LREE-enriched komatiitic basalts are commonly found in association with LREE-depleted komatiites. This association is found in a sequence of late Archaean lavas from Newton Township, Ontario. The komatiitic lavas at Newton Township differ from most late Archaean examples in that both the komatiites and the komatiitic basalts are depleted in Al and in the HREE. The close association and distinctive Al and HREE depletions of the two lava types strongly suggest a common origin, despite their contrasting LREE patterns.

A model is proposed whereby the LREE-depleted komatiites represent the parental magma to the LREE-enriched komatiitic basalts, the two being linked by a combination of crystal fractionation and crustal assimilation. The composition of the contaminant is estimated by comparing the LREE-enriched komatiitic basalts with the evolved part of a thick layered komatiite flow that has similar major element chemistry. The contaminant composition coincides closely with Taylor & McLennan's estimate of the composition of the Archaean upper crust. It is concluded that LREE-enriched komatiitic basalts can be produced from LREE-depleted komatiite parent magmas by combined assimilation and fractionation, and that such a process best explains the geochemistry and Nd isotopic features of most komatiitic basalts.

Lahars and fluvial sediments which buried trees following the 18 May 1980 eruption of Mount St Helens were investigated during August of 1982. Trees buried in older mudflows, dated at a.d. 1885, a.d. 1450–1550, and 36000 years b.p., also were examined. Although many logs clearly were transported, large numbers of trees were buried in growth position. Burial by lahars generally resulted in the death of trees, whereas some trees survived burial by fluvial sediments. Scanning electron microscope studies show that trees buried in lahars are well preserved. Pre-1885 buried woods show incipient silicification, and woods buried 36000 years b.p. show silica impregnation of cell walls.

Features of in situ and allochthonous burial very similar to those seen in southern Washington are found also in Eocene silicified forests in Yellowstone National Park, Wyoming, U.S.A., and in Cretaceous fossil forests in southeastern Alexander Island, Antarctica. Observations of Recent wood from volcaniclastic deposits of Mount St Helens provide valuable insights into processes of burial and silicification of fossil forests of various ages around the world.

Evidence is presented that many of the major strike faults in the British and Irish Caledonides were active as sinistral strike-slip zones in the end-Silurian to pre-mid-Devonian period. Some, such as the Highland Boundary Fault, moved in this way at an earlier stage in the Ordovician. These data allow the Caledonian rocks lying between the Laurentian miogeocline (whose basement is represented by the Lewisian, Moine and possibly the Dalradian) and the Gondwanaland miogeocline (Midland Platform and Welsh Basin) to be re-analysed as a group of disorganized terranes which originated to the southwest in North America and southwest Europe/Africa prior to the Silurian. The Highland Border Terrane and Northern Belt Terrane are interpreted as duplicated pieces of a mid-Ordovician sequence which was a back are to northwest subduction. The Midland Valley Terrane is interpreted as a slice of Laurentian foreland onto which ophiolites were obducted in the lower Ordovician but which became the basement of a continental margin arc to northwest subduction in the mid-Ordovician. The Cockburnland Terrane is inferred to be part of the same arc repeated and then broken up and dispersed by continuing strike slip. The Connemara Terrane is regarded as an allochthonous piece of the Dalradian miogeocline and the South Mayo Terrane as a remnant of an early Ordovician arc and fore arc which in mid-Ordovician times became a back arc/marginal basin to northwest subduction. The Lake District-Wexford Terrane is part of an arc to southeast subduction under Gondwanaland whose activity climaxed in the mid-Ordovician. The Central Terrane is interpreted as a Silurian overstep assemblage which blankets the junction between Laurentian- and Gondwanaland-derived oceanic terranes, and therefore Iapetus is regarded as an Ordovician ocean which closed prior to the Silurian. The model suggests that at the end of the Silurian, a clockwise-rotating Gondwanaland, having carried Laurentia into collision with Baltica, broke free and created a major sinistral strike-slip zone which disrupted the Ordovician palaeogeography in the British Isles/North American sector of Iapetus.

