bluuebunny/crossref_metadata_2025 huggingface dataset:
The origins of late-stage rocks in the St Austell granite—a re-interpretation
Authors: Manning D. A. C. , Exley C. S.
Published: December 2007

The origin of the granite varieties present in the St Austell granite body (biotite granite, megacrystic and non-megacrystic lithium-mica granite, fluorite granite) is discussed in the light of new petrographic, chemical and experimental evidence. There were essentially two stages of magmatic activity—the first represented by the intrusion of several phases of biotite granite, the second by a later intrusion of non-megacrystic lithium-mica granite which crystallized from a volatile-rich residual differentiate of the biotite granite magma. Both intrusions suffered hydrothermal alteration which primarily involved at least two stages of exchange of components via a single aqueous fluid phase. Within the non-megacrystic lithium-mica granite, topaz and lithium-mica were among the first phases to break down, releasing F and Li to this aqueous phase. F was locally fixed as fluorite, removing Ca (derived from the host biotite granite) from the fluid and giving rise to pockets of fluorite granite. Apart from losing Ca from plagioclase, the biotite granite gained Li, expressed as lithium-rich mica, resulting in an aureole of megacrystic lithium-mica granite surrounding the second intrusion. At least one pervasive hydrothermal system, using joints and grain boundaries as fluid pathways, may have been in existence. Water-rock interaction during fluid circulation is believed to have modified considerably the composition of predominantly meteoric water to give rise to the hydrothermal fluid responsible for greisening and mineralization within its sphere of influence. The alteration phenomena initiated by the proposed hydrothermal system may have culminated in the formation of the major kaolinite deposits currently worked in the area.
Structural evolution of the Witch Ground Graben
Authors: Beach Alastair
Published: December 2007

The Witch Ground Graben is a NW-SE trending extensional basin in the central part of the northern North Sea, lying between the Moray Firth and the Viking Graben. NE-SW extension occurred from Triassic to Cretaceous times. Extension took place on listric normal faults along the SW margin, with downthrow to the NE, giving rise to the asymmetry of the basin. One of the principal phases of fault-controlled subsidence occurred in the Lower Cretaceous, from the Valanginian to Albian. Compilation of biostratigraphic data from this interval shows the development of an increasing number of stratigraphic breaks across the basin from SW to NE, these breaks increasing in time span also in this direction. This is a result of rotation of the subsiding fault block on the listric extensional faults. The rotation is recorded in dipmeter data, and occurred in several distinct episodes—the Triassic, the Volgian and the Valanginian to Aptian. In reconstructing a cross-section of the basin, the interpreted fault curvature is constrained by the observed rotation of bedding surfaces, and the amount of extension across the listric faults is constrained by the observed basin subsidence. A depth to décollement for the faults of 4 km below sea level and an extension across the basin of 40% are estimated.
Development of the Porcupine Seabight: use of magnetic data to show the direct relationship between early oceanic and continental structures
Authors: Lefort J. P. , Max M. D.
Published: December 2007

The Porcupine Seabight is a broad, composite rectilinear embayment in the margin of the Irish continental shelf which seems to be floored at least in the southern part by oceanic crust. There is a geometrically straightforward relationship between areas of crustal thinning, rifting, and the position of probable 'oceanic crust'. It is proposed that an important crustal unit here termed the Porcupine Segment includes the Porcupine Bank and the western end of the Slyne Ridge. Interpretation of a new magnetic map indicates that opening in the Porcupine Seabight appears to have taken place along a set of transform faults which 'root' in an older dextral shear fault set oriented at about 130°. Three major zones along the longitudinal axis of the Seabight relate to the geometry of magnetic stripes in the subjacent basement and can be used, in conjunction with opening related to a distant pole of rotation, to explain the overall present physical configuration of the basin. The Porcupine Seabight probably opened no earlier than about 170 Ma, following the establishment of a Jurassic volcanic province in the vicinity of the southern Seabight elbow.
Pyrometamorphism and the contamination of basaltic magma at Tieveragh, Co. Antrim
Authors: Kitchen D.
Published: December 2007

A Tertiary dolerite plug at Tieveragh, Co. Antrim cuts Old Red Sandstone tuffaceous sedimentary rocks. Glassy buchites have formed by disequilibrium partial melting induced by convective heat transfer from the basaltic magma. Variation in bulk rock chemistry has produced glasses coexisting with variable proportions of pyroxene, cordierite, ilmenite, hematite, magnetite, mullite, plagioclase, and alkali feldspar. Mixing of the basaltic magma with acid melt illustrates how up to 10% contamination may occur without radically altering the course of crystallization. Mineral geothermometers indicate maximum temperatures at the contact around 1100°C falling to only 940° at 10 m.
Abor Volcanics: further evidence for the birth of the Tethys Ocean in the Himalayan segment
Authors: Bhat Mohammad Ismail
Published: December 2007

The Himalayan region has a record of several phases of igneous activity which indicate a continued N—S orientated tensional regime since at least late Precambrian time. Though the earlier volcanic eruptions were restricted both in time and space, the Late Carboniferous-Triassic activity was widespread in terms of both the time interval and area covered.

