Showing papers in "Gff in 2016"

U-Pb baddeleyite dating of the Proterozoic Pará de Minas dyke swarm in the São Francisco craton (Brazil) – implications for tectonic correlation with the Siberian, Congo and North China cratons

Abstract: U–Pb baddeleyite ages demonstrate the presence of three dyke generations within the NW-trending Para de Minas swarm (Sao Francisco craton, South America), dated at ca. 1795 Ma (1798 ± 4, 1791 ± 7 and 1793 ± 18 Ma), ca. 1710 Ma (1702 ± 13 and 1717 ± 11 Ma) and 766 ± 36 Ma. These ages have implications for the Sao Francisco-Congo (SF-Congo) craton within the Rodinia and Columbia supercontinent configurations. At 1795 Ma, the Rio de la Plata craton (1790 ± 5 Ma Uruguayan dykes), Amazonian craton (1790–1780 Ma Avanavero LIP), Sarmatia (1789 ± 3 Ma Tomashgorod dyke), North Australia (1790 ± 4 Ma Hart dolerites) and the North China craton (1790–1770 Ma Taihang-Xiong’er event) could potentially have been “next-door neighbours” to the SF-Congo craton. Possible matches at 1710 Ma are the 1750–1700 Ma Bilyakchan-Ulkan complex and coeval dykes in the Siberian craton and the 1730 Ma Miyun dyke swarm in NCC. Given large uncertainties, the 766 ± 36 Ma age may be compared with the age of the 790 Ma Niquelândia c...

The U–Pb zircon and baddeleyite ages of the Neoproterozoic Volyn Large Igneous Province: implication for the age of the magmatism and the nature of a crustal contaminant

Abstract: The Volyn continental flood basalt province is situated on the western margin of the East European platform and constitutes a significant portion of the passive continental margin sequence formed along the Trans-European Suture Zone in response to Rodinia break-up in the Neoproterozoic. In Ukraine, the volcanogenic sequence is subdivided into suites called Zabolottya, Babyne and Ratne, which together with the lowermost terrigeneous Gorbashy suite comprise the Volyn series. Magmatic zircons from one high-Ti basalt sample yielded an age of 573 ± 14 Ma, whereas grains isolated from a rhyolitic dacite yielded an age of 571 ± 13 Ma. Baddeleyite from the olivine dolerite sample gave an older 206Pb/238U age of 626 ± 17 Ma, whereas the 207Pb/206Pb weighted average age of 567 ± 61 Ma is close to the zircon ages. Zircons separated from the other basaltic samples are much older and crystallized at c. 1290, 1470, 1820-1860, 1930-2050 and 2660 Ma. Ages in the 1820-1860 and 1930-2050 Ma time spans correspond to...

Baddeleyite U–Pb ages and gechemistry of the 1875–1835 Ma Black Hills Dyke Swarm across north-eastern South Africa: part of a trans-Kalahari Craton back-arc setting?

Abstract: Eleven new baddeleyite U–Pb crystallisation ages and associated whole-rock geochemistry on NE–NNE-trending tholeiitic dykes cutting across the north-eastern corner of the Archaean Kaapvaal Craton, the overlying Transvaal basin and the Bushveld and Phalaborwa igneous complexes collectively define a 1875–1835 Ma Black Hills Dyke Swarm (BHDS). Dyke ages do not discriminate between dyke trends or geographic location, but subdivide the BHDS into an older set of four more primitive dykes (MgO = 9.4–6.8 wt.%) and a younger set of seven dykes with more differentiated compositions (MgO = 5.6–4.2 wt.%). Despite being emplaced over a c. 40 Myr period, major element compositions are remarkably consistent with a single inversely modelled bulk fractionating assemblage of 57.5% plagioclase, 29.5% augite and 13.0% olivine. This fractionating assemblage requires an additional assimilation of bulk continental crust (at a low r-value of 0.2) for reversed modelling of parental rare earth elements. Even though this cr...

