bluuebunny/crossref_metadata_2025_split huggingface dataset:
Derivation of 500 Ma eclogites from the passive margin of Baltica and a note on the tectonometamorphic heterogeneity of eclogite-bearing crust
Authors: Andréasson Per-Gunnar , Albrecht Lena 
Published: May 2009

AbstractSeveral recent plate reconstructions of the Iapetus Ocean describe the margins of Baltica as passive until Silurian collision with Laurentia. Yet there is a variety of evidence to suggest that the accretion of the Scandinavian Caledonides began by latest Cambrian—early Ordovician subduction and imbrication of the passive continental margin. One such evidence is provided by eclogites occurring in the Seve Nappe Complex. Previous work by others dated the high-pressure metamorphism at 503±14 Ma (Sm—Nd garnet-omphacite age), and the uplift through the c. 500°C isotherm at 491±8 Ma (40Ar/39 Ar hornblende plateau ages). The protolith dolerites of the eclogites have been correlated with Iapetan rift-facies dolerites of the Baltoscandian margin. If valid, such a correlation implies early Caledonian destruction of the margin, and thus modification of those plate reconstructions which require passive margins around Baltica in latest Cambrian-early Ordovician time. This paper provides a substantially improved basis for the concept that the protoliths of eclogites and their host rocks derived from Baltoscandian rift basins. The chemical similarity between coronitic dolerites and dolerites of the rift basins pertains not only to element concentrations and variations but also to the specific T-MORB signature shared by the two groups. The variation of psammitic and pelitic schists, graphitic schists, calc-silicate gneisses and marbles of the eclogite host rocks equates with sequences of sandstones, siltstones, shales, black shale, quartzite, dolomite and limestones of Baltoscandian palaeobasins.At the same time, the paper calls attention to the remarkable preservation of structural and metamorphic contrasts within the eclogite-bearing thrust sheets of the Seve Nappe Complex. Such disequilibrium is generally ascribed to the kinetics of localized deformation and fluid infiltration into dry crust. This paper presents evidence that disequilibrium is found also within inferred subducted sedimentary complexes, which are generally assumed to be pervasively flushed by fluids. Preservation of sedimentary, volcanic and magmatic structures and fabrics, and of both undeformed dolerite dykes and eclogitized dykes demonstrates that neither deformation nor high-pressure metamorphism were pervasive.
Geochemical evidence for the volcanic arc tectonic setting of the Dhanjori volcanics, Singhbhum craton, eastern India
Authors: Alvi Shabber H. , Raza M. 
Published: May 2009

AbstractGeochemical data on the Dhanjori volcanics of the Singhbhum craton indicate that they range from basalt to andesite and show an iron-enrichment trend. Various chemical characteristics suggest that they are differentiated along the trend similar to that of orogenic suites and have a strong affinity with island arc tholeiites. The field relationships as well as other geological information also support this conclusion and indicate their eruption on a thin continental margin. It is inferred that the Dhanjori volcanics were probably erupted as a result of plate convergence in northern Singhbhum with subduction of oceanic crust below the Singhbhum craton.
Non-marine oolitic ironstones in the Lower Cretaceous Wealden sediments of southeast England
Authors: Taylor Kevin G. 
Published: May 2009

AbstractOolitic ironstones are described from the early Cretaceous non-marine Wealden sediments of southeast England. The recognition of non-marine oolitic ironstones in the geological record is rare and, therefore, warrants further study. The oolitic ironstones described take two forms, named here type-I and type-2 ironstones.Type-1 ironstones contain pisoids and ooids of berthierine together with sandstone fragments and detrital quartz grains. The pisoids (up to 0.5 cm in size) vary from subspherical to highly irregular. The smaller ooids (up to 1 mm in size) are generally ellipsoidal but strongly asymmetrical forms are also present. The form of these pisoids and ooids suggest that mechanical accretion was not the dominant mechanism controlling their formation. It is proposed that this ironstone type formed from the reworking and redeposition of local soil material.Type-2 ironstones, of which only one unequivocal example has been studied, is composed of iron oxide ooids set in a detrital matrix. The ooids are most commonly regularly ellipsoidal and exhibit a decrease in iron at their centres. It is proposed here that the ooids suffered post-depositional iron depletion at their centres, in a similar fashion to that proposed for the recent Lake Chad oolites. There is no unequivocal evidence as to the origin of the ooids.This study is important in that it shows that different ironstones can be formed by different processes essentially within the same environment. Comparison of non-marine oolitic ironstones with the better-developed marine examples should prove a valuable exercise.
Significance of radiolarian age data to the Mesozoic tectoni and sedimentary evolution of the northern Pindos Mountains, Greece
Authors: Jones Gregory , de Wever Patrick , Robertson Alastair H. F. 
Published: May 2009

