bluuebunny/crossref_metadata_2025 huggingface dataset:
Geochemical discrimination and petrogenesis of alkalic basalt sequences in part of the Ankara melange, central Turkey
Authors: FLOYD P. A.
Published: December 2007

East of Ankara, Turkey, within the major thrust-bound tectonic units of the Ankara melange (Karakaya and Anatolian Nappes), are numerous blocks of low-grade metamorphosed submarine basaltic lavas. Stable incompatible element geochemistry indicates that the majority of these basalts are alkaline and exhibit a range of characteristics typical of many within-plate oceanic islands (OIB). There is a broad chemical overlap between alkaline basalts from within the Triassic Karakaya Nappes (metamorphic and limestone block melanges) and the Cretaceous Anatolian Nappe (ophiolitic melange). A complete sequence of submarine alkaline basalt lavas was sampled within the ophiolitic melange (the 500 m Kiliclar section) to illustrate the variation and petrogenesis of a small seamount-related crustal segment, prior to tectonic dismemberment. The basalts can be divided into three chemical groups that relate to discrete petrographic segments of the lava pile and probably reflect variable degrees of partial melting of a garnet-bearing enriched source. The chemistry of alkaline basalts within the ophiolitic melange, together with their sedimentary associations, suggests an initial submarine environment adjacent to the volcaniclastite debris-filled moats of oceanic islands or seamounts and their volcanically-active arches. Dismemberment of the seamount-related basalts and pelagic sediments took place during subduction of the oceanic crust and their eventual accretion within a forearc platform.
Implications of Palaeozoic ophiolites from Western Junggar, NW China, for the tectonics of central Asia
Authors: CHI ZHANG , MINGGUO ZHAI , ALLEN M. B. , SAUNDERS A. D. , GUANG-REI WANG , XUAN HUANG
Published: December 2007

Western Junggar, NW China, contains fragments of several Palaeozoic island arcs, accretionary prisms and ophiolites which were accreted to each other, and possibly to the southern margin of Eurasia, by the end of the Palaeozoic. The ophiolites of Western Junggar are important in reconstructions of these events. Four ophiolite belts are recognized, from south to north these are: Tangbale, Mayila, Dalabute and Barleik/Honggulueleng/Hebukesair. They record the production and destruction of oceanic tracts in this region from the Cambrian to the Carboniferous. The following characteristics are representative of them all. (1) The ophiolitic successions are strongly deformed. (2) Sheeted-dyke swarms are not preserved. (3) Basic igneous rocks show a range of chemical characteristics, from depleted, island arc tholeiite to ocean-island basalt. Altered N-type mid-ocean basalts may be represented. (4) Large-scale thrusts, typically directed towards the southeast, lie at the base of many individual ophiolitic bodies. (5) The ophiolites rarely mark the position of possible sutures between arcs; they are normally highly deformed and imbricated with other lithologies. Accretion of Palaeozoic arcs and obduction of ophiolites was complete by the end of the Carboniferous, but the timing and sequence of events remain obscure. No continental collisions occurred during this period of assembly; crustal growth occurred by the amalgamation of non-continental blocks.
Magmatic differentiation
Authors: WILSON MARJORIE
Published: December 2007

During the past 150 years, a wide range of processes have been invoked to explain the mechanism by which magmas differentiate.These may be divided into those which operate essentially in the liquid state, such as liquid immiscibility and thermogravitational diffusion, and those which involve some form of crystal-liquid fractionation. It is now generally accepted that the latter are the most important. Many of the models developed during the past twenty years to explain
magmatic differentiation
have their roots in ideas first proposed in the early years of this century. This review presents some of the historical background to the subject and attempts to summarize some of the more recent developments.

Alfred Harker’s perceptive study of in-situ crystallization within a high-level intrusion (Carrock Fell in the English Lake District), published in volume 50 of the
Quarterly Journal of the Geological Society
in 1893, clearly laid the foundations for many modern theories. In 1900 he introduced the oxide-oxide variation diagram, which is still widely used to depict the geochemical variations within suites of cogenetic igneous rocks. Studies of phase equilibria in synthetic and natural systems by Bowen and his contemporaries, during the 1920s and 30s, provided the theoretical background needed to understand the complex processes involved in fractional crystallization of magmas. In the past decade mathematical modelling has allowed a more quantitative approach to the problem.
Carbonate magmas
Authors: BAILEY D.K.
Published: December 2007

