Late Precambrian Research Articles

Breakup of Rodinia and early stages of evolution of the Paleoasian ocean

The main stages of the evolution of newly formed structural elements are considered against the background of breakup of the epi-Grenville Rodinia supercontinent, which started about 950 Ma ago. The paleomagnetic data on pathways of the traveling of Rodinia’s fragments are analyzed and evidence for their geology, magmatism, and sedimentation are integrated with special emphasis on the evolution of the continental margins. A series of paleotectonic maps with elements of paleogeography for time intervals of 950–900, 850–800, 750–700, 650–630, and 570–550 Ma ago has been compiled on the basis of palinspastic reconstructions with allowance for new paleomagnetic data primarily concerning the position of Siberia in the Late Precambrian. Objects all over the world, not only in Russia, are involved in the analysis, though with less thoroughly described paleogeography. The structural elements of the Paleoasian ocean are included into the system of global paleooceans and framing paleocontinents. The history of the Paleoasian ocean is traced through 400 Ma from the breakup of the Rodinia supercontinent to the origin of the new Paleogondwana supercontinent about 550 Ma ago.

Age and source material of the Kingash ultramafic-mafic massif, East Sayan

The REE distribution patterns and Nd whole-rock and mineral isotope ratios of the Kingash ultramafic-mafic massif enabled us to propose a multistage history for its evolution at 1410 and 875 Ma. These stages reflect the magmatic evolution of the Siberian paleocontinent margin during the Late Precambrian. The age of metamorphism of the massif during collision and accretion in the Early Paleozoic (∼500 Ma) was obtained based on a Sm-Nd mineral isochron from rheomorphic veined albitite. The Nd and Sr isotopic compositions of rocks from the Kingash massif suggest mantle sources for picritic and basic magmas, which are thought to have originated by mixing of different proportions of depleted (PREMA or DM) and enriched (EM) melts. The initial isotope ratios of the parental melts transformed during interaction with Sr-rich material from the host metasedimentary complexes.

Proto-Tethyan remnants in northwest Iran: Geochemistry of the gneisses and metapelitic rocks

The Iran continental crust was metamorphosed, intruded by granitoid magmas, folded and faulted during the Late Precambrian by the Pan-African Orogeny. The basement complex in the Takab Complex (northwest Iran) consists of gneisses, amphibolites, pelitic schists, meta-ultramafic and calc-silicate rocks. Geochemically, the protoliths of the Takab gneisses are slightly peraluminous and medium to high-potassic with calc-alkaline affinity. These gneisses may have been emplaced in volcanic arc tectonic setting. Furthermore, the metapelitic protolith is shale deposited in an active continental margin setting. All these characteristics, and presence of paleo-suture zone and ophiolitic rocks (i.e. serpentines, meta-mafic and meta-ultramafic rocks) around the high grade metamorphic rocks suggest that a continental-margin magmatic arc (Andean-type) formed the Takab Precambrian basement. The basement complexes are extensively overprinted by the Pan-African Orogeny and younger igneous events; this supports the inference that Early Cambrian orogenesis in the Takab Complex region of northwest Iran marks one of the fundamental lithospheric boundaries within Gondwana which belonged to a greater Late Neoproterozoic–Early Paleozoic orogenic system that was active along the Proto-Tethyan margin of the Gondwana supercontinent, extending at least from its Arabian margin to the Himalayan margin of the Indian subcontinent.

The Baykonurian glaciohorizon of the Late Vendian

Glacial deposits occurring close to the Vendian-Cambrian boundary were discovered long ago in several regions of Asia and West Africa. New data of recent years confirm earlier observation and specify age constraints of these deposits. In certain sections, glacial sediments rest on beds with the Upper Vendian fossils (Vendotaenia sp., Sabellidites remains, and microfossils). In the Hoggar Highland, they discordantly overlie magmatic rocks ca. 560 Ma old. In the East Sayan Mts., the Upper Vendian Cloudina sp. have been found in the diamictite matrix. In Central Asia and Northwest China, cap dolomites conformably overlying glacial deposits grade upward into the chert-phosphatic sediments containing fossils (Tarim region), which are characteristic of the Anabarites trisulcatus Zone in the Nemakit-Daldynian Stage. Fossils of this zone have been found directly above glacial deposits in the East Sayan. According to all these data, the respective Baykonurian glaciation, as it is termed in this work, either antedated the Nemakit-Daldynian Age or was concurrent to its commencement. The glaciation is correlative with a significant negative δ13C excursion of −5 to −10‰. Judging from recent paleotectonic reconstructions, the Baykonurian glaciation was not regional in rank and left records in both hemispheres of the Earth. This glaciation resumes the well-known succession of the great glaciation event in the terminal Late Precambrian, i.e., the Sturtian, Marinoan, and Gaskiers glaciations. Hence, one cannot ignore the Baykonurian glaciation when analyzing the geological, biospheric, and biotic history of the Late Precambrian.

