bluuebunny/crossref_metadata_2025 huggingface dataset:
A pre-D2 age for the 590 Ma Ben Vuirich Granite in the Dalradian of Scotland
Authors: TANNER P. W. G. , LESLIE A. G.
Published: December 2007

Regional mapping demonstrates that S2 is the main fabric in the Central Highland Dalradian and that it is this fabric, not S3 as previously thought, which overprints the 590 Ma Ben Vuirich granite. The granite appears to have been emplaced following a tectonic event (D1) in the Dalradian rocks, for a finely-spaced penetrative fabric seen in newly-discovered hornfelses from the granite aureole is overgrown by contact metamorphic cordierite and andalusite porphyroblasts (now both altered) which predate the D2 metamorphism.

The Ben Vuirich granite cuts the Neoproterozoic Dalradian sequence of the Central Highlands of Scotland (Fig.1,

inset), and was first recognized as an
older foliated granite
by Barrow
et al.
1905. Later workers (

Bradbury
et al.
1976

;
Bradbury 1978
) concluded that the granite was intruded between D2 and D3 in the Dalradian rocks (

Harris
et al
. 1976

) and for many years an U-Pb zircon age of 514 ±7 Ma from the intrusion (Pankhurst & Pidgeon 1976) provided a valuable datum point in the evolution of the Scottish Caledonides. However, Dalradian chronology has recently undergone a radical revision following the determination of a precise U-Pb age of 590 ± 2 Ma for the emplacement of the Ben Vuirich granite (

Rogers
et al.
1989

). In accepting the interpretation of Bradbury
et al.
1976 that the regional penetrative fabric which affects the country rocks (and is continuous with that in the granite) is of D3 age, Rogers
et al.
were also guided by the conclusions of Harte
et al.
1984 and Dempster & Harte
Volume balance of erosional loss and sediment deposition related to Himalayan uplifts
Authors: JOHNSON M.R.W.
Published: December 2007

The volume of sediment in the foreland basins, deltas and fans which are a consequence of the collision of India and Asia appears to require the removal of a vertical section
c
.35km high if the sediments were entirely derived from the Himalaya. It is argued that this is implausible and that it is likely that the Himalayan hinterland (e.g. Karakoram and Tibet) was an important source of sediment prior to
c
. 20 Ma when significant Himalayan uplift commenced.
Geochemical evolution of an Ordovician island arc, South Mayo, Ireland
Authors: CLIFT P. D. , RYAN P. D.
Published: December 2007

Early Ordovician volcanic rocks exposed in the South Mayo region of western Ireland document the history of a volcanic arc complex, produced following the initiation of south-dipping subduction within the Iapetus Ocean in the Late Cambrian or Early Ordovician. Lavas of the Lough Nafooey Group (Tremadoc-Arenig) show an eruption history marked by an initial period of tholeiitic basaltic volcanism, followed by a shift to increasing silica compositions of calc-alkaline affinity. Subsequent volcanism of the Tourmakeady Group (Arenig) shows a mixture of rhyolitic and andesitic compositions, which are in turn succeeded by tuffs of the Rosroe and Mweelrea Formations of the Murrisk Group (Llanvirn). These are also rhyolitic and andesitic but are generally more siliceous. The overall increase in silica contents may reflect increasing fractional crystallization due to crustal thickening.

Trace element studies of the basaltic members of the Lough Nafooey Group show a strong subduction zone influence. While absolute levels of immobile trace and rare earth elements decrease upsection, relative depletion of the most incompatible elements decreases over the same time period. These trends suggest decreasing degrees of partial melting of a progressively more depleted source mantle upsection.

The basaltic Bohaun Volcanic Formation (Tremadoc-Arenig ?) shows greater depletion in its incompatible trace elements than the Lough Nafooey Group, so indicating a more depleted mantle source. Comparison with modern arc systems suggests that the Bohaun Volcanic Formation was probably generated shortly after the initiation of subduction and was erupted in a more trenchward position than the Lough Nafooey Group (i.e. in the forearc). This confirms a south-dipping polarity to the subduction zone, and suggests an age of initiation during the Late Cambrian or earliest Ordovician.
Pelagic fluxes and sedimentation
Authors: CRONAN D. S. , WAKEFIELD S. J.
Published: December 2007

In February 1992 there was a two day meeting of the Marine Studies Group dealing with pelagic fluxes and sedimentation. Day 1 covered several topics while Day 2 focused on BOFS (Biogeochemical Ocean Flux Study). The BOFS work is being published elsewhere and so the present thematic set and the meeting summary below concentrates on the topics covered during Day 1.

The floor of the ocean is the site of deposition for particles from any source that are transported down through the water column. The sediments record both about past activities in the overlying waters and about past inputs to those waters. Present-day oceanographic processes are analogues for the past, but the data must be interpreted correctly. To do that some idea is needed of what happens to particles on their way down, and what modifications to the record take place upon sedimentation.

