bluuebunny/crossref_metadata_2025 huggingface dataset:
The nature and origin of lamprophyres: an overview
Authors: Rock Nicholas M.S.
Published: April 2008

Lamprophyres are ‘alkaline rocks’ because they carry high alkalis at a given percentage of SiO2, together with one or more of normativene, lcorac, modal foids, and Na-K-Ti-rich amphiboles or pyroxenes. They reach higher whole-rock H2O, CO2, Sr and Ba contents than other silicate igneous rocks. Contents of related element subsets in amphiboles (Ti, Ba), K-feldspars (Ba, Fe3), phlogopites (Ti, Ba, Fe3) and pyroxenes (Ti, Al, Fe3) include among the highest values known in nature for these minerals. ‘Primitive’ minerals (diopside, forsterite) commonly coexist with ‘evolved’ minerals (albite + orthoclase, quartz). Four well-defined ‘branches’ of the lamprophyre ‘clan’ have distinctive compositions:calc-alkaline (shoshonitic) lamprophyres(minettes etc.), alone among lamprophyres, have mixed alkaline-calc-alkaline affinities;alkaline lamprophyres(camptonites etc.) are basanitic to nephelinitic and, alone among lamprophyres, usually have Na < K;ultramafic lamprophyres(alnöites etc.) are the most Si poor and Ca rich of silicate igneous rocks, and grade into carbonatites;lamproites(orendites etc.) are uniquely rich in K, Rb, Ba, Th, Mg, Cr and Ni at mainly ‘andesitic’ SiO2contents, and carry a suite of diagnostic minerals (wadeite etc.). Each branch comprises at least four rock types which resemble each other much more than rock types of other branches; however, some rock types can be grouped into slightly distinct ‘families’ within one branch (e.g. phlogopitic and madupitic lamproites). A case can be made for including kimberlites as a fifth branch of the lamprophyre clan. Synoptic plots and tables, based on some 5000 major, trace-element and mineral analyses, are presented, to aid identification and classification. Lamprophyres are far more common than generally stated, occurring worldwide in more tectonic settings than many other alkaline rocks and throughout the geological record. They may approximate intratelluric magma compositions. Nearly all represent primitive magmas, and many represent primary magmas. Some represent parental magmas to a wide range of hydrous alkaline intrusive suites: calc-alkaline lamprophyres to potassic pyroxenite-diorite-shonkinite-syenite-granite plutons (Cortlandt); alkaline lamprophyres to hornblendic gabbro-syenite plutons (Monteregian Hills); and ultramafic lamprophyres to ijolite-carbonatite complexes (Fen).
Magma and fluid evolution in the lavas and associated granite xenoliths of Ascension Island
Authors: Harris Chris , Sheppard Simon M.F.
Published: April 2008

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.
Vertical exchange of coarse bedload in desert streams
Authors: Schick A. P. , Lekach J. , Hassan M. A.
Published: April 2008

The tracing of coarse particles, transported by flood events in two ephemeral streams in the Negev Desert, Israel, reveals that a size-independent, highly skewed longitudinal dispersion pattern is accompanied by vertical distribution throughout the scour layer. An analysis of the intermittent burial and resurfacing of these particles from one event to the next leads to the formulation of a concept of equilibrium in vertical exchange. This equilibrium requires that, for the ‘dominant’ flood event (frequency 1–2 years), the number of previously buried particles exposed by that event is balanced by an identical number of previously surficial particles being buried. Such an equilibrium can be maintained only if the scour layer reaches sufficiently deep into the ‘sub-armour’ layer to entrain a requisite number of particles of armour-layer size. The resulting functional relationship between (1) the size of coarse particles in relation to the matrix; (2) the thickness of the armour and ‘sub-armour’ layers and the distribution of coarse particles; and (3) aspects of the flow regime, enables the derivation of hydrological information from studies of the three-dimensional distribution of coarse particles in modern channel bed sediments of arid streams. It also helps to explain the vertical distribution of pebbles in ancient continental clastic sediments.
Plio-Pleistocene raised channel systems of the western Sharqiya (Wahiba), Oman
Authors: Maizels J. K.
Published: April 2008