Late Proterozoic plate tectonics and palaeogeography: a tale of two supercontinents, Rodinia and Pannotia

Abstract The plate tectonic and palaeogeographic history of the late Proterozoic is a tale of two supercontinents: Rodinia and Pannotia. Rodinia formed during the Grenville Event ( c . 1100 Ma) and remained intact until its collision with the Congo continent (800–750 Ma). This collision closed the southern part of the Mozambique Seaway, and triggered the break-up of Rodinia. The Panthalassic Ocean opened as the supercontinent of Rodinia split into a northern half (East Gondwana, Cathyasia and Cimmeria) and a southern half (Laurentia, Amazonia–NW Africa, Baltica, and Siberia). Over the next 150 Ma, North Rodinia rotated counter-clockwise over the North Pole, while South Rodinia rotated clockwise across the South Pole. In the latest Precambrian (650–550 Ma), the three Neoproterozoic continents – North Rodinia, South Rodinia and the Congo continents – collided during the Pan-Africa Event forming the second Neoproterozoic supercontinent, Pannotia (Greater Gondwanaland). Pan-African mountain building and the fall in sea level associated with the assembly of Pannotia may have triggered the extreme Ice House conditions that characterize the middle and late Neoproterozoic. Although the palaeogeographic maps presented here do not prohibit a Snowball Earth, the mapped extent of Neoproterozoic ice sheets favour a bipolar Ice House World with a broad expanse of ocean at the equator. Soon after it was assembled ( c . 560 Ma), Pannotia broke apart into the four principal Palaeozoic continents: Laurentia (North America), Baltica (northern Europe), Siberia and Gondwana. The amalgamation and subsequent break-up of Pannotia may have triggered the ‘Cambrian Explosion’. The first economically important accumulations of hydrocarbons are from Neoproterozoic sources. The two major source rocks of this age (Nepa of Siberia and Huqf of Oman) occur in association with massive Neoproterozoic evaporite deposits and in the warm equatorial–subtropical belt, within 30° of the equator.

Records of Precambrian–Early Palaeozoic volcanic and sedimentary processes in the Central European Variscides: A review of SHRIMP zircon data from the Kaczawa succession (Sudetes, SW Poland)

The early, pre-orogenic stages of evolution in the Variscan belt, i.e. rifting processes, opening of sedimentary basins and associated igneous activities, are often obscure because many successions have yielded little or no biostratigraphic data, have a strong metamorphic overprint and are tectonically deformed and dislocated. The increasing application of SHRIMP zircon dating has provided useful constraints on magmatic and metamorphic processes, helped locate probable source areas for detritus within sedimentary successions and facilitated large-scale palaeogeographic correlations. This methodology has recently thrown considerable light on the age and relationships of the previously poorly constrained rock units of the Kaczawa Complex in the Polish West Sudetes. Thus, recent SHRIMP studies in the Kaczawa Mountains have yielded Early Ordovician ages of the initial rift type bimodal volcanic suites at the bottom part of the Kaczawa Succession: c. 503 Ma for metarhyodacites of crustal derivation, and c. 485 Ma for alkaline metatrachytes of mantle signature. These dates provide a firm temporal constraint on the initial rift magmatism interpreted as related to the continental break-up of the northern peripheries of Gondwana. New SHRIMP data from metavolcaniclastic and metasedimentary rocks of the Kaczawa Complex have yielded results that have provided significantly changed interpretations on their age and relationships. For instance, a siliciclastic sequence interpreted as belonging to the lower part of the Kaczawa Complex (the Gackowa Sandstones) and seemingly sourced (using an array of geochemical and mineralogical evidence) from nearby early Ordovician volcanic rocks has, surprisingly, yielded zircon ages not younger than Precambrian and thus this unit has tentatively been reinterpreted as a possible correlative of the Neoproterozoic Lusatian Graywackes. Felsic metavolcaniclastic rocks embedded in the carbonate succession of the Wojcieszów Limestone have yielded SHRIMP zircon ages of c. 497 Ma, thus supporting a late Cambrian/early Ordovician age for this shallow-water limestone sequence (which had been proposed to be late Ordovician/Silurian or younger in age, based on controversial foraminifera evidence, and more recently assigned to the Mid Cambrian, based on newly found archaeocyathid fragments). And, age spectra of zircon populations in the metamudstone sequence of the Radzimowice Slates of thus far enigmatic stratigraphic position indicate that these deposits may have accumulated during early Palaeozoic rifting, with source areas similar to those known from parts of Gondwana (e.g. NW Africa). Overall, the new SHRIMP zircon studies, both in metavolcanic and metasedimentary sequences, have considerably illuminated their affinities, and should provide encouragement for further application of this technique in this region. In general the results provide support and detail for interpretation of this rock complex as reflecting the early stages of development of the volcanic and sedimentary processes operating along North Gondwana during latest Precambrian and early Palaeozoic times.