Amongst the main themes of the meeting were the variability of the fluxes, and the coupling of the surface waters with the bottom sediments. Although the story that can be gained from isotopes provides extra details, there is still a great deal of information to be gleaned from detailed examination of the trace-metal content of particulates.
R. Chester
showed how the trace-metal composition of particulate material undergoes modification during its journey from the sea surface to the sea-floor. What is essentially a ‘fingerprint’ of solid-state speciation can be determined by looking at which carrier phase contains which trace element. It was shown that organically bound copper in
Microbial and meiofaunal abundance, redox potential, pH and shear strength profiles in deep sea Pacific sediments
Authors: MEADOWS P. S. , REICHELT A. C. , MEADOWS A. , WATERWORTH J. S.
Published: December 2007

Vertical profiles of microbial and meiofaunal abundances, and of redox potential, pH, water content and shear strength have been measured in sediment subcores from seven box cores collected in the southern Central Pacific and the findings have been related to potential effects on pelagic fluxes and sedimentation. Microbial and meiofaunal abundance decreased exponentially with sediment depth. Micro-organisms were found throughout the depth of the core to 40 cm. Meiofauna were only present in the top 5 cm to 10 cm. Meiofaunal metazoans decreased more rapidly than foraminiferans. Redox potential was always positive, decreased below the surface and often rose again deeper in the sediment. pH showed no clear cut trend with depth. Shear strength increased with sediment depth and showed occasional anomalous fluctuations. Water content decreased progressively downwards with the widest scatter in the top 5–10 cm. The most important implications of these results for the water column above are likely to be biologically mediated organic mineralization and nutrient regeneration into the water column, and biodeposition of heavy metals and the binding of particulates at and above the sediment—water interface by extracellular polymeric material, which will in turn lead to differential patterns of sedimentation.
Evidence for Internal Zone unroofing from foreland basin sediments, Betic Cordillera, SE Spain
Authors: LONERGAN L. , MANGE-RAJETZKY M. A.
Published: December 2007

Tertiary sediments at the boundary of the Internal and External Zones in the eastern Betic Cordillera record evidence of compressive deformation in the Internal Zone and its subsequent exhumation. Sediments on the north side of the Sierra Espun̄ associated with the structurally highest unit of the Internal Zone, the Malaguide Complex, are interpreted as a foreland or piggyback basin which formed in response to the deforming Internal Zone. Basin evolution has been reconstructed using an integrated analysis of sedimentary facies, petrography, heavy minerals and structural data. The basin fill contains Eocene shallow-marine carbonate shelf/ramp sediments, early Oligocene fan conglomerates and coralgal reefs transitional to probable slope deposits of the late Oligocene, and a late Oligocene to mid-Miocene deep-water sequence with pelagic faunas. The provenance analyses illustrate an unroofing history of the Internal Zone orogenic edifice. Locally uplifted Mesozoic to Eocene carbonates at the thrust front were the source of lower Oligocene conglomerates. Clastic detritus supplied to the late Oligocene to Aquitanian basinal sediments was mainly derived from the Permo-Triassic and Palaeozoic basement of the Malaguide Complex. The presence of small amounts of first-cycle metamorphic minerals indicates the start of exhumation of the deeper metamorphic Alpujarride Complex. From the Burdigalian onwards a metamorphic terrane dominated the source area of the deep-water sediments, and by the Langhian high pressure minerals from the deepest parts of the Internal Zone orogenic edifice appear in the sedimentary sequence. Rapid exhumation recorded in the early Miocene sediments may be the result of extensional exhumation of the Internal Zone metamorphic complexes.
Was Scotland a Vendian RRR junction?
Authors: SOPER N. J.
Published: December 2007

In Early Palaeozoic time northern Scotland occupied a 120° promontory between the Appalachian and Greenland Iapetus-facing margins of Laurentia. It is proposed that this originated at a triple rift (RRR) jundion between Laurentia and two other continents, possibly Baltica and West Gondwana. The third arm is marked by trans-Caledonoid extensional lineaments that controlled Vendian sedimentation and MORB-type volcanicity in the Grampians. The model accounts for the huge scale of the SE-prograding Moine-Dalradian clastic wedge as compared with Neoproterozoic sequences elsewhere on the E Laurentian margin where an extensional regime persisted throughout the Grenville-Caledonian interval. Evidence currently cited for Precambrian orogeny in the Highlands needs to be reappraised from that viewpoint. Major stretching events are inferred at
c
. 750 and 600 Ma; anorogenic ‘older granite’ magmatism was associated with the latter, which culminated in the opening of Iapetus.
Cadomian
40
Ar/
39
Ar apparent age spectra of detrital muscovites from the Eastern Alps
Authors: DALLMEYER R. D. , NEUBAUER F.
Published: December 2007

Concentrates of detrital muscovite were prepared from the Bischofalm Quartzite (late Ordovician) exposed in the Southalpine Carnic Alps, Austria. Two size fractions (150–175 pm and 85-100 μm) from two samples display similarly discordant
40
Ar/
39
Ar age spectra in which apparent ages systematically increase from c. 300 Ma to c. 600 Ma in low-temperature portions of each analysis. Intermediate- and high-temperature increments are characterized by apparent ages of
c
. 620–645 Ma. Characteristics of the age spectra suggest minor late Palaeozoic, Variscan or combined Variscan/Alpine rejuvenations of argon isotopic systems within detrital grains which originally crystallized within a late Precambrian source terrain. The detrital muscovite ages provide evidence for a Cadomian-age source region for some basement sequences within the Austroalpine/Southalpine sectors of the Eastern Alps. The occurrence of Cadomian-age detritus in the Eastern Alps suggests links with Armorica and other, similarly aged tectonic elements within Alpine chains initiating along northern margins of Gondwana.
Subsidence analysis of Eastern Avalonian sequences: implications for Iapetus closure
Authors: KING L. M.
Published: December 2007

Basin subsidence analysis, employing the backstripping method, indicates that fundamentally different basin-generating mechanisms controlled Silurian basin development in northern England and in Wales. Accelerating subsidence with time suggests that the dominant control on basin formation in northern England was flexure of the Eastern Avalonian lithosphere. Mid-Wales, however, displays dominantly decelerating subsidence, suggesting an extensional control. The errors involved with the backstripping method, particularly concerning Lower Palaeozoic application, have been addressed.

