Well-preserved Radiolaria have been discovered in calcareous silt turbidites and mudstones intercalated with basic extrusives of the Guevgueli Ophiolite, northern Greece. The mudstones contain terrigenous silt, probably derived from adjacent continental basement of the Serbo-Macedonian and/or Paikon units. Volcanic quartz and rare volcanic glass were probably derived from an active continental margin arc (Paikon volcanic arc) to the west. The radiolarian sediments were deposited within fault-controlled hollows in the ophiolitic extrusives, and then covered by massive and pillowed extrusives. The radiolarian assemblage is indicative of an early Late Jurassic (Oxfordian) age, which therefore dates the genesis of the Guevgueli Ophiolite. Our data are consistent with the age of the intrusive Late Jurassic Fanos Granite, believed to be contemporaneous with the Guevgueli Ophiolite. In general, the Guevgueli and related ophiolites of northern Greece are thought to have formed within a transtensional intra-continental marginal basin, generated in response to oblique eastward subduction of older Tethyan oceanic crust (Almopias ocean).

Carbonate-rich sedimentary rocks of the western Anabar region, northern Siberia, preserve an exceptional record of evolutionary and biogeochemical events near the Proterozoic/Cambrian boundary. Sedimentologically, the boundary succession can be divided into three sequences representing successive episodes of late transgressive to early highstand deposition; four parasequences are recognized in the sequence corresponding lithostratigraphically to the Manykai Formation. Small shelly fossils are abundant and include many taxa that also occur in standard sections of southeastern Siberia. Despite this coincidence of faunal elements, biostratigraphic correlations between the two regions have been controversial because numerous species that first appear at or immediately above the basal Tommotian boundary in southeastern sections have first appearances scattered through more than thirty metres of section in the western Anabar. Carbon- and Sr-isotopic data on petrographically and geochemically screened samples collected at one- to two-metre intervals in a section along the Kotuikan River, favour correlation of the Staraya Reckha Formation and most of the overlying Manykai Formation with sub-Tommotian carbonates in southeastern Siberia. In contrast, isotopic data suggest that the uppermost Manykai Formation and the basal 26 m of the unconformably overlying Medvezhya Formation may have no equivalent in the southeast; they appear to provide a sedimentary and palaeontological record of an evolutionarily significant time interval represented in southeastern Siberia only by the sub-Tommotian unconformity. Correlations with radiometrically dated horizons in the Olenek and Kharaulakh regions of northern Siberia suggest that this interval lasted approximately three to six million years, during which essentially all 'basal Tommotian' small shelly fossils evolved.

The six exposures of the Upper tectonic Unit of the Cycladic Massif occurring on the island of Tinos are shown to comprise a metamorphosed dismembered ophiolite complex. The common stratigraphic section consisting of tens-of-metres- thick tectonic slices of mafic phyllites overlain by serpentinites and gabbros is considered to have been derived by a combination of thrusting during obduction and subsequent attenuation by low-angle normal faults. All rock types show evidence of a phase of regional greenschist-facies metamorphism, which in the case of the phyllites is accompanied by penetrative deformation. The greenschist-facies metamorphism in gabbros is preceded by high temperature sea-floor amphibolite-facies alteration, whereas in the serpentinites, the antigorite + forsterite greenschist-facies assemblage overprinted an earlier low temperature lizardite serpentinite. Trace element patterns of the mafic phyllites and a harzburgitic origin of meta-serpentinites suggest a supra subduction zone (SSZ) affinity for the ophiolitic suite. ρ18O values of phyllites, gabbros and serpentinites range from 6 to 15%o. Model calculations indicate that such values are consistent with low temperature (50–200°C) alteration of parent rocks by sea-water at varying water/rock ratios. This would agree with the early low temperature mineralogy of the serpentinites, but the early high temperature alteration of the gabbros would require the presence of 18O-enriched sea-water.The following overall history is suggested for Tinos ophiolitic slices. (1) Oceanic crust was generated at a supra-subduction zone spreading centre with high temperature alteration of gabbros. (2) Tectonic disturbance (its early hot stages recorded in an amphibolitic shear zone at the base of serpentinites) brought the already cooled ultramafics into direct contact with sea-water and caused low-T serpentinization. (3) Tectonism after cooling involved thrusting which caused repetition and inversion of the original order of the oceanic suite. (4) Regional metamorphism of all the ophiolite components at greenschist-facies conditions (−450°C) overprinted the early alteration mineralogy. It was probably induced by continued thrusting and piling up of nappes. The Tinos ophiolite, dated as late Cretaceous and genetically related to other low pressure rock-units of the same age in the Aegean, differs in age and degree of dismemberment and metamorphism from ophiolites in mainland Greece.

