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The crust and upper mantle structure of the Greater and Lesser Antilles Arc provides insights into key subduction zone processes in a unique region of slow convergence of old slow-spreading oceanic lithosphere. We use ambient noise tomography gathered from island broadband seismic stations and the temporary ocean bottom seismometer network installed as part of the Volatile Recycling in the Lesser Antilles experiment to map crustal and upper mantle shear-wave velocity of the eastern Greater Antilles and the Lesser Antilles Arc. Taking the depth to the 2.0 km/s contour as a proxy, we find sediment thickness up to 15 km in the south in the Grenada and Tobago basins and thinner sediments near the arc and to the north. We observe thicker crust, based on the depth to the 4.0 km/s velocity contour, beneath the arc platforms with the greatest crustal thickness of around 30 km, likely related to crustal addition from arc volcanism through time. There are distinct low velocity zones (4.2–4.4 km/s) in the mantle wedge (30–50 km depth), beneath the Mona Passage, Guadeloupe-Martinique, and the Grenadines. The Mona passage mantle anomaly may be related to ongoing extension there, while the Guadeloupe-Martinique and Grenadine anomalies are likely related to fluid flux, upwelling, and/or partial melt related to nearby slab features. The location of the Guadeloupe-Martinique anomaly is slightly to the south of the obliquely subducted fracture zones. This feature could be explained by either three-dimensional mantle flow, a gap in the slab, variable slab hydration, and/or melt dynamics including ponding and interactions with the upper plate.

Deep Earth degassing is a critical forcing factor for atmospheric CO2 variations and palaeoclimate changes in Earth’s history. For the Cenozoic, the key driving mechanism of atmospheric CO2 variations remains controversial. Here we analyse three stages of collision-related magmatism in Tibet, which correspond temporally with the three major stages of atmospheric CO2 variations in the Cenozoic and explore the possibility of a causal link between these phenomena. To this end we present geochemical data for the three stages of magmatic rocks in Tibet, which we use to inform a model calculating the continental collision-induced CO2 emission flux associated with the evolving Neo-Tethyan to continental subduction over the Cenozoic. The correlation between our modelled CO2 emission rates and the global atmospheric CO2 curve is consistent with the hypothesis that the India-Asia collision was the primary driver of changes in atmospheric CO2 over the Cenozoic., “Earth degassing is a critical carbon source, but its contribution to Cenozoic atmospheric CO2 variations is not well known. Here, the authors analyse CO2 fluxes on the Tibetan Plateau and suggest that the India-Asia collision was the primary driver of changes in atmospheric CO2 over the past 65 Ma.”

Uplift of the Tibetan Plateau is thought to be one of the most important orogenic and climate forcing events of the Cenozoic Era, associated with geodynamic changes related to India-Asia collision and subsequent continental lithosphere subduction. However, the fate and scale of the subducted continental lithosphere segments remain highly controversial. Using a comprehensive compilation of the spatiotemporal distribution of postcollisional magmatic rocks across Tibet, together with new geochemical and Sr-Nd-Pb isotopic data and modeling simulations, we propose a holistic, two-stage evolutionary model to explain the link between genesis of the magmas and continental subduction. The magmatism prior to 25 Ma resulted from continuous upwelling of a carbonate-rich upper-mantle plume induced by northward underthrusting of Indian oceanic and continental lithosphere with its cover of Tethyan platform carbonate sediments, whereas magmatism after 25 Ma was related to opposing north-directed and south-directed continental subduction. Our model indicates a transformation in the distribution and nature of the magmatism in Tibet at ca. 25 Ma, which reflects a significant change in the Himalayan-Tibetan orogen and associated mantle dynamic processes in the early Miocene. Understanding this transformation could have important implications for the utility of the Himalayan-Tibetan system as a modern analogue for ancient orogens.

The 2·1 ka Moinui lava flow field, erupted from the southwest rift zone of Mauna Loa, Hawai`i, exhibits striking textural and geochemical variations, that can be used to interpret magma processes pre-, syn- and post-eruption. From this lava flow, the duration of magma storage and storage conditions, the timescales over which magma is transported to the surface, and flow emplacement mechanisms at Mauna Loa are determined. Electron microprobe analysis (EMPA) and diffusion chronometry of olivine crystals identify two distinct crystal populations: a primitive, polyhedral olivine population with core compositions of Fo90–88 and a more evolved, platy olivine population with core compositions of Fo83–82. Fe–Mg diffusion modelling of these olivine populations gives distinct timescales for each population; platy olivines yield timescales of days up to a few weeks, while polyhedral olivines yield timescales of months to years. Despite the nature of a well-insulated pāhoehoe flow, meaning that post-emplacement diffusion continues for some time, a wealth of time information can be retrieved concerning pre-eruptive magmatic processes as well as the processes associated with the lava extrusion. The short timescales obtained from the platy olivine crystals and the observed equilibrium between its cores and ambient melt suggest late-stage nucleation and crystal growth in the shallow conduit and during lava emplacement. Conversely, the longer timescales and olivine-melt disequilibrium of the polyhedral olivine crystals suggests accumulation from a deeper source and subsequent transportation to shallow magma storage beneath the summit of Mauna Loa months, or even years before eruption. The chemical and textural details of the Moinui lava reflect the mode of flow emplacement and may have implications for the interpretation of the distribution of spinifex and cumulate olivine within komatiites; high-temperature, low-viscosity lavas, common in the Archean.

We present a high‐resolution 2‐D P‐wave velocity model from a 225‐km‐long active seismic profile, collected over ~60–75 Ma central Atlantic crust. The profile crosses five ridge segments separated by a transform and three nontransform offsets. All ridge discontinuities share similar primary characteristics, independent of the offset. We identify two types of crustal segment. The first displays a classic two‐layer velocity structure with a high gradient Layer 2 (~0.9 s$^{−1}$) above a lower gradient Layer 3 (0.2 s$^{−1}$). Here, PmP coincides with the 7.5 km s$^{−1}$ contour, and velocity increases to >7.8 km s$^{−1}$ within 1 km below. We interpret these segments as magmatically robust, with PmP representing a petrological boundary between crust and mantle. The second has a reduced contrast in velocity gradient between the upper and lower crust and PmP shallower than the 7.5 km s$^{−1}$ contour. We interpret these segments as tectonically dominated, with PmP representing a serpentinized (alteration) front. While velocity‐depth profiles fit within previous envelopes for slow‐spreading crust, our results suggest that such generalizations give a misleading impression of uniformity. We estimate that the two crustal styles are present in equal proportions on the floor of the Atlantic. Within two tectonically dominated segments, we make the first wide‐angle seismic identifications of buried oceanic core complexes in mature (>20 Ma) Atlantic Ocean crust. They have a ~20‐km‐wide “domal” morphology with shallow basement and increased upper crustal velocities. We interpret their midcrustal seismic velocity inversions as alteration and rock‐type assemblage contrasts across crustal‐scale detachment faults.

Deep water cycle studies have largely focused on subduction of lithosphere formed at fast spreading ridges. However, oceanic plates are more likely to become hydrated as spreading rate decreases.

