The nucleation of barite has been studied in a system involving the doublediffusion of Ba2+ and SO42- in an Na-rich aqueoussolution, through a porous medium. The evolution of the concentration profiles in the medium, which is a column of porous silica gel, has been determined as a function of time by direct chemical analysis of the diffusion-controlled mass transfer. By measuring the pH evolution, a Debye–Huckel treatment of ionic complexing has enabled the supersaturation evolution to be determined. The location of barite precipitation in the column is controlled both by the need to exceed a threshold supersaturation, as well as achieve an ‘equality range’ in which [Ba2+]/[SO42−] is close to unity. The value of the threshold supersaturation is a kinetic parameter and depends on the rate at which supersaturation increases. The experimental system described here has wide application to the study of crystallization phenomena in rocks. Experiments on the effect of additives designed to inhibit nucleation of barite in North Sea oil wells are used to quantify the resultant increase in supersaturation threshold.

The 10 to 20 m resolution of SPOT image data, together with their potential for stereoscopic viewing, provides an excellent base for geological mapping inremote and rugged terrain that is akin to high-level aerial photographs. Their large format (60 × 60 km) also gives the advantage of synoptic coverage that ranks with images from the Landsat series of satellites. Use of stereo pairs of single-band SPOT images has enabled some revision of existing geological maps of the southern Aravalli Hills in Rajasthan at a scale of 1:100000, and has added significantly to knowledge of their complex mid-Proterozoic structure. In particular, many possibly early low-angled faults have been discovered, together with the tectonic nature of a major terrain boundary and much detail of intricate structures has been added in the more remote areas. The potentialfor lithological discrimination of multispectral SPOT data is severely limited by its restricted coverage of geologically important spectral features, and it is far surpassed by that of Landsat Thematic Mapper data, which would have been capable of more comprehensive lithofacies reconnaissance, had they been available.

In the High Himalayan belt of northwest India, crustal thickening linked to Palaeogene collision between India and Eurasia has led to the formation of two main crystalline tectonic units separated by the syn-metamorphic Miyar Thrust: the High Himalayan Crystallines sensu stricto (HHC) at the bottom, and the Kade Unit at the top. These units are structurally interposed between the underlying Lesser Himalaya and the very low-grade sediments of the Tibetan nappes. They consist of paragneisses, orthogneisses, minor metabasics and, chiefly in the HHC, leucogranites. The HHC registers: a polyphase metamorphism with two main stages designated as M1 and M2; a metamorphic zonation with high-temperature recrystallization and migmatization at middle structural levels and medium-temperature assemblages at upper and lower levels. In contrast, the Kade Unit underwent a low-temperature metamorphism. Rb–Sr and U–Th–Pb isotope data point to derivation of the orthogneisses from early Palaeozoic granitoids, while the leucogranites formed by anatexis of the HHC rocks and were probably emplaced during Miocene time.

Most of the complicated metamorphic setting is related to polyphase tectonic stacking of the HHC with the ‘cooler’ Kade Unit and Lesser Himalaya during the Himalayan history. However, a few inconsistencies exist for a purely Himalayan age of some Ml assemblages of the HHC. As regards the crustal-derived leucogranites, the formation of a first generation mixed with quartzo-feldspathic leucosomes was possibly linked to melt-lubricated shear zones which favoured rapid crustal displacements; at upper levels they intruded during stage M2 and the latest movements along the syn-metamorphic Miyar Thrust, but before juxtaposition of the Tibetan nappes along the late- metamorphic Zanskar Fault.

