bluuebunny/crossref_metadata_2025_split huggingface dataset:
Granulites in and around the Bengal anorthosite, eastern India; genesis of coronal garnet, and evolution of the granulite-anorthosite complex
Authors: Bhattacharyya P. K. , Mukherjee Subimal 
Published: May 2009

AbstractGranulites occur in and around an anorthosite massif (commonly referred to as the Bengal anorthosite) in eastern India. The central part of the anorthosite is massive in nature with locally developed ‘fluxion structure’ in isolated blocks. Banding in the peripheral part is typical of that of the massif anorthosites of Anderson & Morin. The granulites bear imprints of polyphase deformation, and the xenoliths of granulites within the anorthositic massif maintain their regional structural alignment. The Anorthosite, syntectonic with the second fold movement in the terrane, has developed second generation structures.Hornblende, orthopyroxene, clinopyroxene, plagioclase and quartz with or without garnet and other minor phases consitute the granulites. In the anorthosites, orthopyroxene, clinopyroxene, hornblende, and magnetite with or without garnet may constitute more than 15% of the mode. Garnet, when present in the rocks, is a late overprint. Symplectitic growth of garnet with one or more of the phases such as ilmenite, quartz, clinopyroxene and sodic plagioclase indicates genesis of garnet through decomposition of orthopyroxene and/or prograde hornblende in the presence of calcic plagioclase. The garnet formed during prograde dehydration reactions, rather than cooling of the complex.Chemical characteristics of the mafic phases in the rocks indicate attainment of exchange equilibrium with respect to the major cations in closely associated hornblende, clinopyroxene, orthopyroxene and garnet. Estimated physical conditions of metamorphism from contrasted assemblages suggest a final equilibration of different granulitic assemblages as well as of the anorthositic assemblages under similar conditions. There was, however, belated building up of water pressure in the anorthositic domains stabilizing a late hornblende during cooling of the complex. Absence of chilled margins in the anorthosite as well as absence of contact aureoles in the enveloping and enclosed granulites indicate emplacement of the anorthosite in a hot environment, prior to cooling of the complex.
Late Caledonian granitoids and timing of deformation in the Iapetus suture zone of eastern Ireland
Authors: Murphy Finbarr C. 
Published: May 2009

AbstractLate Caledonian granitoids, (c. 400 Ma) in the zone of the Iapetus suture provide a datum against which current models for the relative timing of deformation can be tested. One such granitoid adjacent to the proposed suture ‘line’ in eastern Ireland is now buried by an Upper Palaeozoic cover. It is identified geophysically by a negative Bouguer anomaly with no magnetic signature, and geologically by a hypabyssal dyke swarm and hornfels metamorphism. The timing of intrusion of the granitoid is shown to have occurred during the continuing end-Silurian/early-Devonian deformation. Other members of this widespread suite in Ireland show features consistent with diapiric intrusion during the later stages of the deformation. This evidence brackets the age of regional deformation as continuing during granite emplacement and cooling (c. 400 Ma). The unifying characteristics of the straddling granitoid suite, coupled with a sinistrally transpressive deformation, in this broad suture zone are interpreted in terms of a continental collision which climaxed in late-Silurian/early-Devonian time.
On the directional nature of stratigraphic correlation
Authors: Tipper John C. 
Published: May 2009

AbstractIn every correlation exercise the diachroneity of the lithostratigraphic boundaries determines the discrepancy between lithostratigraphic correlation and time-correlation. Here it is shown that diachroneity varies in degree as a simple function of the direction in which correlation is made, and hence that stratigraphic correlation must take that direction into account. When the direction of correlation is not known, a sound strategy for time-correlation of lithostratigraphic boundaries is to assume that each is at its most diachronous.
Burial of trees by eruptions of Mount St Helens, Washington:implications for the interpretation of fossil forests
Authors: Karowe Amy L. , Jefferson Timothy H. 
Published: May 2009