A problem of total glaciations on the Earth in the Late Precambrian

Ideas of global glaciations on the Earth repeatedly emerged in geology since the middle of the 19th century, but they all did not withstand the test of time. The hypothesis of snowball Earth that suggests long-lasted continuous glaciations over the entire land and oceans in the Late Riphean and Vendian became a popular topic of discussions worldwide. These glaciations must last continuously 15 million years or more owing to enormous stability of climate on the “White Earth,” and one can expect their cessation only when CO2 concentration in the atmosphere is getting by several orders of magnitude higher in the course of volcanic eruptions. However, many sections of the Late Precambrian glacial deposits evidence repeated alternations of glacial and interglacial events of variable rank and oscillations of glaciers. Consequently, liquid water existed on the Earth, hydrological cycle had not been interrupted, and development of phototrophic phytoplankton, the eukaryotic organisms inclusive, was always in progress. These facts and results of the climate mathematical simulation are inconsistent with the concept of long continuous glaciations over the globe and their consequences. Paleomagnetic data represent main starting point of the snowball Earth hypothesis, although they are to a great extent still of insufficient validity for the Precambrian. Equitable criticism of the hypothesis weak sides turns sometimes into denying all the glacial periods of the Late Proterozoic, when tillites are regarded in majority as deposits of subaqueous slumps and debris flows accumulated on walls of oceanic rifts prograding along splitting lines into the Rodinia continent. Doubtless marks of glaciations in the sedimentary succession are regarded therewith as heterochronous indications of local mountain glaciers on risen shoulders of prograding rifts. Widespread occurrence of the Late Precambrian discrete glacial horizons on the platforms means, however, that glaciations of that time have been associated not always with the rifts and consisted of separate glacial events. Glaciogenic horizons comparable in thickness and structure with those of the Phanerozoic Eonothem are also indicative of discrete glacial periods in the Precambrian. Being confined predominantly to the Late Precambrian succession of rifts, these horizons characterize a high burial potential of these structures, whereas outside them glacial horizons of lesser thickness could be easily subjected to erosion. The hypothesis of snowball Earth is inadequately consistent with well-known facts and needs in additional substantiation. There are also grounds to think that oceans have not been completely covered with ice at the time of Precambrian glaciations.

Late Precambrian rifting in the geological history of the western slope of the South Urals

The rift-related geodynamic setting of the Late Precambrian geological evolution on the western slope of the South Urals is reconstructed on the basis of localization of lithotectonic complexes of this age, their formation conditions, and the geochemistry of rocks. The Early Riphean stage comprises accumulation of coarse-clastic rocks intercalating with alkaline volcanic rocks of the Navysh Complex, which is a constituent of the Ai Formation, and emplacement of doleritic and picritic intrusions of the Shuida Complex and melanocratic dolerite and gabbrodolerite of the Yusha Complex. The Middle Riphean stage is characterized by wide-spread coarse-clastic terrigenous rocks of the Mashak Formation that intercalate with volcanic rocks of the bimodal basalt-rhyolite association, the Berdyaush pluton of rapakivi granite, the Kusa-Kopan layered intrusive complex, the Lapyshta Complex of dolerites and picrites, and numerous occurrences of gabbrodolerites. The terrigenous rocks of the Vendian stage include conglomerate, gravelstone, and sandstone of the Asha Group, while igneous rocks comprise alkaline volcanics of the Arsha Complex, alkali gabbroids of the Miseli Complex, and melanocratic syenite of the Avashla Complex. The geological evolution of the region is distinguished by local (failed or aborted) rifting. The occurrence of lithotectonic complexes is controlled by dynamic conditions of rifting. A certain inheritance in the evolution may be traced for the Early and Middle Riphean and partly for the Late Riphean and Vendian.

