bluuebunny/crossref_metadata_2025 huggingface dataset:
Time-space relationships of Upper Precambrian volcanic and sedimentary units in the Central Arabian Shield
Authors: Jackson N. J. , Ramsay C. R.
Published: December 2007

Geological and radiometric data are combined to produce time-calibrated stratigraphic columns for 7 provinces in the central Arabian Precambrian shield. Regional correlation suggests a subdivision of the stratigraphy into 3 'sequences' informally named A, B and C.

'Sequence' C (
c
. 1200–950 Ma) includes rocks assigned to the Baish, Ajal, Bahah, Arafat and Hali groups, and typically consists of fine-grained clastic terrigenous sediments interbedded with volcanic/volcanoclastic deposits. These were deposited in a marine basin marginal to but some distance from a continental landmass.

'Sequence' B (
c
. 950–650 Ma) includes rocks assigned to the Jiddah, Halaban, Hulayfah and Samran groups and parts of the Ablah and Urd groups. It comprises several volcano-sedimentary complexes and locally contains allochthonous mafic/ultramafic rocks which have been interpreted as ophiolites. Most of 'sequence' B was deposited in elongated basins adjacent to volcanic arcs.

'Sequence' A (
c
. 650–570 Ma) includes rocks assigned to the Fatima, Murdama, Jibalah and Shammar groups, the Afif and Abt formations, and parts of the Halaban, Hulayfah and Ablah groups. It typically comprises coarse-grained terrigenous clastic sediments, locally interbedded with carbonates, deposited in fluviatile or shallow marine environments, and rhyolite-dominated volcanic units deposited subaerially.

Stratigraphies of similar age and lithological character can also be recognized in E Egypt, NE Sudan and N Ethiopia.
The Lower Cambrian transgression and glauconite-phosphate facies in western Europe
Authors: Brasier M. D.
Published: December 2007

Facies models and successions are outlined for the North American, Avalon, Baltic, Armorican and S European platforms, showing that glauconite and apatite formed partly behind carbonate barriers but mostly on ocean-facing shelves. Phosphogenic rhythms on the Avalon and Baltic platforms are related to transgressions and regressions while the Cambrian phosphogenic episode may be linked with invertebrate evolution, phytoplankton blooms and a rise in the O
2
minimum.
The origin and evolution of Tertiary phosphorites from eastern England
Authors: Balson P. S.
Published: December 2007

Remanié deposits of nodular phosphorite are found at the base of the Coralline Crag (Pliocene) and the Red Crag (Plio-Pleistocene) of Suffolk and Essex. The phosphatic components include phosphorite concretions (nodules), phosphatized vertebrate teeth and bones, and cobbles of sandstone cemented by a phosphatic matrix ('boxstones'). The phosphorite mineral is carbonate fluorapatite (francolite). The source of the phosphorite concretions was the London Clay (Eocene). They formed in unlithified anoxic sediment around sites of organic decay. The source of the 'boxstones' was a Miocene mud-rich sand formation tentatively correlated with the Belgian Anversian. They were formed by concretionary growth in anoxic sediment around sites of organic decay by a similar mechanism to that producing the London Clay concretions. Miocene teeth and bones with a low organic content were phosphatized by an apatite replacement process. This replacement process also enriched the phosphate content of derived phosphatic fossils and phosphorite concretions from the London Clay. Phosphorite formation in eastern England was linked with periods of transgression, while periods of regression were important in concentrating dense phosphatic material as a remanié deposit.
Apatite-glauconite associations off Peru and Chile: palaeo-oceanographic implications
Authors: Burnett William C.
Published: December 2007

Authigenic apatite and glauconite occur within organic-rich diatomaceous sediment from the coastal upwelling zone off Peru and Chile. Both minerals are found as unconsolidated pellets and within indurated phosphate nodules. Uranium disequilibrium dating has shown that the apatite may range from present-day to +100,000 yr (Quaternary) in age. Episodes of phosphorite formation appear to be related to major climatic variations. The occurrence and progressive alteration of glauconite within some of these phosphate nodules also appears to be related to large scale oceanographic phenomena which fluctuated during the Quaternary. A lateral shifting or contraction of the oxygen minimum zone and locus of maximum upwelling in response to climate variations best explains the data.
Ensialic spreading-subsidence in the Mesozoic and Palaeogene Andes of central Chile
Authors: Levi B. , Aguirre L.
Published: December 2007

An oceanic plate subducting eastwards below a continental margin and resulting in crustal shortening can be inferred to have been active in central Chile during the Palaeozoic. However, the geological record for the Mesozoic and Palaeogene shows evidence of a persistent tensional régime and absence of crustal shortening. Although lateral asymmetry, inherent to a subducting model, is shown by this record, a bilateral symmetry is also well substantiated. A coupled action of plate subduction and spreading-subsidence, the latter taking place within the margin of a continental plate, is suggested to explain these features.
Phanerozoic plate boundaries in mainland SE Asia, the Himalayas and Tibet
Authors: Mitchell A. H. G.
Published: December 2007