U–Pb baddeleyite geochronology and geochemistry of the White Mfolozi Dyke Swarm: unravelling the complexities of 2.70–2.66 Ga dyke swarms across the eastern Kaapvaal Craton, South Africa

Abstract: On the south-easternmost Kaapvaal Craton, a NE-trending plagioclase-megacrystic dolerite dyke swarm, herein named the White Mfolozi Dyke Swarm (WMDS), has been identified. New U–Pb baddeleyite ages presented here indicate that the WMDS was emplaced within less than 10 million years, with our three most robust results yielding a weighted mean age of 2662 ± 2 Ma. The WMDS is coeval with the youngest dykes of a 2.70–2.66 Ga radiating dyke swarm already identified further north on the eastern side of the Kaapvaal Craton. This dyke swarm radiates out from the eastern lobe of the ca. 2.05 Ga Bushveld Complex. A clustering of ages from the WMDS and the 2.70–2.66 Ga radiating dyke swarm identify potential magmatic peaks at 2701–2692 Ma, 2686–2683 Ma and 2665–2659 Ma. Geochemical signatures of the dykes do not correlate with these age groups, but are rather unique to specific areas. The northern part of the eastern Kaapvaal Craton hosts relatively differentiated 2.70–2.66 Ga dolerite dykes that could have ...

New U–Pb geochronologic and palaeomagnetic constraints on the late Palaeoproterozoic Hartley magmatic event: evidence for a potential large igneous province in the Kaapvaal Craton during Kalahari assembly, South Africa

Abstract: The volcanic Hartley Formation (part of the Olifantshoek Supergroup, which is dominated by red bed successions) in South Africa recorded depositional and tectonic conditions along the western Kaapvaal Craton during the late Palaeoproterozoic. It formed in association with red bed deposition elsewhere in the cratonic hinterland and along the craton’s northern margin. However, the exact correlation of the Olifantshoek Supergroup with these other red-bed successions is hindered by poor geochronological constraints. Herein, we refine the age and palaeopole of the Hartley Formation, and provide geochronological constraints for large-scale 1.93–1.91 Ga bimodal magmatism on the Kaapvaal Craton (herein named the Hartley large igneous province). We present new age constraints for the mafic and felsic phases of this event at 1923 ± 6 Ma and 1920 ± 4 Ma, respectively, which includes the first reported age dating of the Tsineng Dyke Swarm that has been linked to Hartley volcanism. A mean 1.93–1.91 Ga palaeoma...

Widespread ca. 1.4 Ga intraplate magmatism and tectonics in a growing Amazonia

Abstract: High-quality U–Pb (Isotope Dilution-Thermal Ionisation Mass Spectrometry; ID-TIMS) baddeleyite ages were obtained for the Salto do Ceu (SC) gabbroic sill (1439 ± 4 Ma) and Nova Lacerda mafic dyke swarm (1387 ± 17 Ma) located ca. 150 km apart in the Jauru terrane (Paleo- to Mesoproterozoic) – SW Amazonia. From a geodynamic perspective, the new ages mark widespread Mesoproterozoic extensional tectonics and associated magmatism (e.g. dolerite dykes and rapakivi suites) in a growing continental margin. The SC sill is coeval with the nearby Rio Branco anorogenic rapakivi granite. Intermittent extensional tectonics (1.44 and 1.39 Ga) is widespread in the central and northern portions of the Amazonian Craton, given by co-magmatic charnockites and rapakivi granites, and mafic–ultramafic complexes. On a global scale, this activity may be coeval with a major intra-continental-related igneous event in Laurentia and Fennoscandia, as well as with mafic dykes in NW West African Craton. This suggests large igneo...

The ca. 1.8 Ga mantle plume related magmatism of the central part of the Ukrainian shield