Presence of two plume-related volcanic events along the Indian northern passive margin evidenced by the geochemistry of the Carboniferous Baralacha La dykes, the Permian Panjal traps and the Drakkar Po phonolites

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 113

Deciphering deuterostome phylogeny: molecular, morphological and palaeontological perspectives

Deuterostomes are a monophyletic group of animals that include the vertebrates, invertebrate chordates, ambulacrarians and xenoturbellids. Fossil representatives from most major deuterostome groups, including some phylum-level crown groups, are found in the Lower Cambrian, suggesting that evolutionary divergence occurred in the Late Precambrian, in agreement with some molecular clock estimates. Molecular phylogenies, larval morphology and the adult heart/kidney complex all support echinoderms and hemichordates as a sister grouping (Ambulacraria). Xenoturbellids are a relatively newly discovered phylum of worm-like deuterostomes that lacks a fossil record, but molecular evidence suggests that these animals are a sister group to the Ambulacraria. Within the chordates, cephalochordates share large stretches of chromosomal synteny with the vertebrates, have a complete Hox complex and are sister group to the vertebrates based on ribosomal and mitochondrial gene evidence. In contrast, tunicates have a highly derived adult body plan and are sister group to the vertebrates based on the analyses of concatenated genomic sequences. Cephalochordates and hemichordates share gill slits and an acellular cartilage, suggesting that the ancestral deuterostome also shared these features. Gene network data suggest that the deuterostome ancestor had an anterior-posterior body axis specified by Hox and Wnt genes, a dorsoventral axis specified by a BMP/chordin gradient, and was bilaterally symmetrical with left-right asymmetry determined by expression of nodal.

Mayanian (1100–850 Ma) – Prebaikalian Upper Riphean of Siberia

The Mayanian (1100–850 Ma) is an all-Siberian stratigraphic subdivision of the Neoproterozoic distinguished on the basis of generalized geological and paleontological data for the Maya Group in the southeast of the Siberian craton and for its counterparts in other structure-facies areas (SFA). The Mayanian deposits in all principally different structures of every SFA, such as intracratonic depressions, pericratonic troughs, and marginal parts of blocks with oceanic crust, were incorporated into the craton in the Vendian. It is separated from the surrounding stratigraphic divisions, Baikalian and Aimchanian, by distinct tectonic boundaries coinciding in time with the formation and starting breakup of the supercontinent Rodinia, i.e., with global events. In the adjacent parts of the platform they were reflected in erosion and considerable gaps in sedimentation. These events are well-expressed where they occurred under intense contraction or, on the contrary, are veiled where extension was predominant. The boundary between the Kerpylian and Lakhandinian, subdivisions of the Mayanian, is not bearing on important events and is hardly traceable throughout the region. Therefore, the base of the Kerpylian is preferable to be taken as the lower boundary of the Upper Riphean. There are attempts to establish this boundary, as it is made for the Phanerozoic, from paleontological data in compositionally similar deposits. These attempts, however, fail in matching this boundary with the base of the Lakhandinian. Distinctive transitions in communities of microfossils and stromatolites appear to be linked to the Mayanian-Baikalian and, undoubtedly, Middle Riphean-Aimchanian boundaries. In the Lakhandinian, on the contrary, the changes that appeared in the Kerpylian only tend to continue. Some taxa typical of the Lakhandinian appear as early as the Kerpylian. In the Lakhandinian their amount gradually increases upsection, until the top of this division. Thus, it is reasonable to start the Upper Riphean with the lower boundary of the Mayanian. All the main subdivisions of the Siberian Neoproterozoic — Mayanian, Baikalian, and Vendian — are linked to the most important stages of development of this vast and structurally heterogeneous territory. Each of the above-mentioned stratons can be divided, according to historico-geological and paleontological data, into two or three independent interregional subdivisions. Thus, the stratigraphic scheme of the Neoproterozoic of Siberia seems to be promising for the general scale of this period of the Late Precambrian.

