bluuebunny/crossref_metadata_2025_split huggingface dataset:
Magma and fluid evolution in the lavas and associated granite xenoliths of Ascension Island
Authors: Harris Chris , Sheppard Simon M.F. 
Published: April 2008

Summary

The mildly alkaline lavas of Ascension Island evolved in a H
2
O-undersaturated environment. H
2
O comprises less than 0.4 wt.% of glassy equivalents of the most evolved lavas (comendites) for which there is no evidence that it has been lost by degassing. In contrast, peralkaline granite xenoliths which are chemically similar to the evolved lavas contain silicate glass and fluid inclusions which demonstrate that the original granite body crystallized from a water-saturated magma. Microprobe analysis of the glass inclusions gives low totals suggesting that the H
2
O content was high (5–8 wt.%). Amphibole δD values for the granite xenoliths are significantly different from the whole-rock δD values of the comenditic obsidians while whole-rock δ
18
O values are similar. It is suggested that the high water content in the original granite magma is due to sea-water addition and that this explains these differences. Sea-water interaction with high-level magma chambers on Ascension may have been the cause of the extensive pyroclastic volcanism which occurred on the island.
The Cameroon line, West Africa: a comparison between oceanic and continental alkaline volcanism
Authors: Fitton J. G. 
Published: April 2008

Summary
The Cameroon line is the only known intra-plate alkaline volcanic province which straddles a continental margin. It consists of a chain of Tertiary to Recent volcanoes stretching from the Atlantic island of Pagalu to the interior of the African continent. It therefore provides a unique area in which to study the differences, if any, between the suboceanic and sub-continental mantle sources for alkali basalt. Although the Cameroon line does not have a graben structure, its origin is closely linked to that of the nearby Benue rift.
Geochemical and isotopic data show no significant differences between basaltic rocks in the continental and oceanic sectors. However, the more evolved rocks in the two sectors are quite distinct. The continental magmas evolve towards peralkaline rhyolite, whereas those in the oceanic sector evolve towards phonolite. Progressive crustal contamination of the continental magmas accompanied by crystal fractionation is required to explain this distinction.
The striking geochemical similarity between basaltic rocks in the two sectors implies that their parental magmas had very similar mantle sources. This source must lie in the asthenosphere. The old lithosphere mantle beneath Africa will be chemically and isotopically different from the young Atlantic Ocean lithosphere mantle and its involvement would be readily detected in the geochemistry of the basalts. Recent models of intra-plate alkaline magma genesis have stressed the importance of metasomatic enrichment of the mantle in large-ion lithophile elements (LILE) as a precursor to magmatism. Evidence for mantle metasomatism is provided by lithosphere-derived mantle xenoliths. Since the lithosphere is the only place where large domains of enriched mantle are likely to be preserved for long periods, it follows that the Cameroon line magmas could not have had a metasomatically enriched lithosphere source.
The Cameroon line alkali basalts are chemically and isotopically similar to most other intra-plate (e.g. ocean island and continental rift) basalts which may also, therefore, have an asthenosphere rather than a metasomatized lithosphere source. The asthenosphere is also the source for mid-ocean ridge basalts (MORB) and must have a bulk composition depleted in LILE. Isotopic differences between MORB and intra-plate basalts require this source to be heterogeneous. The Cameroon line and most other intra-plate alkali basalts can be generated by small-degree melting (less than 1%) of a LILE-depleted MORB mantle source containing LILE-rich streaks.
The Ilímaussaq intrusion—progressive crystallization and formation of layering in an agpaitic magma
Authors: Larsen Lotte Melchior , Sørensen Henning 
Published: April 2008