In conjunction with sedimentological and tectonic data, the subsidence patterns for the Windermere Supergroup indicate deposition in a flexurally-loaded foreland basin which migrated southwards with time. Similar data for the Powys Supergroup of Mid-Wales imply an extensional control on Silurian deposition, possibly in a strike-slip setting. However, later Pøídolí and early Devonian Welsh sedimentation may represent the onset of foreland basin deposition. The new data are consistent with a diachronous closure of the Iapetus ocean, related to rotational collision of Eastern Avalonia with Laurentia during the early Silurian. Early Silurian oblique collision produced transtension in Wales. Subsequent rotation of Eastern Avalonia produced more orthogonal collision. Thus, continental flexure was felt initially in northern England in late Wenlock times, followed by migration of the resultant foreland basin to the southeast into Wales in early Devonian times.

The versatility of the backstripping method is shown by this application to Lower Palaeozoic sequences.
Structural synthesis of S Staffordshire, UK: implications for the Variscan evolution of the Pennine Basin
Authors: WATERS C. N. , GLOVER B. W. , POWELL J. H.
Published: December 2007

The presence of strata of Westphalian A to possibly Stephanian age in south Staffordshire, located at the southern margin of the Pennine Basin, provides a rare control on the timing of late Carboniferous deformation in Central England. During Westphalian A—C times, sedimentation in south Staffordshire was controlled by thermal subsidence, with a superimposed influence of synsedimentary normal and reverse faulting. Alkaline basic igneous activity occurred during Westphalian C times, synchronous with uplift of the southern margin of the Pennine Basin. Uplift culminated, during Westphalian C—D times, in a phase of Variscan inversion along basement lineaments and the development of a regional unconformity. Renewed subsidence and deposition of a relatively thick sedimentary sequence of Westphalian D to possibly Stephanian age was followed by a final phase of Variscan deformation, comprising possibly two pulses of uplift, tilting and erosion.

Observations presented for south Staffordshire, in conjunction with studies from other parts of the Pennine Basin, indicate a complex interrelationship between rifting, thermal subsidence and Variscan basin inversion. Pulses of N—S extension and E—W to NW—SE compression are interpreted as the product of regional strike-slip deformation, superimposed upon a regime of thermal subsidence. Alkaline igneous activity is considered to be the product of mantle partial melting in response to limited decompression due to transtensional displacements on crustal thickness faults.
Correlation of Upper Jurassic-Lower Cretaceous continental sequences from the southern Biscay margin, northern Spain
Authors: PLATT N. H. , PUJALTE V.
Published: December 2007

Biostratigraphical data indicate that fresh-water carbonate deposits from the southern part of the the Cantabrian Basin, northern Spain are of earliest Cretaceous (Berriasian) age. Charophyte assemblages enable correlation with fresh-water carbonate rocks of the western Cameros Basin to the south. The charophytes also support correlation with Berriasian marginal marine carbonate sequences in the Jura Mountains, southern England and northern Germany, indicating that these continental carbonate rocks represent fresh-water equivalents of the 'Purbeckian' facies. The biostratigraphical correlation is enhanced by the parallel facies evolution observed in the Upper Jurassic-Berriasian of the Cameros and Cantabrian basins. Interpretation of these successions as non-marine depositional sequences permits member-scale correlation between the two basins. Further strong facies similarities in the overlying alluvial clastic rocks of the two areas suggest a revised correlation of the overlying Valanginian-Albian deposits.

The two basins were connected at least intermittently during the Late Jurassic-Early Cretaceous; only during periods of higher sea-level did facies differences develop as a result of greater marine influence in the north. At other times, vast alluvial, lake and marsh systems extended over the region, leading to the deposition of genetically-related alluvial and lacustrine or palustrine deposits in these linked half-graben systems.

In Cantabria and the Cameros Basin, strong lateral variations in facies, thickness and stratigraphy reflect erosion at the onset of Late Jurassic rifting, differential subsidence in the Early Cretaceous, and erosion beneath Aptian-Albian unconformities.

Improved understanding of the successions described here may assist interpretation of similar Upper Jurassic-Lower Cretaceous sequences on the northern margin of the Biscay rift, such as those in the Wessex and North Celtic Sea basins.
Geofluids '93
Authors: PARNELL J.
Published: December 2007

Report of a conference held at Torquay on 4–7 May 1993. The conference was supported by The Geological Society of London, The Institution of Mining and Metallurgy and The Institute of Petroleum. The organizers were C. Cornford, S. Lawrence and J. Parnell. The conference incorporated a special symposium on Deformation and Fluid Flow organized by R. J. Knipe.
Depletion of gold and LILE in the lower crust: Lewisian Complex, Scotland
Authors: CAMERON E. M.
Published: December 2007

