Mantle plume

Laboratory experiments were performed to study the influence of density and viscosity layering on the formation and stability of plumes. Viscosity ratios ranged from 0.1 to 6400 for buoyancy ratios between 0.3 and 20, and Rayleigh numbers between 105 and 2.108. The presence of a chemically stratified boundary layer generates long-lived thermochemical plumes. These plumes first develop from the interface

Submarine volcanic glass He, Ne and Ar data from the Pacific-Antarctic-Rise at its intersection with the Foundation Seamount Chain have been obtained to further elucidate one of the main outstanding problems in plume-ridge interaction studies: the physical state in which mantle flow and mixing occur in those settings. Mixing of plume mantle and mid-ocean ridge basalt source material may either be in the form of two solids, of solid-melt or of two melt phases only. The positive correlation of 4 He/ 40 Ar* with 206 Pb/ 204 Pb as well as the negative correlation of 4 He/ 40 Ar* with 40 Ar* and 4 He concentrations, combined with a lack of correlation between 4 He/ 40 Ar* and ridge depth or MgO concentration, indicate that the enriched mantle plume material has been extensively degassed prior to mixing with the mid-ocean ridge source material. Degassing of the plume source and mid-ocean ridge source material prior to mixing is also suggested by the He versus Pb isotope systematics. However, the occurrence of degassing processes requires the existence of melts, implying that mixing between the plume and the mid-ocean ridge material occurs in the physical form of melts.

An examination of lithological, geochemical, geochronological, structural and metamorphic P–T path data suggests that the basement of the North China Craton can be divided into Eastern and Western Blocks, separated by major crustal boundaries that roughly correspond with ...

Two geochemically distinct Low-Ti magma types (Gramado and Esmeralda) are distinguished within the flood basalt sequences of southern Paraná on the basis of certain element abundances and ratios. Esmeralda magmas have lower Ce/Sm and ( and higher and ϵNd, than Gramado magmas. Detailed stratigraphical work indicates that there was a temporal progression from Gramado- to Esmeralda-type magmas. This compositional shift cannot simply be explained in terms of a declining extent of crustal contamination of an asthenosphere-derived melt with time, and instead it seems that the two magma types evolved from distinct parental magmas. Although compositional variations of Gramado magmas are dominated by crustal assimilation and fractional crystallisation, geochemical features of the inferred parental magmas [low of < 300 and low of < 0.8, negative ϵNd-values ∼ −3 and high )i 0.708−0.710] are indicative of a significant contribution from the continental mantle lithosphere. Gramado magmas also show a marked regional variation which might imply that the mantle source was compositionally heterogeneous over a length scale of a few hundred km. It is suggested that variations within the Esmeralda magma type are controlled by mixing between an asthenospheric melt with ϵNd > + 4 and a Gramado-type magma, at relatively shallow crustal levels, followed by fractional crystallisation. The asthenospheric component has relatively depleted incompatible element characteristics comparable to MORB. Thus, Low-Ti lava sequences of southern Paraná record a shift from a predominantly lithospheric to an asthenospheric signature with time. Paraná flood basalts appear to have been generated in response to lithospheric extension associated with the opening of the South Atlantic over a region of anomalously hot mantle, attributed to the presence of the Tristan mantle plume. However, the distinctive geochemical signature of the modern Tristan plume, seen on the islands of Tristan da Cunha, Gough and Inaccessible, is not evident in the compositions of Low-Ti magmas in southern Paraná.

The mantle plume hypothesis is now widely known to explain hotspot volcanoes, but direct evidence for actual plumes is weak, and seismic images are available for only a few hotspots. In this work, we present whole-mantle tomographic images under 60 major hotspots on Earth. The lateral resolution of the tomographic images is about 300 km under the continental hotspots and 400-600 under the oceanic hotspots. Twelve plume-like, continuous low-velocity (low-V) anomalies in both the upper and lower mantle are visible under Hawaii,