Hole number CY-4 of the Cyprus Crustal Study Project penetrated the lower sheeted dyke complex, gabbros and ultramafic cumulates of the Troodos ophiolite. The lower part of the drill core sampled a coarse-grained plutonic complex revealing phase and cryptic layering and one major magma chamber replenishment. This magma chamber intruded medium-grained gabbroic rocks showing intricate chemical evolution trends reflecting several magma replenishments. In the upper part of the core, the gabbroic cumulates are intruded by fine-grained dykes, which grade into the sheeted dyke complex and chemically can be correlated with the lavas of the lower pillow sequence. The upper pillow lavas are best correlated with the ultramafic cumulates. A study of coexisting plagioclase (An) and mafic mineral (opx Mg #) compositions in the drill core revealed three main rock types: (1) a primitive group (An95–98, Mg #75–85) represented by the lower, coarse-grained gabbroic and ultramafic cumulates. (2) an intermediate group (An86–95 and Mg #70–78) represented by the upper level gabbroic cumulates, and (3) the lower part of the sheeted dyke complex (An60–80 and Mg #60–70). The plagioclase of the gabbros and ultramafic cumulates have an unusually high An content. Numerical simulation of the expected anhydrous, one atmosphere, crystallization trends show that the Troodos trends cannot be reproduced from known spreading or subduction related glasses. Mineralogical evidence indicates that the extrusives and the cumulate sequences of the Troodos ophiolite are genetically related. Glasses from the extrusives, nevertheless, also fail to reproduce the mineral crystallization trends observed in the plutonics. Attempts to model high PH2O crystallization produced trends more consistent with those observed in the cumulates. The very sodium-poor nature of the plagioclase may therefore mainly reflect high PH2O crystallization. High water content is consistent with the inferred subduction zone basin origin for the Troodos ophiolite.

Rhyolites in the vicinity of Snowdon (North Wales) are intimately associated with the evolution of the Lower Rhyolitic Tuff Formation (LRTF) caldera of Ordovician (Caradoc) age. They occur as deep-seated dykes, sills and small stocks, shallow-level intrusive domes, and domes extruded within a predominantly shallow-marine environment. Extrusion occurred during three main phases, indicating the episodic availability of rhyolite magma. The rhyolites can be divided on their trace element ratios (e.g. Nb/Zr) into five main groups. Extrusive representatives indicate that each group correlates strongly with a single phase of rhyolite extrusion. Within each group, the distribution and variation of intrusive form with stratigraphic level suggests that geochemically similar rocks were emplaced at approximately the same time. Consequently, the groups represent discrete magma compositions tapped from the evolving Snowdon subvolcanic magma system. Differences in distribution of the groups reflect changes in structural controls of emplacement before and after development of the LRTF caldera.

Trace fossils are abundant and diverse in many clastic sequences spanning the Precambrian-Cambrian boundary and may prove to be the most useful palaeontological method for global correlation in this stratigraphic interval. The ichnofaunas of the latest Precambrian (Vendian) rocks include some forms whose range does not extend into the Cambrian (e.g. Bilinichnus, Intrites, Palaeopascichnus, Vendichnus, Vimenites) and others which continue throughout most or all of the Phanerozoic (e.g. Arenicolites, Aulichnites, Cochlichnus, Didymaulichnus, Gordia, Neonereites, Planolites, Skolithos). At least 50 ichnogenera make their first appearance below the lowest trilobites in sections with broad geographic spread. A few of these appear to have a short time range, extending to about the incoming of the trilobites (e.g. Astropolichnus, Didymaulichnus miettensis, Plagiogmus, Taphrhelminthopsis circularis), but the majority continue through most or all of the Phanerozoic.

For correlation of Precambrian-Cambrian boundary sequences it is therefore possible to use both the occurrence of those ichnogenera with a short time range and the incoming of those with an extended range. Three stratigraphical zones can be recognized with respect to the incoming of trace fossils. Zone I is of Upper Vendian age and includes Arenicolites, Bilinichnus, Cochlichnus, Didymaulichnus, Gordia, Harlaniella, Intrites, Nenoxites, Neonereites, Palaeopascichnus, Skolithos, Vendichnus and Vimenites. In Zone II, of Lower Tommotian age, the earliest examples of Bergaueria, Phycodes, Teichichnus and Treptichnus are encountered. Many trace fossils appear in Zone III, which extends from Upper Tommotian to Lower Atdabanian, but the most important are: Astropolichnus, Cruziana, Diplichnites, Diplocraterion, Dimorphichnus, Plagiogmus, Rusophycus and Taphrhelminthopsis circularis.

