bluuebunny/crossref_metadata_2025 huggingface dataset:
Mantled gneiss antiforms and fold nappes in the Rinkian belt, West Greenland: diapiric structures or structures formed in a thrust system?
Authors: GROCOTT J. , VAN DEN EECKHOUT B. , VISSERS R. L. M.
Published: December 2007

In Lower Proterozoic metasedimentary cover rocks of Svartenhuk Halvø, and in their reactivated Archaean basement of tonalitic gneisses, features of the structure and metamorphism are identified which are critical to the understanding of the kinematics and dynamics of the orogenic evolution of the Rinkian belt. These features are as follows: (1) a major ductile shear zone at the gneiss–sediment contact which has a normal-slip displacement and dips steeply NE away from the gneisses in its present orientation, (2) a fan in the main cleavage in the metasediments, in which cleavage steepens over a distance of 15 km from a 30° dip SW towards the contact through the vertical, finally coming to lie parallel with the contact, (3) the existence of a major synform in the sediments near the gneiss–sediment contact which is overprinted by the main cleavage, (4) the orientation and restricted distribution of later crenulation cleavage, and (5) a high temperature–low pressure metamorphic facies series, with thin metamorphic zones close to the basement, reaching anatexis at the gneiss–sediment contact.

The features which relate to the pattern of strain are compared with those to be expected: (1) as a result of the ascent of a diapiric ridge of gneiss into the cover sediments, and (2) as a result of thrusting to the NE, followed by SW-directed backthrusting. It is concluded that gneisses on Svartenhuk Halvø rose diapirically into the cover sediments. This has implications for the origin of flat lying gneiss-cored nappes in areas adjacent to Svertenhuk Halvø. These may respresent, (1) the spreading lobes of mature diapirs, or (2) diapiric antiforms amplified during a subhorizontal shear regime.
Along-strike variation in the stratigraphical and structural profile of the Southern Uplands Central Belt in Galloway and Down
Authors: BARNES R. P. , ANDERSON T. B. , McCURRY J. A.
Published: December 2007

The modern interpretation of the Southern Uplands depends on the recognition of laterally extensive, linear, fault-bounded tracts of northward-younging Lower Palaeozoic sediments with rare volcanics. The tracts become progressively younger southwards and are thought to have been sequentially accreted by northward underthrusting above a subduction zone.

Detailed mapping and tentative correlation of three, well-exposed, coastal profiles through the Central Belt in SW Scotland and NE Ireland offers a new test of the Southern Uplands model. Comparison of the northern parts of each section indicates the presence of strike-parallel tracts with model structure. Moffat Shale outcrops associated with the tract-defining faults demonstrate a diachronous, incremental, southerly decrease in the age of the base of the overlying turbidites, essentially as in the Northern Belt. Southward this becomes markedly less pronounced, particularly in Down where the Central Belt is much wider than in Galloway. Distinctive Hawick Group lithologies permit the correlation of the southern parts of the sections. Large areas of predominantly southward-younging occur in each section, especially in Galloway where the southward-younging area is 12 km across. Here northerly-verging D
1
fold pairs are consistent with observed fault movement opposite in sense to the northerly underthrusting of the model. Back thrusting, in the style of the Pleistocene Cascadia Basin, is invoked to account for the landward-verging structure of these areas and to explain the narrowing of the Central Belt in Galloway.

The structure of the Central Belt thus differs significantly from that of the Northern Belt and the accretionary prism model.
The geochemistry of the Blackstairs Unit of the Leinster Granite, Ireland
Authors: SWEETMAN T. M.
Published: December 2007

The Blackstairs Unit is the southernmost of the five units which comprise the Leinster Granite. Four granite types are recognized, which in order of decreasing age are: Graiguenamanagh Granite, Porphyritic Microcline Granite, Non-Porphyritic Granite and Type 1 Granite. Using whole-rock geochemical data, in conjunction with field and petrographic evidence, a model is proposed whereby partial melting of a Lower Palaeozoic immature sedimentary source produced a parental granitic melt which crystallized without further differentiation to form the Non-Porphyritic Granites of the northeastern end of the pluton. At the SW end, this parental melt fractionated to form the Porphyritic Microcline Granite leaving the melt depleted in K
2
O, which then crystallized to form a slightly more acidic Non-Porphyritic Granite than that at the NE end. A mixture of restite minerals plus melt from this partial melting process crystallized forming the Graiguenamanagh Granite. Fractionation of thorium into restite allanite in the Graiguenamanagh Granite accounts for the low Th and Th/U levels which characterize the Non-Porphyritic Granite and the Leinster Granite in general. A high degree of partial melting of similar source material produced the low volume Type 1 Granite magmas which were subsequently intruded along faults. A similar petrogenesis is envisaged for the whole of the Leinster Granite.
Polarity, stratigraphy and duration of the Tertiary igneous activity of Mull, Scotland
Authors: DAGLEY P. , MUSSETT A. E. , SKELHORN R. R.
Published: December 2007

Previous work left unresolved whether the Tertiary igneous activity had spanned a magnetic polarity succession R-N-R-N-R or, if overprinting had occurred, only R-N-R. New work is reported that demonstrates that thermal overprinting has occurred in a narrow zone, c. 600 m wide, bordering the outermost major intrusions of Centre 2. Criteria for the recognition of overprinting are examined.