Samples of Lewisian granulite ranging from mafic to felsic in composition, average 0.54 ppb Au, well below the crustal abundance of about 4 ppb Au, but higher than granulite from the Bamble shear belt, Norway, with 0.22 ppb Au. Loss of gold during metamorphism requires modification of the primary sulphide phases, mainly pyrrhotite, that host gold in igneous rocks. Pyrite, formed in part by replacement of pyrrhotite, is the principal sulphide mineral of both the Lewisian and Bamble granulites. Conversion of pyrrhotite to pyrite in the Lewisian was by oxidized fluids during Inverian retrogression at temperatures below 625°C. Some grains show replacement of pyrrhotite by pyrite; ion microprobe analyses of such a grain shows an order of magnitude less gold in the pyrite than in the pyrrhotite. Scourie quartz dolerite dykes, which were intruded after Inverian retrogression, but while the Lewisian complex was still deep in the crust, retain pyrrhotite and average 1.47 ppb Au. Transformation of pyrrhotite to pyrite in the Bamble granulites also occurred well after peak metamorphism. Both groups of granulite are depleted in LILE (large ion lithophile elements) and in gold, but the Lewisian samples are more strongly depleted in LILE (median K/Rb of 1909 v. 1079 for Bamble granulite) and Bamble samples are more strongly depleted in gold. For both the Lewisian and Bamble rocks, LILE depletion was associated with prograde metamorphism, whereas loss of gold was a later and essentially unrelated event.
Sill emplacement in wet sediments: fluid inclusion and cathodoluminescence studies at Grunehogna, western Dronning Maud Land, Antarctica
Authors: KRYNAUW J. R. , BEHR H.-J. , VAN DEN KERKHOF A. M.
Published: December 2007

The Grunehogna and Kullen sills of western Dronning Maud Land were emplaced into water-saturated, partially lithified sediments of the mid-Proterozoic Ritscherflya Supergroup. Fluid inclusion and optical and scanning electron microscope cathodoluminescence studies of quartz grains within and adjacent to the sills (the ‘sill-sediment interactive zone’, referred to as the ‘interactive zone’) showed that only a single generation of later fluid inclusions is present. The fluids are Na-K-Ca-Cl aqueous solutions with temperatures of homogenization between 95 and 105 °C. Individual quartz particles in the sedimentary rocks above the interactive zone have unique fluid inclusion populations. Petrographic and cathodoluminescence evidence indicated that there are three types of quartz grains in the interactive zone. These consist of (i) a xenocrystic type, (ii) a high-temperature quartz that crystallized either from the magma or as a result of anatectic melting of xenolithic (and/or xenocrystic) material, and (iii) euhedral quartz crystals that have grown in vugs in the peripheral zone. Cathodoluminescence of xenocrystic grains indicated the existence of healed microfractures along which fluid inclusions were concentrated and secondary and authigenic overgrowths on older generation quartz grains had developed. The presence of zonal luminescence patterns of detrital particles, coupled with evidence for equilibrium polygonization in the sedimentary rocks adjacent to the peripheral zone up to about 8 m above the upper contact of the Grunehogna sill suggested local thermal tempering or sintering and recrystallization. Luminescence of detrital quartz particles in the overlying Hogfonna Formation reflected variation in the characteristics of their source terrains, and showed limited evidence of heating by the magma. The interactive zone fluid inclusions are formation waters which penetrated microfractures during diagenesis. We show that no large-scale hydrothermal circulation system is likely to develop following intrusion into a water-saturated environment.
Significance of early scapolite in greenschist-facies marbles from the Reynolds Range Group, central Australia
Authors: BUICK I. S. , CARTWRIGHT I.
Published: December 2007

Marbles from the Upper Calcsilicate Unit of the Reynolds Range Group, central Australia, experienced regional Proterozoic greenschist-facies metamorphism, but preserve relicts of an earlier, higher temperature assemblage. The earlier assemblage is typified by intermediate scapolite that predates regional metamorphic fabrics, and it is pseudomorphed by assemblages that were stable during regional greenschist facies metamorphism. The scapolite was formed at temperatures in excess of 540–575 °C, far higher than the regional metamorphic temperatures (400–450 ºC). Marbles and associated pelites display variable, but lower δ
I8
O values than are typical of greenschist facies metasediments. Scapolitization and lowering of isotope ratios probably resulted from pre-regional metamorphic contact metamorphism around early granites.
Buffering of melt oxygen fugacity by sulphur redox reactions in calc-alkaline magmas
Authors: MATTHEWS S. J. , JONES A. P. , BEARD A. D.
Published: December 2007

Calculated oxygen fugacities of magmas from Lascar Volcano, Northern Chile, and the 1991 eruption of Mount Pinatubo, Phillipines, are seen to increase with decreasing temperature, relative to the fayalite-magnetite-quartz buffer. Isopleths of constant H
2
S/SO
2
at moderate pressures (2–4 kbar) lie parallel to the data. We conclude that magma mixing prior to eruption liberated large quantities of SO
2
gas by oxidation of dissolved sulphide in the mafic end-member. Oxygen fugacity of the magmas was then buffered by reduction of this gas to H
2
S, leading to increasingly oxidized magmas. Such a process might be used to explain the highly oxidized nature of other sulphur-rich evolved calc-alkaline magmas, notably the El Chichon trachyandesite.
Trace element geochemistry of orogenic igneous rocks and crustal growth models
Authors: TARNEY J. , JONES C. E.
Published: December 2007

Some of the more important constraints on crustal growth mechanisms are reviewed in relation to the trace element composition of orogenic igneous rocks, with specific reference to granitoids, and in relation to tectonothermal models of subduction zones. The wide range of magma types associated with subduction zones is difficult to explain by a single magma-generating mechanism. This variation can be accommodated if some lavas ( are tholeiites and boninites) are formed through wet melting of upwelling asthenosphere at the initiation of oceanic subduction, whereas others are generated from the mantle wedge during the more mature stage of ares, with either hornblende or phlogopite dominated assemblages, where induced convection in the mantle wedge enables source replenishment. Yet other products are generated through melting of the slab (±subducted sediment), but only where the subducting ocean crust is very young and/or warm.