The Abor Volcanic activity in the Eastern Syntaxial Bend of the Himalayan fold belt is coeval with the initiation of Late Carboniferous—Triassic Panjal Trap volcanism of the Western Syntaxial Bend and eruption of the Sikkim lavas. Chemical characters of the Abor Volcanics indicate (1) a within-plate rift tectonic setting of the lava eruptions, (2) a transitional nature between tholeiitic and alkalic compositions, (3) relative enrichment in incompatible elements, and (4) high Fe-Ti values similar to many such basalts erupted in the regions of active lithospheric rifting. The olivine-free nature, a linear trend for MgO and low Co contents of the studied rocks are suggestive of high pressure olivine fractionation for the generation of the bulk chemistry. However, large variations in the incompatible element abundances and ratios of rocks at a similar fractionation stage discount the evolution of the lava chemistry exclusively by simple fractionation or partial melting processes. Instead, these characters together with the low Zr/Nb ratio and within-plate rift tectonic setting suggests the modification of an upper mantle source region, possibly by fluid phase CO
2
immediately before the partial melting and lithospheric rifting. Combining geology of the region with the geochemistry and inferred tectonic setting of these rocks, the lava eruptions are suggested to have been associated with the 'rift valley stage' of the Tethys Ocean formation in the present Himalayan region.
Geodynamic and magmatic stages on a traverse through the Andes between 20° and 24°S (N Chile,S Bolivia, NW Argentina)
Authors: Breitkreuz C. , Zeil W.
Published: December 2007

Metamorphic Precambrian basement is exposed in the Chilean Pre-Cordillera in the form of a narrow horst, and also in the Argentinian Puna. Miogeosynclinal pelites and psammites of Devonian age dominate in the western part of the Andes, while early Palaeozoic clastics occur in the E. Insignificant Triassic sediments were followed by wide-spread shallow-marine Jurassic deposition. Cretaceous and Tertiary strata are predominantly terrestrial deposits. Whereas the Precambrian and Palaeozoic series were intensely deformed by folding, block-faulting dominates in the volcano-sedimentary cover.

Magmatic activity has been significant since the Precambrian. While S-type intrusives and extrusives occurred mainly in the Palaeozoic, I-type magmatic rocks predominated in the Mesozoic. The Cenozoic volcanism in the Cordillera Occidental originated in the lower and upper crust. Magnetotelluric and seismic investigations on the traverse show extremely low electric resistivity values coinciding with a strong absorption of seismic waves below the Chilean Pre-Cordillera and the Cordillera Occidental. This phenomenon can best be explained by the existence of partial melts at depths of more than 10 km. The Bouguer values decrease by about -430 mgal from the coast to the Cordillera Occidental.

In the Precambrian and Palaeozoic, the continent apparently extended farther to the W. The change from a passive to an active continental margin took place at the Palaeozoic/Mesozoic boundary with the break-up of Pangea. In order to grasp better the tectonic and magmatic evolution as well as the recent structure of the crust in the Central Andes, a combined geological-geophysical investigation has been made along a traverse through the Andes between 20° and 24°S (Fig. 1). The following pages present a review of the geological development together with the first results of work from the Chilean section of the traverse.
The Patagonian Batholith S of Tierra del Fuego, Chile: timing and tectonic implications
Authors: Hervé M. , Suárez M. , Puig A.
Published: December 2007

Reconnaissance field studies on the Patagonian Batholith exposed S of Tierra del Fuego, together with 49 new K-Ar and Rb-Sr mineral dates, render new insights into two main aspects of the late Mesozoic geology: the geochronology and composition of the batholith, and time relations between the emplacement of the plutons and deformation. The envelope of the batholith in this area is mainly composed of late Jurassic-early Cretaceous sedimentary and volcanic rocks formed in an island arc-marginal basin system.

Three main plutonic groups have been recognized which have yielded mineral dates, interpreted as crystallization ages, ranging from 141 to 34 Ma. The earliest is a gabbroic assemblage, including gabbros, diorites and hornblendites exposed as relicts within later granitoids and with dates ranging from 141 to 103 Ma. The Canal Beagle Plutonic Group is composed of granitoids characterized by a penetrative syn-magmatic foliation and with dates of 113 to 81 Ma. The Seno Ano Nuevo Plutonic Group is formed of granitoids with dates of 60 to 34 Ma and is mainly exposed in the outer islands, indicating a local oceanwards shift of plutonism with time.

Lower Cretaceous island arc volcanism was contemporaneous with the intrusion of the gabbroic assemblage, and, in part at least, of the Canal Beagle Plutonic Group. Folding of the marginal basin infill, which has been related to the closure of the marginal basin, occurred from about 100 to 93 Ma ago. The gabbroic assemblage was apparently mainly pre-tectonic, whereas the Seno Ano Nuevo Plutonic Group was clearly intruded after this deformational event. A post-tectonic or possibly late-tectonic emplacement for the youngest plutons of the Canal Beagle Plutonic Group and a pre- or syn-tectonic intrusion for the oldest is inferred.
The structural setting and tectonic significance of the Glen Dessary Syenite, Inverness-shire
Authors: Roberts A. M. , Smith D. I. , Harris A. L.
Published: December 2007

The Glen Dessary Syenite crops out in the steep belt of the Northern Highland Moine. It is enclosed within a psammite envelope which was previously assigned to the Glenfinnan Division, but which is here correlated with the psammites of the Loch Eil Division exposed to the E of the Loch Quoich Line. Three episodes of deformation have been identified in the Moine metasediments of the Glen Dessary area, but only the latest (D
3
) is thought to have deformed the syenite, imposing on it a strong, steeply plunging mineral lineation. The syenite is thought to occupy the core of a large, intensely curvilinear, D
3
synform. The D
3
structures which deform the syenite can be correlated with upright structures, which reworked previously flat-lying, already crystalline rocks, recognized at Loch Quoich. Upright reworking must be younger than the Glen Dessary Syenite, dated by U-Pb methods at 456 ± 5 Ma.

