bluuebunny/crossref_metadata_2025_split huggingface dataset:
A palaeomagnetic investigation of the Malvernian and Old Radnor Precambrian, Welsh Borderlands
Authors: Piper J. D. A. 
Published: July 2010

AbstractAlternating field and thermal demagnetization of igneous rocks of the Malvern Hills identifies a number of magnetite‐held components which are characterized by a high blocking temperature (M2) componentD= 283°,I= 47°, and lower blocking temperature (M3) componentD= 269°,I= −43° which is of complex origin or more than one age. Two subordinate components are (M1)D= 7°,I= 56° and (M4)D= 174°,I= 51° in later dolerites. A pervasive hematite‐held remanence with a meanD= 186°,I= −5° is linked to Hercynian palaeofield directions and the uplift/folding of the Malvernian axis. The similarity of the magnetization directions in the Stanner–Hanter (702 Ma) and Malvernian (681 Ma) rocks suggests that folding of the Palaeozoic rocks in the Malvern Hills was achieved by upthrust of the basement and involved little folding of the latter. The Old Radnor sediments possess a post‐folding remanenceD= 117°,I= −13° of probable Cambrian age and a subordinate remanence which may be Hercynian in age.The late Precambrian–Cambrian palaeomagnetic record (ca.700–500 Ma) of England and Wales is compared with data from the Armorican Massif. Although the apparent polar wander (a.p.w.) paths are widely dissimilar prior to 550 Ma, the two regions had similar latitudes and went through similar palaeolatitudinal movements throughout this interval. The palaeomagnetic data support models involving tectonic rotations but little closure across this part of the Hercynian Belt.
Major element composition of the pelites of the Scottish Dalradian
Authors: Atherton M. P. , Brotherton M. S. 
Published: July 2010

AbstractMajor element composition of the available analysed pelitic rocks of the Dalradian of Scotland is examined in relation to geographical position, mineralogy, metamorphic grade and stratigraphy. Some of the large variations found are due to geographical position, possibly reflecting different provenance. A major compositional difference exists between the two lower Barrovian metamorphic zones and the four higher zones. This is considered to be a reflection of the original sedimentary composition and large scale metasomatism is not considered likely.
The stratigraphy of the Drummuck Group (Ashgill), Girvan
Authors: Harper D. A. T. 
Published: July 2010

AbstractThe Drummuck Group of the Girvan district (S. W. Scotland) comprises a varied sequence of marine siliciclastic sedimentary rocks some 350 m thick and ranging in age from early Cautleyan to late Rawtheyan (Ashgill Series). Deposition in an unstable slope environment is envisaged. These upper Ordovician rocks crop out in the centre of the Craighead inlier, north of the Girvan valley and are sporadically exposed and locally very fossiliferous. A detailed revision of the Drummuck rocks has permitted, for the first time, the formal lithostratigraphical division of the group. Four main units, in ascending order, the Auldthorns, Quarrel Hill, Lady Burn and South Threave formations are recognized and, within these, a number of smaller, useful divisions are defined. Detailed geological maps of the Drummuck outcrop are presented. Brachiopods numerically dominate the shelly faunas and all the divisions named are characterized by distinctive brachiopod assemblages. Preliminary correlations are discussed with selected upper Ordovician successions elsewhere but the emphasis of this study is placed upon the establishment of a stratigraphical framework within which future detailed research on the Drummuck faunas may be carried out.
Rb‐Sr isochron study of the Thorr and Main Donegal Granites, Ireland
Authors: O'Connor P. J. , Long C. B. , Kennan P. S. , Halliday A. N. , Max M. D. , Roddick J. C. 
Published: July 2010

AbstractA Rb‐Sr whole‐rock isochron study indicates that the entire Donegal granite suite was emplaced into orthotectonic Caledonian (Dalradian) rocks over a short interval during mid‐Silurian to earliest‐Devonian times. The Thorr pluton, probably the earliest member of the suite, yields an age of 418 ± 26 Myr and initial87Sr/86Sr ratio of 0·7055 ± 4, while the latest member, the Main Donegal pluton has an age of 407 ± 23 Myr and initial87Sr/86Sr ratio of 0·7063 ± 5 (Λ87Rb = 1·42 ± 10−11yr−1). Errors on both the age and initial Sr isotope ratios incorporate botha prioriand geological scatter components and are quoted at the 2‐sigma level. The low and restricted range of initial Sr isotope ratios suggests small but significant differences in the composition of the parental granitic magmas which were derived from a low Rb/Sr, low87Sr/86Sr source.
The Late‐Quaternary deposits of the middle Dyfi Valley, Wales
Authors: Thomas G. S. P. , Summers A. J. , Dackombe R. V. 
Published: July 2010