Field studies on the island of Elba and seismic lines from the Northern Tyrrhenian Sea, Italy, indicate that major extensional displacements were accommodated along east-dipping low-angle detachment faults. The rifting and subsidence in the Northern Tyrrhenian Sea basin have followed convergence and collision of the Corso-Sardinian block and the Apulian microplate. This collisional episode produced the Northern Apennines fold-and-thrust belt. Major extensional faults cut down-section through the stratigraphy and pre-existing west-dipping thrust faults. West-dipping thrusts can also be reactivated and form antithetic faults to the east-dipping detachments. Brittle deformation conditions predominated during the extensional phase. The geometry, internal structure and the fabrics (brittle and penetrative) associated with a well-exposed low-angle extensional detachment in Elba are presented in this paper. A geometrical model for the brittle extensional faulting is presented in which regional extension was accommodated on a system consisting of two sets of simultaneously active antithetic faults. The east-dipping detachment faults appear to have started at steeper angles, based on field and seismic observations, and rotated counter-clockwise to lower dips. Due to this rotation, and for space accommodation, antithetic west-dipping faults formed and rotated clockwise. A tectonic model is proposed whereby slowing of the convergence between Apulia and Corsica, as well as Tethys oceanic crust and Apulian crust subduction, led to the delamination of the Apulian litho-spheric mantle away from the crust. Accompanying asthenospheric upwelling and intrusion at the crust—mantle interface beneath the Tyrrhenian Sea caused late orogenic crustal stretching in the Northern Apennines internal zone.

In this collection of papers, we attempt to document, through interdisciplinary studies in southwest Mongolia, the interlinked evolution of the biosphere and lithosphere over the Neoproterozoic–Cambrian interval. In so doing, we bring together the fruits of two expeditions to the Altay mountains, sponsored by IGCP Project 303 on Precambrian–Cambrian Event Stratigraphy. Both expeditions took place during an interval of great socio-economic change in the region. The first expedition, in 1991, was one of the last in a series of Joint Soviet–Mongolian Palaeontological Expeditions, organized by A. Yu. Rozanov and R. Barsbold, and led by E. A. Zhegallo and A. Yu. Zhuravlev. Scientists from Sweden and the UK also participated. The second, 1993, expedition was one of the first IGCP project meetings organized independently by the Mongolian Academy of Sciences, and was led by M. D. Brasier and D. Dorjnamjaa.

The Antarctic Peninsula is a Mesozoic–Cenozoic magmatic arc built on Palaeozoic and younger basement. It was formed by processes related to the subduction of Pacific ocean floor at its western margin, although subduction has now ceased along most of its length. The peninsula features all the tectonic components commonly associated with a developing arc system: basement, accretionary complex, magmatic arc, arc-related basins, intra-arc extension and post-subduction volcanism. Seventeen thousand kilometres of high resolution aeromagnetic data have recently been collected in a transect across part of the arc, covering an area 290 by 230 km and incorporating examples of most of the above tectonic components. The new map reveals distinct magnetic signatures, which can now be related to each of these components in a way that was not possible with reconnaissance data sets. A characteristic magnetic anomaly pattern for each component is described and comparisons drawn with magnetic studies of other arc regions.