Two, hitherto unrecorded, small bodies of granitic gneiss, similar to the
c.
1020 Ma Ardgour Gneiss, have been identified in psammites of the Loch Eil Division near the W end of Loch Arkaig. Their presence suggests that the Loch Eil Division has a Grenville history, and is part of the 'Old Moine'.
Metamorphic conditions and uplift history in central Perthshire: evidence from mineral equilibria in the Foss celsian–barite–sulphide deposit, Aberfeldy
Authors: Moles Norman R.
Published: December 2007

Interlayered lithologies of diverse mineralogy enclosing the Middle Dalradian syngenetic Ba–Zn–Pb deposit at Foss, S of Loch Tummel in Perthshire, allow estimates of the P–T conditions of metamorphism using several new geothermometers and geobarometers. Temperatures of 500–560°C are derived from calcite–dolomite geothermometry and from data on Mg–Fe partitioning within garnet–hornblende and garnet–biotite pairs. Plagioclase–biotite–garnet–muscovite and biotite–muscovite–chlorite equilibria indicate peak metamorphic pressures of 8–10 kbar which are in agreement with the results of sphalerite geobarometry.

Textural evidence indicates that the rare platy mineral cymrite (BaA1
2
Si
2
O
8
H
2
O) was widespread in mineralized lithologies during penetrative deformation, and has since largely been replaced by celsian. The P–T location of the experimentally determined reaction line, cymrite
ss
= celsian
ss
+ H
2
O, places further constraints on the peak metamorphic conditions and suggests that the final episode of penetrative deformation (D
3
) was followed by a period of near-isothermal uplift during which celsian became stable. The cymrite field appears to have been re-entered during late brittle deformation. Peak metamorphic conditions recorded by mineral equilibria at Foss differ from those of areas 10 km to the N and S.
The Caledonian metamorphic core: an Alpine model
Authors: Bradbury Harry J.
Published: December 2007

The Neogene evolution of the Central Swiss Alps is outlined and used as an analogue for the Grampian Orogeny of Scotland. A direct comparison is made between the three tectonothermal stages which characterize the Alps and similar events in the Scottish Highlands. Geometric similarities are emphasised using geological cross-sections. Kinematic and mechanistic similarities are inferred. A net convergence model which links the metamorphic and uplift–denudation history of the Alps to its tectonic development in a crustal-scale duplex is argued to describe appropriately the history of the Dalradian and northern Scottish Highlands.
Seismic reflection investigations into the stratigraphy and structural evolution of the Worcester Basin
Authors: Chadwick R. A.
Published: December 2007

Two seismic reflection lines recently shot in the western part of the Worcester Basin suggest that its western boundary is defined by major syn-depositional normal faults, downthrowing to the east and active throughout Permo-Triassic times. Farther east, other faults can be seen within the basin. These moved at various times during the Permian and Triassic and profoundly affected patterns of sedimentation. The subsidence history of the basin can be explained by two phases of lithospheric stretching, one in the Permian and the second in early Triassic times, giving a cumulative stretching factor of about 1.2. This is consistent with the regional pattern of normal faulting provided that stretching was either one-dimensional, oblique to the N–S and E–W directions, or two-dimensional.
Volcanic hazards on St Kitts and Montserrat, West Indies
Authors: Baker P. E.
Published: December 2007

Pyroclast flows constitute the main potential hazard on both St Kitts and Montserrat. Radiocarbon dating indicates that the last major series of eruptions on St Kitts occurred about 2000 years ago, although a more recent phase of activity (1620 ± 50 years B.P.) produced the pyroclast surge and blast deposits of the Steel Dust ‘Series’. On Montserrat the last major eruptive episode occurred about 19000 years ago and there is rather uncertain evidence for a small near-historic eruption. Pyroclast flow material tends to have a basaltic andesite composition on St Kitts and that of a more siliceous andesite on Montserrat. At Mt Misery, St Kitts, the present configuration of the summit region has exercised considerable control over the distribution of pyroclast flows and the western side of the volcano remains the most vulnerable to future activity of this kind. The summit morphology of the Soufriere Hills, Montserrat is more complex and appears to have changed during relatively recent times, making the preferred pathway down the eastern slopes over the less populated side of the island. Pyroclast falls, regarded as a negligible hazard on Montserrat, may attain a critical destructive thickness of
c
. 50 cm once every few thousand years on St Kitts. Lava flows, which may be considered unimportant on Montserrat, are also likely to be a minor and very infrequent hazard on St Kitts.
Late Proterozoic schist belts and plutonism in NW Nigeria
Authors: Fitches W. R. , Ajibade A. C. , Egbuniwe I. G. , Holt R. W. , Wright J. B.
Published: December 2007