Extensive alluvial fans bounding the western edge of the Sharqiya (or Wahiba) Sands are characterized by complex palaeochannel systems now forming a series of superimposed gravel ridges. The oldest, most extensive fan system is crossed by numerous sinuous, superimposed palaeochannel courses comprising highly weathered, chert-rich gravels cemented by clear crystalline calcite. Many of these old fan sediments have been chemically altered to depths of over 200 m to form a pink dolomitic clayey deposit here termed barzamanite. The formation of this calcrete-like deposit was probably associated with rising water tables during long-term fan growth. The younger, more limited fan deposit, by contrast, comprises thin, coarse-grained, broad spreads and terrace veneers of weakly cemented ophiolitic gravels. The fans have been subject to entrenchment and deflation, resulting in extensive land-surface lowering and the exhumation of buried palaeochannels. Although no dates are yet available, the older channel systems are likely to date from more humid phases during the Pliocene-Early Pleistocene, whereas the younger, terraced deposits, and the period of subsequent channel exhumation, may date from semi-arid periods of lower sea level during the Pleistocene.
Ancient ephemeral stream deposits: a local terminal fan model from the Bunter Sandstone Formation (L. Triassic) in the Tønder-3, -4 and -5 wells, Denmark
Authors: Olsen H.
Published: April 2008

Facies studies of a
c.
30 m thick fluvial sand body in the Tønder area, Denmark indicate that the sand body was formed by an ephemeral stream system comparable to modern terminal fans. The sediments are grouped into three genetic facies associations (A, B, C) reflecting two styles of channel flow (A and B) and overbank sheet flow (C). The sediments are dominated by parallel laminations (28%) and large-scale cross-bedding (27%) in association with small-scale cross-bedding (17%), massive bedding (14%) and intraformational conglomerates (5%). Mudstones and siltstones form only 9% of the deposits. Anhydrite (formerly gypsum) is associated with the fine-grained deposits which are mainly of overbank origin. It is commonly observed as intraformational clasts in the channel deposits. The inferred ephemeral stream complex probably formed a link between a more permanent braided stream system towards the N (the Skagerrak Formation) and a sabkha system towards the S (the Middle Buntsandstein in northern Germany).
Alluvial fan dissection: relationships between morphology and sedimentation
Authors: Harvey A. M.
Published: April 2008

Aggradation and dissection on alluvial fans influence the coupling between sediment source areas and main channels. Previous alluvial fan models emphasize the influence of drainage area and sediment character on overall fan morphometry and proximal-distal sedimentary variations. A model is proposed for the morphology and sedimentology of dry-region alluvial fans, based on crusted and non-crusted Quaternary fans in SE Spain, differentiating between aggradational and dissectional tendencies. Many fans show fan-head trenching, with the channel emerging on the fan surface at a mid-fan intersection point. Distally either aggradation or headcut development may occur, with headcut development and trenching leading ultimately to the total dissection of the fan surface. Three sets of factors determine intersection-point behaviour: fan surface crusting, discrepancy between fan and channel slopes, and channel morphology. Multiple regression models are developed to account for variations in fan and channel slopes and channel morphology, taking into account source area and sediment variations. Intersection point trenching is most common on steep crusted fans, where the discrepancy between fan slope and channel slope is greatest, and where channel widths are limited. Arid region fans from the American Southwest are contrasted with the Spanish fans. Neither the non-crusted nor the crusted American fans examined show any tendency towards intersection-point trenching. Their fan slope/channel slope discrepancies are less and their channels are wider, the result of both differing Quaternary sequences and contemporary environments between Spain and the American Southwest. The implications are important for the continuity of dry-region fluvial systems and for the stratigraphy of alluvial fan sequences. Simple fans with distal area aggradation show a simple stacked stratigraphy, but fans with a propensity for mid-fan and distal trenching may show a complex inset stratigraphy.
Grain-size variation on a ‘complex’ linear dune in the Namib Desert
Authors: Livingstone I.
Published: April 2008

Against a background of increasing knowledge of regional grain-size variations, particularly in the Namib Desert, this paper reports an investigation of small-scale grain-size variations using 25 sample points on the cross-profile of a single linear dune. The results suggest that a discernible pattern of grain size across the dune does exist, and that changes are gradual rather than discrete. From an examination of the near neighbours of the main study dune there is also some evidence of measurable differences between dunes in the same area. The fact that grain sizes are not randomly distributed across the width of the dune indicates a response to spatial variations of process, but grain size does not seem to control dune form in the way that some earlier workers have envisaged. Furthermore, monthly sampling provides preliminary evidence that grain-size values on this dune respond to a seasonal wind regime, and that time of sampling may therefore be a crucial determinant of the results obtained.
Silica and calcium carbonate replacement of plant roots in tropical dune sands, SE India
Authors: Hendry D. A.
Published: April 2008