The Late Proterozoic Dokhan volcanic suite (620 Ma) of the northern Nubian shield is the product of Late Pan-African volcanism. The suite covers the entire spectrum from basalt to high-silica rhyolite and occurs as two units: a dark-coloured unit containing basalt-andesite-dacite, and a light-coloured unit encompassing dacite-rhyodacite-rhyolite. The latter unit is made up largely of ash flow tuffs and ign-imbrites that are locally interstratified with basalt and andesite lava flows. The suite forms a continuum in composition with a wide range of Si02 (48–77 wt%), CaO (0.1–8.9 wt%), Sr (81–906 ppm), Zr (85–340 ppm) as well as most other elements, and is moderately enriched in incompatible elements, including rare earth elements (REE). The suite exhibits fractionated, subparallel REE patterns that are similar overall to Andean andesites and ignimbrites. Well-defined major and trace element trends and fractionated REE profiles are consistent with a fractionated basalt to rhyolite calc-alkaline magma series. It is a typical calc-alka-line orogenic complex and exhibits mineralogical-geochemical traits of arc-related volcanism. The suite neither resembles products of extensional nor transitional tectonic regimes as previously thought, but was produced in a subduction-related tectonic environment. The mafic nature of the least-evolved rocks of the suite, along with its relatively low initial 87Sr/86Sr ratio (0.7039) are considered to indicate a mantle source. A mantle-derived basaltic magma fractionated, with amphibole and plagioclase dominating the fractionating assemblage, to produce the more felsic varieties, as suggested by major and trace element fractionation modelling.

The Toro Complex is one of the Pan-African Older Granites of Nigeria, first described as a reversely zoned pluton made of a central dioritic mass surrounded by a broad granitic rim. It has been thoroughly reinvestigated both from the petrographic and structural points of view, with the help of systematic anisotropy of magnetic susceptibility (AMS) measurements. The granite main body is a hornblende–biotite porphyritic monzogranite characterized by an early submagmatic fabric displaying a concentric pattern of foliations and west plunging lineations (stage 1). This fabric is overprinted by a later one due to solid-state strain along north-south subvertical dextral shear zones (stage 2). In the vicinity of the diorite, an evengrained granite displays magmatic structures that are contemporaneous with this strike-slip event. The diorite–granite contact is a complex zone where field, petrographic and geochemical data enable recognition of the effects of mixing and mingling between a mafic and a felsic magma. Tonalites cropping out within this contact zone are interpreted as hybrid rocks. The reverse zonation of the diorite itself is also the result of some hybridization process. Magmatic interactions mainly resulted from in situ infiltration of granitic liquid into the dioritic mass. The detailed history of this bimodal intrusion began with the emplacement of the granitic magma acquiring a first stage fabric. Before full crystallization of the granitic core, intrusion of the dioritic magma permitted reheating of the granitic magma that then crystallized with specific structural characters. The second stage structures, whether characterized by magmatic fabric near the diorite or by solid-state strain features in north–south shear zones elsewhere in the granite, are related to late Pan-African dextral strike-slip tectonics in the basement of northern Nigeria. The bimodal Toro Complex is therefore considered as a late Pan-African syntectonic pluton.

Bigganjargga is the classic locality for the Varangian ice age in northern Scandinavia. The presence of two sets of striations on the underlying quartzite basement has been taken as evidence for a glacial origin of the Bigganjargga diamictite. However, soft-sediment surface structures on the underlying sandstone indicate that it was not consolidated when the diamictite was emplaced. Together with structures in the diamictite this shows that the deposit formed as a debris flow moving northwestwards over an unconsolidated substrate. There is no evidence for glacial origin or contribution for the diamictite material.

The deposits of the Pliocene—Quaternary foredeep of the Northern Apennine cover at present an area of 103000 km2. The original boundaries of the basin are not known, since marginal deposits have been eroded, in particular those of the inner, southwestern border. During Pliocene times the basin area was reduced by thrust tectonics and the amount of shortening may be tentatively estimated.