The Lesser Caucasus mountains sit on a transition within the Arabia-Eurasia collision zone between very thin lithosphere (< 100 km) to the west, under Eastern Anatolia, and a very thick lithospheric root (up to 200 km) in the east, under western Iran. A transect of volcanic highlands running from north-west to south-east in the Lesser Caucasus allows us to look at the effects of lithosphere thickness variations on the geochemistry of volcanic rocks in this continental collision zone. Volcanic rocks from across the region show a wide compositional range from basanites to rhyolites, and have arc-like geochemical characteristics, typified by ubiquitous negative Nb-Ta anomalies. Magmatic rocks from the south-east, where the lithosphere is thought to be thicker, are more enriched in incompatible trace elements, especially the light rare earth elements, Sr and P. They also have more radiogenic 87Sr/86Sr, and less radiogenic 143Nd/144Nd. Across the region, there is no correlation between SiO2 content and Sr-Nd isotope ratios, revealing a lack of crustal contamination. Instead, “spiky” mid-ocean-ridge basalt normalised trace element patterns are the result of derivation from a subduction-modified mantle source, which likely inherited its subduction component from subduction of the Tethys Ocean prior to the onset of continent-continent collision in the late Miocene. In addition to the more isotopically enriched mantle source, modelling of non-modal batch melting suggests lower degrees of melting and the involvement of garnet as a residual phase in the south-east. Melt thermobarometry calculations based on bulk-rock major elements confirm that melting in the south-east must occur at greater depths in the mantle. Temperatures of melting below 1200 °C, along with the subduction-modified source, suggest that melting occurred within the lithosphere. It is proposed that in the Northern Lesser Caucasus this melting occurs close to the base of the very thin lithosphere (at a depth of ∼45 km) as a result of small-scale delamination. A striking similarity between the conditions of melting in north-west Iran and the southern Lesser Caucasus (two regions between which the difference in lithosphere thickness is ∼ 100 km) suggests a common mechanism of melt generation in the mid-lithosphere (∼ 75 km). The southern Lesser Caucasus magmas result from mixing between partial melts of deep lithosphere (∼ 120 km in the south) and mid-lithosphere sources to give a composition intermediate between magmas from the northern Lesser Caucasus and north-west Iran. The mid-lithosphere magma source has a distinct composition compared to the base of the lithosphere, which is argued to be the result of the increased retention of metasomatic components in phases such as apatite and amphibole, which are stabilized by lower temperatures prior to magma generation.

Argon isotopic analyses were funded by NERC grant to I. Savov, IP-1690-1116. PJS was funded through a NERC studentship as part of the Leeds York Spheres Doctoral Training Partnership (DTP) (grant number NE/L002574/1). Part of the field work and research were supported by base funding of the Institute of Geological Sciences (IGS) and a thematic project by the Science Committee of the Armenian Ministry of Education and Science (project #18 T-1E368). The post-collisional Syunik and Vardenis volcanic highlands, located in the southern Lesser Caucasus mountains (part of the Arabia-Eurasia collision zone) are host to over 200 monogenetic volcanoes, as well as 2 large Quaternary polygenetic volcanoes in the Syunik highland. The latter are overlain by lavas from the monogenetic volcanoes, suggesting there was a transition in the style of volcanic activity from large-volume central vent eruptions to dispersed small-volume eruptions. 12 new high quality 40Ar/39Ar ages are presented here, with 11 ages calculated by step-heating experiments on groundmass separates, and the final age obtained from total fusions of a population of sanidines. All the ages were younger than 1.5 Ma, except for one ignimbrite deposit whose sanidines gave an age of 6 Ma. While the bulk of the exposed products of post-collisional volcanism relate to Pleistocene activity, it is clear there has been active volcanism in the region since at least the late Miocene. All ages for monogenetic volcanoes in the Syunik highland are younger than 1 Ma, but to the north in Vardenis there is geochronological evidence of monogenetic volcanism at 1.4 and 1.3 Ma. An age of 1.3 Ma is determined for a lava flow from one of the polygenetic volcanoes- Tskhouk, and when combined with other ages helps constrain the timing of the polygenetic to monogenetic transition to around 1 Ma. The new ages illustrate a degree of spatio-temporal coupling in the formation of new vents, which could be related to pull-apart basins focussing ascending magmas. This coupling means that future eruptions are particularly likely to occur close to the sites of the most recent Holocene activity. The polygenetic to monogenetic transition is argued to be the result of a decreasing magma supply based on: (i) volume estimates for Holocene eruptions and for all monogenetic volcanoes and their lava flows in Syunik; and (ii) the volcanic stratigraphy of the Lesser Caucasus region which shows late Pliocene- early Pleistocene continental flood basalts being succeeded by a few large andesite-dacite volcanoes and then the most recent deposits consisting of small-volume scoria cones. The Syunik highland has the highest density of monogenetic centres in the Lesser Caucasus, which is taken to indicate this region has the highest magma flux, and was therefore the last location to transition to monogenetic volcanism, which is why the transition is most clearly seen there. There is no evidence from Sr-Nd-B isotope measurements for the exhaustion of fusible slab components in the mantle source, showing that an inherited slab signature can survive for millions of years after the end of subduction. Although volcanism in the Lesser Caucasus is currently waning, a future pulse of activity is possible. Publisher PDF

This work was supported as part of Patrick Sugden's PhD studentship funded through the Leeds-York SPHERES NERC doctoral training partnership (NE/L002754/1). Part of the fieldwork and research was funded by the University of Leeds, the Carnegie Institution of Washington, the ERASMUS exchange programme (for IS) and the Science Committee of the Armenian Ministry of Education and Science (project #18T-1E368). The majority of the B isotope analytical work was supported by IGG-CNR funds P1600514. We present the first boron abundance and δ11B data for young (1.5-0 Ma) volcanic rocks formed in an active continent-continent collision zone. The δ11B of post-collisional volcanic rocks (−5 to +2‰) from the Armenian sector of the Arabia-Eurasia collision zone are heavier than mid-ocean ridge basalts (MORB), confirming trace element and isotope evidence for their derivation from a subduction-modified mantle source. Based on the low B/Nb (0.03-0.25 vs 0.2-90 in arc magmas), as well as low Ba/Th and Pb/Ce, this source records a subduction signature which is presently fluid-mobile element depleted relative to most arc settings. The heavier than MORB δ11B of post-collision volcanic rocks argues against derivation of their subduction signature from a stalled slab, which would be expected to produce a component with a lighter than MORB B, due to previous fluid depletion. Instead, the similarity of δ11B in Plio-Pleistocene post-collision to 41 Ma alkaline igneous rocks also from Armenia (and also presented in this study), suggests that the subduction signature is inherited from Mesozoic-Paleogene subduction of Neotethys oceanic slabs. The slab component is then stored in the mantle lithosphere in amphibole, which is consistent with the low [B] in both Armenian volcanic rocks and metasomatic amphibole in mantle xenoliths. Based on trace element and radiogenic isotope systematics, this slab component is thought to be dominated by sediment melts (or supercritical fluids). Previously published δ11B of metasediments suggests a sediment-derived metasomatic agent could produce the B isotope composition observed in Armenian volcanic rocks. The lack of evidence for aqueous fluids preserved over the 40 Myr since initial collision supports observations that this latter component is transitory, while the lifetime of sediment melts/supercritical fluids can be extended to >40 Myr. Publisher PDF

Several biotic crises during the past 300 million years have been linked to episodes of continental flood basalt volcanism, and in particular to the release of massive quantities of magmatic sulphur gas species. Flood basalt provinces were typically formed by numerous individual eruptions, each lasting years to decades. However, the environmental impact of these eruptions may have been limited by the occurrence of quiescent periods that lasted hundreds to thousands of years. Here we use a global aerosol model to quantify the sulphur-induced environmental effects of individual, decade-long flood basalt eruptions representative of the Columbia River Basalt Group, 16.5–14.5 million years ago, and the Deccan Traps, 65 million years ago. For a decade-long eruption of Deccan scale, we calculate a decadal-mean reduction in global surface temperature of 4.5 K, which would recover within 50 years after an eruption ceased unless climate feedbacks were very different in deep-time climates. Acid mists and fogs could have caused immediate damage to vegetation in some regions, but acid-sensitive land and marine ecosystems were well-buffered against volcanic sulphur deposition effects even during century-long eruptions. We conclude that magmatic sulphur from flood basalt eruptions would have caused a biotic crisis only if eruption frequencies and lava discharge rates had been high and sustained for several centuries at a time.

