Island. In the majority of the area the sedimentary cover begun with the Vendian clastic deposits described as fluvatile fan deposits (K. Jaworowski, 1979, 1982) gradually passing into the Lower Cambrian deposits. According to K. Łydka et al. (1984) the Vendian series may continue higher on than it was assumed. The Precambrian sedimentary rocks are separated by well marked disconformity from the younger rocks of Caledonian tectonic cycle only locally in the eastern part of the syneclise. Configuration of the Vendian-Lower Cambrian sedimentary basin was reconstructed on the basis of simplified palaeostructural analysis (Fig. 2); the meridional hemianticlinal Oland Elevation, parallel Klaipeda Elevation and diagonal Kaliningrad Elevation were distinguished. The subsidence index for Vendian-Lower Cambrian is 0.91 to 4.1 mm/l000 years, for Lower Cambrian 3.3-15 mm/l000 years. In the Middle and Upper Cambrian took place a structural reconstruction and transformation of early palaeostructures (Fig. 3). The subsidence index in Middle Cambrian was 4.35 to 14.3 mm/1000 years, in Upper Cambrian merely 0.95 mm/l000 years. A next reconstruction of the basin took place in Ordovician (Fig. 4); palaeostructures of Bornholm and Slupsk were well marked. The subsidence axis was displaced north-east and in the Liepaja region an immersion exceeded 2000 m; the subsidence index is small and is equal from 0.75 to 3.7 mm/l000 years. These palaeostructures were transformed as a result of the Lower Silurian subsidence (Fig. 5), which rapidly increased near the marginal zone of the platform. The subsidence index ranges from 11.5 to 115 mm/l000 years. Deepending on the basin bottom continued in Upper Silurian and exceeded 250 mm/l000 years near the marginal zone. The Devonian subsidence was considerably lower, the system of labile zones (Fig. 6) differed from that of Silurian. In the area of the Polish part of the Baltic Syneclise, within the Caledonian structural complex the following structural stages were distinguished: Vendian - Middle Cambrian, CambrianTremadocian and Arenigian - Gedinnian. After W.J. Chajn (1974) in the Caledonian tectonic cycle there were distinguished in the syneclise the early stage represented by the red continental subformation and transgressive terrigenous formation (Vendian - Tremadocian), middle stage - carbonate clayey (Arenigian - Wenlock), late stage - regressive marine terrigenous-clayey and evaporite-carbonate formation. The Baltic Syneclise is a heterogenous structure, differentiately shaped in time. The Masovian- Byelorussian Massif building the SE framework of the syneclise was uplifted as first (Lower – Middle Devonian), the north-western part as the latest (after Visean). The end of differentiated uplifting movements in the eastern part took place in Zechstein - Lower Triassic (F. Stolarczyk, 1980). Influence of the Alpine tectonic epoch expressed mainly by development of discontinuous tectonics was more dominant in the SW near-margin part of the platform than in the central and eastern area. Recently, the Baltic Shield and most part of the Baltic Syneclise influences isostatic uplifting movements, most intensive in the Bothnia Gulf (Fig. 8). The Lower Palaeozoic complex has been divided into 21 strongly dislocated structural units (Fig. 9). There have been distinguished a lot of lineaments that may determine deep fractures in the crystalline basement or in sedimentary cover. A part of lineaments overlaps with fault lines, determined by geophysical-geological survey (Fig. 9). NW - SE and NNE - SSW directions are dominant; the magnitude of the faults thrust in the western part is tremendous (over 3 km) but in the remaining area is multiply smaller - tens to one hundred meters. These faults were formed in various tectonic epochs, often they were regenerated over old structural foundations. The Variscan dislocation are dominant.

