bluuebunny/crossref_metadata_2025_split huggingface dataset:
Pyroxene accumulation in spinifex-textured rocks
Authors: Campbell I. H. , Arndt N. T. 
Published: May 2009

SummaryNatural pyroxene spinifex-textured rocks, free of olivine, may have MgO contents as high as 15%. If these rocks are allowed to crystallize in the laboratory under equilibrium conditions, olivine is the liquidus phase at 1350°C and pyroxene does not appear until the temperature has fallen to 1180°C. A similar problem is encountered in high MgO komatiites. If spinifex-textured and aphyric komatiites from Munro Township are plotted on an MgO-CaO-Al2O3 diagram, olivine control is evident until the MgO content of the melt falls to 15%. At this point the liquid trend moves away from the CaO-MgO boundary, indicating the crystallization of pyroxene. If a high MgO komatiite is crystallized in the laboratory under equilibrium conditions, pyroxene does not appear until the MgO content of the liquid reaches 9%. It is suggested that experimental and natural systems behave differently because pyroxene has crystallized metastably in the natural system under conditions of extreme supercooling. The early metastable crystallization of pyroxene can only affect the liquid trend if the spinifex zone contains a cumulate component.
Rapid estimation of porosity and mineral abundance in backscattered electron images using a simple SEM image analyser
Authors: Pye K. 
Published: May 2009

AbstractImage analysis is rapidly becoming an integral part of scanning electron microscopy. A number of analogue and digital image analysis systems of varying sophistication are now commercially available for the SEM. This paper illustrates how one such relatively simple system, the IMAS image analyser manufactured by Cambridge Technology, can be used to obtain rapid quantitative estimates of porosity and mineral abundance in backscattered electron images of polished rock sections.
The Pusgillian Stage in Wales
Authors: Price David 
Published: May 2009

AbstractOn the basis of previous work in Northern England and in Southern Scotland the Pusgillian Stage is in turn related to the stratigraphical ranges of selected trilobite species and to the standard graptolite zonal scheme. Thus characterized it can be recognized in rocks of ‘shelly’ facies at localities in the northwest, southeast and south-central Berwyn Hills of North Wales and possibly in the Llandeilo area of South Wales, but does not seem likely to be developed in any rocks of graptolitic facies in Wales.
Interpretation of the Middle Jurassic ammonite genus Spiroceras
Authors: Howarth M. K. , Melville R. V. 
Published: May 2009

AbstractThe taxonomy of the Middle Jurassic ammonite genus Spiroceras Quenstedt, 1856, is critically assessed. It is found that the genus was originally proposed conditionally without any included species, and the type species is Spiroceras bifurcatum Hyatt, 1900, by subsequent monotypy. A neotype is now designated for that species, because Hyatt's figured holotype is lost.
Obruchevella from the early Cambrian Meishucun Stage of the Meishucun section, Jinning, Yunnan, China
Authors: Xueliang Song 
Published: May 2009

AbstractWell-preserved, silicified Obruchevella from the earliest Cambrian (Meishucun Stage) sequence from Meishucun section in Yunnan Province, China contains distinct cross-partitioning. This suggests that this helically coiled microfossil represents the remains of an oscillatorialean cyanophyta. Obruchevella is a microfossil that exhibits a variety of form that may be produced through degradation. Obruchevella identified in the Meishucun section includes O: parva, O. parvissima, O. delicata, O. meishucunensis and several other morphs which are yet to be identified.
Silurian volcanism in the central Welsh Borderland
Authors: Sanderson R. W. , Cave R. 
Published: May 2009

SummaryThis study of Silurian volcanism considers three small intrusions near Bishop's Castle, Salop, as possible volcanic feeders. Acid magma intrusive into Silurian calcareous siltstones has resulted in microgranite dykes, hornsfelsed country rock and metasomatized breccia. The intrusions are of late Wenlock age or younger, but they are relatable to other Wenlock igneous rocks in the area, and are in contrast to nearby Carboniferous basaltic rocks.
Basement blocks and tecto-sedimentary evolution in the Pyrenees during Mesozoic times
Authors: Peybernès Bernard , Souquet Pierre 
Published: May 2009