The polarity sequence is now believed to be R-N-R. In conjunction with radiometric dates which show that activity occurred from c. 60 Ma to c. 57 Ma, our data show that the sequence corresponds to chrons 26r-26-25r of the polarity time-scale of Harland
et al.
1982 or 25r, 25 and 24r of Breggren
et al.
1985. Upper and lower limits to the duration of the activity are, respectively, 4.67 Ma and 0.67 Ma, or 4.07 Ma and 0.60 Ma: a duration of about 3.5 Ma is thought probable.

The magnetic evidence supports the view that the Loch Scridain sills correspond to the Early Acid and Intermediate Cone Sheets of Centre 1.
Thrust tectonics of the Dibba zone and the structural evolution of the Arabian continental margin along the Musandam mountains (Oman and United Arab Emirates)
Authors: SEARLE M. P.
Published: December 2007

In the northern Oman Mountains several major thrust sheets of Tethyan oceanic rocks (Dibba zone) and a 12km+ thick complete ophiolite sequence have been thrust westward onto the depressed Arabian continental margin (Musandam zone) during the Turonian to Campanian (late Cretaceous). Early thrusting was dominantly a downward foreland-propagating sequence as the higher earlier thrust sheets are folded around structurally lower and later culminations. Breached roof thrusts, where imbricate thrusts do not always converge with a major roof thrust but in places actually penetrate up into the overlying duplex, are common at outcrop scale affecting adjacent thrust sheets. Late stage ‘breakback’ or ‘leap-frog’ thrusts cut through the whole pile of duplexes on to previously higher, earlier dulexes. Late thrusting in the Musandam zone affects the deeper, previously autochthonous Permian–Cenomanian shelf carbonate sequence and probably also the pre-Permian basement, thrusting these rocks at least 15 km over previously allochthonous Hawasina rocks. Late stage oblique dorsal culmination collapse along the SE margin of the Musandam mountains has downfaulted Dibba zone rocks up to 2.5 km to the SE along a hinged listric normal fault. Strain is transferred from the NE–SW-trending late Cretaceous Dibba zone to the N–S-striking mid-Tertiary Musandam culmination at a major pivot point or transfer zone SW of the shelf carbonate culmination.
Structural evolution of the Shimanto Belt accretionary complex in the area of the Gokase River, Kyushu, SW Japan
Authors: NEEDHAM D. T. , MACKENZIE J. S.
Published: December 2007

The Shimanto Belt of SW Japan is an accretionary complex which developed between Late Cretaceous and Late Miocene. The deeper levels of this complex are exposed along the Gokase River of eastern Kyushu. Deformation in this region took place as a result of southeasterly directed accretionary thrusting, resulting in the development of folds, fabrics and mélanges. All these structures may be attributed to progressive, top to the SE shearing and the mélanges show many similarities to other rock types in fault or shear zones. It is also possible to apply shear zone models to explain extensional structures developed perpendicular to thrust transport direction. It is suggested that deformation took place during underplating beneath the developing prism.
Sulphur diagenesis in freshwater lignites (Hula Basin, Israel): implication for S-C relationships in organic sediments
Authors: BEIN A. , NIELSEN H.
Published: December 2007

A negative correlation between depth and the sulphur content and δ
34
S ratio in organic matter is used to study diagenesis in the freshwater lignites of the Hula Basin. The samples were taken along a 2450 m thick sequence ranging from Miocene to Recent.

A between six and ninefold enrichment in organic sulphur in these lignites compared to that in their living precursors suggests diagenetic control. Since all the water sources of the basin are low in sulphate, oxidation of organic matter must be the main source for this secondary enrichment. The released sulphur reacts with marls interlayered with the lignites, to form gypsum and is taken into the remaining organic matter, probably as ester sulphate. Heavy sulphur isotope composition of the plant material, the gypsum, the humic acids of the lignites at relatively shallow depths and the water sources feeding the basin support this supposition. Negative δ
34
S in the lowermost samples indicate, on the other hand, that further enrichment in sulphur through bacterial sulphate reduction occurs also. The parallel trend of decrease in sulphur content and δ
34
S is explained by the breakdown of ester sulphate-containing compounds with increased maturation and the sulphur derived through bacterial reduction of the ester sulphates and the gypsum is taken into the organic matter.

The transformation of humic acids into kerogens at the very beginning of the maturation stage (up to
c
. 0.3% Ro) involves only a partial breakdown of ester sulphate-containing compounds, as evidenced by the decrease in sulphur content and an insignificant change in δ
34
S. Further transformation at somewhat higher maturation levels (
c
. 0.5% Ro) involves the final breakdown of the ester sulphate-containing compounds and the introduction of bacterially reduced sulphur, probably in a free radical mechanism.

The diagenetic enrichment of organic sulphur in such coals is the sole evidence that the high amounts of organic matter preserved in them actually represent only a fraction of the total organic matter originally formed in the basin.
Metamorphic studies: research in progress
Authors: BRODIE K.
Published: December 2007

Report of a meeting of the Metamorphic Studies Group held at the University of Sheffield, 11 March 1987. The meeting was organized by K. H. Brodie and D. A. Carswell.

A Metamorphic Studies Group meeting on ‘Metamorphic Studies: Research in Progress’ and the sixth Annual General Meeting of the Group were held on 11 March 1987. For the first time the annual meeting was held outside London, in the Department of Geology at the University of Sheffield. It was very well attended, and 10 papers were presented on a range of metamorphic topics, including an invited contribution from
T. Bell
(James Cook University, Australia).