Summary
Agpaitic rocks form the major part of the Ilímaussaq intrusion in S Greenland. The agpaitic magma developed as the narrow top zone in a large stratified basalt-syenite magma chamber at depth. The extreme composition of the agpaitic magma is related to an unusually high crustal position of the cupola. After emplacement, the agpaitic magma developed in an essentially closed system. The magma chamber was shallow and the volatile-rich alkaline magma was light and fluid. Heat loss was mainly through the roof, and the earliest agpaitic rocks crystallized successively downwards from the roof. The magma was probably well mixed in the early stage, but there is evidence for accumulation of residual components in a layer below the roof. This accumulation of low-melting components eventually suppressed the downwards crystallization of the roof rocks. The exposed floor rocks, kakortokites and lujavrites, are younger than the roof rocks, and at this stage the magma had probably developed repeated layering. The layering in the kakortokites, with density-graded units 7 m thick repeated continuously over the whole exposed floor, can be simply explained if they formed from a layered magma by successive upwards crystallization of individual layers. The magma at this stage was nearly volatile saturated, and each layer crystallized in response to the upward loss of a certain amount of volatiles. The lujavrites conformably overlie the kakortokites and formed after a roof collapse which caused the rate of heat loss from the remaining magma to increase. The upward crystallization became faster than the upward transport of residual components, and the successive lujavrites contain more and more of these components which finally gave rise to potentially economic concentrations of U, Be and other rare elements. Finally, a hydrothermal phase was lost from the system.
Alkaline rocks of the eastern part of the Baltic Shield (Kola Peninsula)
Authors: Kogarko L. N. 
Published: April 2008

Summary
The Kola Peninsula contains a remarkable assemblage of alkaline rocks. These can be sub-divided into (a) those of Proterozoic age (1900–1600 Ma), including the Yelet’ozero and Gremiakha-Vyrmes complexes and (b) those of Palaeozoic age (about 365 Ma). The latter include numerous alkaline ultramafic intrusions and the larger agpaitic complexes at Khibina and Lovozero. The Proterozoic activity involves syenites and nepheline syenites in an alkaline gabbroic association. In both the Yelet’ozero and the Gremiakha-Vyrmes complexes the magmas become more salic with time.

In the Palaeozoic associations alkaline gabbroic rocks are absent and the assemblages range from ultramafic (olivinites, pyroxenites, melilitites etc.) to highly differentiated agpaitic rocks and also to carbonatites. Low initial
87
Sr/
86
Sr ratios (0.7035–0.7039) suggest that the magmas were co-genetic and of mantle origin.

Experimental studies coupled with investigation of micro-inclusions suggest that the agpaitic magmas crystallized over a wide temperature interval (more than 300°C) under highly reducing conditions. The associated gas phase had a low
f

H
2
O

but relatively high CH
4
, H
2
and H
2
S fugacities.

The apatite deposits in the Khibina complex are uniquely large and are associated with ijolite, melteigite, urtite, juvite and malignite. The phosphate deposits are interpreted as apatite cumulates produced by suspension of apatite from the host magma after precipitation of clinopyroxene and nepheline.
Faulting in abyssal-plain sediments, Great Meteor East, Madeira Abyssal Plain
Authors: Williams S. R. J. 
Published: April 2008

Summary
Seismic surveys carried out in the Madeira Abyssal Plain have revealed patterns of faulting in abyssal-plain sediments. Shallow faults are concentrated within linear bands which run parallel to the trend of underlying basement topography. The faults are believed to be caused by differential compaction over the basement topography. The reverse throw of many faults is thought to be a result of sediment dewatering. A recent model has invoked preferential dewatering along faults. An alternative mechanism is proposed in which faults act as seals. Overpressurized pore fluids are a feature of both models.
Evidence of pore-water advection in the Madeira Abyssal Plain from pore-pressure and temperature measurements
Authors: Schultheiss P. J. , Noel M. 
Published: April 2008

Summary
Measurements of pore pressure and temperature made in the Great Meteor East region of the Madeira Abyssal Plain (approximately 31°30′N, 24°30′W) are described in this paper. The aim of the study was to establish whether pore-water advection exists in the upper sediment sequence which is important for assessing the area as a potential repository for high-level radioactive waste. The relative merits of deducing advection velocities from pore-pressure or temperature gradients are considered, together with the accuracies that can be obtained from each technique and the data collected to date. Heat-flow measurements and non-linear sediment-temperature profiles indicate that hydrothermal circulation is present in the basement with upward advection occurring within sediments over some basement highs. However, the pore-pressure data imply that advection is generally absent within the area. The discrepancies in these interpretations are discussed with respect to the geological setting and the physical characteristics of the sediments.
The Volcano-Tectonic Setting and Mineralization of the Early Proterozoic Kemiö-Orijärvi-Lohja Belt, SW Finland
Authors: Colley H. , Westra L. 
Published: April 2008