For 40 years, the case for the existence of carbonate magmas rested on field observations of carbonatite intrusions, in which the lack of thermal effects raised an apparent conflict with the high melting temperatures of pure carbonates. Since 1960, the position has changed, with the growth of experimental studies and increasing observations of effusive carbonate rocks. A nephelinite/phonolite volcano in Tanzania is currently erupting Na-Ca-K carbonate magma (around 600°C). This is unlike all other intrusive and effusive carbonatites (350 examples worldwide) which are dominantly composed of Ca, Mg, Fe carbonates, and have negligible alkali contents. Although a number of effusive calcio–carbonatites are considered to be degraded alkali carbonatites, there are several examples (including one magnesio-carbonatite) which are close to their erupted composition, and substantiate the existence of high
T
carbonate magmas lacking essential alkalis at the time of eruption. In these associations silicate magmas are absent (or minor), and in most the effusive carbonatites have been erupted directly from the mantle (with entrained peridotite debris and minerals). They provide a link with the ultramafic association (peridotite and pyroxenite), seen in some carbonatite intrusions, with the commonly associated ultramafic lamprophyres (which may also carry mantle xenoliths), and with carbonate-rich kimberlites. Many carbonatite intrusions also have little or no associated silicate magmas, putting in question a popular view that carbonatites normally form only minor parts of alkaline igneous complexes of nephelinite/phonolite type. The corollary, that the carbonatites are normally differentiates is even less sound, because in alkaline complexes the carbonatite is always last in the eruption sequence. Here the carbonatite may represent the final residua expelled from the source region. Most large carbonatite intrusions seem to have been emplaced at lower
T
than effusives, probably as a near-solidus mush, with the interstitial fluid metasomatizing the country rocks. A wider perspective of carbonate magma genesis is called for, to encompass various kinds of differentiation from alkaline silicate magmas, and primary carbonate magmas from various depths in the mantle (with or without silicate melts). The strongly bimodal composition distribution of calcic and dolomitic carbonatites is a further factor awaiting explanation. Half of the known carbonatites are in Africa, and their timing and distribution indicate that the activity is a response to lateral forces acting across the plate. Carbonate magmatism is waiting to be unleashed. This activity demands attention because it is now clear that carbonate magmatism is a crucial surface expression of deep mantle processes.
An active Neoproterozoic margin: evidence from the Skelton Glacier area, Transantarctic Mountains
Authors: ROWELL A. J. , REES M. N. , DUEBENDORFER E. M. , WALLIN E. T. , VAN SCHMUS W. R. , SMITH E. I.
Published: December 2007

Metamorphosed supracrustal rocks in the central Ross Sea sector of the Transantarctic Mountains are of Neoproterozoic age and not Cambrian. They include pillow basalts with a mantle separation age of 700–800 Ma. In the Skelton Glacier area, these rocks experienced two strong phases of deformation that produced folds and associated foliations. Both rocks and structures are cut by a 551 ± 4 Ma unfoliated quartz syenite (late Neoproterozic). The deformation and limited geochemical data suggest an active Antarctic plate margin whose late Neoproterozoic history is markedly different from that of the temporally equivalent rift to drift transition recorded along the autochthonous western margin of Laurentia. If these two cratons were ever contiguous, separation occurred by
c.
700–800 Ma.
Sediment transport predictions for the English Channel, using numerical models
Authors: GROCHOWSKI N. T. L. , COLLINS M. B. , BOXALL S. R. , SALOMON J. C.
Published: December 2007

Output from an hydrodynanmic numerical model (Salomon & Breton 1991
a, b
) has been combined with various empirical formulae to derive sediment transport rates and directions. The predictions are compared with radioactive sand tracer experiments, demonstrating the applicability of Gadd
et al.’
s (1978) expression. The derived bedload transport pattern for the area is consistent with sedimentological and geomorphological evidence, but provides improved resolution.

A ‘bedload parting zone’ is identified for the central English Channel; this is well defined for sand movement, but seabed material present over this area (gravel) is not predicted to move significantly. The area is ‘non-erosive’, over which fine-grained sediments are being transported. A bedload convergence is located to the southwest of the Dover Strait, not in the Strait itself as proposed previously. A transport pathway, bypassing the convergence, along the French coastline (which is supported by radioactive tracer experiments) could be responsible for sand transfer from the English Channel to the southern North Sea.
Palaeomagnetic rotations and fault kinematics in the Rif Arc of Morocco
Authors: PLATZMAN E. S. , PLATT J. P. , OLIVIER P.
Published: December 2007

Palaeomagnetic and structural investigations in the Rif arc of Morocco indicate that there have been large rotations associated with a pattern of outwardly directed thrusting. Rock magnetic experiments in conjunction with thermal and alternating field demagnetization demonstrate that eight sites in Jurassic and Cretaceous limestones along the Internal/External boundary have a stable remanent magnetization. This is, in most cases, rotated anticlockwise by as much as 100° from the expected Mesozoic declination but in the Tetuan area there are large clockwise rotations. Kinematic indicators from fault surfaces indicate the following. (1) In the eastern Rif there has been predominantly south-directed thrusting, partly overprinted by extensional and sinistral strike-slip faults. (2) At the eastern end of the N 70°E striking Jebha Fault zone there is a pattern of dominant sinistral NE-trending and subsidiary dextral SE-trending strike-slip faults, overprinted by normal faults while at the western end deformation consists largely of south directed thrusting. (3) In the northern section of the chain, where the structural trends are dominantly N-S, thrusting is directed W to NW. It is suggested that the data are best explained by differential motion and rotation of thrust sheets during outwardly directed thrusting around the arc.
Elemental evidence for an enriched small-fraction-melt input into Tertiary Mull basalts, Western Scotland
Authors: KERR A. C.
Published: December 2007

Continental basalts enriched in the incompatible trace elements have recently been interpreted as mixtures of depleted asthenospheric melt and an enriched small-fraction-melt lithospheric component. Hebridean Tertiary basalts are relatively depleted in incompatible trace elements, and it has been suggested that this is because the enriched lithospheric component had already been extracted beneath that region, during the Permo-Carboniferous. It was therefore not widely available to contaminate Tertiary magmas. A flow-by-flow geochemical study of the Mull lava succession has nevertheless revealed the presence of lava flows relatively enriched in the incompatible trace elements, at the base of the succession, in some parts of the island.