The Quaternary coastal sediments of SE India are composed of raised marine deposits capped by aeolianite and red weathered dune sands (Teri). The red weathering associated with these sands is irregular in depth and unusual in the fast rate at which it has developed. Associated at depth with this red weathering is precipitated silica and calcium carbonate which replace plant roots and act as a cement. In places the plant root cells are prefectly replaced to form rhizoliths which occur in the soil profile with and without surrounding cement. Ultimately, the silica source is silicic acid (H
4
SiO
4
) that is a product of the
in situ
weathering of unstable minerals in the overlying dune sands during the last 20 000 years. The calcium carbonate is thought to have had a much more localized source. Plants and the litter they produce play at least two possible roles in the silica cycle. First, silica-organic complexing occurs as the Teri sands are leached by organic acids derived from decomposing litter. Secondly, the plant roots create a favourable micro-environment for silica and calcium carbonate precipitation and replacement by altering partial gas pressures and hence the pH of soil solutions through their respiratory activity.
The application of a solution-mineral equilibrium model to the diagenesis of Carboniferous sandstones, Bothamsall oilfield, East Midlands, England
Authors: Warren E. A.
Published: April 2008

Within the fluviatile reservoir sandstones of the Bothamsall oilfield, authigenic cements of early quartz, kaolinite and illite predominate. Detrital feldspars show varying degrees of alteration and muscovite micas are commonly altered to intergrowths of kaolinite and illite.

Solution-mineral equilibria in the idealized system of K
2
O-Al
2
O
3
-SiO
2
-H
2
O are used to relate the simple silicate mineral assemblage observed to possible pore-fluid compositions. Previous studies considered aluminium to be immobile in this system. Petrographic evidence of the paragenetic sequence at Bothamsall indicates that aluminium is mobile, albeit over short distances. This study therefore considers aluminium mobility in the solution-mineral equilibria. Graphical plots of total aluminium versus pH for given potassium ion activity, at a temperature of 298 K, are constructed to illustrate the stability fields of the phases identified. These show that aluminium can be an important component, together with pH and potassium-ion activity, in affecting mineral stabilities in this system.

The existence of several co-stable phases at Bothamsall considerably reduces the possible permutations for pore-fluid chemistry; kaolinite-illite alteration of mica proves particularly sensitive in this respect. The relative merits of two different pore-fluid models are compared: one, where the pore-fluid composition is considered to be constant in an open system, and another where the pore-fluid composition is assumed to vary in response to
in situ
mineralogy in a closed system. The former would require a very restricted range of pore-fluid compositions to result in the diagenetic modifications observed, whereas a wide range of pore fluids could evolve in a closed system driven by feldspar dissolution. This pore-fluid evolutionary model is preferred.
Petrology (including fluorescence microscopy) of cherts from the Portlandian of Wiltshire, UK—evidence of an episode of meteoric water circulation
Authors: Astin T. R.
Published: April 2008

Chert nodules from the Wockley Member of the Portland Stone (Upper Jurassic) in Wiltshire, England are comprised of three stages of chert cementation and sediment replacement: (1) opal-CT lepisphere cement and grain replacement, (2) length fast chalcedony grain and matrix replacement and cement, and (3) quartz cement. These are readily distinguished by their fluorescence, the first and last stages being non-fluorescent, the second stage showing internal zonation changing from moderate to bright fluorescence with time.

Some carbonate grain dissolution took place prior to silica precipitation. The first stage silica cement formed mainly as pore-linings, with minor replacement of grain margins. Its botryoidal, lepisphere habit implies it was opal-CT, now inverted to quartz. Extensive grain replacement took place while chalcedony cements formed. Chalcedony cements show an increase in inclusions of brown organic particles with time, joined at a late stage by iron oxide inclusions.