The range and variation of trace-element patterns displayed by basic lavas is also prevalent in more siliceous compositions, implying that the mantle component in many granitoids may be much higher than is commonly assumed. As well as the traditional I-type, S-type and A-type granites with generally low-Ba and low-Sr characteristics, there is another important group of granitoids that has high-Ba and high-Sr concentrations, and that consistently has about 10 trace-element characteristics that are complementary in nature to the first groups. This type of granitoid is common in the Arehaean, rare in the Proterozoic, but is surprisingly abundant amongst post-Cretaceous to Recent orogenic volcanic and plutonic rocks. Trace element patterns may be inherited from mineralogically-controlled element fractionation occurring deep in the lithosphere; major element characteristics are more dependent on mineral-fluid control during magma generation.
Tectonomagmatic evolution of the Kenya rift valley: some geological perspectives
Authors: MACDONALD R. , WILLIAMS L. A. J. , GASS I. G.
Published: December 2007

The Cenozoic Kenya rift valley has developed in an area of great lithospheric complexity, partly related to late Proterozoic orogenic events, which has strongly influenced the regional patterns of faulting, subsidence, uplift and magmatism. The mantle sources of the magmas are poorly understood, but seem to be heterogeneous as regards trace elements and isotopes, and to have undergone one or more depletion-enrichment events. Mafic magmas erupted in the rift zone may have both asthenospheric and lithospheric components, but the evidence is equivocal. Uncertainties about geothermal gradients and the abundances and speciation of volatiles in the melting zone make it difficult to constrain the depths and degrees of partial melting. Partial fusion may, however, have taken place over the depth range 100+ to 23 km. The largest volumes of basalt last equilibrated at pressures <l5 kb and have trace element characteristics consistent with generation in the spinel-garnet peridotite transition zone.

The thickness and internal structure of the lithospheric mantle and attempts to interpret rift evolution in terms of lithosphere-asthenosphere interactions are poorly constrained. Evidence in favour of active (plume-driven) rifting includes the long wavelength gravity and topographic anomalies, the volume of eruptive rocks, the relative timing of magmatism and rifting, the distribution of geophysical anomalies, and the pattern of the tectonomagmatic evolution of the rift. A passive role for the asthenosphere is suggested by the continent-scale correlation between episodes of rifting and magmatism and by fault geometry. Active and passive mechanisms may have acted together in rift development.
Basin structure and Tertiary magmatism on Skye, NW Scotland
Authors: BUTLER R. W. H. , HUTTON D. H. W.
Published: December 2007

The emplacement of igneous material into upper crustal rocks of sedimentary basins is likely to be strongly controlled by the geometry of the pre-existing basin structures. These controls are investigated using examples from the Tertiary igneous complexes of Skye, part of the Sea of Hebrides basin of NW Scotland. The basin consists of an array of half-graben related to SE-dipping normal faults. These pre-volcanic, Mesozoic structures are traced near the igneous complexes using geological relationships preserved unconformably beneath the widespread basaltic lava fields. The unconformity represents a period of Cretaceous uplift and denudation of the basin and its flanks, entirely pre-dating the Tertiary volcanism of NW Scotland. This unconformity seals stratigraphically the major basin faults, preserving field relationships that permit the tracing of these faults in the country rocks to the Tertiary intrusions. The major Camasunary fault is separated from the Raasay fault via a series of minor graben, linked by a series of steep, NW–SE-trending faults that transfered Mesozoic displacements between the principal fault strands.

A broad range of igneous material of various compositions was intruded into part of the Mesozoic Sea of Hebrides basins and their flanks during Palaeocene times. Different emplacement styles and different structural controls are found. The major gabbroic centres do not appear to be controlled by upper crustal structures, having been emplaced into the footwalls of major faults. However, minor synmagmatic displacements on the basin faults may have been sufficient to generate dilatational sites in these footwall positions, thereby facilitating emplacement. In contrast, the granitic melts have been emplaced as sheets and domed into the sediments and overlying lava pile, reactivating segments of the basin fault network. Doming occurred from an array of sills, the stratigraphic levels of which can be reconstructed using structural relationships preserved in the roofs and walls of the intrusions. The sill levels and their transgressive forms are strongly related to inferred Mesozoic basin structures. The major fold structures of Tertiary age in southern Skye are interpreted as accommodating granitic emplacement rather than crustal shortening. The NW-SE Mesozoic transfer fault trend appears to have strongly influenced the segmentation of the granite domes. These interpretations are illustrated using field relationships mapped in the vicinity of the Coire Uaigneich granophyre. It is concluded that although the higher parts of the basin faults were reactivated to facilitate the doming of granitic intrusions, the deeper levels of the Mesozoic faults show no evidence of substantial reactivation.
Tectonothermal evolution of the Neoproterozoic Grampian and Appin groups, southwestern Monadhliath Mountains, Scotland
Authors: PHILLIPS E. R. , KEY R. M. , CLARK G. C. , MAY F. , GLOVER B. W. , CHACKSFIELD B. C.
Published: December 2007

