bluuebunny/crossref_metadata_2025_split huggingface dataset:
Crustal magnetization variations across the Iapetus Suture Zone
Authors: Kimbell G. S. , Stone P. 
Published: May 2009

AbstractThe Iapetus Suture (Solway) line coincides with a magnetic low, which lies between magnetic highs over southwestern Scotland and the Lake District-Isle of Man region. Although topography on deep magnetic basement can account for these long wavelength geophysical variations, an explanation which involves lateral basement magnetization contrasts is preferred on the basis of (a) correlations between inferred magnetization boundaries and major structures delineated from other evidence, and (b) the apparent westward continuation of the Solway low through Ireland and Newfoundland across areas with very different subsidence histories but similar position with respect to the collision of Laurentia and Avalonia. In the preferred model, relatively magnetic continental crust beneath the Southern Uplands and Lake District terranes is separated by a zone of less magnetic crust interpreted as sedimentary rock of Avalonian affinity carried to deeper structural levels within the Iapetus Suture Zone. The magnetic unit beneath the Southern Uplands is bounded to the south by the northward-dipping Iapetus Suture and to the north by a structure which may have been reactivated in late Caledonian times to produce the Moniaive Shear Zone in the overlying rocks; this unit may represent the ‘missing’ arc terrane inferred from provenance studies. Alternatively, the two magnetic basement domains may have originally been part of the same terrane, with that portion beneath the Southern Uplands rifting from the Avalonian continent during its northwards drift and being subsequently trapped in the hanging wall of the Iapetus Suture. The southern margin of the Lake District domain appears as a discontinuity in the magnetic anomaly pattern, with long wavelength anomalies to the south having a southeast ‘Tornquist’ trend.
Post-depositional processes on a buried Cambrian sequence in southern Israel, north Arabian Massif: evidence from new K–Ar dating of Mn-nodules
Authors: Segev A. , Halicz L. , Steinitz G. , Lang B. 
Published: May 2009

AbstractThe Cambrian sedimentary sequence in Israel and adjacent countries marks the beginning of the Phanerozoic sedimentation on the Arabian–Nubian craton. The maximal burial of this sequence, in the southernmost part of Israel, was approximaly 2.5 km. Manganese nodules hosted by shales of the marine Cambrian Timna Formation, Timna Valley, were subjected to K–Ar analysis in order to date their Mn-mineral formation. In addition, the <2 μm clay fraction in the host rock was dated by K–Ar and Rb–Sr methods. The K–Ar ages (average 365 ± 4 Ma) and Rb–Sr isochron (381 ± 10 Ma) of the illitic clay fraction yielded a Middle/Late Devonian age. The results imply that K–Ar and Rb–Sr systems of <2μm illites in the Cambrian host rocks, as well as those enclosed in the Mn nodule insoluble residues, were completely resetin a Middle/Late Devonian thermo-tectonic event, coeval with the beginning of a stratigraphically recorded epeirogenic uplift. The Mn-nodules which were studied fall into two types: (1) nodules constituted by massive, well-crystallized hollandite and pyrolusite; and (2) younger nodules of poorly crystallized massive hollandite and coronadite solid-solutions. Type-1 nodules yielded a calculated Early Cretaceous age of 112 ± 11 Ma, whereas type-2 nodules yielded calculated apparent dates of 20 and 49 Ma. The first age suggests a first stage of manganese nodule formation within the Timna Formation, in Early Cretaceous time, possibly genetically connected with the shallow basic intrusions and volcanic explosive activity in the area. The much younger K–Ar dates of type-2 Mn nodules may be due to late manganese remobilization and mineralization processes. This activity is interpreted as being related to the nearby Tertiary Dead Sea rifting, which was accompanied by low temperature hydrothermal processes.
Radioelement distribution in the Tertiary Lundy granite (Bristol Channel, UK)
Authors: Thorpe Richard S. , Tindle Andrew G. , Williams-Thorpe Olwen 
Published: May 2009

