bluuebunny/crossref_metadata_2025 huggingface dataset:
Geochemistry of Dalradian pelites from Connemara, Ireland: new constraints on kyanite genesis and conditions of metamorphism
Authors: FERGUSON C. C. , AL-AMEEN S. I.
Published: December 2007

Whole-rock and mineral analyses of Dalradian pelites from W Connemara show that kyanite pelites are not unusually Mg-rich as previously supposed. Their chemistry is transitional, especially in Ca content, between overlying Fe-rich staurolite pelites and underlying Ca-rich rocks. The flysch basin in which they were deposited was established relatively early in the west, its sediment fill including carbonate-rich influxes derived from an adjacent swell to the east which later subsided to form an enlarged basin. Kyanite has a very restricted spatial distribution because its development was controlled by Fe deficiency associated with the carbonate-rich influxes, which in turn led to relatively high X
Mg
during metamorphism. P–T estimates for the metamorphic peak are 650° ± 25 °C and ~8kbar, about 50 °C and 3kbar greater than for similar rocks in the east. The difference can be related to structural level with respect to regional-scale folds, which may have developed as a huge backfolded complex during the emplacement of the Connemara allochthon.
Glacial sediments and erosional pavements near the Cambrian—Precambrian boundary in western Henan Province, China
Authors: BAODE GUAN , RUITANG W. U. , HAMBREY M. J. , WUCHEN GENG
Published: December 2007

Exceptionally well-preserved glaciated pavements and a variety of glacigenic sediments comprising the Luoquan Formation in Henan Province, central China, are believed to have formed close to the Cambrian–Precambrian boundary. They may therefore be younger than the better known Sinian tillites representing the Changan and Nantuo ice ages. They perhaps belong to a distinct glacial epoch that is recorded elsewhere in China and in other continents, but for which there is less evidence than for earlier Late Proterozoic glacial periods.

Lithofacies represented in the Luoquan Formation are massive diamictite (signifying deposition as lodgement or melt-out till), bedded diamictite (waterlain till or glaciomarine/glaciolacustrine sediments with dropstones), rhythmites with dropstones (glaciolacustrine or fjord deposits, possibly varves), tabular conglomerates and sandstones (outwash, deltaic or sub-aqueous mass-flow deposits), lens-shaped breccia deposits (sub-aqueous mass-flow deposits) and gravel-sand wedges (frost contraction crack infillings).

The glaciated pavements reveal well-preserved striations, grooves (often intersecting), 'plastically moulded' forms, crescentic gouges and steps. On a regional scale the linear features are of variable orientation, but they indicate a dominant ice flow direction towards the southeast. All this evidence points to upland glaciation with erosion followed by deposition in a terrestrial glacial and glaciolacustrine environment.
Neotectonics
Authors: HANCOCK P. L. , WILLIAMS G. D.
Published: December 2007

The study of neotectonics is carried out by workers from a wide variety of disciplines, including structural geology seismology, regional geophysics, geomorphology and engineering geology. Landsat MSS imagery is a particularly effective tool for the investigation of neotectonic structures. The aim of the meeting was to bring together workers from the various contributory disciplines in order to discuss the development of modern techniques in each subject area. These modern approaches were exemplified by descriptions of neotectonically active regions ranging from NW Europe to China.

In order to stimulate a debate on the temporal scope of neotectonics, the meeting commenced with a short discussion paper by Hancock. He argued that the selection of an arbitrary Neogene or Quaternary date to be applied globally for the beginning of the period during which neotectonic structures form is unhelpful. He proposed as an alternative the notion that for a given region the neotectonic phase might be regarded as having begun when the present-day configuration of relevant plate boundaries and motions was established.

Blenkinsop suggested modifying Hancock’s proposal by defining the neotectonic phase as having commenced when the contemporary stress field of a region was established. Blenkinsop’s proposal possesses the great practical merit of permitting the geologist to compare regionally significant directions of horizontal compression and extension inferred from structures, known from field evidence to have been active during an appropriate time frame, with those determined from in situ measurements or fault plane solutions.

Recent earth movements give rise to geomorphological features indicative of the
Fault rocks of the Kaltungo lineament, NE Nigeria, and their relationship with Benue Trough tectonics
Authors: MAURIN J. C. , BENKHELIL J. , ROBINEAU B.
Published: December 2007

In the upper part of the Benue Trough of Nigeria, several major NE–SW lineaments are essential components in the structure of both the Pan-African basement and the Cretaceous sediments. In order to estimate their influence on the geodynamics of the Trough, detailed structural analyses have been carried out on the fault rocks of one of them, the Kaltungo lineament. It cross-cuts a basement inlier emerging from Lower to Middle Cretaceous sediments. This inlier of granitic rocks exhibits a wide fault zone trending N50E which displays an anastomosing network of both ductile and brittle shear zones. The former are expressed by mylonitic bands generated by a Pan-African dextral wrenching on a regional scale; the latter are represented by fault rocks of the cataclasite series related to a sinistral wrenching which occurred during the Mesozoic. The fault zone extends into the cover where it is seen as a thin crush zone characterized by conjugate strike-slip and reverse faults. From a computer dynamic analysis of microfaults, an earlier compression (N–S shortening axis) was found only within the basement, whereas a second one (N140E shortening axis) appears in both sediments and basement. The first palaeostress field corresponds to cataclasite genesis which preceded or coincided with the formation of pull-apart basins along the sinistral Kaltungo wrench fault, which initiated the Upper Benue Trough during the Lower Cretaceous. Sedimentary basins and uplifted zones (Kaltungo Inlier) were formed on releasing and restraining oversteps of such a fault. The above statement may be extended to other similar faults at the scale of the entire trough. The second palaeostress field corresponds to the Late Cretaceous compression responsible for the large scale folding in the sediments and the formation of fault-breccia along the sinistral Kaltungo lineament.
Mineral weathering processes in podzolic soils on granitic materials and their implications for surface water acidification
Authors: WILSON M. J.
Published: December 2007