The Argolis Peninsula, southern Greece, is believed to form part of a Pelagonian microcontinent located between two oceanic basins, the Pindos to the west and theVardar to the east, in Triassic to Tertiary time. In eastern Argolis, two important units are exposed: (i) the Ermioni Limestones cropping out in the southwest; (ii) the Poros Formation, observed on an offshore island in the northeast, and on the adjacent mainland. Both these units comprise late Cretaceous (Aptian-Maastrichtian) pelagic limestones, calciturbidites, lenticular matrix- and clast-supported limestone conglomerates and slump sheets. However, the Poros Formation is distinguished from the Ermioni Limestones by the presence of bituminous micritic limestones and an increasing proportion of shale up sequence. These successions are deep-water slope carbonates that once formed the southeast-facing passive margin of the Pelagonian platform (Akros Limestone). Beyond this lay a late Cretaceous ocean basin in the Vardar Zone. This ocean was consumed in an easterly-dipping subduction zone in latest Cretaceous (?) to early Tertiary time, giving rise to an accretionary complex (Ermioni Complex). During early Tertiary (Palaeocene-Eocene) time the passive continental margin (Pelagonian Zone) collided with the trench and accretionary complex to the east. As the suture tightened, former lower-slope carbonates (Ermioni Limestones) were accreted to the base of the over-riding thrust sheets and emplaced onto the platform. Farther west, bituminous upper slope carbonates (Poros Formation) flexurally subsided and passed transitionally upwards into calcareous flysch and olistostromes in a foreland basin. These sediments were then overridden by the emplacing thrust stack and themselves underplated. Late-stage high-angle faulting then disrupted the tectonostratigraphy, in places juxtaposing relatively high and low structural levels of the complex.

Element, carbon and oxygen isotope fractionations have been studied in laminated deposits, manganese nodules and phosphorite lenses in clastic sediments of the Cambrian Timna Formation, southern Israel. Comparisons of element abundances show that only manganese, phosphorus and uranium were mobilized and concentrated during diagenesis: element fractionations among the diagenetic manganese components particularly resemble those observed in present-day suboxic pelagic marine sediments. Epigenetic processes involving reducing solutions reversed the diagenetic geochemical trends, with the redox-sensitive elements manganese and uranium showing considerable depletions in nodules, and iron and lead showing corresponding enrichments. The manganese oxides of nodules have calculated δ18O compositions varying from –1.6 to 3.1 ‰. These compositions are significantly lower than a δ18O value of 7.9 ‰ obtained for the manganese oxides of an ocean floor nodule and are taken to largely represent temperature differences between the ocean floor and the shallow marine Timna conditions. Slight decreases in δ18O accompany the transformation from diagenetic type A to epigenetic type B manganese nodules; corresponding shifts to lower δ18O are also inferred for the structural carbonate of apatites. Such shifts are compatible with re-equilibration accompanying a slight temperature increase and/or low temperature meteoric-water alteration. δ13C values of –7 to –12‰ obtained for the apatites of laminated deposits and phosphorite lenses are consistent with phosphorite diagenesis in suboxic to anoxic pore-waters. Early diagenetic redox conditions are indicated to be major controls governing the development of the assemblages.

There are two thrust systems in the Southwest Pyrenees: a NW-SE trending, thin-skin system exposed in the post-Triassic cover and a larger, thick-skin system of NE-SW thrusts in the Palaeozoic basement. The ‘cover’ thrust system propagated and migrated both southward and westward in response to the non-orthogonal collision of Iberia with Europe during Palaeogene mountain building. The ‘basement’ thrust system is interpreted to be a longer-lived structure, initiated during the extensional tectonic regime in mid Cretaceous time, and inverted during the main episode of Pyrenean collision. A model in which interaction of the two thrust systems controlled the timing and magnitude of thrust-induced, flexural subsidence is presented. The development of the basement thrust system caused regional subsidence along the South Pyrenean foreland margin that was subsequently halted by local uplift associated with the west-migratingcover thrust system.

Perim Island is an eroded fragment of the southwest flank of a late Miocene (10.5 ± 1.0 Ma) volcano whose centre lay on the southwesternmost tip of Arabia. The volcano is the westernmost of the E–W line of six central vent volcanoes (the Aden Line) that extends 200 km along the south coast of Arabia from Perim to Aden. Major oxide and trace element abundances are given for 35 Perim specimens and these show that the volcano has within-plate trace element characteristics and consists of a petrographically and geochemically simple suite of alumina-poor olivine basalts, andesites, and transitional andesite–trachyandesites. Six specimens, however, are markedly enriched in Al2O3 and CaO, and contain abundant (20–30 mode %) highly calcic (An77–83) plagioclase phenocrysts. Geochemical modelling suggests that the main Perim volcanic sequence was produced by the fractional crystallization (o1 + cpx + Ti-mt + plag) of a silica saturated (SiO2 c. 45%) basic melt. The high A1, high Ca, magmas appear to be mixing products of plagioclase-enriched basic magmas with more evolved melts. Perim is the oldest volcano of the Aden line, which becomes increasingly younger and alkalic eastward. It is suggested that the volcanism is related to an eastwards-propagating rift produced before the most recent stage of sea-floor spreading in the Gulf of Aden (4.5 Ma–present).