AbstractThe Alpine tectonics in the Pyrenean Range is controlled by motions of basement blocks (High Primary Range, North Pyrenean blocks). These blocks belong to different tectonic units; they are amygdaloid, wedge-shaped, squeezed at their roots, upthrust and displaced with their cover (detached or not) in a convergent shear zone between the European and Iberian plates. These basement blocks are distributed within isopic (isofacial) units with diachronous terrigenous prisms. The stratigraphical analysis of their Mesozoīc cover reveals their relative position during the Mid-Cretaceous continental break-up which induced the separation of the two previously joined plates.
A geometrical approach to percolation through random fractured rocks
Authors: Rivier N. , Guyon E. , Charlaix E. 
Published: May 2009

AbstractThe permeability of rocks fractured by random, planar cracks, is expressed as a classical bond percolation problem on a random lattice, by Voronoi partition of space. The percolation threshold is determined as a function of the statistical characteristics of the cracks, or of their traces on an arbitrary face of the rock, by using an empirical quasi-invariant of percolation theory.
Hybridization and the petrogenesis of composite intrusions: the dyke at An Cumhann, Isle of Arran, Scotland
Authors: Kanaris-Sotiriou R. , Gibb F. G. F. 
Published: May 2009

AbstractThe composite dyke at An Cumhann, Arran consists of a central unit of quartz-feldspar porphyry flanked by narrow marginal dolerites containing xenocrysts of quartz, plagioclase and alkali feldspar identical to the phenocrysts in the porphyry. The chemistry of the marginal dolerites indicates that they formed by the crystallization of a hybrid magma produced at depth by the incorporation of the porphyritic acid magma in a basic liquid. During the intrusion of the hybrid magma, flow caused a differential distribution of the xenocrysts across the initial basic dyke. Subsequent intrusion of the quartz-feldspar porphyry magma along the still unconsolidated centre of the basic dyke produced the wide central unit. A similar origin is proposed for other composite intrusions of the same type in the region. Whilst it is unlikely that the coexistence of the highly contrasting magmas necessary for the formation of these intrusions is entirely coincidental there is little likelihood that the two liquids were related as members of the same fractionation series.
Zonation, mixing and eruption of silica-undersaturated alkaline magma: a case study from Tenerife, Canary Islands
Authors: Wolff John A. 
Published: May 2009

AbstractThe mineralogy and petrology of banded pumice clasts from a mixed-magma non-welded ignimbrite on Tenerife, Canary Islands, is described. Phenocrysts, which frequently show signs of resorption, are grouped into three assemblages in mutual disequilibrium; these assemblages were originally precipitated by alkali basalt magma (assemblage 1, titanaugite + olivine + calcic plagioclase + high-Mg Fe–Ti oxides), trachyandesitic magma (assemblage 2, salite + kaersutite + sodic plagioclase + hauyne + medium-Mg Fe–Ti oxides) and phonolitic magma (assemblage 3, aegirine-augite + biotite + sodic sanidine + sodalite + nepheline + sphene + low-Mg magnetite) respectively. Phenocryst types (e.g. clinopyroxene, feldspar) which are common to two or all assemblages are compositionally distinctive in each. The excess silica content of nepheline in assemblage 3 indicates a temperature of 770 °C for phonolitic magma, while Fe–Ti oxides and clinopyroxene-ilmenite give temperatures of 915 °C and 907 °C respectively for the trachyandesite. Assemblage 1 and 2 phenocrysts are set in mafic glass; assemblage 3 phenocrysts are only found in a matrix of salic (phonolitic) glass. Analyses of the volumetrically dominant phonolitic glass define a compositional trend consistent with fractionation of assemblage 3 phenocrysts. Analyses of the mafic glass scatter about a straight-line trend produced by mixing of phonolitic liquid with homogeneous intermediate (trachyandesitic) magma. These relations are interpreted as the result of two distinct magma mixing events, consequent on the invasion of a stably zoned magma chamber containing phonolitic liquid stratified above denser trachyandesitic magma, by intruding basaltic magma. High Ca content of assemblage 1 plagioclase suggests precipitation of the basaltic phenocrysts under locally high partial pressure of water. The preferred model involves, in chronological sequence: (i) generation of a differentiated phonolitic cap above trachyandesitic magma, through wall crystallization and boundary-layer uprise of phonolite; (ii) pooling, convective heat loss, and partial crystallization of dense, hydrous alkali basalt magma injected into the chamber and forming a liquid layer beneath the trachyandesitic magma; (iii) volatile saturation of the residual basalt liquid through crystallization, consequent buoyant uprise into, and efficient mixing with, trachyandesite; (iv) eruption of chamber contents and partial mixing of the mixed basalt–trachyandesite with phonolite, although the precise relative timing of eruption and the second mixing event is not known.
Cretaceous-Palaeogene sub-basaltic and intrabasaltic sediments of the Kangerdlugssuaq area, Central East Greenland
Authors: Higgins A. C. , Soper N. J. 
Published: May 2009