The present volume of Pliocene and Quaternary sediments may be inferred with good approximation from the maps of the base of the Pliocene and of the Quaternary (base of the Hyalinea balthica Zone) successions. The Pliocene volume has been corrected adding the estimate of the underthrust sediments, while no correction has been attempted for the eroded marginal deposits. The estimates of 97000 and 95000km3. reflecting the present volume of the Pliocene and Quaternary deposits, are therefore significantly less than the volumes originally deposited.

Present volumes have been transformed in ‘net’ (0% porosity) volumes, in order to obtain the relative net supply rates: 0.021 (Pliocene) and 0.047 (Quaternary) km3/a. Other unmeasurable factors (volume variations due to the weathering of silicates, loss of leached carbonates) may induce a probably unimportant underestimate of the supply rates.

Available data allow an approximate estimate of the range of the net volume of the Holocene sediments deposited during the last 6000 a BP (221–276km3) and of the relative net supply rate (0.037–0.046km3/a), that is not significantly different from the Quaternary one. Applying a porosity correction, these supply rates may be related to the Quaternary source area (128000km2) to obtain the relative denudation rates: 0.41–0.46mm/a (Quaternary) and 0.36–0.51 mm/a (Holocene). Present supply and denudation rates, deduced from the direct measurements of the bed load and suspended load of the apenninic and alpine rivers, do not differ significantly from the Quaternary and Holocene ones.

Available data do not allow a reliable estimate of the Pliocene source area, and consequently of the Pliocene denudation rate. However, a minimum of 160000–177 000 km3 has been eroded during Pliocene and Quaternary times. Assuming, as a working hypothesis, that the Pliocene source area did not significantly differ from the present one, an average thickness of 1240–1390 m could have been eroded since the beginning of Pliocene. This estimate is in agreement with the values obtained from the measurements of coalification of vegetal organic matter in the outcrops, and suggests that post-orogenic successions and ‘higher’ thrust sheets may have been completely removed in vast areas.

The Antarctic Peninsula is a Mesozoic–Cenozoic magmatic arc built on Palaeozoic and younger basement. It was formed by processes related to the subduction of Pacific ocean floor at its western margin, although subduction has now ceased along most of its length. The peninsula features all the tectonic components commonly associated with a developing arc system: basement, accretionary complex, magmatic arc, arc-related basins, intra-arc extension and post-subduction volcanism. Seventeen thousand kilometres of high resolution aeromagnetic data have recently been collected in a transect across part of the arc, covering an area 290 by 230 km and incorporating examples of most of the above tectonic components. The new map reveals distinct magnetic signatures, which can now be related to each of these components in a way that was not possible with reconnaissance data sets. A characteristic magnetic anomaly pattern for each component is described and comparisons drawn with magnetic studies of other arc regions.

This paper provides a framework for the stratigraphy and correlation of key sections through the Neoproterozoic–lower Cambrian of the Govi-Altay area, western Mongolia, studied by members of IGCP Project 303. Dzabkhan Formation volcanic rocks at the base of the succession are suggested to have formed during rifting of the Baydrik–Tarbagatay microcontinent in the Baykalian interval (c. 850–650 Ma). Pre-Vendian accretion transformed this rift and adjacent microcontinental fragments into a marginal marine back-arc basin, in which accumulated relatively uninterrupted Neoproterozoic and earliest Palaeozoic sediments. Lithostratigraphy for the latter is defined, with detailed maps, at five sections (Bayan Gol northern and southern blocks, Tsagaan Gol, Salaany Gol and Tayshir). The Tsagaan Oloom Formation lower member contains two diamictites of possible Sturtian age. Middle and upper carbonates are compared with the Yudomian (Vendian) of Siberia, the former ending with a unit of Boxonia grumulosa stromatolites. In the upper member, phosphorite-shale is followed mainly by limestones, with the earliest skeletal fauna. The Bayan Gol Formation comprises siliciclastic–limestone intercalations with increasingly diverse invertebrate biota. Correlation of lithological marker beds across the study area produces a composite stratigraphic section to which the earliest skeletal fossil occurrences are tied. The Anabarites trisulcatus Zone of the Nemakit-Daldynian Stage begins near the base of the Tsagaan Oloom Formation upper member (unit 11). The Purella antiqua Zone begins near the base of the Bayan Gol Formation (unit 18). Assemblages typical of the Nochoroicyathus sunnaginicus Zone (base of the Tommotian Stage) range through units 19 to 23. Elements typical of the Dokidocyathus regularis Zone (middle Tommotian) appear in unit 24 near the top of the Bayan Gol Formation.