Fractional crystallization, contamination by Archean and Moinian crust, variation in the degree of mantle melting and an asthenospheric, ocean island basalt-like source, have all been ruled out as possible mechanisms of enrichment in favour of 5–10% contamination by an enriched, fusible, small-fraction-melt from the lithospheric mantle. The overall lack of basalts enriched in trace elements in the British Tertiary Igneous Province, argues strongly against the
widespread
presence of an enriched Tertiary lithospheric mantle below the Hebrides. The origin of the a sodic (rather than potassic) contaminant within the Hebridean lithosphere is also discussed.
U-Pb zircon ages from the Bindal Batholith, and the tectonic history of the Helgeland Nappe Complex, Scandinavian Caledonides
Authors: NORDGULEN Ø. , BICKFORD M. E. , NISSEN A. L. , WORTMAN G. L.
Published: December 2007

The Helgeland Nappe Complex forms part of the Uppermost Allochthon in the central Scandinavian Caledonides. It consists of metasupracrustal rocks of assumed Precambrian age which are juxtaposed with metasupracrustal rocks present as non-conformable cover sequences to subjacent fragments of Early Ordovician ophiolitic rocks. All of these rock units have been intruded by the Bindal Batholith, which ranges in composition from mafic gabbro to leucogranite. In this paper, U-Pb zircon ages from five plutons in the southern part of the batholith are presented. These are a porphyritic granite of the Andalshatten pluton (447 ± 7 Ma), a tonalite from the Kråkfjellet pluton (443 ± 7 Ma), a granodiorite west of Gåsvassfjellet (437 ± 4 Ma), the Kalvvatnet monzonite (435 ± 10 Ma), and a monzodiorite near Tosbotn (430 ± 7 Ma). The dates are essentially concordant and are considered to reflect the crystallization ages of the intrusions.

Based on regional correlations and the age of the oldest of the dated plutons, it is concluded that internal accretion and polyphase tectonometamorphism of the Helgeland Nappe Complex took place in Ordovician times. The final Silurian (Scandian) thrusting of the nappe stack across the continent of Baltica appears to post-date emplacement of the Bindal Batholith.
Plate-moving mechanisms: their relative importance
Authors: ZIEGLER P.A.
Published: December 2007

The lithosphere forms the upper boundary layer of the Earth's mantle convection system which facilitates escape of thermal energy from the Earth's interior. Motion and interaction of lithospheric plates is probably governed by the combination of drag-forces excerted on the base of the lithosphere by the convecting mantle and by plate boundary forces. Phanerozoic movements and interaction of major continental blocks are difficult to explain in terms of conventional plate moving mechanisms, such as slab-forces, ridge-push and deviatoric tensional stresses developing over upwellings of the asthenosphere and in response to lithospheric over-thickening in orogenic belts. Circumstantial evidence suggests that shear-traction exerted by the convecting asthenosphere on the base of the lithosphere plays an important, and at times even a dominant role as a plate moving mechanism. The relative importance of the different processes contributing to the motion of lithospheric plates probably varies during the assembly and break-up of Pangaea-type megacontinents. Continents assemble in areas of downwelling branches of the asthenospheric convection system. Development of major orogens along the trailing edges of drifting continents and subduction-progradation from the megasuture of colliding continents to their distal margins suggests that plate convergence is controlled, apart from slab-forces and ridge-push, also by shear-traction. Assembly of a Pangaea has an insulating effect on the downwelling convection cells, governing its suturing, causes their decay and a reorganization of the global upper-mantle convection system. Development of new upwelling and outflowing asthenospheric cells under mega-continents gives rise to tensional stresses in the lithosphere, causing its extension. Following crustal separation, the asthenosphere advects passively into the space opening between diverging plates. Progressive opening of oceanic basins is coupled with the development of ridge-push forces, contributing to plate divergence. Activity along sea-floor spreading axes can terminate abruptly if far-field stresses, resulting from plate interaction, impede further plate divergence. The nearly contemporaneous decay of sea-floor spreading axes in often distant areas reflects changes in plate interaction. Assuming a finite globe, generation of new lithosphere at sea-floor spreading axes has to be compensated for elsewhere by subduction of commensurate amounts of oceanic lithosphere and/or shortening and subduction of continental lithosphere. Plate interaction, driven largely by shear-traction of the mantle convection systems and their changes, ridge-push and slab forces, plays probably an all-important role in the development of intra-continental rift systems, the opening of new oceanic basins and the inception of and activity along subduction zones. A two-layered mantle convection system is envisaged, that may be coupled to a greater degree during the break-up of mega-continents than during periods of dispersed continents.
Granite magmatism
Authors: ATHERTON MICHAEL P.
Published: December 2007