Tectonics versus eustatic control on supersequences of the Zagros Mountains of Iran

At least 12 km of strata ranging in age from the latest Precambrian to the Recent are exposed in the Zagros Mountains of Iran. This sedimentary cover is characterized by distinct stratal packages separated by major unconformities forming twelve supersequences. They are informally named as: (1) Late Precambrian – Cambrian Hakhamanesh Supersequence, (2) Ordovician Kourosh Supersequence, (3) Silurian Camboojiyeh Supersequence, (4) Devonian Darioush Supersequence, (5) Mississippian – Pennsylvanian Khashayar Supersequence, (6) Permian – Triassic Ashk Supersequence, (7) Jurassic Farhad Supersequence, (8) Early Cretaceous Mehrdad Supersequence, (9) Late Cretaceous Ardavan Supersequence, (10) Paleocene – Oligocene Sassan Supersequence, (11) Oligocene – Miocene Ardeshir Supersequence, and (12) Miocene – Pleistocene Shapour Supersequence. These supersequences and their correlatives in neighboring areas have been used to infer tectonic events. The dominant interpretation has been that local or regional epeirogenic movements were responsible for the formation of these supersequences. Unconformities are considered as indications that epeirogenic movements associated with tectonic events affected the area. The present investigation provides an alternative to the established view of the Phanerozoic supersequences of the Zagros Mountains. A good correlation exists between the lithofacies of supersequences in the Zagros Mountains and the second-order eustatic sea-level changes. Deposition of deep-water, marine shales occurred during periods of eustatic sea-level rise. Platform-wide unconformities coincided with eustatic sea-level lows. In fact, supersequences of the Zagros Mountains are nearly identical to those described from the North American Craton and the Russian Platform suggesting that these stratal packages are global. These observations suggest that supersequences of the Zagros Mountains formed by second order eustatic sea-level changes and not by local or regional epeirogenic movements. Although tectonic events did not produce supersequences of the Zagros Mountains, they influenced regional lithofacies patterns through the formation of intrashelf depressions such as the Hormoz Salt Basin during the Precambrian and the Dezful Embayment and the Lorestan Basin during the Mesozoic. Tectonic events also affected sedimentation during the Tertiary collision of Arabia and the Central Iran microplate through uplift, erosion, and the formation of the Zagros Foreland Basin. The results of this investigation necessitate a re-evaluation of the role and the significance of pre-Tertiary tectonic events commonly used to interpret the geological evolution of the Zagros Mountains.

Cambrian explosion: Birth of tree of animals

Excluding the sponges the Kingdom Animalia is usually divided into three subkingdoms: Diploblasta, Protostomia and Deuterostomia. The Cambrian Explosion consists of three major episodes, two of which were in the early Early Cambrian (one represented by the small skeletal fossils “SSFs” at the base of the Cambrian and the other represented by the succeeding Chengjiang faunas “CFs”), and the other episode as their prelude took place in the “Eocambrian” (i.e. the latest Precambrian), represented by the Ediacaran faunas. This unique Big Bang of life has been recognized as giving birth to the entire morphological Tree Of Animals (or metazoans), in short the TOA. Its “seed” in the deep Precambrian, represented by some sort of protist from which the complete TOA must have grown, remains unknown paleontologically. However, the fossil evidence suggests that the three major episodes of the Cambrian Explosion are responsible for the earliest radiations of the three subkingdoms of animals respectively. While the observed Ediacaran fauna might constitute only a small part of the whole Ediacaran biota, our evidence supports that it was dominated by diploblasts (the “trunk” of the TOA) with only a few possible stem-group triploblasts. The Early Cambrian in turn in two phases explosively yielded almost all the major triploblastic crown-branches (Bilateria: the huge “crown” of the TOA), which include the other two subkingdoms: first the extremely diverse protostomes in the Meishucunian Age and then followed by a nearly entire lineage of early deuterostomes from the Chengjiang, including even its most derived member – the earliest true vertebrates. Among the four most significant milestones of morphological origins and radiations in animal history, the first one (i.e. appearance of metazoans) took place in the Ediacaran Period or earlier times, and the other three can be seen in the windows available from the Chengjiang and the Meishucunian fossil assemblages. The newly discovered extinct Phylum Vetulicolia, which has primitively segmented body with simple gill slits in its anterior division, most probably represents one of the roots of the deuterostome subkingdom. Showing a mosaic of basic features possessed in both the bilateral vetulicolians and some primitive echinoderms, the soft-bodied vetulocystids are best regarded as one of the roots of the extant pentamerous echinoderms. Standing on the “top” of the deuterostome super-branch in the early Cambrian TOA are the “the first fish” Myllokunmingia and Haikouichthys, which bear paired eyes and salient proto-vertebrae. These animals represent the real root of the remainder of the vertebrates or craniates. On the contrary, yunnanozoans, including Yunnanozoon and Haikouella, possess neither eyes nor unequivocal vertebrae, and may have nothing to do with the craniates, let alone the vertebrates. Those enigmatic creatures share a similar body-plan with vetulicolians and should be treated as a side-branch within the lower deuterostomes.

