Description: Publication year: 2011 Source: Precambrian Research, Available online 26 December 2011 Małgorzata Moczydłowska, Konstantin E. Nagovitsin A new assemblage of morphologically complex, ornamented and large organic-walled microfossils (acritarchs) from the Ura Formation in the Patom Uplift, East Siberia, is identified and taxonomically described as a continuation of the previous studies (Nagovitsin et al., 2004; Sergeev et al., 2011). Nine new species and three new genera are recognised; ten other species are known from occurrences in the Ediacaran strata of Australia, China, the Siberian Platform (SP), and the East European Platform (EEP). The new species are morphologically advanced phenotypes, showing innovative elements of vesicle ornamentation and variable symmetry, and internal bodies within zygotic cyst-like vesicles that are indicative of the sexual reproduction stage of the microorganism. The morphological features and the preliminarily recognised cell wall ultrastructure are characteristic of phytoplankton; thus the described microfossils are identified as green microalgae. The relative age of the Ura assemblage, by comparison to microfossil records elsewhere and estimated in the regional geologic context of the Dalniaya Taiga (containing the Ura Formation) and the Zhuya (overlying) groups, is suggested to be early Ediacaran. Radiation of the Ura-type microorganisms pre-dates the records of Ediacaran microbiota from Australia (the Pertatataka association/Ediacaran Complex Acanthomorphic Palynoflora), the Siberian Platform (Khamaka-type assemblages), and the EEP (Vychegda association) but is preceded by the appearance of certain species recorded in China at the base of the Doushantuo Formation. The stratigraphic ranges of genera co-occurring in these palaeocontinents (Appendisphaera, Cavaspina, Ceratiosphaeridium, Gyalosphaeridium, Labruscasphaeridium, Multifronsphaeridium, Tanarium, andVariomargosphaeridium) are extended by their lower record in the Patom Uplift and may span a major portion of the Ediacaran Period. The refined stratigraphic distribution of microfossils may provide a means for zoning the entire Ediacaran System in addition to the upper zones established in Australia. Highlights ► We identified a new assemblage of Ediacaran organic-walled microbiota from the Ura Formation, East Siberia ► We recognised nine new species and three new genera ► Phenotypic features and zygotic cyst-like vesicles with reproductive cells support the affinity within Chlorophyta ► We inferred the early Ediacaran age of the Ura assemblage from its relative stratigraphic position

Description: Publication year: 2011 Source: Precambrian Research, Available online 22 December 2011 M. Reuschel, V.A. Melezhik, H. Strauss Sulfide sulfur isotope values and iron speciation data are reported here for sediments from the 2.0 Ga old Pilgujärvi Sedimentary Formation, Pechenga Greenstone Belt, NW Russia. Sulfides are abundant throughout the c. 1400 m thick turbiditic sandstone-siltstone-shale succession and present as pyrite and pyrrhotite. A stratigraphic trend is discernible with respect to sulfide mineralogy and sulfur isotopic compositions, changing up-section from highly to less variable and positive δS values. The occurrence of pyrrhotite, particularly high in the upper part of the section, is thought to be the result of decomposition of pyrite during low-grade metamorphism. This is evidenced by the crystallographic habit and sulfur isotopic composition of the different sulfide minerals. The δS of both sulfur species is quite variable in the lower part of the formation (-9.5 to 18.7 ‰) while the upper part displays a smaller fluctuation in δS (4.4 to 13.3 ‰). The pyrrhotite is generally depleted inS by approximately 3.6 ‰ (average isotopic difference between pyrrhotite and pyrite) compared to the pyrite; this shift in the sulfur isotopic composition results from thermal decomposition of pyrite.The overall spread in δS is indicative for the microbial reduction of sulfate, but δS values suggest a sulfate limited environment in the upper part of the formation. We note, however, that the sulfate limitation is restricted to the pore water realm, caused by high sedimentation rates that inhibited a sufficiently high sulfate flux from the overlying water column into the sediment.Based on iron speciation data, a previous characterization of the depositional environment as non-euxinic can be specified more precisely. We note that the siliciclastic succession of the Pilgujärvi Sedimentary Formation was deposited under anoxic and ferruginous water column conditions derived from high ratios of FeHR/FeTand low ratios of Fepy/FeHR. Highlights ► We measured sulfur isotopic compositions and iron speciation in Paleoproterozoic strata. ► Sulfur isotope patterns and sulfide mineralogy reveal a distinct stratigraphic trend. ► Iron speciation is homogenous throughout the succession. ► The results of the study indicate ferruginous water column conditions at 2.0 Ga.