The morning session concentrated on high grade and regional metamorphic topics.
C. R. L. Friend,
in a joint contribution with
A. P. Nutman and V. R. McGregor,
presented the results of a reinvestigation of an area of amphibolite and granulite facies metamorphism in SW Greenland, involving systematic isotopic studies. The area can be divided into zones with significantly different tectono-metamorphic histories that have been tectonically juxtaposed prior to amphibolite acies metamorphism and deformation which affected the whole area.
P. J. Treloar
continued the topic of
P–T
histories of granulite facies terranes in a joint paper with
J. Carney
and
M. J. Crow,
involving an example from NE Zimbabwe. Attempts have been made to date two distinct metamorphic events that have affected the rocks, the second of which involved decompression associated with uplift of the area.
D. A. Carswell
discussed the reality of the postulated inflection
Peridotite sills and metasomatic gabbros in the Eastern Layered Series of the Rhum complex
Authors: BÉDARD J. H. , SPARKS R. S. J. , RENNER R. , CHEADLE M. J. , HALLWORTH M. A.
Published: December 2007

Mapping of the Eastern Layered Series (ELS) of the Rhum ultrabasic complex on the northern flank of Hallival shows that peridotite and allivalite (troctolite or gabbro) layers are laterally discontinuous and vary both in thickness and lithology. Peridotite generally has sharp upper and lower contacts against the allivalites, which sometimes cut across the layering in the allivalite. Reaction, dissolution and hybridization effects between peridotite and allivalite are developed locally. Some troctolite layers terminate as isolated, fingered blocks in peridotite. There are many small peridotite bodies which are clearly intrusive into allivalite and have previously been identified as distinct peridotite sheets and plugs. They are petrographically almost identical to the major stratiform peridotites and in some cases are apophyses from them. We propose that many of the peridotite layers in the ELS formed as thick sills of picritic magma emplaced into a partly solidified, layered troctolite complex. The stratiform gabbros of the ELS are heterogeneous, layered rocks that commonly contain relicts of troctolite and anorthosite. Wavy (metre-scale) contacts between gabbro and troctolite cut across pre-existing grain-size, modal and rhythmic layering with little disruption. These metasomatic gabbros mimic the textures, grain-size and rhythmic layering of their troctolitic protoliths. We propose that many of the ELS gabbros formed as a result of interaction between porous troctolites and a low-temperature basaltic melt. Residual basaltic melt segregated from solidifying peridotite may have caused this metasomatism.
Triassic evaporites and Plio-Quaternary diapirism in western Greece
Authors: UNDERHILL J. R.
Published: December 2007

Wide Triassic evaporite outcrops run parallel to the strike of Hellenide thrust faults in western Greece. They crop out along the leading edges of thrust sheets emplaced as part of the foreland-migrating fold-and-thrust belt. The evaporites acted as the plane of décollement for thrust-sheet emplacement over much of the External Hellenides. Plio-Quaternary diapirs occur locally within the broad outcrops of Triassic evaporite. They are the result of local mobilization of evaporite after its emplacement. Their rise may have been initially triggered by Hellenide thrust-sheet loading and subsequently accentuated by Plio-Quaternary subsidence. Geophysical records and onland exposures show that diapiric rise controlled local sedimentation patterns above the sites of earlier Hellenide thrusts. The subsequent intrusion of diapirs into these cover sediments caused localized extensional structures to develop. This is best seen in SE Zakinthos, where strain-hardened, conjugate faults developed in friable Pliocene sandstones early in the intrusion history. Principal strains calculated from these sets demonstrate patterns of extension sub-orthogonal to intrusion margins. They are comparable with patterns known from other field and experimental examples, but contrast with some recent studies on diapir-related strains.
The lithostratigraphy of the upper Ordovician of central Portugal
Authors: YOUNG T. P.
Published: December 2007

A lithostratigraphic scheme for the upper Ordovician of central Portugal is described incorporating previously-erected local lithostratigraphic terms and new units. Thirteen formations (10 new), including 11 members (eight new), with one named bed, are arranged into four groups (three new). A description of each unit is given, together with comments on its age and correlation. Deposition in a storm-dominated, shallow-shelf environment through Llandeilo and Caradoc time is indicated. Differential subsidence occurred to the NE and SW of the Dornes/Maçâo ‘rise’ particularly during the Caradoc. Lower to mid-Ashgill sequences in the north are dominated by volcanic rocks with subordinate carbonate deposition. Farther south a hiatus preceded the regressive deposits associated with the onset of the upper Ordovician glaciation. Hirnantian glaciomarine deposits occur across most of the area, except for the regions of carbonate and volcanic build-ups, and appear to represent the period of waning of the glaciation. The Ordovician/Silurian boundary is difficult to place, but probably lies close to or within a sheet sand-body which underlies a condensed Llandovery sequence. Lithostratigraphic correlation of the study area with W. Brittany, France, is generally good; the relationship of some of the upper Ordovician units in central Portugal aids the interpretation of the relationships of the equivalent deposits in Brittany.
Joint three-dimensional inversion of gravity and magnetic data from Jombo Hill alkaline complex, Kenya
Authors: DINDI E. W. , SWAIN C. J.
Published: December 2007

Jombo alkaline complex is the largest of the alkaline complexes in Kenya. It has been the subject of several geological and geochemical studies. However, the surface geology puts few constraints on the subsurface shape of the intrusion which we here attempt to determine by simultaneous inversion of gravity and aeromagnetic data.