Recently available geological data on Tibet, Yunnan and Burma facilitate recognition of Phanerozoic subduction systems and two continental fragments of Gondwanaland which extend through mainland SE Asia. A Cambrian subduction system and 5 Mesozoic-early Caenozoic collision belts are identified in the region. Indochina, eastern Thailand and Central Tibet accreted to China in the early Triassic; western SE Asia and S Tibet separated from Gondwanaland in the Permian or early Triassic and collided with Asia in the late Triassic, and the western Burma island arc system collided with Asia in the Jurassic. Upper Triassic flysch and schist in the eastern Indoburman Ranges were accreted to western Burma in the Jurassic-early Cretaceous during eastward subduction and formation of a back-arc thrust belt. Following collision of Greater India with arc complexes to the N in the Cretaceous, and with Tibet in the early Eocene, western Burma moved northwards about 450 km along the Sagaing-Namyin ridge-thrust transform fault. Formation of the 3 major belts of tin granites and the porphyry copper deposits in the region took place in specific tectonic settings within the subduction systems and collision belts.
Processes in magma chambers beneath spreading axes: evidence from magmatic associations in the Oman Ophiolite
Authors: Browning P. , Smewing J. D.
Published: December 2007

Ophiolite complexes, where complete, consist of tectonized ultramafic rocks overlain by layered gabbros and peridotites, isotropic gabbros and plagiogranites, a sheeted dyke swarm and pillowed volcanics. This pseudo stratigraphy has been interpreted as oceanic crust and upper mantle (
Dewey & Bird 1971
;
Moores & Vine 1971
), the base of the crust being drawn at the base of the layered series (
Malpas 1978
). In Oman, this succession is particularly well exposed in a 500 km long, 14 km thick ophiolite nappe of Upper Cretaceous age (

Glennie
et al.
1973

). Magmatic associations in its lower crustal and upper mantle portions (oceanic layers 3 and 4) are currently being studied to determine the processes of magma supply and fractionation in oceanic magma chambers. In the first section of this report we list the evidence for the greater part of the plutonic sequence of the ophiolite forming in a dynamic, long-lived, open system magma chamber beneath a spreading axis. In the second section the remainder of the plutonic sequence is described as a series of later, short-lived, possibly closed-system, off-axis plutons. A fuller treatment of magma chamber processes during the spreading event is presented elsewhere (
Smewing, in press
).

The earliest and volumetrically most significant magmatic event is represented by up to 3.5 km of layered gabbros and peridotites and an overlying unit of up to 200 m of isotropic gabbros and plagiogranites intruding the base of the sheeted dyke swarm. The layered series rests with a sharp contact
Magma chamber below Poàs volcano, Costa Rica
Authors: Thorpe R. S. , Locke C. A. , Brown G. C. , Francis P. W. , Randal M.
Published: December 2007

Direct field evidence for links between volcanic rocks and contemporaneous subvolcanic intrusions are difficult to establish. Such links might, however, be inferred by making physical measurements on a youthful active volcano. Poás is a small composite volcano (elevation 2700 m, diameter 20 km) which has evolved to its present form by a sequence of caldera- and crater-forming episodes, possibly during the last 5 × 10
4
y. There has been a high level of historic activity from the active crater, which is 1 km in diameter and 300 m deep, and which contains a hot lake. Lavas and dominant pyroclastic rocks exposed in the active crater calc-alkaline basaltic andesites and andesites. Poás volcano is therefore typical of those in island arcs and young continental margins. Gravity measurements show that the volcano has a regional negative Bouguer anomaly, of amplitudes 100–200 gu, upon which is superimposed a closed positive anomaly, in the active crater area, about 2 km in diameter and with a maximum amplitude of 100 gu. This is thought to indicate a 1 km radius cylinder of solid rock of basaltic andesite or andesite composition with a shallow upper surface and extending to a depth of several kilometers. The concept of a large shallow magma chamber, with a diameter similar to that of the volcano, appears not to be appropriate for this volcano.
Manganese nodules and other ferromanganese oxide deposits from the Indian Ocean
Authors: Cronan D. S. , Moorby S. A.
Published: December 2007

Ferromanganese oxide concretions from the Indian Ocean exhibit considerable mineralogical and chemical variation, often differentiated on a regional basis. Nodules richest in Mn, Ni, Cu and Zn are found in sub-equatorial basin areas, but enrichments of some of these metals also occur in nodules elsewhere, particularly where there is evidence of major biogenic contributions to their underlying sediments. Fe, CO and Pb are most enriched in concretions from sea-mounts and the mid-ocean ridge system, but also attain high values in some basin areas. Local variations in concretion composition have been found off southern Africa, and are due to both biological and continental influences.

There are several major controls on the compositions of the concretions. Source variations influence composition in that Mn, Ni and Cu show evidence of diagenetic enrichment in deposits after being cycled through the biogenic component of sediments, whereas, Fe, CO and Pb appear to be supplied mainly by normal seawater. Environmental influences, including bottom water characteristics, affect the degree of dissolution of biogenic remains and this affects the sediment type and the supply of metals to the deposits and also their mineralogy, which in turn affects trace metal uptake. In the Central Indian Basin, source and environmental influences combine to produce ‘ore grade’ nodules.
The metamorphic environment of the Sgurr Beag Slide; a major crustal displacement zone in Proterozoic, Moine rocks of Scotland
Authors: Powell D. , Baird A. W. , Charnley N. R. , Jordan P. J.
Published: December 2007

New evidence from the SW Northern Highlands of Scotland establishes the southerly extension of the Sgurr Beag Slide. Interpretation of changes in the mineralogy of calcsilicate rocks, particularly the use of the anorthite content of plagioclase feldspar as an index of metamorphic grade, together with consideration of the texture and mineralogy of pelitic rocks, suggests that the slide juxtaposes crustal segments of different metamorphic grade and history.Movement on the slide post-dates a major phase of regional metamorphism and two phases of regional deformation of Precambrian age, was synchronous with a regional fold-forming event and regional metamorphism at or before
c.
467 Ma (Caledonian), and was followed by regional folding.