The deformed, low-grade, metasedimentary-volcanic schist belts of NW Nigeria, and the voluminous granitoid plutons which invaded them, are expressions of late Proterozoic-early Phanerozoic activity in the terrain separating the W African and Congo cratons. Recent interpretations of Nigerian geotectonic evolution have invoked two generations of schist belt, one a product of Kibaran (
c
. 1100 Ma) ensialic processes, the other due to Pan-African (700–450 Ma) marginal basin development. The detailed histories of the Anka Belt (Pan-African), Maru Belt (Kibaran) and Birnin Gwari Belt (unknown age), and the plutons emplaced in them, are documented here on the basis of new field, chemical and isotopic data.* Each belt represents a dominantly quiet-water sedimentary environment but volumetrically minor lithologies reveal important differences between the belts. The volcanic rocks and early minor intrusions have strong affinities with those of destructive plate margins. The schist belts were deformed congruently and simply before
c
. 750 Ma, following the development of flat-lying, possibly thrust-related structures in the Maru and Birnin Gwari Belts. Subsequent plutonism, beginning
c
. 750 Ma ago, has the calcalkaline, I-type characteristics of subduction zones but younger plutons have mildly peralkaline compositions due to thickening crust.
Fission track dating of alluvial zircons and heavy mineral provenance in Northeast Tasmania
Authors: Yim W. W.-S. , Gleadow A. J. W. , van Moort J. C.
Published: December 2007

Three alluvial zircon concentrates from the Blue Tier region have been fission track dated in a heavy mineral provenance study of Cenozoic sediments in NE Tasmania. Zircons in one concentrate are euhedral and are fine grained (mostly 0.1 to 0.3 mm). Dating gave a combined age of 367 ± 15 Ma confirming their derivation from Upper Devonian granites. Zircons from the other concentrates are rounded and coarser-grained (mostly 2–5mm). They gave a combined age of 46.7 ± 0.6 Ma. This agrees with the K-Ar radiometric dates obtained from three Blue Tier basalts (Sutherland & Wellman, pers. comm.). Some of the geological implications of this study are outlined in this paper.
Thin, fine-grained storm layers in a muddy shelf sequence: an example from the Lower Jurassic in the Stenlille 1 well, Denmark
Authors: Pedersen Gunver Krarup
Published: December 2007

Thin storm-sand layers are a characteristic feature of many sandy shelf sequences, while little evidence of storm layers is preserved in muddy shelf deposits where bioturbation is often total. The present study shows that storm-generated depositional processes governed sedimentation in a muddy shelf environment. The studied sequence is dominated by blackish mudshale and silt-streaked mudshale with thinner intervals of pale, fine-grained heterolithic sandstone. It represents the Lower Jurassic Fjerritslev Formation in the eastern part of the Danish Basin and has been studied in four cores from the Stenlille 1 well. Within the muddy outer shelf association four sedimentary facies and 14 types of laminae, 0.5–10 mm thick, may be distinguished. Most of the laminae share the characteristics of graded bedding, erosive bases and occasional fading ripples with storm layers of sand grade. The structures indicate that the mudshale laminae formed by settling of the suspension load carried by storm-generated currents. This kind of episodic deposition of fine-grained storm layers may be common, though rarely preserved, in outer shelf environments.
A long pollen record from Laghi di Monticchio, southern Italy: a preiminary account
Authors: Watts William Arthur
Published: December 2007

A pollen diagram from Lake Monticchio, a volcanic crater lake in S Central Italy east of Naples, shows a long vegetation record for the Holocene and the later portion of the Last (Weichselian) Glacial Period. The upper 735 cm of a 2550-cm core is assigned to the Holocene. It begins with expansion of
Betula
(birch) and
Quercus
(oak), followed by
Fagus
(beech),
Tilia
(lime) and other mesic trees. In the next phase the flora is dominated by oak with an increase in herb pollen. In the later Holocene the flora is again diversified by the appearance of
Abies
(fir),
Curpinus
(hornbeam) and other trees. In the latest Pleistocene portion of the core the vegetation is dominated by grasses,
Artemisia
(sage-brush) and chenopods, indicating a treeless environment. The base of the core shows continued dominance by steppe-like vegetation with 'interstadial' episodes in which trees, especially oak, beech and fir, were abundant. The climatic interpretation is that the late Pleistocene was very dry, at least in the summer growing season, and probably, although not necessarily, colder than now. In the Holocene increased precipitation favoured expansion of tree populations. For a brief period there was a return of water stress in the growing season marked by a loss of tree species diversity and an increase in herbs. In the later Holocene a progressive diversification of tree species indicates a return to a more humid and perhaps cooler climate. The vegetation history and climatic inferences are closely similar to records available from Greece. The available radiocarbon dates, which show a consistent pattern, indicate that the core is much older than 40000 years at the base but not old enough to include the major post-Eemian interstadials. The 'Holocene' birch expansion began nearly 20000 years ago. These dates appear to be systematically too old in the light of present knowledge. The preliminary character of the study is emphasized. Further radiocarbon dates, especially of the 'Holocene' section, are needed, as are more detailed studies of sedimentation processes and tephra occurrences.
Alluvial fan and playa sedimentation in an Andean arid closed basin: the Pacencia Group, Antofagasta Province, Chile
Authors: Flint S.
Published: December 2007