The major feature of the gravity map is an elliptical high >800g.u. in amplitude centred near Jombo Hill. When the filtered magnetic map is reduced to the pole and pseudo-gravity transformed, a strikingly similar anomaly is revealed, suggesting a common source. Using an iterative least-squares technique, joint inversion of the gravity and unfiltered magnetic data for a three-dimensional model established that both data sets can be adequately modelled by a thick slab tapering upwards from
c
. 29 km depth and striking approximately E-W. Only the upper
c
. 18 km is magnetized which we interpret as an indication of the depth to the Curie isotherm. The body is predicted to be of ultramafic composition capped by the exposed syenites and ijolites.
The Crummock Water aureole: a zone of metasomatism and source of ore metals in the English Lake District
Authors: COOPER D. C. , LEE M. K. , FORTEY N. J. , COOPER A. H. , RUNDLE C. C. , WEBB B. C. , ALLEN P. M.
Published: December 2007

The Crummock Water aureole, an ENE-trending elongate zone of bleached and recrystallized Skiddaw Group rocks, 24 km in length and up to 3 km wide, is a zone in which pervasive metasomatism has modified the composition of the dominantly siltstone and mudstone lithologies. The bleached rocks show a substantial net gain of As, B, K and Rb and loss of Cl, Ni, S, Zn, H
2
O and C. Carbon loss is responsible for the bleaching. There are smaller and more localized net losses of Cu, Fe, Li and Mn, and gains of Ca, F and Si, whilst Co, Pb and REE are at least locally redistributed. Many chalcophile elements show evidence of initial widespread depletion and subsequent local enrichment.

The mineralogy of the rocks is little affected by the geochemical changes. Like their counterparts outside of the bleached zone, the metasomatized rocks consist essentially of quartz, chlorite, muscovite, paragonite and rutile. Small aggregates and porphyroblasts of white mica and chlorite are developed. The metasomatism, which was accompanied by tourmaline veining, is superimposed on a contact metamorphic event. It post-dates the main Caledonian cleavage but pre-dates late Caledonian minor folds.

Rb-Sr whole rock isochrons suggest that the metasomatic event occurred at
c.
400 Ma and was thus associated with the Lower Devonian Shap-Skiddaw granite magmatism and not the earlier Eskdale Granite or Ennerdale Granophyre magmatic events. Modelling of Bouguer anomalies indicates that geological and geochemical constraints are most simply satisfied if the metasomatism is attributed to a buried, elongate, highly evolved granitic body intruded along the northern margin of a major granitic-granodioritic component of the Lake District batholith. The bleached zone is associated with a major lineament, which may reflect basement control on the location and form of the buried intrusion. Loss of metals from the bleached rocks is related to penecontemporaneous and subsequent hydrothermal vein mineralization and demonstrates that Skiddaw Group sedimentary rocks were a source of ore metals in the Lake District.
The geology and geochemistry of Upper Proterozoic granitoids from the Red Sea Hills, Sudan
Authors: KLEMENIC P. M. , POOLE S.
Published: December 2007

Granitoid plutons are common throughout the late Proterozoic basement complex of north-east Sudan, frequently accounting for 60% of the outcrop. Geological, petrological and isotopic studies indicate that two distinct types of granitic plutonism can be recognized. An older diorite to granodiorite assemblage was emplaced between
c.
815 and 724 Ma and a younger predominantly alkali feldspar granite assemblage consisting of numerous discrete high level intrusions was emplaced between
c.
717 and 555 Ma. These granitoids have a calc-alkaline character similar to the volcanic rocks of the basement complex to which they are spatially and temporally related. The geochemical and isotopic data suggest a destructive plate margin rather than a within-plate palaeotectonic association. It is suggested that the granitoids represent an evolutionary trend from early diorite to granodiorite intrusions emplaced above a late Proterozoic subduction zone to later, post-tectonic intrusions emplaced during a late stage of volcanic arc development. The magmas of the early granitoids were probably mantle-derived while the later post-tectonic intrusions may have been derived from magmas produced by partial refusion of the volcanic arc protolith.
The cooling history of Connemara, western Ireland, from K-Ar and Rb-Sr age studies
Authors: Elias E. M. , Macintyre R. M. , Leake B. E.
Published: December 2007

