SW England Research Articles

Assessing habitat disturbance using an historical perspective: The case of Salisbury Plain military training area

Chalk grasslands are a habitat with high European importance for both flora and fauna. The largest known expanse of unimproved chalk grassland in north-west Europe lies within the Salisbury Plain training area (SPTA) in SW England, where sole use of the land for military training since the end of the nineteenth century has limited the ecologically damaging impacts of modern intensive agriculture. Organisational changes in the British Army may now paradoxically be threatening this unique ecological resource. In this study, historical aerial photograph analysis was carried out for SPTA using images from the 1940s through to the mid 1990s. Image analysis software enabled the creation of a model that analysed the extent and pattern of high intensity military disturbance on SPTA at a local landscape scale. Although trends in disturbance vary across SPTA for the time period under investigation, the average annual increase in bare ground since WWII has been in the region of 25·5ha. These trends indicate that disturbance is occurring at a greater rate than natural regeneration, representing a significant threat to the chalk grassland through habitat loss and fragmentation.

Abnormal historic sea-surface fluctuations, SW England

Abstract This paper presents a series of documentary accounts of abnormal sea surface changes that have affected the coastline of SW England during historic times. An attempt is made to consider possible origins for individual events although it is recognised that certain episodes of coastal flooding appear to have no simple explanation. However, certain coastal floods appear to have been generated by tsunamis triggered by far-field offshore earthquakes.

A central European Variscide source for Upper Carboniferous sediments in SW England: 40 Ar/ 39 Ar detrital white mica ages from the Forest of Dean Basin

40 Ar/ 39 Ar detrital white mica ages from a Westphalian C/D sediment in the Varisan foreland Forest of Dean Basin (SW England) range from 309±12 to 371±2 Ma. Combining these ages with chemical data from the detrital white micas constrain their provenance to two sources: reworked Westphalian A to C, and, more controversially, the central European Variscides. Taking into account the palaeogeography of the Late Devonian to Late Carboniferous, the sediment derived from the central European Variscides must have resided in a holding basin proximal to their source for 30–40 Ma. During the Mid- to Late Carboniferous sediment was transported longitudinally from east to west along the Variscan front. Deposition and mixing of the two source components took place in the Forest of Dean Basin at approximately 307 Ma.

Pre-emplacement structural history recorded by mantle peridotites: an example from the Lizard Complex, SW England

The Lizard Complex of SW England includes thrusted units of peridotites that were initially exhumed from upper mantle ( c. 52 km) to lower crustal ( c. 24 km) depths during a period of Early Devonian rifting and break-up. This basin closed during the Late Devonian, when the Lizard Complex was thrust towards the NNW along a major low-angle detachment and became incorporated within a series of Variscan thrust nappes. In the peridotites, a primary high- T and high- P spinel lherzolite mineral assemblage ( c . 1119°C and c. 15.7 kbar) was progressively exhumed and re-equilibrated to conditions of lower T and P ( c . 991–1010°C and c . 7.5 kbar) during the development of kilometre-scale mylonitic plagioclase- and amphibole-bearing mantle shear zones. These fabrics demonstrably pre-date emplacement related structures. The new structural and geochemical evidence from the peridotites also strongly suggests that the Lizard Complex formed in a rifted, non-volcanic continental margin setting, possibly in a pull-apart basin, rather than at a mid-ocean ridge. The P–T and textural evolution of the Lizard peridotites supports growing evidence that shear zones in the lithospheric upper mantle may to some extent accommodate large-scale displacements associated with crustal extension and continental breakup.