Many areas where concern has arisen over the recent acidification of their surface waters contain podzolic soils developed upon granitic materials. The mineral weathering processes in these soils are reviewed and the ways in which such processes could be altered by acid deposition are considered. The most weatherable primary minerals are plagioclase feldspar, biotite and fine-grained dioctahedral mica and their decomposition, which is mediated by organic acids, leads to the formation of proto-imogolite allophane, vermiculite and smectite. Other clay minerals occurring in podzols such as kaolinite and halloysite, as well as gibbsite, are probably inherited from previous weathering cycles, although a present-day pedogenic origin should not be ruled out entirely. The possibility is considered that the partial replacement of soil organic acids by mineral acids introduced through precipitation could bring about significant differences in mineral weathering, leading to more acid soils with more dissolved ionic aluminium in soil and stream waters. The weathering products involved in the mobilization of aluminium from podzols are likely to be proto-imogolite allophane and hydroxy Al-interlayered vermiculite or smectite. Another possibility is that sand and silt-sized plagioclase feldspars are more reactive than previously realized.
Review of oceanic transform activity and development
Authors: GARFUNKEL Z.
Published: December 2007

Oceanic transforms generally have a complex morphology and structure and they do not conform to the ideal concept of simple fractures that exactly follow small circles centered on Euler poles of relative plate motions. It is proposed that the special properties of transforms arise because they are inherently leaky, i.e. new plate area forms along them. As transforms have finite widths the adjacent ridge tips are at different distances from the Euler poles, and the plate boundaries along transforms are slightly oblique to the direction of plate motions, such that new plate area forms along them. However, along transforms the normal process of seafloor spreading is interrupted and new plate area forms by a different process. Transforms are underlain by crust consisting of variable proportions of (a) material emplaced in the transform domains and (b) stretched ridge-generated crust. The transform structure is further modified by shearing, deformation related to irregularities of lateral faults and because of propagation of adjacent ridges. Transform leakiness is not considered to result merely from changes of plate motions (though such changes will influence transform structure), but it reflects the segmented structure of mid-oceanic ridges. The persistence of transforms and their migration in the absolute reference frame, with a component perpendicular to their strikes, shows that the ridge structure is decoupled from flow in deeper levels. Thermal anomalies along transforms, especially enhanced heat losses, stabilize the segmented ridge structure.
Behaviour of Rb-Sr whole rock and U-Pb zircon systems during partial melting as shown in migmatitic gneisses from the St Malo Massif, NE Brittany, France
Authors: PEUCAT J. J.
Published: December 2007

The age of regional metamorphism and partial melting in the Late Proterozoic succession of the St Malo area has been established at around 540 Ma. Whole-rock homogenization of the Sr isotope systems has been achieved only in the anatectic granites, whereas the associated metatexitic and diatexitic gneisses remain highly heterogeneous. At lower grade than the sillimanite isograd, metasedimentary gneisses contain detrital zircons which record the regional metamorphic event as a U-Pb lower intercept age (536 ± 14 Ma). In the sillimanite zone U-Pb zircon lower intercept ages are around 500 ± 10 Ma, and suggest a recent loss of radiogenic Pb from the observed overgrowths. Zircons from the anatectic granites do not yield U-Pb intercept ages due to the presence of inherited radiogenic Pb, but the finer fractions define a discordant array whose upper intercept on Concordia yields 555 ± 13 Ma, an age significantly older than the partial melting event indicated by the lower grade zircon ages. The lower intercept defined by the fine grained anatectic zircons yields a date in the future (negative ordinate value). In their regional context, the St Malo migmatites can be considered as resulting from late stage crustal melting in the Cadomian orogenic belt, following genesis of calc-alkaline magma at an active margin around 600 Ma ago. An age of 540 Ma for partial melting in the St Malo migmatites places the end of the Cadomian orogeny near the Precambrian-Cambrian boundary.
Processes and controls involved in the transfer of fluviatile sediments to the deep ocean
Authors: COLLINS M. B.
Published: December 2007

Sediment transport processes are described in relation to land surfaces, river systems, estuaries, deltas and continental shelves. Complex interrelationships between variables controlling transport, within each of the environments, are summarized. Extreme rates of mechanical denudation are presented and can be as high as 44 × 10
3
tonnes/km
2
/a (for Taiwan); they compare with mean European rates of around 30 tonnes/km
2
/a. Comparison is made between some representative rates of mechanical and chemical denudation. River inputs are examined, within the context of deltaic sedimentation patterns. It is suggested that fluviatile sediments can be transported over continental shelf areas and out into the oceans as hyperpycnal discharges. Organic material contained within the sediments is important geologically, in terms of its burial within marine sediments.
Interpretation of the gravity and magnetic anomalies over the Mull Tertiary intrusive complex, NW Scotland
Authors: BOTT M. H. P. , TANTRIGODA D. A.
Published: December 2007

A 50 mGal local positive gravity anomaly over the Mull intrusive complex is interpreted as caused by an underlying dense body of mafic (basic and/or ultrabasic) rocks extending to a depth of between 6.5 and 13 km and occupying a volume of between 2000 and 3600 km
3
, depending on density contrast. The granites produce smaller superimposed negative gravity anomalies, which indicate that the granites of the late Loch Bà centre extend to a depth of about 2 km and those of the early Glen More centre occupy a significantly smaller volume. The granites occupy between 5 and 9% of the total volume, consistent with a derivative origin.