Summary
The Kemiö-Orijäirvi-Lohja region of SW Finland, which consists of metabasic rocks, metagreywackes, metavolcanic rocks of felsic composition (leptites) and syngenetic gabbro-tonalite bodies, is a westerly striking portion of the early Proterozoic (1900 Ma) Svecofennian Belt. This belt, which extends westward into Central Sweden is flanked by large post-kinematic granites. The grade of metamorphism decreases westward from amphibolite-granulite transitional conditions around Orijärvi to greenschist conditions in Central Sweden.
The volcanic sequence begins with submarine basic-intermediate lavas followed by turbiditic metasediments and ends with the deposition of felsic pyroclastic rocks. Sheets of gabbro related to larger synkinematic intrusive bodies occur within the felsic rocks. Disseminated Cu-Fe sulphides occur in the lower basic rocks and massive Cu-Pb-Zn ore bodies, of probable volcanogenic origin, are found in the upper felsic rocks. Mg-alteration and the formation of cordierite-anthophyllite rock characterizes the latter ore deposits.
The geochemistry of the metavolcanic rocks indicates a transition between alkaline and subalkaline compositions, and the geochemistry of intrusive bodies suggests they are comagmatic with the volcanic rocks they intrude. In more strongly deformed areas mobility of Fe, Si and K is indicated. The origin of some high Fe basic lavas is unclear. After consideration of the geology and geochemistry of the Svecofennian Belt it is suggested that it may represent an ancient analogue of present day New Zealand where ‘back-arc’ rifting is occurring in both oceanic areas and mature arc crust.
Early Proterozoic Volcanic Regimes in Southwestern North America
Authors: Condie K. C. 
Published: April 2008

Summary
Five early Proterozoic volcanic terranes of the following ages occur in the southwestern United States: 1760–1800 Ma, 1730–1740 Ma, 1720 Ma, 1680–1700 Ma, and 1650 Ma. Volcanics are typically bimodal in composition and, for the most part, represent submarine eruptions. In terms of the least mobile incompatible element distributions, basalts from the 1760–1800 Ma and 1720 Ma terranes are similar to basalts in evolved oceanic arcs and associated incipiently opened back-arc basins. Basalts from the other three terranes resemble basalts from continental-margin arcs and associated back-arc basins. Andesites are important volcanic components at only two localities and they are similar chemically to andesites from evolved oceanic and continental-margin arcs. Felsic volcanics define two major geochemical populations. Those from the 1760–1800 Ma and 1720 Ma terranes, which have low HFSE and heavy REE contents, are similar in composition to felsic volcanics from modern arc systems while those from the other three terranes, which have relatively high HFSE and heavy REE contents, resemble felsic volcanics from rifts or back-arc basins in or near continental crust.
The 1760–1800 Ma terrane appears to reflect a succession of oceanic arcs accreted to the Archaean craton from the southeast and the 1730–1740 Ma terrane, an incipient back-arc basin formed within the 1760–1800 Ma terrane. The 1720 Ma terrane is interpreted as an evolved back-arc basin in which depleted mantle is tapped. The 1680–1700 Ma and 1650 Ma terranes are remnants of an extensive continental-margin arc system and associated back-arc basins. Bimodal volcanism may reflect differences in preservation of arc and back-arc successions.
Early Proterozoic Alkaline Igneous Rocks, District of Keewatin, Canada: Petrogenesis and Mineralization
Authors: LeCheminant A. N. , Miller A. R. , LeCheminant G. M. 
Published: April 2008

Summary

Extensive potassic alkaline magmatism accompanied early stages in the formation of intracratonic basins in central Keewatin. Subaerial trachyandesite and trachyte flows, minette, syenite and bostonite dykes and small pyroxenite-syenite stocks comprise the
c.
1.8 Ga igneous suite. High Mg, Cr and Ni contents in mafic flows and mica-rich lamprophyres indicate crystallization from mantle-derived magmas. Enhanced LIL element content and LREE enriched noninflected REE patterns are consistent with an enriched upper mantle source containing high-Ti micas. Enrichment in Sr, K, Rb, Ba and Th is interpreted as due to mobilization by water-rich fluids. Polymodal zoned clinopyroxene and mica macrocrysts in lavas and dykes record combined effects of crystal fractionation, disaggregation of wall rock xenoliths and magma mixing during rapid magma ascent through extending continental lithosphere. High Fe-Ti-Ba xenocryst micas may have originated metasomatically in the upper mantle where they were incorporated as partly melted crystals near the site of subsequent magma generation. Inhomogeneous lithospheric reworking and cracking during
ç
1.9–1.85 Ga collisions may be responsible for post-tectonic extension and the voluminous alkaline magmatism in central Keewatin.