Post-collisional (25–8 Ma) ultrapotassic mafic magmatic rocks occur to the north of the India–Asia collision zone within the Lhasa terrane of the southern Tibetan Plateau, forming a near 1000 km long semi-continuous igneous belt. They include both extrusive and intrusive facies, although lava flows dominate. To understand their petrogenesis, the mineral chemistry of olivine phenocrysts and xenocrysts and whole-rock major and trace element and Sr–Nd–Pb isotope data are presented for the most primitive mafic magmatic rocks (MgO > 6 wt %) from west to east. The studied samples are characterized by high MgO (6·28–15·75 wt %), K2O (4·76–8·89 wt %), SiO2 (46·44–59·74 wt %), Ba (1368–14076 ppm), Th (69–336 ppm) and Ni (106–527 ppm) contents. Chondrite-normalized rare earth element (REE) patterns show enrichment in light rare earth elements (LREE), flat heavy REE (HREE) patterns and negative Eu anomalies. These REE patterns have a very distinctive inverted ‘spoon shape’, which appears to be a common characteristic of collision-related ultrapotassic magmas. Primitive mantle-normalized incompatible trace element patterns exhibit strong enrichments in large ion lithophile elements (LILE) relative to high field strength elements (HFSE) and strong negative Ta–Nb–Ti anomalies, which are typical of subduction-related magmas. The ultrapotassic magmatic rocks studied have extremely radiogenic initial Sr isotopic compositions (0·712379–0·737616) and low (143Nd/144Nd)i (0·511662–0·511984). Combined with their Pb isotope compositions [(206Pb/204Pb)i = 18·30–18·92; (207Pb/204Pb)i = 15·65–15·87; (208Pb/204Pb)i = 39·02–39·76] these data are consistent with the involvement of a subducted continental crustal component in their petrogenesis. The Sr–Nd–Pb isotope compositions exhibit linear trends between depleted mid-ocean ridge basalt (MORB)-source mantle (DMM) and Indian continental crust. The extreme enrichment of the upper mantle below south Tibet is considered to result from the addition of components derived from subducted Indian continental crust to the overlying mantle wedge during northward underthrusting of Indian continental lithosphere beneath the Lhasa terrane since India–Asia collision at ∼55 Ma. The post-collisional K-rich mafic magmas in south Tibet were generated by partial melting of pyroxenite in a mantle source region that was created by reaction of hydrous fluids and siliceous melts from subducted granulite–eclogite-facies Indian continental crustal rocks with the surrounding peridotitic mantle. A continuous process from slab roll-back, through break-off, to detachment of the slab may have induced partial melting of the pyroxenites. Cessation of the post-collisional ultrapotassic magmatism at ∼8 Ma may be linked to the onset of flat slab subduction beneath southern Tibet and the elimination of the wedge of Tibetan subcontinental lithospheric mantle and underlying asthenosphere; geophysical data indicate that at the present day eclogite-facies Indian continental crust directly underthrusts the crust of the Lhasa terrane with no intervening mantle wedge. The proportion of the Indian continental crustal component in the mantle source of the ultrapotassic mafic magmas decreases eastward, as do the ages and volumes of the magmatic rocks. There are no outcrops of post-collisional K-rich mafic magmatic rocks (MgO > 6 wt %) to the east of 87°E in the Lhasa terrane, which may indicate a change in subduction geometry at this longitude.

The last Plinian-type eruption of Volcán de Colima, Mexico, occurred in 1913; this resulted in the removal of the top 100 m of the edifice and the deposition of a tephra layer that blanketed the slopes of the Colima Volcanic Complex (CVC). Road-cuts on the flanks of the nearby Nevado de Colima edifice expose pre-1913 air-fall tephra, pyroclastic flow and ash-rich surge deposits resulting from numerous highly explosive events throughout the Holocene. The majority of the pumice and scoria fallout deposits are medium-K subalkaline basaltic andesite and andesite in composition, defining a clear major element differentiation trend. In contrast, three newly discovered scoria fallout deposits are high-K subalkaline, transitional to alkaline, basaltic andesite in composition and are characterized by the presence of phlogopite; these deposits have high MgO (up to 7·9 wt %), K2O (up to 2·6 wt %) and P2O5 (up to 0·67 wt %) contents. They are also strongly enriched in fluid- and melt-mobile large ion lithophile elements (LILE; Rb, Ba, K, Sr and Th) and light REE (LREE; La, Ce, Pr and Nd) relative to the majority of the Colima tephra fallout deposits. Strontium and Nd isotope systematics reveal that the high-K mafic scoria have more radiogenic Sr (87Sr/86Sr = 0·70365–0·70408) and less radiogenic Nd (143Nd/144Nd = 0·51279–0·51294) compared with the majority of the subalkaline tephras (87Sr/86Sr = 0·70338–0·70371 and 143Nd/144Nd = 0·51290–0·51295). Two-component mixing models, using whole-rock geochemical data, indicate the importance of magma mixing in the petrogenesis of the Colima magmas, with addition of up to 50% by volume of an alkaline mafic magma component in the most potassic magmas. This is supported by mineral chemistry and textural data, which reveal multiple episodes of decompression and magma mingling within a shallow crustal magma storage region. The presence of these potassic tephra fall deposits among the otherwise prevailing medium-K subalkaline stratigraphy indicates that pulses of K-rich alkaline mafic magmas periodically enter the CVC plumbing system on timescales of a few thousand years and may trigger Plinian explosive eruptions.

article i nfo Post-collisional potassium-rich mafic magmatism occurred in the northwestern part of the Tibetan Plateau, close tothewesternsyntaxisoftheHimalayanorogen,from8.3Matothepresent.Thismagmatismisassociatedwitha