The complicated aeromagnetic anomalies have been simplified by applying the pseudogravimetric transformation. The most conspicuous positive pseudogravimetric anomaly occurs over the rocks of centres 1 and 2. This is of much smaller areal extent than the positive gravity anomaly, and it is attributed to highly magnetic rocks extending to a depth of about 2 km beneath centres 1 and 2, including cone sheets. The magnetization of the deeper parts of the complex is much weaker and has not been resolved. A partial ring of linear or arcuate negative magnetic anomalies occurs along the SE, S and W margins of the deep-seated intrusive complex and may mark an unexposed early ring dyke with strong reverse magnetization.

It is estimated that up to a quarter of the volume occupied by the intrusive complex may have been provided by various types of forceful intrusion but that most of the upper crustal space has been made available by ring subsidence and other types of stoping. This is feasible because the magma was demonstrably lower in density than the Moine and Lewisian. The mantle-derived magma probably rose through the more ductile lower crust by diapirism, thereby causing a slight thickening of the underlying crust which may provide isostatic support for the excess mass of the intrusive complex and its associated topography.
The determination of true shear senses from the deflection of passive markers in shear zones
Authors: WHEELER J.
Published: December 2007

The sense of movement in shear zones can often be determined from the reorientation of pre-existing planar or linear features within the zone. This cannot always be done simply by examining the change in strike or trend of these features: sometimes these will show the opposite sense of rotation to the true sense of shear in the zone. This is illustrated for a generalized oblique-slip zone. Using the deflection method' described here, the true movement sense can be deduced from the apparent rotation, the emphasis being on a three-dimensional approach. Consideration of the movement on a large-scale shear zone (the Laxford Front) underlines the importance of being aware of this aspect of shear zone geometry. The method may be applied to brittle fault offsets as well as to deflections in ductile shear zones.
Two-phase flow in crust and mantle
Authors: ROBINSON D.
Published: December 2007

The fortieth William Smith meeting of the Geological Society was a one-day lecture meeting held at Burlington House on 30 April 1986, with the William Smith Lecture presented by Professor R. K. O'Nions. The meeting was convened by Dr K. Coe (University of Exeter) and Professor D. McKenzie (University of Cambridge).

The meeting’s theme ‘Two-phase flow in crust and mantle’ relates to the interaction between, and relative movements of, the ‘solid’ and ‘fluid’ phases within the crust and mantle. The terms solid and fluid, while being convenient terms to distinguish two such phases in upper levels of the crust, become increasingly imprecise at greater depths. It then becomes better to refer to differences between two phases of sharply contrasting viscosities; a matrix of high viscosity and an ‘intergranular’ component of much lower viscosity. The behaviour of the two phases can be described in terms of fluid dynamics, and application of this technique has much potential for producing major advances across the spectrum of the geological sciences. There is also potential for the field and laboratory based geologist to integrate with the theoretical fluid dynamicist for the advancement of the science. This set of papers aims to demonstrate this link and there is much to be gained from this thematic set for both types of specialist. This theme is particularly topical in the geological sciences at the present time with active interest in the field of fluid processes and fluid-rock interaction.

The seven papers presented at the meeting form this thematic
The segregation and emplacement of granitic magmas
Authors: WICKHAM S. M.
Published: December 2007

The segregation of granitic magma from residual crystals at low melt-fraction is strongly dependent on the viscosity of the melt. Theoretical considerations imply that for the typical range of granitic melt viscosities (10
4
Pas to 10
11
Pas) only very limited separation will be possible by a compaction mechanism over the typical duration of a crustal melting event (
c
. 10
6
years). Small-scale segregations (millimetre to metre) of the type observed in migmatite terranes may be generated by compaction (possibly assisted by continuous deformation), or by flow of melt into extensional fractures, but low melt-fraction liquids are unlikely to be extracted to form large (kilometre-size) granitic plutons because of the limited separation efficiency. At higher melt-fractions (>30%) the rapid decrease in strength and effective viscosity during partial melting allows other segregation processes to operate. Calculations and experiments indicate that in granitic systems the effective viscosity of partially melted rocks, having a very narrow melt fraction range of 30–50% will fall rapidly to levels at which convective overturn of kilometre-thick zones can occur. Convective motion within anatectic regions is capable of generating large (kilometre-size) homogeneous, high crystal-fraction, crustally-derived magma bodies, which are orders of magnitude greater in size than low melt-fraction segregates. Before convective instability is reached, small (centimetre- to metre-sized) pods of granitic liquid may rise buoyantly through, and pond at the top of such partly molten zones; such a process is consistent with the observation that some granulites appear to be residue rocks, chemically depleted in a minimum melt component. The effective viscosity (and hence the susceptibility to convection) of a partially melted zone within the crust, is strongly dependent on the water content of the system at a given pressure and temperature, because this controls both the quantity of melt generated and also the viscosity of the melt. The intrinsic water content of most crustal lithologies is incapable of promoting the high percentages of partial melting, or the low liquid viscosities, required to form large kilometre-sized granitic plutons by convective homogenization, at typical crustal temperatures. This suggests that the anatexis involved in the generation of large crustally-derived magma bodies has in many cases been promoted by an influx of externally derived aqueous fluid. These magma segregation processes are illustrated with respect to the petrogenesis of three different types of granitoid pluton from a Hercynian low-pressure, metamorphic-anatectic terrane in the Pyrenees.
Late Ordovician shallow marine environments due to glacio-eustatic regression: Scrach Formation, Mid-Wales
Authors: WOODCOCK N. H. , SMALLWOOD S. D.
Published: December 2007