Abstract: Palaeoproterozoic (ca. 1.8 Ga) mafic and ultramafic dykes are widely distributed within the whole Sarmatian segment of the East-European craton. This paper focuses on new geochronological, geochemical and isotope data obtained for mafic and ultramafic dykes of the Ingul terrain. Geochronological data available for these dykes indicate ages around 1800 Ma. We provide a new U–Pb zircon age of 1810 ± 15 Ma obtained for a dolerite dyke in the Kirovograd area. Geochemical and petrographical data allow identification of three groups of dykes: (1) kimberlites, (2) high-Mg# subalkaline rocks (picrite, camptonite, subalkaline dolerite etc.) and (3) tholeiite dolerite. Rocks of these groups were probably derived from different sources. eNd1800 values of studied rocks vary from 0.7 to 2.8. The highest values were obtained for mantle xenoliths and their kimberlite host (eNd1800 = 2.5–2.8). Rb–Sr data yield a regression age of 1729 ± 20 Ma with an initial 87Sr/86Sr = 0.70366 ± 41 (MSWD = 10.8). The whole-rock lead isotope data scatter, but data for sub-groups of samples can tentatively be fitted to parallel 1.8 Ga isochrons. The geochemical data indicate rocks to have formed by partial melting and the degree of melting is thought to be a function of formation depth, the latter ranging from sub-lithospheric to lower-crustal levels; we assume that melting was caused by a mantle plume. Dyking in the Ingul terrain was closely associated in time and space with metasomatic albitites that host numerous economic U deposits.

A Devonian >2000-km-long dolerite dyke swarm-belt and associated basalts along the Urals-Novozemelian fold-belt: part of an East-European (Baltica) LIP tracing the Tuzo Superswell

Abstract: Two dolerite dyke swarms are recognized along and paralleling the Ural Mountains, Russia. The Uralian swarm is 1400-km long (2300-km long if traced from its inferred plume centre). Further north, the Pay-Khoy swarm can be traced through the Pay-Khoy–Novaya Zemlya fold belt for a distance of c. 250 km (800-km long if traced from its inferred plume centre). An Upper Devonian age for volcanism associated with the Pay-Khoy swarm is well constrained by isotopic data. A Devonian age for the Uralian swarm was until now supported mainly by broad geological field relationships between the dykes and host rocks, i.e. dykes locally cut Proterozoic, Ordovician to Devonian sedimentary rocks, but never Carboniferous sequences. Rare isotopic age determinations also support an Upper Devonian age for the weakly altered dykes. Herein, a new precise U–Pb baddeleyite age determination of 377.2 ± 0.9 Ma is reported from a large (>50-m wide) gabbro–dolerite dyke cutting the uppermost Proterozoic in the Middle Urals. Thi...

New U–Pb baddeleyite age, and AMS and paleomagnetic data for dolerites in the Lake Onega region belonging to the 1.98–1.95 Ga regional Pechenga–Onega Large Igneous Province

Abstract: A new U–Pb baddeleyite age of 1970 ± 3 Ma for the Unoi dolerite sill of the Onega structure of Karelia craton matches other 1.98–1.95 Ga units across the Kola craton (Pechenga) and widely separated parts of the Karelian craton, including the Lake Onega sill area and a extensive NW-trending dolerite dyke swarm. Herein these coeval units are combined into the Pechenga–Onega Large Igneous Province. The sills in the Lake Onega area exhibit similar geochemical patterns, although the Unoi dolerite sills appear less contaminated and less differentiated than the Pudozhgora intrusion, Gabnev sill and Koikary-Svatnavolok and Palieyeozero sills but are similar to other doleritic sills in the northern part of the Onega structure. New AMS data from sills are consistent with emplacement along the same NW–SE trend as the dykes, consistent with the dykes acting as their feeder system. Paleomagnetic data obtained on 1.98–1.95 Ga magmatic rocks result in a new robust paleopole for the Karelian craton and accentuate...

New U–Pb ages for mafic dykes in the Northwestern region of the Ukrainian shield: coeval tholeiitic and jotunitic magmatism

Abstract: The palaeoproterozoic Northwestern region of the Ukrainian shield hosts two compositional types of mafic dykes and associated magmatism that intruded at c. 1800–1760 Ma: (1) high-Ni dolerite dykes and layered intrusions of tholeiitic affinity and (2) high-Ti dolerite dykes of jotunitic affinity associated with anorthosite–mangerite–charnockite–granite (AMCG) suites. The jotunitic dykes represent initial melts for basic rocks of the Korosten AMCG plutonic complex, whereas tholeiitic dykes may reflect emplacement of a mantle plume and formation of a large igneous province (LIP). New U–Pb baddeleyite ages indicate that both compositional types can be coeval: the jotunitic Rudnya Bazarska dyke was emplaced at 1793 ± 3 Ma, and the Zamyslovychi tholeiitic dolerite dyke at 1789 ± 9 Ma. In our model, the mantle plume-derived tholeiitic melts (underplate) supplied heat required for melting of the mafic lower crust and the production of jotunitic melts. As formation of the jotunite melts requires pressures ...