AbstractThe surficial deposits and landforms of the middle Dyfi Valley, Wales, are examined and a sequence of glacial, glacio‐fluvial and glacio‐lacustrine sediments is identified. This is considered to relate to a late stage in the wastage of the Late‐Devensian Merioneth ice‐cap. A sequence of terrace gravels overlying the giacigenic succession represents dissection during early Flandrian times.
Geophysical and radiometric studies of the Northern Units of the Leinster Intrusion
Authors: Cassidy J. , Locke C. A. 
Published: July 2010

AbstractThe Leinster intrusion, S.E. Ireland, is one of a group of Caledonian granites, sensu lato, of Siluro‐Devonian age, which typically exhibit strong geophysical and geochemical contrasts with their host rocks. New gravity data supplement previous road‐based data and define, over the Northern Units of the intrusion, a closed residual anomaly (‐160 g.u.) with steep gradients. The Northern Units are shown to comprise a large volume of low‐density material extending to a depth of 8 km, and hence to constitute a significant contribution to Caledonian crustal growth. The sub‐surface form of the intrusion is essentially vertical‐sided having no broad root, in contrast with other Siluro‐Devonian granites (s.l.).Radioelement concentrations in the Northern Units were mapped quantitatively by in situ gamma‐ray spectrometry. The mean concentrations of 4·2 per cent K2O, 3·0 ppm U and 8·1 ppm Th measured are lower than those typical of the Siluro‐Devonian granites (s.l.), suggesting depletion associated with high‐level fractionation, and in addition a possible Th‐depleted source region. Complex areal radioelement distributions reflect both the multiple intrusive event, and later secondary remobilization, particularly of U. This is demonstrated using fission‐track radiography which shows that U, at least, is located in both primary and secondary minerals.
Interpretation of field relationships of Diorites and associated rocks with particular reference to northwest Guernsey, Channel Islands
Authors: Topley C. G. , Brown M. , Power G. M. 
Published: July 2010

AbstractIn northwest Guernsey, excellently exposed sections of coast reveal a wide variety of diorites and associated rocks which are particulariy instructive in understanding field relationships in diorite complexes generally. In this area, three groups of rocks may be distinguished. A Diorite Group consists of even grained diorite, acicular diorite and appinite, believed to be related to each other by fractionation. This group was intruded by a Granodiorite Group which contains diorite xenoliths. In turn, both of these were intruded by an Inhomogeneous Suite of rocks varying from meladiorite through to tonalite. Field evidence and geochemistry show that this last, highly variable suite of rocks was formed by the unusual process of fractional crystallization coupled with disruption and partial redigestion of early solid fractionates by derivative liquids, leading to the marked inhomogeneity. Such a mechanism may be more generally applicable but clear evidence for it seems to be restricted to rocks of dioritic composition. The margins of the Inhomogeneous Suite usually consist of tonalite which has piped into the surrounding diorites of the Diorite Group, and the two rock groups are thought to have been at least partially liquid at the same time. The exact shape of pipes may prove to be a useful indicator of physicai conditions at the time of magma emplacement in this and other intimate associations of magmas of contrasted composition. Although the Diorite Group was emplaced first, dark (apparently chilled) margins sometimes occur in diorites of this Group where they are in contact with the later Inhomogeneous Suite. In such intimate associations as this, dark margins must be interpreted with extreme caution.
Middle carboniferous marine transgression, Bjørnøya, Svalbard: facies sequences from an interplay of sea level changes and tectonics
Authors: Gjelberg J. , Steel R. 
Published: July 2010