Cancer as a consequence of the rising level of oxygen in the Late Precambrian

The origin of multicelled animal life required collagen-family molecules whose own formation depended on the availability of molecular oxygen. Cancers, by contrast, are characterized by their low use of oxygen. In discussing the relationship between the origin of multicelled life and the origin of cancer, it is useful to think in terms of tissues rather than individual cells or complete animals. When animal tissues are disturbed, their constituent cells may be partially released from the constraints of multicellularity. This permits or obliges cells to reactivate anaerobic metabolic ways used by their single-celled ancestors in the oxygen-deficient Precambrian seas. Inhibition or loss of cell respiration under such circumstances may cause reversion to glycolytic fermentation, a less efficient metabolic style that generates waste products that are retained, thereby producing excess cell-growth. Distortion of tissue architecture may ensue with impairment of cell-to-cell adhesion, thereby liberating individual cells. Cells freed from tissue constraints undergo Darwinian variation which leads to loss of differentiation and produces cell types that are incompatible with the normal functioning of tissues. These steps, which may manifest themselves as carcinogenesis, are not reversible by restoration of oxygen and in effect constitute a demergence from the metazoan state. The existence of cancer among diverse phyla and especially among domesticated animals, suggests that the risk of cancer may be an initial condition of complex multicellular life and that it remains preferentially associated with newly modified designs. If so, there would be therapeutic strategies that have not yet been adequately considered. □Cambrian explosion, cancer, cell differentiation, collagen, glycolysis, hard parts, metazoan origins.

Early Cambrian Yangtze Plate Maotianshan Shale macrofauna biodiversity and the evolution of predation

The large number of soft-part preserved fossils from the Lower Cambrian Maotianshan Shale (Chengjiang) Lagerstätten suggests particularly favourable conditions for a rich life on the shelf-zone sea bottom of the Yangtze Plate, China. This high degree of biodiversity opens an excellent window into the early radiation phase of Metazoa and represents a significant data source for the study of adaptive strategies among early animals. Feeding and locomotion are the main life strategies of organisms we investigated with regard to two major benthic macrofaunal components of the Maotianshan Shale biota, the nemathelminths and the arthropods. Our attempt was to test whether food, feeding and locomotory strategies of the benthic Lower Cambrian shallow-water communities were as diversified as it appears from the morphological diversity of the organisms present. Two major types of feeders can be discerned: suspension/micro-particle feeders – mostly epibenthic sedentary taxa – and larger-particle feeders, living in benthic to bentho-pelagic realms. Scant evidence exists for exclusive vegetarians, fungi eaters, biomat utilizers and for in-faunal vertical burrowers or grazers (bioturbators). Predation, in a wider sense, seems to be a, if not the, major feeding mode among metazoans. Nemathelminths and arthropods are amongst the best examples. In the benthic shallow-water regime, as exposed by the Maotianshan Shale biota, animals and their ontogenetic stages were the most suitable and readily available food source. At least for arthropods, we propose that improvement of predatory strategies was paralleled by the enhancement of locomotory and food manipulation structures. Accumulating evidence of late Precambrian to Early Cambrian metazoans exposing diverse morphologies and life styles indicates that, on the large scale, phylogenesis progressed gradually in the Late Precambrian. This renders an “explosive” radiation of Metazoa unlikely.