Metasedimentary rocks of the Neoproterozoic Grampian and Appin groups exposed in the Glen Roy district (Monadhliath Mountains, Scotland) were strongly folded during an initial progressive tectonothermal event of the Caledonian orogeny. The attitude and style of the first folds varies from open, upright NE-trending structures in the west (e.g. the Appin Synform) to recumbent NW-facing nappes in the east (e.g. the Treig Synform). These early nappes occur only south of the Corrieyairack Complex and their amplitude decreases towards the NW. Ductility contrasts induced sliding such as that locally recorded along the Grampian-Appin group lithostratigraphical boundary. Subsequently the early structures were deformed by tight, upright ‘D2’ folds. NW-thrusting along the Eilrig Shear Zone, during ‘D2’, is interpreted as occurring in response to a transfer of bulk transport to deeper structural levels relative to ‘D1’. Prograde metamorphism accompanied deformation, with the thermal peak in the Glen Roy district
(T =
500–600 °C;
P
= 7.0–8.0 kbar) coinciding with ‘D2’ folding. It is suggested that the regional variation in the style of deformation was controlled, at least in part, by pre-existing major structures, most notably fundamental, transverse basement lineaments. Temporal changes in the style of deformation occurred in response to changing crustal architecture during progressive deformation. The study demonstrates the diachronous nature of deformation and also illustrates how its is possible to preserve areas with simple deformation histories in the centre of a major orogenic belt.
Tectonic implications of Late Cenozoic sedimentation from the Coastal Cordillera of northern Chile (22–24ºS)
Authors: HARTLEY A. J. , JOLLEY E. J.
Published: December 2007

Late Cenozoic sediments from northern Chile (22–24ºS) are exposed on the coastal plain between the Coastal Cordillera to the east and tilted fault blocks of the Mejillones Peninsula to the west. During the mid-Miocene to Pliocene (?up to mid-Pleistocene) a shallow marine basin developed unconformably over basement. Sedimentation was initially restricted to a small half-graben on the western margin of the Mejillones Peninsula during the Miocene. Expansion of the basin during the early Pliocene resulted in widespread shallow marine sedimentation across the study area. Alluvial, aeolian and beach sediments were restricted to the basin margins, where sediment was supplied from the Coastal Scarp (a major cliff-line bounding the western margin of the Coastal Cordillera) and isolated (islands) fault blocks. Areas of restricted clastic input were characterized by carbonate deposition.

Marine planation surfaces or terraces (0.5–600 m elevation) and associated palaeo-sea cliffs cut into Miocene–Pliocene sediments and basement rocks, developed along the coastline of northern Chile in the ?late Pliocene to late Pleistocene. A similar age for a number of late Pleistocene terraces (125 000 years) now at different elevations, suggests that they were cut during interglacial highstands (oxygen isotope stage 5). Variations in terrace elevation are attributed to faulting.

Regional scale uplift (over hundreds of kilometres) of shallow marine sediments beneath an extensive pre-mid Pleistocene shoreline is considered to be related to variations in subduction zone geodynamics. The favoured hypothesis is that of aseismic ridge subduction.
Geochemistry and tectonic environment of Ordovician meta-igneous rocks in the Rudawy Janowickie Complex, SW Poland
Authors: WINCHESTER J. A. , FLOYD P. A. , CHOCYK M. , HORBOWY K. , KOZDROJ W.
Published: December 2007

A bimodal suite of Ordovician metavolcanic rocks in the Rudawy Janowickie Mountains and the Lasocki Ridge (Sudetic region, SW Poland) is dominated by voluminous epidote amphibolites and greenschists derived from basalt, dolerite and gabbro precursors. Less abundant felsic components comprise both mylonitized granitic sheets that intrude the mafic rocks, and minor interlaminated volcanic rocks ranging from andesite to rhyolite in composition. These rocks are collectively referred to as the Rudawy Janowickie Complex. In chemical terms the mafic volcanic rocks define three north-south-trending provinces (western, central and eastern) which are separated tectonically by broad major mylonite zones. The western province contains all the alkali basalts in the study area and minor tholeiites, all exhibiting within-plate features: specifically high Zr/Y (>4) and enriched chondrite-normalized incompatible element patterns. The central province contains two fractionated tholeiitic suites (Leszczyniec and Okraj groups) with low Zr/Y ratios (2.0-3.5), depleted to flat REE patterns, and incompatible element compositions ranging from normal MORB to slightly enriched MORB. The eastern province has some features in common with the central province, notably metabasalts with normalized flat REE patterns, although it is also characterized by metagabbro bodies with enriched light REE patterns ((La/Yb)
N
4-7), but depleted high field strength element contents. In tectonic terms the Rudawy Janowickie complex is interpreted as a former intracratonic rift floored by oceanic crust and bounded to the west by an attenuated continental margin. Existing data do not allow a definitive conclusion about the exact nature of the eastern margin of the basin: it might, in part reflect the presence of a distant subduction zone.
Early sediment fabrics
Authors: KEMP A. E. S.
Published: December 2007

The papers in this thematic set arise from a meeting sponsored by the British Sedimentological Research Group and held at Burlington House on 3 November 1992. About 50 people heard 14 papers of which seven are published in this thematic set.