AbstractThe Svalbard Platform, in middle Carboniferous time, was dominated by a series of NNW‐SSE oriented, asymmetric rift basins. The Landnørdingsvika Formation represents the infill of one such basin in the Bjørnøya (Bear Island) area and consists of red beds deposited during a regional rise of sea level. The basin was filled mainly from the west and southwest across a Carboniferous fault zone, the West Bjørnøya. Fault, which bounded the deep edge of the basin.The basin succession is dominated by floodplain and coastal plain deposits in its lower part and fanglomerates interbedded with shallow marine clastics and carbonates in its upper part. The marine facies gradually increase in volume upwards and culminate in the overlying, carbonate‐dominated Kapp Kåre Formation (Moscovian). This continental‐marine transition, which has also been identified in the other Svalbard basins, thus reflects an important middie‐late Palaeozoic transgression. Analysis of the facies sequences shows that there are repeated submergence‐emergence events which are superimposed on the longer term transition. These are interpreted in terms of repeated basin floor tilting and sinking against the upland block.
The Monian ‘Penmynydd Zone of Metamorphism’ in Llŷn, North Wales
Authors: Gibbons W. 
Published: July 2010

AbstractIn the Mona Complex of Llŷn, a narrow, steep belt of fine grained schists known as the Penmynydd Zone of Metamorphism lies between a deformed melange (Gwna Group) and a unit of gneisses and plutonic rocks (Sarn Complex). Rocks within and adjacent to this Zone are characterized by mylonitic textures. The highest grade Penmynydd rocks on Llŷn reached greenschist and, locally, glaucophanitic greenschist fades conditions. The schist belt is interpreted as a major shear zone formed as the Sarn Complex moved up against the lower grade Gwna Group. A previous interpretation of the schists (Shackleton 1956) as being the result of a prograde metamorphism from sub‐greenschist facies Gwna Group to anatectic granitic and gneissic rocks of the Sarn Complex is rejected.
Metasomatism and the formation of greisen in Grainsgill, Cumbria, England
Authors: Roberts D. E. 
Published: July 2010

AbstractA section of hornfelsed Skiddaw Slate adjacent to the margin of the Grainsgill Greisen is described and chemical analyses of the various rock types presented. The results confirm that hydrothermal fluids liberated K and Na from the granite to produce the greisen. This gave rise to K metasomatism of the adjacent hornfels and to retrogressive metamorphism over a radius of 200m from the intrusion. Na leached from the hornfels near the intrusion and that liberated during the formation of the greisen are probably the sources of the Na in the brines recorded in fluid inclusions within the nearby quartz‐tungsten veins.
The skipton rock fault–‐an Hercynian wrench fault associated with the Skipton Anticline, northwest England
Authors: Arthurton Russell S. 
Published: July 2010

AbstractA zone of deformation characterized by en échelon faults and folds extends along the Skipton Anticline. The zone is well exposed on its northern margin at Haw Park, where it involves early Dinantian strata. Here a high‐angle reverse fault–‐the Skipton Rock Fault–‐is broadly coincident with the axial surface of the Skipton Anticline. This fault and its associated subsidiary faults and folds form a structural pattern consistent with an origin in dextral convergent wrenching or transpression.
On the occurrence of an unusual form of monazite in panned stream sediments in Wales
Authors: Cooper D. C. , Basham I. R. , Smith T. K. 
Published: July 2010

AbstractAn unusual nodular form of monazite has been found to account for abnormally high levels of cerium in panned heavy mineral concentrates from stream sediments in several areas in Wales and pan of Exmoor. Textural features indicate a pre‐metamorphic, diagenetic origin within Lower Palaeozoic shale or siltstone host‐rocks but REE patterns show only minor variation, including relative enrichment in europium, from ‘average’ granitic monazite. The similarities between these occurrences, described for the first time from the British Isles, and those previously described from Brittany and elsewhere are discussed.
The distribution of hydrocarbon minerals in the Welsh borderlands and adjacent areas
Authors: Parnell J. 
Published: July 2010

AbstractSolid hydrocarbon minerals occur in small quantities in the Lower Palaeozoic Welsh Basin, where Ordovician igneous intrusions mobilized them from local organic‐rich source rocks. Hydrocarbon minerals are widespread in the Wenlockian and Carboniferous Limestones, and at least in the Carboniferous Limestone they show a close and probably genetic relationship with lead‐zinc mineralization. The association of hydrocarbon minerals with lead‐zinc‐copper ores in Ordovician and Longmydian rocks' in the West Shropshire mining region is however largely coincidental. The hydrocarbon minerals in that region are residual from Carboniferous reservoir hydrocarbons. Reservoir hydrocarbon minerals in a breccia at Row Brook include crystallites of iron sulphides and manganese oxides. Hydrocarbon minerals in siderite nodules in the Coal Measures are spatially related to metal sulphides.
Low‐grade metamorphism within the Welsh sector of the paratectonic Caledonides
Authors: Bevins R. E. , Rowbotham G. 
Published: July 2010