AbstractLahars and fluvial sediments which buried trees following the 18 May 1980 eruption of Mount St Helens were investigated during August of 1982. Trees buried in older mudflows, dated at a.d. 1885, a.d. 1450–1550, and 36000 years b.p., also were examined. Although many logs clearly were transported, large numbers of trees were buried in growth position. Burial by lahars generally resulted in the death of trees, whereas some trees survived burial by fluvial sediments. Scanning electron microscope studies show that trees buried in lahars are well preserved. Pre-1885 buried woods show incipient silicification, and woods buried 36000 years b.p. show silica impregnation of cell walls.Features ofin situand allochthonous burial very similar to those seen in southern Washington are found also in Eocene silicified forests in Yellowstone National Park, Wyoming, U.S.A., and in Cretaceous fossil forests in southeastern Alexander Island, Antarctica. Observations of Recent wood from volcaniclastic deposits of Mount St Helens provide valuable insights into processes of burial and silicification of fossil forests of various ages around the world.
Origin of finger structures in the Rhum Complex: phase equilibrium and heat effects
Authors: Morse S. A. , Owens Brent E. , Butcher Alan R. 
Published: May 2009

AbstractThe finger structures described earlier by Brown and later by Butcher, Young & Faithfull involve dissolution of troctolite during crystallization of olivine, followed by crystallization of pyroxene around olivine grains in the fingers. However, the ingestion of troctolite takes the liquid away from pyroxene saturation rather than toward it. The pyroxene field can be encountered metastably, and pyroxene caused to crystallize, by supercooling the olivine-rich liquid against the troctolite. The melt corrosion represented by the fingers, and other field relations, suggest that the mafic layers were emplaced as sills of mafic magma into nearly solid troctolites. Melting at the base of mafic liquid layers was impeded by a bed of olivine crystals releasing light solute upward, causing compositional convention and rapid heat transfer to the top of the layer, where melting demonstrably occurred. Recognition of this process introduces the novel concept of a magmatic heat pump driven by compositional convection. The crystallization path ol–px–pl(–sp) is also found next to xenoliths in the Kiglapait Intrusion where the magma was normally saturated only in ol+pl, directly demonstrating the effect of supercooling on the crystallization sequence.
Continuous out-of-sequence ductile thrusting in the Norwegian Caledonides
Authors: Rice A. H. N. 
Published: May 2009

AbstractPeak metamorphic conditions (up to sillimanite grade) in a unit of rocks beneath the evolving Kalak Nappe Complex (Finnmarkian Caledonides) developed prior to its incorporation as a nappe within the complex. The presence of displacement textures in peak metamorphic por-phyroblasts indicates that they grew in a hydrostatic stress regime and thus that the emplacement of tectonically higher units caused little or no deviatoric stress in the footwall rocks except close to the thrust plane.The developing geotherm in the incipient nappe was disrupted by the onset of thrusting (D2 deformation), which placed hot rocks on colder rocks. This led to the cessation of prograde metamorphism and accounts for the ubiquitous association of nappe emplacement and the meta morphic peak. Later deformation (D3/D4) occurred at lower grades (biotite/chlorite). Consideration of nappe translation rates and the likely erosion/cooling rates indicates that nappe movements must have occurred ‘continuously out-of-sequence’ within the Kalak Nappe Complex.
Jurassic clay mineral assemblages and their post-depositional alteration: upper Great Estuarine Group, Scotland
Authors: Andrews Julian E. 
Published: May 2009

AbstractClay minerals from Middle Jurassic lagoonal mudrocks, siltstones and silty fine-grained sandstones of the upper Great Estuarine Group (Bathonian) are divided into four assemblages. Assemblage 1, the most common assemblage, is rich in mixed-layer illite–smectite with attendant illite and kaolinite. Assemblage 2 is dominated by smectitic clay. These assemblages are indicative of primary Jurassic deposition. Illite and kaolinite were probably derived from the weathering of older rocks and soils in the basin hinterland and were deposited in the lagoons as river-borne detritus. The majority of smectite and mixed-layer illite–smectite is interpreted as the argillization product of Jurassic volcanic dust, also deposited in the lagoons by rivers. Near major Tertiary igneous intrusions these depositional clay mineral assemblages have been altered. Assemblage 3 contains smectite-poor mixed-layer illite–smectite, whilst Assemblage 4 contains no smectitic clay at all. Destruction of smectite interlayers occurred at relatively shallow burial depths (< 2500 m) due to enhanced geothermal gradients and local convective hot-water circulation cells associated with the major Tertiary igneous intrusions.
Enriched komatiitic basalts from Newton Township, Ontario: their genesis by crustal contamination of depleted komatiite magma
Authors: Cattell Alan 
Published: May 2009

