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Variations in the intensity of high-latitude Northern Hemisphere summer insolation, driven largely by precession of the equinoxes, are widely thought to control the timing of Late Pleistocene glacial terminations. However, recently it has been suggested that changes in Earth's obliquity may be a more important mechanism. We present a new speleothem-based North Atlantic marine chronology that shows that the penultimate glacial termination (Termination II) commenced 141,000 [+ or -] 2500 years before the present, too early to be explained by Northern Hemisphere summer insolation but consistent with changes in Earth's obliquity. Our record reveals that Terminations I and II ate separated by three obliquity cycles and that they started at near-identical obliquity phases.

The Neoproterozoic Dalradian Supergroup is a dominantly siliciclastic metasedimentary succession in the Caledonian orogenic belt of Scotland and Ireland. Despite polyphase deformation and greenschist-to upper amphibolite-facies metamorphism, carbonate units distributed throughout the Dalradian record marked [[delta].sup.l3][C.sub.carbonate] excursions that can be linked to those associated with key environmental events of Neoproterozoic time. These include: (1) tentative correlation of the Ballachulish Limestone with the c. 800 Ma Bitter Springs anomaly; (2) the presence of the pre-Marinoan Trezona anomaly and 635 Ma Marinoan-equivalent cap carbonate sequence in rocks of the middle Easdale Subgroup; (3) the terminal Proterozoic (c. 600--551 Ma) Wonoka--Shuram anomaly in the Girlsta Limestone on Shetland. These linkages strengthen previously inferred correlations of the Stralinchy--Reelan formations and the Inishowen--Loch na Cille--MacDuff ice-rafted debris beds to the respectively 635 Ma Marinoan and 582 Ma Gaskiers glaciations, and suggest that the oldest Dalradian glacial unit, the Port Askaig Formation, represents one of the c. 750-690 Ma Sturtian giacial episodes. These [[delta].sup.13]C data and resulting correlations provide more robust constraints on the geological evolution of the Dalradian Supergroup than anything hitherto available and enhance its utility in helping refine understanding of Neoproterozoic Earth history.

In SE Siberia, carbonate formations with [[delta].sup.13][C.sub.carb] values ranging between -12%o and -7%o (V-PDB) and Sr concentrations of up to 2.5% occupy an area of 40 000 [km.sup.2]. Several successions exceed 1000 m in thickness and represent the world's largest known exposures of sedimentary carbonates exhibiting extreme depletion in [sup.13]C. The carbonates were deposited on a carbonate platform evolving from a mixed carbonate-siliciclastic ramp to a carbonate ramp, and then from a peritidal rimmed shelf to a deep-water open shelf. All sequences reveal a facies-independent, upward rise in marine [[delta].sup.13][C.sub.carb] from -12% to -7%o. The trend and magnitude of the values mimic those that are characteristic of the 600-550 Ma Shuram-Wonoka isotope event. A coincident stratigraphic rise in [sup.87]Sr/[sup.86]Sr from 0.70802 to 0.70862 in several sections of limestones, containing 4400 [micro]g [g.sup.-1] Sr on average, is considered to be by far the best available constraint on a temporal variation of seawater isotopic composition through the Late Ediacaran. If the greatest temporal rate of change in seawater [sup.87]Sr/[sup.86]Sr observed in the Cenozoic is applied to the Siberian sections, the calculated minimum duration for the Suram-Wonoka event is l0 Ma. Supplementary material: XRF and ICP-AES analysis, C, O and Sr isotopic data are available at http://www.geolsoc.org.uk/SUP18324.

The Shetland Islands in Scotland contain a c. 12 km thick, mostly siliciclastic Neoproterozoic succession that experienced deformation and greenschist-to amphibolite-facies metamorphism during the Ordovician-Silurian Caledonian Orogeny. Correlating these rocks with Neoproterozoic successions elsewhere has remained problematic. However, four marble units punctuate the succession; these are, in stratigraphic order, the Weisdale, Whiteness, Girlsta and Laxfirth Limestones, two of which record hallmark C-isotopic excursions diagnostic of Neoproterozoic time. The Whiteness Limestone is lithologically similar to the 635 Ma Marinoan post-glacial cap carbonate sequence and displays a [[delta].sup.13][C.sub.carbonate] profile in which values decline from a range of 0 to +2%o to a nadir of -5%o and then rise towards and oscillate around 0%o. The Girlsta Limestone is a 700-900 m thick marble unit with values of -9%o to -11%o and is interpreted as recording the late Neoproterozoic Shuram-Wonoka event. These correlations permit linking the Shetland succession to Neoproterozoic sections elsewhere and also show that rift-related rocks in Shetland post-date the Shuram-Wonoka event (post c. 600 Ma-pre c. 550 Ma) thereby supporting the concept of a diachronous phase of extensional tectonism, from c. 700 Ma in east central Laurentia to near the close of the Proterozoic in northeastern Laurentia, prior to the opening of the Iapetus Ocean.