Models of the thermal effects of displacement across syn-metamorphic shear zones are investigated with particular reference to the roles of thermal relaxation and levels of shear strain. A model based on the development of a ductile, syn-metamorphic, asymmetric shear zone best explains the metamorphic patterns associated with the Sgurr Beag Slide. An original low to moderate easterly dip is implied, with north-westerly upthrusting of the eastern crustal block.
Cyclicity of Devonian fluvial sedimentation, Cunningsburgh Peninsula, SE Shetland
Authors: Allen P. A.
Published: December 2007

The Cunningsburgh area, SE Shetland, contains approximately 1000 m of fluvial sediments deposited in four sub-environments. Ripple cross-laminated siltstones and sandstones represent levee-type sediments, horizontally- and solitary cross-stratified sandstones were deposited by overbank sheet floods; red mudstones with sandstone lenses represent floodbasin sediments and grouped cross-stratified sandstones are substratum sediments deposited in the channellized zone as transverse bars. point bars and megaripple channel-fills.

Transition, entropy and substitutability analyses indicate that in the S of the Cunningsburgh area cyclicity was controlled by two processes: one was that of channel migration, cut-off or avulsion, producing scoured surfaces overlain by substratum cross-stratified sandstones. The other was that of overbank flooding, producing an interbedding of overbank sheet flood deposits, levee-type sediments and floodbasin fines. Further to the N, distant from the main channellized zone, overbank sheet flood deposits are more common and are interbedded with levee and floodbasin deposits. Cyclicity in this zone was, therefore, controlled essentially by overbank flooding alone but not by channel migration or shifting.

The fluvial system was characterized by a central zone containing channellized high-sinuosity flow which was bordered, within the same alluvial valley, by low-lying floodplains subject to flooding by both crevassing and overtopping of levees. The main stream only rarely migrated into this outer floodplain.
Wrench and thrust tectonics along a Mesozoic–Cenozoic continental margin: Antalya Complex, SW Turkey
Authors: Woodcock N. H. , Robertson A. H. F.
Published: December 2007

The segment of the Antalya Complex W of Antalya Bay displays the results of latest Cretaceous to Miocene deformation on a previously passive Mesozoic continental margin. The 300 km
2
area detailed here includes parts of, from W to E, a Mesozoic carbonate platform, its bordering sediment margin and the adjacent marginal oceanic crust with sediment cover. The platform is folded on N–S axes and has been overthrust westward by its sediment margin. This thin sediment sequence has been detached from its basement in a spectacular W-verging imbricate thrust belt. Similar, though less organized, westward thrusting occurs in the thin sediment cover to the marginal oceanic crust, and may also affect the igneous basement itself. However, this part of the margin is dominated by an array of anastomosing subvertical screens of serpentinite up to several kilometres wide. These are thought to be cool, upward protrusions of the hydrated ultramafic substratum to the marginal crust into a then active, braided, strike-slip fault system. The fault strands now divide the intact marginal rocks into large lozenges, some of which have oblique and sigmoidal structures and steeply plunging folds. The thrust and wrench tectonics may have operated in concert in a regionally transpressive regime, or thrusting may have accompanied only the initiation of wrench tectonics. The local interplay of transpression and transtension was partly responsible for the present complexity of the area. Even more important was the original mechanical inhomogeneity of the margin, particularly the inclusion of rifted-off continental fragments in the marginal ocean crust and the strong basement/cover contrasts on the margin.
Deep boreholes, seismic refraction lines and the interpretation of gravity anomalies in Norfolk
Authors: Chroston P. N. , Sola M. A.
Published: December 2007

Deep boreholes and seismic refraction data are reviewed and used to constrain the interpretation of gravity anomalies in Norfolk. The sub-Carboniferous rocks range from ? Precambrian to Devonian in age, with the Palaeozoic sediments showing shales, mudstones and siltstones, often steeply dipping and cleaved. Seismic refraction lines show that compressional wave velocities in the sub-Mesozoic floor range from 3.5 to 6.0 km s
−1
. High velocities (5.6–6.0 km s
−1
) are found in N and NW Norfolk, low velocities (3.5–4.5 km s
−1
) in W Suffolk and Cambridgeshire, and intermediate velocities in the rest of the area. A broad gravity low is found in W Norfolk, and two interpretive models are considered: either a thick sequence of Palaeozoic sediments, or granitic intrusions in the metamorphic basement. The latter is preferred.
Extensional tectonics in Neogene and Quaternary sequences at the western margin of the Axios basin, northern Greece
Authors: Brooks M. , Williams G. D.
Published: December 2007