The mid-Tertiary Pacencia Group comprises over 2 km of continental sediments. It was deposited in a half-graben basin thought to have been active since early Tertiary times. Conglomerates along the western basin margin were deposited as longitudinal bars, rare transverse bars and channel fills with subsidiary debris flows, all in a distal fan pebbly braided stream environment. No proximal fan sediments are exposed. Petrographic and palaeocurrent analysis indicates these sediments were derived from Cretaceous and possibly Jurassic source areas to the northwest. To the east, within the basin, a succession of sands, silts, muds, thick evaporites and rare carbonates were deposited in playa sub-environments. Saliferous muds (central salt pan) are overlain by interbedded red mudstones and thin, high-energy ribbon and sheet sands (marginal dry mudflat and sandflat). Certain sand bodies carry stratiform Cui/Agi/Au mineralization. Lakes existed at various stages and were fed by permanent fluvial systems. The playa sediments were derived from crystalline source areas to the east. Proximal fan facies and sediment sources are now buried under the volcanic Andes.
Geometry of submarine slides in the Bowland Basin (Dinantian) and their relation to debris flows
Authors: Gawthorpe R. L. , Clemmey H.
Published: December 2007

Sedimentary slide deformation structures in the Lower Carboniferous (Dinantian) sediments of the Bowland basin show a spoon-shaped morphology with extension in the head region and compression in the toe. The particular structures developed depend on the rheological properties of the sediment involved. Slides in soft sediment have deformation distributed throughout the slide mass, with folding being the dominant deformation style. Slides within more lithified sediment have deformation concentrated along slip planes. Both types of deformation may be present within a particular slide, depending on diagenetic factors and the depth to which slide planes cut down. Disaggregation of slides to yield material for sediment gravity flows is explained by processes operating within the slide unit.
Geomorphological controls on the distribution of placer deposits
Authors: Sutherland Donald G.
Published: December 2007

The objective of this paper is to show that the global distribution of placer deposits is largely a product of variation (both at present and in the recent geological past) in geomorphological processes acting at the earth's surface, given suitable primary mineral sources. The distribution of known placers of diamonds and gold as well as certain littoral placers is considered in relation to the occurrence and tectonic setting of the primary sources of the various minerals. Climatic controls of the many variables that are relevant to placer formation (e.g. weathering, rate of erosion, nature of sediment supply, opportunities for sediment reworking) are discussed in terms of broad morphogenetic regions. Five such regions are identified on the basis of distinctive combinations of the geomorphological processes responsible for placer formation. Global changes in climate and accompanying changes in the surface processes responsible for placer formation during both the Tertiary and, more particularly, the Quaternary have been important in producing the alternation between erosional and depositional conditions that is especially suitable to placer formation. Primary source tectonic setting and morphogenetic processes all interact to allow the identification of regions in which placer formation is favoured.
Very low-grade metamorphic Sr isotopic resettings of magmatic rocks and minerals: evidence for a late Givetian strike-slip division of the Brabant Massif, Belgium
Authors: André L. , Deutsch S.
Published: December 2007

Rb-Sr isochron ages recorded by the Ashgillian ignimbrites from Deerlijk and Harelbeke and the Caradocian quartz diorites from Quenast and Bierghes significantly post-date their emplacement. At Quenast, two mineral isochrons (biotite-WR-plagioclase: 372 ± 13 Ma; K-feldspar-WR-epidote: 375 ± 14 Ma) reveal that the Brabantian branch of the Caledonian belts was affected by a post-Siegenian, pre-Tournaisian event. This overprint turns out to be contemporaneous with the development of the Oudenaarde–Bierghes fault zone which is dated to 373 ± 11 Ma and 379 ± 13 Ma on foliated rocks from Bierghes, Deerlijk and Harelbeke. This
c
. 375 Ma cluster of events is interpreted as representing late Givetian stresses developed along the Oudenaarde–Bierghes fault zone for which the balance of geological data favours strike-slip movements. Since this compressive phase of deformation is posterior to the Givetian opening of the Borain trough, the Brabant Massif appears to have been affected by successive crustal extension and shortening events in the Middle Devonian.

Petrological evidence demonstrates the very low-grade character of the metamorphism in the metasediments on both sides of the Oudenaarde–Bierghes fault zone. From this, we deduce that Sr isotopic homogenization of whole-rock ignimbrite and the loss of radiogenic Sr from biotite and K-feldspar could be operative even at temperatures below 250°C.
Magnetostratigraphic correlation of Palaeogene sediments in the Hampshire and London Basins, southern UK
Authors: Townsend H. A. , Hailwood E. A.
Published: December 2007