Description: Publication year: 2011 Source: Precambrian Research, Available online 21 December 2011 D.I. Schofield, M.S.A. Horstwood, P.E.J. Pitfield, M. Gillespie, F. Darbyshire, ... The Reguibat Shield of N Mauritania and W Algeria represents the northern exposure of the West African Craton. As with its counterpart in equatorial West Africa, the Leo Shield, it comprises a western Archaean Domain and an eastern Palaeoproterozoic Domain. Much of the southern part of the Archaean Domain is underlain by the Tasiast-Tijirit Terrane and Amsaga Complex which, along with the Ghallaman Complex in the northeast, preserve a history of Mesoarchaean crustal growth, reworking and terrane assembly. This study presents new U-Pb and Sm-Nd data from the Tiris Complex, a granite-migmatite-supracrustal belt, that intervenes between these units and the Palaeoproterozoic Domain to the northeast.New U-Pb geochronology indicates that the main intrusive events, broadly associated with formation of dome-shaped structures, occurred at around 2.95 to 2.87 Ga and 2.69 to 2.65 Ga. This study also recognises younger regional metamorphism and intrusion of syn-tectonic granites located within major shear zones at around 2.56 to 2.48 Ga. Sm-Nd depleted mantle model ages indicate that magmatism involved recycling of crustal source components older than at least 3.25 Ga in age. Comparison with other Archaean units in the Reguibat Shield and in the Leo Shield illustrate the importance of deformation and tectonism of a regional greenstone-sedimentary province prior to around 3.00 Ga as well as subsequent magmatic episodes broadly equivalent in age to those in the Tiris Complex. Highlights ► In this study we combine new survey data with isotopic analyses from the Tiris Complex. ► We identify a main magmatic episode dated between 2948 and 2875 Ma. ► Subsequent granite magmatism is dated at around 2691 to 2654 Ma. ► The youngest granitic rocks record fault movements at around 2487 to 2482 Ma. ► Nd T DM ages are interpreted to reflect an older crustal source component.

Description: Publication year: 2011 Source: Precambrian Research, Available online 21 December 2011 Anna K. Ksienzyk, Joachim Jacobs, Steven D. Boger, Jan Košler, Keith N. Sircombe, ... The Pinjarra Orogen, stretching along the western coast of Western Australia, is one of the least understood orogens on the continent. Exposure is limited to fault-bound basement blocks within the Phanerozoic Perth Basin, which have been interpreted as either remnants of a Mesoproterozoic orogen or as allochthonous blocks within a Neoproterozoic orogen. Thus even the timing of orogenesis is controversial. However, since the orogen is in a critical position for reconstructions of both Rodinia and Gondwana, a better understanding is highly desirable. New U-Pb ages of detrital zircon cores, metamorphic zircon rims and metamorphic monazites from the Northampton Complex, the northernmost and largest of the exposed basement blocks, provide better insights into the sedimentary provenance and metamorphic history of the metasediments exposed in this part of the orogen. Both the monazite ages and the youngest metamorphic zircon rims constrain metamorphism in the Northampton Complex to 1090-1020 Ma. Additionally, a large number of older metamorphic zircon rims pre-date this event and were inherited from the Albany-Fraser Orogen in southwestern Australia, which is also identified as the main source region for the detrital zircon cores. Additional sources of detrital zircons are the Mawson Continent (the South Australian Craton and parts of East Antarctica) and possibly the North Australian Craton, as well as synsedimentary volcanic rocks. However, the combination of ages from the cores and inherited metamorphic rims shows that zircons from the Mawson Continent/North Australian Craton were not directly transported to the Northampton Complex but arrived there after being incorporated into the Albany-Fraser Orogen and then eroded from there. The dataset records therefore two orogenic cycles: (1) erosion of the Mawson Continent/North Australian Craton, deposition along the craton margins, collision between the Mawson Continent and North-West Australian Craton and metamorphism in the Albany-Fraser Orogen and (2) erosion of the Albany-Fraser Orogen, deposition along Australia's western margin and metamorphism during the Pinjarra Orogeny. This highlights the care that must be taken in detrital zircon studies, since many of the detrital cores record a second-order provenance. It also explains how zircons from the Mawson Continent could be transported across the Albany-Fraser Orogen that should normally have been a barrier to northwest-directed sediment transport. Finally, in light of these new and previously published data, we support the interpretation that the Pinjarra Orogen is a Mesoproterozoic orogen formed along Australia's western margin. Highlights ► 900 new U-Pb zircon and monazite ages from the Northampton Complex. ► Metamorphic zircon and monazite growth between 1090-1020 Ma. ► The Albany-Fraser Orogen is the main source for detrital zircons. ► Additional sources are the Mawson Continent and synsedimentary volcanism. ► Detrital zircons can be traced through two orogenic cycles.

