bluuebunny/crossref_metadata_2025_split huggingface dataset:
Postcumulus processes in layered intrusions
Authors: Sparks R. S. J. , Huppert H. E. , Kerr R. C. , McKenzie D. P. , Tait S. R. 
Published: May 2009

AbstractDuring the postcumulus stage of solidification in layered intrusions, fluid dynamic phenomena play an important role in developing the textural and chemical characteristics of the cumulate rocks. One mechanism of adcumulus growth involves crystallization at the top of the cumulate pile where crystals are in direct contact with the magma reservoir. Convection in the chamber can enable adcumulus growth to occur to form a completely solid contact between cumulate and magma. Another important process may involve compositional convection in which light differentiated melt released by intercumulus crystallization is continually replaced by denser melt from the overlying magma reservoir. This process favours adcumulus growth and can allow adcumulus growth within the pore space of the cumulate pile. Calculations indicate that this process could reduce residual porosities to a few percent in large layered intrusions, but could not form pure monomineralic rocks. Intercumulus melt may also be replaced by more primitive melt during episodes of magma chamber replenishment. Dense magma, emplaced over a cumulate pile containing lower density differentiated melt may sink several metres into the underlying pile in the form of fingers. Reactions between melt and matrix may lead to changes in mineral compositions, mineral textures and whole rock isotope compositions. Another important mechanism for forming adcumulate rocks is compaction, in which the imbalance of the hydrostatic and lithostatic pressures in the cumulate pile causes the crystalline matrix to deform and intercumulus melt to be expelled. For cumulate layers from 10 to 1000 metres in thickness, compaction can reduce porosities to very low values (< 1%) and form monomineralic rocks. The characteristic time-scale for such compaction is theoretically short compared to the time required to solidify a large layered intrusion. During compaction changes of mineral compositions and texture may occur as moving melts interact with the surrounding matrix. Both compaction and compositional convection can be interrupted by solidification in the pore spaces. Compositional convection will only occur if the Rayleigh number is larger than 40, if the residual melt becomes lower in density, and the convective velocity exceeds the solidification velocity (measured by the rate of crystal accumulation in the chamber). Orthocumulates are thus more likely to form in rapidly cooled intrusions where residual melt is frozen into the pore spaces before it can be expelled by compaction or replaced by convection.
Metamorphic rocks of an intermediate facies series juxtaposed at the Start boundary, southwest England
Authors: Robinson D. 
Published: May 2009

SummaryX-ray diffraction analysis of metamorphic rocks from the Start peninsula shows that the dominant mineral assemblage in the Start schists is quartz-muscovite-chlorite ± paragonite ± albite. The assemblage in the Devonian phyllites to the north of the Start boundary is quartz-muscovite-chlorite and the crystallinity of the white mica there is indicative of greenschist fades. Theb0parameter of white micas indicates that the metamorphism was of an intermediate facies series (Barrovian type), but of a lower pressure variety to the north of the Start boundary juxtaposed against a higher pressure variety to the south. No evidence exists of the high pressure metamorphism needed to support tectonic models in which the Start boundary is identified as a suture representing a Variscan subduction margin.
Some geochronological observations on the Precambrians of Chotanagpur, Bihar, India
Authors: Ghose N. C. , Shmakin B. M. , Smirnov V. N. 
Published: May 2009

SummaryK–Ar measurements of the Chotanagpur granite and associated intrusive rocks are found to be younger than the geological events in the Singhbhum region of Bihar. The history of metamorphism, granitic activities and igneous intrusions in the Chotanagpur area continued for a period from over 1000 Ma to 185 Ma. The relevant geological sequences and correlations are briefly discussed.
Late Cretaceous climatic trends, faunas, and hydrography in Britain and Ireland
Authors: Reid R. E. H. 
Published: May 2009

SummaryChanges in the Upper Cretaceous faunas of northeastern Ireland point to rising Cenomanian temperatures, and to progressive falls in temperatures from early Santonian to Maastrichtian times. At first sight, these changes fit the Cretaceous climatic trends claimed from isotopic data; but comparison with faunas seen in England suggests that this fit is illusory. It is suggested that faunas were more directly controlled by the influence of contrasting Atlanto-West Tethyan and Russo-Germanic water masses, and by changes in their relative distribution. Changes seen through the Irish Senonian may reflect the progressive submergence of a Scotland—Pennines barrier, permitting an increasing westward spread of the Russo—Germanic (‘boreal’) water and fauna.
Evidence of co-genesis of the Troodos Lavas, Cyprus
Authors: Desmet A. P. 
Published: May 2009