Paleomagnetism and chronology of B-1 marginal sills of the Bushveld Complex from the eastern Kaapvaal Craton, South Africa

Abstract: The Rustenburg Layered Suite (RLS) of the Bushveld Complex in South Africa is the largest mafic–ultramafic-layered complex on Earth. The RLS is associated with marginal sills that penetrate into the ~2.2 billion-year-old sedimentary strata of the Pretoria Group. These sills are in contact and share some geochemical similarities with different zones of RLS and are classified in terms of chemical composition, which suggests their derivation from distinct parental magma compositions (so-called B-1, B-2 and B-3 parental magmas). Existing paleomagnetic constraints for the Bushveld Complex originate from the upper Critical to Upper zones of the RLS, which are associated with B-2 and B-3 marginal sills. Geochemically, verified B-1 marginal intrusions are here used as a proxy for constraining the paleomagnetism and chronology of the Lower and lower Critical zones of the RLS. We identified a dual-polarity magnetic component with a paleopole (Latitude = 13.1°N, Longitude = 44.0°E, A95 = 14.3, N = 7) that is...

New U–Pb age and paleomagnetic constraints from the Uitkomst Complex, South Africa: clues to the timing of intrusion

Abstract: The Uitkomst Complex is considered to be coeval with, and genetically linked to the Rustenburg Layered Suite (RLS) of the Bushveld Large Igneous Province. This study reports new paleomagnetic, geochemical and geochronological results from the Uitkomst Complex and a crosscutting dyke at the Nkomati Mine. Primary magnetisations for the complex and the dyke are statistically indistinguishable. This, together with the geochemical signature of the dyke, suggests a late-magmatic link with the Uitkomst Complex. A Virtual Geomagnetic Pole calculated for the complex at Longitude 28.7°N and Latitude 58.5°E (dp = 6.2; dm = 9.4; N = 3) differs from the 1900 Ma and younger-aged poles for the Kaapvaal Craton. It, however, shares similarities with the poles from the 2058 to 2054 Ma RLS and other ~2.0 Ga paleopoles for the Kaapvaal Craton. Moreover, a new U–Pb baddeleyite age of 2054 ± 7 Ma given by a coarse-grained gabbroic sample from Nkomati’s underground mine provides a minimum age constraint on the crystalli...

Root zone of a continental rift: the Neoproterozoic Kebnekaise Intrusive Complex, northern Swedish Caledonides

Abstract: Mafic magmatic rocks formed between ca. 615 and 560 Ma along the Neoproterozoic margins of Baltica and Laurentia are classically attributed to continental rifting heralding the opening of the Iapetus Ocean. We report new data for the Kebnekaise Intrusive Complex (KIC) exposed in the Seve Nappes in northern Sweden. The KIC consists of sheeted dolerite dykes and gabbroic bodies with mutually intrusive relations. Major and trace element data exhibit a transitional- to normal-mid-ocean-ridge basalt-type geochemical signature. Differentiation processes and late-stage liquid immiscibility of a tholeiitic melt are considered to account for the compositional range exhibited by the KIC igneous rocks. U/Pb SIMS geochronological data from zircon rims yield an emplacement age of 578 ± 9 Ma. The KIC is thus younger and more depleted than coeval mafic rocks found in the Seve Nappe, and is interpreted to represent a high-level magma plumbing system in a late-stage continental rift. The composition and volume of ...