Magnetostratigraphic studies of the late Palaeocene and early to middle Eocene formations in the Hampshire and London Basins indicate that the geomagnetic field maintained a predominantly reverse polarity throughout the period of deposition of these sediments. Eight discrete normal polarity magnetozones have been identified within this dominantly reverse polarity interval. Three of these lie within the Bracklesham Group, three within the London Clay Formation, one in the Oldhaven 'Formation', and one within the lowermost part of the Thanet Formation. Assuming that the magnetozone boundaries represent absolute time planes (except where they correspond with stratigraphic hiatuses), they may be used to provide a time framework for chrono-stratigraphic correlation between sections, and for investigating the dynamics of early Palaeogene sedimentation patterns in this region. The results indicate that the majority of the marine transgressive and regressive events which form the basis of
King's (1981)
subdivisions of the London Clay occurred very rapidly, and may be regarded as being effectively 'instantaneous'. In marked contrast, the boundaries between the sedimentary cycles of the Bracklesham Group (represented by the Wittering, Earnley, Selsey and Huntingbridge Divisions) are significantly diachronous. In relation to the absolute time planes provided by the magnetozone boundaries it is clear that the Bracklesham Group transgressive events occurred systematically earlier in the east than in the west, as might be expected for westward-directed transgressions. Certain biostratigraphic zonal boundaries within the Bracklesham Group appear also to have been influenced by the changing environmental conditions which accompanied these transgressive-regressive cycles.
Magnetic fabric arising from paramagnetic phyllosilicate minerals in mudrocks
Authors: Hounslow M. W.
Published: December 2007

The magnetic fabric of mudrock samples from the Westbury Member of the Penarth Group (Rhaetian), the Blue Lias Formation (Hettangian to Sinemurian) and the Oxford Clay (Callovian to Oxfordian) has been measured, using a high field torque magnetometer. The magnetic susceptibility anisotropy of these mudrocks originates from paramagnetic phyllosilicate minerals, of which the most important is considered to be detrital chlorite. The magnetic fabric preserves both a lineation and a foliation, which are of a primary style. This magnetic fabric is sensitive to the effects of recent weathering, due to the ease with which chlorite is oxidized during weathering. A model is proposed utilizing compaction, to explain the acquisition of this magnetic fabric. This model infers that the magnetic fabric represents, in both degree and direction, a measure of the preferred crystallographic orientation of the paramagnetic phyllosilicate minerals. The magnetic fabric data suggest that the Kmax direction corresponds with the quartz grain long-axis preferred orientation, and therefore may be used as a palaeocurrent indicator as in titanomagnetite-bearing sediments. Evidence is presented to suggest that paramagnetic susceptibility anisotropy may be common in other mudrocks.
Magnetic fabric in some granitic aureoles, Southern Uplands, Scotland
Authors: Kafafy A. M. , Tarling D. H.
Published: December 2007

The Loch Doon, Cairnsmore of Fleet and Criffell granites and aureoles show considerable variation in their magnetic mineralogy and fabrics. The Loch Doon area has magnetite as the dominant magnetic mineral, while the Cairnsmore of Fleet area shows magnetite and hematite in different areas. The Criffell granite and immediately adjacent aureole are characterized by magnetite, but hematite predominates in the outer aureole and country rocks. Despite such differences, the magnetic fabric only shows effects of granitic emplacement within the thermal aureole, mostly within 200–300 m of the contact. Considerable variation in magnetic fabric is observed over small distances, but the granitic contacts are generally marked by a strong foliation parallel to the granite contact.
Particle-size related, mineral magnetic source sediment linkages in the Rhode River catchment, Maryland, USA
Authors: Oldfield F. , Maher B. A. , Donoghue J. , Pierce J.
Published: December 2007

Characterization of both the source materials and the bottom sediments of an estuary by magnetic characters, on a particle size basis, allows the source of the bottom sediments to be determined and indicates a shift in sediment sources, corresponding to changing land-use patterns. The major source of the sediments in the lower part of cores from the middle of the estuary is eroding shorelines. Soil-derived particulates, from the uplands, dominate the upper part of the cores. This change in source occurred in the early part of the 19th century. The magnetic character of the most recent sediments suggests a progressive depletion, through time, of the upland soils in the finest fractions.
Barbuda—an emerging reef and lagoon complex on the edge of the Lesser Antilles island are
Authors: Brasier Martin , Donahue Jack
Published: December 2007

The Pliocene to Holocene limestones of Barbuda have formed on a wide, shallow, outlying bank of the Lesser Antilles island arc, some 50 km east of the older axis of the Limestone Caribbees and 100 km east of the newer axis of the active Volcanic Caribbees. Contrasts with neighbouring islands of similar size include the lack of exposed igneous basement or mid-Tertiary sediments, the dominance of younger flat-lying carbonates, and the greater frequency of earthquake shocks.

The history of emergence of the island has been studied through aerial reconnaissance, mapping, logging, hand coring, facies and microfacies analysis. These show a pattern of progressively falling high sea level stands (from more than + 50 m down to the present level) on which are superimposed at least three major phases of subaerial exposure, when sea levels were close to, or below, their present level. This sequence can be summarized as follows: 1, bank edge facies (early Pliocene Highlands Formation) deposited at not more than c. 50–100 m above the present sea level; 2, emergence with moderate upwarping in the north, associated with the Bat Hole subaerial phase forming widespread karst; 3, older Pleistocene transgression with fringing reefs and protected bays formed at + l0 to + l5 m high sea level stands (Beazer Formation); 4, Marl Pits subaerial phase with widespread karst and soil formation; 5, late Pleistocene transgression up to +6 m high stand with fringing and barrier reefs, protected backreefs and bays (Codrington Formation Phase I); 6, gradual regression resulting in emergence of reefs, enclosure of lagoons, and progradation of beach ridges at heights falling from
c
. 5 m to below present sea level (Codrington Phase II); 7, Castle Bay subaerial phase produced karst, caliche and coastal dunes that built eastwards to below present sea level; and 8, Holocene transgression producing the present mosaic, with reefs, lagoons and prograding beach ridge complexes, with the present sea level reached before
c
. 4085 years BP.