Fourteen early Ordovician (Tremadoc-Llandeilo) graptolite sequences from around the world are precisely (infrazonally) correlated, based on the stratigraphic ranges of 130 species and species groups. The composite standard sequence (CSS) of graptolites has been determined from the six best regional sequences by a nonparametric graphic correlation. Two data sets were selected: one comprised first appearance events of 103 taxa, the other, first and last appearance events of 45 taxa. The results of the two runs accord well and reveal respectively 66 and 73 successive bioevents in early Ordovician time. Event spacing averages 0.7–0.8 Ma and enables fine subdivision, correlation and homotaxial testing for diachroneity. The strong correlation between each of the six regional sequences and the CSS indicates the high level of accordance among graptolite successions around the world.

Arenig (Ordovician) clastic sediments crop out in the Harlech Dome region (North Wales), and are placed in a single stratigraphic unit: the Allt Lwyd Formation. This unit records a marine transgression onto an erosion surface produced during late Tremadoc arc volcanicity. Four discrete petrofacies are denned, and reflect differing proportions of detritus derived from Tremadoc-type basic-intermediate igneous rocks, and the local sedimentary basement. Initial shallow marine siliciclastic sandstones and conglomerates are overlain by extensive deep water mud-rich units. These generally shallow up into a complex arc-apron deposit, with sediments derived from the eroding Tremadoc arc, as well as from similar, synchronous volcanics. Predominantly epiclastic sandstones and conglomerates were deposited in deltaic and tidal environments in an arc-apron complex, and capped by condensed mudstones and an ironstone, deposited as sea level rose across these systems. Sediments were ponded in north–south orientated troughs and derived from uplifted blocks. Facies and petrofacies distribution were controlled by syn-sedimentary north-south and northeast–southwest faults. The Allt Lwyd Formation was ponded in a fault-controlled basin (the Dolgellau Basin), one of a series of interconnected sub-basins flooded by the Arenig transgression. The sediments preserved reflect deposition during the transgression of a volcanic arc, prior to the extrusion of marginal basin-type volcanics.

The position of a coastline in time and space is determined by (1) the vertical displacement and/or tilting of the depositional surface, (2) the rate of sediment accumulation or erosion across that surface, and (3) variation of sea-level. All three rates of change may vary through time. We present computer simulations of coastline movements that illustrate the interaction of the above variables, with (1) and (2) held at various defined levels whilst (3) is varied according to the late Quaternary glacio-eustatic sea-level curve. The Corinth Canal section in central Greece exposes uplifted late Quaternary coastal transgressive cycles, each of which may be related to a radiometrically dated, c. 100 ka duration, cycle of sea-level change. Observed stratigraphic sequence geometries are predicted by forward modelling based on the known glacio-eustatic history over the last 430 ka. Milankovitch orbital parameters are calculated for the Carboniferous period. The obliquity and precession parameters are found to have been significantly shorter than at present. A simulation is presented of the effects of sea-level changes across low gradient, fluvio-deltaic environments such as existed in northern England and other parts of the Laurentian continental margin during Upper Carboniferous time.

Metabasalts of the Upper Ordovician Solund-Stavfjord Ophiolite Complex of the westernmost Norwegian Caledonides, show N-to E-MORB affinity, with high Th/Ta (or Nb) ratios giving evidence of subduction influence. The Solund–Stavfjord Ophiolite Complex is overlain by a heterogeneous assemblage of sedimentary and volcanic rocks, the Stavenes Group, of which the Heggøy Formation of metasandstones and phyllites conformably overlies the metabasalts of the Solund–Stavfjord Ophiolite Complex. The Heggøy Formation contains, in places, abundant metabasalt pillow lavas and minor intrusions, geochemically similar to those of the Solund–Stavfjord Ophiolite Complex, and basic metavolcaniclastites of island arc tholeiite (IAT) composition. This indicates that the Solund–Stavfjord Ophiolite Complex and Heggøy Formation developed in a marginal basin between a continental margin and an active subduction system, for which the present-day Andaman Sea may provide a realistic model. The other magmatic rocks of the Stavenes Group, showing both calc-alkaline and alkaline affinities, are less well time-constrained, but they are thought to represent an advanced stage of the island arc development, and ocean island build-up, respectively.