The two mass extinctions of the Late Permian, end-Guadalupian and end-Changhsingian events, coincide with intense volcanic paroxysms located both in subpolar and tropical realms. For the Siberian Traps eruptions in particular, thermal destabilization of methane hydrates in extensive bipolar reservoirs initiated catastrophic release of CH 4 that may have triggered a subsequent runaway greenhouse culminated through the Permian-Triassic (P-Tr) boundary interval. In addition, increased geothermal heat and volcano-ice interactions since the early Late Permian resulted finally in unusual high-latitude warmth via catastrophic methane release. This combination of factors suggests excessive permafrost melting with a consequent inflow of heated and low-salinity waters. This is postulated to have provoked shutdown of the ocean thermohaline system, and thus to have caused the superanoxic event of the P-Tr boundary timespan, even if many global feedbacks and interactions were acting earlier in concert towards progressive warming and destabilized oceanic circulation and biogeochemical cycling. Both larger than hitherto assumed, Chinese and Siberian volcanic cataclysms are considered as the main driving force of profound P-Tr oceanic stagnation and superanoxia, nutrient stress and bioproductivity crash, in action in the geodynamically activated activity (due to plate boundary reorganization and accelerated rifting processes), fragile ecosystem. This end-Permian crisis acme was recorded in the Late Changhsingian-Griesbachian Chert Gap and overall 'fossilization low' through ~10 Ma, and the consequent biogeochemical and ecological peculiarities of the 'dead'/survival intervals. In contrast, a specific volcanogenic/tectonoeustatic scenario for the end-Guadalupian crisis may be focused on the Phanerozoic sea-level minimum and/or an extraordinary concomitant uplift of the Gondwana supercontinent.

The mantle plume concept is currently being challenged as an explanation for North Atlantic Igneous Province formation. Alternative models have been suggested, including delamination, meteorite impact, small-scale rift-related convection, and chemical mantle heterogeneities. We review available datasets on uplift, strain localization, age and chemistry of igneous material, and tomography for the North Atlantic Igneous Province, and compare them with predictions from the mantle plume and alternative models. The mantle plume concept is quite successful in explaining formation of the NAIP, but unexplained aspects remain. Delamination and impact models are currently not supported. Rift-related small-scale convection models appear to be able to explain volcanic rifted margin volcanism well. However, the most important problem that non-plume models need to overcome is the continuing, long-lived melt anomaly extending via the Greenland-Faeroe Ridges to Iceland. Mantle heterogeneities, resulting from an ancient subducted slab, are included in plate tectonic models to explain the continuing melt production as an alternative to the mantle plume model, but there are still uncertainties related to this idea that need to be solved.

The timing and duration of surface uplift associated with large igneous provinces provide important constraints on mantle convection processes. Here we review geological indicators of surface uplift associated with five continent-based magmatic provinces: Emeishan Traps (260 million years ago: Ma), Siberian Traps (251 Ma), Deccan Traps (65 Ma), North Atlantic (Phase 1, 61 Ma and Phase 2, 55 Ma), and Yellowstone (16 Ma to recent). All five magmatic provinces were associated with surface uplift. Surface uplift can be measured directly from sedimentary indicators of sea-level in the North Atlantic and from geomorpholocial indicators of relative uplift and tilting in Yellowstone. In the other provinces, surface uplift is inferred from the record of erosion. In the Deccan, North Atlantic and Emeishan provinces, transient uplift that results from variations in thermal structure of the lithosphere and underlying mantle can be distinguished from permanent uplift that results from the extraction and emplacement of magma. Transient surface uplift is more useful in constraining mantle convection since models of melt generation and emplacement are not required for its interpretation. Observations of the spatial and temporal relationships between surface uplift, rifting and magmatism are also important in constraining models of LIP formation. Onset of surface uplift preceded magmatism in all five of the provinces. Biostratigraphic constraints on timing of uplift and erosion are best for the North Atlantic and Emeishan Provinces, where the time interval between significant uplift and first magmatism is less than 1 million years and 2.5 million years respectively. Rifting post-dates the earliest magmatism in the case of the North Atlantic Phase 1 and possibly in the case of Siberia. The relative age of onset of offshore rifting is not well constrained for the Deccan and the importance of rifting in controlling magmatism is disputed in the Emeishan and Yellowstone Provinces. In these examples, rifting is not a requirement for onset of LIP magmatism but melting rates are significantly increased when rifting occurs.