Neogene and Quaternary sequences close to the western margin of the Axios basin, northern Greece, have been studied to investigate the nature and extent of their neotectonic deformation. Synsedimentary deformation is indicated by conjugate growth faults and shear zones, and by multiple angular unconformities. Where possible, palaeo-stress analysis has been carried out and the majority of the synsedimentary faults are shown to be dynamically controlled and to have developed in response to horizontal NNE-SSW extension. This result, which is consistent with recent earthquake focal mechanism studies in the region, suggests a similarity in the neotectonic evolution of the northern and southern Aegean areas during late Cenozoic times.
Copper, lead, zinc and cobalt mineralization in the English Lake District: classification, conditions of formation and genesis
Authors: Stanley C. J. , Vaughan D. J.
Published: December 2007

The copper, lead, zinc, and minor cobalt mineralization of the English Lake District has been classified on the basis of mineralogy, spatial distribution and geological setting of the veins, and age of mineralization. The two major types of mineralization are: (1) ‘chalcopyrite-pyrite-arsenopyrite’, of Lower Devonian (Caledonian) age, and (2) ‘galena sphalerite’ of early Carboniferous age. When the former is closely associated with granite intrusions, more complex mineral assemblages are found involving tungsten or molybdenum bearing phases. Also distinguished from these two major types are later assemblages which probably result from their alteration. A later (Upper Carboniferous–Permian) baryte mineralization is also present. Minor graphite and stibnite deposits, and the major haematite mineralization found in the area, are incorporated into the classification but are not discussed, since they have been dealt with by other authors.

Using data from fluid inclusions and experimental data on particular mineral assemblages occurring in the major types of mineralization, estimates have been made of the temperatures and compositions of the ore-forming fluids. The Lower Devonian veins formed at temperatures in the range of 350–200°C with typical
a
S
2
, values of 10
−10
–l0
−16
atm and
a
O
2
of ~10
−25
–10
−45
atm or even less. The solutions were probably brines of low to moderate salinity (~5–10 equiv. wt% NaCl). The Lower Carboniferous veins formed at temperatures in the region of 110–130°C from highly saline brines (~23 equiv. wt% NaCl).

Theories regarding the origins of the mineralization are also discussed. The distribution of Lower Devonian veins shows a clear relationship to the underlying composite granite batholith; the Lower Carboniferous galena-sphalerite veins show no such relationship, but tensional tectonic activity in the early Carboniferous may have provided a mechanism for the opening of fissures. The source of the mineralizing fluids forming the Lower Devonian veins may have been the Borrowdale Volcanic Group, with contributions from fluids associated with the granites, which are also likely to have provided a source of heat. The high salinities of the fluids producing the Lower Carboniferous veins suggest that seawater may have been involved in leaching the metals from basement rocks.
The Cenozoic subduction history of the Pacific margin of the Antarctic Peninsula: ridge crest–trench interactions
Authors: Barker P. F.
Published: December 2007

New magnetic anomaly identifications W of the Shackleton Fracture Zone show 5 spreading sections, separated by fracture zones. In the 2 most southerly, the ridge crest collided with a trench at the margin of the Antarctic Peninsula only 6.5 and 4 Ma ago, the latest of a series of collisions starting at the base of the peninsula SO Ma ago. Following each collision, spreading and subduction both stopped. Opposite the South Shetland trench and actively extending Bransfield Strait, spreading also stopped 4 Ma ago, but before the spreading centre reached the trench. The tendency of the subducting plate to continue sinking probably initiated the opening of the Bransfield Strait.

The parallelism between fracture zone orientation on the descending plate and convergence direction, previously thought responsible for the tectonic segmentation of the peninsula, was effective for only 10–30 Ma before collision. Before that, either the fracture zones did not extend to the trench, or the subducting ocean floor formed at the Farallon–Phoenix (Nazca–Aluk) boundary, with fracture zones oblique to the subduction direction. The Aleutian Arc–Kula Ridge model, in which arc magmatism virtually ceases when ocean floor younger than 25–30 Ma occurs beneath the arc, fits the distribution of Antarctic Peninsula radiometric dates, explaining a 50–60 Ma gap between collisions and youngest ages in the S, and possibly the migration of the youngest activity towards the trench. Thus, gaps in the geologic record of arc magmatism need not imply cessation of subduction. The progressive steepening of the peninsular margin towards the 4 Ma collision site suggests tectonic erosion of the fore-arc as the ridge crest approached.
Thrust tectonics of S Devon
Authors: Coward M. P. , McClay K. R.
Published: December 2007

The tectonics of the area from Start Point to Chudleigh, SW England, are shown to be dominated by northward-verging thrust nappes. Two major thrusts are postulated. The ‘Dartmouth antiform’ is considered to be a major thrust structure, not a fold structure as previously interpreted. The Torquay limestones are allochthonous and part of a klippe of a major thrust nappe with a minimum displacement of 13 km northwards. A progressive and diachronous sequence of deformation has been established with deformation starting in the S and moving N. Many of the folds and cleavages are probably related to the strains developed locally in thrust tips and may have no regional correlation. More intense and complex deformation is found in the S compared to the N. The diachroneity is also reflected in the K-Ar mineral ages. In the S, at Start Point, southward-verging backfolding is the last significant Hercynian deformation phase folding the thrust nappes.
Neomorphic spar versus cement in some Jurassic grainstones: significance for evaluation of porosity evolution and compaction
Authors: Bathurst R. G. C.
Published: December 2007