The Yangtze Platform (Yangtze Palaeoplate) drifted into the area of southern China following late Silurian tectonism. In late Palaeozoic to early Mesozoic time the Yangtze Platform was subjected to strong extensional movements in its southeastern region within Yunnan, Guizhou, Guangxi and Hunan provinces, and along its northwestern margin in the Songpan-Ganzi area. Taphrogenesis (intracontinental extension) began in Devonian times, climaxed with the late Permian eruption of the Emeishan basalts, and ended in mid Triassic times. Therefore, the senior author (LZL) has named this extension the ‘Emei Taphrogenesis’, a phenomenon that was constrained by the neighbouring tectonic units of the Yangtze Platform. The platform has been substantially affected by the early Palaeozoic south China fold zone along its eastern margin, and by the late Palaeozoic opening of the Tethys Ocean on the northwestern margin. This paper delineates the tectonic patterns associated with the Emei Taphrogenesis.

The late Precambrian–early Palaeozoic rocks of Ny Friesland, which have been subjected to Caledonian deformation and metamorphism, constitute part of the Eastern Province or Terrane of Svalbard. The Harkerbreen group and other divisions of the Stubendorffbreen supergroup form a high-grade and intensely deformed core complex to this terrane which is bounded to the west by the Billefjorden Fault Zone and to the east by a major north–south shear zone. The Stubendorffbreen rocks exhibit two gneissic foliations, one axial planar to a large scale, F1 fold nappe closing to the east and the other axial planar to kilometre-scale upright F2 folds subsidiary to the Atomfjella Arch. Metamorphism in the mid-amphibolite facies range coincided with generation of these folds, and F3.crenulation folding was accompanied by waning P–T conditions. A significant proportion of the gneisses within the Harkerbreen group display silica–major element covariation patterns consistent with their position in the granodiorite field on the AFM plot. Incompatible, immobile element ratios Zr/Ti v. Nb/Y indicate affinities with rhyolites to rhyodacites which is also suggested by their REE profiles. Normalized multi-element plots of the gneisses are similar to those of granites from attenuated within-plate settings such as Mull and Skaergaard. The amphibolites which were intruded in the D1–D2 interval appear to be derivatives of fractionated basalts. They plot across the calk-alkaline tholeiite boundary on the AFM diagram, and the calc-alkaline character of some of the amphibolites is further suggested by their Yb-normalized Ce-Ta abundances. Zr-Ti-Y and REE abundances would support their derivation from a related suite of fractionated basalt liquids. On the Zr/Y v. Zr discrimination diagram the amphibolites appear to have compositions transitional between Mid Ocean Ridge and Within-Plate basalts whilst the Zr-Nb-Ta plot indicates Volcanic Arc Basalt affinities. Th-Hf-Ta and multi-element plots, however, indicate a marginal to back-arc basin setting possibly above a mature subduction zone. The late Caledonian Chydenius granite is an adamellite with mixed within-plate and syn-orogenic characteristics typical of post-collisional granites.

Upper Ordovician – lower Silurian sequences of the Bohemian Massif (in the Moldanubian Zone and Barrandian preserved in the Prague Basin; Rožmitál area; metamorphic ‘islets’ in the mantle of Central Bohemian Pluton; and in the Železné hory area), as well as along the northern margin of the Bohemian Massif in the Saxothuringian and Lugian (= West Sudetian) zones, show evidence of regressive–transgressive facies changes which can be related to glacio-eustatic sea-level fluctuations recognized elsewhere. Glacio-marine diamictites of late Ordovician (Hirnantian) age occur in the Prague Basin and Rožmitál area, and in north Bavaria and Thuringia. They provide evidence of temporary cooling of the region. Facies changes caused by synchronous sea-level changes allow stratigraphic correlation of even the faulted, weakly metamorphosed and biostratigraphically poorly dated sections in the Bohemian Massif. Correlation is based on sections in the Prague Basin in which the changes are best recorded and dated.