This vertical zonation of trace fossils allows an attempt at world-wide correlation, from which the most significant conclusions are that the Vendian/Tommotian boundary can probably be placed: (i) near the middle of the McNaughton Formation in the Rocky Mountains, Canada; (ii) at the base of the Deep Spring Formation or in the underlying Reed Dolomite in the White Inyo Mountains, California, U.S.A.; (iii) low in the Chapel Island Formation in the Burin Peninsula, Newfoundland, Canada; (iv) at or close to the base of the Candana Quartzite in North Spain; (v) at or below the base of the Breivik Member in Finnmark, Norway; and (vi) near or below the base of the Zhongyicun Member at Meischucun, China.

The sections in the Burin Peninsula, Newfoundland and Meischucun, China are favoured candidates for the global stratotype for the Precambrian-Cambrian boundary. In the Burin Peninsula, the trace fossils suggest that the Tommotian/Atdabanian boundary may be within or at the base of the Random Formation, thereby implying that the Tommotian may include a thickness of 500 m of sediment comprising at least most of the Chapel Island Formation. At Meishucun, the ichnofaunal evidence implies that the Tommotian/Atdabanian boundary is probably no higher than the top of the Zhongyicun Member. The thickness of the Tommotian is therefore possibly only about 20 m here, implying a very condensed sequence, a conclusion consistent with an abundance of phosphorites. Two stratotype reference points for the Precambrian-Cambrian boundary have been suggested in this section. The lower point (0.8 m above the base of the Xiawaitoushan Member) may be near the Vendian/Tommotian boundary or younger, while the higher point (base of Unit 7 of the Zhongyicun Member) is probably Upper Tommotian or even Lower Atdabanian. The higher point would place the boundary above the world-wide dramatic increase in trace fossil abundance and diversity but probably before the first trilobites. This would almost certainly have advantages for correlation. The inference that the Meishucun section is younger than most Chinese work suggests should not therefore, by itself, prejudice its adoption as global stratotype.

In general, where comparative data are available, the trace fossil correlations agree well with pre-existing proposals based on small shelly fossils. The degree of resolution of the two methods would appear at present to be similar but trace fossils, being found mainly in clastic facies, may benefit from more frequent occurrence.

Microscopic soft-sediment deformation structures in a 30 × 25 × 12 mm hand specimen of glaciogenic silty mudstone from the Permo-Carboniferous Dwyka Formation of northern Natal in South Africa are illustrated by serial sections. The processes these structures imply are interpreted using palinspastically restored sections and isopachytes reconstructed from them. The sedimentation of silts, muds, and microrhythmites is found to have been punctuated by episodes of hydraulic activity which resulted in thirteen bodies with the mixed fabrics of micromelanges. These are divisible into five melange types: (1) subconcordant intrusive; (2) disconcordant intrusive; (3) subconcordant extrusive; (4) near-surface with asymmetric internal structures, and (5) conformable near-surface with internal structures symmetric about the palaeovertical.

Remarkable similarities between isopachs for pairs of micromelanges at different levels in the specimen suggest that intrusive melanges at depth fed contemporaneous extrusive melanges on the sea floor. Each couplet of melanges with matched isopachs is linked by faults which are interpreted as having acted as hydraulic vents despite only parts of them being infilled by intrusive melange. A significant proportion of the succession could consist of sediment recycled from depth by hydraulic extrusion. Repeated hydraulic intrusions along, and extrusions from, the same disturbed interface suggest that this interface acted as the distal leaking end of one of the hydraulic sills recently described by von Brunn & Talbot (1986). Pulses of pressurized water were transmitted through a prograding marine slope of quickclay by the tidal pumping of a marine ice sheet periodically grounding upslope.

The results of this analysis are extrapolated to processes operative in subduction complexes. All four fabrics described from large-scale hydraulic melanges in modern and ancient accretionary prisms are matched on a much smaller scale in our sample from a proglacial submarine slope. Dismemberment by faults is the only fabric element described from accretionary complexes which is missing from the micro-analogues described here.