AbstractRadiolarians were extracted from siliceous sediments of the northern Pindos Mountains, in an attempt to establish the chronology of tectonic and stratigraphic events related to the evolution of the Pindos ocean basin. Three separate phases of siliceous sedimentation were identified: (i) (mid-) late Triassic; (ii) mid-late Jurassic and (iii) mid-late Cretaceous. The first two phases are also known from the Pindos and Sub-Pelagonian zones of southern and central Greece, and elsewhere in the Dinarides andHellenides. However, the occurrence of Cretaceous radiolarites in the west central Tethyan region is somewhat unusual. Field observations suggest thatfrom the mid-late Triassic through to the mid Jurassic, radiolarites were deposited on volcanic basement, or were interbedded with sediments associated with the late rifting/spreading stages in the development of the Pindos ocean. Radiolarites of mid-late Jurassic age are commonly interbedded with clastic sediments of ophiolitic derivation. This coincides with a phase of significant compression within the Hellenides, which caused intra-oceanic deformation of the Pindos ophiolite. The ophiolite was subsequently emplaced onto the margin of the Pelagonian microcontinent in latest Jurassic time (Kim-meridgian-early Tithonian), as evidenced by transgressive marine carbonates. However, the Pindos basin survived in reduced form until the early Tertiary, allowing radiolarites to accumulate again within Cretaceous post-tectonic clastic sequences.
Lower Cambrian acritarchs from northern Spain: the Precambrian-Cambrian boundary and biostratigraphic implications
Authors: Palacios Teodoro , Vidal Gonzalo 
Published: May 2009

AbstractAcritarchs are reported from basal Cambrian rock units inthe Cantabrian region of northern Spain that are known to contain archaeocyathan and trilobite faunas. Biostratigraphic correlation of the Iberian sequences with other regions has been hampered by the strong provincialism of these faunas. However, this report of evidently cosmopolitan acritarch taxaestablishes the time equivalence of early Cambrian trilobite faunas from Iberia, Baltoscandia and the East European Platform (EEP). Our data suggest that the detrital deposition of the Lower Cambrian Herreria Formation embraces at least three (and possibly four) Lower Cambrian acritarch zones previously identified in the EEP, eastern Siberia, Baltoscandia, Scotland, Greenland, Svalbard and western North America. The early Cambrian transgression in northern Spain was probably initiated in Talsy times (Schmidtiellus mickwitzitrilobite Zone in Baltoscandia and the EEP), in part corresponding to theDokidocyathus regularisarchaeocyathian Zone of the Middle Tommotian in Siberia.
Cambrian stratigraphy and stage subdivision in the Holy Cross Mountains, Poland
Authors: Orlowski Stanislaw 
Published: May 2009

AbstractThe lithostratigraphic and biostratigraphic subdivisions of the Cambrian sequence in the Holy Cross Mountains of Poland are briefly presented and discussed. In accordance with recent progress in Cambrian stratigraphy and correlation in Great Britain and Scandinavia, a new proposal for chronostratigraphic stages is made. It is suggested that these stages are suitable for the Cambrian areas of central and northwest Europe, comprising Great Britain, Scandinavia and Poland. The formal establishment of the stages, a task for the Commission on Cambrian Stratigraphy, could be made using British or Scandinavian stratotypes.
Superposed structures in the Mona Complex at Rhoscolyi Ynys Gybi, North Wales
Authors: Roper H. 
Published: May 2009