Pre-Riphean metapelites of the Yenisei Range: Chemical composition, sources of eroded material, and paleogeodynamics

The paper reports data on metapelites (gneisses and crystalline schists) from various Early Precambrian lithostratigraphic units of the Yenisei Range (Kan and Yenisei complexes, Garevka Unit, and Teya Group). A representative selection of 57 samples from the examined vertical section were analyzed for major oxides, LILE, transition elements, HFSE, REE, radioactive elements, and other trace and minor elements. The most important distinctive geochemical feature of metapelites composing most of the vertical section is their elevated concentrations of Al, which are higher than in the Post-Archean Australian Shale (PAAS), K, Rb, Ba, Ga, Sc, Pb, Th, Nb, Y, and REE. These data and the systematics of elements in certain discriminant diagrams testify that the metasedimentary complexes in the sedimentation basin and the rocks associations composing the erosion area (crystalline massifs of the ancient basement of the Siberian craton) were strongly geochemically differentiated and mature. The composition of the eroded crustal material was close to the average composition of the post-Archean continental crust and PAAS. The metapelites of the Kuzeeva Unit in the Kan Complex and Penchenginskaya Formation in the Teya Group were the only ones that could have basic rocks as an additional source of material, as also follows from the elevated Cr concentrations of the rocks and their REE systematics. The three types of REE patterns of the metapelites largely correspond to the composition of the rocks that composed the ancient drainage areas and the degrees of the differentiation and averaging of their terrigenous material. The first predominant type corresponds to PAAS and the averaged composition of the upper continental crust. The second and third types of the REE patterns with high and low LaN/YbN ratios, respectively, and with or without negative Eu anomalies provide evidence of the contribution of tonalite-trondhjemite-granodiorite (TTG) or basite sources typical of granite-greenstone provinces. In certain diagrams, the compositional fields of Early Precambrian metapelites in the Yenisei Range almost exactly coincide with the compositional fields of regional fine-grained Riphean terrigenous rocks. This suggests that the Late Precambrian sedimentary rock sequences inherited their geochemical features from more ancient rocks. The reproduced ancient geodynamic environments in which the Early Precambrian metaterrigenous complexes of the Yenisei Range were accumulated correspond to ensialic continental marginal basins. The rocks of the Kan and Yenisei complexes and the Garevka Unit were formed mostly on active continental margins and, less frequently, on passive margins. The protoliths of metasedimentary rocks of the Teya Group were accumulated in a subplatform environment at a passive margin.

Comment on “Abiotic Pyrite Formation Produces a Large Fe Isotope Fractionation”

Guilbaud et al. (Reports, 24 June 2011, p. 1548) suggest that the geologic record of Fe isotope fractionation can be explained by abiological precipitation of pyrite. We argue that a detailed understanding of the depositional setting, mineralogy, and geologic history of Precambrian sedimentary rocks indicates that the Fe isotope record dominantly reflects biological fractionations and Fe redox processes.

Effects of phosphate mining activity on groundwater quality at Wadi Queh, Red Sea, Egypt

The groundwater in Wadi Queh exists in two main hydrogeological units; fractured Precambrian basement and sedimentary rocks with high contribution for groundwater recharge. To study the impacts of phosphate mining activities on the groundwater quality in the area, three groundwater samples that represent all water wells in the area were collected and analysed for major ions and some heavy metals. In addition, three bulk samples representing the phosphatic sediments collected from upstream and downstream of the drainage basin were collected and analysed to understand the source of groundwater contamination. The total concentrations of dissolved solids suggest that the groundwater in the area grades from fresh to brackish water (961–1,580 mg/l), and is characterized by sodium–calcium–sulphate–chloride and sodium–magnesium–sulphate–chloride chemical types. The results showed high concentrations of the heavy metals in well nos. 1 and 2 in downstream parts compared to well no. 3 in upstream part reflecting their influence by the mining activities.