AbstractThe radioelement distribution and content of the Lundy granite, a coarse-grained megacrystic granite of Tertiary age, has been measured using a portable gamma-ray spectrometer in order to assess fractionation and alteration processes in the granite. Results indicate a systematic variation of K, Th and U (with a few notable exceptions) that follows a partially concentric distribution to lower concentrations inland. The plateau region of the island (particularly the southern half) is relatively depleted in all radioelements. Over the island, measurements of K vary from 1.3–4.9 wt %, Th varies from 5.0–20.3 ppm and U varies from 2.0–12.5 ppm. A petrographic, electron microprobe and autoradiography examination of the granite indicates that the radioelements mainly reside in discrete major and accessory minerals, of which K-feldspar (K), biotite (K), monazite (Th), xenotime (U), tungsteniferous columbite (U) and uraninite (U) are the most important. Uraninite is rare, being preserved only in fresh samples which come mainly from abandoned quarries. Mass balance modelling indicates that up to 76.6% of uranium could reside in uraninite and where this has been leached by secondary processes such as hydrothermal alteration or weathering then the present radioelement content no longer reflects the original rock composition. Fission track evidence is presented to show the pathways along which uranium has been mobilized from or within the granite. Secondary sites of radioelements include fractures cross-cutting all major minerals (but especially quartz), grain boundaries, altered cores of plagioclase feldspar and occasionally yellowy brown mixed chlorite/smectite replacement product after biotite. Biotite itself may exhibit secondary tracks along cleavage traces. Combined effects of crystal fractionation (primary variation) and secondary alteration best explain the distribution of radioelements, with K controlled by fractionation of the major phases K-feldspar and biotite, Th by fractionation of the accessory mineral monazite (±xenotime and uraninite) and U contents by uraninite and tungsteniferous columbite. Secondary processes have removed much of the uraninite leaving behind indeterminate Fe—U material along fractures and residual U (and Th) enrichment within altered major minerals. There is some evidence to suggest that late radioelement-bearing fluids precipitated monazite and uraniferous zircon along fractures during the waning stages of magmatic activity.
Palaeozoic palaeomagnetic studies, in the Welsh Basin-recent advances
Authors: Channell J. E. T. , McCabe C. , Torsvik T. H. , Trench A. , Woodcock N. H. 
Published: May 2009

AbstractIn the last two years, new palaeomagnetic data from Wales have resulted in radical revision of the Ordovician palaeogeography of Eastern Avalonia, part of the southern margin of the Iapetus Ocean. Combined with Palaeozoic palaeomagnetic data from Laurentia and Gondwana, these data suggest that Eastern Avalonia was a peri-Gondwanide high latitude continental fragment during at least part of Ordovician time, with a palaeolatitude of about 62° S and 51° S in Arenig and Llanvirn time, respectively. This implies a latitudinal width of the early Ordovician Iapetus Ocean between Eastern Avalonia and Laurentia of at least 30°. Geological evidence for the proximity of Eastern Avalonia and Laurentia suggests that the intervening Iapetus Ocean closed during Silurian time, from late Llandovery to early Ludlow. Recent palaeolatitude data from the Iapetus bordering continents are consistent with closure by middle to late Silurian time. New pre-Acadian early Devonian palaeomagnetic data from the Old Red Sandstone places the Welsh Basin at about 17° S, consistent with a palaeogeography in which Laurentia, Baltica, Avalonia, Armorica, and possibly Gondwana, were part of a single supercontinent. Pervasive late Carboniferous/early Permian remagnetization affects the Welsh Basin. The remagnetization is probably associated with fluids emanating from the Variscan thrust front. We do not observe remagnetization associated with Acadian orogeny and the remagnetizations, which have been studied in more detail in North America, appear to be a unique feature of the Variscan-Hercynian-Alleghenian orogeny.
Geochemical and isotope composition of pebbles from the Caban Conglomerate Formation and their bearing on the source of Welsh Palaeozoic sedimentary rocks
Authors: Evans J. A. 
Published: May 2009

AbstractMajor, trace and REE geochemistry, together with Nd isotope determinations, provide new insights into the provenance of seven pebbles of igneous origin from an early Silurian facies of the Caban Conglomerate Formation from central Wales. The geochemical composition of the pebbles supports formation in a subduction related environment on an active continental margin. Biotite whole-rock Rb-Sr systematics constrain the age of a granite pebble to 650±38 Ma. The pebbles provide discrete examples of Avalonian basement which contributed to the isotope composition of sedimentary rocks in the Welsh Basin and one granite pebble provides a rare example from the Welsh Basin of an igneous rock with a Palaeoproterozoic depleted mantle model age.
Melt-enhanced deformation during emplacement of gabbro and granodiorite in the Sunnhordland Batholith, west Norway
Authors: Andersen Torgeir B. , Nielsen Peter , Rykkelid Erling , Sølna Hanne 
Published: May 2009