A wide spectrum of uranium mineralization and minor gold and base metal occurrences are found in rocks of the alkaline suite. Syngenetic mineralization is limited to disseminated REE+U+Th in bostonite dykes. Minor U±Cu±Ag±Pb mineralization results from thermal and hydrothermal effects of dyke and pluton intrusion. Epigenetic uranium mineralization is associated with localized propylitic alteration of the alkaline rocks and with crosscutting quartz veins, fracture systems and fault zones. Important metal associations are U+Cu+Pb+Ag±Mo±Zn, U+Cu+Se+Ag+Au and Pb+Cu±Zn±Cd±Ag±Bi.
Trace Element Geochemical Correlation in the Reworked Proterozoic Dalradian Metavolcanic Suites of the Western Ox Mountains and NW Mayo Inliers, Ireland
Authors: Winchester J. A. , Max M. D. , Long C. B. 
Published: April 2008

Summary
Metavolcanic rocks occur within late Proterozoic metamorphic successions in the NW Mayo and western Ox Mountains inliers of western Ireland. Geochemical studies have shown that most of these basic metavolcanic rocks have continental tholeiite affinities, and have confirmed that most form separated portions of the same suite. Within the Ox Mountains Succession, which forms the western part of the Ox Mountains Inlier, all but the Callow metavolcanics can be correlated with those in the NW Mayo Inlier. Thus the Ox Mountains Succession itself can now be equated with rocks in the NW Mayo Inlier, which have in turn been correlated with the late Proterozoic Argyll Group of the Dalradian Supergroup. By contrast, the Callow metavolcanics show affinities with amphibolites from the Westport Inlier and the Kill Inlier on Clare Island, and all have a chemistry consistent with extrusion in an ‘oceanic’ setting similar to some metabasalts from the Scottish Highland Border Complex. These correlations revealed by geochemical studies have important implications for the interpretation of the former positions of discrete terranes within the Caledonide orogen in Ireland, and illustrate the value of geochemistry in resolving stratigraphic problems in complex areas.
Geochemical Evidence for the Tectonic Setting of Late Proterozoic Volcanic Suites in Central England
Authors: Pharaoh T. C. , Webb P. C. , Thorpe R. S. , Beckinsale R. D. 
Published: April 2008

Summary
The late Proterozoic basement of central England is largely concealed by a cover of Phanerozoic rocks. Proterozoic volcanic rocks are exposed as a result of localized uplift/folding in the eastern Midlands (the Charnian Supergroup) and adjacent to major crustal lineaments which include the Church Stretton and Pontesford-Linley Fault Zones (the Uriconian volcanic group) and the Malvern lineament (the Warren House volcanic group). The geochemistry of these volcanic associations has been examined through midocean ridge basalt (MORB)-normalized multi-element diagrams which emphasize the contrasted and variable relationships between large-ion lithophile (LIL) elements and high field strength (HFS) elements within/between these volcanic groups.

The
Charnian Supergroup
comprises over 2.5 km of bedded volcaniclastic and sedimentary rocks locally with interbedded lavas and high-level intrusions (termed ‘porphyroids’) of andesitic to rhyodacitic composition. The more mafic lavas are characterized by enrichment of the LIL elements K, Rb, Ba and Th, while HFS elements (Nb, Ta, Zr and Y) are depleted relative to MORB, and suggest that the Charnian lavas resulted from calcalkaline volcanism in an island arc. The
Charnwood diorites
, a minor intrusive suite emplaced into the bedded series, reflect increasing maturity of this same arc. The
Uriconian volcanic group
comprises a bimodal association of volcanic and pyroclastic rocks ranging from basalt to rhyolite in composition. They are enriched in HFS elements relative to MORB, which may indicate a within-plate component to the source, although their high LIL/HFS element ratios imply a subduction-related environment. The tectonic environment of eruption may have been a post-subduction, ensialic faulted basin between the Church Stretton and Pontesford-Linley Fault Zones. The
Warren House group
is in tectonic contact with the Malvernian (calcalkaline) plutonic complex and has LIL-element concentrations enriched in relation to MORB, but shows slight depletion of HFS elements similar to that exhibited by some marginal basin basalts and island arc tholeiites. These lavas may represent relics of marginal basin lithosphere trapped and uplifted in a Proterozoic suture between the Uriconian and Charnian terranes.