The Kula province of Western Turkey provides an excellent example of an alkali basalt province in an area of active rifting. This paper establishes the relationship between the generation of the basalts and the extension of the region. The wide, shallow, terrestrial basin, formed by early extension, was subsequently cut through by narrow, fault-bounded grabens. The genesis of alkali basalt magmas began soon after the concentration of the extension into two grabens ˜2 Ma. The basalts, in the form of some 80 small cinder cones and associated lava flows and fields, have a total volume of 2.3 km3. This small volume reflects the small amount of extension in the region (β < 1.2 where β = final length of crust: initial length of crust).The intercalation of sediments and basalts in the Kula area enables the establishment of a relationship between the extensional activity and the generation of the basalts. This is demonstrable using radiometric and stratigraphic techniques, notably Ar-Ar dating of amphibole phenocrysts.

Ninella tarimensis from the Lower Cambrian Yurtus Formation of western Xinjiang, China, is shown to be a relative of the halkieriids and siphogonuchitids. These are known principally from dispersed sclerites, presumably secreted by slug-like organisms that were an abundant component of the Cambrian benthos. The sclerites of N. tarimensis are relatively small, strongly recurved and usually possess an upper surface ornamented with transverse sculpture. The lower surface is more or less smooth and is joined to the upper surface via steep faces that may be grooved. The apertural region is quadrate in outline and bears a circular foramen. Ninella is fairly widespread and is known from elsewhere in China (Guizhou and Shaanxi Provinces) and from Kazakhstan. A brief description of the siphogonuchitid scleritome is also presented, based on dispersed sclerites and shells from the Lower Cambrian of Shaanxi Province, China. Formal synonymy of the sciotaxa Siphogonuchites, Dabashanites, Lopochites, and Maikhanella as Siphogonuchites is proposed.

Diamictites, many of glacial origin, are globally distributed in the Neoproterozoic. Recently, two relatively thin diamictites in the Maikhan Uul Member at the base of the Neoproterozoic Tsagaan Oloom Formation from the Zavkhan Basin of western Mongolia have been identified as being of glacial origin. The Mongolian diamictites form a series of backstepping units within the transgressive systems tract of two major depositional sequences associated with sea-level changes. In each case the diamictites of the transgressive systems tract are abruptly overlain by deeper water, upward shoaling highstand systems tracts consisting of thinly bedded sandstones and shales in sequence 1 and thinly bedded, dark carbonates in sequence 3. The fact that the sequences conform closely to depositional models established at other localities suggests that all are related to major ice ages and that the depositional sequences they have generated provide a valuable tool for global correlation in this part of the stratigraphic column. Available stratigraphic and isotope geochemical information presented by Brasier et al. (1996, this issue) suggests that both diamictites are likely to be of Sturtian age. A riftogenic setting and Sturtian age for the diamictites provide a link with eastern Australia and western America. It is possible, therefore, that these diamictites formed during the breakup of a supercontinental assembly including Siberia, Australia and Laurentia c. 750–725 Ma BP.