Description: Publication year: 2011 Source: Precambrian Research, Available online 22 December 2011 Leila Battison, Martin D. Brasier The ∼1000 Ma Torridon Group of northwest Scotland are here shown to contain a rich diversity of organic walled microfossils preserved in exceptional detail within sedimentary phosphate. The phosphatic nodules and bands in which they occur are autochthonous, as are the organic fossils they contain. The associated sedimentology of this phosphate is shown to be consistent with a lacustrine setting. Informal taxonomic treatment of the microfossils allows statistical assessment of the relative abundances of different morphotypes across a range of lacustrine environments. Exceptional preservation of pristine organic-walled eukaryotes and prokaryotes, together with a taphonomic spectrum in both physicochemical and biological degradational conditions allows a picture to be built of the earliest lacustrine ecological communities. Highlights ► We identify bedded and nodular sedimentary phosphate throughout lacustrine portions of the 1 Ga-old Torridon Group of NW Scotland. ► A diverse organic microfossil assemblage is found preserved exceptional three dimensional detail within these phosphates. ► The assemblage contains informally classified eukaryotes and prokaryotes, as well as a distinct taphonomic spectrum. ► Such a varied ecological assemblage is rarely found in rocks of this age, and is afforded by the exceptional preservation of the phosphates. ► This work contributes unprecedented detail to the growing picture of a verdantly colonised Precambrian terrestrial environment.

Description: Publication year: 2011 Source: Precambrian Research, Available online 21 December 2011 Miao Lu, Maoyan Zhu, Junming Zhang, Graham Shields-Zhou, Guoxiang Li, ... The DOUNCE (Doushantuo negative carbon isotope excursion), characterized by a large shift in δCcarbfrom +5‰ down to -12‰ in the upper part of the Ediacaran Doushantuo Formation, was previously known only from a few sections mainly in the Yangtze Gorges area, western Hubei, South China. As a consequence, it remains uncertain whether the DOUNCE is of diagenetic origin, of local significance only, or corresponds to the worldwide mid-Ediacaran Shuram-Wonoka anomaly. Here we report the widespread occurrence of the DOUNCE in nine sections from various depositional settings ranging from coastal shallow basin, offshore intra-shelf basin to deepwater slope on the Yangtze platform. The results indicate that the DOUNCE starts in the upper part of Doushantuo Sequence 2 and ends at the base of the Dengying Formation. The variable appearance of the DOUNCE is facies dependent and any absence of the DOUNCE is shown to be the result of stratigraphic hiatus. The fall and recovery of δCcarbvalues that bracket the DOUNCE occur over a relatively short stratigraphic interval and were possibly of shorter duration than the nadir of δCcarbvalues which remain stable for a much longer interval from the transgressive systems tract to the condensed portion of the sedimentary sequence. Both the fall and the recovery of δCcarbvalues are gradual and independent of lithology, suggesting a non-diagenetic origin for the DOUNCE which is further supported by the lack of any systematic diagenetic positive covariation between δCcarband δOcarbdata, and systematic differences in absolute isotope values between dolostone and limestone units. Field investigations and sedimentary analysis suggests the occurrence of a major sequence boundary at the basal part of the DOUNCE and reveals that previous reports of an interval of positive δC values within the DOUNCE resulted from either post-depositional sliding or faulting. Similarities between the evolutionary pattern and stratigraphic occurrence of the DOUNCE and global Shuram-Wonoka anomalies demonstrate that the DOUNCE represents a global change in seawater chemistry during the mid-Ediacaran which was closely related to the evolution of macroscopic metazoans.