SummaryCretaceous-Palaeogene sediments of the Kangerdlugssuaq area on the continental margin of Central East Greenland were deposited in an embayment of an extensive pre-NE Atlantic shelf sea. Pre-Sparnacian sediments are thin (150 m), incomplete and of siliciclastic type, formed in shallow marine waters. Sparnacian times saw the onset of vigorous basaltic vulcanicity, marking the initial rifting episode between Greenland and Eurasia. Uplift immediately prior to the vulcanicity is evidenced by an unconformity at the base of the Sparnacian, above which basement-derived arkosic sandstones and conglomerates are followed by about 1.5 km of coarse volcaniclastics, basaltic flows of dominantly picritic composition, pro-grading hyaloclastite wedges and thin siltstones with abundant organic detritus. Very rapid subsidence accompanied this early phase of vulcanicity, maintaining the top of the pile close to sea level and allowing the deposition of a further kilometre of waterlain tuffs in the embayment. Sedimentation extended northwards and eastwards on to basement rocks at this period, with the formation of a non-marine sequence which includes coals.The overlying plateau tholeiites overlap the earlier volcanics; their depositional area was extremely extensive along the East Greenland margin and bears no relationship to the Kangerdlugssuaq sedimentary embayment, although their thickest development, 4 km or more, was attained in that region. Eruption rate of the pile exceeded subsidence for a period and it is dominantly subaerial.This sequence of events is compared with the similar history of sedimentation and basaltic vulcanicity on the west coast of Greenland, and it is inferred that just as the East Greenland sequence records the initiation of spreading between Greenland and Rockall-Faeroe at anomaly 24−25 time, so that of West Greenland marks the propagation of the Labrador Sea spreading axis through the Davis Strait into Baffin Bay at anomaly 26−27 time.
A marine fauna from the Dartmouth Beds (Lower Devonian) of Cornwall
Authors: Evans K. M. 
Published: May 2009

SummaryA brachiopod fauna is described from a marine horizon within the Dartmouth Beds at Bull Cove, Cornwall. The fauna indicates a Middle or Late Siegenian age and aspects of the palaeoecology are discussed. The Lower Devonian succession of south Devon and north Cornwall is discussed and the terms Meadfoot facies and Standon facies are proposed to replace the Meadfoot Beds and Staddon Grits of the old terminology.
Structural and photogeological evidence for the boundaries to an East Arabian' block
Authors: Hancock P. L. , Al-Khatieb S. O. , Al-Kadhi A. 
Published: May 2009

SummaryComputer-enhanced Landsat images reveal a 65 km arcuate lineament linking the concealed fault zone beneath Wadi Al-Batin to the central Arabian graben and trough system. Detection of the link allows a continuous 1000 km arc of faults and troughs cutting the Mesozoic-Cenozoic rocks of the Arabian Shelf to be recognized. The arc, here called the East Arabian fault zone, is interpreted as forming the western and southern boundaries of the East Arabian block, a tectonic unit underlying at least 200 000 km2 of the Arabian Peninsula.Stratigraphical relationships within the 560 km central Arabian graben and trough system demonstrate that its faults were active during the late Cretaceous-early Palaeogene, movements ceasing when the Red Sea began to open and the Arabian subplate was established. The principal hydrocarbon fields of the Arabian Peninsula lie within the boundaries of the East Arabian block.
On polygenetic myrmekite
Authors: Phillips Evan R. 
Published: May 2009

SummaryA brief review of Becke's (1908) replacement and Schwantke's (1909) exsolution models for myrmekite genesis and further consideration of the morphology and spatial distribution of myrmekite as outlined by Phillips (1974) lead to the conclusion that both hypotheses have a place in explaining the origin of myrmekite. The Schwantke model is generallybest applied, for example, to high-level undeformed massive granitoids and the Becke model to deformed metamorphic rocks. The latter hypothesis and a third model involving interaction between exsolution and replacement (Ashworth, 1972) may be used especially to explain the association of muscovite and myrmekite in rocks whose mineral assemblages have undergone retrograde modification, involving either redistribution of Na, Ca and K by local metasomatic reactions among the mineral grains, or metasomatic change of bulk rock composition.
Revision of the Dinantian stratigraphy of the Craven Basin, N England
Authors: Fewtrell M. D. , Smith D. G. 
Published: May 2009