K-Ar age determinations are reported on 26 hornblendes, 17 coexisting muscovites and biotites, nine muscovites and nine biotites from the Dalradian Connemara schists, metagabbros and gneisses. These are supplemented by 19 Rb-Sr whole-rock-mica ages and four
40
Ar/
39
Ar incremental heating ages on micas. A long (at least 75 Ma) and complex cooling history without a simple pattern of ages has been obtained. The ages are not correlated with the grain-sizes or the chemical compositions of the analysed minerals, or the topographic elevation from which the samples came, but there is an irregular increase of hornblende K-Ar ages (to 480 ± 10 Ma) with increase of metamorphic grade southwards towards the metagabbros and gneiss. The preservation of the complex cooling pattern is ascribed to the action of syn- and post-metamorphic slides or faults which moved slices of the massif upwards by varying amounts. Closely spaced samples give results which rule out simple tilting of the massif. Rapid cooling (
c.
30° Ma
−1
) at 480 ± 10 Ma (Arenig-Llanvirn) during the F3 fold phase was followed by slower cooling until 455 ± 10 Ma (Caradoc), when renewed rapid uplift occurred, possibly correlated with the southward thrusting of Connemara and with synchronous renewed uplift in the Scottish Dalradian rocks. Widespread biotite K-Ar and Rb-Sr ages of 440 ± 5 Ma (end-Ordovician) throughout Connemara and the Dalradian rocks of Scotland are ascribed to the major Taconic tectonic event, which docked the Dalradian rocks against the flanking folded Ordovician successions of the Midland Valley and the South Mayo Trough. Biotites giving K-Ar ages of about 415 Ma may have been reset during the late Silurian folding of the cover rocks. The importance of very close sampling integrated with structural studies is emphasized.
Dalradian slides and basin development: a radical interpretation of stratigraphy and structure in the SW and Central Highlands of Scotland
Authors: ANDERTON R.
Published: December 2007

Movements on syndepositional listric faults are responsible for many of the lateral variations in Dalradian stratigraphy. Sedimentary facies and thickness variations show that the surface traces of these faults correspond, in many cases, to what have previously been interpreted as tectonic slides, although normal faults, thrusts and sedimentary contacts may also mark their position.

Three main episodes of faulting are described in the Islay to Lochaber area. Appin Group sedimentation was controlled by a fault now represented by the Loch Skerrols Thrust-Fort William Slide-Ossian steep belt. In the Islay-Jura area an anomalously thick Islay Subgroup succession accumulated in a half-graben bounded by a listric normal fault to the NW and terminated by transfer faults at each end. The deposition of a thick Easdale Subgroup section was controlled by movements on what is now the Benderloch-Ballachulish Slide. This listric fault terminates southwards in the Scarba Fault and probably extends north into the Ossian steep belt.

The thinness of the Appin and lower Argyll Group succession between the Ericht-Laidon and Bridge of Balgie Faults is an original feature. The stratigraphic gap represented by the Boundary Slide here is reinterpreted as a disconformity. However, further east the Boundary Slide may well be a listric fault, movement on which accommodated the thick succession around Schiehallion.

The development of the major structure was influenced by the geometry of the sedimentary basins. The Schiehallion-Tummel area is reinterpreted as lying on the lower limb of the Ben Lui fold. This resolves the problem of the emplacement of the Tay nappe which is likely to be a gravity structure derived from above the Ossian steep belt.
A reappraisal of the facing confrontation in north Cornwall: fold- or thrust-dominated tectonics?
Authors: ANDREWS J. R. , BARKER A. J. , PAMPLIN C. F.
Published: December 2007

The problem of conflicting fold or thrust solutions to the interpretation of the structure of north Cornwall is discussed via new detailed mapping of the coastal sections. In addition to facing evidence, shear criteria have been extensively applied in order to sustain new interpretations. The facing confrontation at Polzeath is confirmed but the high strain zone centred upon Tintagel is ascribed to later superimposed northwards directed ductile shearing. Peak metamorphism was synchronous with this event. Subsequent northwards displacement became confined to discrete thrust planes dismembering the south-facing structures which developed during an intermediate phase of backthrusting. The major structures are thrust-dominated, though large recumbent folds were generated during the early ductile phases. The facing confrontation is probably allochthonous having been carried northwards on a blind thrust. It is suggested that the important Rusey Fault Zone is the roof thrust to a triangle zone in which the lower grade rocks of the Culm Basin are uplifted over an underlying antiformal stack.
Short Paper: The subaerial setting of the Ordovician Borrowdale Volcanic Group, English Lake District
Authors: BRANNEY M. J.
Published: December 2007

Most Ordovician volcanic rocks of southern Britain were produced in volcanic island settings and the Borrowdale Volcanic Group (BVG) of the Lake District has traditionally been interpreted in this way. However, a review of lithofacies associations in the BVG indicates a subaerial environment. Pyroclastic surge deposits, thin welded tuff beds, lava flow morphologies, abundant erosional unconformities, and fluviatile and laharic facies indicate widespread subaerial emplacement. Volcano-tectonic subsidence aided preservation of subaerial facies, although a preservation bias favoured lacustrine deposits. Erosional unconformities above subaerial aggradational volcanic sequences are to be expected, so the sub-Windermere Group unconformity does not necessarily imply tectonic uplift.
A stable isotope study of black shale-hosted gold mineralization in the Dolgellau Gold Belt, North Wales
Authors: BOTTRELL S. H. , SPIRO B.
Published: December 2007

Isotopic determinations have been made on minerals from mineralized veins (including inclusion fluids), altered wall-rocks and host shales from the Clogau-St Davids gold mine and smaller mines in the Dolgellau Gold Belt. The Clogau Formation shales hosting the mineralization contain calcite (mean δ
13
C −11.5‰PDB) and graphite (mean δ
13
C −22.2‰PDB) yielding an equilibrium isotopic exchange temperature of 300-315 ±20 °C, a likely metamorphic temperature. Mineralized vein calcites are isotopically lighter than calcite in the shale with mean δ
13
C of -12.9‰PDB. Mineralized vein quartz oxygen isotopic compositions (9.6-11.0‰ SMOW) are similar to the host shale quartz (9.4-13.6‰ SMOW). Calcites are always too enriched in
18
O to be in equilibrium with quartz in the same sample. It is not possible to distinguish whether this is a primary feature or whether the calcite compositions have been reset subsequent to formation. Chalcopyrite and pyrrhotite in one sample from the Clogau-St Davids mine give an equilibrium formation temperature of 300 ± 50 °C. A relationship exists between the composition of mineralized vein sulphur and that in the host shales, though the ore sulphur is always depeleted in
34
S relative to the shale host. This is most probably due to a mixing of shale sulphur with a lighter, introduced component during ore formation. Deuterium/hydrogen ratios in fluid inclusion water may also indicate the introduction of a fluid into the shales during mineralization.

