bluuebunny/crossref_metadata_2025_split huggingface dataset:
A new record of early Silurian land plant spores from the Paraná Basin, Paraguay (Malvinokaffric Realm)
Authors: Gray Jane , Boucot A. J. , Grahn Yngve , Himes Gregory 
Published: May 2009

AbstractThe first conclusive evidence for Silurian rocks in the Paraná Basin, Brazil, was reported in 1985. This evidence was based on organic-walled microfossils, principally spore tetrads, and associated phytoplankton (‘acritarchs’ and prasinophytes), the first recovered from the Vila Maria Formation on the northeastern rim of the Basin. The spore assemblage was typical of Gray's Microfossil Assemblage Zone I; size-frequency data for the tetrads suggested an early Silurian (early Llandovery; Rhuddanian) age. We now document a new MA Zone I occurrence of spore tetrads together with a few single trilete spores, from the southwestern rim of the Paraná Basin, Paraguay. The single spores are among the earliest known to have normally dissociated from a tetrahedral tetrad. Size-frequency data for the tetrads suggest a late Llandovery, pre-C5(mid-Telychian) age. These spores, together with phytoplankton and chitinozoans, were recovered from beds laterally equivalent to the Vargas Peña Shale at the type locality from which Llandovery diplograptids and monograptids are known. Llandovery age rocks on the southwesternandnortheastern borders of the Paraná Basin,c. 1400 km apart, suggest that early Silurian rocks were extensively distributed throughout southern Brazil and adjacent Paraguay. Close conformity between the age reference obtained with graptolites and spore tetrads again demonstrates the stratigraphic utility of early Silurian spores in providing reliable age determinations for otherwise unfossiliferous rocks. Spore tetrad assemblages available from the Malvinokaifric Realm (South America, Africa, Arabia) represent a distinct phytogeographic unit contrasted with spore tetrad assemblages from the North Silurian Realm of North America and parts of Europe.
Coal—magma interaction: an integrated model for the emplacement of cylindrical intrusions
Authors: Kent R. W. , Ghose N. C. , Paul P. R. , Hassan M. J. , Saunders A. D. 
Published: May 2009

AbstractOlivine-bearing lamproite magmas intruded into Permian coal seams in northeast India occur as root-like cylinder stockworks, extending for up to several kilometres up-dip along the bedding planes of their sedimentary host. Clusters of eight or more conduits are typical, linked by thin tubular cross-branches. Cylindrical geometry may arise by injection of hot, low-viscosity fluid through a slot, with the development of multiple tube-like instabilities at the interface between the moving fluid and a higher-viscosity host. This behaviour appears more complex than the models of Chouke, van Meurs & van der Pod, and Saffman & Taylor, which predict the development of a single dominant tube in porous or layered flow. Cylinder emplacement may be an essentially passive process, in which the sediment column is reduced by expulsion of heated pore fluids at the head of the moving intrusion, creating a space into which the melt can propagate. Generation of a superheated vapour envelope by non-nucleated film boiling of these fluids around the hot lamproite magma (the Leidenfrost effect) allows melt flow to be maintained in a lengthening tube thermally insulated from the surrounding coal, in a manner analogous to submarine lava tubes. Cooling of the magma through the Nukiyama temperature (the temperature at which maximum evaporation of the heated fluid occurs) may give rise to violent surface boiling and the formation of large vapour bubbles at the magma–coal interface. Implosion of these bubbles could then result in the formation of shock breccias, comparable to hyaloclastites produced by bubble or surface film collapse in the vicinity of pillow lava tubes. The operation of such a process around lamproite magma is suggested by the presence of complex breccias composed of highly fragmented coal, sandstone, and lamproite, at the termini of certain cylinders.Surface and subsurface exposures of the cylinders reveal the presence of a carbonate–chlorite–clay halo surrounding each intrusion, resulting from the alteration of mafic mineral phases by fugitive volatiles released from the protective vapour jacket. The coal seams proximal to intrusion clusters are relatively undeformed, with no evidence of assimilation by the invading melts. The coals have experienced extensive carbonization, probably as a result of slow conductive heating from the cooling lamproite bodies, or fluids derived therefrom. Field observations indicate that these thermal effects are not merely confined to the coal–melt interface, but occur for some considerable distance away from the intrusions, producing large areas of naturally coked coal.
Marine molluscan constraints on the age of Cretaceous fossil forests of Alexander Island, Antarctica
Authors: Kelly S. R. A. , Moncrieff A. C. M. 
Published: May 2009