SummaryPetrological and geochemical features of recently discovered basalts of the Troodos Massif, Cyprus — a dyke in the Basal Group and altered pillow lavas in the Lower Pillow Lavas — suggest that all the lavas are part of a single volcanic suite.
Clastic dykes from the Precambrian Porsangerfjord Group, North Norway
Authors: Williams D. M. 
Published: May 2009

SummaryClastic dykes are described from a late Precambrian sequence of shallow water sediments — the Porsangerfjord Group of Finnmark, North Norway. The dykes are from one stratigraphic member of the Group and only occur where this member outcrops in a zone where sedimentation was thought to have been affected by recurrent earthquake shock. The dykes consist of poorly defined sandstone clasts in a muddy matrix. They exhibit no significant internal sedimentary structures. It is suggested that the dykes were intruded due to remobilization of sediment affected by a combination of tectonic shock and hydrostatic load. The dykes bear no genetic relationship to tectonic structures (cleavage and jointing) and evidence suggests that dewatering was virtually complete before the formation of the first cleavage.
Phengite spherules from the Dalradian Bonahaven Formation, Islay, Scotland: glauconitized microfossils?
Authors: Fairchild I. J. 
Published: May 2009

SummaryAbundant spherules of phengite, 50—150 μm in diameter and with a radial texture, occur in mudstones at the top of the lowest member of the Bonahaven Formation. They have smooth, sharp boundaries, show flattening next to desiccation cracks and exhibit a restricted, bi-or tri-modal size distribution. It is inferred that the spherules represent infilled voids in a sediment which was deposited entirely from suspension. Accordingly, it is considered likely that the spherules are microfossils, although characteristic morphological features are lacking. They were probably infihled by glauconite, which was transformed to phengite during diagenesis and low-grade metamorphism.
Dating of adularia and the relationship of hydrothermal events in the Lizard complex, Cornwall
Authors: Seager A. F. , Fitch F. J. , Miller J. A. 
Published: May 2009

SummaryA new total degassing argon-40/argon-39 age determination is given for crystals of fresh adularia from hydrothermal veins within the amphibolites near Porthkerris Point, the Lizard, Cornwall. The apparent age of 211±2 Ma obtained is regarded as a close minimum age for crystallization of the adularia. There appears to have been only one phase of crystallization of primary hydrothermal adularia, about 210–220 Ma, in the granulites and amphibolites in the vicinity of Porthkerris Point and in the gabbro of Dean quarry. Another hydrothermal event occurred c. 160–170 Ma. Younger apparent ages have been attributed to hydrothermal activity at one or more subsequent periods.
The Ballantrae ophiolite
Authors: Church W. R. , Gayer R. A. 
Published: May 2009

SummaryThe Ballantrae complex, variously interpreted in the light of plate tectonics, is reappraised on the basis of comparisons with ophiolites of the Newfoundland Appalachians and the southwestern Pacific. All the main components of a typical ophiolite sequence can be recognized although the sequence has been greatly disturbed by post-emplacement Caledonian deformation. The presence of a garnet granulite-amphibolite ‘aureole’ beneath the peridotite member of the complex is confirmed, thus establishing the affinity of the complex to other ophiolite-amphibolite associations of the Appalachian and Alpine systems. Rock units previously described as agglomerates are re-interpreted as olistostromes, and the important Pinbain Beach ‘shaly group’ containing fragments of ophiolitic debris, garnet-ariegite, and glaucophane schist, is considered to be a Wildflysch unit of Middle Arenig age. The olistostromes were deposited following obduction of oceanic lithosphere represented by the ultramafic and mafic rocks of the Ballantrae complex. Recognition of the complex as overthrust ophiolite removes certain constraints on the position of a Lower Paleozoic subduction zone within the Southern Uplands.
Ultramafic and mafic protoclastic intrusions in the Dahomeyan gneiss of Ghana
Authors: Holm R. F. 
Published: May 2009