Kwangsian crustal anatexis within the eastern South China Block: Geochemical, zircon U–Pb geochronological and Hf isotopic fingerprints from the gneissoid granites of Wugong and Wuyi–Yunkai Domains

Gneissoid granites were traditionally thought to be the components of the Precambrian basement in the eastern South China Block, but twenty-four gneissoid granite samples from the Wugong, northern Wuyi, southern Wuyi and Yunkai Domains gave zircon U–Pb ages of 424–456Ma, 410–457Ma, 426–438Ma and 415–450Ma, respectively. The existence of the abundant Kwangsian gneissoid granites, which have incorrectly been assigned to the Precambrian basement, indicates that the “so-called” Precambrian stratigraphic sequence of the Cathaysia Block should be termed “Complex” rather than “Group”. These gneissoid granites are peraluminous granites with relatively high Al2O3, MgO, TiO2, FeOt and CaO/Na2O but low CaO, Al2O3/(MgO+FeOt) and Rb/Sr ratios. They exhibit strongly negative Ba, Sr, Nb, P and Ti and positive Pb anomalies in primitive mantle-normalized diagrams, and have initial 87Sr/86Sr ratios ranging from 0.70924 to 0.72935 and negative εNd(t) values from −6.4 to −11.4, similar to those of paragneissic enclaves and Precambrian sedimentary rocks in the eastern South China Block. The zircons crystallized in the Kwangsian gneissoid granites gave εHf(t) values ranging from +2.4 to −19.4 with the peak at −4 (almost all grains clustering at −1 to −12) and Hf model ages of 1.1–2.1Ga. The synthesis of these geochemical and in-situ zircon Hf isotopic data indicates that the Kwangsian gneissoid granites dominantly originated from Proterozoic metapelitic and meta-igneous components with insignificant input of juvenile mantle-derived magmas. A model of two-stage crustal anatexis is proposed for the Kwangsian granitic magma in the eastern South China Block, with (1) the formation of 460–430Ma granite through the breakdown of hydrous minerals under the condition of the doubly thickened crust, and (2) the generation of the 430–400Ma granite accompanying promoted melting along a path of isothermal decompression due to the increasing thermal weakening for the collapse of the thickened crust. The crustal anatexis has probably occurred in an intracontinental tectonic regime that was geodynamically linked to the far-field response to the assembly of the Australian–Indian plate with the Cathaysia Block during middle Paleozoic (Kwangsian) time.

Chapter 27 Geology and petroleum potential of the north and east margins of the Siberian Craton, north of the Arctic Circle

Abstract The Siberian Craton consists of crystalline rocks and superimposed Precambrian sedimentary rocks deposited in rift basins. Palaeozoic rocks, mainly carbonates, were deposited along the margins of the craton to form an outwardly younger concentric pattern that underlies an outward-thickening Mesozoic sedimentary section. The north and east margins of the Siberian Craton subsequently became foreland basins created by compressional deformation during collision with other tectonic plates. The Tunguska Basin developed as a Palaeozoic rift/sag basin over Proterozoic rifts. The geological provinces along the north and east margins of the Siberian Craton are immature with respect to exploration, so exploration-history analysis alone cannot be used for assessing undiscovered petroleum resources. Therefore, other areas from around the world having greater petroleum exploration maturity and similar geological characteristics, and which have been previously assessed, were used as analogues to aid in this assessment. The analogues included those of foreland basins and rift/sag basins that were later subjected to compression. The US Geological Survey estimated the mean undiscovered, technically recoverable conventional petroleum resources to be approximately 28 billion barrels of oil equivalent, including approximately 8 billion barrels of crude oil, 103 trillion cubic feet of natural gas and 3 billion barrels of natural gas liquids.

Dating sedimentary rocks using in situ U-Pb geochronology of syneruptive zircon in ash-fall tuffs <1 mm thick

Absolute ages for sedimentary rocks are required to construct a temporal framework in which to decipher Earth9s history. The most reliable method for dating Precambrian sedimentary rocks is U-Pb geochronology of zircon in intercalated volcanic rocks. However, extracting sufficient zircons involves destructive mineral separation procedures, often requiring several kilograms of sample. We have dated felsic tuffs 207 Pb/ 206 Pb dates between ca. 2600 Ma and ca. 2680 Ma, all consistent with their stratigraphic position and available age constraints. The dates represent the timing of syndepositional volcanism and provide reliable estimates for the age of deposition. Zircons in a 0.5 mm tuff band 15 mm above the Jeerinah impact layer provide an age of 2632 ± 7 Ma, which represents a close approximation for the timing of a major asteroid impact, the strewn field of which is distributed across two Archean cratons. In situ geochronology of zircon in felsic tuffs requires only small volumes of sample and is especially useful for drill core samples where material is limited. It avoids the destruction of samples associated with mineral separation, retaining textural information and avoiding potential contamination. This approach adds significantly to the number of sedimentary rocks that can be dated accurately.