AbstractLREE-enriched komatiitic basalts are commonly found in association with LREE-depleted komatiites. This association is found in a sequence of late Archaean lavas from Newton Township, Ontario. The komatiitic lavas at Newton Township differ from most late Archaean examples in that both the komatiites and the komatiitic basalts are depleted in Al and in the HREE. The close association and distinctive Al and HREE depletions of the two lava types strongly suggest a common origin, despite their contrasting LREE patterns.A model is proposed whereby the LREE-depleted komatiites represent the parental magma to the LREE-enriched komatiitic basalts, the two being linked by a combination of crystal fractionation and crustal assimilation. The composition of the contaminant is estimated by comparing the LREE-enriched komatiitic basalts with the evolved part of a thick layered komatiite flow that has similar major element chemistry. The contaminant composition coincides closely with Taylor & McLennan's estimate of the composition of the Archaean upper crust. It is concluded that LREE-enriched komatiitic basalts can be produced from LREE-depleted komatiite parent magmas by combined assimilation and fractionation, and that such a process best explains the geochemistry and Nd isotopic features of most komatiitic basalts.
Asymmetric extensional structures and their implications for the generation of melanges
Authors: Needham D. T. 
Published: May 2009

AbstractStudies of melanges in the Shimanto Belt of southwest Japan have shown that many of the included blocks exhibit asymmetric geometries, similar to boudins and inclusions in medium grade metamorphic rocks which have been subjected to markedly non-coaxial strains. Deformation is accomplished by mesoscopically ductile processes such as pore-pressure-controlled or diffusionally-controlled grain boundary sliding, although localized fracture and cataclasis also occurs. It is suggested that the melanges developed by the propagation of extensional displacement zones, analogous to shear bands, through sandstone beds, so progressively dismembering them. Studies of the detailed internal geometries of melanges along with the deformation mechanisms active during their formation may help resolve the mode of formation of these problematic units.
The significance of large-scale sedimentary structures in the Silurian succession of western Ireland
Authors: Williams D. Michael , Nealon T. 
Published: May 2009

AbstractLarge-scale sedimentary structures are described from the Lough Muck Formation (Wenlock) of north Galway, Ireland. They consist of bed packages separated by truncation surfaces of varying orientations. They had previously been interpreted as rotational slumps generated on a shelf–slope interface. It is shown that the structures formed in a tidally influenced environment far removed from a slope. They bear strong similarities to structures described from other tidal channel environments and it is proposed that they represent deposition on aggradational margins of such channels. The supposed slope–shelf transition is therefore not present at this level in the Silurian succession and much of the Lough Muck Formation may be of a dominantly shallow-marine origin.
Strike-slip terranes and a model for the evolution of the British and Irish Caledonides
Authors: Hutton Donald H. W. 
Published: May 2009

AbstractEvidence is presented that many of the major strike faults in the British and Irish Caledonides were active as sinistral strike-slip zones in the end-Silurian to pre-mid-Devonian period. Some, such as the Highland Boundary Fault, moved in this way at an earlier stage in the Ordovician. These data allow the Caledonian rocks lying between the Laurentian miogeocline (whose basement is represented by the Lewisian, Moine and possibly the Dalradian) and the Gondwanaland miogeocline (Midland Platform and Welsh Basin) to be re-analysed as a group of disorganized terranes which originated to the southwest in North America and southwest Europe/Africa prior to the Silurian.The Highland Border TerraneandNorthern Belt Terraneare interpreted as duplicated pieces of a mid-Ordovician sequence which was a back are to northwest subduction. TheMidland Valley Terraneis interpreted as a slice of Laurentian foreland onto which ophiolites were obducted in the lower Ordovician but which became the basement of a continental margin arc to northwest subduction in the mid-Ordovician. TheCockburnland Terraneis inferred to be part of the same arc repeated and then broken up and dispersed by continuing strike slip. TheConnemara Terraneis regarded as an allochthonous piece of the Dalradian miogeocline and theSouth Mayo Terraneas a remnant of an early Ordovician arc and fore arc which in mid-Ordovician times became a back arc/marginal basin to northwest subduction. TheLake District-Wexford Terraneis part of an arc to southeast subduction under Gondwanaland whose activity climaxed in the mid-Ordovician. TheCentral Terraneis interpreted as a Silurian overstep assemblage which blankets the junction between Laurentian- and Gondwanaland-derived oceanic terranes, and therefore Iapetus is regarded as an Ordovician ocean which closed prior to the Silurian. The model suggests that at the end of the Silurian, a clockwise-rotating Gondwanaland, having carried Laurentia into collision with Baltica, broke free and created a major sinistral strike-slip zone which disrupted the Ordovician palaeogeography in the British Isles/North American sector of Iapetus.
A shallow detachment beneath the North Greenland fold belt: implications for sedimentation and tectonics
Authors: Soper N. J , Higgins A. K. 
Published: May 2009