Read’s two presidential addresses to the Geological Society (1948 Geological Society (1949) heralded the end of the coherent rearguard action by the ‘granitizers’ against the ‘magmatists’. They were the distillation of his thoughts on the genesis of granite and culminated in his concept of the ‘Granite Series’. In this, he identified a continuity from metamorphic through migmatitic rocks to granite. Although he was wrong on granitization, the general idea remains intact and granites are produced by high-temperature metamorphism leading to partial melting. However the role of migmatites is still contentious. Not all granites belong to the granite series as he presented it; this particularly applies to Cordilleran (Andean) type granites. The genesis of this type will be discussed in the context of the earlier, classic work of
Nockolds (1940)
on the Garabal Hill complex where he demonstrated fractional crystallization was a major process in producing the diversity present and was similar to that seen in volcanic rocks, which had clearly been liquids. He also proposed that the source was basaltic with the implication, strongly supported by modern isotopic studies, that granites of this type are essentially mantle derived. Nockolds contribution was a geochemical confirmation of Bowen’s belief in the importance of closed-system fractional crystallization in the differentiation of plutonic rocks. Most authorities today would accept this but would not necessarily follow Bowen in the belief that major granitic batholiths formed by differentiation of basalt or that the system was closed. Recent models of the generation of the two main types of granite are presented, incorporating many of the ideas of Nockolds and Read. The discussion focuses on high-level differentiation, partial melting and intrusion in an extensional regime, high-
T
/low-
P
metamorphism associated with the magmatism, and the relation of granite to the plate tectonic setting.
Cretaceous–Cenozoic tectonic events in the western Myanmar (Burma)–Assam region
Authors: MITCHELL A. H. G.
Published: December 2007

Correlation of late Mesozoic mafic and ophiolitic rocks suggests that in the late Lower Cretaceous a northeasterly-facing mafic arc was emplaced onto southeast Borneo, western Sumatra, western Myanmar (Burma) then 1100 km south of its present position, the Mogok belt of Myanmar and the Denqen–Bangong Co ophiolite zone in Tibet, all then situated on the southwestern margin of Asia. Reversal in tectonic polarity and eastward subduction generated a late Cretaceous magmatic arc in western Myanmar and Sumatra, and its continuation through the Mogok belt into the Gandise batholith of Tibet. Magmatism, interrupted in the latest Cretaceous, probably resumed in the western Myanmar–Tibet and Sumatra arc segments in the early Palaeogene. It was again interrupted in the Mid-Eocene when an east-vergent ophiolite nappe overrode the Indo-Burman Ranges, deforming Lower Eocene turbidites derived from the magmatic arc to the east. Following renewed northward and eastward subduction of ocean floor in the latest Eocene, India collided with northern Myanmar, and with Tibet along a suture south of the Jurassic–Triassic flysch and adjacent Yarlung ophiolite. In the Mogok segment of the late Cretaceous arc, crust thickened in the early Cretaceous orogeny underwent extension to form a metamorphic core complex as Myanmar, west of the Sagaing Fault moved northward. More than half of this northward movement preceded the well-documented 450 km of post-early Miocene dextral displacement on the Sagaing Fault and related spreading in the Andaman Sea. Generation of late Cretaceous to early Eocene I- and S-type granites in and east of the Mogok belt was related either to crustal shortening, or more probably, to subsequent extension in the Mogok belt.
Palaeosols as indicators of geomorphic stability in two Old Red Sandstone alluvial suites, South Wales
Authors: MARRIOTT S. B. , WRIGHT V. P.
Published: December 2007

Palaeosols (fossil soils) are abundant in the alluvial late Silurian–early Devonian Moor Cliffs Formation and Rat Island Mudstone (Freshwater West Formation) of south Dyfed, South Wales. The palaeosols occur in thick mudstones deposited on floodplains under a seasonal, semi-arid climate, and represent Vertisol-type soils. Simple, single profiles are rare and most of the mudstone intervals exhibit complex overprinted profiles. However, a major difference exists between the palaeo-Vertisols in each of the two units studied. The Moor Cliffs Formation contains thick floodplain intervals with evidence of frequent erosion and reactivation of earlier soil horizons. The associated coarser channel deposits mainly represent ephemeral streams. The alluvial system was unstable, flashy and prone to extensive stripping of stored floodplain sediment. Arroyos (unstable ephemeral stream channels) probably developed during this interval. Evidence for any marine influence on soil development is missing and previous speculations that the formation may represent deposition in an upper-intertidal or supratidal system are not supported.

The Rat Island Mudstone represents a complete contrast in that there was little or no stripping of floodplain sediments. The channel deposits are typically of the fining-upwards type representing more stable, low sinuosity channels and with ephemeral-stream floods.