An approximately 250 m thick polydeformed and polymetamorphosed, isotopically unusual, variegated marble (locally termed the 'Leivset marble') shows a great lateral extent in the Scandinavian Caledonides. Its extremely [sup.13]C-depleted primary nature (-7.9 [+ or -] 1.2 [per thousand] on average, n = 93) makes the Leivset marble unique. This, together with a high Sr content (up to 8740 ppm) that buffered [sup.87]Sr/[sup.86]Sr ratios between 0.70802 and 0.70872, suggests correlation with the worldwide Shuram--Wonoka isotopic event occurring within the 600-550 Ma time interval during the Ediacaran (Vendian) period. Despite a high-grade deformation and metamorphism, the Leivset marble has retained its original carbon and strontium isotope ratios. A combination of the variegated colour with unusually low [[delta] sup.13][C.sub.carb] can potentially be used for stratigraphic correlations in high-grade, nonfossiliferous, marble-dominated terranes across the Caledonian orogenic belt in Baltica and Laurentia. Isotope chemostratigraphy has identified a prominent cryptic stratigraphic discontinuity and suggests that the Ediacaran Leivset marble was tectonically juxtaposed above low-grade, Llandovery-age, fossiliferous marbles during the Scandian orogeny. Keywords: Caledonides, Ediacaran, marble, carbon, strontium, isotopes.

Two distinct Neoproterozoic glacial episodes are known for the Dalradian Supergroup in the British-Irish Caledonides, the Port Askaig Formation and the Inishowen-Loch na Cille ice-rafted debris (IRD) beds. Here we describe a third, intermediate between those two, the Stralinchy-Reelan formations, composed of diamictite and IRD. Developed directly above these rocks is the Cranford Limestone, which consists of a basal, 1-6-m-thick, tan-gray dolostone overlain by a variably developed, but as much as 340-m-thick, sequence of thin-bedded limestone and dolostone. This unit exhibits a C isotopic trend that begins negative in the basal dolostone, reaches a nadir of -7[per thousand], and then rises to 0[per thousand]-2[per thousand]. These characteristics match strikingly those of Marinoan-style cap carbonates. Consequently, we interpret the Port Askaig Formation, the Stralinchy-Reelan units, and the Inishowan-Loch na Cille beds as equivalents of the ca. 700 Ma Sturtian, the 635 Ma Marinoan, and the ca. 580 Ma Gaskiers glacials, respectively. Two additional observations are noteworthy. Carbonate rocks below the Port Askaig Formation record a [[delta].sup.13]C decline to -6[per thousand] that implies that such downturns may occur in both pre-Sturtian and pre-Marinoan strata. In addition, the Bonahaven Dolomite is not a cap carbonate to the Port Askaig Formation, but exhibits a [[delta].sup.13]C rise to 12[per thousand], which we correlate with the inferred global Keele peak. These data further document the utility of Neoproterozoic glacial-cap carbonate sequences in global correlations and denote the base of the Cranford Linestone as the Cryogenian-Ediacaran boundary. Keywords: Dalradian, Neoproterozoic, cap carbonate, glacial, Caledonides.