AbstractNew evidence, provided by bracketing marine molluscan faunas, suggests a Late Albian age for fossil forests in the upper part of the Fossil Bluff Group of Alexander Island. These in situ forests are underlain by Late Aptian and Late Albian strata containing faunas including ammonites (the last of which is Lechites), inoceramid bivalves, dimitobelid belemnites and trace fossils. The tree-bearing levels occupied a fluvial dominated environment, subject to periodic major floods. They were transgressed during late Albian time by returning marine conditions with diverse faunas, including hamitid and puzosiid ammonites. The increased precision in dating of the fossil forests will improve palaeobotanical contributions to climate and environmental studies.
Glacial or non-glacial origin for the Bigganjargga tillite, Finnmark, northern Norway
Authors: Jensen P. A. , Wulff-Pedersen E. 
Published: May 2009

AbstractBigganjargga is the classic locality for the Varangian ice age in northern Scandinavia. The presence of two sets of striations on the underlying quartzite basement has been taken as evidence for a glacial origin of the Bigganjargga diamictite. However, soft-sediment surface structures on the underlying sandstone indicate that it was not consolidated when the diamictite was emplaced. Together with structures in the diamictite this shows that the deposit formed as a debris flow moving northwestwards over an unconsolidated substrate. There is no evidence for glacial origin or contribution for the diamictite material.
Neoproterozoic shear zone tectonics within the Icartian basement of Guernsey and Sark, Channel Islands
Authors: Tribe I. R. , Strachan R. A. , D’Lemos R. S. 
Published: May 2009

AbstractThe Channel Islands of Guernsey and Sark are amongst the few localities within the Neoproterozoic, Cadomian orogenic belt where Palaeoproterozoic basement is exposed. Basement units collectively referred to as ‘Icartian’ comprise orthogneisses (e.g. the c. 2000 Ma Icart granite gneiss), metasediments and amphibolites. On Guernsey, the protolith to the Icart granite gneiss was intruded into metasediments already deformed during a D1 deformation event. Both were variably deformed during a D2 event within an approximately north—south trending, steeply dipping, oblique dextral shear zone. On Sark, metasediments and amphibolites carry a D1 fabric (not necessarily correlatable with that on Guernsey) which was deformed during D2 to form recumbent—gently inclined folds and tectonic fabrics which locally carry top-to-the-south kinematic indicators. Early Cadomian quartz diorites which intruded the Icartian basement carry variably developed magmatic and moderate temperature (400–550 °C) LS solid-state fabrics which are similarly oriented to D2 basement structures. This indicates that the quartz diorites were emplaced and deformed during a protracted D2 event. Published 40Ar/39Ar amphibole cooling ages of c. 600 Ma from Guernsey and Sark quartz diorites and a host Icartian amphibolite are consistent with reworking of basement units during the Cadomian Orogeny. The contrasting kinematic pattern during Cadomian deformation implies that a flat-lying shear zone on Sark may have accommodated oblique dextral strike-slip movement on Guernsey with an overall southerly direction of tectonic transport. Deformation most likely occurred in the hanging-wall of a major south-dipping Neoproterozoic subduction zone.
The tectono-metamorphic evolution of a dismembered ophiolite (Tinos, Cyclades, Greece)
Authors: Katzir Yaron , Matthews Alan , Garfunkel Zvi , Schliestedt Manfred , Avigad Dov 
Published: May 2009