Thinly-bedded Upper Bathonian grainstones with shales, in Wiltshire, UK, have intergranular spar of 2 kinds, a primary calcite cement and a neomorphic spar that has replaced an earlier fibrous cement. The earlier cemented grainstones have been less compacted than those cemented by a late calcite spar (fewer broken grains, fewer grain contacts, more intergranular spar). The crystals in the neomorphic intergranular spar are mostly subequant, have irregular intercrystalline boundaries, patchy extinction, some dusty relics of earlier cement fringes and, with cathodoluminescence, show bright orange, needle-like relics of earlier cement fabric. Most of this subequant neomorphic spar is indistinguishable from the neomorphic spar that has replaced the aragonite of molluscan shells. It is concluded that calcitization of an early cement and of the aragonite shells, along with the precipitation of the true cement spar, were delayed until after the carbonate sands had undergone some compactive fracture and grain-grain pressure-solution.
The trace element and isotope geochemistry of the Sabaloka Igneous Complex, Sudan
Authors: Harris N. B. W. , Duyverman H. J. , Almond D. C.
Published: December 2007

The Sabaloka Igneous Complex comprises 2 igneous centres of 'within-plate' magmatism, intruded into Precambrian gneisses in northern Sudan. The Cauldron Complex is the larger centre and was emplaced at 383 ± 14 Ma. The earliest felsic magmas are highly fractionated, peraluminous rhyolites with isotope and trace element characteristics indicative of either a biotite tonalite–granodiorite source or of amphibole fractionation from a metalumi-nous melt. The rhyolites are overlain by ignimbrites, which have similar trace element characteristics to the rhyolites but are less aluminous. A mica granite boss intrudes both rhyolites and ignimbrites and has an unuaual trace element geochemistry, which is ascribed to volatile loss during emplacement. The Cauldron Complex is circumscribed by a microgranite ring dyke, which has isotopic and trace element characteristics which allow the microgranite magma to be the source from which the rhyolites fractionated; the microgranite itself requires a monzonite or syenite source. The subsidiary Tuleih Complex was emplaced at 465 ± 14 Ma and is largely composed of a silica saturated–marginally undersaturated syenite. Trace elements and isotopes from the syenite indicate a garnet eclogite source of mantle origin. The Sabaloka intrusions provide an example of several periods of magmatism contributing to the same igneous complex, although not necessarily to the same igneous centre.
Tectonic evolution of SW Wales during the Upper Palaeozoic
Authors: Dunne William M.
Published: December 2007

During the Upper Palaeozoic the development of several strike faults strongly influenced the deformation history of SW Wales. This aspect of the deformation is emphasized in this new description of the tectonic evolution of the region.

After the Caledonian orogeny, several strike faults caused crustal extension by normal dip-slip and partially controlled the subsidence of depositional basins. These faults ceased moving before the initiation of Variscan deformation in SW Wales. Two of the earlier faults were reactivated with reverse-slip translations during Variscan deformation, when three strike faults were initiated as a result of gravitational spreading. Two of the new faults formed sole thrusts for northward-translating nappes, and the other developed as a subsidiary lag fault. The sole thrusts transected some of the pre-existing faults, causing along-strike variation in their motion. The strike faults in SW Wales were therefore initiated at different times and display contrasting displacement histories.
Petrochemistry and tectonic significance of the Highland Border Suite mafic rocks
Authors: Ikin N. P.
Published: December 2007

The metamorphosed mafic rocks of the ophiolitic Highland Border Suite on Arran and Bute are of 2 types: greenschist facies pillow lavas and epidote-amphibolite facies hornblende schists, respectively. Both types have been affected by post-magmatic alteration processes, including spilitization (i.e. Na-enrichment coupled with Ca-depletion). On the basis of 'immobile element' chemistry and other criteria, the mafic rocks are split into 2 (magmatic) groups: a tholeiitic one which shows the effects of extensive low-pressure fractionation (e.g. Fe- and Ti-enrichment), and a less-fractionated alkaline group that contains relict Al- and Ti-rich clinopyroxenes. The tholeiitic rocks are similar to modem ocean-floor or marginal-basin basalts, and the alkaline rocks show affinities with 'within-plate' basalts. This 'magma-type' and 'tectonic-setting' dichotomy is found at other Highland Border localities and is present in both the pillow lavas and hornblende schists. The Highland Border Suite mafic rocks may have formed as tholeiitic ocean-basin crust upon which alkaline ocean-island lavas had been erupted, or alternatively, as is suggested in this paper, they may have formed during the early stages of development of a pre-Grampian marginal basin of Cambrian (or Cambro-Ordovician) age.
Shallow marine and continental conglomerates derived from coral reef complexes after desiccation of a deep marine basin: the Tortonian-Messinian deposits of the Fortuna Basin, SE Spain
Authors: Santisteban C. , Taberner C.
Published: December 2007

Some Miocene basins in SE Spain show an unusual arrangement of sedimentary facies associated with desiccated deep basins. Tortonian reefs and deltaic deposits are located around the basin margins but younger Messinian evaporites are exposed in central areas. These Messinian sediments contain thick conglomerates rich in coral debris. Although located hundreds of metres below the Tortonian reefs, they show subaerial features, including mud-cracks, vertebrate tracks, and rain-pits. In the coral reef derived conglomerates there are aragonitic coral fragments with aragonitic fibrous cement preserved by early cementation with calcium sulphate. The existence of these conglomerates within the Messinian evaporites helps to identify normal salinity and desiccation stages of the 'Messinian Salinity Crisis Event'.
Fluid-absent metamorphism
Authors: Thompson A. B.
Published: December 2007

Many interpretations of metamorphic parageneses tacitly assume that a free fluid phase was always present during metamorphic equilibration (fluid-present metamorphism), as opposed to the circumstance that, while volatile species are certainly released from minerals during metamorphism, these species may not saturate the grain boundaries to produce free fluid (fluid-absent metamorphism). H
2
O-activity has meaning for both fluid-present and fluid-absent metamorphism and may be evaluated if an equilibrium may be written involving H
2
O and the temperature and rock-pressure of equilibration are known. P

H
2
O

has only a physical meaning for fluid-present metamorphism.