The evidence suggests that slight uplift took place in the north of the island after early Pliocene times. Subsequent shoreline fluctuations are consistent with glacio-eustatic changes in sea level, indicating that the island has not experienced significant uplift during the Quaternary.
The geochemistry, metasomatism and petrogenesis of the granites of the English Lake District
Authors: O’Brien C. , Plant J. A. , Simpson P. R. , Tarney J.
Published: December 2007

New geochemical data for the Carrock, Threlkeld, Ennerdale, Shap, Skiddaw and Eskdale granites of the Lake District are presented and discussed with particular reference to the metasomatism and petrogenesis of the intrusions.

The Caledonian granites of the Lake District have more associated hydrothermal activity and mineralization than their equivalents north of the Iapetus suture in Scotland. Pervasive high-temperature metasomatism which affects the Shap, Skiddaw and Eskdale intrusions is accompanied by remobilization of the large ionic lithophile elements (K, Rb, Sr, U) and Li and B, although high field strength elements, including the rare earth elements, remain unaffected except near greisen and mineral veins. The Threlkeld intrusion appears to have suffered loss of a Ba and Sr during a low-temperature event. However, there is no consistent relationship between granite composition, hydrothermal activity and mineralization which could be used to support the granite cupola model for mineralization in the province. Rather, the episodic mineralization which affects the Lake District may be related more to the broad geothermal field consequent upon the emplacement of the deeper Lake District batholith, identified from geophysical data.

Despite the variable metasomatic perturbations of their primary chemistry, three distinct groups of intrusions can be recognized on geochemical grounds. The Carrock granophyre is a product of tholeiitic fractionation and the Threlkeld and Ennerdale bodies are more typical calc-alkaline intrusions, whereas the Eskdale, Shap and Skiddaw plutons show a progressive evolution towards more complex geochemical patterns, suggesting decreasing hornblende but increasing plagioclase control of fractionating processes. The culmination of this trend may have been the emplacement of a large, high heat production granite beneath the Lake District at the end of the Caledonian orogeny. The suggestion made on geophysical grounds that the Eskdale intrusion is a cupola of the Lake District batholith cannot be simply reconciled with geochemical modelling. None of the Lake District granites has geochemical or isotopic characteristics which are convincingly S-type (i.e. formed by partial melting of a sedimentary protolith), and a model of subcrustal magmagenesis beneath an evolving arc or continental margin appears to be more appropriate.
The Variscan structure of South West England and related areas: conference report and introduction to published papers
Authors: CHAPMAN T. J.
Published: December 2007

The Department of Geological Sciences at Plymouth Polytechnic hosted a two day meeting of the Tectonic Studies Group on October 27 and 28, 1984. On the first day of the meeting 17 papers were presented on tectonic aspects of the Variscan orogen in SW England and related areas. On the second day two field excursions ran concurrently to the north Cornish coast (leaders G. E. Lloyd, J. S. Whalley and D. J. Sanderson) and the Roseland area of south Cornwall (leaders J. R. Andrews and R. P. Barnes).

The success of the conference in terms of papers presented and number of participants (well over 100) is a reflection of the continuing interest in the structural geology of SW England. It was the first conference for a number of years dealing specifically with the Variscan tectonics of SW England. A short symposium on the structure of SW England (co-ordinated by
W. R. Dearman
) was held in 1971 (see
Proceedings of the Ussher Society
2
, Part 4, 1971) and in 1977 a meeting entitled ‘Tectonic evolution of the Variscides of the British Isles’ was held at University College, Swansea (see
Hancock 1983
). In 1981 the thirty-fifth William Smith Lecture meeting included papers relating to the Variscan (or Hercynian) of SW England (
Journal of the Geological Society
134
, Part 4, 1982). Following this in 1982 the Department of Geology at Trinity College, Dublin hosted a conference entitled ‘Variscan tectonics of the North Atlantic Region’ which included several papers on SW England (see
Hutton
A model for the tectonic evolution of south Cornwall
Authors: HOLDER M. T. , LEVERIDGE B. E.
Published: December 2007

A stratigraphy for south Cornwall is combined with an analysis of the chronology of thrust nappe emplacement. The resulting model suggests that northward overthrusting of nappes of flysch and olistostrome, ophiolite, and continental basement is related to the closure of an oceanic basin in south Cornwall involving southward subduction throughout the Devonian. The subsequent continental collision between the Normannian High and part of Laurentia occurred in the Upper Devonian with deformatian continuing until late Carboniferous times.
Correlation of the Rhenohercynian Variscides
Authors: HOLDER M. T. , LEVERIDGE B. E.
Published: December 2007

Recent structural and stratigraphical data from the Rhenohercynian Variscides of Belgium, Germany and South Cornwall indicate tectonic continuity within this zone prior to disruption along a late Carboniferous dextral transcurrent fault. This extends from the Bristol Channel to the Vosges, parallel to similar lineaments of Variscan age in Europe and North America. Consequently, a tectonic model for south Cornwall is extrapolated to Europe and a plate tectonic setting for the evolution of the north European Variscides by the closure of an oceanic basin is discussed.
Rates of Neogene and Quaternary tectonic movements in the Southern Banda Arc based on micropalaeontology
Authors: AUDLEY-CHARLES M. G.
Published: December 2007