Middle Cambrian and Caradoc metabentonites and associated mudrocks are described from the northern part of the Welsh Lower Palaeozoic basin. They occur in areas of slight deformation, distant from thick piles of contemporaneous volcanics. The < 2 μm fraction from anchizonal Middle Cambrian metabentonites consists dominantly of 1Md micas (with up to 5% randomly interstratified smectite) with lesser 1M and 2M1 muscovite, corrensite, quartz, chlorite and kutnohorite. Interbedded mudrocks are similar but contain, additionally, minor paragonite and regular (6:4) paragonite-muscovite. Late diagenetic grade Caradoc bentonite consists of rectorite, pyrophyllite and small amounts of chlorite. Interbedded mudrocks are similar but contain, additionally, 1Md mica with up to 10% smectite interlayers, minor kaolinite and quartz.

Metabentonites contain high concentrations of the trace elements Ce, La, Hf, Nb, Ta, Th, Y and Zr in comparison with associated mudrocks. Discriminant diagrams indicate derivation from trachyandesite–dacite–rhyolite magmas in a within-plate setting. Major element variations in Cambrian metabentonites compare closely with those described in Silurian examples whereas the Caradoc metabentonite is closely similar to rhyolitic tuffs in the Caradoc Llewelyn Volcanic Group.

Both geochemical and field evidence indicate that whereas Middle Cambrian mudrocks were derived from bentonitic material adulterated variably by a terrigenous input. Lower Cambrian mudrocks were derived from a strongly depleted source such as basic Mona Complex rocks. Geochemical and field evidence suggest Caradoc mudrocks southeast of the Aber–Dinlle fault are largely volcanogenic; and the geological setting of Tremadoc, Arenig and Llanvirn mudrocks demands some volcanogenic input.

The Caradoc volcanic succession of Snowdonia is represented by a single bentonite northwest of the Aber–Dinlle fault. The fault system was either a highly effective contemporaneous barrier or, more probably, a site of post-Caradoc transcurrent movement.

The western part of the Fintona Block is divided into four fault-bounded segments that contain red-bed sediments formerly assigned to the Lower Old Red Sandstone.Dating by miospores indicates the presence of deposits of early Devonian age in the Irvinestown Segment, late Viséan–early Silesian age in the Tempo–Lisbellaw Segment, and late Viséan–early Silesian and late Silesian ages in the Milltown Segment. Northward migration of the early Carboniferous marine transgression in the northern part of Ireland coincided with the sequential propagation of back-stepping faults and resulted in the development of diachronous facies belts between late Courceyan and Arundian times. Tectonic uplift, of a possible southwesterly extension of the Tyrone Igneous Complex, gave rise to the deposition of Asbian to Pendleian red-beds to the south of a massif. An interface between these red-beds and contemporaneous marine sediments farther to the south is recognized and dated. A new non-marine basin, containing Brigantian and Pendleian red-beds, also developed to the north of the massif A waterlogged floodplain that developed during Westphalian A times may be coeval with more widespread coal-bearing sequences elsewhere in Ireland. Alluvial fans prograded southwards over this plain during Westphalian B times when faults bordering a northern landmass were reactivated.

The fossiliferous section at Meishucun of Yunnan, China, is a candidate stratotype section for the Precambrian–Cambrian boundary. Early diagenetic dolomites and phosphorites have been sampled across the boundary interval here, and in the correlated section at Maidiping in Sichuanand Valiabad in Iran, for comparison of their carbon and oxygen isotopes. This is the first such study that is calibrated by biostratigraphy in the interval from the earliest (pre-Tommotian) skeletal fossils to trilobites. Although negative oxygen isotopes indicate a diagenetic signal in the Zhongyicun Member and basal Badaowan Member phosphorites, two carbon-isotope cycles are clearly present and can be correlated in dolomitic rocks between the two sections. The first appearance datum (FAD) of the earliest skeletal assemblage (zone I, Marker A), FAD of diverse micromolluscs (zone II, Marker B) and FAD of Chinese trilobites (zones IV, V) and Marker C appear at similar points on the carbon-isotope curve in the two Chinese sections. Integrated carbon-isotope and early skeletal fossil biostratigraphy is shown to have the potential to correlate further afield, with sections in Iran, as well as with India, Siberia, Morocco and Australia. We suggest that a distinctive positive excursion provides a global marker for the interval between Marker B and C in China and just below the Tommotian Stage of Siberia.