Regional deglaciation and postglacial lake development as reflected in a 74 m sedimentary record from Lake Vättern, southern Sweden

Abstract: The withdrawal of the Late Weichselian ice sheet and rapid isostatic uplift in southern Scandinavia led to the entrainment of large volumes of melt water within the proglacial Baltic Ice Lake (BIL). The eventual western outpost of BIL, Lake Vattern, has been a focal point for studying the dynamic retreat history of the Late Weichselian ice sheet in south central Sweden. This part of the deglacial history is described from an abundance of terrestrial studies, but, to date, no complimentary long sediment cores from Lake Vattern have been available. Here, we present the results from a unique, 74 m borehole in southern Lake Vattern that recovered a Late Pleistocene to Holocene sedimentary sequence. Physical and chemical analyses of the sediment and pore water, together with geophysical mapping, reveal glacial as well as postglacial imprints implying an oscillating ice sheet margin, evidence for neotectonic activity and one or more marine incursions into the lake during deglaciation. We attribute the glaciotec...

Mafic magmatism in the Bakhuis Granulite Belt (western Suriname): relationship with charnockite magmatism and UHT metamorphism

Abstract: The Bakhuis Granulite Belt (BGB) is a metamorphic terrain within the Guiana Shield that experienced ultrahigh-temperature (UHT) metamorphism at 2.07–2.05Ga. In the southwest of the BGB, the Kabalebo charnockites were emplaced at ca. 1.99Ga and thus postdate UHT metamorphism by at least 60Myr. Two generations of gabbroic intrusions have been recognized within the BGB, which could act as a heat source for the two UHT events. A younger generation of tholeiitic “Charlie” gabbros yields a baddeleyite U/Pb age of 1971±15Ma. The presence of a metamorphic overprint indicates that the hornblende-bearing “Moi–Moi” metagabbros predate the Charlie gabbros. Large zircons with complex zoning patterns are found in a Moi–Moi metagabbro sample. The main growth domains of these zircons give an age of 1984±4Ma, which is indistinguishable from the surrounding charnockites. Matching trace element and Hf isotope characteristics indicate that the complex zircons are derived from the charnockites. We argue that the emplacement of the metagabbros and charnockite magmatism were contemporaneous and that zircon grains from the charnockitic melt were mechanically transferred to the gabbroic bodies during magma mingling. The new ages for the gabbroic bodies in the BGB confirm that they are contemporaneous with, and the likely heat source for, charnockite magmatism, but that they are not associated with the 2.07–2.05Ga UHT event. Furthermore, the new ages and recognition of the Moi–Moi metagabbros as an Alaskan-type complex provide the first direct evidence for late Transamazonian subduction zone magmatism in the Guiana Shield.

Darriwilian (Middle Ordovician) worms of southern Sweden

Abstract: The record of scolecodonts (polychaete jaws) from the Ordovician of Sweden is very poor. In this paper, we document a Darriwilian (Middle Ordovician) assemblage recovered from palynological samples from the “orthoceratite limestone” (Lanna and Holen limestones) of Mount Kinnekulle, Vastergotland, southern Sweden. The collection of diminutive specimens forms an assemblage taxonomically composed mainly of simple placognath (mochtyellids, xanioprionids) and ctenognath (tetraprionids) taxa, whereas labidognaths (polychaetaspids) and taxa with other evolutionary grade-type apparatuses are very rare or absent. In addition, putative priapulid (penis worm) teeth were identified, possibly representing the first fossil representatives recorded in Sweden. The highest scolecodont abundance coincides with the lower to middle part of the “Taljsten” interval (lower Kunda Baltoscandian Stage). These strata are interpreted as having been formed during a marked regressional phase, suggesting that the palaeobathymet...

Mesoproterozoic dykes in the Timmiarmiit area, Southeast Greenland: evidence for a continuous Gardar dyke swarm across Greenland’s North Atlantic Craton

Abstract: During the Proterozoic, several mafic dykes with variable trends and mineralogies intruded the Archaean basement of Southeast Greenland. Some of the younger ENE-trending dykes are interpreted to represent a prolongation of the Mesoproterozoic Gardar Province, and have been termed Timmiarmiit dykes. Extrapolations of their trends across the inland ice sheet coincide with the northernmost so-called brown dykes (BD’s) which are part of the Gardar Province. Baddeleyite U–Pb ID-TIMS analyses for three ENE-trending Timmiarmiit dykes give ages of 1277 ± 4, 1275 ± 3 and 1268 ± 4 Ma, which are slightly younger than the oldest (BD0 = 1284–1279 ± 3 Ma, Upton 2013) and only dated generation of dykes in the Gardar Province, and thereby indirectly provide a possible age for the two younger dyke generations (BD1 and BD2). The Timmiarmiit–Gardar correlation is strengthened by a rigorous multivariate statistical analysis, on the basis of all major and trace elements. Thus, a coherent ENE-trending trans-Greenlandic...