AbstractMineral assemblages within metabasites from the paratectonic Caledonides of Wales show that the grade of metamorphism was predominantly within the prehnite‐pumpellyite facies, although green‐schist fades assemblages occur in the more central parts of the region. General agreement with this pattern of metamorphism is seen in recent studies of illite crystallinity and conodont colour. Comparison with other regions suggests that all of the paratectonic Caledonides of the British Isles has suffered a low‐grade of regional metamorphism during late Caledonian times.
The Avalon Zone: A Pan‐African terrane in the Appalachian Orogen of Canada
Authors: O'Brien S. J. , Wardle R. J. , King A. F. 
Published: July 2010

AbstractThe Precambrian sequences of the Avalon Zone in Canada (southeastern margin of the Appalachian Orogen) are interpreted as a Pan‐African orogenic belt incorporated into the Appalachian Orogen during Palaeozoic times as its southeastern margin. The Precambrian evolution of the Avalon Zone was genetically unrelated to subsequent Palaeozoic evolution. The Avalon Zone shows marked similarities in age, tectonic history, and facies development to the Pan‐African belts adjacent to the West African Craton.Precambrian evolution of the zone began with circa 800 Ma rifting of a sialic gneissic basement and deposition of a Middle Proterozoic(?) carbonate‐clastic cover sequence. Early crustal rifting was associated with localized partial melting and metamorphism. Limited crustal separation led to the restricted development of circa 760 Ma oceanic volcanics. Continued rifting and subsequent closure of these narrow ocean basins led to the eruption of widespread subaerial volcanic suites, block faulting, granite plutonism, and local, late Proterozoic sedimentary basin formation. Precambrian evolution of the zone terminated with the Avalonian Orogeny (circa 650‐600 Ma), a deformational event, the affects of which are most evident locally along the northwestern margin of the zone.The controlling features of the Proterozoic evolution of the Avalon Zone are a series of linear intracratonic troughs and small ocean basins that formed during thinning and separation of the crust by ductile spreading, rupture, and delamination (cf. Martin and Porada 1977). The variation in degree of crustal separation led to subsequent variation in orogenesis during late Proterozoic compression.The zone marks the original westward limit of Pan‐African activity and displays no apparent genetic link with the Appalachian Orogen in Canada until Devonian times.
Basement massifs in the Appalachians: their role in deformation during the Appalachian Orogenies
Authors: Hatcher Robert D. 
Published: July 2010

AbstractBasement is constituted of rocks which belong to a previous orogenic cycle which have been reactivated and incorporated into a younger cycle. Basement massifs may be classified according to their relative position in an orogen as external or internal massifs. They may also be categorized according to their role in deformation, as thrust‐related, fold‐related and composite massifs.All Appalachian external massifs were transported following their removal from the overridden edge of the ancient North American continental margin. Most of the internal massifs are also probably transported, but several (Pine Mountain and Sauratown Mountains) may be present as windows exposing parautochthonous basement beneath the main thrust sheet. The latter reside immediately west of the low (west) to high (east) gravity gradient which probably outlines the old edge of Grenvillian crust. Reactivated crustal material generated during early Palaeozoic orogeny plays the same mechanical role in reactivation as basement from the previous Grenville cycle. The domes of the Bronson Hill anticlinorium cored with Ordovician or older gneisses illustrate this behaviour.Basement (Grenville) massifs are distributed throughout the Appalachians as a belt of external massifs (Blue Ridge, Reading Prong, Hudson and Berkshire Highlands, Green Mountains, and Long Range Mountains) along the western edge of the crystalline metamorphic core. Additionally, internal massifs are also present (Pine Mountain belt, Tallulah Falls and Toxaway domes, Sauratown Mountains anticlinorium, State Farm gneiss dome, Baltimore Gneiss domes, Mine Ridge anticline, and Chain Lakes massif).Basement internal massifs probably served to localize thrusts by causing them to ramp over and around the massifs. Their antiformal shape may in part be as much related to thrust mechanics as to folding.
Basement‐cover relations in the Appalachian fold and thrust belt
Authors: Thomas William A. 
Published: July 2010