Strontium and carbon isotopic data from amphibolite-facies marbles of the Falkenes formation, Soroy succession (structurally upper Kalak Nappe Complex) in West Finnmark, northern Norway, indicate that the marbles were deposited between 760 and 710 Ma. Marbles of similar age have previously been identified only in the Uppermost Allochthon in north-central Norway, where they are considered to have a Laurentian ancestry. A similar origin and tectonostratigraphic position appears likely for the Falkenes formation. In contrast, structurally lower units within the Kalak Nappe Complex appear to have Baltican affinities. This dichotomy requires that commonly held ideas regarding the tectonostratigraphy of the West Finnmark Caledonides be revised. Combined with recent U--Pb dating of zircon and monazite from rocks associated with the marbles, the isotopic data suggest that the Soroy succession of the Kalak Nappe Complex is not a continuous, depositional sequence as previously thought, but rather consists of a number of disparate and probably unrelated thrust-sheets, assembled during Late Silurian, Scandian orogenesis. This work shows that the West Finnmark Caledonides share a generally similar tectonometamorphic history with certain other parts of the Scandinavian Caledonides, and contributes to already existing data that may allow 'chemostratigraphic' correlation and reconstruction of the entire Caledonian belt.

Injected sandstones occurring in the Kimmeridgian of NE Scotland along the bounding Great Glen and Helmsdale faults formed when basinal fluids moved upward along the fault zones, fluidizing Oxfordian sands encountered at shallow depth and injecting them into overlying Kimmeridgian strata. The orientation of dykes, in addition to coeval faults and fractures, was controlled by a stress state related to dextral strike-slip along the bounding fault zones. Diagenetic studies of cements allow the reconstruction of the fluid flow history. The origin of deformation bands in sandstone dykes and sills was related to the contraction of the host-rocks against dyke and sill walls following the initial stage of fluidized flow, and these deformation bands are the earliest diagenetic imprint. Early non-ferroan calcite precipitated in injection structures at temperatures between 70 and 100 [degrees]C, indicating that it precipitated from relatively hot basinal fluids that drove injection. Coeval calcite-filled fractures show similar temperatures, suggesting that relatively hot fluids were responsible for calcite precipitation in any permeable pathway created by dextral simple shear along the faults. During progressive burial, percolating sea water was responsible for completely cementing the still relatively porous injected sandstones with a second generation of ferroan calcite, which contains fluid inclusions with homogenization temperatures below 50 [degrees]C. During this phase, depositional host sandstones were also cemented. Keywords: Kimmeridgian, sandstone dykes, carbonates, cements, deformation.

Three small picritic dykes, intruded at a late stage in the evolution of the Rum basic--ultra-basic complex, Inner Hebrides, shed new light on the nature of the magmas responsible for the main complex. The magmas are of transitional (mildly alkalic) type, generated by relatively small-fraction (6-7%) melting of a depleted mantle source. Melting is tentatively deduced to have commenced at [+ or -] 100 km, straddling the garnet--spinel transition. Of the three samples, one (M9) is remarkable for the preservation of very primitive characteristics, with olivines of Fo93 containing highly aluminous spinels; these appear unique within the British Tertiary Volcanic Province. Sr, Nd and Pb isotopes indicate only minor ([less than or equal to] 4%) contamination with Precambrian crustal lithologies, reflecting the rapidity of ascent of the magma batches. The Rum picrites have [sup.187]Os/[sup.188]Os ratios and trace element characteristics comparable to those of recent picrites from Iceland, suggesting minimal temporal change of the depleted parts of the Iceland plume over 60 Ma. Movements of the Long Loch Fault may have been instrumental in causing decompression melting of the spreading Iceland plume-head and facilitating ascent of the melts to near-surface levels. Keywords: picrite, magmas, Iceland, mantle, plumes.

Carbon and strontium isotope stratigraphy has been applied to constrain the depositional ages of high-grade marble sequences in the Ofoten district of the North-Central Norwegian Caledonides. Two marble formations hosted by diverse schists from the Bogen Group, all previously correlated over long distances with a Late Ordovician--Early Silurian, low-grade, fossiliferous succession, have been studied for carbon, oxygen and strontium isotopes. The least altered [sup.87]Sr/[sup.86]Sr ratios ranging between 0.7062 and 0.7068, and the best preserved [delta][sup.13]C values falling between +5.0 and +6.5% obtained from two marble formations, are consistent with a seawater composition in the time interval 700-600 Ma. The results obtained do not support the previously proposed correlation of the Bogen Group with an Ordovician-Silurian lithostratigraphic succession further north. The apparent depositional ages suggest that the tectonostratigraphic succession studied is inverted and that the tectonostratigraphy of the region requires revision. The Neoproterozoic depositional ages combined with the palaeogeographic position of Baltica imply that carbonates were initially accumulated in seas on a continental shelf, probably Laurentia, and were tectonically transported onto Baltica during Early Silurian, Scandian collision, at c. 425 Ma. Prospecting for new dolomite marble deposits of the Hekkelstrand type and carbonate-hosted manganese--iron ores should be restricted to 700-600 Ma sequences in the Uppermost Allochthon of the Norwegian Caledonides. Keywords: Caledonides, Neoproterozoic, marble, carbon, strontium, isotopes.