By integrating the palaeosols, a more detailed picture of the geomorphic history of these units can be formulated. Comparison with contemporary drainage basins indicates that a change in climate was a possible cause for the differences between the two units, less frequent and more severe storms occurring during the deposition of the Rat Island Mudstone.
Stratigraphy and structure of the Maltese graben system
Authors: DART C. J. , BOSENCE D. W. J. , McCLAY K. R.
Published: December 2007

The Maltese graben system is a series of Miocene–Quaternary extensional basins located in the foreland of the Sicilian Apennine–Maghrebian fold and thrust belt. The tectono-sedimentary development of the graben was determined from the first fully integrated analysis of the area to include onshore field and hydrological borehole data, together with offshore reflection seismic data and exploration wells. The NW–SE- and ENE–WSW-trending rifts have a coeval, four phase tectono-sedimentary evolution revealed by a succession of platform and pelagic carbonates. The pre-rift phase (>21 Ma) is followed by a early syn-rift phase (21–6 Ma) characterized by evidence for relatively minor extensional faulting including the development of neptunian dykes. Major fault activity occurred during the succeeding late syn-rift phase (<5 Ma) which is characterized by growth faults and fault control on facies and the areas of deposition/non-deposition. This was followed by passive graben infilling during the post-rift phase (probably < 1.5 Ma). Despite low measured extension factors (
β
= 1.03–1.17) across the rift structures, major basin bounding extensional faults have throws of up to 2.2 km. Fault slip data indicate that both NW–SE and ENE–WSW rift trends were generated in response to N–S stretching. These results are in accord with Argnanis (1990) description of a major N–S orientated transfer fault located 50 km west of the present study area.
Nature and tectonic setting of accreted basalts from the Mino terrane, central Japan
Authors: JONES G. , SANO H. , VALSAMI-JONES E.
Published: December 2007

The Mino tectono-stratigraphic terrane of central Japan is dominated by thrust-deformed Jurassic terrigenous clastic sediments, thought to have formed as a subduction–accretion complex at an active margin. However, in the central Mino terrane, basic volcanic rocks, together with stratigraphically overlying Permian shallow-water platform and slope carbonates and radiolarian cherts (Okumino Group), are present as tectonic intercalations. These Permian units have previously been interpreted as representing a large oceanic seamount-type body, formed in a distal oceanic setting. In addition, basaltic volcanic and volcaniclastic rocks of unknown age and affinities are also found as tectonic slivers within the adjacent Jurassic trench clastic sequences (Neo Formation). New petrological and geochemical investigations of all these basalts show that two main compositional groups are present: (i) alkali basalts (Mid-Permian) which are highly enriched in incompatible trace elements and the light REE (Group 1); (ii) tholeiitic basalts (mainly Lower Permian) which are moderately enriched in incompatible trace elements and light REE, relative to N-type MORB (Group 2). These characteristics suggest derivation from an enriched mantle source, such as a mantle plume. The compositions of Neo Formation basalts closely resemble those of the Okumino Group, suggesting that they were probably derived from the latter by both tectonic and sedimentary processes. Geochemical comparison with modern oceanic plume-influenced basalts, notably using La/Sm
n
–Sm/Yb
n
ratios, suggests that the older Group 2 basalts are more akin to volcanics erupted along a constructive plate boundary (P- or E-type MORBs), than to tholeiitic basalts of intraplate oceanic islands (OIB). The sedimentary evidence for bathymetric elevation further indicates that the setting for this volcanism was most likely a spreading axis-centered oceanic plateau or ridge (e.g. Iceland, Ninety-East Ridge). The younger alkali basalts probably formed in an off-axis position, either due to normal lateral spreading processes, or perhaps more complex spreading centre–plume interactions. Accretion of this plateau onto the southern margin of the Asian landmass in the Mid–Late Jurassic apparently involved significant subduction-erosion of the plateau, evidenced by reworking of the Permian units into adjacent trench clastic sequences.
The Grenville Province within Antarctica: a test of the SWEAT hypothesis
Authors: STOREY B. C. , PANKHURST R. J. , JOHNSON A. C.
Published: December 2007

Geochronological, isotopic and aeromagnetic data, including a new Rb-Sr isochron of 1076 ± 7 Ma for volcanic rocks at Bertrab Nunatak, Coats Land, confirm the existence of rocks of Grenvillian age within both East and West Antarctica and support the juxtaposition of Antarctica and western North America in Neoproterozoic reconstructions.
Timing of Barrovian metamorphism and ‘Older Granite’ emplacement in relation to Dalradian deformation
Authors: ROBERTSON S.
Published: December 2007

Detailed studies of the relationship between structure and metamorphism in the southern Scottish Highlands and new Rb-Sr isotope data for the Rough Craig Granite have resulted in a revision of the tectonothemal history of the Dalradian. Two main events are recognized. The first (Dl), probably at c. 590 Ma, produced a regiondy inverted stratigraphic succession on the underlib of the Tay Nappe. The second event, possibly between 520 and 470 Ma, encompassed regional D2 and D3 deformation phases and was synchronous with peak Barrovian metamorphic conditions.
Hollow natrocarbonatite lapilli from the 1992 eruption of Oldoinyo Lengai, Tanzania
Authors: CHURCH A. A. , JONES A. P.
Published: December 2007

A new style of layered tuff deposit in the SW of Oldoinyo Lengai’s active crater consists of bedded spherical lapilli 8–12 mm in diameter with hollow cores and vesicular rims. Lapilli of this type have not previously been reported from extrusive carbonatites and similar deposits are apparently unreported amongst siliceous pyroclastics. Some lapilli are solid and are either vesicular throughout or contain vitreous crystal cores. A mechanism for the eruption and a sequential model for the formation of hollow lapilli are suggested.
Petrographic analyses of organo-mineral relationships: depositional conditions of the Oxford Clay Formation (Jurassic), UK
Authors: BELIN S. , KENIG F.
Published: December 2007

Five samples representing the range of variability of palynofacies and organic geochemical parameters of the organic-rich facies of the Peterborough Member (formerly Lower Oxford Clay) and one sample from the Stewartby Member (Middle Oxford Clay) were studied using optical microscopy (transmitted light and fluorescence) and scanning electron microscopy (backscattered electron mode).