Diversity of platinum-group minerals in podiform chromitites of the late Proterozoic ophiolite, Eastern Desert, Egypt: Genetic implications

Podiform chromitites are frequently distributed as lensoidal pods in the central and southern parts of the Eastern Desert, Egypt. They are, in most cases, hosted by fully serpentinized peridotite which is a part of dismembered ophiolite complexes of the Pan–African belt of Late Precambrian age. Serpentinites are the predominant components in the ophiolitic mélange, either as matrix or as variably sized blocks, and are derived from harzburgite and subordinate dunite. The central Eastern Desert (CED) chromitites have a wide compositional range from high-Cr to high-Al varieties, whereas those of the southern Eastern Desert (SED) have a very restricted compositional range. The Cr# of spinel ranges from 0.5 up to 0.8 in the former, while it is around 0.8 in the latter. Platinum-group element (PGE) mineralization has been recently reported in podiform chromitites from the late Proterozoic Pan–African ophiolite of the Eastern Desert of Egypt. The populations of platinum-group minerals (PGM) in the studied CED and SED chromitites are quite distinguishable; they are mainly sulfides (Os-rich laurite) in the former, and Os–Ir alloy in the latter. Sulfarsenides and arsenides are also found in subordinate amounts from both chromitites. The most abundant base metal sulfides (BMS) in the Eastern Desert chromitites of Egypt are millerite, heazlewoodite, pentlandite, chalcopyrite and pyrite. The sulfur fugacity and temperature conditions are the main controller of PGE mineralogy in the host chromitite at the initial stage within the upper mantle. Os-rich laurite is stable at high sulfur fugacity and low temperature conditions, whereas Os–Ir alloy is stable at lower sulfur fugacity and higher temperature conditions. The diversity of PGE mineralogy combined with the differences in petrological characteristics of chromian spinels from CED chromitites to SED ones suggests different degrees of partial melting of the mantle rocks of this ophiolite which, in turn may be attributed to different tectonic settings. During the post-magmatic processes, i.e., serpentinization, the primary PGM, e.g., Os-rich laurite and Os–Ir alloy, can be modified at low temperatures to secondary PGM.

The Egersund dykes (SW Norway): a robust Early Ediacaran (Vendian) palaeomagnetic pole from Baltica

SUMMARY Palaeomagnetic data for Baltica in the Late Precambrian are highly ambiguous, and have therefore, given rise to different interpretations concerning the need to explain Varangerian glaciations through snowball Earth conditions. We present new palaeomagnetic data from the 616 ± 3 Ma (U-Pb) Egersund dykes in SW Norway, which yield a palaeolatitude of 53 ◦ +16 ◦ /−13 ◦ for the studied location. This would indicate that the Baltica plate spanned latitudes between 50 ◦ and 75 ◦ S in the Early Ediacaran. The pole position (31 ◦ N, 44 ◦ E, dp/dm = 15/17) confirms earlier studies, but the primary nature of the remanence is now supported by two positive contact tests. A new 40 Ar/ 39 Ar biotite age of 600 ± 10 Ma from one of the dykes suggests that remanence acquisition in the dykes took place between 600 and 616 Ma. The Egersund palaeomagnetic data demonstrate that Baltica was located at relatively high latitudes at the time of the Varangerian glaciations. Additional palaeomagnetic sites in the Rogaland Igneous Complex yield a pole position (46 ◦ S, 238 ◦ E, dp/dm = 17/19) that confirms previous studies. The age of this remanence has traditionally been quoted as c. 930 Ma based on U-Pb ages from the complex. However, previous 40 Ar/ 39 Ar hornblende ages of around 870 Ma are now supported by a new 40 Ar/ 39 Ar biotite age of 869 ± 14 Ma obtained from a noritic dyke, and we argue that this represents an uplift/cooling age which better represents the age of the remanence in SW Norway.