Geochemical Constraints from Zoned Hydrothermal Tourmalines on Fluid Evolution and Sn Mineralization: an Example from Fault Breccias at Roche, SW England

Hydrothermal fluid evolution north of the St Austell granite, southwest England, has been studied through geochemical analysis of tourmaline from a fault breccia of <2 cm width within massive quartz–tourmaline rocks at Roche. Brecciated tourmaline grains have overgrowths of <400 μm width [Fe/(Fe + Mg) = 0·31–0·99] with four chemically distinct zones (1–4, towards the margins). Variations in overgrowth composition were caused by episodic mixing between Mg-, Al-rich magmatic hydrothermal fluids (dominant in zone 1), with an increasing component of more oxidizing, Fe-rich formation waters (zones 2 and 4). More oxidizing conditions are supported by high Sn contents in zone 2 (<0·35 wt %), with Sn probably present as Sn4+ rather than Sn2+, the usual form in hydrothermal fluids. From X-ray maps, zones 1 and 3 occur exclusively as overgrowths on pre-existing grains, indicating that overgrowth formation was kinetically favoured over tourmaline nucleation. In zones 2 and 4, nucleation and growth occurred, possibly as a result of supersaturation with respect to tourmaline during increased mixing with formation waters. Tourmaline is associated with the main episode of mineralization in many important mineral deposits, often unaffected by alteration. This method of studying hydrothermal fluid evolution may therefore have uses in exploration, particularly for tourmaline-breccia-hosted ores in Cu-porphyry deposits.

Sulphur, carbon and oxygen isotope studies of early Variscan mineralisation and Pb-Sb vein deposits in the Cornubian orefield: implications for the scale of fluid movements during Variscan deformation

The strongly deformed Middle Devonian-Lower Carboniferous metasedimentary-volcanic successions of the Trevone Basin (SW England) contain stratiform and Pb-Sb vein deposits that reveal a wide variation in δ34S and δ13C, reflecting mineral deposition during diagenesis, regional metamorphism and basin inversion. Pre-Variscan metasedimentary sulphide (δ34S=−33.7 to −26.7‰) and metabasite sulphide (δ34S=+4.0 to +10.8‰) suggest two accessible source reservoirs for sulphur which were available for Sb-As-(Au) and Pb-Zn-(Ag) mineralisation (δ34S=−3.3 to −15.0‰) during late Variscan semiductile-brittle shear. On the basis of pressure-corrected fluid inclusion temperatures, the calculated composition of fluid sulphur reveals an enrichment in δ34SH2S in the individual vein parageneses and depletion of the fluid sulphur reservoir during evolution of the vein systems. Carbonates in the same veins are partly contemporaneous with Pb-Sb mineralisation and late tensional deformation; their isotopic composition (δ13C=−3.2 and −13.4‰) appears strongly influenced by the host formation. Fluid inclusions in post-tensional quartz show a marked reduction in CO2, suggesting that episodes of CO2 degassing in response to punctuated reductions in pressure during uplift and brittle deformation was an important mechanism for vein carbonation. An origin for the Pb-Sb mineralisation involving local remobilisation of sulphur from the mixed metasedimentary-volcanic succession is probably inseparable from processes connected with Variscan metamorphism and deformation. Although the N Cornish Variscan deformation is part of a spatially large-scale event, the isotopic evidence suggests compartmentalisation of sulphur and carbon isotope features and short distances between sources and sinks.

Environmental variables in a holistic evaluation of land contaminated by historic mine wastes: a study of multi-element mine wastes in West Devon, England using arsenic as an element of potential concern to human health