It is generally considered that the Outer Banda Arc islands are composed in part of strongly deformed sediments that accumulated at the north Australian continental margin. The onset and completion of folding, nappe emplacement, imbrication and uplift that produced the islands of the Outer Banda Arc have been dated by means of planktonic foraminifera. The dating of the onset of uplift of Timor, from a deep submarine position at the end of the Neogene nappe emplacement, to mountains now reaching nearly 3 km high, indicates the rate of post-collision uplift to have been initially 3 mm/yr, then it slowed to about 1.5mm/yr. Where the Australian continental margin meets the eastern end of the present Java Trench there are indications that the Australian margin has overridden the Trench in the Timor region by 240 km. Micropalaeontological dating of the Neogene collision between the Australian continental slope–proximal rise and the trench provides constraints for checking the overriding hypothesis by calculating rates of convergence derived from plate motions (7.5cm/yr) against the rates based on the tectonic model (7.4cm/yr). After nappe emplacement (possibly faster than 6.25 cm/yr) shortening of the continental crust appears to have migrated towards the Australian continent so that the shelf itself has become involved in the imbrication with shortening of the cover rocks between the nappes and the present shelf edge amounting to about 40 km during the last 2 Ma
Variscan facies and structure in central SW England
Authors: SELWOOD E. B. , THOMAS J. M.
Published: December 2007

The character and distribution of middle Devonian to upper Carboniferous facies in central SW England is described. These may be interpreted in terms of the simple model of a shelf extending from a continent to the north, across most of SW England in earliest Devonian times. Within it a major E–W trending basin developed during the middle Devonian, possibly as a half graben structure with an active northern hinge. On this shelf in mid and late Devonian times, argillite successions accumulated to the north and a carbonate complex on the now starved platform to the south of the basin. In the early Carboniferous, basinal conditions extended northwards over the shelf, but to the south, at first flysch and then paralic sediments with a southern origin were ponded against the carbonate complex. Flysch penetrated through and across the platform into the intra-shelf basin. This flysch prograded northward to cover the whole area by early late Carboniferous times.

The western part of the main basin suffered structural inversion in late Visean times and semiconsolidated sediments spread northward across its margin. The eastern extension of the basin was not deformed until the late Namurian, at which time successions were folded and disrupted into nappe structures showing northward transport. The most far-travelled nappes are represented at the highest structural levels.

The stratigraphy of the coastal section of N Cornwall, which has been used to devise a structural model involving a major autochthonous south-facing overfold, is reassessed. The nappe structures and successions recognized inland are identified on the coast, and the overfold model is disproved. The whole of central SW England forms part of southerly derived thrust and nappe terrane.
Later Palaeozoic Radiolaria in thrust belts of Nevada
Authors: HOLDSWORTH B. K.
Published: December 2007

The use of later Palaeozoic Radiolaria as age indicators in N Nevada has made possible stratigraphic comparisons between a small part of the Golconda Allochthon—the Schoonover Sequence, the structurally underlying Roberts Mountains Allochthon and sequences which formed in a foreland in a foreland basin as the Roberts Mountains Allochthon was emplaced at the North American continent margin (

Miller
et al.
1984

).

Emplacement of the Cambrian to Devonian deeper water Roberts Mountains Allochthon rocks occurred in Kinderhook–Osage time (
Johnson & Pendergast 1981
) and is usually viewed as a consequence of latest Devonian to early Mississippian Antler orogenesis. The late Devonian to mid-Permian Golconda Allochthon rocks were probably emplaced in the early Mesozoic.

Radiolaria are essential to the elucidation of the older Golconda succession. Hitherto unsuspected similarities between events recorded by these older Golconda rocks and those recorded in the Antler foreland basin seem at variance with the proposal that the Golconda Allochthon is a relatively far-travelled accretionary prism which became part of North America in the early Mesozoic (
Speed & Sleep 1982
). It might rather seem that the Schoonover basin was the exterior (oceanward) analogue of the Antler foreland basin: it originated during the first, late Devonian, volcaniclastic supply but shared with the foreland basin a Mississippian siliciclastic source in the Roberts Mountains Allochthon and suffered the same intra-Pennsylvanian movements that are recorded in successions. The Schoonover basin existed, therefore, at the continent margin adjacent to the 'Antler volcanic arc' postualated by several authours and to the eroding Roberts Mountains Allochthon.
Geological setting and genesis of coticules (spessartine metapelites) in the Lower Ordovician of the Stavelot Massif, Belgium
Authors: LAMENS J. , GEUKENS F. , VIAENE W.
Published: December 2007

The Lower Ordovician in the Caledonian Stavelot Massif (Belgium) contains a coticule member, probably Arenig in age, composed of red hematite-rich shales alternating with thin yellowish coticule beds. These coticules are fine-grained highly manganiferous metamorphic rocks mainly composed of spessartine garnet, mica and quartz. Similar rocks of about the same age occur in several localities along the Appalachian and Caledonian fold belts. The Belgian coticules are thought to have originated from marly sediments which had been deposited rapidly, probably by density currents, and in which CaCO
3
had been diagenetically replaced by MnCO
3.
During metamorphism, spessartine garnets were formed from this MnCO
3
and mica. Manganese and iron are thought to have originated from volcanic-exhalative processes.