Prograde metamorphic reactions involving the growth of phyllosilicates and accompanying cleavage development have been investigated in Dalradian metasediments from the biotite zone of eastern Scotland. Crystallization of muscovite within the psammites of the Southern Highland Group is linked to the replacement of plagioclase porphyroclasts. This reaction is triggered by significant alkali metasomatism during active deformation and plays an important role in the formation of a prominent spaced cleavage within the psammites. The Si content of most of these early-formed muscovites is partially buffered by the quartz content of the rock, although close to the Highland Boundary Fault, evidence of greater influence from externally derived fluids on muscovite compositions is preserved. Locally higher fluid fluxes adjacent to the fault are also indicated hy a relatively high [delta][sup.18][O.sub.(SMOW)] signature in the rocks. The biotite-producing reaction in these greenschist-facies rocks is linked to the later production of a celadonite-poor muscovite which formed as overgrowths around preexisting white micas. This reaction is sensitive to the initial composition of muscovite and preferentially occurs in quartz-rich metasediments containing a celadonite-rich muscovite. A systematic increase in the progress of the biotite-producing reaction northwards across the biotite zone confirms the presence of high geothermal gradients along the southern margin of the Dalradian block, adjacent to the Highland Boundary Fault.

Infiltration of a metabasite sill from Islay, Scotland by an [H.sub.2]O-C[O.sub.2] fluid caused (1) modification of [delta][sup.18]O and (2) carbonation at the sill margins. Maps of [delta][sup.18]O and reaction progress were constructed from a 20 x 47.7 metre sample grid across the sill. The grid consisted of 300 samples, spaced at m, dm and cm intervals, many of which were analysed for both [delta][sup.18]O and reaction progress. The [delta][sup.18]O was determined by laser fluorination of whole rock silicate powders and reaction progress was determined by rapid field-based measurement of % calcite ('fizz-o-meter', Skelton et al. 1995). Reaction and isotope fronts outlined tube-like features that emanate from the sill margin and discrete nodes that, although detached from the sill margin in two dimensions, are thought to represent sections through similar tubes in three dimensions. We envisage that these protrusions are the fossil record of metamorphic 'fluid pathways' whereby fluid permeated the sill. Isotope and reaction fronts are found to correlate spatially as predicted by a modified form of the chromatographic equation which describes this envisaged geometry, that is where isotopic and reactive transport in the fluid phase are facilitated by advection along specific fluid pathways and transverse diffusion in the surrounding rock. These fluid pathways consist of bundles of anastomosing grain boundary channels or micro-cracks, which are thought to propagate through transient cyclic infiltration, reaction, porosity enhancement and fracturing. This mechanism is self-perpetuating and accentuates random perturbations at the sill margin to form the observed tubes. We argue that this is the earliest stage of the infiltration process which has affected metabasites of the SW Scottish Highlands and that subsequent shear deformation of the reacted rims of these pathways, has caused their re-orientation and juxtaposition to form the reacted sill margins described by Skelton et al. (1995).

Research on delta(13)C lipid values was conducted to find evidence supporting the theory that hydrothermal vent ecosystems in the deep sea rely on certain dietary nutrients generated by photosynthetic organisms. The data obtained are consistent with previous studies which originated the theory.