Post-collisional (23–8 Ma), potassium-rich (including ultrapotassic and potassic) mafic magmatic rocks occur within the north–south-trending Xuruco lake–Dangre Yongcuo lake (XDY) rift in the Lhasa terrane of the southern Tibetan Plateau, forming an approximately 130-km-long semi-continuous magmatic belt. They include both extrusive and intrusive facies. Major and trace element and Sr–Nd–Pb isotopic data are presented for all of the known exposures within the XDY rift. The potassium-rich, mafic igneous rocks are characterized by high MgO (5.9–10.8 wt.%), K2O (4.81–10.68 wt.%), Ba (1,782–5,618 ppm) and Th (81.3–327.4 ppm) contents, and relatively high SiO2 (52.76–58.32 wt.%) and Al2O3 (11.10–13.67 wt.%). Initial Sr isotopic compositions are extremely radiogenic (0.712600–0.736157), combined with low (206Pb/204Pb) i (18.28–18.96) and (143Nd/144Nd) i (0.511781–0.512046). Chondrite-normalized rare earth element patterns display relatively weak negative Eu anomalies. Primitive mantle-normalized incompatible trace element patterns exhibit strong enrichments in large ion lithophile elements relative to high-field-strength elements and display strongly negative Ta–Nb–Ti anomalies. The combined major and trace element and Sr–Nd–Pb isotopic characteristics of the K-rich igneous rocks suggest that the primitive magmas were produced by 1–10 % partial melting of an asthenospheric mantle source enriched by both fluids and partial melts derived from Indian passive continental margin sediments subducted into the shallow mantle as a consequence of the northward underthrusting of the Indian continental lithosphere beneath Tibet since the India–Asia collision at ~55 Ma. The best-fit model results indicate that a melt with trace element characteristics similar to those of the K-rich rocks could be generated by 8–10 % partial melting of a metasomatized mantle source in the south and 1–2 % melting in the north of the XDY rift. Trace element and Sr–Nd–Pb isotopic modeling indicate that the proportion of fluid derived from the subducted sediments, for which we use as a proxy the Higher Himalayan Crystalline Sequence (HHCS), in the mantle source region increases from north (rear-arc) to south (front-arc), ranging from 0 to 5 %, respectively. Correspondingly, the proportion of the melt derived from the subducted HHCS in the source increases from north (2 %) to south (15 %). The increasing proportion of the fluid and melt component in the mantle source from north to south, together with a southward decreasing trend in the age of the K-rich magmatism within the XDY rift, is inferred to reflect rollback of the subducted Indian lithospheric mantle slab during the period 25–8 Ma. Slab rollback may be linked to a decreasing convergence rate between India and Asia. As a consequence of slab rollback at 25 Ma beneath the Lhasa terrane, its geodynamic setting was transformed from a convergent (55–25 Ma) to an extensional (25–8 Ma) regime. The occurrence of K-rich magmatism during the period 25–8 Ma is a consequence of the decompression melting of an enriched mantle source, which may signal the onset of extension in the southern Tibetan Plateau and provide a petrological record of the extension process.

article i nfo Late Oligocene-Miocene leucogranites within southern Tibet form part of an extensive intrusive igneous province within the Himalayan orogen. The main rock types are tourmaline leucogranites (Tg) and two-mica leucogranites (2 mg). They have high SiO2 (70.56-75.32 wt.%), Al2O3 (13.55-15.67 wt.%) and ( 87 Sr/ 86 Sr)i (0.724001-0.797297), and low MgO (0.02-0.46 wt.%) and ( 143 Nd/ 144 Nd)i (0.511693-0.511906). Chondri- te-normalized rare earth element (REE) patterns display strong negative Eu anomalies. Whole-rock major and trace element and Sr-Nd isotope data for the leucogranites suggest that their source region was a two-component mixture between a fluid derived from the Lesser Himalayan (LH) crustal sequence and the bulk crust of the Higher Himalayan (HH) sequence. Trace element and Sr-Nd isotope modeling indicate that the proportion of fluid derived from the LH sequence varied from 2% to 19% and the resulting metasomatised source experienced 7-16% melting. The amount of fluid derived from the LH sequence increases from north to south. Northward underthrusting of the Indian continent resulted in infiltration of the LH-derived fluid into the overlying HH sequence. Subsequent decompression melting of this metasomatised crust, mostly during the Miocene (25-9 Ma), generated the leucogranites. This may be linked to steepening of the subducted slab of Indian lithosphere beneath the orogenic belt.

We analyze the first-order observations, basic concepts and explicit/implicit assumptions built into the three major hypotheses for the enriched component(s) in the source of ocean island basalts (OIB) in terms of incompatible trace elements: (1) subducted ocean crust (SOC), (2) subducted continental sediments, and (3) mantle metasomatism. SOC is compositionally too depleted (i.e., [La/Sm] N

Volcanic rocks erupted at Mt Pelée (Martinique; central Lesser Antilles) and the Quill volcano (Statia; northern Lesser Antilles) define distinct differentiation trends, each of which can be accounted for largely by fractional crystallization of plagioclase, amphibole and Fe–Ti oxides. This assemblage is seen commonly in associated cumulate nodules, although the petrography of the lavas is pyroxene + plagioclase + Fe–Ti oxides. Thus differentiation is controlled, in part, by cryptic amphibole fractionation. At a given degree of differentiation incompatible trace element abundances tend to be higher at Mt Pelée, and REE patterns are more fractionated than is the case at the Quill. Isotopic ratios of Sr and Pb correlate with indices of differentiation (e.g. wt % SiO2) at both volcanoes (more convincingly at Mt Pelée), indicating that differentiation is also an open-system process. When the differentiation trends for the two volcanoes are compared they do not converge towards a single parental magma composition, suggesting that the primary magmas for the Quill and Mt Pelée are different. This difference is most probably due to mantle source variations. The source of the Mt Pelée magmas appears to be more enriched in incompatible elements, consistent with a greater proportion of admixed subducted sediment. This observation is in agreement with previous studies that have documented an increasing sediment contribution southwards along the arc. However, comparison with available data for other volcanoes along the arc does not reveal a consistent along-arc trend, suggesting that a model of sediment–source mixing is oversimplified and that additional factors such as variable fluid contributions from the subducted slab may be important.

Based on an evaluation of major and trace element data for ocean island basalts (OIB), we demonstrate that oceanic lithosphere thickness variation, which we refer to as the lid effect, exerts the primary control on OIB geochemistry on a global scale. The lid effect caps the final depth (pressure) of melting or melt equilibration. OIB erupted on thick lithosphere have geochemical characteristics consistent with a low extent and high pressure of partial melting, whereas those erupted on thin lithosphere exhibit the reverse; that is, a high extent and low pressure of melting cessation. This observation requires that mantle melting beneath intra-plate volcanic islands takes place in the asthenosphere and results from dynamic upwelling and decompression. Melting beneath all ocean islands begins in the garnet peridotite facies, imparting the familiar 'garnet signature' to all OIB melts (e.g. [Sm/Yb](N) > 1); however, the intensity of this signature decreases with increasing extent of melting beneath thinner lithospheric lids as a result of dilution. The dilution effect is also recorded in the radiogenic isotope composition of OIB, consistent with the notion that their mantle source regions are heterogeneous with an enriched component of lower solidus temperature dispersed in a more refractory matrix. High-quality data on the compositions of olivine phenocrysts from mid-ocean ridge basalt and global OIB sample suites are wholly consistent with the lid effect without the need to invoke olivine-free pyroxenite as a major source component for OIB. Caution is necessary when using basalt-based thermobarometry approaches to estimate mantle potential temperatures and solidus depth because OIB do not unequivocally record such information. For plate ages up to similar to 80 Ma, we demonstrate that the geophysically defined base of the growing oceanic lithosphere corresponds to both an isotherm (similar to 1100 degrees C) and the pargasite (amphibole) dehydration solidus of fertile mantle peridotite. As pargasite in H2O-CO2-bearing mantle peridotite is stable under conditions of T 1100 degrees C and P 3 GPa (similar to 90 km), this solidus is essentially isothermal (i.e. dT/dP similar to 0 in P-T space) with T similar to 1100 degrees C) at depths 90 km, but becomes isobaric (i.e. dP/dT similar to 0 in P-T space) at the similar to 90 km depth. The latter explains why older (> 70 Ma) oceanic lithosphere cannot be thicker than similar to 90 km without the need to invoke physically complex processes such as convective removal.