The stable isotope data show that mineralization in the Dolgellau gold belt probably involved the introduction of an externally derived fluid into the shales of the Mawddach Group and that it interacted to a significant extent with the shales. This interaction makes it difficult to be certain of the original isotopic 'signature' of the ore-forming fluid, but is was probably derived from an igneous source, either as a primary magmatic fluid or as the result of fluid interacting with igneous material during metamorphism.
Short Paper: The depositional age of the Dalradian Supergroup: U-Pb and Sm-Nd isotopic studies of the Tayvallich Volcanics, Scotland
Authors: HALLIDAY A. N. , GRAHAM C. M. , AFTALION M. , DYMOKE P.
Published: December 2007

Zircons from a keratophyre associated with the Tayvallich Volcanics in the Dalradian rocks of the SW Scottish Highlands have been dated by U-Pb methods, yielding an age of 595 ± 4 Ma. This age indicates that most or all of the Dalradian is Precambrian in age, and that Dalradian sedimentation may have lasted for about 200 million years. The age also constrains the time interval between cessation of Dalradian sedimentation and subsequent crustal thickening and regional metamorphism during the Grampian Orogeny. Sm-Nd isotopic data for the Tayvallich Volcanins and related metadolerite sills yield initial ºNd values of +2 to +4, which are thought to reffect the effects of melting of lithospheric mantle.
Mass extinctions: the view of a sceptic
Authors: HOFFMAN ANTONI
Published: December 2007

The concept of periodic mass extinctions by extraterrestrial impacts has contributed very strongly to the current revival of interest in the problem of mass extinctions. In this context, several radical claims have been made, asserting that the newly emerging picture of mass extinctions as a separate class of biological phenomena, caused by a separate class of processes, demands a far-reaching revision of the the neodarwinian paradigm of evolution and implies a vindication of the Cuvierian perspective on Earth's history. The validity of these claims, however, is questionable at all levels: from the interpretation of particular events, to the concept of mass extinctions as a special class of phenomena, to their proposed causation by impacts, and to their alleged historical pedigree and implications for current evolutionary theory. Mass extinctions may actually be clusters of more or less accidentally aggregated-in-time but causally distinct extinction episodes.
Mechanics of fossil vertebrates
Authors: McNEILL ALEXANDER R.
Published: December 2007

All the principal branches of mechanics have been used in studies of fossil vertebrates, including kinematics, statics, solid dynamics, strength of materials, hydrostatics and hydrodynamics. They have been applied to problems concerning jaws, posture, running, swimming, flight and blood circulation.
Ordovician intrusions of the Strumble Head-Mynydd Preseli region, Wales: lateral extensions of the Fishguard Volcanic Complex
Authors: BEVINS R. E. , LEES G. J. , ROACH R. A.
Published: December 2007

A major doleritic and gabbroic sill complex, with minor microtonalitic components, cuts Ordovician strata to the east of Fishguard. Their age has long been equivocal. New geochemical data, combined with petrographic analyses, indicate that the intrusions represent an eastward extension of the Ordovician (Llanvirn) Fishguard Volcanic Complex lavas and high-level intrusions of the Strumble Head region. In addition similar doleritic intrusions crop out to the west of Strumble Head, as far as Traeth Llyfn.

The data presented here confirm earlier conclusions that the FVC has tholeiitic affinities and that compositional variations largely result from low-pressure fractional crystallization. The petrogenetic relationships of the microtonalites are, however, as yet equivocal. Incompatible element discriminant diagrams indicate that the parental basic magmas were derived by melting of a slightly depleted mantle source similar to N-type MORB, to which had been added a minor supra-subduction zone LIL element component. The data are consistent with emplacement of the magmas in a marginal basin environment.
Late-orogenic alkaline/subalkaline Silurian volcanism of the Skomer Volcanic Group in the Caledonides of south Wales
Authors: THORPE R. S. , LEAT P. T. , BEVINS R. E. , HUGHES D. J.
Published: December 2007

Basaltic, andesitic and rhyolitic volcanism was widespread during Ordovician time in the Welsh, Basin. Minor Silurian volcanic rocks occur in the Welsh Borderland and in Pembrokeshire (Dyfed), where they represent the youngest volcanic activity associated with the Caledonian orogeny in Wales. The Skomer Volcanic Group comprises
c
. 760 m of lavas and pyroclastic rocks with minor sedimentary rocks which are best exposed on Skomer Island but crop out over 43 km from the Smalls and Grassholm (in the Irish Sea) to mainland Pembrokeshire. New chemical analyses have been used to characterize these rocks and to compare them with earlier Ordovician volcanic rocks. The Skomer Volcanic Group comprises hawaiite/mugearite lavas and rhyolitic flows and pyroclastic rocks. The rhyolitic rocks are subdivided into a high-Zr group, (originally of peralkaline composition) and an unrelated low-Zr group. The hawaiite/mugearite lavas and high-Zr rhyolites are considered to form a single group, related by fractional crystallization, analogous to basalt–hawaiite–mugearite–comendite associations erupted in within-plate oceanic and continental settings. However concentrations of high field strength elements (Ti, Zr, Nb, Ta, Hf) suggest that the parent magmas may have been derived from a within-plate (ocean island basalt) source modified by the effects of earlier or contemporaneous Caledonian subduction.
Pre-Mesozoic geology of the western and north-western Irish continental shelf
Authors: TATE M. P. , DOBSON M. R.
Published: December 2007