An Ashgill to Llandovery offshore marine sequence in Mid-Wales is punctuated by a shallow marine sequence (the Scrach Formation) attributed to Late Ordovician glacio-eustatic regression. The base of the regressive sequence is gradational and suggests progressive fall in sealevel during mid-Hirnantian (late Ashgill) time. An abrupt top to the Scrach Formation marks a rapid sealevel rise, probably within the
persculptus
Zone (late Hirnantian). Interbedded lenticular thin sands and muds dominate the Scrach Formation and represent shallow shelf environments, with evidence of NW-directed tidal ebb flow and storm currents. Sporadic medium-bedded sand sheets represent the strom events. Locally, lenticular thick-bedded sand units embedded in the fine facies probably represent barrier facies in an area of high clastic input.

The Scrach Formation thins SE into the present NE-striking Myddfai steep belt. This is underlain by NE-striking faults that actively controlled late Ordovician to early Silurian sedimentation. The faults probably defined an emergent but low relief coast during the glacial regression. Offshore, to the NW, the Scrach passed into parallel laminated mudstones, then across a fault-influenced slope (the present Tywi lineament) into coarse clastic sediments deposited by gravity flows.
Age and origin of the New Zealand Orocline in relation to Alpine Fault movement
Authors: KAMP P. J. J.
Published: December 2007

The Alpine Fault, and the recurved arc structures (orocline) in the late Palaeozoic-Mesozoic Rangitata Orogen, are two of the first-order structures of the New Zealand subcontinent. A commonly held view is that the orocline originated as a gigantic drag fold with Alpine Fault movement. In view of the post-Oligocene age of the Alpine Fault, however, this concept, involving a single recurved arc, cannot reconcile the two published sets of palaeomagnetic data on tectonic rotations within the Rangitata Orogen and its cover rocks. These data indicate at least 62° of dextral rotation during each of the Mesozoic Rangitata Orogeny and the late Cenozoic Kaikoura Orogeny. A new model presented here postulates that there are actually two dextral recurved arcs. A Western Arc, which lies mainly outside the Australia–Pacific plate boundary zone, has a multiple origin having formed chiefly during the late Triassic phase of the Rangitata Orogeny by the subduction accretion of the exotic Rakaia terrane. An Eastern Arc coincides with the area of the present plate boundary zone. It formed as a mega, brittle–ductile, shear zone with the late Cenozoic development of the modern Australia–Pacific plate boundary, which includes the Alpine Fault. This solves the problem of the nature and location, north of the Alpine Fault, of the relative plate movement evident as 480 km of displacement on the Alpine Fault. A major implication of this model for plate tectonic reconstruc­tions of the SW Pacific is that only approximately 500 km of dextral displacement has occurred through New Zealand during the late Cretaceous–Cenozoic compared with the commonly adopted figure of 1000 km.
The Southern Uplands as an accretionary prism: the importance of analogues in reconstructing palaeogeography
Authors: LEGGETT J. K.
Published: December 2007

Fore-arc regions, whether accretionary, erosional or composite, vary greatly both along strike and through time, in the geometries which they assume in response to subduction. There is no well-studied modern/Neogene margin which can be established as the 'type' accretionary prism. All are individual in some respect or other. Nor is there an example of a well-studied modern/Neogene accretionary margin which can be compared closely to the Southern Uplands, though there are several good analogues for particular stages of its history. Accretion took place along the Southern Uplands margin for
c.
50 million years. The range of geometries on modern/Neogene accretionary active margins, and the extent to which the behaviour of those margins can change during periods as long as 50 million years, is reviewed, emphasizing the accretionary prisms of Mexico, SW Japan and the Makran. The interpretation of the Southern Uplands as an accretionary prism, as presented in the late 1970s and early 1980s, is reassessed in the light of advances made subsequently in understanding modern/Neogene prisms.