Silicification and carbonate dissolution occurred during a period of meteoric water penetration, probably as a result of soil formation at the top of the Portland Stone. Silica was apparently derived as opal-A from adjacent marine limestones, redistributed by groundwater flow, and precipitated as opal-CT and chalcedony. The organic inclusions probably come from the developing soil horizon, the iron oxides marking the migration of an oxidation front away from the soil horizon into the sediment. The later quartz cement lacks inclusions and was formed during later burial after stagnant groundwater conditions were re-established.
Diagenesis of carbonate cements in Permo-Triassic sandstones in the Wessex and East Yorkshire-Lincolnshire Basins, UK: a stable isotope study
Authors: Bath A. H. , Milodowski A. E. , Spiro B.
Published: April 2008

The
13
C/
12
C and
18
O/
16
O ratios of carbonate cements in Permo-Triassic fluvio-deltaic sandstones differ between the Wessex and East Yorkshire-Lincolnshire Basins. The isotopic data indicate different conditions of deposition and early post-depositional carbonate precipitation which supports petrological and mineralogical studies of the same sandstones focussed on the development of reservoir properties. Isotopic compositions of carbonate cements vary across the Wessex Basin; they indicate early derivation of dissolved carbonate during recrystallization of detrital limestone. Within the basin, calcite cements show little variation in δ
13
C but considerable differences in δ
18
O between early calcrete calcites and isotopically-light later calcites; δ
18
O decreases to values close to isotopic equilibrium with present water while δ
13
C is not in isotopic equilibrium with bicarbonate in present-day brines. Two distinct generations of replacive dolomite, early non-ferroan and later ferroan, are found in the basin centre and have different δ
18
O but similar δ
13
C values.
34
S/
32
S and
18
O/
16
O ratios of anhydrite cements could indicate a remote Zechstein source of sulphate or a mixed marine sulphate-sulphide source. The Permo-Triassic sandstones of the East Yorkshire-Lincolnshire Basin, in contrast, contain dominantly dolomite cement. δ
18
O and δ
13
C values are relatively heavy and indicate early dolomitization of calcite over large areas. δ
18
O values of minor calcites indicate that dedolomitization is continuing in the present-day groundwater regime. The isotopic differences between carbonate cements in the two basins is tentatively attributed to the differing early post-depositional conditions and to the differences in carbonate chemistry between calcite dissolution/precipitation reactions and dolomitization reactions.
Secondary porosity in hydrocarbon-bearing transgressive sandstones on an unstable Lower Palaeozoic continental shelf, Welsh Borderland
Authors: Parnell J.
Published: April 2008

Coarse sandstones were deposited during each of four transgressions across the shelf of the Lower Palaeozoic back-arc Welsh Basin. Hydrocarbon shows record porosity in the deposits of the Cambrian, Arenig, Caradoc and Llandovery transgressions. Hydrocarbons migrated from two sources: from Lower Palaeozoic source rocks within the basin, and later from Carboniferous rocks.

Diagenesis and reservoir potential is related to the mineralogy of the sandstones, which reflects a balance between reworking of shelf sediment and an input from the eroded substrate. The substrate included Precambrian volcanic and plutonic rocks formed at the southern margin of the Iapetus Ocean. Quartz arenites are quartz-cemented or exhibit thick pore-linings of clay which restrict permeability. They have only minor reservoir potential. Arkoses do not exhibit macroporosity but have effective microporosity in the immediate vicinity of source rocks. Subarkoses have optimal reservoir potential, consisting of primary porosity about non-quartz grains (feldspars, glauconite) and secondary (intragranular) porosity in feldspars and lithics. The secondary porosity formed by leaching of clay alteration products in feldspars. Subarkoses retained sufficient framework stability to preserve the secondary porosity, whereas arkoses experienced widespread alteration and framework collapse. Limited secondary porosity after calcite dissolution may be due to surficial weathering of the Lower Palaeozoic rocks during Devonian/Lower Carboniferous exposure.
Deposition and diagenesis in an extensional basin: the Corallian Formation (Jurassic) near Oxford, England
Authors: de Wet C. B.
Published: April 2008

The mixed carbonate-siliclastic Corallian Formation was deposited on horsts and in grabens which were produced during Jurassic extensional tectonics. The high areas became sites for sandstone and carbonate deposition in shallow subtidal environments while clays and allochthonous debris accumulated in the depressions. Tectonic activity, perhaps confined to single fault blocks, produced depositional variation distinct to each area.

The basin has been subdivided into four major east-west trending fault blocks. The most northerly of these is the Oxford Shallows, interpreted as a small accretionary-margin slope. The depositional facies and diagnesis of this area are discussed relative to the basin as a whole.