We report new major element, trace element, isotope ratio, and geochronological data on the Galfipagos Archipelago. Magmas erupted from the large western volcanos are generally moderately fractionated tholeiites of uniform composition; those erupted on other islands are compositionally diverse, ranging from tholeiites to picritic basanitoids. While these volcanos do not form a strictly linear age progressive chain, the ages of the oldest dated flows on any given volcano do form a reasonable progression from youngest in the west to oldest in the east, consistent with motion of the Nazca plate with respect to the fixed hotspot reference frame. lsotope ratios in the Galfipagos display a considerable range, from values typical of mid-ocean ridge basalt on Genovesa (87Sr/86Sr: 0.70259, end: +9.4, 206pb/204pb: ! 8.44), to typical oceanic island values on Floreana (87Sr/86Sr: 0.70366, oeNd: +5.2, 206pb/204pb: 20.0). La/Sm N ranges from 0.45 to 6.7; other incompatible element abundances and ratios show comparable ranges. Isotope and incompatible element ratios define a horseshoe pattern with the most depleted signatures in the center of the Galfipagos Archipelago and the more enriched signatures on the eastern, northern, and southern periphery. These isotope and incompatible element patterns appear to reflect thermal entrainment of asthenosphere by the Galfipagos plume as it experiences velocity shear in the uppermost asthenosphere. Both north-south heterogeneity within the plume itself and regional variations in degree and depth of melting also affect magma compositions. Rare earth systematics indicate that melting beneath the Galfipagos begins in the garnet peridotite stability field, except beneath the southern islands, where melting may occur entirely in the spinel peridotite stability field. The greatest degree of melting occurs beneath the central western volcanos and decreases both to the east and to the north and south. Sis. 0, FeB. 0, and NaB. 0 values are generally consistent with these inferences. This suggests that interaction between the plume and surrounding asthenosphere results in significant cooling of the plume. Superimposed on this thermal pattern produced by plume-asthenosphere interaction is a tendency for melting to be less extensive and to occur at shallower depths to the south, presumably reflecting a decrease in ambient asthenospheric temperatures away from the Galfipagos Spreading Center.

The Tana basin is situated on the northwestern plateau of Ethiopia, west of the Afar depression. The basin is perched on a topographic high. New data from digital elevation modelling and satellite imagery analysis confirm the basin's location at the junction of three grabens: the Dengel Ber (buried), Gondar (exposed by erosion) and Debre Tabor (reactivated). This structural complex was notably active during the build-up of the mid-Tertiary flood basalt pile, into which the Tana basin is impressed. Fault reactivation occurred in the Late Miocene-Quaternary, accompanied locally by predominantly basaltic volcanism. Fault-slip indicators are consistent with crustal subsidence centred on the present morphologic basin. Concentric and radial dike patterns in the Tana region indicate that diking and basin formation were contemporary. Tana rifting and magmatism occurred above the inferred western side of the Afar mantle plume-head.

In the orogenic belts of the Central Asian Orogenic Belt (CAOB), many mafic and felsic plutons are temporally and spatially associated with orogen-scale strike-slip faults. The CAOB is a huge and complex orogenic collage of accreted terranes that was formed in the Early to Mid-Palaeozoic. In the CAOB, orogen-scale strike-slip faults extend for 100-1000 s of kilometres marking the boundaries of tectonic units and terranes. I use examples from southern Siberia and NW China to illustrate the important role that strike-slip faults have in localising intraplate magmatism and associated metallogeny. Cases from the Altay-Sayan in southern Siberia, the Altay and Tianshan orogens in NW China, are compelling for providing good evidence of the control that strike-slip structures exert for the emplacement of magmas and related mineral systems. These strike-slip faults controlled the emplacement of mafic-ultramafic intrusions, alkaline mafic and felsic magmatism in the period 280-240 Ma, which coincides with mantle plume(s) activity that led to the emplacement of the Tarim and Siberian large igneous provinces (LIPs). Mineral systems that are associated with these LIPs include magmatic Ni-Cu in sill-like intrusions, concentrically zoned mafic-ultramafic intrusions (e.g. Kalatongke, the second largest Ni-Cu sulphide deposit in China, after Jinchuan), epithermal systems, breccia pipes, polymetallic hydrothermal veins, granitoidrelated greisen and rare earth pegmatites, as well as kimberlite fields. In the Altay-Sayan and NW China regions, orogen-scale translithospheric strike-slip faults provided the channels for the emplacement of magmas, resulting from lateral flow of mantle melts along the base of the lithosphere. This lateral flow is interpreted to have resulted from the impingement of mantle plumes to the base of the lithosphere of what was, to all intents and purposes, a stationary plate. Lateral flow from mantle plumes head was sustained or facilitated, during stages of extension and movements along orogen-scale strike-slip faults. In the Altay-Sayan and NW China, decompression melting of the mantle material produced mafic-ultramafic magmas that were emplaced along the comparatively narrow conduits of the strike-slip zones, forming concentrically zoned complexes that locally, where favourable conditions allowed it (e.g. crustal contamination), host magmatic Ni-Cu sulphides. Flow of mantle melts into translithospheric strike-slip structures also caused partial melting of a thinned and metasomatised lithosphere, resulting in alkaline magmatic products and a wide range of related mineral systems, from polymetallic veins to greisens. Partial melting of the lower crust also produced A-type granitic magmas that locally vented to the surface, forming calderas hosting epithermal and porphyry systems, as observed in NW China.