Description: Publication year: 2011 Source: Precambrian Research, Available online 18 December 2011 Lorenzo Cremonese, Graham Shields-Zhou, Ulrich Struck, Hong-Fei Ling, Lawrence Och, ... The Precambrian-Cambrian boundary is claimed as one of the most crucial biological breakthroughs on our planet, when changes in chemical and physical conditions, together with key biological innovations, helped to trigger a biodiversity “explosion”. The Yangtze Platform (South China), mainly characterized by continuous and unaltered successions of this age, is ideally suited to high-resolution, palaeomarine investigations of this event. In this study, δN and δCorg records from Xiaotan Section (Yunnan, SW China) were investigated in order to provide insight into variations in primary productivity, ecological developments and marine environment. The Ediacaran-early Cambrian Xiaotan section is characterised by relatively high sedimentation rates and variable lithologies (carbonates, cherts, phosphorites, sandstones and siltstones) that alternate through nearly 600 meters thickness. Organic carbon isotope values vary between -36‰ and -21‰, tightly following and helping to complete published δCcarb trends. The base of the Cambrian is characterised in this section by a significant drop from -25‰ to -35‰, mirroring the δCcarb fall observed at this and other sections worldwide. Higher in the section in the Dahai Member, values increase to -20‰, again demonstrating communication between DOC and DIC pools during the early Cambrian on 10-10year time scales. From the base of the overlying Shiyantou Formation, δCorg values remain around -30‰ until the end of the section, testifying to an interval of more modest change in DIC δC that would be consistent with biostratigraphic correlations. Nitrogen isotope values vary independently from carbon isotope trends, exhibiting several major δN cycles, with values fluctuating between +9‰ and -1‰. Robust trends in δN within the Dahai Member testify to changes in the ratio between nitrogen fixation and denitrification, possibly reflecting fluctuations in the water column chemocline as δC values reached their global acme. Up section, nitrogen isotope values show dampened cyclicity with values remaining between 0‰ and +5‰, indicating establishment of an equilibrium state in marine biogeochemistry and nutrient cycles. The overall variability in δN closely resembles that of recent marine sediments, while the observed cyclicity is defined by several samples in all cases. These observations argue for good preservation of original isotopic signatures which bodes well for future N isotope studies of the Precambrian-Cambrian boundary.

Description: Publication year: 2011 Source: Precambrian Research, Available online 21 December 2011 Feiko Kalsbeek, Pascal Affaton, Barth Ekwueme, Robert Frei, Kristine Thrane The westernmost part of the Pan-African (c. 600 Ma) Dahomeyide orogen in Togo and Benin consists of a complex stack of thrust sheets, mainly composed of Palaeoproterozoic (‘Eburnean’, c. 2000 Ma) granitoid rocks and younger volcanic and metasedimentary successions. This thrust stack comprises the suture between the West-African craton and the Benino-Nigerian province to the east. This eastern province consists largely of migmatites, supracrustals and granitoid rocks. Rb-Sr and Sm-Nd isotope data for four granites from the western domain confirm the Palaeoproterozoic age of these rocks found by earlier studies. One of these granites yielded a zircon U-Pb age of c. 2060 Ma. Twenty samples from the eastern region yield Rb-Sr and Sm-Nd data indicating that both migmatites and granitoid rocks are of Neoproterozoic (Pan-African) age. Zircon U-Pb data on six of these samples yielded ages ranging from c. 650–c. 550 Ma. Suggestions that parts of the migmatites might be of Eburnean age could not be confirmed. The Neoproterozoic granitoid rocks may represent a Pan-African arc overlying an eastward dipping subduction zone, and Sm-Nd data indicate that they contain significant proportions of older crustal material. Age probability diagrams for detrital zircons from three metasedimentary rocks (one from Nigeria) and one S-type quartz diorite from the eastern region show prominent peaks at c. 1900 Ma and 2400–2200 Ma. A provenance of the zircons from sources in the Borborema province in NE Brazil or lateral equivalents thereof is suggested. A few Neoproterozoic zircons are present in three of the samples. On the basis of Th/U ratios a distinction can be made between Neoproterozoic zircons of metamorphic and detrital origin. The zircon age probability diagram of a quartzite from northern Benin exhibits one peak, at c.1900 Ma, different from the other metasedimentary samples. This quartzite might be correlated with sedimentary strata of the Volta basin that overlie the Eburnean basement in Ghana, Togo and Burkina Faso. Correlation of the rocks in the study area with those in the Borborema province is discussed.