AbstractIn northern Greenland in early Palaeozoic time a turbidite trough (the eastward extension of the Hazen trough of Arctic Canada) was flanked to the south by a carbonate platform. The trough was deformed during the mid-Palaeozoic Ellesmerian orogeny to form the E–W trending North Greenland fold belt. This fold belt was deformed further by Eurekan (Tertiary) structures, important among which is a major fault complex, the Harder Fjord fault zone (HFFZ). The suggestion has been made that this fault zone controlled early Cambrian sedimentation, even though the fault trace does not coincide with the trough–platform facies transition in sediments of that age; this has led to some controversy.We report new information from a mapping programme by the Geological Survey of Greenland which has established the thin-skinned nature of Ellesmerian deformation at the trough-platform transition and implies that much of the fold belt is underlain by a shallow detachment. This in turn implies that the HFFZ exists in the hanging-wall of the detachment while the early Cambrian trough-platform transition is located autochthonously in the foot-wall. We adduce evidence to show that the latter was probably controlled by syndepositional faulting with actively eroding fault scarps and suggest that these basement structures were reactivated in a dextral strike-slip mode in early Tertiary time to form the HFFZ as now observed.
The emplacement of geochemically distinct groups of rhyolites during the evolution of the Lower Rhyolitic Tuff Formation caldera (Ordovician), North Wales, U.K.
Authors: Campbell S. D. G. , Reedman A. J. , Howells M. F. , Mann A. C. 
Published: May 2009

AbstractRhyolites in the vicinity of Snowdon (North Wales) are intimately associated with the evolution of the Lower Rhyolitic Tuff Formation (LRTF) caldera of Ordovician (Caradoc) age. They occur as deep-seated dykes, sills and small stocks, shallow-level intrusive domes, and domes extruded within a predominantly shallow-marine environment. Extrusion occurred during three main phases, indicating the episodic availability of rhyolite magma. The rhyolites can be divided on their trace element ratios (e.g. Nb/Zr) into five main groups. Extrusive representatives indicate that each group correlates strongly with a single phase of rhyolite extrusion. Within each group, the distribution and variation of intrusive form with stratigraphic level suggests that geochemically similar rocks were emplaced at approximately the same time. Consequently, the groups represent discrete magma compositions tapped from the evolving Snowdon subvolcanic magma system. Differences in distribution of the groups reflect changes in structural controls of emplacement before and after development of the LRTF caldera.
Cretaceous diapiric plutonism in the southern cordillera, Chile
Authors: Suarez M. , Herve M. , Puig A. 
Published: May 2009

AbstractThe Castores and probably the Santa Rosa plutons of north-west Isla Navarino, southern Chile, have been emplaced by in situ diapirism into metasedimentary rocks of the Upper Jurassic (?)–Lower Cretaceous Yaghan Formation. For the former, this model is consistent with the concentric foliation paralleling the margin of the pluton and the foliation and stratification planes in the metamorphic aureole. Only the southern part of the Santa Rosa Pluton is preserved, and it has some structures similar to those of the Castores Pluton, which can also be interpreted as produced by an inflating diapir. The main intrusive rocks of these plutons are quartz-monzodiorites and quartz-diorites with synmagmatic foliation. They were preceded by minor bodies of hornblende gabbros, and followed by dykes and small bodies of non-foliated granodiorites. Non-foliated to weakly foliated granodiorites, forming the centre of the Castores Pluton, probably represent a younger intrusive pulse.Twelve K–Ar mineral dates from 10 specimens of plutonic rocks, interpreted as near crystallization ages, span the period 80–90 Ma. These dates do not show the sequence of intrusion of the different rock-types, which may suggest that all of them were intruded and cooled in a short period of time. The timing of emplacement of these plutons in relation to tectonism is difficult to determine; however, a post-tectonic emplacement for at least the Castores Pluton, is proposed.
Stratigraphy and morphotectonics of Karoo deposits of the northern Selous Basin, Tanzania
Authors: Wopfner H. , Kaaya C. Z. 
Published: May 2009