SummaryNew work on the sedimentology and palaeontology of the classic area of the ‘basinal’ facies of the Carboniferous Limestone requires that a formal lithostratigraphic framework be established to rationalize the existing system of local rock units and regional lithofaunal ‘groups’ and ‘series’. Five formations, based on the existing ’groups‘ in the Clitheroe area, are defined for the whole Craven Basin outcrop. They are, from the base, the Chatburn Limestone Formation, the Clitheroe Limestone Formation (which includes the ’reef-knolls’ of the Clitheroe area), the Worston Shale Formation, the Pendleside Limestone Formation (to include the knolls of the Craven reef belt) and the Bowland Shale Formation. The chronostratigraphy, in terms of the six British Dinantian Stages, and a provisional micropalaeontological biostratigraphy are outlined.
Volcanic pebbles in Pleistocene gravels of the Thames in Buckinghamshire and Hertfordshire
Authors: Green C. P. , Hey R. W. , McGregor D. F. M. 
Published: May 2009

SummaryThe incidence of volcanic pebbles in the gravels of the middle Thames basin is noted. The detailed petrography of volcanic clasts at two sites is described and found to be broadly similar to that of material from sites in East Anglia, thought to be of Welsh origin. The frequency of volcanic pebbles in the two areas is also similar. This evidence suggests that volcanic material was brought into East Anglia via the ancestral Thames.Records of volcanic rocks in the drift deposits of the upper Thames basin provide additional evidence of this route.
Age of the Cedarberg Formation, South Africa and early land plant evolution
Authors: Gray J. , Theron J. N. , Boucot A. J. 
Published: May 2009

AbstractThe first occurrence of Early Paleozoic land plants is reported from South Africa. The plant remains are small, compact tetrahedral spore tetrads. They occur abundantly in the Soom Shale Member of the Cedarberg Formation, Table Mountain Group. Marine? phytoplankton (sphaeromorphs or leiospheres) occur with the spore tetrads in all samples. Rare chitinozoans are found in half the samples. Together with similar spore tetrads from the Paraná Basin (Grayet al.1985) these are the first well-documented records of Ashgill and/or earlier Llandovery land plants from the Malvinokaffric Realm, and from the African continent south of Libya. These spore tetrads have botanical, evolutionary, and biogeographic significance. Their size in comparison with spore tetrads from stratigraphic sections throughout eastern North America, suggests that an earliest Llandovery age is more probable for the Soom Shale Member, although a latest Ordovician age cannot be discounted. The age of the brachiopods in the overlying Disa Siltstone Member has been in contention for over a decade. Both Ashgillian and Early Llandovery ages have been proposed. The age of the underlying Soom Shale Member based on plant spores and trilobites (earliest Llandovery or latest Ashgillian) suggests that the Disa Siltstone Member is also likely to be of Early Llandovery age, although the distance between the Soom Shale Member spore-bearing locality and rocks to the south yielding abundant invertebrate body fossils at one locality is great enough to permit diachroneity.
Randomness in the pattern of ‘mass extinctions’ and ‘waves of origination’
Authors: Hoffman Antoni , Ghiold Joe 
Published: May 2009

Abstract‘Mass extinctions’ and ‘waves of origination’ are defined as peaks in the average probability of family extinction and origination per geologic stage, respectively. A simple neutral model is proposed, assuming only that the average probabilities of family extinction and origination have varied independently and had equal chances of going up and down from one stage to another. As a null hypothesis, this model is not rejected by the available palaeontologic data on marine animal and nonmarine invertebrate families in the Phanerozoic. The only discrepancy between the model and the empirical pattern is due to a systematic bias introduced by Fossil-Lagerstätten. The alleged periodicity of Late Phanerozoic mass extinctions follows inevitably from the model.
The Cenomanian hiatus in the Southern Benue Trough, Nigeria
Authors: Amajor L. C. 
Published: May 2009