The Pitts Head Tuff Formation, of Ordovician (Caradoc) age, was emplaced as a thick (c. 700 m) intracaldera sequence and two outflow units comprising welded acidic ash-flow tuff. The Pitts Head pyroclastic flows were erupted subaerially but the lower and most extensive of the outflows crossed a shoreface, and continued for several kilometres offshore. The flow entered the sea without disruption and, following deflation and tuff emplacement, displaced the shoreface several kilometres to the east and northeast. Post-eruption subsidence in the northeast resulted here in the rapid establishment of environments deeper than had previously existed.

The lower outflow tuff is parataxitically to eutaxitically welded in both the subaerial and marine environments. The extremely regular plane-parallel welding foliation of the subaerial tuffs, however, contrasts with the locally highly deformed foliation of the tuff deposited beyond the shoreface. The deformed foliation, associated with irregular zones of intense siliceous nodule development, is ascribed to the upward streaming of water vapour generated at the tuff/sediment boundary. Elsewhere rheomorphism within the tuff was caused by instability resulting from emplacement on slopes related to faulting. Continued movement initiated extensive brecciation, detachment, and local gravity sliding of large rafts of tuff.

Upper Cretaceous carbonate sequences contain omission surfaces, hardgrounds and intercalations of ‘clays’. These ‘clays’ are largely associated with submarine biogeochemical carbonate dissolution that was caused by high biological productivity in the pelagic zone. The Maastrichtian-Danian ‘boundary clays’ probably accumulated during a maximum productivity that led to the exhaustion of nutrients, development of phenomena comparable to present-day red tides, and mass mortality of marine biota. There was a type of ecological break in the seas and oceans at the time of the Maastrichtian-Danian boundary.

Studies of melanges in the Shimanto Belt of southwest Japan have shown that many of the included blocks exhibit asymmetric geometries, similar to boudins and inclusions in medium grade metamorphic rocks which have been subjected to markedly non-coaxial strains. Deformation is accomplished by mesoscopically ductile processes such as pore-pressure-controlled or diffusionally-controlled grain boundary sliding, although localized fracture and cataclasis also occurs. It is suggested that the melanges developed by the propagation of extensional displacement zones, analogous to shear bands, through sandstone beds, so progressively dismembering them. Studies of the detailed internal geometries of melanges along with the deformation mechanisms active during their formation may help resolve the mode of formation of these problematic units.

The finger structures described earlier by Brown and later by Butcher, Young & Faithfull involve dissolution of troctolite during crystallization of olivine, followed by crystallization of pyroxene around olivine grains in the fingers. However, the ingestion of troctolite takes the liquid away from pyroxene saturation rather than toward it. The pyroxene field can be encountered metastably, and pyroxene caused to crystallize, by supercooling the olivine-rich liquid against the troctolite. The melt corrosion represented by the fingers, and other field relations, suggest that the mafic layers were emplaced as sills of mafic magma into nearly solid troctolites. Melting at the base of mafic liquid layers was impeded by a bed of olivine crystals releasing light solute upward, causing compositional convention and rapid heat transfer to the top of the layer, where melting demonstrably occurred. Recognition of this process introduces the novel concept of a magmatic heat pump driven by compositional convection. The crystallization path ol–px–pl(–sp) is also found next to xenoliths in the Kiglapait Intrusion where the magma was normally saturated only in ol+pl, directly demonstrating the effect of supercooling on the crystallization sequence.

The Aguero fanglomerate body developed in late Oligocene to early Miocene time at the northern margin of the Ebro Basin where the emergent southern Pyrenean thrust front created a topographic high. Tectonic activity in the thrust belt strongly influenced the sequences and structures within the fan deposits. The fan deposits display an initial coarsening-up sequence. Intraformational unconformities subdivide the proximal sediments into a series of wedges. These result from a continued uplift along the thrust front during the initial stages of fan development. A major intraformational unconformity marks the top of this sequence and the start of a fining-up sequence. Further tectonic activity in the thrust front is indicated by a syn-depositional synclinal fold which decreases in amplitude up sequence. Rejuvenation of fan sedimentation to form a second coarsening-up sequence reflects renewed activity in the thrust front. This second sedimentation event resulted in a plus 200 m thickness of massive conglomerates. The geographical limits of fan sedimentation can be determined because the fan deposits are lithologically distinct from the other Ebro Basin molasse in the area. The area of the drainage basin of the fan can also be estimated by consideration of the clast types present in the fan deposits. The fan and drainage basin areas are estimated to be 6 km2 and 10 km2 respectively.