Global Biogeochemical Changes at Both Ends of the Proterozoic: Insights from Phosphorites

The distribution of major phosphate deposits in the Precambrian sedimentary rock record is restricted to periods that witnessed global biogeochemical changes, but the cause of this distribution is unclear. The oldest known phosphogenic event occurred around 2.0 Ga and was followed, after more than 1.3 billion years, by an even larger phosphogenic event in the Neoproterozoic. Phosphorites (phosphate-rich sedimentary rocks that contain more than 15% P(2)O(5)) preserve a unique record of seawater chemistry, biological activity, and oceanographic changes. In an attempt to emphasize the potentially crucial significance of phosphorites in the evolution of Proterozoic biogeochemical cycles, this contribution provides a review of some important Paleoproterozoic phosphate deposits and of models proposed for their origin. A new model is then presented for the spatial and temporal modes of occurrence of phosphorites along with possible connections to global changes at both ends of the Proterozoic. Central to the new model is that periods of atmospheric oxygenation may have been caused by globally elevated rates of primary productivity stimulated by high fluxes of phosphorus delivery to seawater as a result of increased chemical weathering of continental crust over geological timescales. The striking similarities in biogeochemical evolution between the Paleo- and Neoproterozoic are discussed in light of the two oldest major phosphogenic events and their possible relation to the stepwise rise of atmospheric oxygen that ultimately resulted in significant leaps in biological evolution.

Assessment of undiscovered petroleum resources of the north and east margins of the Siberian craton north of the Arctic Circle

Abstract The Siberian craton consists of crystalline rocks and superimposed Precambrian sedimentary rocks deposited in rift basins. Palaeozoic rocks, mainly carbonates, were deposited along the margins of the craton to form an outwardly younger concentric pattern that underlies an outward-thickening Mesozoic sedimentary section. The north and east margins of the Siberian craton subsequently became foreland basins created by compressional deformation during collision with other tectonic plates. The Tunguska Basin developed as a Palaeozoic rift/sag basin over Proterozoic rifts. The geological provinces along the north and east margins of the Siberian craton are immature with respect to exploration, so exploration-history analysis alone cannot be used for assessing undiscovered petroleum resources. Therefore, other areas from around the world having greater petroleum exploration maturity and similar geological characteristics, and which have been previously assessed, were used as analogues to aid in this assessment. The analogues included those of foreland basins and rift/sag basins that were later subjected to compression. The US Geological Survey estimated the mean undiscovered, technically recoverable conventional petroleum resources to be c. 28 billion barrels of oil equivalent, including c. 8 billion barrels of crude oil, 103 trillion cubic feet of natural gas, and 3 billion barrels of natural gas liquids.

Geochemistry of Precambrian sedimentary rocks used to solve stratigraphical problems: An example from the Neoproterozoic Volta basin, Ghana

The Neoproterozoic Volta basin of Ghana (∼115,000 km 2; depth up to 5–7 km) consists of flat-lying sedimentary rocks, mainly sandstones that unconformably overlie the crystalline basement of the West-African craton. The stratigraphical column has been subdivided into three main units, in upward succession the Bombouaka, Oti and Obosum Groups, but poor exposure has resulted in major disagreements on stratigraphical correlations and on the areal extents of these units. Geochemical data (major and trace element concentrations as well as Rb–Sr, Pb and Sm–Nd isotope data) on siltstones and mudstones, intercalated with the sandstones from the different units, were used in an attempt to solve some of these problems. Siltstones and mudstones from the Bombouaka Group can be unequivocally distinguished from similar rocks from the Oti and Obosum Groups by higher K 2O and Rb, larger Eu anamalies, higher 87Sr/ 86Sr, and more negative ɛ Nd values. Geochemical distinction between samples from the Oti and Obosum Groups is ambiguous because published geological maps differ with respect to the relative extents of the Oti and Obosum Groups. Rb–Sr isotope data, combined with high degrees of correlation between the concentrations of K and Rb, and Ca and Sr, indicate that mobility of these elements did not significantly change their concentrations during surface weathering. The clear geochemical distinction between mudstones and siltstones from the Bombouaka Group and similar rocks from the Oti and Obosum Groups is used to solve one of the outstanding controversies regarding the stratigraphy of the Volta basin.