Epithelial cell moulds preserved in the earliest acrotretid brachiopods from the Cambrian (Series 2) of the Three Gorges area, China

Abstract: Distinctive moulds of epithelial cells are known to occur on valve interiors in early Palaeozoic acrotretid brachiopods but morphological variation and distributional patterns of the epithelial cel ...

New advances in using large igneous provinces (LIPs) to reconstruct ancient supercontinents

Abstract: The remarkable fit between continents on the opposite side of the Atlantic was noticed already by early explorers of the 15th century as the first maps of the known world were being constructed. Fo...

Metamorphism and deformation of a Palaeoproterozoic polymetallic sulphide–oxide mineralisation: Hornkullen, Bergslagen, Sweden

Abstract: The Hornkullen mineralisation is situated in the westernmost part of the Bergslagen ore province, south-central Sweden. Here, polymetallic sulphides and oxides are hosted by an inlier of Svecofennian, c. 1.9 Ga skarn-bearing metavolcanic units, enclosed in the c. 1.8 Ga Filipstad granite belonging to the Transscandinavian Igneous Belt. The Ag- and Au-bearing mineralisation is dominated by veins and impregnations of magnetite, pyrrhotite, galena, chalcopyrite and arsenopyrite with subordinate pyrite, sphalerite, ilmenite, lollingite, Pb–Fe–Ag–Cu–Sb sulphosalts and rare gudmundite, pentlandite and molybdenite. Overall, a detailed textural and mineralogical study of the ore assemblages suggests significant deformation and remobilisation at high temperature, which is corroborated by sulphide geothermobarometry. The arsenopyrite geothermometer yields an average temperature of c. 525 °C, which is likely to be the result of metamorphic re-equilibration. Sphalerite geobarometry gives peak pressures of c. ...

Systematics, biostratigraphy and biogeography of brachiopods and other fossils from the middle Cambrian Nelson Limestone, Antarctica

Abstract: An assemblage dominated by acrotretid brachiopods (Prototreta millsi Brock and Percival, 2006) is described from the Nelson Limestone of the Transantarctic Mountains, Antarctica. The formation also includes paterinids (Micromitra sp. cf. M. nerranubawu Kruse, 1990 and Dictyonina australis Roberts and Jell, 1990), lingulids (Oepikites?haimantensis Reed, 1910) and acrothelids (Acrothele sp. cf. A. vertex Reed, 1910). The macrofauna also contains orthids (Diraphora dyunyin Kruse, 1990). The associated fauna include sponge spicules present in vast abundance. Other recognisable small shelly fossils (including chancelloriids and hyolithelminths) make up a small section of the fauna. Trilobites belong to taxa previously described from the Nelson Limestone and support the Drumian age proposed for the formation. Palaeobiogeographic analysis indicates that the brachiopod fauna corroborates previously described strong links with Australasia and also suggests a strong link to Indian material, corroborating pr...

Reproductive cyst and operculum formation in the Cambrian-Ordovician galeate-plexus microfossils

Abstract: Unicellular organic-walled microfossils from the Cambrian-Ordovician transition in Estonia (ca. 490-480 million years ago) exhibit rare characters reflecting their function as reproductive algal cy ...