Arguments on the periodicity of mass extinction have been marred by incorrect statistical arguments. Mass extinction patterns can be best interpreted as chance fluctuations over time.

Durvilleoceras woodmani of debated Late Permian or Early Triassic age is accompanied by other ammonoids, deemed to belong to Araxoceratidae, Xenodiscidae, and Goniatitida, of chiefly Late Permian age. The ammonoids are found principally in the Deserter Bay and especially Tongue Point Members (new names) of the newly named South Arm and Wells Arm formations of the Greville Subgroup, which are overlain, apparently in normal stratigraphic succession, by the Waiua Formation and newly named formations of the Te Mokai Group, with three successive faunas of Permian appearance.

Late Caledonian granitoids, (c. 400 Ma) in the zone of the Iapetus suture provide a datum against which current models for the relative timing of deformation can be tested. One such granitoid adjacent to the proposed suture ‘line’ in eastern Ireland is now buried by an Upper Palaeozoic cover. It is identified geophysically by a negative Bouguer anomaly with no magnetic signature, and geologically by a hypabyssal dyke swarm and hornfels metamorphism. The timing of intrusion of the granitoid is shown to have occurred during the continuing end-Silurian/early-Devonian deformation. Other members of this widespread suite in Ireland show features consistent with diapiric intrusion during the later stages of the deformation. This evidence brackets the age of regional deformation as continuing during granite emplacement and cooling (c. 400 Ma). The unifying characteristics of the straddling granitoid suite, coupled with a sinistrally transpressive deformation, in this broad suture zone are interpreted in terms of a continental collision which climaxed in late-Silurian/early-Devonian time.

Uranium enrichments (up to 4000 ppm) occur in the manganese and phosphorite assemblages of the Lower Cambrian clastic marine sedimentary sequence, Timna Basin, Israel. Two types of mineralization assemblages can be defined. Sedimentary stratabound assemblages consist of uranium-enriched stratiform manganese and phosphatic laminae, diagenetic (type A) manganese nodules composed of pyrolusite and hollandite laminae and phosphorite lenses. Fission-track maps show that the uranium is homogeneously distributed within host manganese and phosphatic minerals of these assemblages. Epigenetic assemblages are mainly composed of manganese- and phosphorite-bearing veins and secondary (type B) manganese nodules with a coronadite dominant mineralogy. Uranium is depleted in these assemblages, relative to the sedimentary stratabound assemblages.

The distribution of manganese and phosphorite assemblages has a marked bimodal character. Alternation between manganese and phosphatic laminae in the stratiform deposits reflects cycles of oxidizing and reducing conditions brought about by mixing and stratification of the waters in the Timna semi-closed depositional basins. Compaction of wet sediments led to remobilization and the formation of uranium-enriched manganese nodules at the aerated sediment–water interface, and uranium-enriched phosphorite lenses below the interface in reducing conditions. Epigenesis occurred through the passage of solution fronts which recrystallized the manganese and phosphatic minerals and remobilized metallic elements, particularly uranium which was leached away and is still being remobilized today.

The mechanism of uranium uptake in manganese phases is shown most probably to involve adsorbtion of [(UO2)3. (OH)5]+ complexes on precipitating minerals. Uranium is enriched in both the pyrolusite and hollandite laminae of type A nodules, but is particularly concentrated in the former (4000–10000 ppm). Thermodynamic calculations of the relative stabilities of pyrolusite and hollandite suggest that the pH conditions of hollandite formation were close enough to the pH limit of efficient uranium adsorption to inhibit its uptake relative to pyrolusite.

Granulites occur in and around an anorthosite massif (commonly referred to as the Bengal anorthosite) in eastern India. The central part of the anorthosite is massive in nature with locally developed ‘fluxion structure’ in isolated blocks. Banding in the peripheral part is typical of that of the massif anorthosites of Anderson & Morin. The granulites bear imprints of polyphase deformation, and the xenoliths of granulites within the anorthositic massif maintain their regional structural alignment. The Anorthosite, syntectonic with the second fold movement in the terrane, has developed second generation structures.