The 1783–1784 AD Laki flood lava eruption commenced on 8 June 1783 and released 122 Tg of sulphur dioxide gas over the course of 8 months into the upper troposphere and lower stratosphere above Iceland. Previous studies have examined the impact of the Laki eruption on sulphate aerosol and climate using general circulation models. Here, we study the impact on aerosol microphysical processes, including the nucleation of new particles and their growth to cloud condensation nuclei (CCN) using a comprehensive Global Model of Aerosol Processes (GLOMAP). Total particle concentrations in the free troposphere increase by a factor ~16 over large parts of the Northern Hemisphere in the 3 months following the onset of the eruption. Particle concentrations in the boundary layer increase by a factor 2 to 5 in regions as far away as North America, the Middle East and Asia due to long-range transport of nucleated particles. CCN concentrations (at 0.22% supersaturation) increase by a factor 65 in the upper troposphere with maximum changes in 3-month zonal mean concentrations of ~1400 cm−3 at high northern latitudes. 3-month zonal mean CCN concentrations in the boundary layer at the latitude of the eruption increase by up to a factor 26, and averaged over the Northern Hemisphere, the eruption caused a factor 4 increase in CCN concentrations at low-level cloud altitude. The simulations show that the Laki eruption would have completely dominated as a source of CCN in the pre-industrial atmosphere. The model also suggests an impact of the eruption in the Southern Hemisphere, where CCN concentrations are increased by up to a factor 1.4 at 20° S. Our model simulations suggest that the impact of an equivalent wintertime eruption on upper tropospheric CCN concentrations is only about one-third of that of a summertime eruption. The simulations show that the microphysical processes leading to the growth of particles to CCN sizes are fundamentally different after an eruption when compared to the unperturbed atmosphere, underlining the importance of using a fully coupled microphysics model when studying long-lasting, high-latitude eruptions.

Cluster analysis, a classification technique used to group data in many fields, is developed here as a tool to study magma mixing and mixed crystal populations in volcanic rocks. The method is based on the quantification of the chemical degree of similarity among populations of mineral chemistry data, which allows identification of discrete clusters. In order to apply the technique for the particular problem of mixed crystal populations, the mineral chemistry of a given crystalline phase is represented by a vector with “n” coordinates, in which each coordinate is a real number that represents the amount of a given component in cations per formula unit present in the phase. These vectors are in a set, which is a subset of Rn, the real vector space of n dimensions. Because mineral chemistry data are a particular case of compositional data (i.e. the components sum to a constant value, usually 100% or the numbers of cations per formula unit), the conventional Euclidean distance cannot be used to quantify how similar the data are, in order to apply cluster analysis. To avoid this predicament, Aitchison's metric is proposed to measure similarities instead. Here, average linkage, a hierarchical clustering technique, combined with the Aitchison metric and stoichiometrical constraints, is applied to mineral chemistry data. This approach is evaluated using well-characterized lava samples from the Vancori period of activity (26–13.8 ky) of Stromboli volcano, Italy, in which magma mixing has been identified between a basaltic andesite–latite, hosted in the magma chamber and a less evolved basaltic recharge magma. The results are in agreement with previous interpretations of magma mixing, which validates the use of the cluster analysis technique in the context of magma mixing relationships, and opens the possibility to expand this methodology to other aspects of igneous petrology.

Post-collisional (26.2 to 10.1 Ma) adakites occur within the Lhasa terrane of the southern Tibetan Plateau in an E–W trending, 1500 km long, magmatic belt. Outcrops are small and restricted within N–S-trending rift zones (grabens); they include both extrusive and intrusive facies. The adakites have high SiO2 (59–70 wt.%), Al2O3 (15–18 wt.%) and Sr (317–1133 ppm) contents and Sr/Y ratios (44–162), and low Y (4.2–12.9 ppm) and HREE (e.g. Ybb0.9 ppm) concentrations. Their MORB-normalised incompatible element patterns exhibit strong enrichments in large ion lithophile elements (LILE) relative to high field strength elements (HFSE). The combined trace element and Sr–Nd–Pb isotope characteristics of the adakites suggest that their source was mafic-intermediate lower crust formed during a preceding stage (153–40 Ma) of active continental margin magmatism. Lower crustal melting was primarily induced by the conduction of heat from contemporaneous potassic–ultrapotassic magmas produced by partial melting in the asthenosphere or lower lithosphere. Trace element modelling calculations suggest that the adakites are the products of 5–10% partial melting of garnet-bearing amphibolite facies meta-igneous rocks. Differences in the LILE contents and Sr–Nd isotope compositions of adakites sampled to the east and west of 86° E can be attributed to variable degrees of partial melting of the lower crust and mixing between potassic–ultrapotassic magmas and lower crustal melts. Extensional collapse of the Tibetan Plateau may have contributed to partial melting of the lower crust and the formation of the potassic–ultrapotassic magmatism by decompression melting of a thin asthenospheric mantle wedge above a subducted slab of Indian continental margin lithosphere. The oldest age of the post-collisional adakites and contemporaneous potassic–ultrapotassic magmatism in the Lhasa terrane may, therefore, provide constraints on the timing of initiation of tectonic collapse in the southern part of the plateau.

Cenozoic, post-collisional, potassic and ultrapotassic igneous rocks in the North Qiangtang, Songpan–Ganzi and North Kunlun terranes of the northern Tibetan Plateau are distributed along a semi-continuous, east–west-trending, volcanic belt, which is over 1200 km in length. Spatially, there is a close association with major strike-slip faults, thrust faults and pull-apart basins. The ages of these magmatic rocks range from 45 Ma to the present (the youngest known eruption occurred in 1951); they are shoshonitic, compositionally similar to K-rich subduction-related magmas, and range in SiO 2 from 44 to 66 wt %. There is a relative enrichment of large ion lithophile elements (LILE) and light rare earth elements (LREE) in the most primitive magmatic rocks (MgO >6 wt %) in the North Qiangtang terrane compared with those in the Songpan–Ganzi and North Kunlun terranes; correspondingly, the primitive magmas have higher 87 Sr/ 86 Sr and 206 Pb/ 204 Pb, and lower 143 Nd/ 144 Nd ratios in the North Qiangtang terrane than in the Songpan–Ganzi and North Kunlun terranes. The dominant factors that control the geochemical characteristics of the magmas are an enriched asthenospheric mantle source composition, the degree of partial melting of this source, and the combined processes of crustal assimilation and fractional crystallization (AFC). Enrichment of the asthenosphere is considered to have occurred by incorporation of subducted sediments into the mantle wedge above a subducted slab of Indian lithosphere during India–Asia convergence. Continental lithospheric mantle, metasomatically enriched during earlier episodes of subduction, may have also contributed a source component to the magmas. Trace element modelling indicates that the mantle source of the most primitive magmas in the North Qiangtang terrane contained higher amounts of subducted sediment (0·5–10%) compared with those in the Songpan–Ganzi and North Kunlun terranes (