AbstractThe six exposures of the Upper tectonic Unit of the Cycladic Massif occurring on the island of Tinos are shown to comprise a metamorphosed dismembered ophiolite complex. The common stratigraphic section consisting of tens-of-metres- thick tectonic slices of mafic phyllites overlain by serpentinites and gabbros is considered to have been derived by a combination of thrusting during obduction and subsequent attenuation by low-angle normal faults. All rock types show evidence of a phase of regional greenschist-facies metamorphism, which in the case of the phyllites is accompanied by penetrative deformation. The greenschist-facies metamorphism in gabbros is preceded by high temperature sea-floor amphibolite-facies alteration, whereas in the serpentinites, the antigorite + forsterite greenschist-facies assemblage overprinted an earlier low temperature lizardite serpentinite. Trace element patterns of the mafic phyllites and a harzburgitic origin of meta-serpentinites suggest a supra subduction zone (SSZ) affinity for the ophiolitic suite. ρ18O values of phyllites, gabbros and serpentinites range from 6 to 15%o. Model calculations indicate that such values are consistent with low temperature (50–200°C) alteration of parent rocks by sea-water at varying water/rock ratios. This would agree with the early low temperature mineralogy of the serpentinites, but the early high temperature alteration of the gabbros would require the presence of18O-enriched sea-water.The following overall history is suggested for Tinos ophiolitic slices. (1) Oceanic crust was generated at a supra-subduction zone spreading centre with high temperature alteration of gabbros. (2) Tectonic disturbance (its early hot stages recorded in an amphibolitic shear zone at the base of serpentinites) brought the already cooled ultramafics into direct contact with sea-water and caused low-Tserpentinization. (3) Tectonism after cooling involved thrusting which caused repetition and inversion of the original order of the oceanic suite. (4) Regional metamorphism of all the ophiolite components at greenschist-facies conditions (−450°C) overprinted the early alteration mineralogy. It was probably induced by continued thrusting and piling up of nappes. The Tinos ophiolite, dated as late Cretaceous and genetically related to other low pressure rock-units of the same age in the Aegean, differs in age and degree of dismemberment and metamorphism from ophiolites in mainland Greece.
The Kula Volcanic Field, western Turkey: the development of a Holocene alkali basalt province and the adjacent normal-faulting graben
Authors: Richardson-Bunbury J. M. 
Published: May 2009

AbstractThe Kula province of Western Turkey provides an excellent example of an alkali basalt province in an area of active rifting. This paper establishes the relationship between the generation of the basalts and the extension of the region. The wide, shallow, terrestrial basin, formed by early extension, was subsequently cut through by narrow, fault-bounded grabens. The genesis of alkali basalt magmas began soon after the concentration of the extension into two grabens ˜2 Ma. The basalts, in the form of some 80 small cinder cones and associated lava flows and fields, have a total volume of 2.3 km3. This small volume reflects the small amount of extension in the region (β < 1.2 where β = final length of crust: initial length of crust).The intercalation of sediments and basalts in the Kula area enables the establishment of a relationship between the extensional activity and the generation of the basalts. This is demonstrable using radiometric and stratigraphic techniques, notably Ar-Ar dating of amphibole phenocrysts.
Zircon geochronology of anatectic melts and residues from a highgrade pelitic assemblage at Ihosy, southern Madagascar: evidence for Pan-African granulite metamorphism
Authors: Kröner A. , Braun I. , Jaeckel P. 
Published: May 2009

AbstractWe report U—Pb and207Pb/206Pb zircon ages for a granulite facies gneiss assemblage exposed in a large quarry at Ihosy, southern Madagascar. The granulites are derived from pelitic to arkosic sediments and attained equilibrium conditions at 650–700°C and 4–5 kbar. HigherP—Tconditions of 750–800°C and 6 kbar in the presence of low water activities have led to dehydration melting processes. The formation of granitic melts, which (partly) moved away from their source region, intruded into upper parts of the metapelitic gneisses as small granitic veins and left behind granulitic garnet-cordierite-quartz bearing rocks. Detrital zircons in a sample of metapelite and a sample of quartzofeldspathic gneiss yielded ages between ˜720 and ˜1855 Ma, suggesting a chronologically heterogeneous source region and a depositional age of less than ˜720 Ma for these rocks. High-grade metamorphism and anatexis are documented by zircon ages between 526 ±34 and 557 ±2 Ma with a mean age of about 550 Ma. The broad lithologies, metamorphic grades and ages recorded in the Ihosy rocks are similar to those in the Wanni Complex of northwestern Sri Lanka and in high-grade assemblages of southernmost India and support the contention that all these terrains were part of the Mozambique belt which formed as a result of collision of East and West Gondwana in latest Precambrian time.
Potassium–argon ages of volcanic rocks from northeast of Lake Turkana, northern Kenya
Authors: McDougall I. , Watkins R. T. 
Published: May 2009