The recent alternative model of Stone
et al.
for the Southern Uplands envisages that the terrane evolved by the opening and closure (by arc-subduction) of a marginal basin. The new model draws heavily on arc-type detritus derived from the oceanward side of the ancient Southern Uplands trough. There are areas along the SW Japan fore-arc where oceanward-derived tubidites are accumulating in a trench (Nankai Trough), and where active submarine arc-type volcanism is occurring outboard of a currently growing accretionary prism. Such features in an ancient terrane like the Southern Uplands do not necessarily mean that an alternative model must be sought involving whole new arc systems. Furthermore, analysis of the life-spans and sedimentary fills of modern/Neogene marginal basins begs the conclusion that the Southern Uplands' strata were not deposited in a marginal basin. It is most unlikely that the
c.
50 Ma history of pelagic sedimentation preserved there can be manifesting anything other than the closure of a sizeable ocean basin, of a scale exceeding any known western Pacific-type marginal basin, and with a different sedimentation history from any such marginal basin.
The Northern Belt of the Longford-Down Inlier, Ireland and Southern Uplands, Scotland: an Ordovician back-arc basin
Authors: MORRIS J. H.
Published: December 2007

The Northern Belt of the Southern Uplands/Longford-Down zone contains two compositionally distinct clastic petrofacies suites each ranging from mid- to late Ordovician age. They extend throughout the length of the belt and are generally separated by a strike fault. The northwestern Metaclast Petrofacies suite is characterized by felsic igneous and metamorphic detritus derived from a continental source to the NW, whereas the southeastern Basic Clast Petrofacies suite contains abundant calc-alkaline detritus derived from an island arc and plutonic/metamorphic root zone complex to the S or SE. A similar source, but located to the north, is indicated for younger clastic sequences of similar composition in the northern part of the Central Belt immediately south of a major sinistral shear zone which defines the boundary between the Northern and Central Belts. It is concluded, in light of other facets of regional geology and possible palaeogeographic configurations, that this pattern reflects a back-arc basin in the Northern Belt, margined by a volcanic arc massif which once occupied the space now represented by the shear zone, and a fore-arc basin to the SE (Central and Southern Belts). The Northern Belt back-arc basin is postulated to have been closed and deformed at the end Ordovician and subsequently overthrust northwestward by the allochthonous, thrust stack imbricated Central and Southern Belts fore-arc sequence at the end Silurian. This, in the Northern Belt, resulted in the development of a major recumbent structure, downward facing on regional (end Silurian) cleavage.
The age of blueschist metamorphism in Anglesey, North Wales: evidence from
40
Ar/
39
Ar mineral dates of the Penmynydd schists
Authors: DALLMEYER R. D. , GIBBONS W.
Published: December 2007

The 'Penmynydd' schists of SE Anglesey include several high
P
/low
T
lithologies including: (1) metasedimentary quartz–phengite schist; and (2) mafic blueschist with barroisite/crossite assemblages that appear to have developed from an earlier actinolitic greenschist facies protolith. One phengite and three amphibole concentrates have been analysed by incremental-release
40
Ar/
39
Ar dating methods and display discordant age spectra with anomalously young apparent ages recorded in low
T
increments. Ages increase systematically throughout intermediate
T
portions of the analyses to define high
T
plateaux of
c
. 550–560 Ma (barroisite/crossite-rich concentrates) and
c.
580–590 Ma (actinolite-rich concentrate). The phengite concentrate also displays an internally discordant spectrum with intermediate
T
increments defining ages similar to those recorded by barroisite/crossite and high
T
increments giving ages similar to those of actinolite. The amphibole and phengite ages are interpreted as dating post-metamorphic cooling following a regional low-
P
Ml greenschist metamorphism (
c
. 580–590 Ma) and a high-
P
M2 metamorphism (
c
. 550–560 Ma).
40
Ar/
39
Ar ages suggest multiple metamorphism of oceanic crust before the arrival of the blueschist terrane and its accretion to the late Precambrian basement of southern Britain immediately prior to development of the Welsh Basin. An age of
c.
550–560 Ma for blueschist metamorphism on Anglesey provides a likely maximum age for ignimbrites within the pre-late Lower Cambrian Arfon Group exposed in NW Wales.
The provenance of granite boulders in conglomerates of the Northern and Central Belts of the Southern Uplands of Scotland
Authors: ELDERS C.
Published: December 2007

Rb–Sr whole rock isochrons from several suites of granite boulders which occur in Ordovician and Silurian conglomerates in the Southern Uplands of Scotland yield ages of around 1200 Ma, 600 to 700 Ma and 470 to 490 Ma. The plutonic and tectonic history recorded by these clasts corresponds more closely with that of NW Newfoundland than with that of Scotland. The petrography of the clasts can also be matched with exposed granite plutons in Newfoundland. Since the conglomerates were derived from the NW it is concluded that the Southern Uplands lay to the SE of Newfoundland. A total of 1500 km strike-slip displacement occurred between the Caradoc and the Early Devonian, consistent with oblique subduction of the Iapetus Ocean below the North American continental margin.
Preliminary interpretation of upper crustal structure across the Midland Valley of Scotland from two East–West seismic refraction profiles
Authors: CONWAY A. , DENTITH M. C. , DOODY J. J. , HALL J.
Published: December 2007