The (Mid-) Late Devonian to Early Carboniferous was a time of widespread rifting on the East European Craton (EEC) and its margins. The most prominent basin among these and, accordingly, the best documented is the Dniepr-Donets Basin (DDB) in Ukraine and southern Russia. The DDB is associated with voluminous rift-related magmatism and broad basement uplift. Two other large, extensional, basin systems developed along the margins of the EEC at the same time: the East Barents Basin (EEB) and its onshore prolongation the Timan-Pechora Basin (TPB), and the Peri-Caspian Basin (PCB). Rifting, associated magmatism, and possible domal basement uplift are also reported elsewhere within the EEC, suggesting a common, 'active', rifting process, involving a cluster of thermal instabilities (or generalized thermal instability) at the base of the lithosphere beneath widely separated parts of the EEC by Mid-Late Devonian times. The DDB is an intracratonic rift basin, cutting across the Archaean-Palaeoproterozoic structural grain of its basement and, as such, differs from the EBB-TPB and PCB, which are pericratonic rift basins developed on reworked and juvenile crystalline basement accreted to the EEC during the Neoproterozoic. The DDB opened into a deep basin, possibly having oceanic lithospheric affinity, to the SE, in the area where it adjoins the southern PCB, suggesting the possibility that rifting led to (limited?) continental break-up in this area at this time. Post-rift compressional tectonic reactivations and basin inversion in the DDB, leading to the formation of its prominent Donbas Foldbelt segment, are related to Tethyan events (Cimmerian and Alpine orogenies) occurring on the nearby southern margin of the EEC. Post-rift compressional inversions in the PCB and TPB, which lie closer to the Urals margin of the EEC, are related to Uralian tectonics. © The Geological Society of London 2006.

Anorogenic intra-plate magmatism was widespread in Europe from early Tertiary to Recent times, extending west to east from Spain to Bulgaria, and south to north from Sicily to central Germany. Magmatism is spatially and temporally associated with Alpine-Pyrenean collisional tectonics, the development of an extensive lithospheric rift system in the northern foreland of the Alps, and, locally, with uplift of Variscan basement massifs (Massif Central, Rhenish Massif, Bohemian Massif). The volcanic regions vary in volume from large central volcanoes (e.g. Cantal, Massif Central;Vogelsberg, central Germany), to small isolated plugs (e.g. Urach and Hegau provinces in southern Germany). Within the Mediterranean region, the Dinarides, the Pannonian Basin and Bulgaria, anorogenic volcanism locally post-dates an earlier phase of subduction-related magmatism. The major and trace element and Sr-Nd-Pb isotope characteristics of the most primitive mafic magmatic rocks (MgO > 6 wt%) provide important constraints on the nature of the mantle source and the conditions of partial melting. These are predominantly sodic (melilitites, nephelinites, basanites and alkali olivine basalts); however, locally, potassic magma types (olivine leucitites, leucite nephelinites) also occur. In several localities (e.g. Sicily; Vogelsberg and the Rhine Graben, Germany; Calatrava, central Spain) olivine and quartz tholeiites form a significant component of the magmatism. The sodic magmas were derived by variable degrees of partial melting (c. 0.5-5%) within a transitional zone between garnet-peridotite and spinel-peridotite mantle facies, close to the base of the lithosphere; the potassic magma types are interpreted as partial melts of enriched domains within the lithospheric mantle. Mantle partial melting was induced by adiabatic decompression of the asthenosphere, locally in small-scale, plume-like, diapirs, which appear to upwell from c. 400 km depth

Stratigraphic changes in bulk-rock and mineral chemistry within a volcanic sequence can be used to understand the evolution of the magma storage system beneath the volcano; in particular, changing eruptive styles and major changes in magma chemistry can be linked to open- or closed-system behaviour. Changes in phenocryst mineral chemistry, often expressed as core–rim zonation patterns or the development of reaction rims, combined with thermodynamic analysis of appropriate mineral equilibria, can provide powerful insights into the timing of inputs of new, more primitive magma. The Vancori period (26–13.8 ky) of the eruptive activity of Stromboli has a relatively simple stratigraphy, comprising three sub-periods, Lower, Middle and Upper Vancori, each one beginning and ending with an explosive volcanic event, marked by breccias. Electron microprobe analyses of phenocryst and groundmass minerals (olivine, clinopyroxene, plagioclase, Fe–Ti oxides) in representative samples from the Lower, Middle and Upper Vancori phases of activity allow tracking of the evolution of the magma chamber system with time. Thermodynamic analysis of olivine–clinopyroxene mineral equilibria is used to constrain changes in temperature within the magmatic system. The main stratigraphic discontinuities within the Vancori sequence are clearly reflected by changes in bulk-rock and mineral chemistry, and evidence of textural and thermodynamic disequilibrium (calculated temperatures) in the eruptive products. On the basis of these data the Lower and Middle Vancori periods of the eruptive history of Stromboli appear to be characterised by an open magma chamber system repeatedly re-filled with basaltic magma, which was continuously being erupted. At the end of the Middle Vancori period the system became closed, corresponding to a period of dormancy of the volcano, and fractional crystallisation dominated. Following a major flank collapse of the volcanic edifice, the Upper Vancori eruptive products show the most extreme range in bulk-rock chemistry as the chamber was progressively flushed with new basaltic magma. D 2005 Elsevier B.V. All rights reserved.

We present an analysis of a unique 3D survey that allows us to relate the deep structure of the crystalline crust to the shallow structure of the overlying, potentially hydrocarbon-rich sedimentary basins. The survey is located over the Gjallar Ridge, Mid-Norway, and extends from a Moho-level reflector at around 15 km depth to polygonal faulting and diapiric structures at or near the seabed. 3D visualization techniques using seismic workstations and the Cave immersive environment have been used to illustrate the geometries of these features. The deep reflector is correlated with the top of a deep, high-density, high-velocity body that is interpreted to indicate the presence of magmatic underplating and is intimately related to localized uplift of the Gjallar Ridge. Abundant high-amplitude reflectors in the deep Cretaceous sections of the survey are interpreted as sills emplaced during the Palaeocene magmatic event and are therefore interpreted to be coeval with the magmatic underplate. In contrast, the shallow parts of the survey have numerous gas-charged mud diapirs and an extensive network of polygonal faults extending to the seabed. Study of such very deep or very shallow features is not standard industry practice. However, the intention here is to demonstrate that, by utilizing the full volume of 3D seismic data, it is not only of scientific interest but also results in a greater understanding of the tectonic history of a hydrocarbon prospect.

An extensive rift system developed within the northern foreland of the Variscan orogenic belt during Late Carboniferous-Early Permian times, post-dating the Devonian-Early Carboniferous accretion of various Neoproterozoic Gondwana-derived terranes on to the southern margin of Laurussia (Laurentia-Baltica; Fig. 1). Rifting was associated with widespread magmatism and with a fundamental change, at the Westphalian-Stephanian boundary, in the regional stress field affecting western and central Europe (Ziegler 1990; Ziegler & Cloetingh 2003). The change in regional stress patterns was coincident with the termination of orogenic activity in the Variscan fold belt, followed by major dextral translation between North Africa and Europe. Rifting propagated across a collage of basement terranes with different ages and thermal histories. Whilst most of the Carboniferous-Permian rift basins of NW Europe developed on relatively thin lithosphere, the highly magmatic Oslo Graben in southern Norway initiated within the thick, stable and, presumably, strong (cold) lithosphere of the Fennoscandian craton. The rift basins in the North Sea, in contrast, developed in younger Caledonian age lithosphere, which was both thinner and warmer than the lithosphere of the craton to the east.