SummaryCharacteristics of mineralogy and texture, together with systematic changes in the enstatite content of orthopyroxene and in the anorthite content of plagioclase, indicate that intrusions of pyroxenite, norite and gabbro, and diorite and leucocratic diorite in the Dahomeyan gneiss of Ghana are genetically related, and that they belong to a subalkaline comagmatic series derived from a parental mafic magma. Partial recrystallization (deuteric) of the igneous rocks during and after intrusion and protoclasis produced minerals characteristic of temperatures higher than those that existed during the medium-grade regional metamorphism of the Dahomeyan gneiss, thus indicating that emplacement of the igneous rocks occurred late during, or perhaps after, the metamorphism. Contact metamorphism appears to have resulted in higher than normal anorthite contents of the plagioclase in the surrounding Dahomeyan gneiss.
Controls of mylonitic compositional layering during non-cataclastic ductile deformation
Authors: Vernon R. H. 
Published: May 2009

SummaryLenticular compositional layering and/or rodding in blastomylonitic rocks, in the simplest situations, can be controlled by (i) the presence of more than one mineral in the parent rock and (ii) relative large grain-size of the parent rock. Deformation and recrystallization during non-cataclastic ductile deformation can produce compositional layering, without chemical segregation.
Fine-scale layering in igneous intrusions: a possible mechanism for a non-depositional origin
Authors: Kanaris-Sotiriou R. 
Published: May 2009

SummaryExamination of layered rocks from the Currywongaun-Doughruagh Intrusion, Connemara, Eire, has suggested a possible connexion between adcumulate crystallization and the formation of thin, yet very persistent igneous layering. It is thought that as a result of adcumulate crystallization proceeding under undisturbed conditions near the top of a pile of cumulus crystals, a zone of liquid, supersaturated in those components not required for adcumulate growth, could be generated in the magma just above the cumulate pile. Crystallization from such a supersaturated zone would result in the cumulate phases, inter-directly beneath. The rhythmic deposition is lacking, could by this mechanism build up a sequence of cumulate layers interbedded with fine-scale layers of a non-depositional origin.
Pleistocene gravels of the River Axe in south-western England, and their bearing on the southern limit of glaciation in Britain
Authors: Green C. P. 
Published: May 2009

SummaryThe source of far-travelled pebbles in the Pleistocene gravels of the lower Axe is investigated. Reference is made to the possible introduction of far-travelled material by glacial meltwater, flowing into the basin of the Axe through a ‘gap’ in the present watershed at Chard. Gravels of the lower Axe are described and compared with gravels of the upper Axe and of neighbouring streams. Far-travelled material is found in almost all of the gravels examined. The source of the material is identified in early Tertiary gravels which occur within the present basins of the rivers in question. A glacial origin for the far-travelled material is rejected.
Geochemistry of post-Dharwar basic dikes and the Precambrian crustal evolution of peninsular India
Authors: Naqvi S. M. , Rao V. Divakara , Satyanarayana K. , Hussain S. M. 
Published: May 2009

SummaryThe post-Dharwar basic dikes from Chitaldrug and Shimoga schist belts have been analysed to determine their major, minor and trace-element composition. The comparison of the results from these formations (1.4–0.95 b.y.) with those of the early Precambrian metabasalts (2.4 b.y.), which are indicative of a thin crust at the time of their extrusion shows significant differences between them. Utilizing this and results of similar studies on corresponding formations it is suggested that the present thickness of this part of the crust might have been obtained after the extrusion of the early Precambrian basalts and before the intrusion of the middle Precambrian dolerites.
Calcareous nannoplankton and the age of the Globigerina silts of the western approaches of the English Channel
Authors: Martini E. 
Published: May 2009

SummaryCalcareous nannoplankton from the Globigerina silts of the western approaches of the English Channel indicate the presence of standard nannoplankton zones NN2, NN 4 and NN 5, part of which can be correlated with Neogene beds in Belgium and Northern Germany. The calcareous nannoplankton support the possibility of a connexion between the North Sea Basin and the Atlantic Ocean through the English Channel during part of the Lower Miocene.
Tectonic control of graphite mineralization in Sri Lanka
Authors: Silva K. K. M. W. 
Published: May 2009

SummaryThe localization of graphite veins in well-defined regions within anticlinal structures of the SW sector of the island, where the rocks have been migmatitized partially by a reactivation, is very significant. The graphite mineralization seems to have taken place during the above mentioned reactivation and has followed regions of structural weakness developed during the folding of the metasedimentary rocks.
Jointing along the Swansea Valley Disturbance between Clydach and Hay-on-Wye, South Wales
Authors: Weaver J. D. 
Published: May 2009