The study of early sediment fabrics is an interdisciplinary field in which practitioners are required to interpret the combined effects of a very wide range of physical, chemical and biological processes. Not surprisingly, scientists from several different backgrounds are involved in this area of research, including biologists, civil engineers, environmental scientists and geographers as well as geologists. This broad spectrum of researchers was well represented at the meeting, and it was particularly gratifying to see the development of interactions between scientists from different disciplines.

A superbly illustrated overview of the potential and limitations of scanning electron microscopy in sediment fabric analysis was presented by
A. T. Kearsley
(Oxford Brookes University) who highlighted the expansion of the use of backscattered electron imagery (BSEI) in sedimentary petrography. He emphasized commonly overlooked limitations on the interpretation of BSEI resulting from the effects of complex beam-specimen interactions and variation in instrument parameters. Compositional ambiguities arising from BSEI can be resolved by the concurrent use of digital X-ray mapping and he further outlined the power of the more newly developed technique of scanning cathodoluminscence imagery for differentiating grain cores and overgrowths.
J. Macquaker
(University of Manchester) continued the theme of BSEI by presenting a synthesis of fabric variation within Mesozoic mudstones. He demonstrated that,
Subaqueous shrinkage cracks and early sediment fabrics preserved in Pleistocene calcareous concretions
Authors: DUCK R. W.
Published: December 2007

Septarian cracks within concretions are widely believed to have formed by dehydration of initially soft concretion interiors. Other workers believe that such cracks form as tensile fractures in response to stresses induced during burial by 50 m and more of overburden. Herein attention is drawn to the fabrics of Pleistocene, calcareous concretions which have been subjected to a maximum overburden thickness of 10–12 m. These bodies, up to 10.5 cm in maximum dimension, are common in the silty clays of the Late Devensian (18000–13500 years BP), glacimarine Errol Beds of east-central Scotland. Within the concretions occur open fractures; microscopic lenticular, macroscopic, margin-parallel lenticular and septarian-like (radial) forms. These are interpreted as a product of subaqueous shrinkage during very shallow burial. This mode of formation is supported by SEM studies of the clastic fabrics preserved in the concretions which are indicative of deposition from originally flocculated suspensions. Spherical cavities, 30–35
μ
m in diameter, are interpreted as former gas bubbles derived from the decay of proteinaceous organic matter. This is believed to have promoted localized Ca
2+
precipitation, in the form of a soap of fatty acids, through a rise in pH. The shrinkage cracks are thought to have been induced by the density increase which accompanies the conversion of Ca soap to CaCO
3
. The former gas cavities exert some control on the position of microcracks. Moreover, the delicate open fabrics preserved confirm that the concretions are of early diagenetic origin and formed close to the sediment water interface.
A Devonian auriferous hot spring system, Rhynie, Scotland
Authors: RICE C. M. , ASHCROFT W. A. , BATTEN D. J. , BOYCE A. J. , CAULFIELD J. B. D. , FALLICK A. E. , HOLE M.J. , JONES E. , PEARSON M. J. , ROGERS G. , SAXTON J. M. , STUART F. M. , TREWIN N. H. , TURNER G.
Published: December 2007

The Early Devonian Rhynie hot spring system is the oldest known and is of the low sulphidation type. It extends for at least 1.5 km along a major fault zone defining the western margin of an outlier of fluvial and lacustrine sediments, plant-bearing sinters and andesitic lavas. The age of sedimentation and hydrothermal activity has been determined by palynological (Pragian) and radio-metric (396 ± 12 Ma) techniques. The outlier is a half graben with a complex stepped western margin.

The Devonian rocks show intense hydrothermal alteration along the fault zone. The main alteration minerals are quartz, K-feldspar, calcite, hematite and illitic and chloritic clays. Multiple chert veining and brecciation are widely developed, and geyserite and vent material are also present. Pyrite occurs in veins and all alteration facies. Sinters and altered rocks contain high concentrations of Au, As, Sb, Hg, W and Mo. Gold occurs in arsenian pyrite and as sub-micron particles in oxidized rocks.

The fluid(s) responsible for most hydrothermal alteration were near neutral with low sulphur and oxygen activities and dominated by meteoric water. However, incursions of high temperature (300–440°C) magmatic fluids occurred with δD–65‰ and δ
18
O around +8.5‰. δ
34
S (pyrite) and initial
87
Sr/
86
Sr ratios (vein calcite) lie mainly within the ranges +3.4‰ to +8.5‰ and 0.71138 to 0.71402 respectively. These data indicate that late Proterozoic Dalradian metasediments are a likely source for S and Sr but other sources are possible. δ
13
C values for caliche and vein calcite imply derivation of carbon from non-organic sources.