The Okenyenya gabbro-syenite complex, one of a number of intrusive igneous complexes of late-Mesozoic age in northwestern Namibia, was emplaced at the time of opening of the South Atlantic Ocean. The 5-km-diameter complex comprises a wide variety of rock types that can be subdivided into two contrasting magmatic suites, one tholeiitic and the other alkaline, which were emplaced in close proximity over a time-span of ˜5 Ma. The tholeiitic suite of rocks includes picritic gabbro, olivine gabbro through quartz monzodiorite and syenite, whereas the alkaline suite includes alkaline gabbro, essexite, nepheline syenite and a range of lamprophyric rock types. Detailed petrographic, mineralogical and bulk rock geochemical data show that the earliest, saucer-shaped, intrusion of olivine gabbro-quartz monzodiorite rocks can be subdivided into an Inner Zone and an Outer Zone (each comprising three distinct intrusive units). The individual units can be readily distinguished on the basis of bulk rock geochemical variations, together with cryptic and modal mineralogical variations. An unusual feature of the intrusive body is that bulk rock and mineral compositions become more evolved with apparent depth, within the body as a whole and within each unit. Compositional variation within the individual intrusive units requires a complex interplay between in situ crystallization, variable expulsion of interstitial melt, magma recharge, and re-equilibration of primocrysts with trapped interstitial melt. Cross-cutting dykes of picritic gabbro (MgO= 13–21 %) have compositions consistent with olivine control. Incompatible trace element ratios (e.g. Zr/Nb= 12.5 ± 1.3) suggest that the picritic gabbro magmas were derived from a distinct source region compared to that giving rise to the tholeiitic olivine gabbros (Zr/Nb = 6.8 ± 1.1).

Alkaline gabbro occupies the central region of the complex and, on the basis of major, trace and rare earth element variations, can be subdivided into four distinct intrusive bodies, interpreted as remnant magma chambers, each having experienced variable degrees of crystal accumulation. In places, magma chamber processes have given rise to centimetre-scale rhythmic layering. Incompatible trace element ratios (e.g. Zr/Nb = 4.4 ± 1.2) serve to distinguish the source region of the alkaline gabbro magmas from those giving rise to the tholeiitic suite of magmas. Younger rocks of both the tholeiitic and alkaline suites show strong evidence of the effects of extensive crystal fractionation. The quartz syenite is characterized by a strong negative Eu anomaly indicative of substantial feldspar fractionation and also shows evidence for direct contamination by earlier gabbro, whereas the syenite shows evidence for feldspar accumulation. Both syenites have geochemical characteristics suggesting consanguinity with the Outer Zone rocks of the olivine gabbro-quartz monzodiorite intrusion. In contrast, the essexite and nepheline syenite compositions are qualitatively consistent with derivation from one of the alkaline gabbro magmas by extensive fractionation of plagioclase, clinopyroxene, olivine and amphibole. The final stage of magmatism is represented by a suite of alkaline and ultramafic lamprophyres emplaced as dykes and diatremes, the latter carrying a variety of megacrystic and xenolithic material, including mantle nodules. The alternation between tholeiitic and alkaline magmatism evident within the Okenyenya complex is similar to that characteristic of the evolution of many ocean island volcanoes.

Radiolichas has been a poorly known and exclusively Devonian genus of very limited diversity. Radiolichas raydaviesi sp. nov. and R. davedaviesi sp. nov. are the best-known members of the genus, and extend its stratigraphic and geographic range to the Wenlock of the central Canadian Arctic. A further Silurian occurrence is an unnamed species from the Wenlock Slite Beds of Gotland, Sweden. Radiolichas is most similar to Dicranogmus. Together, the genera lack any of the robust trochurine apomorphies (linear pygidial postaxial ridge, prominent pygidial border, prominent cranidial fossula) and appear to fall outside the phylogenetic structure of that subfamily. They are similar to members of the Tetralichinae, particularly in pygidial features, but lack the composite cranidial lobe characteristic of that subfamily. Their systematic position is presently unresolved.

Upper Palaeozoic conodonts are described for the first time from the North Greenland Wandel Sea Basin. In eastern Peary Land, the Moscovian species Idiognathodus incurvus and the Kasimovian—Gzhelian I. magnificus occur in the Upper Carboniferous Foldedal Formation, while an assemblage from the lower part of the succeeding Kim Fjelde Formation suggests deposition in the Upper Artinskian Neostreptognathodus pequopensis—N. clarki Zone. These datings confirm the existence in the northern part of the Wandel Sea Basin of the pronounced early Permian hiatus previously recognized in Holm Land and Amdrup Land in the southern part of the basin. The single conodont specimen found at Prinsesse Ingeborg Halvø further corrobates the local absence of this regional hiatus in the central part of the Wandel Sea Basin.