A seismic refraction survey was carried out over the Central Coalfield Syncline in the Midland Valley of Scotland during May 1984, to determine depth to sub-Carboniferous basement and to locate intra-basement contrasts and structure. Three refractors were identified and mapped. The shallowest is thought to be the top of the Lower Old Red Sandstone, so that the top layer of the crustal model defines the Carboniferous and Upper Old Red Sandstone which varies in thickness from 1.5 to 3.5 km. Deeper refractors are interpreted as the top of the crystalline basement and an intra-basement horizon. The depth of the top basement refractor limits the combined thickness of Lower Old Red Sandstone and Lower Palaeozoic sedimentary rocks to between 0 and 5 km. No obvious correlations with the Bathgate gravity and magnetic anomaly are found in the seismic data. Gravity modelling based on the seismic interpretation suggests that the likeliest cause of the anomaly is thick extrusive basic igneous rocks of Carboniferous and/or Lower Old Red Sandstone age.
Geometrical control of subduction-related magmatism: the Mesozoic and Cenozoic plutonic history of Western Colombia
Authors: ASPDEN J. A. , McCOURT W. J. , BROOK M.
Published: December 2007

Radiometric age data from Western Colombia, combined with geological evidence, allow five main plutonic episodes, ranging in age from Triassic to Tertiary, to be recognized. It is suggested that variation in the convergence angle of the oceanic plate relative to the continental edge was an important factor affecting the timing and spatial distribution of plutonic activity. On a regional scale, major breaks in activity are probably best attributed to either low-angle/parallel convergence or periods of accretion along the convergent margin.
The relationship between K-Ar mineral ages, mica grainsizes and movement on the Moine Thrust Zone, NW Highlands, Scotland
Authors: KELLEY S.
Published: December 2007

Muscovite and biotite separated from Moine schists close to the Moine Thrust Zone yield a range of K-Ar ages incompatible with the simple cooling regime previously proposed for the Caledonides of northern Scotland. Differences of only 1 Ma between muscovite and biotite of the same grainsize contrast with differences of 12–13 Ma between the 125–200
μ
m and 250–500
μ
m grainsize fractions. This is contrary to the normally accepted dominance of mica type over grainsize effects. Dynamic recrystallization of the micas during early deformation associated with the Moine Thrust Zone is thought to play a role as important as temperature in determining the K-Ar ages. The conclusions drawn have important implications for the use of uplift and cooling rates derived from the absolute K-Ar closure temperatures of micas close to thrust zones.

Although the K-Ar ages from micas with diameters greater than 200
μ
m are not reproducible, the data can be used to show that deformation associated with thrusting commenced between
c
. 443 Ma and
c
. 435 Ma, and that the thrust zone was active until at least 422 Ma ago.
Oncoids as environmental indicators in the Much Wenlock Limestone Formation of the English Midlands
Authors: RATCLIFFE K. T.
Published: December 2007

The Much Wenlock Limestone Formation of the West Midlands was deposited in a mid-shelf setting and is divisible into three members; the Upper and Lower Quarried Limestone members being separated by the more argillaceous Nodular Limestone Member. Oncoids, composed predominantly of micritic fabrics with
Rothpletzella
and
Girvanella,
occur commonly in the Lower Quarried Limestone Member. These oncoids vary from subspherical bodies up to 5 mm in diameter to forms with a highly irregular and branched upper surface which reach 70 mm across. Each form is indicative of a different depositional environment, which is also reflected in the sediment enclosing the oncoid. Equidimensional oncoids in peloidal packstones were formed by continuous rolling, whereas the larger, branched forms enclosed in loosely packed wackestones developed in quieter conditions below wave base. The distribution of oncoid morphotypes in the Lower Quarried Limestone Member shows that small variations in relative sea level were superimposed on the overall middle to late Wenlock regressive episode during which the Much Wenlock Limestone Formation was deposited. The uniformity of the formation throughout the West Midlands indicates that the sea floor was essentially planar over a large area. Vertical variation of oncoid morphology within the Lower Quarried Limestone Member can be traced throughout the area, allowing accurate correlation of relative sea-level variations.
Mineral Deposits Studies Group annual meeting, 1986
Authors: FOSTER R. P.
Published: December 2007

Report of a meeting of the Mineral Deposits Studies Group held on 15–16 December 1986, at the University of Southampton. The meeting included a thematic session on fluid inclusions which is reported elsewhere in this volume.

Six presentations encompassed epithermal or volcanic-hosted precious-metal deposits and mineralized breccias. The Tambo deposit, near El Indio in Chile, was described by
Bernard
as a typical hot spring system with bedded sulphates, sulphate sinters, explosion breccias and native sulphur o sulphur overlying hydrothermal breccias and veins. Gold occurred as relatively coarse grains in the silicified wallrocks and also finely dispersed throughout barite. Mixing of ascending auriferous fluids with near-surface acid-sulphate waters was postulated to explain the mineral paragenesis. A similar mixing model was presented by
Redwood
to account for the disseminated and vein-type Au–Ag–Cu–Pb–Zn mineralization of the Altiplano in Bolivia. The mineralization was associated with silicic flow-dome complexes, and high-K calc-alkaline and shoshonitic magmatism; oxygen and hydrogen isotope data indicated an important magmatic input to the hydrothermal system.