Although the effects of an aqueous fluid on displacing reactions in P–T space are well known, as are the enhanced kinetic and deformational effects of excess fluid, there is actually little compelling evidence to suggest that the presence of aqueous fluids is common during metamorphism of the middle and lower crust, and the mantle, for the duration of most dynamic processes, including partial melting. Possible exceptions are discontinuous dehydration reactions which affect large compositional ranges of metamorphic rock over narrow pressure–temperature intervals and which could locally produce a free liquid if production rate exceeds escape rate.

Evidence from stable isotopes and fluid inclusions obviously points to the accumulation of free aqueous fluid in definite locations for limited duration, but cannot, unfortunately, be used as evidence for generalized fluid transport during regional or contact metamorphism. Both mineralogical and textural criteria presently used to infer ubiquitous fluid, must be examined in terms of crustal volume and duration of dynamic processes to deduce the presence or absence of free fluid. If volatile components ever saturate grain boundaries, then the fluid-phase may only exist locally and perhaps only for short periods of time. Flow of metamorphic fluid may be focused into narrow regimes that could be quite widely spaced. Fluid-present metamorphism should not be considered as typical of whole crust evolution.
The Upper Proterozoic ophiolite mélange zones of the easternmost Arabian shield
Authors: Al-Shanti A. M. , Gass I. G.
Published: December 2007

The Arabian-Nubian shield was produced by the episodic cratonization and accretion of intra-oceanic arc systems over the period 1100–500 Ma BP. The easternmost part of the Arabian shield is dominated by the N-S Al Amar fault zone, long believed to represent an easterly-inclined Upper Proterozoic suture between either two island arc systems or oceanic and continental lithosphere. In the area studied there are two N–S ophiolitic mélange zones which separate a central belt of metamorphosed trench sediments (Abt Schists) from flanking granitoid zones that represent the plutonic cores of arc systems. The mélange blocks are of layered gabbro, chromitite, homogeneous gabbro with dykes, plagiogranite, sheeted dykes and pillow lavas set in a sheared serpentinite matrix. The blocks are large (
c.
1 km) and gabbroic in the western zone and smaller (1–100 m) and of fine-grained basic rocks in the eastern zone; all are compositionally similar and petrographically typical of rock types found in ophiolite complexes elsewhere. However, the Zr, Ti and Nb content of all these rocks is too low even for the back-arc, marginal basin setting commonly invoked for ophiolite genesis. To explain the unusually low high field strength element abundances, it is suggested that either these rocks were generated in or near a transform fault zone and then deposited in an oceanic trench when they began to be subducted, or that their boninitic character identifies them as the products of fore-arc igneous activity and, by analogy with the tectono-magmatic evolution of the W. Phillipine-Mariana region in the W Pacific, they mark the site of arc separation prior to back-arc sea-floor spreading; the latter explanation is preferred.
Crustal structure beneath southern England from deep seismic reflection profiles
Authors: Chadwick R. A. , Kenolty N. , Whittaker A.
Published: December 2007

Seismic reflection events from recently acquired profiles in southern England are interpreted in terms of stratigraphic and structural interfaces between rock units. In particular, evidence is brought forward that a series of reflection events, dipping between 22° and 27°, with azimuth averaging 160°, represents 2 major thrust zones occurring in the Variscan floor beneath Mesozoic cover. A geological model based upon borehole information and regional geological considerations is presented. The tentative recognition of crystalline basement overlain and overstepped by late Proterozoic rocks of variable thickness suggests that basement in this area was unstable from late Precambrian times. The Mesozoic structural evolution can be related to tectonic re-activation of major concealed thrust zones in the Palaeozoic 'basement'.
The petrology and geochemistry of dykes of the Lizard Ophiolite Complex, Cornwall
Authors: Kirby G. A.
Published: December 2007

Dykes from the coastal section of the Lizard ophiolite, SW England, have been analysed for both major and trace elements. The dykes vary from fresh olivine dolerites to lower amphibolite grade metadolerites. All are basaltic and tholeiitic and possess broadly oceanic characteristics.