The ca. 2.0-Ga-old, 600-m-thick upper Zaonezhskaya Formation near Lake Onega, NW Russia, contains unusually high concentrations of [C.sub.org] (up to 98%), averaging around 25%. The formation contains an estimated 25 x [10.sup.10] tonnes of organic carbon accumulated within an area of 9000 [km.sup.2]. Organic material is represented by shungite, which forms a black, dense, amorphous or nanocrystalline mass consisting of C with traces of N, O, S, and H. Autochthonous shungite occurs as disseminated organic material (0.1-50% [C.sub.org]) which, when mixed with migrated bitumen (now pyrobitumen), appears as coal-like seams and lenses of semilustrous and semimat layer-shungite rocks (oil shales, 50-75% [C.sub.org]). The migrated bitumen (originally petroleum), represented by the lustrous vein- and layer-shungite, conformably fills interbedding spaces or cross-cutting joints and usually contains 80-98% [C.sub.org]. The shungite-bearing rocks of the upper Zaonezhskaya Formation represent one of the most richest accumulations of organic material reported from the Palaeoproterozoic, and one of the geologically earliest stages of petroleum generation. The sediments of the Zaonezhskaya Formation were initially deposited in brackish water in a non-euxinic, lagoonal environment. The high C/S ratio (8-1000) with a zero intercept on the C-S cross-plot indicates that deposition occurred in sulphur-poor water. Intensive synchronous volcanism may have contributed to both the enhanced delivery of nutrients and elevated sedimentation rate, and eventually to the high degree of preservation of organic material. The integrated data suggest that the organic material has a biogenic origin, most likely algal or bacterial. The organic material suffered complex catagenetic and metamorphic alteration which is reflected in: (1) the four-modal distribution of [C.sub.org] content (with maxima at 5, 30, 65 and 95%); (2) highly variable [[Delta].sup.13][C.sub.org] (-45% to - 17%); (3) bimodal distribution of [[Delta].sup.13][C.sub.org] (with maxima at -28 and -39%); and (4) low H/C ratios (0.005-0.2). Abundant diagenetic carbonates associated with shungite rocks ([[Delta].sup.13] [C.sub.carb] = -5 to - 26%) and the presence of pyrite ([[Delta].sup.34]S -22 to + 31%), reflects substantial loss of organic matter via bacterial reduction of sulphate during diagenesis. The shungite rocks are characterised by a further substantial loss (> 50%) of biologically produced organic material in the course of thermal maturation and by a depletion in 12C (> 10%). The isotopic composition of carbonate concretions does not indicate the involvement of fermentative diagenesis. Conservative estimates give [[Delta].sup.13][C.sub.org] -34%, as the best value of the initial biomass. Lustrous vein- and layer-shungite containing more than 80% [C.sub.org] are considered to be allochthonous, migrated bitumen (originally petroleum). The semilustrous and semimat layer shungite rocks containing 55-75% [C.sub.org] represent oil shales with both migrated bitumen (originally petroleum) and autochthonous kerogen residues. The oil source rocks were apparently hosted in the Zaonezhskaya Formation. The generated oil has migrated both vertically and laterally with the highest concentration in cupola structures. The locality at Shunga represents the most significant volume of trapped petroleum from the study area. Keywords: palaeoproterozoic; black shale: oil shale; carbonates; sulphides; kerogen; bitumen; carbon; oxygen; sulphur; isotopes; hydrocarbon indicators; petroleum accumulation; diagenesis; metamorphism

Wood-grained chert is an unusual type of banded nodular chert that is characterized by alternating thin dark-coloured bands and thicker light-coloured bands, which give the chert a texture reminiscent of the growth rings of wood. The wood-grained texture found in some chert nodules in the Portland Limestone Formation of southern England is the result of variations in the concentration of calcite inclusions within the chert. Light-coloured bands contain more abundant calcite inclusions than adjoining darker coloured chert bands. The Portland Limestone wood-grained cherts formed by the late diagenetic replacement of a highly compacted carbonate sand, in which abundant siliceous sponge spicules were the silica source. Oxygen and hydrogen isotope data ([[Delta].sup.18]O = 27.0 to 29.3[per thousand]; [[Delta].sup.18]O = -71 to -91[per thousand] V-SMOW), indicate that either chert formation or, more likely, the opal-CT to quartz transformation, occurred in meteoric or mixed marine and meteoric pore waters at temperatures in the range of 20-60 [degrees]C. The wood-grained texture likely formed by a self-organization process. A feedback mechanism is proposed for the origin of the texture in which calcium and carbonate ions released during the replacement of the host limestone resulted in calcite supersaturation along the boundaries of growing chert nodule. Calcite inclusion-rich chert bands formed within the calcite supersaturated zone and inclusion-poor bands formed outside of the calcite-supersaturated zone. Wood-grained chert provides another example of how the coupling of reaction and transport can produce repetitive patterns in otherwise unordered sediments or rock.