AbstractThe Bedded Series of the Mona Complex at Rhoscolyn comprises two groups of clastic metasediments: the Holy Island Group, consistingof quartzites, impure psammites and pelites, with well-preserved bedding, is overlain conformably by the New Harbour Group, which is for the most parthomogeneously semi-pelitic without surviving bedding. Both groups have undergone the same two major tectono-metamorphic episodes, but with differing response. In the Holy Island Group the first episode (Dx) produced nearly upright and upward-facing folds (Fx) with an axial planar foliation (Sx), which varies from an anastomosing or rough-spaced cleavage in quartzites to a penetrative phyllitic schistosity in pelites. In the New Harbour Group Dxhas generally obliterated original bedding surfaces, replacing them with a composite foliation (Sx) of fine compositional banding and a penetrative schistosity, together with a stretching lineation (Lx), the latter being at a high angle to the Fxaxial direction. The Dxstructures are attributed to a major episode of compressional tectonics.The structures attributed to the second deformation (Dy) includestrata-bound sets of quartz-filled tension fractures (attributed by most previous authors to an earlier episode), abundant NNW-verging asymmetric folds (Fy) of Sx, and a sporadically developed set of shear fractures which constitute a crenulation cleavage (Sy) axial planar to the folds. It is suggested that all these structures were produced by a single agency. One interpretation is that the observed shear fractures and folded tension fractures correspond fairly closely to and provide a natural analogy of those obtained in the classical simple shear experimentsof Riedel. In this case all the Dystructures can be accounted for by the action of a large-scale simple shear couple (Cy), whose vergence and shallow dip were both towards the NNW. Such a mechanism may imply a gravity-dominated regime of net horizontal extension in a NNW-SSE direction, with extension being less constrained to the north than to the south. J. W. Cosgrove has suggested an alternative interpretation, that all the Dystructures can be explained as reverse kink bands; the simple shear interpretation is here preferred because the angle between Syand the estimated direction of Pmaxduring Dywas < 45°; the kink band model would require an angle > 45°.The fact that cleavage vergence boundaries for both Sxand Syoccur close to the hinge zone of the Rhoscolyn Antiform is consistent with either Dxor Dyage for the initiation of this fold. However, when fold limb length (or limb rotation) vergence is considered, the presence of an Fx0vergence boundary but absence of an Fxyvergence boundary (and by implication of an Fy0boundary) is consistent with a Dxage but difficult to reconcile with a Dyage.
Ammonite correlation of the uppermost Campanian of Western Europe, the U.S.Gulf Coast, Atlantic Seaboard and Western Interior, and the numerical age of the base of the Maastrichtian
Authors: Kennedy W. J. , Cobban W. A. , Scott G. R. 
Published: May 2009

AbstractIf the base of the Maastrichtian Stage is placed at the first appearance ofthe belemniteBelemnella lanceolatain Western Europe, the boundary between the Campanian and Maastrichtian stages in the U.S. Western Interiormay lie between the zones ofBaculites jenseniandBaculites eliasi. TheB. jenseniZone can be shown to be latest Campanian in age on the basis of the occurrence ofNostoceras (N.) hyattiStephenson andJeletzkytes nodosus(Owen), which range to the top of the Campanian in the well-dated Vistula Valley sequence in Poland, but disappear before the first appearance ofB. lanceolata. So defined the numerical age of the Maastrichtian lies between 73.2 ± 0.7 and 70.1 ±0.7 Ma, based upon dates from high temperature chronometers in bentonites in the Western Interior.
The influence of strain, lithology and stratigraphical depth on white mica (illite) crystallinity in mudrocks from the vicinity of the Corris Slate Belt, Wales: implications for the timing of metamorphism in the Welsh Basin
Authors: Roberts B. , Merriman R. J. , Pratt W. 
Published: May 2009

AbstractLow and very low grade metamorphism in a mudrock-dominated succession, ranging from Upper Cambrian to Wenlock, is characterized by the white mica (illite) crystallinity technique. Kubler indices indicate that grade ranges from late diagenetic to low epizonal. The prograde reaction most easily observed by XRD study is the progressive ordering of K/Na micas; diagnostic2M1polytypic reflections become recognizable at 0.4 Δ 2θ and strengthen as grade advances. Grade generally correlates with thickness of overburden in the > 9 km thick succession. However, strain exercises additionally a profound influence on grade, and the distribution of strain was related to décollement resulting from inhomogeneities and ductility contrasts within the succession. Strain energy is seen as an additional and important variable in the development of white mica crystallinity in slates. The distribution of metamorphic grade implies that the Lower Palaeozoic succession was augmented by several kilometres of Lower Devonian (Old Red Sandstone) strata at the metamorphic culmination. The culmination was syntectonic and represents an Acadian rather than an early diastathermal event. Subsequent dextral movement on the Bala Fault displaced the regional isocryst pattern byc.4 km. Retrogressive smectite appears to be localized to the vicinity of the Bala Fault and is probably of Variscan age.
A U/Pb age for the Shetland Islands oceanic fragment, Scottish Caledonides: evidence from anatectic plagiogranites in ‘layer 3’ shear zones
Authors: Spray John G. , Dunning Gregory R. 
Published: May 2009