A silver-dominated vein-type mineralization comprising silver sulphosalts, native silver, argentite, minor sulphides, calcite, and baryte at Caracoles, northern Chile, was described by
Fontbote, Schmidt
&
Flint,
who emphasized the localization of high-grade ore at the intrusive contact between a porphyritic granite and bituminous siltstones and limestones. The role of decompressive shock and hydraulic decrepitation in the generation of hydrothermal breccias and lodes was reviewed
by Halls
&
Allman-Ward.
Mineralized hydromagmatic breccias of Hercynian age were identified as the source of the gold
The Dent Fault System, northern England—reinterpreted as a major oblique-slip fault zone
Authors: UNDERHILL J. R. , GAYER R. A. , WOODCOCK N. H. , DONNELLY R. , JOLLEY E. J. , STIMPSON I. G.
Published: December 2007

The Dent Fault System forms part of the Pennine Boundary Fault System in northern England and consists of a series of vertical to steeply west-dipping, anastomosing fault strands generally striking 010°–020°. Maximum throw on the faults occurs in the south, where an easterly downthrow juxtaposes Carboniferous and Lower Palaeozoic rocks. The amount of easterly downthrow decreases northward, and westerly downthrow occurs along its northernmost extension. An east-facing and slightly overturned monocline is dissected by the fault strands along the length of the fault system, irrespective of the amount or sense of displacement across the zone.

A number of features suggest that the observed structural relationships are not
simply
the results of E-W directed, post-Carboniferous compression as previously thought. The most significant are: (1) the variable amount and sense of displacement along the length of the sub-vertical fault system, (2) east-directed reverse faults exhibiting oblique- and strike-slip slickenside striations, that locally may form a partial- or half-flower structure, (3) folds developed both sub-parallel and en echelon to the main fault zone, (4) the existence of conjugate sets of shear fractures, (5) the occurrence of structures analogous to strike-slip duplexes, and (6) regional considerations.

Instead, it is suggested that the fault system represents an Early Palaeozoic lineament, reactivated during Carboniferous and into early Permian time. It accommodated both the formation and N–S directed shortening (inversion) of early Carboniferous basins and later NE–SW directed extension during the formation of the Vale of Eden half-graben, by oblique-slip movement along its length. Younger movements may also have occurred but are difficult to document and are not thought to be large.
Trace fossils in coal-bearing sequences
Authors: POLLARD J. E.
Published: December 2007

In the past decade trace fossils have been recorded extensively from coal-bearing sediments, differing widely in facies, age and location. Westphalian or Stephanian 'coal-measures' in Britain, Europe and Eastern Canada contain an ichnofauna produced by invertebrates and/or vertebrates in upper delta plain sediments. This contrasts with the marine-related lower delta plain ichnofaunas known from Pennsylvanian rocks of the United States and Permian Gondwana 'coal-measures' of South Africa. Deltaic complexes of Middle Jurassic age in the North Sea basin and Upper Cretaceous age in North America contain marine trace fossils and dinosaur footprints in coastal coal-bearing facies. These case histories illustrate the importance of trace fossils both in facies analysis of coal-bearing sequences and in recording the presence of animals rarely known as body fossils in such clastic sediments.
Ordovician volcano-tectonics in the English Lake District
Authors: BRANNEY M. J. , SOPER N. J.
Published: December 2007

Although the major late Caledonian (Acadian) deformation of the British slate belts south of the Iapetus suture took place in early Devonian time, two intra-Ordovician unconformities are present in the Lower Palaeozoic succession of NW England which have been interpreted as due to regional compressional events. These unconformities are at the base and top of the Caradocian Borrowdale Volcanic Group (BVG) and have been associated respectively with a pre-BVG orogenic cycle affecting the early Ordovician Skiddaw Group (SG) and with ‘pre-Bala’ folding of the BVG. A problem has been that no consistent sets of minor structures attributable to these deformations have been demonstrated in the Skiddaw Group.

Recent advances in understanding of the volcanic history of the Lake District lead to a radical reinterpretation of the significance of these two unconformities. In the ‘pre-Bala’ (intra-Caradocian) deformation of the BVG, flexuring is now seen to be much less important than volcano-tectonic faulting and block tilting associated with caldera collapse and the eruption of voluminous ash flows in the upper part of the pile. Foundering of the volcanic edifice accounts for the preservation of this thick subaerial sequence beneath the overstepping late Ordovician marine deposits.

More tentatively, we suggest that the unconformity beneath the Borrowdale Volcanic Group reflects regional uplift due to buoyancy effects associated with the generation of andesitic melt by Iapetus subduction and its rise through the over-riding wedge of continental lithosphere; several such mechanisms have been proposed to account for the topographic elevation of present day continental margin magmatic arcs.

The new model removes the evidence for Ordovician N–S regional compression from the geological record of NW England and this has geotectonic implications: the evidence for late Ordovician collision is weakened and the direction of Iapetus subduction beneath southern Britain may not have been southward.
The Leka Opholite Complex, central Norwegian Caledonides: field characteristics and geotectonic significance
Authors: FURNES H. , PEDERSEN R. B. , STILLMAN C. J.
Published: December 2007

The Leka Ophiolite Complex (LOC) of Lower Ordovician age (U-Pb zircon age of 497 ± 2 Ma) contains the following components: (1) A lower, strongly deformed harzburgite unit, representing the depleted upper mantle. This unit becomes progressively richer in dunite towards (2) a sequence of ultramafic cumulates, ‘unconformably’ overlying the harzburgite tectonite. The crystallization sequence in the ultramafic cumulates is: olivine→chromite→clinopyroxene→orthopyroxene. In the south-western part of the ultramafic cumulate sequence, layers of metagabbro appear interlayered with banded dunite/wehrlite. The above-mentioned rock sequences may be in tectonic contact with (3) layered metagabbros which give way upwards to laminated and vari-textured metagabbro. Within the laminated/vari-textured metagabbro is the first appearance of (4) metabasalt dykes, which increase upwards in abundance from scattered single dykes to a 100% sheeted dyke swarm. Associated with the vari-textured metagabbro and metabasalt dykes are minor acid intrusions. Within the upper part of the dyke swarm is the first appearance of (5) pillow lavas, which are associated with thick volcaniclastic metasediments at apparently higher stratigraphic levels.