AbstractThe Caledonian Sunnhordland Batholith comprises calc-alkaline plutons that have been assigned to three units according to their relative age and composition: a gabbro-diorite unit, a granodiorite unit and a later granodiorite-granite unit. The batholith was emplaced into an envelope including ophiolite and island-arc complexes, sediments and volcanites of early Ordovician age that were developed in a zone of plate convergence. Continued convergence resulted in the formation of a mature magmatic arc and a thickened crust; the late granitoids (unit 3), which commenced their crystallization at pressures around 6 to 7 kb, rose as permitted diapiric intrusions. The ingress and ascent of the magmas in this setting is considered to have been facilitated by the presence of major shear zones developed in relation to plate convergence. In this model, plastic instabilities were formed in an otherwise elastic middle and upper crust. Non-coaxial deformation was accelerated by the emplacement of magmas and the formation of abundant partial melts in water-rich sediments of the envelope. The deformation, which was accelerated by magma and melt lubrication in aureoles, controlled both the shape and internal structure in the gabbro and granodiorite plutons.
Palynology of the Stonehaven Group, Scotland: evidence for a Mid Silurian age and its geological implications
Authors: Marshall J. E. A. 
Published: May 2009

AbstractPalynomorphs from the Stonehaven Group indicate a late Wenlock to early Ludlow age. This is older than the currently accepted Pridoli age based on fish and arthropods. Reasons for this discrepancy are discussed. A previous correlative of the Stonehaven Group has been the Old Red Sandstone of Kerrera, Argyll. However, palynological assemblages from both Kerrera and the adjacent succession at Oban are in fact younger than that from Stonehaven in being of latest Silurian to earliest Devonian age. The palynomorphs from Kerra and Oban lie beneath the important geochronological tie-point of the Lorne Lavas and suggest that the Silurian-Devonian boundary is older than currently accepted.
B1 subdivisions in thin komatiites at Kambalda, Western Australia
Authors: Thomson B. 
Published: May 2009

AbstractB1 subdivisions are narrow foliated zones of stubby, skeletal olivine blades, situated at the top of the granular olivine cumulates (B2) in ponded komatiite lavas. They developed at a late stage in pond crystallization as a result of compaction-related circulation of intercumulus liquids through and along the top of the cumulates. The total thickness of a B1 and its degree of blade parallelism are related to lateral position within ponded lavas. The deeper, hotter and longer-lived core regions generated a thick B1 with a high degree of blade parallelism (ordered B1), whereas the shallower, peripheral regions produced a narrow B1 with a poor degree of blade parallelism (disordered B1), or failed to develop one at all.
The origin of the Neihart Quartzite, a basal deposit of the Mid-Proterozoic Belt Supergroup, Montana, U.S.A.
Authors: Schieber J. 
Published: May 2009

AbstractThe Neihart Quartzite is the basal quartz arenite unit (≍270m thick) of the Mid-Proterozoic Belt Supergroup of western North America. Petrographic studies indicate a source area with plutonic granitic, metamorphic and felsic volcanic rocks. Extreme textural maturity and bimodality indicate an episode of aeolian transport for the detrital quartz grains. The lower 80% of the Neihart Quartzite were probably deposited by braided streams, whereas the upper 20% were deposited in shoreline environments. Residual material that was ‘stored up’ on the pre-Beltian cratonic surface and underwent aeolian reworking was the likely source material for most of the Neihart Quartzite. Less mature sediments in the top portion of the Neihart Quartzite indicate uplift and erosion of new source material during Neihart deposition. Other known cratonic quartz arenites, such as the St. Peter Sandstone (Ordovician), Lamotte Sandstone (Cambrian) and Flathead Quartzite (Cambrian), are thin (tens of metres thickness) and exhibit sheet-like geometry. In contrast, the Neihart Quartzite and its probable lateral equivalents are considerably thicker and increase in thickness towards the central portions of the basin. It thus appears that Belt sedimentation began with accumulation of a basal quartz arenite unit, and that sand for that unit was transported by braided streams from the surrounding craton to a gradually subsiding Belt basin.
The sedimentary expression of regional tectonic events during the Miocene-Pliocene transition in the southern Cyprus basins
Authors: Orszag-Sperber F. , Rouchy J. M. , Elion P. 
Published: May 2009