AbstractBasal Turonian Ezeaku Sandstone ridges along the Cenomanian hiatus in Afikpo area contain sandstone pebbles, cobbles, and boulders randomly distributed in nearly all the coarsening upward subfacies of the host units. Contacts with the host sandbody are sharp and, in places, lined with solution grooves. Generally, the angular to subrounded clasts have their 2 mm rims ferruginized, are fine to coarse grained and very strongly bioturbated. They are compositionally and texturally more similar to the adjacent Albian Asu River Group sandunits. These characteristics suggest an extrabasinal origin for these marine sandstone clasts. Albian Asu River Group sediments uplifted by the Cenomanian deformation are the most likely source. The general absence of similar clasts towards the eastern palaeostrandline where the primary Cretaceous source rocks lay is consistent with the interpretation. The remarkable parallelism between the trend of the clast outcrops and the axis of the Santonian anticlinorium (NE–SW) suggests the same axial direction for both tectonic phases.
An application of the second derivative as a tool in tectonic analysis in the Qattara Depression area, Egypt
Authors: El-Hussaini A. , Youssef M. , Ibrahim H. 
Published: May 2009

AbstractThe second derivative of gravity anomalies of the Qattara area was analysed and statistically studied for determining the tectonic elements. Zones of zero second derivative were considered as the locations of possible faults. The analysis of a constructed tectonic map portrays the predominance of N45°W, N85°E and N45°E fault trends in addition to less pronounced N15°E and N–S faults. The NW–SE faults are very old and inherited from the basement structures. They acted as first order right-lateral wrench faults during the Alpine tectonism. Second and higher orders of faults, developed as a consequence of these movements, are represented by the N85°E and other less abundant trends. Vertical movements along the existing fault system, in addition to the horizontal displacement, is supported by the analysis of the pronounced anomalies of the second derivative map. The subsurface structural picture of the area is composed of uplifted and downfaulted adjoining blocks.
Conglomerate-hosted copper mineralization in Cretaceous Andean molasse: the Coloso Formation of northern Chile
Authors: Flint S. , Clemmey H. , Turner P. 
Published: May 2009

AbstractAlluvial fan sediments of the Lower Cretaceous Coloso Formation of northern Chile were deposited during the evolution of a half-graben, arid, intra-arc basin. The sediments were derived from an andesitic volcanic arc and show compositional variation which can be interpreted as an unroofing sequence. Copper mineralization is stratigraphically restricted to the upper 150 m of the sequence in conglomerates of predominantly granodioritic composition. The presence of mineralized clasts at this level indicates the erosion of bedrock mineralization. Lead isotopic signatures of ore sulphide support the sedimentological evidence for derivation of the sediments and metals from an island arc volcano-plutonic complex to the west of the basin.A genetic model is proposed in which uneconomic metal concentrations were transported into the basin as detrital phases and released by diagenetic reactions. The precipitation of copper minerals followed authigenic formation of quartz, feldspar, analcime and calcite and also some secondary parosity generation. Copper minerals include the oxychloride atacamite, and the suphides bornite, digenite, djurleite and covellite. Sulphur isotope values for the sulphides are relatively heavy isotope-enriched (δ34S = −3.6 to −12.8‰). Although these results could be interpreted in terms of closed-system bacterial sulphide reduction there is no known occurrence of organic material within the Coloso basin. An alternative explanation involving the mixing of diagenetically derived copper-rich brines and sulphur-rich hydrothermal fluids is proposed to account for the Coloso mineralization.
Graptolites from the type Arenig Series
Authors: Zalasiewicz J. A. 
Published: May 2009

AbstractThe graptolites recovered from the Carnedd Iago Formation of Arenig Fawr, Gwynedd, North Wales are described. They originate from the type section of the Arenig Series and comprise the species Didymograptus aff. simulans Elles & Wood, D. aff. deflexus Elles & Wood, D. cf. praenuntius Törnquist, Azygograptus cf. eivionicus Elles and Tetragraptus reclinatus Elles & Wood.
The parental magma on Rhum: evidence from alkaline segregations and veins in the peridotites from Salisbury's Dam
Authors: Kitchen D. E. 
Published: May 2009

AbstractFeldspathic peridotites on Rhum contain teschenite segregations and veins. Bulk rock and mineral compositional trends define a path of alkali and iron enrichment related to discontinuous isolation of evolving intercumulus melts. Compositionally zoned and skeletal crystals of actinolite, zircon, apatite, sphene, epidote and chlorite are enclosed by clear analcime, and appear to have grown directly from a water-rich fluid at some undercooling, and below dry equilibrium solidus temperatures. The occurrence of late stage residua with undersaturated alkalirich characteristics in the Rhum ultrabasic complex suggests that the patental magma was alkaline rather than tholeiitic, and had similarities to the Plateau Magmas on Skye.