Exploration wells drilled offshore west and north-west of Ireland penetrated a
c
. 3.7 m cumulative thickness of Permo-Carboniferous sediments overlying Dalradian metasediments intruded by Caledonian granitic plutons. In the north, late Tournaisian to early Visean clastics were deposited coeval with the Rheic transgression upon an emergent craton in a continental to marginal marine environment. They record an increasing marine influence upwards with the establishment of a carbonate shelf in the Asbian. Clastic/carbonate sedimentation, which became cyclical in late Visean to early Namurian, temporally correlates with the Yoredale cyclothems. Younger Namurian sediments are not preserved. Sedimentation in early Westphalian progressed from prodelta to coal-enriched delta-top and abandonment facies, and is conformably succeeded by brackish to freshwater sediments that contain evidence for local marine incursions of Stephanian age. An Autuno-Silesian section, that is preserved offshore, may also have been deposited onshore prior to erosion. Seismic data enable partial delineation of N-S and NE-SW early sub-basins marginal to the Main Porcupine Basin. These sub-basins which contain Silesian sediments, may have styled successive extensional episodes in the basin and augment support for rift models of Carboniferous basin development in the North Atlantic/NW European domain. BIRPS data provide supporting evidence for the offshore prolonga­tion of pre-Mesozoic basement fault-zones.
The Outer Hebrides fault: a major Proterozoic structure in NW Britain
Authors: LAILEY M. , STEIN A. M. , RESTON T. J.
Published: December 2007

Mylonites in the Outer Hebrides fault zone on Scalpay, Hams, are cut by metabasite sheets which are probably of Scourie dyke age (2400–2200 Ma) and by pegmatites of late Laxfordian age (c. 1750 Ma). The mylonites contain relicts of a gneissose banding which probably formed in the Archaean
c
. 2700 Ma ago. These age constraints suggest that the fault formed in the Inverian event c. 2500 Ma ago, not in the Caledonian (Silurian-Devonian) as widely reported. The fault is a long-lived structure which was reactivated as an extensional fualt in the Laxfordian, late Proterozoic (Torr-idonian) and the Carboniferous/Early Mesozoic, and as a thrust in the Caledonian. In a regional context this interpretation has important implications for the structural evolution of the Lewisian complex, for the nature of the Caledonian event in the Laurentian craton (Caledonian foreland) and for the age and geometry of the sedimentary basins in the hanging wall of the Outer Hebrides fault.
The sedimentology and diagenesis of organic-rich sediments
Authors: MACQUAKER J. H. S. , CURTIS C. D.
Published: December 2007

Despite comprising more than 60% of the sedimentary record, mudrocks have received disproportionately little study, both in terms of their sedimentology and diagenesis. This is perhaps surprising considering their economic importance as the source rock for most commercial hydrocarbons. To readdress this imbalance, a British Sedimentological Research Group/Petroleum Group meeting entitled ‘The Sedimentology and Diagenesis of Organic-Rich Sediments’ was held at Sheffield University on the 24–25 September 1987. The overall intentions of the meeting were: (a) that it should be interdisciplinary, (b) bring together research workers interested in diverse aspects of mudrock sedimentology and diagenesis (from both organic and inorganic backgrounds) and (c) to expand the generally sketchy mudrock data base.

With these aims in mind some 60 people gathered in Sheffield to hear two days of talks and examine a variety of posters dealing with topics that ranged from palaeoenviron-mental interpretation to inorganic geochemistry of mudrock sequences.

In a keynote address,
I. N. McCave
(Cambridge) presented a stimulating talk on mud deposition in modern environments. He particularly emphasized the important role of particulate organic matter as a clotting agent. He showed that the chemistry of this organic clotting agent influenced its ‘clotting ability’, and that, in turn, its chemistry was dependent on total oxygen concentrations in the water column. As a result he noted that the organic/inorganic ratio in mudrock sequences from different localities varied despite them having similar overall accumulation rates.