AbstractA well-known tectonic event affecting the East Mediterranean region, generally referred to as ‘the’ Miocene–Pliocene phase, occurs at, or near the Miocene–Pliocene boundary. Recent sedimentological studies in Cyprus indicate that this ‘event’, in fact, is complex. The Tortonian–Lower Pliocene period is marked by a stress involving an N20 extension in the Polemi and Pissouri basins and by an N100 extension in the Psematismenos basin. Sedimentological studies have demonstrated three tectonic pulsations during Messinian time, prior to the Pliocene transgression. These are expressed by two episodes of seismic brecciation, and a palaeo-emersion is indicated by palaeosols and detrital discharges. These phenomena suggest brief tectonic instability during Messinian time. Microtectonic studies in the South Troodos basins of Cyprus reveal that the main change in tectonic stress does not coincide with the Miocene–Pliocene contact but occurs at the end of Lower Pliocene time. The authors conclude that the so-called Miocene–Pliocene ‘event’, in reality, is a series of subtle tectonic pulsations recorded clearly by sedimentary parameters.
Hylaeosaurus, Polacanthus, and the systematics and stratigraphy of Wealden armoured dinosaurs
Authors: Pereda-Suberbiola Javier 
Published: May 2009

AbstractTwo genera of armoured dinosaurs (Ankylosauria) are present in the Lower Cretaceous Wealden Beds of England.PolacanthusOwencan be distinguished fromHylaeosaurusMantell on the basis of significant differences in the pectoral girdle, hindlimb and dermal armour. A comprehensive review of the dinosaurs from the Wealden Series of England and contemporary basins of western Europe adds further evidence of a biologicalseparation between Lower Wealden (Hastings Beds, containingHylaeosaurusarmatus)and Upper Wealden faunas (Wealden Group of the Isle of Wight-containingPolacanthus foxii-and nearly equivalent units in Belgium, France, Germany and Spain).
Steep-sided carbonate mud mounds in the Middle Devonian of the eastern Anti-Atlas, Morocco
Authors: Wendt J. 
Published: May 2009

AbstractDuring the Eifelian and early Givetian, isolated mud mounds were established in a shallow basin, predominantly characterized by calcareous mudstone deposition, in the eastern Anti-Atlas of Morocco. The shapes, lithologies and faunas of these mounds stand in marked contrast with those of nearby contemporaneous, more widespread stromatoporoid/coral biostromes and small carbonate platforms adjacent to shallow shelves. With one exception, the mounds are totally exhumed, perfectly exposing their original morphologies. The smaller mounds are asymmetrical, with steeper northeastern and eastern (35–75°) than southwestern and western flanks (30–50°). The largest mound is almost circular and symmetrical, with scattered stromatoporoids, tabulate and rugose corals that do not form a rigid framework. Frame-builders are much rarer or absent in the smaller mounds. The nature and geographical distribution of the mounds reflects a bathymetric gradient, indicating that they formed on low-angle ramps which sloped gently into a shallow basin. A large reef mound was constructed at moderate depth, while small mud mounds accumulated in deeper water. Decrease of frame-builders and increase in pelagic organisms in the latter document the transition to a pelagic platform with reduced sedimentation at a short distance from the deepest mound.
Structure of veins in a gold–pyrite deposit in banded iron formation, Amalia greenstone belt, South Africa
Authors: Vearncombe J. R. 
Published: May 2009

AbstractFibrous quartz veins in deformed banded iron formation of the Amalia greenstone belt, southwestern Transvaal, are spatially related to gold–pyrite mineralization in both wallrock and vein inclusions. Poles to quartz vein orientations show a general parallelism with mineral elongation and fold plunges of the principal deformation in the wallrock. Quartz vein fibres show a consistent anticlockwise rotation, late components being subparallel to the elongation lineation, suggesting veining was probably synchronous with the principal deformation. Antitaxial fibrous veins, which dominate the mineralized banded iron formation, formed by the process of crack–seal which channelled mineralizing fluids along the vein walls, increasing the potential for fluid–wallrock interaction. Gold mineralization in quartz veins occurs in wall-parallel slivers of banded iron formation which have been plucked off the vein wall during antitaxial fibre growth. Mineralization can be explained by a process of fluid–wallrock interaction with sulphidation and gold precipitation.
Metamorphic fluids and transtension in the Cantabrian Mountains of northern Spain: an application of the conodont colour alteration index
Authors: Raven J. G. M. , Van Der Pluijm Ben A. 
Published: May 2009