Geotechnical and sedimentary evidence for thick-grounded ice in southern Lake Vättern during deglaciation

Abstract: A 74-meter Late Pleistocene to Holocene sedimentary sequence was recovered from southern Lake Vattern in the autumn of 2012. At ∼54 m below the lake floor, shear strength and high-resolution bulk density measurements suggest the presence of an unconformity in the varved proglacial clays. Incremental load consolidation tests reveal highly overconsolidated sediments below this level. Preconsolidation pressures for the underlying sediments are between 1250 and 2100 kPa, up to ∼1700 kPa more than the current in-situ effective stress. The highly overconsolidated sediments indicate either substantial erosion (the removal of 215–360 m of sediment), or consolidation under a large grounded ice mass sitting up to 230 m above paleo-lake level. Glaciotectonic deformation in underlying sediments supports the interpretation of a grounded ice mass. It is likely that this horizon is either contemporaneous with or older than the Levene moraine, formed between 13.4 and 13.8 ka. In the ∼30 m of overlying proglacial clays, t...

U–Pb SIMS dating of some granitoids from eastern Blekinge, southern Sweden

Abstract: Zircons from seven granitoids in eastern Blekinge have been dated using secondary ion mass spectrometry. The analyzed rocks include one Smaland granitoid from north of the Smaland-Blekinge Deformation Zone (SBDZ), two samples of megacrystic “Filipstad-type” granite from south of that zone and one sample each of the “Smaland-type” Rodeby, Almo, Tjurko and Jamjo granites. The results yield a crystallization age of 1778 ± 5 Ma for the Smaland granitoid, and crystallization ages between 1770 ± 4 and 1758 ± 5 Ma for the other granitoids, in most cases substantially older than previous TIMS ages. These data show that the “Smaland-type” granitoids in eastern Blekinge are similar in age to the surrounding Tving granitoids, and the more felsic of them may represent late-stage differentiates belonging to the same magmatic suite. As the Tving granitoids show differences both in degree of deformation, in geochemistry and in age, compared to the Smaland granitoids north of the SBDZ, it is suggested that these ...

Proterozoic magmatism in the southern Scandinavian Caledonides, with special reference to the occurrences in the Eikefjord Nappe

Abstract: Caledonian nappes of southern Norway comprise basement slices and sedimentary cover, and generally have close affinity with autochthonous terranes in the Fennoscandian foreland. The nappes can be distinguished in those having Gothian (1700–1600 Ma) character and those of Telemarkian (1550–1480 Ma) type. There were also several periods of variously intense magmatism between these early accretionary events and the end of the Sveconorwegian Orogeny. Most prominent features are the anorthositic-noritic massifs, which formed mainly during the Telemarkian period and in the late stages of the Sveconorwegian Orogeny between 980 and 920 Ma. In this paper, we review the main events affecting the crystalline parts of the nappes and report new results from the Eikefjord Nappe. An augen gneiss yields a crystallization age of 1191 ± 2 Ma, which is an event rarely seen in nappes or the autochthon. An amphibolitic gneiss formed at about 1500 Ma and underwent high-grade metamorphism at 986 ± 3 Ma, followed by lowe...

Traces of the Transscandinavian Igneous Belt in the central Scandinavian Caledonides: U-Pb zircon dating and geochemistry of crystalline basement rocks in the Middle Allochthon

Abstract: Basement-slices are frequent components in the lower nappes of the central Scandinavian Caledonides. New geochronological and geochemical data provide evidence that three of these basement-slices in the Middle Allochthon are derived from the Transscandinavian Igneous Belt. Dated samples in this study comprise a quartz monzonite from the Stalon Nappe Complex and a quartz monzodiorite and a monzonite from the Ammarnas Nappe Complex. A Ce/Yb-Ta/Yb-plot of the mafic rocks from the Ammarnas Nappe Complex suggests a calc-alkaline to shoshonitic geochemical character. Low Ni- and Cr-contents and low Mg# indicate that the mafic rocks originated from an already evolved magma. Enrichment of Fe-Ti oxides and V>200 ppm indicate presence of cumulus phases to various degrees. These basement-derived rocks in the Middle Allochthon yielded concordant U-Pb zircon SIMS ages of 1799 ± 10 Ma, 1787 ± 6 Ma and 1797 ± 5 Ma. They are therefore interpreted to represent rocks detached from the Palaeoproterozoic Transscandinavian Ig...