Composition and geodynamic nature of the protoliths of diamondiferous rocks from the Kumdy-Kol deposit of the Kokchetav metamorphic belt, northern Kazakhstan

The distribution of rare earth elements was analyzed in the Early Cambrian diamondiferous calcsilicate rocks and gneisses, calciphyres, and marbles of the Kumdy-Kol deposit. These data were compared with the lithogeochemical characteristics of the sedimentary assemblages of weakly metamorphosed Late Precambrian graphite-bearing sedimentary rocks of the Kokchetav metamorphic belt. The obtained results allowed us to suppose that the protoliths of the Kumdy-Kol rocks were compositionally similar to the Late Precambrian graphite-bearing terrigenous-carbonate and sand-shale sequences of the continental shelf of the Kokchetav microcontinent, some of which were transformed in a subduction zone into diamondiferous rocks.

Direct dating of paleomagnetic results from Precambrian sediments in the Amazon craton: Evidence for Grenvillian emplacement of exotic crust in SE Appalachians of North America

We apply a new diagenetic dating technique to determine the age of magnetization for Precambrian sedimentary rocks in the SW Amazon craton. Two new paleomagnetic poles are reported from the rocks of the Aguapeí Gp.: red beds of the Fortuna Fm. (Plat=59.8°N, Plon=155.9°E, A95=9.5, K=14, 18 sites, N/n 128/115, Q=5) and the reverse-polarity mudstones of the overlying Vale da Promissão Formation (Plat=49.5°N, Plon=89.3°E, A95=12.5, K=30, 6 sites, N/n=94/80, Q=4). The Fortuna Fm. magnetization is hosted by massive, interstitial hematite cement and constitutes a post-depositional remanence. The age of diagenesis of the red beds is well-constrained by the 1149±7 Ma U–Pb age of authigenic xenotime rims on detrital zircons determined by SHRIMP analysis. The magnetite-hosted remanence of the Vale da Promissão Fm. may be detrital in origin, but the age of deposition is poorly constrained. The reliable and precisely-dated Fortuna Fm. paleomagnetic pole fixes the paleogeographic position of the Amazon craton near the SE Appalachians portion of North America at 1.15 Ga. These data demonstrate a mobile Grenvillian link between these two cratons, and support the recent identification of Amazon crust in the Blue Ridge province region of North America.

Non-destructive method to detect the cycle of lamination in sedimentary rocks: rhythmite sequence in Neoproterozoic Cap carbonates

Abstract Environmental changes were flagged by lamination within Precambrian sedimentary rocks sequences using a high-resolution method. A continuous sample was collected and embedded in epoxy resin to form sample plates in order to investigate 2D laminations on sample surfaces. Elemental distributions in laminated samples were determined and recorded as XRF profiles, using the scanning X-ray analytical microscope (SXAM) and an image processing lamination tracer. These methods were used to study a rhythmite sequence at the base of a Neoproterozoic cap carbonates, which immediately overlies glacial deposits in Namibia, to determine profiles of Si, Ca, Mn, Ti, Fe, and Sr. At the base and top of each interval, the concentrations of six elements were relatively constant. In the middle of the interval, Ca–Sr and Mn, which reflect presence of calcite and dolomite respectively, fluctuated. This reflects seventeen cycles. The cyclic fluctuations of Ca–Sr and Mn were interpreted as calcite–dolomite cycles. Strontium also recorded two cycles. Furthermore, the fluctuations of Ca–Sr and Mn in calcite–dolomite cycles were laterally traceable. Possibly, these calcite–dolomite cycles were deposited synchronously and regionally. New methods allow the extraction of data that record cyclic- and event-phenomena from long sequences of laminated sedimentary rocks; this is not isolated to the Neoproterozoic but also throughout Earth's history.