Hornblende, orthopyroxene, clinopyroxene, plagioclase and quartz with or without garnet and other minor phases consitute the granulites. In the anorthosites, orthopyroxene, clinopyroxene, hornblende, and magnetite with or without garnet may constitute more than 15% of the mode. Garnet, when present in the rocks, is a late overprint. Symplectitic growth of garnet with one or more of the phases such as ilmenite, quartz, clinopyroxene and sodic plagioclase indicates genesis of garnet through decomposition of orthopyroxene and/or prograde hornblende in the presence of calcic plagioclase. The garnet formed during prograde dehydration reactions, rather than cooling of the complex.

Chemical characteristics of the mafic phases in the rocks indicate attainment of exchange equilibrium with respect to the major cations in closely associated hornblende, clinopyroxene, orthopyroxene and garnet. Estimated physical conditions of metamorphism from contrasted assemblages suggest a final equilibration of different granulitic assemblages as well as of the anorthositic assemblages under similar conditions. There was, however, belated building up of water pressure in the anorthositic domains stabilizing a late hornblende during cooling of the complex. Absence of chilled margins in the anorthosite as well as absence of contact aureoles in the enveloping and enclosed granulites indicate emplacement of the anorthosite in a hot environment, prior to cooling of the complex.

In northern Greenland in early Palaeozoic time a turbidite trough (the eastward extension of the Hazen trough of Arctic Canada) was flanked to the south by a carbonate platform. The trough was deformed during the mid-Palaeozoic Ellesmerian orogeny to form the E–W trending North Greenland fold belt. This fold belt was deformed further by Eurekan (Tertiary) structures, important among which is a major fault complex, the Harder Fjord fault zone (HFFZ). The suggestion has been made that this fault zone controlled early Cambrian sedimentation, even though the fault trace does not coincide with the trough–platform facies transition in sediments of that age; this has led to some controversy.

We report new information from a mapping programme by the Geological Survey of Greenland which has established the thin-skinned nature of Ellesmerian deformation at the trough-platform transition and implies that much of the fold belt is underlain by a shallow detachment. This in turn implies that the HFFZ exists in the hanging-wall of the detachment while the early Cambrian trough-platform transition is located autochthonously in the foot-wall. We adduce evidence to show that the latter was probably controlled by syndepositional faulting with actively eroding fault scarps and suggest that these basement structures were reactivated in a dextral strike-slip mode in early Tertiary time to form the HFFZ as now observed.

An assemblage of problematical microfossils of Precambrian–Cambrian boundary age is redescribed from the Chert–Phosphorite Member, at the base of the Lower Tal Formation of Maldeota in the Lesser Himalaya of India. This assemblage has previously been ascribed to various ages, from Precambrian to Cretaceous, but is held by us to contain: Maldeotaia bandalica, Protohertzina anabarica group, trumpet-shaped elements, acicular elements A & B, ?Conotheca sp., Ovalitheca cf. multicostata, allathecid sp. A, Barbitositheca ansata, Hexangulaconularia cf. formosa, Coleoloides aff. typicalis, Hyolithellus aff. insolitus, H. cf. isiticus, H. vladimirovae, Spirellus shankari and Olivooides multisulcatus. These compare closely with assemblages found above the base of the first, Anabarites trisulcatus–Protohertzina anabarica Zone in China and in the second, Pseudorthotheca costata Zone of southern Kazakhstan. The stratigraphic setting of the Krol–Tal succession is reviewed and several similarities are noted between the Precambrian–Cambrian boundary successions of Lesser Himalaya in India and of Yunnan and Sichuan in Southwest China, indicating that correlation between them is possible at several levels.

The relationship between disposition of metamorphic isograds and early strain history is examined for the Sulitjelma Fold Nappe of the central Scandes. Evidence for stratigraphic inversion and the widespread inversion of the garnet isograd is reviewed, and new evidence for the nature of early Scandian deformation is presented. A two-stage model is presented which explains the inversions by (i) an initial phase of horizontal compression resulting in crustal thickening by vertical stretching during closure of a marginal basin, followed by (ii) continued horizontal compression and crustal thickening by eastward-directed nappe emplacement. The emplacement of the Sulitjelma Fold Nappe during the latter phase of progressive simple-shear-dominated deformation was accompanied by the development of sheath fold geometries and the overturning of isotherms to produce the widespread inversion of the garnet isograd.