The organic matter assemblage of the Oxford Clay Formation is dominated by amorphous organic matter of marine origin (75 to 95%). Whatever their organic content, the sediments display a highly microbioturbated texture, with edge-to-face disposition of clay minerals and numerous microburrows, signifying that the sea floor was never completely anoxic. The presence of abundant organic matter in the sediment has enhanced diagenetic transformations, and in particular is related to the formation of pyrite, calcite and authigenic quartz. The nature of diagenetic changes, particularly those involving silica species, provides information on the interstitial environment. The sediment displays, at the micrometric scale, numerous microenvironments with varying reducing conditions, resulting in different diagenetic products. Pyrite displays two habits, framboidal and euhedral. Euhedral pyrite, which characterizes more anoxic interstitial environments than framboids, is found in the most organic-rich sediments. The abundance of large coccolith-rich faecal pellets in the most organic-rich sediments is correlated with chemical evidence of intense trophic activity.
A lithofacies study of the Peterborough Member, Oxford Clay Formation (Jurassic), UK: an example of sediment bypass in a mudstone succession
Authors: MACQUAKER J. H. S.
Published: December 2007

Thirteen samples from the Peterborough Member (Oxford Clay Formation, Middle Jurassic), from Quest Pit, Stewartby, central England, were examined in detail using a combination of optical microscopy and backscattered electron imagery. The majority of the samples were clay-rich mudstones, two were silt-rich mudstones and one was a muddy sandstone. The dominant components of these samples are fine-grained clays (illite, mixed layer illite/smectite, kaolinite) and amorphous organic matter; minor components include authigenic pyrite (framboidal and euhedral), silt-sized quartz grains, calcareous nannoplankton (mainly disarticulated coccolith plates) and a variety of early carbonate cements. Some of the mudstones were found also to contain abundant foraminifera. Temporal trends between samples in grain size, foraminiferal abundance and total organic carbon contents were observed.

These mudstones were deposited in a shallow epeiric sea, where productivity was high and the bottom waters were oxic. Relative sea-level change, coupled to a lack of available accommodation space within the basin at low sea-level stands is invoked to explain the observed mudstone stacking patterns.
A late Caledonian melange in Ireland: implications for tectonic models
Authors: WILLIAMS D. M. , HARKIN J. , ARMSTRONG H. A. , HIGGS K. T.
Published: December 2007

The Clew Bay area in western Ireland contains the remnants of a Caledonian terrane which separates the Dalradian of North Mayo from the Ordovician of South Mayo to the south. Rocks of the area have been variously interpreted as representing part of the Dalradian succession, as a Cambrian to Ordovician rifted margin to subduction related basin, and as a shear carpet derived from an accretionary wedge overridden by an ophiolite in the early Ordovician. The lithologies present include discontinuous outcrops of ultrabasic and basic rocks, together with quartzose and semipelitic schists on the south shore of Clew Bay. These are in contact to the north with a sedimentary and volcanic sequence exposed both on the mainland and on Clare Island for which a detailed stratigraphy had previously been established. We reinterpret this latter sequence as a melange containing blocks up to 500 m long of sandstone, conglomerate, chert and volcanics.

Although microfossil evidence had previously shown that a chert block on Clare Island was of Late Llanvirn age, new fossil control shows that this date must be extended upwards. Microfossils extracted from a variety of lithologies within the melange show that the cherts range from Middle Ordovician to at least Caradoc. Further, spores extracted from the melange matrix show that the formation of the melange took place in the Silurian, probably in the Wenlock or later. These data indicate that the melange is unrelated to any early Ordovician obduction event and in fact represents the effects of a significant tectonic episode in the late Silurian of the British and Irish Caledonides.
Pan-African granulite-facies metamorphism in the Mozambique Belt of Tanzania: U-Pb zircon geochronology
Authors: MUHONGO S. , LENOIR J.-L.
Published: December 2007

Two new U-Pb concordia zircon ages are reported for granulite-facies rocks from the Mozambique Orogenic Belt in eastern Tanzania. These rocks are thought to have been generated during collision-related crustal thickening process and exhumed by deep erosion. A two-pyroxene granulite sample from the Pare Mountains has yielded a concordia upper intercept age of 645 ± 10 Ma, whereas a granulite-facies meta-anorthosite sample from the Uluguru Mountains has given an upper intercept age of 695 ± 4 Ma. Evidence for the involvement of older crustal material (pre-Late Proterozoic) was not found. These zircon data indicate crystallization during the peak of granulite-facies metamorphism during the Pan-African orogeny (
c
. 950 to
c
. 550 Ma) consistent with a subduction-collisional environment.
Bioturbation in deep sea Pacific sediments
Authors: MEADOWS A. , MEADOWS P. S.
Published: December 2007