These new geochemical data suggest that the late Proterozoic basement of central England comprises a heterogeneous mosaic of geochemically-distinct volcanic terranes, formed in separate volcanic arc/marginal basin environments and juxtaposed by subsequent accretionary tectonism in the Avalonian/Cadomian Orogeny (cf. Thorpe
et al.
1984).
Fluid flow due to sediment loading—an application to the Arabian Gulf region
Authors: Magara Kinji 
Published: April 2008

Summary

Fluid pressure in excess of hydrostatic pressure can be generated during sediment loading. The rate of increase in the excess fluid pressure (psi Ma
−1
) can be calculated from the sedimentation rate (ft Ma
−1
) of a formation and its average bulk density. Further, the inferred directions of horizontal migration of the compaction fluid can be shown by means of a contour map based on the rate of increase in the excess fluid pressure, the first derivative of which gives the intensity of the horizontal motion of the compaction fluid (psi Ma
−1
mile
−1
). The horizontal fluid movement may be essential for driving hydrocarbons towards a trapping position during the primary stages of migration.

Application of this technique using the regional isopach maps of the Jurassic, Cretaceous and Tertiary formations in the Arabian Gulf region suggests that the major oil fields are concentrated in areas of relatively strong horizontal fluid movement. The area N of the Qatar Peninsula, where horizontal migration of fluid was relatively weak during the Cretaceous period when both the Jurassic and Cretaceous source rocks reached their early stages of oil generation, seems to have relatively low concentrations of oil. At present there are relatively few oil fields in this part of the region.
Some aspects of the thermal regime and hydrodynamics of the western Canadian sedimentary basin
Authors: Jones F. W. , Majorowicz J. A. 
Published: April 2008

Summary
Approximately 65 000 bottom-hole temperatures from about 36 000 wells in the Prairies Basin of western Canada have been used to investigate the thermal regime there. Both lateral and depth variations in temperature gradients and heat-flow densities occur, and these are related to the hydrodynamics which is controlled by the topography. The vertical heat-flow density in the Mesozoic-Cenozoic formations is less than in the Palaeozoic formations in recharge areas, but is greater in discharge areas. A zone in which heat flow is approximately constant with depth exists in the central part of the basin between recharge and discharge areas. It is clear that basin-wide redistribution of heat by groundwater motion takes place. The relationship between geothermal gradients and hydrocarbon occurrences is studied for southern Alberta and it is found that the mean geothermal gradient in the Mesozoic and Cenozoic sediments is greater for areas where Mesozoic gas and oil pools exist than for areas where they do not occur.
Regional groundwater flow in sedimentary basins in the U.K.
Authors: Downing R. A. , Edmunds W. M. , Gale I. N. 
Published: April 2008

Summary
The U.K. contains seven groundwater provinces. Regional groundwater flow systems occur in these provinces above an essentially impermeable basement of varying age. The pattern of flow in the U.K. is strongly influenced by the distribution of the Permo-Triassic sandstones. Intermediate flow systems are encouraged by the scarp and vale topography that has developed as a consequence of the alternating sequence of aquifers and aquicludes in the Mesozoic.
Regional changes in groundwater chemistry reveal the direction of flow paths, and isotopic ratios and inert gas contents give an indication of the residence time of water in the flow systems. The composition of the groundwater in deep systems has been modified by shale-membrane filtration. Density settling may be a feature in thick relatively homogeneous aquifers.
Thermal aspects of the East Midlands aquifer system
Authors: Wilson N. P. , Luheshi M. N. 
Published: April 2008

Summary
A case study of a heat flow anomaly in the E Midlands of England is reported. The anomaly has been suggested to be an effect of water movement at depth within the E Midlands basin, with recharge to the Lower Carboniferous limestones in their outcrop, eastward movement and ascent of water up a steep faulted anticline at Eakring where the heat flow measurements were made.
Numerical modelling of heat and fluid flow has been undertaken for a section running from the Peak District through Eakring to the coast. The results indicate that, although an anomaly is expected for reasonable values of hydrological parameters, its magnitude is less than that observed. The geological structure at Eakring is such that three-dimensional flow is likely to be important, and this could easily account for the discrepancy between the modelling results and the observations.
The regional water flow regime has other effects on heat flow, notably the depression of heat flow above the Sherwood Sandstone aquifer.