Early Cretaceous (146–115 Ma) magmatism in the region of Mt. Hermon, Northern Israel, is part of an extensive Mesozoic igneous province within the Levant associated with the evolution of the Neotethyan passive margin of Gondwana. The initial stages of activity were characterised by the emplacement of tholeiitic dykes (146–140 Ma) which were uplifted and eroded prior to the eruption of a sequence of alkali basalts, basanites and more differentiated alkaline lavas and pyroclastics from 127 to 120 Ma. The latest stages of activity (120–115 Ma) were highly explosive, resulting in the emplacement of diatreme breccias. Trace element and Sr-Nd-Pb isotope data for the most primitive Early Cretaceous mafic igneous rocks sampled suggest that they were derived by mixing of melts derived by variable degrees of partial melting of both garnet- and spinel-peridotite-facies mantle sources. Though isotopically heterogeneous, the source of the magmas has many similarities to that of HIMU oceanic island basalts. Earlier Liassic (200 Ma) transitional basalts and Neogene–Quaternary (15–0 Ma) alkali basalts erupted within northern Israel also have HIMU affinities. The petrogenesis of the Early Cretaceous and Cenozoic basalts is explained by partial melting of a lithospheric mantle protolith metasomatically enriched during the Liassic volcanic phase, which may be plume-related.

The mineralogy and mineral chemistry of olivine melilitites from Rhine-graben the Late Tertiary Urach and Hegau volcanic provinces of SW Germany, and from Mahlberg Castle, north of the Kaiserstuhl carbonatite complex within the Upper Rhinegraben, provide important constraints on the petrogenesis of these rather rare magma

Detailed field mapping in the Guvem area in the Galatia province of NW Central Anatolia, Turkey, combined with K–Ar dating, has established the existence of two discrete Miocene volcanic phases, separated by a major unconformity. The magmas were erupted in a post-collisional tectonic setting and it is possible that the younger phase could be geodynamically linked to the onset of transtensional tectonics along the North Anatolian Fault zone. The Early Miocene phase (18–20 Ma; Burdigalian) is the most voluminous, comprising of over 1500 m of potassium-rich intermediate-acid magmas. In contrast, the Late Miocene volcanic phase (ca. 10 Ma; Tortonian) comprises a single 70-m-thick flow unit of alkali basalt. The major and trace element and Sr–Nd isotope compositions of the volcanics suggest that the Late Miocene basalts and the parental mafic magmas to the Early Miocene series were derived from different mantle sources. Despite showing some similarities to high-K calc-alkaline magma series from active continental margins, the Early Miocene volcanics are clearly alkaline with higher abundances of high field strength elements (Zr, Nb, Ti, Y). Crustal contamination appears to have enhanced the effects of crystal fractionation in the petrogensis of this series and some of the most silica-rich magmas may be crustal melts. The mantle source of the most primitive mafic magmas is considered to have been an asthenospheric mantle wedge modified by crustally-derived fluids rising from a Late Cretaceous–Early Tertiary Tethyan subduction zone dipping northwards beneath the Galatia province. The Late Miocene basalts, whilst still alkaline, have a Sr–Nd isotope composition indicating partial melting of a more depleted mantle source component, which most likely represents the average composition of the asthenosphere beneath the region.

Pb isotope compositions of acid-leached clinopyroxene and amphibole mineral separates from spinel peridotite mantle xenoliths entrained in Tertiary-Quaternary alkali basalts from the Carpathian-Pannonian Region of eastern Europe provide important constraints on the processes of metasomatic enrichment of the mantle lithosphere in an extensional tectonic setting associated with recent subduction. Principal component analysis of Pb-Sr-Nd isotope and rare earth element compositions of the pyroxenes is used to identify the geochemical characteristics of the original lithospheric mantle protolith and a spectrum of infiltrating metasomatic agents including subduction-related aqueous fluids and silicate melts derived from a subduction-modified mantle wedge which contains a St. Helena-type (HIMU) plume component. The mantle protolith is highly depleted relative to mid-ocean ridge basalt-source mantle with Pb-Nd-Sr isotope compositions consistent with an ancient depletion event. Silicate melt infiltration into the protolith accounts for the primary variance in the Pb-Sr-Nd isotope compositions of the xenoliths and has locally generated metasomatic amphibole. Infiltration of aqueous fluids has introduced radiogenic Pb and Sr without significantly perturbing the rare earth element signature of the protolith. The Pb isotope compositions of the fluid-modified xenoliths suggest that they reacted with aqueous fluids released from a subduction zone which had equilibrated with sediment derived from an ancient basement terrain. We propose a model for mantle lithosphere evolution consistent with available textural and geochemical data for the xenolith population. The Pb-Sr-Nd isotope compositions of both alkaline mafic magmas and rare, subduction-related, calc-alkaline basaltic andesites from the region provide important constraints for the nature of the asthenospheric mantle wedge and confirm the presence of a HIMU plume component. These silicate melts contribute to the metasomatism of the mantle lithosphere rather than being derived therefrom.

The Adamawa volcanic uplift in central Cameroon forms the eastward termination of the Cameroon Volcanic Line(CVL) in West-Central Africa. This line is unique among intra-plate volcanic provinces in that it straddles a continental margin and has both oceanic and continental volcanic centres. The age of these volcanoes ranges from Tertiary to Recent and there is no apparent age progression along the length of the line. The Adamawa plateau is characterised by altitudes ranging from 800 m in its northern part to 1200 m in its southern part. The uplift is characterised by a long-wavelength negative Bouguer anomaly similar in shape and amplitude to those of other African basement uplifts. A gravity profile derived from the Bouguer gravity map of Cameroon, perpendicular to the anomaly associated with the uplift shows a broad negative and an axial positive Bouguer anomaly. This profile is used as a basis for a 2D gravity modelling. The broad negative and central positive anomalies beneath the Adamawa uplift are interpreted as a consequence of lithospheric thinning (40 km) and crustal thinning (10 km), respectively. Compared to the Kenya dome, the Adamawa uplift may be in an early stage of continental rifting, along the site of a pre-existing basement weakness, the Central African Shear Zone (CASZ). This shear zone has acted as a major pathway for magmas to reach the surface.

A near synchronous Early Jurassic (200 Ma) magmatic event along the North American and West African passive continental margins can be explained by the upwelling of a large-scale mantle plume (super-plume) beneath the West African craton. Lateral deflection of the outflow from the plume to the NE by the ambient sub-lithospheric mantle flow could account for tholeiitic basaltic magmatism extending over a distance of nearly 5000 km from Brazil to southern Spain. Continental break-up is considered to occur above the stagnation streamline along which cooler material eroded from the base of the continental lithosphere by the plume is returned to the convecting mantle. The dynamic model is a scaled version of that recently proposed for the Hawaiian plume and satisfactorily explains the apparent lack of involvement of thermally anomalous mantle in the generation of Central Atlantic oceanic crust.

Early Triassic-Cretaceous rifting and magmatism occurred along the western edge of the Arabian Peninsula from Egypt in the south to Syria in the north, associated with the development of the Neotethyan passive continental margin which is preserved in the present-day Eastern Mediterranean. A change in the direction and rate of extension during the Early Cretaceous had a profound effect upon the developing continental margin and controlled the pattern of rifting, magmatism and sedimentation. Extensive Late Jurassic-Early Cretaceous volcanism occurred throughout the region and is clearly rift-related. Major and trace element (including rare earth element) and Sr–Nd isotopic compositions suggest derivation of the most primitive mafic magmas from an enriched mantle source with minimal evidence of crustal contamination. The magmas appear to have ascended rapidly to the surface through large-scale faults. Isopach maps have been constructed for a series of time intervals during the Mesozoic, using well data from Egypt, Israel, Iraq, Jordan, Lebanon and Syria. These reveal the distribution and orientation of the main depocentres. The main tectonic feature from the Late Triassic until the Late Jurassic was a basin which extended northeastwards from northern Israel to the Palmyrides in central Syria. By the Early Cretaceous, however, deposition switched to a N–S oriented basin in central and northern Israel and Lebanon. The distribution of Late Jurassic-Early Cretaceous magmatism coincides with this new basin and indicates the location of the major extensional fault zones.