SummaryAlong the length of the Swansea Valley Disturbance, from Clydach (near Swansea) to Hay-on-Wye (Herefordshire), 4 main joint sets are developed. They are NE—SW, NW—SE, N—S and E—W in trend. On the NW and SE sides of the main fault zone only the N—S and E—W sets tend to be developed, whereas within this fault zone only the NE—SW and NW—SE sets are found. It is suggested that the NE—SW and NW—SE sets are shear joints developed at the same time as the main fault zone and the N—S and E—W sets have been developed in response to the release of residual strain energy after the end of the Variscan orogeny.
An essexitic sill at Mbalevuto, Viti Levu, Fiji
Authors: Colley H. 
Published: May 2009

SummaryThe Mbalevuto sill intrudes sedimentary rocks on the SW flank of Tavua volcano, Viti Levu, Fiji. It appears to have been intruded at shallow depth in a semi-molten state. The sill is of essexitic composition and petrochemical evidence suggests that magmas forming the sill and lavas of the Tavua volcano are consanguineous.
Regulaecystis devonica, a new Devonian pleurocystitid cystoid from Devon
Authors: Paul C. R. C. 
Published: May 2009

SummaryRegulaecystis devonica, which is probably from the Meadfoot Beds (Siegeman— Emsian) near Torquay, Devon, is characterized by smooth thick thecal plates and a deeply sunken pectinirhomb. Coopericystis Parsley from the Middle Ordovician of Tennessee is considered a junior synonym of Regulaecystis from the Lower Devonian of Europe.
Syndepositional tectonism in the Kvamshesten district (Old Red Sandstone), western Norway
Authors: Bryhni I. , Skjerlie F. J. 
Published: May 2009

SummaryContinental Middle Devonian sediments of the Kvamshesten district have recently been interpreted as a thrusted block. However, although basal contacts in the E of the district are mylonitized, contacts in the W can be interpreted as depositional unconformities. Coarse alluvial-fan breccias (Markavatn Formation) in the district's northern margins interdigitate with fluviatile sandstones (Heilefjell Formation) towards the basin's centre. Syndepositional tectonism is demonstrated by the geometry of the coarse marginal deposits.
Zircon U–Pb ages of granulites from the Central Region of the Lewisian, northwestern Scotland
Authors: Pidgeon R. T. , Bowes D. R. 
Published: May 2009

SummaryThe overprinting of the high grade rocks of the Scourian episode, during the Inverian and Laxfordian episodes, has so far prevented a reliable estimate being made of the age of the Scourian granulite–facies metamorphism in the Central Region of the Lewisian of northwestern Scotland. In an attempt to solve this problem the U–Pb isotopic systems of zircons from two samples of hypersthene granulite from the Kylesku group have been examined. These zircons have discordant apparent ages complicating the interpretation of the results. However, by measuring a number of size fractions from each of the two zircon suites it has been possible to demonstrate that a major isotopic event or events took place approximately 2700 m.y. ago in these granulite–facies rocks. It has also been shown that the two discordance trends found for the two zircon populations can be explained in terms of episodic disturbances of the U–Pb isotopic systems at times which correspond to the known ages of ‘Laxfordian’ and ‘Caledonian’ events.
Size-graded, igneous layering in an Icelandic intrusion
Authors: Roobol M. J. 
Published: May 2009

SummaryThe Vesturhorn Intrusion of SE Iceland contains a tilted block consisting largely of size-graded, igneous-layered, eucritic gabbro, which is described in some detail. Size-graded, igneous layering is remarkably uncommon and at present there appear to be only two known occurrences. The physical properties of basalt magma suggest that most igneous layering is likely to form from currents moving by laminar flow. Considerations of flow in such a simple system do not satisfactorily account for the scarcity of this structure. It is suggested that size-graded, igneous layering may reflect the availability of convecting magmas containing crystals with appreciable proportions of different grain sizes of each mineral species. Such a grain size distribution, together with other features considered here, may indicate primocryst growth outside the chamber of deposition.
The significance of toolmarks on a Silurian erosional furrow
Authors: Bridges P. H. 
Published: May 2009