AbstractThe external massifs along the Appalachian orogen include Precambrian basement rocks with attached cover. To the northwest (cratonward), in the Appalachian foreland fold and thrust belt, Palaeozoic sedimentary rocks, but no basement rocks, are exposed; that belt was the subject of the classic debate about thin‐skinned (deformed cover rocks detached from undeformed basement) and thick‐skinned (basement deformed with attached cover) structural styles. Presently available data indicate detached cover rocks and thin‐skinned style in the fold and thrust belt: large‐scale thrusting occurred late in the orogenic history. In the external basement massifs, late Precambrian graben‐fill sedimentary and volcanic rocks indicate early basement faults; and within the craton, steep basement faults bound graben blocks of Cambrian age. Distribution of known basement faults suggests that basement rocks beneath the fold and thrust belt may also be faulted. Local episodic synsedimentary structural movement through much of the Palaeozoic is documented by stratigraphy in the fold and thrust belt. Axes of early synsedimentary structures are approximately coincident with axes of late folds and thrust fault ramps, but stratigraphic data show that magnitude of the early structures was much less than that of the late structures. These relations suggest the interpretation that early low‐magnitude structures formed in cover rocks over basement faults and that the early structures, or the basement faults, significantly influenced the geometry of later detachment structures during large‐scale horizontal translation.
Observations and speculations on the northeast Towy ‘axis’, mid‐Wales
Authors: James D. M. D. 
Published: July 2010

AbstractLate Ashgill and Llandovery sedimentation across the Towy ‘axis’ southeast of Rhayader is interpreted within the context of the rapid southeasterly thickening of the upper portion of the Ashgill best demonstrable in the Chwefru valley. This thickening sequence overlies deep‐water middle‐fan arenites and is interpreted as a northwest prograding shelf/slope association. The slope eventually became non‐depositional and was onlapped by the basinal facies of the Llandovery during relative sea‐level rise. The non‐depositional slope and associated major slump‐scars at the shelf‐edge explain many of the non‐sequences‐ in the region previously ascribed to erosion related to episodes of tectonism. The regional strain history is thus simplified.
A Pleistocene succession from beneath Chelford Sands at Oakwood Quarry, Chelford, Cheshire
Authors: Worsley P. , Coope G. R. , Good T. R. , Holyoak D. T. , Robinson J. E. 
Published: July 2010

AbstractA thin sequence of in situ fossiliferous gravels and silts overlain by a glacigenic bed is described from a temporary exposure in a working silica sand quarry. The sequence directly overlies unweathered Mercia Mudstone. Locally the bedrock surface forms a shallow palaeovalley some 15 m deep and almost 1 km wide. The sequence in question lies close to the valley axis. The silts, pollen, plant macrofossils, Mollusca, Coleoptera, and Ostracoda assemblages each suggest a similar environment of sedimentation: a shallow pool within a treeless open landscape. Much of the bedrock within the working quarry appears to be draped by the Oakwood till and this unit is co‐extensive with the glacigenic sediments over the silts and gravels. In parts at least, the till has been subject to redeposition and its upper surface has scattered ventifacts. Above this wind‐deflation horizon lies the Chelford Sands Formation, in the middle of which is the Chelford Interstadial stratotype, the Farm Wood member. The biota preserved in the gravels and silts suggest a stadial prior to the Early Devensian Chelford Interstadial. This is the first unambiguous stratigraphic evidence from Cheshire of a glacial event antedating the Chelford Interstadial.
Discovery of Arundian and Holkerian faunas from a Dinantian platform succession in North Wales
Authors: Somerville Ian D. , Strank Angela R. E. 
Published: July 2010

AbstractFor the first time Arundian and Holkerian faunas have been recognized from the lower part of the Dinantian succession in North Wales. These limestones and sandstones, hitherto regarded as Asbian, have a macrofauna and microfauna confirming their older age, and this discovery' has necessitated a revision of the palaeogeography in the North Wales region. Biostratigraphical correlations have been made with contemporaneous platform successions in the Central, Northern, and Southwest Provinces of Britain. A new genus and species of foraminifer, Groessensella moldensis, is described.