The evidence is reviewed for the timing of collision between the microcontinent of Eastern Avalonia (southern Britain and adjacent areas) and the Laurentian continent. Recent palaeomagnetic results placing Eastern Avalonia in a high (50°) southern latitude in mid Ordovician time are now consistent with faunal evidence for the first time. The resulting apparent polar wander path is evaluated and suggests that Eastern Avalonia detached itself from a southern peri-Gondwanan latitude in the early Ordovician, moved northwards, and approached Laurentia by the late Ordovician. Its western corner probably impinged on Laurentia in the early Silurian and it docked against the Laurentian margin during Silurian and early Devonian time with a component of anticlockwise rotation.

This kinematic history is supported by a compilation of sediment dispersal patterns on Eastern Avalonia. A low-volume Ordovician and earliest Silurian supply from within the microcontinent was overwhelmed in late Llandovery time by a large volume of southwest-derived turbidites, probably from the uplifting impact zone to the west. This source was later augmented by a high-volume clastic supply to the north margin of the microcontinent. Eastward migration of this source through Wenlock and Ludlow time reflects the progressive anticlockwise docking of Eastern Avalonia against the Laurentian margin. The earliest sign of a large-volume supply from Baltica is in the late Wenlock, arguing against any earlier hard collision.

New radiometric age data are reported for alkaline centres in southern Namibia, and are discussed together with published age data in terms of models put forward to account for post-Karoo (Mesozoic–Recent) alkaline magmatism within the African plate. Agreement between K–Ar and Rb–Sr ages indicate emplacement of the Dicker Willem carbonatite in southern Namibia at 49 ± 1 Ma. Alkaline rocks associated with the Gross Brukkaros volcano show a discordant radiometric age pattern, but the best estimate for the age of this complex is 77 ± 2 Ma, similar to that obtained for the neighbouring Gibeon carbonatite-kimberlite province. The Dicker Willem carbonatite is therefore younger than the Luderitz alkaline province (133 ± 2 Ma), and the Gross Brukkaros volcano, but is older than the Klinghardt phonolite field (29–37 Ma). The new age data argue against a distinct periodicity in alkaline igneous activity in southern Africa, thereby ruling out possible controls by episodic marginal upwarping of the subcontinent. Although the available age data do not appear to be consistent with the passage of one or even two hotspots under southern Namibia, it is argued that the surface expression of hotspots under continents may be so large and overlapping that within-plate magmatism attributed to these thermal anomalies need not necessarily be confined to narrow linear belts or show an age progression. The role of hotspots in continental alkaline magmatism is most likely one of melt generation, while local crustal structure probably controls the distribution and timing of eruption. Major tectonic boundaries in the Precambrian basement underlying southern Namibia seem to have controlled the development of Tertiary alkaline centres in that region.

The proposed correlation of Lower Old Red Sandstone dacitic ignimbrites between the Northern Midland Valley and Grampian terranes of central Scotland has been investigated in a combined palaeomagnetic and geochemical study. Statistically indistinguishable late Silurian palaeolatitudes (Grampian terrane 27.0° S±3.5°; Northern Midland Valley terrane 22.9° S± 1.8°), comparable remagnetization histories, a shared ‘normal’ polarity for the primary magnetization and equivalent gyroremanence acquisition, favour correlation across the Highland Boundary Fault. Similar chemistry for biotite phenocrysts further corroborates correlation which was initially made on the basis of a similar field appearance and petrography.

This correlation, together with evidence from provenance studies, suggests that the separation of the Northern Midland Valley and Grampian terranes inferred for the Ordovician period was largely removed by late Silurian times, with scope for relative displacements of the order of only tens of kilometres after Lower Old Red Sandstone deposition.