Latest Precambrian to Early Cambrian U–Pb zircon ages of augen gneisses from Calabria (Italy), with inference to the Alboran microplate in the evolution of the peri-Gondwana terranes

In situ U–Pb dating of zircons from five samples of Calabrian augen gneisses shows that their protoliths are Latest Precambrian to Early Cambrian in age (562 ± 15, 547 ± 7, 540 ± 4, 539 ± 16 and 526 ± 10 Ma), and contain Archaean (3.1 Ga), Palaeoproterozoic (1.7–2.4 Ga) and Neoproterozoic (0.6–0.9 Ga) inheritance. Geochemical signature of augen gneisses is typical of high-K calc-alkaline post-collisional magmatism. Their Sr–Nd isotopic compositions [0.7093 < (87Sr/86Sr)i < 0.7139; −3.2 < ɛNd(t) < −5.4; 1.5 < T DM < 1.7 Ga] indicate the involvement of a crustal component in significant proportions. The Calabrian augen gneisses have, therefore, to be distinguished from the orthogneisses of Sardinia and northern Algeria, and from the porphyroids of Sicily, which are Middle Ordovician. By contrast, the Calabrian augen gneisses show a close similarity to the Pan-African post-collisional granitoids of the northern edge of the West African craton (e.g. the Moroccan Anti-Atlas). This suggests a peri-Gondwana origin and corroborates previous palaeogeodynamic reconstructions attributing the Alboran microplate to the northern margin of the West African craton.

New insights into Arctic paleogeography and tectonics from U‐Pb detrital zircon geochronology

To test existing models for the formation of the Amerasian Basin, detrital zircon suites from 12 samples of Triassic sandstone from the circum‐Arctic region were dated by laser ablation‐inductively coupled plasma‐mass spectrometry (ICP‐MS). The northern Verkhoyansk (NE Russia) has Permo‐Carboniferous (265–320 Ma) and Cambro‐Silurian (410–505 Ma) zircon populations derived via river systems from the active Baikal Mountain region along the southern Siberian craton. Chukotka, Wrangel Island (Russia), and the Lisburne Hills (western Alaska) also have Permo‐Carboniferous (280–330 Ma) and late Precambrian‐Silurian (420–580 Ma) zircons in addition to Permo‐Triassic (235–265 Ma), Devonian (340–390 Ma), and late Precambrian (1000–1300 Ma) zircons. These ages suggest at least partial derivation from the Taimyr, Siberian Trap, and/or east Urals regions of Arctic Russia. The northerly derived Ivishak Formation (Sadlerochit Mountains, Alaska) and Pat Bay Formation (Sverdrup Basin, Canada) are dominated by Cambrian–latest Precambrian (500–600 Ma) and 445–490 Ma zircons. Permo‐Carboniferous and Permo‐Triassic zircons are absent. The Bjorne Formation (Sverdrup Basin), derived from the south, differs from other samples studied with mostly 1130–1240 Ma and older Precambrian zircons in addition to 430–470 Ma zircons. The most popular plate tectonic model for the origin of the Amerasian Basin involves counterclockwise rotation of the Arctic Alaska–Chukotka microplate away from the Canadian Arctic margin. The detrital zircon data suggest that the Chukotka part of the microplate originated closer to the Taimyr and Verkhoyansk, east of the Polar Urals of Russia, and not from the Canadian Arctic.

The Permian of the Verkhoyansk–Okhotsk region, NE Russia

This report summarizes data on the Permian deposits of the Verkhoyansk–Okhotsk region (tectonics, paleogeography, fauna, flora and stratigraphy). The Verkhoyansk–Okhotsk region covers the area of the Verkhoyansk fold-and-thrust belt. The Permian deposits of this region were formed on the passive continental margin of the Angarida Continent, the ancient Siberian Continent, situated within the Siberian Platform. This continental margin developed as the result of continental break-up in the Late Precambrian. Late Precambrian to Jurassic (including Permian) sediments were deposited on a passive margin with a continental shelf extending out into an ocean towards the northeast. During the Permian, exclusively terrigenous sediments were deposited over a vast territory in the Verkhoyansk paleobasin forming part of this continental margin. The Permian deposits are divided into six regional stratigraphic units ranked as horizons (=regional stages), which in turn are subdivided into faunal zones and beds serving as the basis for intra-regional correlation. In the Late Jurassic–Cretaceous the continental margin sediments were converted into a foreland fold-and-thrust belt due to the collision of the Siberian Platform with the Kolyma–Omolon and Okhotsk continental blocks.