SummaryThe toolmark pattern on an erosional furrow from the Llandovery shelf sequence in the Welsh Borderland indicates that there was:— (i) local helical flow within the furrow similar to that found in sinuous alluvial streams and (ii) a moderately powerful current which moved towards the shore. Erosion by a shallow water turbidity current or storm surge tidal ebb flow therefore seems unlikely. A flood tidal current super-imposed on storm wave oscillatory disturbance of the sea-floor provides a possible mechanism.
The tectonic evolution of the the southern portion of the Benue Trough, Nigeria
Authors: Nwachukwu S. O. 
Published: May 2009

SummaryA reconstruction of the tectonic history of part of the Benue Trough is made from a study of the marine transgressions and regressions in SE Nigeria. Two periods of deformation involving essentially the interaction of continental plates are suggested. The oceanic crust produced by crustal attenuation in the Benue rift is very minimal. Unstable tectonic conditions in the Benue Trough may have created conditions for the development of an evaporite sequence.
Potassium–argon ages from the Cheviot Hills, northern England
Authors: Mitchell J. G. 
Published: May 2009

SummaryPotassium–argon ages of a variety of rocks and minerals from the Cheviot Hills, northern England, lie in the range 390 m.y. to 370 m.y. The Cheviot granite, dated at 380 m.y., has an age similar to that of many of the younger members of the British late Caledonian granites.
The origin of cordierite—anthophllite rocks in the Land's End aureole
Authors: Chinner G. A. , Fox J. S. 
Published: May 2009

SummaryThe prevalence of low-variance assemblages, the frequency of the buffered association anthophyllite—cordierite—cummingtonite—plagioclase, and the ubiquity of aluminium-rich clots of diaspore incompatible with the ortho-amphibole of the rock suggest that the cordierite—anthophyllite rocks of the Land's End aureole are not the product of metasomatism, but resulted from the thermal metamorphism of pre-existing rock-compositions. The origin of many kyanite-, amphibole- and cordierite-rich bodies by pre-metamorphic alteration is of economic significance in regions where such curious silicate assemblages accompany sulphide deposits.
Basaltic volcanism in south-east Terceira, Azores
Authors: Rosenbaum M. S. 
Published: May 2009

SummaryThe Terceira Rift is a subsidiary zone of crustal generation in the Azores to the E of the Mid-Atlantic Ridge. The geology of the island of Terceira, which straddles the rift, is thought to give an indication of the nature of the volcanic activity along this zone of crustal separation. Terceira is composed of 4 large strato-volcanoes associated with trachytic or trachybasaltic activity which have since been intruded by basaltic magmas to form monogenic cinder cones and lava flows on the flanks of these large volcanoes. There is little direct evidence of eruptions along fissures although lines of cinder cones may indicate zones of weakness in the crust.
Hornblende-Granulite Subfacies Mineral Assemblages from Areas in Ceylon
Authors: Hapuarachchi D. J. A. C. 
Published: May 2009

AbstractHornblende and biotite occur very commonly in the granulite facies rocks of the areas mapped, and both appear to occur as stable phases. Thus, these hornblende- and biotite-bearing rocks find accommodation only in the hornblende-granulite subfacies. It appears that most of the rocks in the areas mapped would fall into this subfacies and the mineral assemblages of these rocks are recorded in this paper under the different chemical classes. Hornblende and biotite are very commonly present in the pelitic, quartzo-felspathic, basic and magnesian rocks; phlogopite and sphene are commonly present in the calcareous rocks. A tentative ACFK tetrahedral phase diagram has been constructed for these rocks.
An Outline of the Geology of the Juan Fernandez Archipelago
Authors: Baker P. E. 
Published: May 2009

AbstractThe Juan Fernandez Archipelago, located off the coast of Chile, comprises a group of three small, considerably eroded volcanic islands, probably of late Tertiary age. Traces of old craters or calderas have been recognized on Robinson Crusoe Island which is built mainly of basaltic lava flows, many of them picrite-basalts. Coarse-grained ejected blocks, in which olivine predominates, are fairly common on Robinson Crusoe and there are also a number of exposures of dolerite showing pronounced banding. The only acid rock reported from the group is a soda-trachyte from Alexander Selkirk Island. Pumice washed ashore on the islands during the past year is possibly from a submarine eruption near the South Sandwich Islands in 1962.