AbstractThe Asille Group, a 1500 m succession comprising dominantly basaltic lavas but with interbedded sediments and silicic volcanics, lies unconformably upon metamorphosed basement northeast of Lake Turkana. Concordant K–Ar ages on alkali feldspars from pantelleritic units in the succession show that the Langaria Formation erupted 26.9±0.3 Ma ago in late Oligocene time, that Gum Dura Formation volcanism occurred at 15.8±0.2 Ma (early Miocene), and that there was widespread middle Miocene silicic volcanism at 13.0±0.3 Ma. Younger basaltic volcanism producing the flood lavas of the Gombe Group commenced at least 6.16±0.09 Ma ago in late Miocene time. These results, based upon 20 K–Ar age determinations, provide good numerical age control for much of the sequence developed within a little known sector of the Eastern Rift of Africa between the Ethiopian and Kenyan domes.
Multiphase magnetic overprints in the Moine Thrust Zone
Authors: Torsvik T. H. , Sturt B. A. 
Published: May 2009

AbstractPalaeomagnetic studies from the southern part of the Moine Thrust Zone (MTZ) indicate a complex pattern of four major secondary remanence components (A–D). Their relative ages appear to vary from mid-Ordovician to Recent/Tertiary. The oldest remanence (A) is tentatively interpreted as associated with the formation of the Lochalsh Syncline (D1) and/or D2. However, the consistency between component A and the regional southeast-plunging lineation, makes it uncertain to what extent component A reflects the structural grain rather than a true palaeofield record. Component B appears to be associated with early Devonian uplift along the MTZ relating to localized shearing/high level brittle thrusting, and is almost synchronous with the emplacement of lamprophyre and felsic dykes.In the inverted limb of the Lochalsh Syncline, and beneath the Balmacara Thrust, possible Permian (component C) and Recent/Tertiary magnetic overprints are recognized. Component C is partly of high magnetic stability and may indicate localized shears in the inverted limb, usually considered as Caledonian shearing. It is uncertain to what extent the youngest component (D), essentially recorded in rocks close to the Balmacara Thrust in mylonites and fault breccias, reflects important tectonic reactivation and magnetic resetting. This component is governed by low stability magnetic phases with blocking temperatures below 200 °C.
Mineralogy and geochemistry of Mesozoic black shales and interbedded carbonates, southeastern Sicily: evaluation of diagenetic processes
Authors: Azzaro E. , Bellanca A. , Neri R. 
Published: May 2009

AbstractUpper Triassic/Lower Jurassic organic-rich shales and interbedded carbonates (Rhaetian → Sinemurian) are widespread in the subsurface of southeastern Sicily where important oil fields have been found hosted in Triassic reservoirs. Core samples from wells drilled offshore and onshore were studied from petrographie and geochemical viewpoints.In the Hettangian/Sinemurian shale-carbonate sequences, which accumulated in a rapidly subsiding basin, the micritic aragonitic mud is still largely preserved. Mixed-layer I/S has remained randomly interstratified to a depth > 4000 m. Diagenetic carbonates are non-stoichiometric finely crystalline, pore-filling dolomite and/or calcite. The carbonate component exhibits a high Sr content and fair amounts of Fe and Mn. Carbon and oxygen isotopic values suggest a subsurface interstitial formation for the digenetic carbonates in an essentially closed system. Based on all accumulated data it is suggested that anoxic marine waters were retained in the sediment pores for a long time after deposition, thus enhancing the preservation of significant amounts of the original organic matter.In contrast, Rhaetian tidal-flat deposits hosting black shales display a clay component characterized by ordered illite-rich I/S and a carbonate mineralogy dominated by low-Mg calcite in the uppermost beds and by near-stoichiometric dolomite in the lowermost ones. Petrographie, chemical and isotopic data indicate early cementation in an oxidizing phreatic environment and lower down in the sequence pervasive dolomitization in a sabkha-type environment.
Major southeast-directed Caledonian thrusting and folding in the Dalradian rocks of mid-Ulster: implications for Caledonian tectonics and mid-crustal shear zones
Authors: Alsop G. I. , Hutton D. H. W. 
Published: May 2009