Bioturbation structures have been recorded in sediment cores from 5000 m to 5300 m water depth collected from the Pacific Ocean using subcores from spade box cores. Shear strength and Eh profiles were also taken. The results have been related to pelagic fluxes and sedimentation. Bioturba-tion structures consist of holes and chambers of various sizes running vertically and horizontally, whose abundance, size and shapes have been measured. Values did not decrease exponentially with sediment depth but end abruptly at depths ranging from 30 cm to 45 cm. In many instances there were subsurface peaks of vertical and horizontal burrows. There was also evidence of lateral galleries. Large deep in filled burrows were observed. These consisted of light and dark brown sediment having different shear strengths. Eh profiles showed that the sediment was aerobic, with slightly lower values near the sediment surface, possibly associated with meiofaunal and bacterial activity. The most important implications of this work for pelagic processes are likely to be increased transfer of dissolved and particulate material between the sediment and the water column due to the physical network of burrows within the sediment and increased surface roughness caused by burrow openings. Mucus binding of particulates and faecal pelletization are also likely to be important. All of these effects will be most noticeable at high burrow densities.
Geological and hydrological controls on the producibility of coalbed methane
Authors: KAISER W. R. , HAMILTON D. S. , SCOTT A. R. , TYLER ROGER , FINLEY R. J.
Published: December 2007

Geological and hydrological comparison of two United States coalbed methane basins, the prolific San Juan Basin and the marginally producing Sand Wash Basin, indicates that coal distribution and rank, gas content, permeability, ground-water flow, and depositional and structural setting are critical controls on coalbed methane producibility. A complex interplay, and moreover, a synergy amongst these controls determines high productivity. This paper proposes a basin-scale explanation for the prolific and marginal production in the two basins and that can be applied to evaluation of coalbed methane potential in coal basins worldwide. High productivity is governed by (1) thick, laterally continuous coals of high thermal maturity, (2) basinward flow of ground water through coals of high rank and gas content orthogonally toward no-flow boundaries (regional hingelines, fault systems, facies changes, and/or discharge areas), and (3) conventional trapping along those boundaries to provide additional gas beyond that sorbed on the coal surface.
Vitrinite reflectivity and the structure and burial history of the Old Red Sandstone of the Midland Valley of Scotland
Authors: MARSHALL J.E.A. , HAUGHTON P.D.W. , HILLIER S. J.
Published: December 2007

Vitrinite reflectivities from Lower Old Red Sandstone (ORS) sediments of the Strathmore region in the Midland Valley of Scotland give values of 1.2% for the Stonehaven Group (Kincardineshire), 0.9% for the Arbuthnott Group (Angus) and 1.3% for the Strathmore Group (Perthshire). These values are corroborated by clay mineral diagenesis studies. The Stonehaven Group reflectivity values are considerably lower than expected under the 9 km thickness of sediments aggregated for the Lower Old Red Sandstone of Kincardineshire and demonstrate a maximum burial of some 3 to 5 km. These results together with detailed vitrinite reflectivity measurements in the Dundee Formation, consideration of post-Devonian burial history and kinetic burial modelling, show that the present level of Lower ORS thermal maturation results from Late Carboniferous maximum burial. This is confirmed by comparison with a vitrinite reflectivity gradient from the Strathclyde Group of Fife.

To reconcile the vitrinite reflectivity data with the stratigraphy and structure of the Lower Old Red Sandstone, burial models invoking structural repetition, depocentre migration and structurally assembled stratigraphy are considered. It is concluded that the Lower Old Red Sandstone is a structurally assembled series of basins with the Cowie Formation of the Stonehaven Group being part of a separate depositional succession and structurally juxtaposed into its present position as part of an apparently contiguous sequence.
Structure and metamorphism of blueschist–eclogite facies rocks from the northeastern Oman Mountains
Authors: SEARLE M. P. , WATERS D. J. , MARTIN H. N. , REX D. C.
Published: December 2007

The northern part of the Saih Hatat window, Oman, shows high-pressure metamorphic rocks derived from shelf sediments and pre-Premian continental basement, and is atypical of sub-ophiolite metamorphism elsewhere. The high-pressure rocks are divided into structural units originally bounded by foreland-propagating thrusts formed during ophiolite obduction, although now many contacts are backthrusts, normal faults or extensional shear zones. Metamorphic breaks exist across many unit boundaries. The deepest unit (As Sifah) has eclogite-facies assemblages in metabasites and metapelites which record evolution of
P
–
T
conditions along a clockwise path culminating al 23±2.5kbar, 540±75°C, contrasting markedly with overlying units (5–10 kbar, 200–500°C), although separated by <l0km on the ground. The dominant penetrative structures in the eclogites predate exhumation, but broad zones in the enveloping and overlying schists show a later, greenschist-facies extensional fabric. Phengites from eclogite-facies schist show discordant
40
Ar/
39
Ar apparent ages. Our tectonic model relates all the high-
P
units to a single convergent event in the Late Cretaceous. The As Sifah eclogites were exhumed in two stages: (i) tectonic emplacement against other units at
c
. 20–25 km depth, and (ii) exhumation of the entire high-
P
zone by culmination collapse after obduction.
Structural evolution of the Gander-Avalon terrane boundary: a reactivated transpression zone in the NE Newfoundland Appalachians
Authors: HOLDSWORTH R. E.
Published: December 2007