Upper Devonian strata are distributed widely throughout Iran. A review of all available literature on the Devonian, together with information from recent studies in the Kerman region, provides a basis for correlation and description of facies successions for the whole country. In most sections the local base of the Upper Devonian sits disconformably on older rocks, whilst the upper contact with the Carboniferous is almost everywhere transitional. There is everywhere an apparent hiatus spanning a probable early and mid Devonian interval. Lower and Middle Devonian fossils have not yet been discovered, but for the Upper Devonian there are abundant fossil data indicating the widespread occurrence of Frasnian and Famennian shallow marine deposits. A palaeogeographical map for the Upper Devonian of Iran is constructed.

The Okenyenya gabbro–syenite complex, one of a number of intrusive igneous complexes of late-Mesozoic age in northwestern Namibia, was emplaced at the time of opening of the South Atlantic Ocean. The 5-km-diameter complex comprises a wide variety of rock types that can be subdivided into two contrasting magmatic suites, one tholeiitic and the other alkaline, which were emplaced in close proximity over a time-span of ~5 Ma. The tholeiitic suite of rocks includes picritic gabbro, olivine gabbro through quartz monzodiorite and syenite, whereas the alkaline suite includes alkaline gabbro, essexite, nepheline syenite and a range of lamprophyric rock types. Detailed petrographic, mineralogical and bulk rock geochemical data show that the earliest, saucer-shaped, intrusion of olivine gabbro–quartz monzodiorite rocks can be subdivided into an Inner Zone and an Outer Zone (each comprising three distinct intrusive units). The individual units can be readily distinguished on the basis of bulk rock geochemical variations, together with cryptic and modal mineralogical variations. An unusual feature of the intrusive body is that bulk rock and mineral compositions become more evolved with apparent depth, within the body as a whole and within each unit. Compositional variation within the individual intrusive units requires a complex interplay between in situ crystallization, variable expulsion of interstitial melt, magma recharge, and re-equilibration of primocrysts with trapped interstitial melt. Cross-cutting dykes of picritic gabbro (MgO = 13–21 %) have compositions consistent with olivine control. Incompatible trace element ratios (e.g. Zr/Nb= 12.5 ± 1.3) suggest that the picritic gabbro magmas were derived from a distinct source region compared to that giving rise to the tholeiitic olivine gabbros (Zr/Nb = 6.8 ± 1.1).

Alkaline gabbro occupies the central region of the complex and, on the basis of major, trace and rare earth element variations, can be subdivided into four distinct intrusive bodies, interpreted as remnant magma chambers, each having experienced variable degrees of crystal accumulation. In places, magma chamber processes have given rise to centimetre-scale rhythmic layering. Incompatible trace element ratios (e.g. Zr/Nb = 4.4 ± 1.2) serve to distinguish the source region of the alkaline gabbro magmas from those giving rise to the tholeiitic suite of magmas. Younger rocks of both the tholeiitic and alkaline suites show strong evidence of the effects of extensive crystal fractionation. The quartz syenite is characterized by a strong negative Eu anomaly indicative of substantial feldspar fractionation and also shows evidence for direct contamination by earlier gabbro, whereas the syenite shows evidence for feldspar accumulation. Both syenites have geochemical characteristics suggesting consanguinity with the Outer Zone rocks of the olivine gabbro–quartz monzodiorite intrusion. In contrast, the essexite and nepheline syenite compositions are qualitatively consistent with derivation from one of the alkaline gabbro magmas by extensive fractionation of plagioclase, clinopyroxene, olivine and amphibole. The final stage of magmatism is represented by a suite of alkaline and ultramafic lamprophyres emplaced as dykes and diatremes, the latter carrying a variety of megacrystic and xenolithic material, including mantle nodules. The alternation between tholeiitic and alkaline magmatism evident within the Okenyenya complex is similar to that characteristic of the evolution of many ocean island volcanoes.