Unusual and unexpected concentrations of a number of elements were identified in samples of house dust, that indicated the presence of mine wastes in an area where they were not expected. In pursuing this matter, several other very unusual observations and practices, involving highly contaminated mine wastes, were also identified. Here, using an available, but not a custom-made database, the matter is pursued. In England and Wales, the usual framework within which hazards are assessed, starts with an identification of those levels of exposure to elements which are considered to be safe and acceptable. At the other extreme, levels that are considered to be hazardous are identified together with procedures for dealing with them for different situations; for example, the manner in which contaminated land is to be used. The level at which an identification of sites and their use rests with the Local Authority, such as District Councils. Although new legislation is pending; at present for the non-occupationally exposed population there are no firm values to define which levels are acceptable and those that are not acceptable. One of the elements in the samples is arsenic and, because of its well-known toxicity, this element is selected to be of prime concern. However, simultaneous exposure to the general public is from a number of other elements, such as copper, lead, zinc, antimony, molybdenum, tin, selenium and mercury which are present in the mine wastes. The collective impairment to human health, if it should occur, is far too complex to evaluate, hence a need to focus attention upon arsenic which by any standard is present in some samples at very high concentrations, for example >1000 mg/kg. Irrespective of any changes in permitted levels of exposure to the general public in the study area, together with those that are occupationally exposed, it is the past exposure that will give rise to the present patterns of morbidity and mortality. Because of a latent period between exposure and effects for the induction of cancers, of between 5 and 20 years, past exposures cannot be ignored. They need to be evaluated before changes are made in legislation. In England and Wales, concern to health is, in practice, invoked when there is clinical evidence of harm. With the exception of a few accidents in the study area of SW England, this is not identified, hence it is accepted, in practice, to conclude that no harm accrues following acute or chronic exposure to the mine wastes. There are reasons for questioning this, but if eventually supported, then the current very high costs for remediation of land may not be necessary and brown field sites can be used for a number of purposes. The primary concern is the lack of adequate scientific and clinical data, in relation to exposure to the wastes for the past 100 years or so. For many elements, compounds and substances, the general public is being made aware of potential risks to health but often the levels are extremely low. Using basic geochemical data for any area, it is possible to evaluate the expected background levels for elements. They should serve to identify levels that are acceptable, i.e. it is impractical or uneconomic to reduce them. Within the environment, simple tests are also available that can be used to rank risks to fauna and flora. There are also well-structured clinical evaluations of harm to humans available, which can also be ranked. All three need to be considered in the establishment of safe levels of exposure. It may not be practical to identify universal levels for exposure, rather each site, area or region needs to be considered separately in order to rank the potential risks. For the study area the exposures can be extremely high; compared with the general population, the number who are exposed is probably small. It is surprising that no effects upon human health have been noted.

A comparison of the geochemistry of Fe–Mn coatings on pebbles and Potamopyrgus ( Hydrobia) jenkinsi shells, Allen River, SW England, UK

An investigation of the chemistry of surface precipitates on stream pebbles and Potamopyrgus ( Hydrobia) jenkinsi (Smith) shells has been undertaken on samples collected from a contaminated river in SW England, UK. Samples were analysed by LA-ICP-AES to evaluate the technique as a suitable method to use on small environmental samples, and to determine if the Pb/Fe and Zn/Mn ratios of the coatings on the two substrates were comparable. The ratios in the coatings are similar in these samples types, but are generally higher on the pebble samples. Results have been compared to a previous investigation of pebble coating chemistry in the river, and are shown to have the same spatial distribution but very different absolute values due to changes in environmental conditions in the river over time.

Late Carboniferous tectonic subsidence in South Wales: implications for Variscan basin evolution and tectonic history in SW Britain

Detailed stratigraphic data have been used to backstrip seven sections from the Carboniferous South Wales coal basin. Resulting tectonic subsidence curves for the interval 319–305 Ma (Namurian–Westphalian D) are convex‐up, indicating increasing subsidence rate with time, with rates between 130 and 250 m Ma −1 , suggesting a foreland basin setting. Forward modelling of subsidence due to flexural loading in front of a propagating orogenic wedge shows that an orogenic load migrating in a north‐northeasterly direction across SW England between 319 and 305 Ma could have generated the backstripped subsidence patterns. Sensitivity tests show that while many of the forward model parameters are poorly constrained, and the model results non‐unique, backstripped subsidence patterns allow reasonable constraint on the different model cases, so that model predictions can be treated as one possibility in a limited range. The predicted load evolution is consistent with current knowledge of tectonic and stratigraphic features of SW England. These results suggest that subsidence in other Late Carboniferous UK basins may also have been influenced by flexure due to a propagating orogenic load.