AbstractLate Permian Karoo deposits of the northern Selous Basin in south-central Tanzania comprise conglomerates and diamictitic boulder beds of alluvial and scarp-foot fan origin. These merge with grey to greenish sandstones, siltstones and black shales of deltaic and lacustrine environments. Microflora assemblages indicate a late Permian age. Lateral changes and interfingering of various lithofacies units are common. Depositional development was controlled by syndepositional faulting and variations of gradients resulting from fault movements. The position of the fault scarp separating the basin area from the elevated basement horst to the west roughly corresponded with the present boundary between the Selous Basin and the Precambrian metamorphics of the Uluguru Mountains.The late Permian Karoo succession of this part of the Selous Basin apparently overlaps older Karoo deposits contained in the north-northeast trending narrow graben structures. It is therefore regarded as a new depositional event which was initiated by renewed tensional tectonism in late Permian time. During this tectonic episode the narrow early Karoo graben structures were expanded into much broader rift basins. Material eroded from the rift shoulders and associated highlands was literally dumped across the fault scarps, forming debris aprons and scarp-foot fans. Rivers emanating from the highlands formed large alluvial fans and, further afield, deltas issued into freshwater lakes.Some of these late Permian faults were rejuvenated by late Cretaceous to early Tertiary tectonism. Thermal waters circulating along fractures converted feldspars, biotites and hornblendes to prehnite. Further tectonic adjustments in mid Tertiary time led to the present-day morphology.
Cathodoluminescence of Recent biogenic carbonates: environmental and ontogenetic fingerprint
Authors: Barbin V. , Ramseyer K. , Debenay J. P. , Schein E. , Roux M. , Decrouez D. 
Published: May 2009

AbstractCathodoluminescence (CL) examination of Recent biogenic carbonates shows that they are often luminescent regardless of their mineralogical composition (calcite v. aragonite), habitat (marine v. fresh water), way of life (sessile v. vagile) or environment (hyper- v. hyposaline water). Thus, the presence of luminescence in biogenic particles is not a reliable indicator of diagenetic alteration as some authors have suggested. In addition, CL can reveal variations in the mineralogy of shell material (e.g. regenerated calcitic v. primary aragonitic) and can highlight growth-related structures. Manganese (Mn2+) is the most likely activator of this luminescence, and its content in the shells of benthic organisms seems to be linked to growth rate, ontogeny, open sea conditions, bathymetry and salinity. In neritic environments the Mn2+content and the CL of molluscs and foraminifera appear to increase with decreasing salinity. This study indicates that CL may be an important tool for the determination of environmental and ontogenetic parameters in biogenic carbonates in addition to its current use indiagenetic studies.
Modelling of chalk diagenesis (Eldfisk Field, Norwegian North Sea) using whole rock and laser ablation stable isotopic data
Authors: Maliva R. G. , Dickson J. A. D. , Råheim A. 
Published: May 2009

AbstractLaser ablation techniques permit the determination of the stable isotopic ratios of finely crystalline calcite cements in chalks for the first time. Modelling of fluid–rock interaction using whole rock and laser ablation stable isotopic data indicates that carbonate mineral diagenesis in the Eldfisk Field consisted largely of the dissolution and reprecipitation of calcite with little associated loss of porosity. Cementation by calcite derived from stylolites apparently occurred throughout the Eldfisk Field chalk, but had only a subsidiary effect on whole rock isotopic ratios. Oxygen isotopic data indicates a pore water temperature of 50–80 °C during the bulk of chalk recrystallization. Increases in whole rock δ13C values with depth are likely the result of bacterial methanogenesis during chalk recrystallization.