AbstractThe dominant structure controlling the disposition of Dalradian stratigraphy in mid-Ulster has hitherto been regarded as a southeast-facing gently inclined F1 anticline, a gross geometry modelled on, and thought to be a possible correlative of, the Tay Nappe in Scotland. Remapping of the supposedly inverted southern limb of this major fold reveals that much of it is in fact the correct way up. However, a stratigraphie repetition coupled with a reversal in younging does occur in the Sperrin Mountains, much further south than previously realized. This hitherto unrecognized upward southeast-facing isoclinal Sperrin Nappe is, however, a D2 structure, traceable for at least 40 km along strike and responsible for a regional stratigraphie inversion over an area of 300 km2. Following D2, a major 10 km thick D3 ductile shear zone resulted in translation towards the east-southeast. In the south, this deformation carried the Dalradian over Ordovician volcanics of the Tyrone Igneous Complex along the Omagh Thrust. Penecontemporaneity of magmatism with deformation clearly demonstrates that D3 is Caledonian (Arenig-Llanvirn). This deformation correlates with similar southeast-directed Caledonian thrusting in southern Donegal and Connemara. The apparent absence of Dalradian deformation of this age in southwestern Scotland may imply that Caledonian collision of outboard terranes with the miogeoclinal margin was initiated in Ireland and/or subsequent strike-slip has removed the evidence for deformation of this age from southwestern Scotland. The D3 shear zone in the Sperrin Mountains affects a very large volume of psammitic rocks. Within this shear zone the strain is not markedly higher than surrounding areas; however, its existence is demonstrated by the reorientation of mineral lineations over a large area. Such broad zones of only moderate strain may, we believe, be typical of translatory tectonics in areas of the mid-crust where there is little lithological diversity.
Deltaic sedimentation in an unstable tectonic environment – the Lower Limestone Group (Lower Carboniferous) of East Fife, Scotland
Authors: Fielding C. R. , Al-Rubaii M. , Walton E. K. 
Published: May 2009

AbstractThe Lower Limestone Group, which outcrops extensively on the east coast of Fife, southeast Scotland, consists of interbedded sandstones, siltstones and claystones with occasional coal seams, and limestone beds. Several of the coals have been mined economically in the past.The sequence was deposited in a shallow-water, deltaic setting. Three facies associations have been distinguished; (1) laminated claystones, and marine bioclastic limestones, which were deposited in prodelta and marine shelf environments, (2) crudely coarsening-upward sandstone units, interpreted as delta front deposits of fluvially-dominated though wave-influenced, shallow-water deltas, and (3) interbedded clastic sedimentary rocks and coals, deposited in delta plain environments. The sequence as a whole accumulated by repeated progradation and abandonment of deltaic complexes.Palaeosol profiles of four types are superbly preserved within the delta plain facies association. These are (1) organic soils (peats), now coal seams, (2) gley palaeosols, which formed in persistently waterlogged conditions, (3) freely drained soils which formed on a quartz sand-rich substrate, and (4) similar profiles formed by free drainage through variable substrates and involving B horizon accumulation of concretionary iron oxides. Compound palaeosols are common.Deposition of the Lower Limestone Group was influenced by the tectonic and volcanic instability of the Fife area during Visean times. Frequent seismic events centred on active fault lines caused local uplift of parts of the normally waterlogged delta plain environment, allowing the formation of well-drained soils. Peats (coals) were best developed and least affected by oxidation in an elongate, fault bounded zone of enhanced subsidence.
A Caradoc Failed-Rift within the Ordovician Marginal Basin of Wales
Authors: Campbell S. D. G. , Howells M. F. , Smith M. , Reedman A. J. 
Published: May 2009

AbstractThe dolerite and basalt intrusions within the Lower Palaeozoic sequence of northwest Wales are largely restricted to the outcrop of Ordovician strata. Their distribution and close association with known volcano-tectonic structures were controlled by a tectonic framework of deep-seated fractures. In central and northern Snowdonia, volcanism during Caradoc times was related to the evolution of a fracture-controlled trough. An increase in the extensional stress across the trough with time is reflected in the progressive increase in basaltic magma movement during the volcanic cycle. The trough represents an attempted rift in the lithospheric plate, which failed to create an ocean basin and was subsequently aborted.