Spatial and temporal variation in settlement and recruitment of the intertidal barnacle Semibalanus balanoides (L.) (Crustacea: Cirripedia) over a European scale

Variation in the level of settlement and recruitment in the intertidal barnacle Semibalanus balanoides was studied using a hierarchical sampling programme. The effect of three spatial scales, 10s of metres (sites), 1000s of metres (shores) and 100s of kilometres (locations), was determined. The largest spatial scale represented the distance between four widely separated locations, Sweden, the Isle of Man , SW Ireland and SW England, covering a large part of the range of S. balanoides in Europe. Temporal variation was determined by comparison between two years, 1997 and 1998. The settlement period of S. balanoides varied in length and timing, being earlier and shorter at the most northerly location, Sweden. The duration of settlement showed little difference among shores within locations, but the pattern of settlement did vary. Estimates of total settlement throughout the settlement period and of recruitment at the end of this period both showed substantial variation among locations which was dependent on the year of study. There was little consistency in the ranking of locations between the two years. Recruitment showed significant variation at the lower spatial scales of shore and site. In addition, examination of variance components showed a high degree of variation between replicates within sites in 1997. There was a significant relationship between settlement and recruitment at three of the four locations. Across all locations variation in settlement explained between 29 and 99% of variation in recruitment. However, locations showed distinct differences in the level of post-settlement survival.

U–Pb ages and geochemistry of granite pebbles from the Devonian Menaver Conglomerate, Lizard peninsula: provenance of Rhenohercynian flysch of SW England

The geochemical composition of some garnet-bearing biotite granite pebbles within the Menaver Conglomerate were investigated, together with the U–Pb isotope systematics of zircons extracted from two samples. The granites have suffered low-grade metamorphism as shown by the development of a secondary mineral assemblage of chlorite, Fe-oxides, muscovite and stilpnomelane. In terms of their determined ages, the older granitic pebble exhibits a strong cataclastic fabric with orthoclase megacryst `augen', whereas the younger granitic pebble is less deformed. Chemical discrimination of the initial emplacement environment of the granites is equivocal, although a syn-collisional setting during closure of the Ordovician Rheic ocean is preferred. The granites were finally exposed and released as clasts after rapid uplift and erosion into the remainder of Lizard–Harz ocean during the Frasnian. The zircons of both granitic pebbles show different degrees of discordance. Their lower intercept ages with the concordia curves of 373±6 Ma and 422±4 Ma, respectively, are interpreted as generation ages of the granites. The upper intercept ages with the concordia curves of 2606±40 Ma and 2445±19 Ma, respectively, indicate the presence of a late Archaean/Early Proterozoic source terrane. The Normannian High, the northern part of the Armorican plate, is proposed as the initial source region of the granites and the pebbles derived from them.

Midwife Attitudes and Advice to Pregnant Smokers

Midwives (N = 138, response rate = 51%) from two regions (SW England and London/SE England) responded to a questionnaire designed to assess (a) their attitudes to giving anti-smoking advice to pregnant smokers, and whether they perceived this as part of their professional role; and (b) the types of advice they gave to pregnant smokers as part of their routine practice. Midwives' own smoking status was also recorded. Attitudes towards giving anti-smoking advice were generally positive, and almost all reported routinely explaining the health dangers of smoking to pregnant smokers. Midwives who had never smoked (63%) reported attitudes and behaviour which did not differ significantly on average from those of the remainder of the sample. Overall, those who gave more anti-smoking advice also perceived this to be a more important part of their professional role. The form of this relationship depended somewhat on midwives' own smoking status. Among midwives who had never smoked, those who held role attitudes tha...

A comparison of triple superphosphate and Gafsa ground rock phosphate fertilisers as P-sources for grass-clover swards on a poorly-drained acid clay soil