AbstractHigh precision U/Pb data obtained from zircons extracted from plagiogranite within the gabbro unit of the Shetland Islands oceanic fragment of northeast Scotland yield an age of 492 ± 3 Ma. Field relations indicate that the plagiogranites were generated by the partial melting of amphibolitized gabbros within high-temperature shear zones formed due to crustal deformation and fluid infiltration occurring in proximity to a spreading centre. The U/Pb data therefore constrain the crystallization age of the Shetland complex. This age is similar to U/Pb ages obtained from the Leka (497±2 Ma), Karmoy (493+7-4 Ma) and Gulfjellet (489±3 Ma) oceanic fragments of the Norwegian Caledonides, and the Pipestone Pond (4943-2 Ma) and Betts Cove (4893-2 Ma) oceanic fragments of the Canadian Appalachians.
Geochemistry of the Start Complex greenschists: Rhenohercynian MORB?
Authors: Floyd P. A. , Holdsworth R. E. , Steele S. A. 
Published: May 2009

AbstractThe meta-igneous greenschists of the Start Complex, SouthDevon, are composed of a mineralogically uniform, but texturally variable, actinolite-epidote-albite assemblage with retrogressed variants containing chlorite, muscovite, sphene, carbonate and oxidized opaque minerals. Geochemically they represent a suite of relatively primitive tholeiites, exhibiting mild differentiation, depleted incompatible element abundances, and variable light rare-earth-element-depletion patterns comparable to modern basalts from normal spreading ridge segments (N-MORB). As the Start greenschistsexhibit a number of chemical similarities to the nearby Upper Palaeozoic Lizard ophiolite, and MORB-type clasts within the Rhenohercynian Zone generally, they may also represent local Variscan ocean crust, which floored smalloceanic basins that separated the Old Red Sandstone continent from the Armorican microplate to the south. The Start Complex could thus represent a previously unrecognized oceanic component to the Variscan orogenic belt (Rhenohercynian Zone) of Northwest Europe.
The timing of Ordovician magmatism in the English Lake District and Cross Fell inliers
Authors: Hughes Richard A. , Kokelaar Peter 
Published: May 2009

AbstractVolcanic and hypabyssal intrusive rocks of the Lower Palaeozoic English Lake District and Cross Fell inliers are elements of the Ordovician destructive plate margin system of microcontinental Avalonia. Two igneous sheets within the marine sedimentary Skiddaw Group of these inliers, previously described as lavas, are reinterpreted as sills. Sedimentary rocks enclosing these sills are of late Tremadoc-early Arenig (c. 493 Ma) and early Llanvirn (> 476 Ma) age, and breccias along the upper contacts of both were produced by steam explosivity and fluidization ahead of theadvancing tips of the intrusions. Previous interpretation of the breccias on the older sheet, as sediment deposited on the eroded top of a lava flow, implied an early Ordovician onset of arc magmatism. Such early magmatism would have been virtually coincident with the latest Tremadoc initiation of arc magmatism in Wales, but evidence for such a near synchronous response tothe putative onset of subduction is lacking. Respective onsets of magmatismwere probably separated by at least 17 m.y., and possibly by as much as 29 m.y. The apparent contemporaneity of mid and late Ordovician volcanic episodes in England and Wales, and similarities in extensional tectonic style, suggest that the two areas then were part of the same subduction system responding similarly to plate-scale magma-generating and tectonic processes. The early Ordovician situation is uncertain, but the absence of arc volcanic rocks of this age in the English Lake District suggests that this area and Wales are not tectonically juxtaposed elements of a former simple linear arc.
Emplacement and thermal metamorphism associated with the post-orogenic Strath Ossian Pluton, Grampian Highlands, Scotland
Authors: Key R. M. , Phillips E. R. , Chacksfield B. C. 
Published: May 2009