The distribution of volcanism in the Late Devonian Pripyat-Dnieper-Donets rift within the East European Platform, based upon the results of deep drilling, indicates that pre-existing basement structures and the major rift-bounding faults played an important role in channelling the magmas towards the surface. Major- and trace-element and Sr Nd isotopic studies of the most primitive basic and ultrabasic magmas, combined with estimates of the amount of extension (β = 1.1 to 1.3), strongly suggest that magmatism was triggered by the upwelling of a thermally and geochemically anomalous mantle plume from the deep mantle during the Late Frasnian. The peak of the magmatism occurred in the Famennian, coincident with the maximum amount of lithospheric extension. Magmatism, rifting and domal basement uplife were contemporaneous at several localities within the EEP, suggesting that the thermal and geodynamic evolution of the platform could have been influenced by a cluster of mantle plumes during the Late Devonian.

Teleseismic tomography is now widely recognised as a very powerful tool for studying the velocity structure of the lithosphere-asthenosphere system on both local and regional scales. A prime objective is to identify velocity perturbations and to constrain their location and intensity. Here we use the arrival times of P-waves from teleseismic events, recorded by a dense network of mobile and permanent stations within the French Massif Central, to derive a 3D P-velocity model for an area of 425 km × 300 km. Combined with knowledge of the basement geology of the area and the timing and geochemical characteristics of Tertiary-Quaternary volcanic activity, we consider that the velocity model provides unequivocal evidence for the ascent of a thermally and chemically anomalous mantle plume beneath the Massif Central during Tertiary times. Our model is consistent with regional Bouger gravity anomalies, documented patterns of Neogene-Recent uplift and deformation fabrics within lithosphere-derived mantle xenoliths exhumed by the volcanics.

Silica-poor, calcium-rich melilitites form a chemical and isotopic end-member of the spectrum of mafic magmas of the Tertiary-Quaternary volcanic province of western and central Europe. We propose that these unusual magmas are derived by partial melting of the thermal boundary layer (TBL) at the base of the European lithosphere. The processes involved in the evolution of the TBL have been constrained using major and trace element and Nd-Sr-Pb isotope data for Tertiary melilitites from the Urach, Hegau, and Rhine graben regions of Germany. The initiation of boundary layer evolution is limited in time by a major phase of Permo-Carboniferous rifting and associated magmatism postdating the Hercynian orogeny which would have destroyed the existing TBL by delamination. Model calculations indicate that the isotopic composition of the melilitite source cannot develop within the TBL over geologically reasonable periods of time (250–300 Million years) if the TBL evolves solely by incorporation of small degree (

The distribution of volcanism in the African plate is the surface expression of a variety of processes, many of which are poorly understood, involving interaction between the lithosphere and the underlying convective mantle. Despite the maturity of the plate tectonic paradigm, our knowledge of the processes involved in the breakup of continents and the formation of new ocean basins remains limited. The African Rift system provides a unique natural laboratory to study the transition from continental breakup to seafloor spreading. Thus, it is important to explore the similarities among the volcanic provinces of the Saharan zone, Cameroon volcanic line, Angola and Namibia, and the East African Rift system. The aim of this volume is to bring together recent and updated contributions on African volcanism (and associated mantle xenoliths), providing multidisciplinary contexts that include volcanology, geochemistry, petrology, geophysics, and structural geology, for a better understanding of the geological evolution of the African lithosphere.

The nature of the driving forces for plate tectonics has been a subject for intense, often heated debate, for more than a quarter of a century. On 15 April 1992 Peter Ziegler delivered the annual William Smith lecture to the Geological Society on the subject of Plate-Moving Mechanisms, followed by a series of lectures on related themes. This thematic set of papers, resulting from the meeting, covers some of the major issues discussed. For more than a quarter of a century Earth Scientists have debated the nature of the forces which drive the lithospheric plates. One of the major problems has been whether mantle convection, including the activity of mantle plumes, dominates or whether boundary and plate forces, such as slab pull and ridge push, exert the most important controls on plate motions. Clearly the relative importance of the different forces involved may vary, both in space and time, and they may either reinforce or counterbalance each other depending upon the tectonic setting. On 15 April 1992 Peter Ziegler delivered the annual William Smith Lecture to the Geological Society on the subject of Plate-Moving Mechanisms. This was followed by a further eight presentations on related themes by an international group geoscientists from Europe and North America. Of the nine papers presented at the meeting, six are included in this thematic set (Ziegler, Bott, Wilson, Searle et al., Doglioni, Pavoni). The other speakers were Randall M. Richardson, University of Arizona, Tuscon, USA, John F. Dewey, Oxford and Sierd Cloetingh, Free University

The magmatic response to extension of the continental lithosphere, with the resultant formation of sedimentary basins, varies widely. Quantitative models (e.g. McKenzie and Bickle, 1988), based upon an assumption of anhydrous partial melting in both lithosphere and asthenosphere, suggest that the temperature of the sub-lithospheric upper mantle, upwelling beneath the zone of extension, exerts a fundamental control on both the chemistry and volume of the erupted magmas. High volcanicity rifts are, in general, only likely to occur when the asthenospheric potential temperature is ca. 100–300°C greater than normal, associated with the activity of mantle plumes. In addition, such models also predict a strong correlation between the volume of magma produced and the amount of lithospheric stretching (β). More complex models (e.g. Gallagher and Hawkesworth, 1992) include partial melting of volatile-rich mantle source regions with lower than normal solidus temperatures. These may reside within mantle plumes or within enriched domains within the continental lithospheric mantle. Quantitative modelling of the magma generation process in the latter case is limited by our rather poor understanding of volatile-present partial melting of the range of potential source components. Geochemical and isotopic (Sr—Nd—Pb—O) studies of the igneous rocks emplaced within a basin are of fundamental importance in evaluating the relative roles of lithospheric versus asthenospheric source components in the petrogenesis of the magmas and the depth and degree of partial melting. In addition, these data may provide important constraints on the amount of crustal contamination which has occurred, particularly by partial melting of the lower crust. Knowledge of the latter is important in the development of models to explain crustal thinning. In many basins the bulk of the mantle-derived magmas may never actually reach the surface, but are underplating and intruding the lower crust instead. Identification of such a mafic underplate is critical if the amount of crustal thinning is to be evaluated correctly. The location of eruptive sites within individual basins is frequently determined by pre-existing basement structures. Within the extensive Mesozoic rift system of West and Central Africa, Pan-African lithospheric shear zones and deep basement faults have acted as foci for rift basin development, with the faults acting as magma pathways to the surface. In western and central Europe, Neogene—Recent extension has generated an extremely varied magmatic response which can be correlated with the Hercynian structural grain of the lithosphere. The Rhine graben, trending at a high angle to the Hercynian terrane boundaries is largely amagmatic. In contrast the Ohre rift of Czechoslovakia, which trends parallel to a major (Saxothuringian—Moldanubian) terrane boundary, is a classic example of a pure shear rift with major volcanic complexes located along the axial zone.