Organic farming practice prohibits the use of triple superphosphate (TSP) as a source of phosphorus. As basic slag is not now generally available, interest is focused on the relative value of ground rock phosphate (GRP). A comparison of TSP and Gafsa GRP was made during 1988–92 as to their ability to increase DM production under cutting from newly sown grass/white clover swards established in 1987 on an acid clay soil in SW England. Averaged over the different P fertiliser inputs and years, the DM yield was 8.0 t ha-1 y-1 (range 6.93–9.81) compared to 6.3t ha-1y-1 (range 6.00–7.71) without added P. Lime was added at either 3 or 6t ha-1 in 1987, and at half these rates in 1990. Whereas the yield improved by 45% with P at the lower rate of lime, it improved only by 12% at the higher rate. When P fertiliser was applied annually at 30 kg ha-1, TSP was superior to GRP, but when applied in one initial dose of 120 kg ha-1, GRP was superior over the subsequent 4 year period. There was no consistent effect of the addition of either P or lime on the clover content of the sward. The alkaline bicarbonate soil test (Olsen P) was a good predictor of available P within a given year; there was a general reduction of P availability over the course of the experiment. Mechanisms to explain the longer term effectiveness of GRP are postulated and discussed. It is concluded that farmers who are limited to using GRP rather than TSP would suffer a yield penalty over the longer term of 11.5%, and that P fertilizer for the ley phase in a rotation should be incorporated in one dose at the outset.

Evidence for the importance of `small' faults on block rotation

Field and earthquake data are presented which show that small faults can be important during block rotation. `Small' here means those faults which are at least an order of magnitude smaller than the largest faults at a particular scale of observation. The NW–SE-trending dextral faults of SW England have millimetres to kilometres of displacement. The largest of these faults, such as the Sticklepath–Lustleigh fault, do not appear to cause more than a few degrees of rotation from the general E–W strike of beds. Tens of degrees of rotation are often visible, however, on faults with millimetres of displacement. Similarly, the apparent rotation of beds in the Badajoz–Cordoba Shear Zone, Spain, increases as the resolution of the faults is increased. The power-law scaling relationship of earthquake magnitudes in the San Andreas fault zone illustrates that small faults can also be important in regions of active block rotation. `Small' faults can allow deformation within rotating blocks and can allow high displacement gradients to occur on the block-bounding faults. This would reduce the need for void creation, which is a requirement of rigid block rotation models. A tentative model is presented which incorporates the concept that fault behaviour is fractal, emphasising the contribution of small faults. It is suggested that future studies of block rotation should rigorously test the contribution of `small' faults, and that particular care is needed when comparing palaeomagnetic data with regional-scale structures.

Comparative study of the reproduction of Calanus helgolandicus in well-mixed and seasonally stratified coastal waters of the western English Channel

Field estimates of the production and viability of eggs of the copepod Calanus helgolandicus were conducted weekly during 1993 and 1994 at two fixed stations in the western English Channel. The stations were located in the coastal zone off Roscoff (NW France) and Plymouth (SW England). These areas are characterized by homogeneous and seasonally stratified waters, respectively. In these two different environments, the egg production rate varied seasonally and was significantly correlated with chlorophyll a concentration, but not with surface temperature. Significant correlations between fecundity and particulate carbon and nitrogen were found only off Plymouth. The seasonal pattern of fecundity differed between the two sites. Off Plymouth, spawning occurred earlier, lasted longer and was significantly higher than in Roscoff waters. In both localities, fecundity was generally below the maximum specific egg production rate (∼60 eggs female −1 day −1 implying that adult females were permanently under, or close to, limiting food conditions. Egg viability showed seasonal variations in both localities, ranging between 20 and 100%. However, in 1994, the amplitude of egg viability variation was lower in stratified waters. The results show that the potential fecundity of C.helgolandicus was different between mixed and stratified waters of the western English Channel. Food quality as well as quantity appear to be important for the reproductive success of this copepod species.

Composition of trioctahedral micas in the Karlovy Vary pluton, Czech Republic and a comparison with those in the Coruubian batholith, SW England