AbstractConodont colour alteration index (CAI) values from Upper Paleozoic rocks in the Cantabrian zone of northern Spain show that temperatures during Hercynian metamorphism locally exceeded 300 °C. Various temperature domains have been defined, which are generally separated by fundamental structures. These domains do not correspond with the tripartite subdivision based on stratigraphic analysis.The observed CAI values of conodonts are in general agreement with the mineral paragenesis. Areas with high CAI values display extensive alteration and mineralization, and where CAI values exceed 4–4.5 (>200 °C) slaty cleavage has developed.The Cantabrian zone is an area of very low grade metamorphism, where peak conditions were reached in Upper Carboniferous to Lower Permian times. The characteristics of the metamorphism and its spatial relationship with major faults suggest that fluids were the main source for regional heating and that fluid transport was focussed along crustal-scale structural features.The overall deformation regime in this part of the Variscan orogen of western Europe is interpreted to be large-scale transtension. This is in agreement with earlier proposed models for the formation of Upper Palaeozoic basins in this area.
The ultrabasic lava flows of Mashhad, North East Iran
Authors: Majidi B. 
Published: May 2009

SummaryUltrabasic and basic lavas are interbedded with metamorphosed Lower-Carboniferous sediments in the northern slope of the Alborz mountains, NE Iran. In the outcrop area at least 15 individual units of ultrabasic lava have been observed. Flow units range in thickness from a few metres up to about 70 m. The inner parts of the flow units are holo-crystalline, showing a poikilitic texture with rounded small crystals of serpentinized olivine surrounded by large crystals of clinopyroxene (‘wehrlitic facies’). The upper portions of the thicker units are olivine-free, and pyroxene, sometimes accompanied by brown hornblende, is set in a groundmass of fine-grained epidotized plagioclase (‘doleritic facies’). In the upper and lower margins of flows the groundmass is devitrified to chlorite and tremolite. Small-scale differentiation and igneous lamination is observable in transition zones between the wehrlitic and doleritic ‘facies’. The upper doleritic facies and other individual basic units have a tholeitic chemistry. In contrast, the chemical composition of wehrlitic rocks (which predominate amongst the exposed rocks in the area) is comparable with Archaean ultrabasic lava flows in Canada and southern Africa.
Overbank deposits of the Narrator Brook, Dartmoor, England
Authors: Gomez B. , Sims P. C. 
Published: May 2009

SummaryOverbank deposits of the Narrator Brook, exposed by the excavation of a drainage ditch, are described. The deposits, which consist of coarse (sand and gravel), fine (silt and clay) and peat units, were laid down in a floodbasin environment. The coarse units are interpreted as crevasse-splay deposits, rather than being classified as floodbasin sediments along with the fine units. Three stages of development in the evolution of the floodbasin are identified. The first stage is represented by the development of peat at a time when the area was not subjected to flooding, the second by deposition of the fine and coarse sediment units and the third by an upper peat horizon. No precise date for the formation of the deposits is at present available, but it is thought that they may be the result of man's activity in the catchment.
Implications for the age of South African Devonian rocks in which Tropidoleptus (Brachiopoda) has been found
Authors: Boucot A. J. , Brunton C. H. C. , Theron J. N. 
Published: May 2009

SummaryThe Devonian brachiopod Tropidoleptus is recognized for the first time in South Africa. It is present in the lower part of the Witteberg Group at four widely separated localities. Data regarding the stratigraphical range of the genus elsewhere, combined with information on recently described fossil plants and vertebrates from underlying strata of the upper Bokkeveld Group, suggest that a Frasnian or even Givetian age is reasonable for the lower part of the Witteberg Group. The recognition of Tropidoleptus in a shallow water, near-shore, molluscan association, at the top of the South African marine Devonian sequence, is similar to its occurrence in Bolivia, and suggests a common Malvinokaffric Realm history of shallowing, prior to later Devonian or early Carboniferous non-marine sedimentation. It is noteworthy that Tropidoleptus is now known to occur in ecologically suitable environments around the Atlantic, but is absent from these same environments in Asia and Australia. Tropidoleptus is an excellent example of dispersal in geological time — first appearing in northern Europe and Nova Scotia, then elsewhere in eastern North America and North Africa, followed by South America and South Africa, while continuing in North America.