Description: Publication year: 2011 Source: Precambrian Research, Available online 14 December 2011 Nils Lenhardt, Patrick G. Eriksson The volcanic rocks of the Rooiberg Group represent the uppermost unit in the Palaeoproterozoic Transvaal Supergroup and form one of the largest provinces of silicic volcanic rocks in the world. Although stratigraphically associated with the Transvaal basin-fill, the Rooiberg Group is petrogenetically linked with the larger Bushveld magmatic event for which emplacement was preceded by the extrusion of the vast Rooiberg lava flows in the northern part of the Kaapvaal Craton. Like many silicic-dominated Large Igneous Provinces (LIPs), which are increasingly recognised in the rock record, the Rooiberg Group volcanics are intracontinental, subaerial, and are dominated by voluminous lava flows. Originally, the Rooiberg Group is inferred to have covered an area of more than 200,000 kmof which, after erosion, an area of 50,000 to 67,000 kmremains. The lava flows form a stratigraphic succession up to 6 km thick, and are divided into four formations in ascending order: Dullstroom, Damwal, Kwaggasnek and Schrikkloof. Due to a scarcity of reliable geochronologic data, the temporal span of the Rooiberg Group is poorly understood. The Rooiberg Group consists of basaltic to rhyolitic lava erupted from fissural volcanism with estimated eruption temperatures of the rhyolitic lavas exceeding 1000 °C. Minor explosive eruptions are represented by pyroclastic rocks, and subordinate sedimentary interbeds originated from sandy fluvial and lacustrine processes. The rocks are essentially undeformed and have not been buried so that their original textures are well preserved. The Bushveld Complex and the associated Rooiberg Group lava flows are proposed to have formed as a result of partial melting of subcontinental lithosphere and lower crust by a mantle plume. This thorough review of the geochronology, physical volcanology, and geochemistry of the Rooiberg Group enables construction of a geodynamic model. Highlights ► intracontinental, subaerial volcanism dominated by voluminous lavas ► high temperature, low viscosity, high eruption rates caused widespread distribution ► alternating effusive and explosive eruptions concomitant with clastic deposition ► eruptions from several vents along a fissure.

Description: Publication year: 2011 Source: Precambrian Research, Available online 17 December 2011 Stanislaw Mazur, Robert Anczkiewicz, Jacek Szczepanski, Jeroen A.M. van Gool, Matthew Thirlwall Lu–Hf and Sm–Nd garnet geochronology combined with geothermobarometric calculations allowed us to gain new insight into the metamorphic evolution of the Northern Nagssugtoqidian orogen (NNO), in West Greenland. Three samples were collected from the southern (SM 310 and SM 316) and northern (SM 339) parts of the NNO. They revealed amphibolite-grade Palaeoproterozoic metamorphism under peak temperature conditions decreasing from c. 810-840 °C at 7.5 kbar (SM 310, 316) to c. 690 °C at 10 kbar (SM 339) in the southern and northern part of the NNO, respectively. Our garnet dating defines two episodes: (1) at c. 1780 Ma, determined by Lu-Hf in sample SM 316 and Sm-Nd in sample SM 339, most likely approximating the time of garnet growth, and (2) at c. 1750 Ma defined by the remaining Sm-Nd dates, reflecting time of cooling below isotopic closure. Original garnet growth patterns preserved by HREE (SM 316) and clearly seen zonation of Sm and Nd (SM 339) suggest that the garnets may not have been completely reset and thus, the obtained age of 1780 Ma could be fairly close to the time of garnet crystallization.The growth of peak metamorphic mineral assemblages in the southern NNO took place after the end of deformation, as demonstrated by sample SM 316 that was collected from an undeformed mafic dyke cutting across the regional fabric. In contrast, sample SM 339 revealed synkinematic growth of peak metamorphic paragenesis that crystallised in a low angle top-to-SW shear zone overprinted on the pre-existing fabric. Our results are consistent with the interpretation of the NNO as an Archaean crustal block, the Aasiaat domain, that was affected by Palaeoproterozoic metamorphism and localised deformation at 1780 Ma i.e., 70 Ma after the Nagssugtoqidian collision. This event can be compared to the formation of steep belts (major left-lateral strike-slip shear zones) in the central part of the Nagssugtoqidian orogen. The Aasiaat domain is considered to be an independent microplate separating the Nagssugtoqidian and Rinkian orogens which define respectively its southern and northern margins. The interpretation presented implies the existence of two discrete tectonic sutures to the north and south of the Aasiaat domain. Highlights ► Northern Nagssugtoqidian Orogen (NNO) was overprinted by amphibolite-grade Paleoproterozoic metamorphism decreasing northward. ► Lu-Hf and Sm-Nd geochronology suggest that peak metamorphism conditions were attained not much earlier than 1780 Ma. ► Paleopretorozoic metamorphism overprinted an Archean regional fabric that avoided penetrative deformation during the Nagssugtoqidian orogeny. ► The NNO represents an Archean block that experienced Proterozoic metamorphism and localised deformation well after the Nagssugtoqidian collision at 1850 Ma.