Continuing with aspects of mudrock deposition,
P. B. Wignall
(Birmingham) documented the importance of storm events in
The stratigraphic and palaeogeographic setting of the late Cenomanian ‘anoxic’ event
Authors: HART M. B. , LEARY P. N.
Published: December 2007

The late Cenomanian carbon isotope excursion can be shown to be synchronous over most areas of the globe where micropalaeontological data are adequate. The sedimentological variations seen in these different areas are thought to be due to palaeobathymetry. The relationship between this ‘anoxic’ event and the major faunal change in the late Cenomanian is described.
Petrographic examination and re-interpretation of concretionary carbonate horizons from the Kimmeridge Clay, Dorset
Authors: FEISTNER K. W. A.
Published: December 2007

Sections through two concretionary carbonate horizons (Yellow Ledge and Maple Ledge) from the Kimmeridge Clay type locality of the Dorset coast have been examined using back-scattered imaging electron microscopy, X-ray microanalysis and X-ray diffraction. The Yellow Ledge has experienced a complex cementation history involving precipitation of several carbonate phases. Samples from both the centre and margins of this ledge contain juxtaposed low Mg, low Fe calcite and ferroan dolomite. This indicates that the ledge did not simply grow from the centre outwards during progressive burial. By contrast, the Maple Ledge was found to be compositionally and texturally homogeneous, being cemented throughout by interlocking subhedral ferroan dolomite crystals. The observations made in this study indicate that a detailed petrographic examination of concretionary layers must be undertaken if geochemical and isotopic data obtained from the analysis of bulk samples are to be interpreted correctly.
Displaced terranes in Britain and Ireland
Authors: SOPER N. J. , GIBBONS W. , McKERROW W. S.
Published: December 2007

A meeting entitled Displaced Terranes in Britain and Ireland was held at Burlington House on 2 November 1988.

This was the third ‘terranes’ meeting held in Britain since the inception of International Geological Correlation Project No 233 ‘Terranes in the Circum-Atlantic Palaeozoic Orogens’, the others having been held at Cardiff in 1986 and Glasgow in 1987. IGCP 233 grew out of IGCP 27 (‘The Caledonide Orogen’), many of the developments promoted by which are recorded within two Special Publications of the Geological Society (

Harris
et al.
1979

; Harris & Fettes1988). Over the last few years research on displaced terranes in the British Caledonides has accelerated, with new data now challenging what have become traditional applications of plate tectonic theory to the British Caledonides. The reports of work in progress on Caledonian terranes, and it was thought appropriate to take advantage of the rapid publication facilities afforded by the Short Papers scheme. Because of the four-page limit on Short Papers, much introductory material has had to be omitted
so
that we provide here an introduction to the concept of displaced terranes and its applications to British Palaeozoic geology.

Displaced terranes.
The essential value of the terrane concept lies in its recognition of the likelihood of large scale lateral fault movements occurring along active plate margins. The concept of terrane tectonics is therefore no more than a fuller appreciation of the kinematic implications of plate tectonic theory. The impact of this recognition upon our perception of geotectonic processes in general, and the
Short Paper: Arc detritus in the Southern Uplands: mineralogical characterization of a ‘missing’ terrane
Authors: STYLES M. T. , STONE P. , FLOYD J. D.
Published: December 2007

Within the late Ordovician-early Silurian Southern Uplands thrust belt, quartzo-feldspathic greywackes derived from the north are interbedded with volcanic-rich greywackes of southerly provenance. Abundant fresh clinopyroxenes which occur in the latter, both as single crystals and phenocrysts in lithic fragments, have compositions that indicate a calc-alkaline island arc origin.
Short Paper: Cadomian terrane tectonics and magmatism in the Armorican Massif
Authors: STRACHAN ROBIN A. , TRELOAR PETER J. , BROWN MICHAEL , D’LEMOS RICHARD S.
Published: December 2007

The North Armorican composite terrane, NW France, is a collage of displaced terranes which result from the amalgamation of Cadomian continental arcs and marginal basin complexes by sinistral transpression along a continental margin above a subduction zone. Early Cadomian arc activity occurred at
c.
700-650 Ma, but terrane accretion did not occur until
c.
540 Ma, and post-tectonic magmatism persisted well into the Palaeozoic. Cadomian events thus span a considerably greater period than previously supposed.
Crustal structure of the Goban Spur continental margin, Northeast Atlantic, from deep seismic reflection profiling
Authors: PEDDY C. , PINET B. , MASSON D. , SCRUTTON R. , SIBUET J. C. , WARNER M. R. , LEFORT J. P. , SHROEDER I. J. ,   BIRPS,   ECORS
Published: December 2007

Deep reflection data across the UK continental margin are used to test theories concerning the origin of a pervasive reflective fabric in the lower continental crust seen on many deep reflection profiles worldwide. The origin of the reflections is at present little understood, but clearly represents an important crustal process. The Western Approaches Margin (WAM) line was designed to determine the behaviour of the lower crustal reflections in crust which has undergone significant extension. The WAM profile across the continental shelf is characterized by bright, continuous lower crustal reflections, some of which reach 30 km in length and have minimum reflection coefficients of 0.1. Deep water multiples in the area of the continental slope partially obscure reflections from the lower crust, but detailed analyses show that reflectors are present in the lower crust on the continental slope. Adjacent to the continent-ocean boundary, the lower crust on the WAM profile is well-imaged and free of noise and multiples. This very thin crust exhibits the same reflectivity pattern as is observed on the continental shelf, i.e. the upper half of the crust is blank and the lower half contains sub-horizontal reflections. The lower crustal reflections adjacent to the continent-ocean boundary are less continuous than the reflections seen on the shelf, and are interpreted as being the remnants after extension of the lower crustal reflectors observed on the continental shelf. The lower crustal reflectors are thus interpreted to pre-date the extensional episode that led to the opening of the Atlantic. The preservation of thinned sections of both transparent upper crust and reflective lower crust adjacent to the continent-ocean boundary suggests that extension by pure shear was the mechanism by which the crust was thinned in the Goban Spur area.