Geochemical and isotopic constraints on the origin of Paleoproterozoic red shales of the Gamagara/Mapedi Formation, Postmasburg Group, South Africa

The occurrence of red shales in Precambrian sedimentary rocks has been used to infer the presence of free O 2 in the atmosphere. Red shale samples from the ~2.2 Ga Gamagara/Mapedi Formation of the Postmasburg Group, South Africa were analyzed for mineralogy, geochemistry, and stable isotope composition (Fe and organic C) to constrain the timing and mechanism of red shale formation. These data suggest that the red shales were detrital accumulations of lateritic soils that were produced from moderate to intense chemical weathering of intermediate to felsic source rocks. Such weathering occurred under stable climatic conditions in low-latitude (sub)tropical regions. The data also suggest that the red shales were potentially influenced by hydrothermal activity. Hematite, a pigmenting agent of the red shales, was probably formed during the lateritic weathering of the source rocks ~2.2 Ga ago, rather than by the modern oxidation/weathering of pre-existing Fe 2+ -bearing minerals in the shales. Dissolution, remobilization, and reprecipitation of Fe and U during the diagenetic stage of shales are suggested from variations in the Fe 3+ /Ti, Fe 2+ /Ti, Fe isotope compositions, and U/Th ratios. Leaching of Fe 2+ -bearing silicate and/or reductive dissolution of Fe 3+ -oxide minerals, probably by organic acids, resulted in the discoloration of several red shales (to pale-green/white) during red shale diagenesis. Oxidation of Fe 2+ on the seafloor that was supplied by hydrothermal discharge and migration (upward diffusion) of leached Fe 2+ in sediments was additional source of Fe 3+ for red shale formation. Geochemical evidence thus supports the existence of an oxygenated atmosphere and shallow ocean system at least ~2.2 Ga ago.

A balanced cross section across the Himalayan frontal fold-thrust belt, Subathu area, Himachal Pradesh, India: thrust sequence, structural evolution and shortening

The structural evolution of the frontal fold-thrust belt in the NW Himalayas has been deciphered through the construction of a balanced cross section, which incorporates all the Tertiary and the Precambrian sedimentary rock units belonging to the Sub-Himalaya Zone and the Lesser Himalaya Zone, respectively. Towards the foreland the structural geometry is rather simple with widely spaced thrust ramps and simple fault-bend folds in the hangingwall. In the central sector, the structural geometry seen at the surface is essentially controlled by a thrust system that can be approximately described as a buried hinterland-dipping duplex. The structural geometry becomes extremely complex in the northeastern sector (i.e. towards the hinterland) owing to out-of-sequence thrusting, low ramp spacing leading to dislocation of older thrusts and related structures by younger thrusts, folded thrusts, and breached horses. Stacked-up horses, occupied by Lesser Himalaya Zone formations, dominate the structural geometry in this sector. An in-sequence thrusting event followed by out-of-sequence thrusting in an approximately break-back style best describe the structural evolution of the fold-thrust belt in this area. A total slip of about 96 km occurred along the detachment and the shortening partitioned within the fold-thrust belt is about 72 km or about 71%.