AbstractTrioctahedral micas in the Karlovy Vary pluton range in composition from Fe-biotites in the granites of the Older Intrusive Complex (OIC) through siderophyllite and lithian siderophyllite to zinnwaldite in the granites of the Younger Intrusive Complex (YIC). Li + AlVI + Si would appear to substitute for Fe2+ + AlIV in biotite with a formula similar to that given in Henderson et al. (1989), but Li + Si appears to substitute for Fe2+ + AlIV in the Li-micas. In mica vs. host rock plots, Rb and F show positive linear covariation except for the Li-mica granites, but femic constituents and tFeO/(tFeO + MgO) have separate trends for OIC and YIC granites and micas. Further differences between OIC and YIC granite micas are seen in their Ti and Mg contents and in plots like V vs. SiO2, AlIVvs. Fe/(Fe+Mg) and Li vs. total iron as Fe2+ and in the results of discriminant analysis. These reveal a geochemical hiatus between OIC and YIC granite micas that coincides with a major temporal hiatus.Biotite compositions in the YIC granites are similar to those in the granites of the Cornubian batholith and reveal a similar magmatic evolution and genesis in which later biotites evolve to lithian siderophyllites with some enrichment in trace alkalis and F. It is suggested that the biotite granites in the YIC were derived from the products of partial fusion of the OIC granites. A less well-marked geochemical hiatus exists between YIC biotites and zinnwaldites. In some plots (e.g. Si vs. Li, Li vs. tFe) apparent continuity between biotite and the Li-micas suggests continuous evolution, but in others (e.g. Rb vs. TiO2, Rb(biotite) vs. Rb(rock)), Li-mica data points stand apart from the biotites suggesting, like the whole rock data, a separate evolution. Comparison with the more abundant data for Li-micas of the Cornubian batholith suggests derivation of the Li-mica granites by partial fusion of the OIC/YIC granite residues.

Neotectonic and seismic activity in the Armorican and Cornubian Massifs: Regional stress field with glacio-isostatic influence?

In Brittany and SW England, evidence for low magnitude Quaternary seismicity can be found in sand pit exposures and beach sections. Deformation is especially well seen in alluvial and estuarine complexes resting on Late Pliocene sands or thick saprolite. The deformations are shallow, dominantly hydroplastic (high water-table) and led to overconsolidated sands, silts or clays. They reveal normal loading at micro (millimetric) and macro (decametric) scales as controlled by the local rheological properties of the sediments, by strikeslip faults associated with positive flower structures, folding, and intraformational water expulsion or hill slope sliding with reverse microfaulting. All the sites where these features occur, are in the vicinity of presumed active faults or steep slopes in highly fractured Proterozoic basement rocks indicating a possible shear zone. In most cases, these features are not associated with synsedimentary deformation, as strong superficial red soils are generally reworked by them. All these features are reworked by microfaulting after overconsolidation. Additional periglacial phenomena are superimposed on them and are often confused with them. Deformation occurred after the development of Holstenian peats (isotopic stage 11,400 ka BP) at Crozon (Brittany), after 317 ka BP (beginning of isotopic stage 9) in the Vilaine estuary, and in most other sites before the last rubified pedogenesis in the Middle Pleistocene (presumed isotopic stage 9). These dates correspond to the same episode that gave rise to the last main reactivation of the fossil cliff around 300 ka BP and to local uplift. One or several seismic clusters have taken place, probably due to delayed crustal rebound after a major glacial event (stage 10) and to rapid loading resulting from younger ice sheet growth (stage 8). Similar events occurred in late stage 7 and late stage 5. These events might have locally amplified the crustal deformation of the old Brittany and Cornubian Hercynian massifs resulting from a regional stress field especially from 400 ka to 200 ka BP. Stratigraphical, geodynamical and paleoclimatological implications are discussed.

Use of a flowing helium atmosphere incubation technique to measure the effects of denitrification controls applied to intact cores of a clay soil