AbstractThe multiphase Strath Ossian Pluton was intruded into metasedimentary rocks of the Neoproterozoic Grampian and Appin groups (Grampian Highlands, Scotland) during Silurian or early Devonian times. Emplacementfollowed the main ductile tectono-thermal history of the area and took place during post-orogenic regional uplift and cooling. Early emplacement of dioritic magma in the northern part of the pluton resulted in migmatization of its immediate country rocks with the generation of new ductile structures. The main granodiorite was then emplaced with magma migrating towards the southeast where wall-rock stoping took place. Elsewhere the pluton created its own space with little stoping or veining. Thermal metamorphism caused by granodiorite emplacement resulted in the progressive development of the assemblage quartz+ plagioclase + biotite+ cordierite +andalusite ± K-feldspar in the metapelitic country rocks. Six prograde mineral assemblage zones are identified in the aureole. Final emplacement of a marginal porphyritic microgranite was accompanied by the release of alkaline fluids into the thermal aureole. This produced sillimanite (fibrolite) in association with hydrous phases such as chlorite and white mica. The development of andalusite and cordierite-bearing assemblages is estimated to have occurred at temperatures of 650±50 °C at an estimated pressure of 3.2±0.5 kbars. An approximately isobaric temperature change of 300±50 °C across the width of the main aureole is deduced. The migmatization close to the plutons margins took place at temperatures of about 700 °C. An estimated depth of emplacement of about 11 km is obtained for the Strath Ossian Pluton. This implies considerable regional uplift both prior to, andimmediately after its emplacement. Thus it has been estimated that at the peak of regional metamorphism, probably during the Ordovician Period, the country rocks were at a depth of 15 to 18.5 km, whereas the early Devonian dykes of the Etive dyke swarm, which cut the Strath Ossian Pluton, were emplaced at, or near surface.
A new look at Belgian aeromagnetic and gravity data through image-based display and integrated modelling techniques
Authors: Chacksfield B. C. , De Vos W. , D'Hooge L. , Dusar M. , Lee M. K. , Poitevin C. , Royles C. P. , Verniers J. 
Published: May 2009

AbstractDigital processing and image-based display techniques have been used to generate contour and shaded-relief maps of Belgian aeromagnetic data at a scale of 1:300000 for the whole of Belgium. These highlight the important anomalies and structural trends, particularly over the Brabant Massif. North and vertically illuminated shaded-relief plots, enhanced structural belts trending west–east to northwest–southeast in the Brabant Massif and west–east to southwest–northeast in the core of the Ardennes. The principal magnetic lineaments have been identified from the shaded-relief plots and tentatively correlated to basement structures. Most short lineaments are correlated with individual folds while the more extensive lineaments are correlated with large scale fault structures. Magnetic highs within the Brabant Massif are attributed to folded sediments of the Tubize Group. The magnetic basement in the east of Belgium is sinistrally displaced to the north by an inferred deep NNW–SSE crustal fracture. The Bouguer anomaly map of Belgium identifies the Ardennes as a negative area, and the Brabant Massif as a positive area, with the exception of a WNW–trending gravity low in its western part. The southern margin of the Brabant Massif is defined by a steep gravity gradient coincident with the Faille Bordiere (Border Fault). Trial modelling of the gravity and magnetic data, carried out along profiles across the Brabant and Stavelot massifs, has identified probable acid igneous intrusions in the western part of the Brabant Massif, and a deep magnetic lower density body underlying the whole Ardennes region, which is thought to be a distinctive Precambrian crustal block.
The Rügen Caledonides, northeast Germany
Authors: Katzung G. , Giese U. , Walter R. , Von Winterfeld C. 
Published: May 2009

AbstractBorehole evidence from the Isle of Rügen, and geophysical data from the adjacent Baltic Sea suggest a Caledonian deformation front that caused NNE-directed thrusting on the East European Platform. It is thought to mark a Caledonian suture zone which was reactivated in syn- to post-Variscan events to form the Rømiø-Møn Fault Zone.