Argon isotope fractionation induced by stepwise heating

Noble gas isotopes are widely used to elucidate the history of the rocks in which they have been trapped, either from distinct reservoirs or by accumulation following radioactive decay. To extract noble gases from their host rocks, stepwise heating is the most commonly used technique to deconvolve isotopically different components, e.g., atmospheric, in situ radiogenic, or excess radiogenic from mantle or crustal reservoirs. The accurate determination of the isotopic composition of these different components is of crucial importance, e.g., for ages obtained by 40Ar- 39Ar stepheating plateaus. However, diffusion theory-based model calculations predict that the stepwise thermal extraction process from mineral phases induces isotope fractionation and, hence, adulterates the original composition. Such effects are largely unconsidered, as they are small and a compelling experimental observation is lacking. We report the first unequivocal evidence for significant mass fractionation of argon isotopes during thermal extraction, observed on shungite, a carbon-rich Precambrian sedimentary rock. The degree of fractionation, as monitored by 38Ar/ 36Ar and 40Ar/ 36Ar ratios, very well agrees with theoretical predictions assuming an inverse square root dependence of diffusion coefficient and atomic mass, resulting in easier extraction of lighter isotopes. Hence, subatmospheric 40Ar/ 36Ar ratios obtained for argon extracted at low temperatures may not represent paleoatmospheric argon. Shungite argon resembles modern atmospheric composition, but constraints on the timing of trapping appear difficult to obtain, as shungites are multicomponent systems. In 40Ar- 39Ar stepwise heating, the isotope fractionation effect could cause systematic underestimations of plateau ages, between 0.15 and 0.4% depending on age, or considerably higher if samples contain appreciable atmospheric Ar. The magnitude of this effect is similar to the presently achieved uncertainties of this increasingly precise dating technique. Our results also indicate the importance of thermally activated diffusion as a possible fractionation mechanism, e.g., for hydrothermal gas exhalations, or for carbonaceous carrier phases such as “Q” in meteorites that have been suggested as carriers of highly fractionated noble gas residues from the early solar nebula.

Re–Os isotopic composition and PGE contents of proterozoic carbonaceous argillites, Virginia Formation, Northeastern Minnesota

Carbonaceous, sulfidic argillites from the Proterozoic Virginia Formation in Minnesota have been investigated in terms of platinum group elements (PGE) distribution and Re–Os isotopic signatures. Kerogen from protoliths and slightly metamorphosed equivalents is characterized by Re and Os enrichment factors relative to whole rock concentrations that range from 4 to 272. Re–Os isotopic values of kerogen in samples that show no evidence of extensive recrystallization fall near a 1.85 Ga reference isochron. This age is consistent with that estimated for the Virginia Formation, and suggests that dating of organic-rich Precambrian sedimentary rocks and the retrieval of data pertinent to the Os isotopic evolution of seawater may be possible using the Re–Os isotopic system.

Cambro‐Ordovician stratigraphy of Bjørnøya and North Greenland: constraints on tectonic models for the Arctic Caledonides and the Tertiary opening of the Greenland Sea

The southern tip of Bjørnøya is composed of poorly known sedimentary rocks of Precambrian and Lower Palaeozoic age. Biostratigraphic and sedimentological investigations have demonstrated that the sediments of the Younger Dolomite are of Early, but not earliest, Ordovician age and that those of the conformably overlying Tetradium Limestone are of Black Riveran (Mid‐Ordovician) age. A significant hiatus lies between the base of the Younger Dolomite and the underlying Vendian glacial diamictites of the Sørhamna Formation. Comparison of this distinctive stratigraphic context with other sequences in Arctic Laurentia shows that a similar hiatus is present only in eastern North Greenland, where the Early Ordovician Wandel Valley Formation rests unconformably on a variety of Proterozoic units. Consideration of other geological characters, including the presence of west‐directed Caledonian thrusting and the location of the platform‐trough boundary in the Franklinian Basin, indicates that Bjørnøya was an integral part of the Franklinian Basin during Early Palaeozoic deposition and was attached to eastern North Greenland until early Tertiary rifting. A corollary of this interpretation of the Phanerozoic location of Bjørnøya indicates that any Caledonian strike‐slip, orogen‐parallel, displacements must have been located outboard of the combined Bjørnøya–Greenland craton. Furthermore, it provides strong constraints on interpretations of the geometry of Greenland Sea opening during early Tertiary time, and of the Vestbakken Volcanic Province and pull‐apart basin with its bounding faults, the Hornsund and Senja fracture zones.