Three magma groupings can be defined geochemically on the basis of trace element contents, particularly rare earth elements and P/Y ratios. These magma groupings are thought to have been generated by differing degrees of partial melting and from differing source rocks. Each group is thought to have undergone fractional crystallization. The spatial and suggested temporal separation of these groups supports a pulsatory magma chamber model for the spreading centre at which the Lizard ophiolite developed.
Major crustal reflectors beneath SW England
Authors: Brooks M. , Doody J. J. , Al-Rawi F. R. J.
Published: December 2007

Strong, wide-angle reflection events were detected during a crustal seismic experiment carried out in SW Britain in 1979. Three crustal reflectors have been identified, referred to as Rl, R2 and R3. Rl lies at a depth of about 8 km and appears to be restricted to the interior of the granite batholith underlying SW England. The nature of this reflector is unknown. R2 is a mid-crustal reflector lying at a depth of 10–15 km and interpreted as rising gradually northwards. It coincides locally with the base of the granite as predicted by gravity modeling, but extends well beyond the northern limit of the granite body. It is suggested that R2 may represent a major thrust of late Variscan age. R3 is the deepest reflector identified and lies at a depth of 27–30 km. R3 is interpreted as the base of the crust and its computed depth is in close agreement with the local depth to the Moho derived from an earlier seismic experiment in SW Britain.
Origin of some mixed-magma and net-veined ring intrusions
Authors: Marshall L. A. , Sparks R. S. J.
Published: December 2007

Most of the ring complexes in the British Tertiary Igneous Province include at least one net-veined or mixed-magma ring-dyke. Net-veined ring-dykes consist of a granophyre host rock containing mafic pillows, with areas of granophyric net-veined mafic rock. Mixed-magma ring-dykes contain mafic micropillows dispersed in a silicic host rock. The ‘mafic’ component covers a wide compositional range with basaltic-andesite to andesite abundant. These magmas represent both fractionates of basaltic magma and hybrids of silicic, intermediate and mafic magma. Analogous compositional diversity in extrusive rocks of Hekla and Askja, Iceland, indicate that the Hebridean composite ring-dykes originate from compositionally zoned magma chambers. Density relationships suggest that zoning in chambers of tholeiitic systems can only result for magmas more evolved than ferrobasalt. Zonation can result from crystallization on the sidewalls and roof of the chamber (producing a boundary layer flow of less dense fluid), wall melting and magma replenishment. Hybrid mafic pillows originate by mixing within the chamber, especially when lower-density olivine tholeiite is emplaced beneath ferrobasalt magma. Subsidence of a crustal block into an underlying compositionally and thermally zoned chamber results in commingling of silicic, intermediate and mafic magmas within both the reservoir and the ring-dyke, producing pillows and net-veining. Mixing is incomplete due to the contrasted viscosities and solidus temperatures of the magmas. Mixing and net-veining is also enhanced by volatile exsolution and explosive emplacement of magma at sub-volcanic levels, with mixed magma eruptions sometimes occurring at the surface.
Applications of electron microscopy in metamorphic and structural geology
Authors: Brodie K. H.
Published: December 2007

Report on a joint meeting of the Metamorphic Studies Group and Tectonic Studies Group at Burlington House on 24 November 1982.

The meeting was opened by two introductory talks on the techniques of electron microscopy and microanalysis. Knipe outlined the basic principles involved in various formsof electron microscopy(scanning, transmission and high resolution) and the range of applicability of such methods for tackling geological problems. The combination of microstructural, crystal-lographic and geochemical information which can be obtained simultaneously from samples makes these techniques ideal for application to metamorphic rocks. Champness introduced the method of thin film analysis, contrasting this with conventional electron probe microanalysis and noting some of the problems involved and how they could be minimized. As an example of the applications of this technique she described work to examine the cause of brown staining in an olivine. TEM and microanalysis showed that particulate phases of diopside, chlorite and Fe oxide were all included in the olivine.

Lloyd described the method of backscattered SEM, both for atomic number contrast and electron channelling. He outlined the optimum sample preparation methods and illustrated someof the applications of both methods. Atomic number differences in the order of 0.1 can be imaged, enabling complex chemical zoning and inclusions to be studied, and indexing of electron channelling patterns allow variations in crystallographic orientations to be investigated between, for example, subgrains. between, for example, subgrains.

Two talks illustrated the usefulness of electron microscopy in studying experimental charges, where very small amounts of material are involved and
Topographic control on Precambrian weathering in the Elliot Lake uranium district, Canada
Authors: Kimberley M. M. , Grandstaff D. E. , Tanaka R. T.
Published: December 2007

The voluminous uraniferous conglomerate beds at Elliot Lake, Canada lie directly above or within several metres of a palaeosol that developed on Archaean granite and greenstone (metabasalt). This Lower Proterozoic paleosol provides evidence for the environment in which the uraniferous conglomerate accumulated. Previous investigators have interpreted a few analyses of the palaeosol to indicate palaeoweathering under an atmosphere devoid of free oxygen. However, we have independently analysed in detail three oxidized palaeosols that formed contemporaneously with reduced palaeosols. The degree of oxidation correlates well with palaeoelevation.
Possible microbial pathways in the formation of Precambrian ore deposits
Authors: Dexter-Dyer Betsey , Kretzschmar Manfred , Krumbein Wolfgang E.
Published: December 2007

Extant micro-organisms and their products interact with heavy metals in a variety of ways. Consideration of these activities provides a background against which possible influences of microbes on the mobilization, accumulation and deposition of heavy metals in modern and ancient environments may be examined. Several different interactions between micro-organisms and their products and heavy metals are described. Experiments on the uptake and metabolism of gold, uranium, iron and other heavy metals are reported. Several examples from recent studies and younger fossil deposits are reported and discussed concerning their applicability to Precambrian conditions.
Subduction of pelagic sediments: implications for the origin of Ce-anomalous basalts from the Mariana Islands
Authors: Hole M. J. , Saunders A. D. , Marriner G. F. , Tarney J.
Published: December 2007