In the Appalachian Orogen of NE Newfoundland, the Gander and Avalon zones are separated by the 1–2 km wide, dextral, Dover Fault Shear Zone. This structure overprints sinistral transpressive deformations that affect broad regions either side of the Gander-Avalon boundary and is post-dated by ?Alleghenian (or younger) brittle dextral faults that are not thought to record large regional displacements. In ?Lower Palaeozoic sedimentary rocks of the Gander Lake Subzone, a steeply dipping sinistral shear zone (up to 20 km wide) was the focus of syn-tectonic sillimanite grade metamorphism, migmatization and granitic plutonism. In lower greenschist facies late Precambrian sedimentary and volcanic rocks of the Avalon Zone, early sinistral displacements were focused into a high strain zone (
c
. 2km wide) along the boundary. Although developed at different metamorphic grades, these early sinistral transpressive shear zones can be correlated and are thought to record the main phase of sinistral-oblique accretion and regional orogenesis during the mid-Silurian (
c.
415-435 Ma). The switch to dextral displacements forming the Dover Fault Shear Zone appears to have initiated during later stages of syntectonic plutonism in the Gander Lake Subzone, suggesting that it may also be Silurian in age.

No evidence links the Gander and Avalon zones prior to sinistral transpression. Sinistral fabrics overprint flat-lying structures in the Gander Lake Subzone that are thought to be related to mid-Ordovician accretion of the superjacent Exploits (Dunnage) arc-ophiolite terrane to the west. The Gander and Dunnage allochthons were probably accreted to the Laurentian margin prior to the sinistral-oblique collision of Western Avalonia during the Silurian and they overlie the suture that separates Laurentia from the Central Lower Crustal Block of Avalonian affinities. The Gander-Avalon boundary in NE Newfoundland would then form the steeply-dipping surface expression of a collision-linked sinistral shear zone that allowed lateral decoupling of the Avalon and Central Lower Crustal Blocks during or following oblique collision.

Along the margin of the Gander Lake Subzone, the shear zone-hosted belt of sheeted granites and melting increasingly became the focus of deformation during the later stages of sinistral transpression. This suggests significant rheological softening of the crust that would account for the ability of transpressive terrane boundaries to accommodate large displacements. Furthermore, the estab-lishment of a thermal anomaly would have a long-lived weakening effect that could explain why such terrane boundaries are prone to reactivation.
Non-marine bivalve assemblage in the Asker Group, Oslo Graben and its correlation with a late Pennsylvanian assemblage from North America
Authors: EAGAR R. M. C.
Published: December 2007

A non-marine bivalve band in the Asker Group of Semsvik, Asker, usually regarded as being of Permian age, has been re-examined. The fauna, an assemblage ofAnthraconaia protractaEagar, is widely and continuously variable. The Semsvik assemblage is identical in shell height and length toA. protractaabove the Benwood Limestone, middle Monongahela Group, of Pennsylvania, and toA. protracta2700 km further west in central Arizona, with which the Pennsylvania band has been correlated. The ranges of variation within the Oslo, Pennsylvania and Arizona assemblages are closely similar. When their horizons are broadly correlated, the Semsvik band is placed in the upper Virgilian Stage of the upper Pennsylvanian and in the upper Stephanian C of Europe. This age, which is applicable to the thin succession of the Asker Group in general, pre-dates formation of the Oslo Graben and is in conformity with the radiometric age of the overlying basal basalt lava. The age of the Knabberud Limestone in the Asker Group is at present under discussion. The extremely widespread, evidently rapid dispersal of faunas of smallAnthraconaiain Stephanian time may have been aided by palaeoniscoid fish.
Fractography in geology
Authors: AMEEN M. S.
Published: December 2007

The first thematic meeting on fractography in geology, held on 13 September at Burlington House, London, was jointly sponsored by the Petroleum Group and the Tectonic Studies Group of the Geological Society. It was attended by 50 participants from Australia, Canada, France, Germany, Holland, Norway, Sweden, United Kingdom and the United States of America, a gathering which included a wide spectrum of specialists from the oil, environmental, engineering and mining industries and from universities and colleges. Three posters and 14 papers were presented, including two keynote addresses.

A one-day field trip led by Ameen, the convenor, preceded the conference during the course of which the fracture characterization of the Upper Chalk in the Thanet monocline, Kent, was discussed.

The purpose of the conference was to discuss those aspects and applications of fractography which have evolved into an invaluable discipline within the vast field of fracture characterization of rocks. Fractography can be defined as the science which deals with the diagnosis, description, analysis and interpretation of fracture surface morphology or topography, and links them to the causes and mechanisms of the fractures. The term fractography was first used by the metallurgist Car1 A. Zapffe (Zapffe & Clogg 1944), and introduced to geology by Byron Kulander in the 1970s (

Kulander
et al.
1979

).

An introduction by the convenor outlined several factors which prompted the meeting.

The discipline of fractography in geology is new, even though
Woodworth (1896)
was the first to describe joint surface morphology from field observations. It is hardly mentioned