Geochemically the metabasalt dykes and associated pillow lavas range from predominant island arc tholeiite (IAT) to MORB character, and typical metaboninites are also present. This range of geochemical composition, as well as the crystallization sequence of the cumulates, strongly suggests that the first stages in the formation of the LOC was by spreading above a subduction zone. The pillow lavas associated with volcaniclastic rocks, are of MORB or WPB type, and these probably represent the later spreading stages of the LOC in a back-arc basin, but at sufficient distance to be unaffected by the geochemical ‘fingerprints’ of the subduction process.
Holarctic correlation of non-marine Palaeocene–Eocene boundary strata using mammals
Authors: DASHZEVEG D.
Published: December 2007

A new biostratigraphic correlation is suggested for the Palaeocene–Eocene boundary strata in the Holarctic. Great significance is attached to the Thanetian hiatus which has critical importance in the placing of the Palaeocene–Eocene boundary in Western Europe. Intercontinental correlation of the Palaeocene–Eocene boundary in non-marine strata by means of mammals reveals the duration of the Thanetian hiatus in the Paris Basin. It corresponds almost entirely to the Clarkforkian Land Mammal Age of North America and possibly to the Zhigden and Naran Members of the Naran-Bulak Formation of Mongolia, which suggests a Palaeocene age for the Clark's Fork fauna. The Gray Bull and Bumban faunas appear to be practically contemporaneous and are correlated with the Sparnacian fauna of West European Meudon. The Palaeocene–Eocene boundary in North America is thus traced beneath the Gray Bull zone (Wasatchian Land Mammal Age), while in Central Asia it is believed to underlie the Bumban Member of the Naran-Bulak sections where there is evidence of the simultaneous appearance of perissodactyls, primates (Omomyidae), rodents, condylarths of the genus
Hyopsodus
and others.
The mineralogy and metamorphic geology of low-grade metasediments, Northern Range, Trinidad
Authors: FREY M. , SAUNDERS J. , SCHWANDER H.
Published: December 2007

Forty-one low-grade metasediments from the Northern Range of Trinidad and from the neighbouring island of Tobago, West Indies, were investigated by microscopic, X-ray powder diffraction, Guinier camera technique and electron microprobe methods. The following minerals were encountered (in order of decreasing abundance): quartz, muscovite, chlorite, paragonite/muscovite mixed-layer, paragonite, organic material, calcite, pyrophyllite, dolomite, albite, hematite, chloritoid, pyrite, rutile, titanite, rectorite, stilpnomelane.

The chemical composition of muscovite is dependent on the mineral assemblage, with Si = 6.1–6.2 per formula unit in two high alumina assemblages, versus Si = 6.6 in a low alumina assemblage. Microprobe analyses indicate that muscovite, paragonite and pyrophyllite are sometimes intergrown on a micrometre scale. Chlorite composition estimated from X-ray data shows good correspondence with microprobe data for two Al-poor samples but is less satisfactory for an Al-rich sample.

Observed mineral assemblages in AKNa are indicative of the pyrophyllite-paragonite-albite fades. For the first time, chemical data for coexisting pyrophyllite, chlorite (M = MgO/(MgO+ FeO) = 0.377) and chloritoid (M = 0.110) are given. This assemblage is indicative of the low or medium pressure range of the lower greenschist facies.

Pressure conditions during metamorphism are not well constrained but a minimum pressure of 2 kbar, corresponding to 5–6 km of overburden, can be estimated from the Na
M4
content of actinolite. Temperatures derived from the calcite-dolomite geothermometer range between 300 and 350°C and are consistent with the presence of the assemblage pyrophyllite + calcite + quartz.
U-Pb zircon ages of basic rocks and gneisses intruded into the Dalradian rocks of Cashel, Connemara, western Ireland
Authors: JAGGER M. D. , MAX M. D. , AFTALION M. , LEAKE B. E.
Published: December 2007

New isotopic ages are presented from the syntectonic Cashel-Lough Wheelaun basic intrusion and an associated K-feldspar granite gneiss of the Connemara gneiss complex from Lettershinna Hill. The linear, normally discordant distribution of data-points from the Cashel-Lough Wheelaun metagabbro indicates that the zircons in this intrusion have no significant inherited radiogenic Pb. They yield an upper concordia intercept age of 477
+25
−6
Ma, and a more precise
207
Pb/
206
Pb age of 490 ± 1 Ma. In contrast, zircons from the K-feldspar granite gneiss show distinct reverse discordance. The non-magnetic fractions show a good linear arrangement yielding a lower intercept age of 454
+6
−14
Ma and a very uncertain upper intercept age of 1807
+646
−570
Ma. The lower intercept age is younger than the 477 ± 9 Ma K-Ar age of hornblendes in gneisses from Lettershinna Hill (

Elias
et al.
1988

) and therefore reflects a Pb loss episode younger than igneous crystallization. The imprecise Proterozoic upper intercept age indicates contamination by older inherited and only partially reset zircons.