Attempts to assess the significance of subducted sediments in the genesis of island arc magmas have been limited by the lack of trace element data on pelagic oozes. Accordingly, we have analysed a series of pelagic clays and nannofossil oozes from the Nazca Plate for REE and other trace elements. A calculated average––Pacific Authigenic Weighted Mean Sediment (PAWMS)––exhibits light REE-enrichment (La

n

/Yb

n

∼4.5), high contents of Ba and Sr, but low abundances of Rb, Nb and Ta. Most significant, however, is the occurrence of large, negative Ce anomalies (Ce/Ce*~0.2). We have attempted to model the contribution of PAWMS-type material to the source of the magmas of the Mariana Island Arc, an intra-oceanic arc far removed from the effect of continent-derived detritus. Only small amounts of pelagic sediment, between 0.3 and 0.5% of the source, are required to develop the small negative Ce anomalies, high Ba/La ratios, and high LIL/HFS element ratios characteristic of these and other arc lavas. However, a small fluid contribution from the dehydrating subducted oceanic crust is required to produce the high Rb/Ba observed in several Mariana suites. The ternary mixing between sediment, mantle host and dehydrating oceanic crust also produces very low Nb and Ta abundances in the arc lava source. However, the very high abundance of Sr (>1000ppm) and the estimated high
87
Sr/
86
Sr ratio (~0.7150) of PAWMS, results in a model
87
Sr/
86
Sr ratio of 0.7070. This is far higher than the measured ratios in the Mariana arc lavas (0.7033–0.7040) and may suggest that the subducted sediment has a lower Sr content (<200 ppm) or a lower
87
Sr/
86
Sr ratio, or that the carbonate-rich component is not involved in source contamination.

Volumetrically it would appear that much of the sediment approaching the Mariana trench (~45 km
3
Ma
−1
per km of arc) may be recycled into the deeper mantle. This will have the effect of introducing high Ba, Sr, Th,
87
Sr/
86
Sr,
208
Pb/
204
Pb and
207
Pb/
204
Pb material into the mantle. However, incorporation of such material cannot alone satisfactorily account for the trace element and isotope chemistry of ocean islands; oceanic sediments have LIL/Ta, Nb ratios far too high to produce the trace element characteristics of most intraplate magmas.
Contemporaneous faulting, and the eruption and preservation of the Lough Guitane Volcanic Complex, Co. Kerry
Authors: Avison Mark
Published: December 2007

Interbedded with the clastic fluvial sediments of the Munster Basin, the Lough Guitane Volcanic Complex represents a rare volcanic episode in the Upper Devonian of SW Ireland. From three distinctly separate volcanic centres, an exclusively rhyolitic suite of thick lava flows and tuffs were erupted. The largest of the centres possessed a system of contemporaneous faults producing a subcircular depressed area, itself enclosed within a graben 1.5 km across. These faults lead to rapid thickness variations in many volcanic units, and a pooled lava flow reaches a thickness of 300 m. The site of a major elongate vent lies along one such fault, and rapid erosion of its poorly lithified walls generated coarse sandstone boulder agglomerates and is related to the absence throughout the complex of normal air-fall tuffs unmixed with terrigenous sand.

The containment of the bulk of the volcanics by contemporaneous faults, and subsequent rapid burial by terrigenous sediments has led to preservation of the volcanic succession virtually intact.
Potassium-Argon ages from the Arequipa Segment of the Coastal Batholith of Peru and their correlation with regional tectonic events
Authors: Moore N. D.
Published: December 2007

Detailed geological mapping of the Ica-Pisco valleys in southern Peru has provided a strict control on K-Ar ages obtained from the plutonic rocks of the Arequipa Segment of the Coastal Batholith of Peru. It is concluded that although the batholith in the Ica-Pisco region was emplaced over a total time span of up to 26.5 Ma active plutonism was confined to two relatively short lived episodes between 107–97 Ma and 82.5-80.5 Ma. This is in contrast to the four main episodes of intrusion in the Lima Segment that spanned about 60 Ma. Correlation of the K-Ar ages and mineral fabrics contained within the plutonic rocks of the Arequipa Segment indicates that emplacement was associated with periods of regional compressive tectonism which, in turn, may be linked to variable rates of subduction.
The Jebel Thurwah Upper Proterozoic Ophiolite Complex, western Saudi Arabia
Authors: Nassief M. O. , Macdonald R. , Gass I. G.
Published: December 2007

The Jebel Thurwah manc-ultramafic igneous complex occupies some 40 km
2
in W-central Saudi Arabia and is a largely inverted, structurally dismembered mass of tectonized harzburgite with dunitic lenses, layered mafic and ultramafic plutonic rocks, varitextured gabbros, tonalites and plagiogranites, sheeted dykes and pillow lavas. Major oxide and trace element geochemistry suggest that the complex was originally oceanic lithosphere formed above a subduction zone. During a subsequent collision event, this ophiolite was thrust on to an Upper Proterozoic island arc sequence composed largely of andesitic-rhyodacitic volcanic rocks of tholeiitic transitional to calc-alkaline affinity. Regional studies indicate that the Thurwah mass lies in a NE-SW zone of ophiolite complexes extending from Bir Umq, 150 km to the NE, to the Khor Nakasib and Wadi Amur bodies in the NE Sudan, that marks a suture between island arc systems subsequently accreted to form the Upper Proterozoic continental crust of the Arabian-Nubian Shield.