Patterns of evolution of N 2O and N 2 due to denitrification in intact cores of a clay loam soil were measured using a He O 2 atmosphere “flow-over” incubation system housed in a temperature-controlled room. Square section cores were taken from a grassland site in SW England under extensive grazing management and assembled into composite turves, each comprising 25 cores in a 5 × 5 array, which were placed in each of six incubation vessels. After replacement of N 2 in the soil pores with He, the headspace gas above each turf was continuously flushed with a stream of 20% O 2 in He, which was directed to either waste or dual gas chromatographs. The effects of the major controls on denitrification were investigated while simulating the application of NO 3 − fertilizer to the sward made via a N 2-free irrigation assembly placed above each incubation vessel. Denitrification increased with increasing NO 3 added within the range equivalent to 0–150 kg ha −1, and with increasing water-filled pore space within the range 70–90%. The denitrification response to variation in the other controls did not agree well with the results of previous studies: although the initial rate of denitrification increased with a Q 10 of 2 within the range 5–30°C, there was no clear trend in the total N denitrified at temperatures above 10°C; denitrification decreased with increasing soil pH within the range 5.1–9.4. The N 2O-to-N 2 ratio increased with increasing NO 3 −, and with decreasing water content, pH and temperature. Antecedent soil aerobicity also had a large effect on the N 2O-to-N 2 ratio: after 7 d of either aerobic or anaerobic conditioning, the ratio was 1.74 or 0.15, respectively. In most of the experimental runs, less than 100%, and sometimes less than 50%, of the added N could be accounted for in gaseous products. The results indicate the need to develop and apply techniques that enable concurrent measurement of all relevant processes of N transformation, such as assimilatory NO 3 − reduction, nitrification and plant uptake, if prediction of denitrification in field soils is to be improved.

Deformation and emplacement of the Lizard Ophiolite Complex, SW England, based on evidence from the Basal Unit

The Basal Unit of the Lizard Ophiolite Complex preserves evidence of a protracted deformation and accretion history. The earliest recognizable events record the construction of ocean crust and the generation of the Lizard Ophiolite Complex in a slow-spreading ridge-axis environment. The initial stages of deformation involved the development of a sub-horizontal crustal-scale shear zone and its progressive deformation by ductile thrusting and folding. Subsequent deformation progressed with the out-of-sequence stacking of ocean lithosphere in which detachment, initiated at mid-crustal levels, culminated in mantle decoupling as thrust slices were derived from progressively deeper crustal levels. The progressive emplacement of a coherent slice of ocean lithosphere over previously accreted units and the generation of a dynamothermal aureole are consistent with models suggesting rapid overthrusting of young ocean lithosphere. Evidence from the dynamothermal aureole indicates that syn-emplacement extensional collapse of the complex occurred in response to thrust loading. Foreland-directed offsliding and lateral thrusting mark the final stages in the collapse of the complex.

Geochemistry and U-Pb and 40 Ar- 39 Ar geochronology of the Man of War Gneiss, Lizard Complex, SW England: pre-Hercynian arc-type crust with a Sudeten-Iberian connection

The Man of War Gneiss is a variably deformed meta-igneous body that ranges in composition from gabbro to tonalite and comprises part of the basal structural slice of the Lizard ophiolite complex, SW England. U-Pb dating of zircon demonstrates that the parental magmas crystallized in the earliest Ordovician (499t.+ s 3 Ma), but 40 Ar- 39 Ar step-heating analysis of amphiboles suggests that at least the northernmost zone of the gneiss was subjected to an amphibolite-grade metamorphic event at c. 374 Ma, probably associated with thrust emplacement of the ophiolite. The gneiss exhibits major, trace and rare earth element variations in conformity with an origin through partial melting of an amphibolitic source containing hornblende and plagioclase, probably with minor garnet and zircon. The elemental abundances and variations are characteristic of granitoids formed in supra-subduction zone environments. A granitoid sill within the adjacent Old Lizard Head Series schists, previously inferred to be related to the Man of War Gneiss, is trondhjemitic and exhibits higher SiO 2 , TiO 2 and MgO, but lower Zr, V, Ba, and Sr contents, and therefore is probably not genetically related to the latter. The sill may record local melting during ophiolite emplacement. The Man of War Gneiss probably represents a structural inlier of pre-Hercynian basement incorporated in the Hercynian nappes of SW England during NNW-directed thrusting. We emphasize the geological and geochronological similarities between variably deformed granitoid units of the Lizard complex, the Sudeten Massif of southern Poland, and the allochthonous complexes of northern Portugal and NW Spain. These granitoid rocks may represent dispersed remnants of Tremadoc arcs that formed oceanward from the northern margin of Gondwana. This is further convincing evidence that at this time Eastern Avalonia was separated from northern Gondwana by an ocean basin that may have been connected with the Tornquist Sea.