The diagenesis of the Corallian sediments has been divided into three phases of cementation: pre-burial, shallow burial and deep burial. Phase 1 spars include marine precipitates and non-ferroan, neomorphic and drusy spars. Phase 2 cements are ferroan, precipitating during shallow burial, including an intermediate ferroan (mauve) spar which may represent development of a redox front. Phase 3 spar post-dates vuggy dissolution which probably took place during Barremian uplift. The cement precipitated during the ensuing burial phase.

Geochemistry and stable isotope data show an overall fluid evolution path for the first two spars, although Sr
2+
values in replaced corals reveal variability in the degree of meteoric flushing. The chemistry of the late spars suggests low water/rock ratios, where local dissolution provided the cations for the cement. The oxygen isotope values suggest, however, that meteoric and connate waters were mixed at slightly elevated temperatures.
A hot-spot model for early Tertiary volcanism in the N Atlantic
Authors: White R. S.
Published: April 2008

The initial stages of rifting of the N Atlantic were accompanied by massive extrusion of volcanic rocks and the accretion to the lower crust of even greater volumes of igneous rock. Emplacement of the bulk of the igneous rocks along the 2000 km long continental rift occurred in as little as 2–3 My. The pattern and timing of magmatic activity can be explained by partial melting of anomalously hot asthenosphere as it welled up to fill the space left by the stretched and thinned continental lithosphere. Asthenosphere temperatures were elevated regionally by 100–150°C over a 2000 km diameter region by the mushroom-shaped head of hot material fed by a mantle plume, which at present is centred beneath Iceland.
Deep crustal structure and magmatic processes: the inception of seafloor spreading in the Norwegian-Greenland Sea
Authors: Mutter J. C. , Zehnder C. M.
Published: April 2008

An extensive investigation into the deep crustal structure of the conjugate Norwegian and E Greenland margins utilizing two-ship Expanded Spread and Wide Aperture CDP Profiling has led to the identification of a unique suite of crustal structures developed during the onset of seafloor spreading. On both margins, a region of low velocity crust was found proximal to marginal escarpments. Immediately seaward of the low velocity crustal block seaward-dipping reflectors occur within thick crust that is interpreted to be wholly igneous, a product of voluminous melt production enhanced by convective partial melting processes. This earliest formed oceanic crust is actually several kilometres thicker than the low velocity crust or the adjacent continental crust. Normal oceanic crustal thicknesses are not produced until about 5 My after the initiation of spreading when convective partial melting has abated and the magma budget has reduced. The extrusive unit reduces to normal thicknesses long before the intrusive and plutonic section, indicating that the thickening or thinning of oceanic crust is accomplished primarily by variations in the latter.
Early Cainozoic evolution of the Norwegian volcanic passive margin and the formation of marginal highs
Authors: Skogseid J. , Eldholm O.
Published: April 2008

The final continental break-up between Greenland and Norway which took place during the late Palaeocene/early Eocene was accompanied by extensive subaerial volcanism along parts of the plate boundary lasting for about 3 My. Subsequent subsidence and change to normal seafloor spreading have left behind marginal highs partly underlain by seaward-dipping reflector sequences composed of basaltic extrusives. We propose a model in which the formation of the marginal highs and the emplacement of the dipping series are related to the crustal configuration prior to the early Cainozoic plate break-up, causing an initial uplift if the rifting occurs within much thinned continental crust. This forms a ‘volcanic’ type rifted margin characterized by a small amount of crustal extension compared with a ‘normal’ margin segment that has experienced a large amount of extension and rapid initial subsidence. Recent drilling of the Vøring marginal high suggests that the plate boundary volcanism and the N Atlantic Volcanic Province may be manifestations of the same regional volcanic surge.
Dipping reflector styles in the NE Atlantic Ocean
Authors: Parson L. M. ,   the ODP Leg 104 Scientific Party
Published: April 2008

The geometries and structure of volcanic rifted continental margins, that is, those with an ocean-continent transition masked by thick sequences of inclined tholeiitic basaltic flows, are examined. Small- and large-scale differences in sequence seismostratigraphy along strike, down-dip, and between conjugate configurations are discussed. Detailed appraisals of deep drilling results and seismic data for the Vøring Plateau and western Rockall Plateau are used to contrast margin architecture and demonstrate difficulties in interpolating data between sites of dipping reflector formation.