The Rhynie cherts were deposited from a low salinity fluid of probable meteoric origin (δ
18
O
chert
+ 13.1‰ to +16.5‰) which had interacted with the basement rocks and sediments (high Xe/Ar, Br/Cl and I/Cl ratios). Plant-bearing chert yielded an
40
Ar/
39
Ar ratio (292.1± 0.6) significantly less than that of modem air and may be the first valid determination of a sample of ancient atmosphere.
Ordovician palaeogeography of Siberia and adjacent continents
Authors: TORSVIK T. H. , TAIT J. , MORALEV V. M. , McKERROW W. S. , STURT B. A. , ROBERTS D.
Published: December 2007

Ordovician palaeomagnetic data from the upper reaches of the Lena River, Southern Siberia, confirm and refine the earlier reported data sets. Ordovician palaeomagnetic poles from Siberia define a systematic southwesterly apparent polar wander (APW) trend during Ordovician times (mean south poles: 500 Ma: 42°N, 310°E; 467 Ma: 27°N, 314°E; 460 Ma: 23°N, 313°E; 448 Ma: 22°N, 301°E and 437 Ma: 0N, 290°E). A primary or early magnetization age is verified by the reversal stratigraphy.

Siberia was geographically inverted at low southerly latitudes during the Latest Cambrian and Early Ordovician and drifted slowly northward and across the equator at an average palaeo-latitudinal velocity of c. 5cm/year. In Early Ordovician times, Avalonia and the European Massifs (e.g. Armorica and Bohemia) were located together with Gondwana in high southerly latitudes, Laurentia was positioned in equatorial latitudes whereas Baltica was located at intermediate southerly latitudes. Siberia was probably located north of Baltica in latest Cambrian–Early Ordovician times. Subduction-related, eclogite-facies metamorphism in latest Cambrian–Early Ordovician time in the Scandinavian Caledonides occurred in an ocean-continent transition zone marginal to Baltica but facing northern Siberia, and thus throws doubt on traditional Baltic-Laurentian correlations during this particular time period. With Baltica rotating counterclockwise during the Ordovician, the plate scenario allows for a Siberian source for Late Ordovician sedimentation in some areas of maritime Laurentia and perhaps even northern Norway. It also helps to explain the imposition of a deep-seated, sinistral strike-slip, fault regime between the obliquely converging Baltica and Laurentia, a transcurrent system which may have led to the permissive ascent of calc-alkaline granitoid magmas in favourable sites prior to the main stages of Scandinavian orogenic deformation.

Recent proposals that Laurentia formed a conjugate margin to the South American part of Gondwana during Ordovician times are permissible from palaeomagnetic data, but a tight continental fit during the entire Ordovician is contradicted by biogeographic data. The tight palaeomagnetic fit could perhaps be an artefact of inaccuracies in the palaeomagnetic record for Gondwana.
Geochronology, stratigraphy and structural evolution of Silali volcano, Gregory Rift, Kenya
Authors: SMITH M. , DUNKLEY P. N. , DEINO A. , WILLIAMS L. A. J. , McCALL G. J. H.
Published: December 2007

Silali, 1°10'N, 36°12°E, is the largest Quaternary caldera volcano in the northern Gregory Rift and is composed predominantly of peralkaline trachytic lavas and pyroclastic deposits, and mildly alkaline to transitional basalts. Detailed mapping combined with radiometric
40
Ar/
39
Ar age determinations is used to constrain the evolutionary development of Silali. Activity commenced at
c.
400–220 ka with the construction of a low relief lava shield whose summit area was subsequently modified by alternating periods of faulting, subsidence and infilling associated with two major periods of explosive activity. This activity ceased around 133–131 ka and was probably the result of fracturing and decompression of a high level magma chamber by regional extension and the injection of basaltic dykes below the volcano. Later eruptions (c. 120 ka) along the western flanks migrated eastward with time and culminated in the eruption of viscous trachyte lavas from a circumferential fissure zone. The emplacement of a basic dyke swarm to shallow crustal levels beneath Silali resulted in the formation a broad volcanic rift zone within which large volumes of fluid basalts were erupted to mantle the flanks of the volcano. This activity mainly pre-dated, but probably also overlapped with, incremental subsidence and asymmetric downsagging of the summit area and the propagation of a circumferential fissure zone. Basalt and trachyte lavas, erupted along the circumferential fissure zone and from a major meridional summit fissure, moved outward to form a series of flat summit benches, and ponded in summit depressions before overflowing. The continuing inward collapse of the summit area and the lateral drainage of magma from a high-level reservoir finally culminated in the formation of a large caldera at c. 63 ka. Post-caldera activity utilized pre-existing weaknesses within the caldera, erupting basalt and trachyte lavas until around 7 ka, and was contemporaneous with the eruption of trachyte lava domes (9–7 ka) on the eastern flanks.
Junggar, Turfan and Alakol basins as Late Permian to ?Early Triassic extensional structures in a sinistral shear zone in the Altaid orogenic collage, Central Asia
Authors: ALLEN M. B. , ªENGÖR A. M. C. , NATAL’IN B. A.
Published: December 2007

The Junggar, Turfan and Alakol basins in northwestern China and Kazakhstan formed as Late Permian to ?Early Triassic extensional structures in a broad sinistral shear zone between large strike-slip faults that separate two main domains of the Altaid orogenic collage. This extension was in response to an inferred large (> l000 km) sinistral motion of the East European craton with respect to the Angaran craton during this time. Deformation associated with the formation of the basins was taken up in part by counter-clockwise rotations of crustal blocks with respect to the Altaid orogenic collage and to the Angaran craton. This event is the only important phase of extension in a region otherwise dominated by compressional tectonics throughout the Phanerozoic. The basement rocks of these basins formed by Altaid subduction–accretion through the latter half of the Palaeozoic and the region was subsequently thrown into compression again during the Mesozoic Cimmeride and Cenozoic Alpide evolution.