Siphuncular structures in Calciosiphonate nautiloid orders Actinocerida, Orthocerida and Barrandeocerida (Cephalopoda)

Abstract: The calciosiphonate type of the connecting ring was probably present in the oldest known nautiloid cephalopods, plectronocerids, from the Late Cambrian. This type also occurs in the nautiloid orders Actinocerida, Orthocerida and Barrandeocerida that appeared in the Ordovician. The calciosiphonate connecting ring is composed of two calcareous layers: the outer, spherulitic–prismatic layer, and the inner, calcified-perforate layer. The calcified-perforate layer contains cavities that are separated by calcified walls. The cavities open into the shell chamber by pore canals. In Adamsoceras holmi (actinocerid) and Orthoceras scabridum (orthocerid), the cavities have large diameters and thin calcareous walls, whereas in Archigeisonoceras folkeslundense (orthocerid), and probably also in Danococeras gotlandense (orthocerid) and Boionautilus sp. (barrandeocerid), the cavities have small diameters and thick calcareous walls. The structure of the connecting rings is discussed and reconstructed.

First occurrence of Lesueurilla prima (Gastropoda) from the Middle Ordovician (Darriwilian) of Iran

Abstract: The near planispiral euomphaloid gastropod Lesueurilla prima (Barrande in Perner) is a typical part of the cold water Mediterranean fauna in the Lower and Middle Ordovician. It is a signature species in strata of this age in the Prague Basin in the Czech Republic, Montagne Noire in France, and Central Morocco. The species is reported for the first time from the Darriwilian of Iran where it occurs in the upper part of the Shirgesht Formation of the Derenjal Mountains north of Tabas in east Central Iran. It occurs with a small species of the bellerophontoid gastropod Sinuites sp., as well as the calymenid trilobite Neseuretinus birmanicus (Reed) and the pliomerid trilobite Ovalocephalus kanlingensis (Zhang). The biogeographic distribution of Lesueurilla suggests that the genus had its origin in cold water, high-latitude peri-Gondwana areas during the Lower Ordovician, spreading to other peri-Gondwana areas and low-latitude areas of North China, Tarim, and finally Baltica, Laurentia and Australia in ...

Mass occurrence of the large solitary rugose coral Phaulactis angusta at the boundary Lower/Upper Visby Formation in the Silurian of Gotland, Sweden: palaeoecology and depositional implications

Abstract: The boundary between the Lower and Upper Visby formations on Gotland (Sweden), which roughly correlates with the Llandovery–Wenlock boundary, is characterised by a mass occurrence of the large solitary rugose coral Phaulactis angusta. This coral-rich layer can be followed for some 50 km along the north-west coast of Gotland. The mass occurrence coincides with the onset of a pronounced, globally recognised δ13C and δ18O excursions in the lower Sheinwoodian. Altogether 31 specimens of P. angusta from eight localities were examined. We demonstrate that the corals preferred to grow in an upright position. Attachment structures were not observed, and therefore a liberosessile life strategy is suggested. Some corallites show an abrupt change in growth direction accompanied by a decrease in diameter. These features mostly occur in later growth stages and may be explained by disturbances in the growth due to toppling or tilting of the corals. Many specimens were able to change their growth axes, but final...

Cambrian stratigraphy of the Tomten-1 drill core, Västergötland, Sweden

Abstract: The Tomten-1 drilling at Torbjorntorp in Vastergotland, southern Sweden, penetrated 29.85 m of Cambrian Series 2, Cambrian Series 3, Furongian, and Lower–Middle Ordovician strata. Lithostratigraphically, the succession includes the File Haidar, Borgholm and Alum Shale formations, and the Latorp and Lanna limestones. The drill core succession is described herein for the first time, with special focus on the biostratigraphy of the Cambrian Alum Shale Formation. In the Cambrian Series 3, through Furongian Alum Shale Formation, agnostoids and trilobites have been identified to species level and the succession is subdivided into nine biozones (in ascending order): the Ptychagnostus gibbus, Ptychagnostus atavus, Lejopyge laevigata, Agnostus pisiformis, Olenus gibbosus, Parabolina spinulosa, Ctenopyge tumida, Ctenopyge bisulcata and Ctenopyge linnarssoni zones. The succession is interrupted by numerous stratigraphic gaps of variable magnitudes, as is evident from the biostratigraphy and conspicuous uncon...