Large areas of central Asia and mainland China have been affected by intraplate anorogenic magmatism, linked to mantle activity from the Palaeoproterozoic to present day. This magmatism is manifested by layered maficultramafic intrusions, flood basalts, bimodal volcanic rocks, dyke swarms, I-and A-type granitoids, alkaline complexes, carbonatites and kimberlites. In this paper we discuss a selection of these thermal events, namely East Asia Mesozoic-Cenozoic, Siberian Traps, Emeishan, Tarim (Bachu), Yakutsk and South China. Mineral systems that are associated with the Asian intraplate magmatic events include Ni-Cu-PGE in mafic-ultramafic intrusions, porphyry Cu-Mo and skarn deposits, polymetallic veins and auriferous lode deposits generally hosted in granitoids, rare earths and rare metals in alkaline complexes and diamondiferous kimberlites. The anorogenic intraplate igneous rocks originate from mantle sources or mixed mantle and crustal sources and are related, directly or indirectly, to deep mantle plumes or to asthenospheric upwellings that occur in response to lower crust and subcontinental lithosphere delamination linked to collision and subduction processes.

Intraplate tectono-magmatic phenomena, including the emplacement of layered intrusions, and the giant dyke swarms, anorogenic (hotspot) volcanism, oceanic plataeux, rifting processes, basin formation, and geomorphological features are discussed in the context of the mantle plume theory. A review of the relationships between mantle plumes and ore deposits focuses on direct links, proxied by the emplacement of mafic-ultramafic magmas (e.g. PGE and Ni-Cu sulphides associated with flood basalts) and indirectly in rift systems where high geothermal gradients are set up in the crust above the plume, induce large scale circulation of hydrothermal fluids, which result in the generation of a wide range of ore deposits.

In the Skagafjorður district, North Iceland, an unconformity with a sedimentary layer divides the strata pile into Neogene and Pleistocene volcanic successions. Neogene volcanism faded out 4-5 Ma when the area drifted out of the volcanic zone near the diverging boundaries of the North American and Eurasian crustal plates, after which erosion took over for about a million years. Volcanism started again at the beginning of the Pleistocene. Lava flows covered the eroded Neogene landscape and an unconformity was formed. It is proposed that the Pleistocene volcanic rocks above the unconformity all belong to a short-lived axial rift zone, the Skagafjorður zone. Eruptive sites belonging to this zone can be found dispersed in the area between the Hofsjokull central volcano and the mouth of Skagafjorður. The oldest volcanic formations of the Pleistocene rock series are from about 1.7 Ma, when rifting in the area started. The activity culminated in the early Pleistocene but declined in the la...

Precambrian and younger basins re¯ect the interaction of sediment supply and subsidence; the latter is generally ascribed to tectonic, magmatic and related thermal processes. The interplay of supply and subsidence is further modi®ed by eustasy and palaeoclimate. Problems and enigmas inherent in analysis of Precambrian basin-®lls include: a spectrum of ideas on the maximum age of Phanerozoic-style plate tectonics in the rock record; Archaean heat¯ow up to two to three times present values; changes in magmatism over time (including global magmatic events); the evolution of atmospheric composition and of life and their in¯uence on weathering, erosion and sediment supply rates; degree of preservation, deformation and metamorphism, and preservational bias (especially of intracratonic basins which would lack evidence for early plate tectonics); a limited rock record; poor age constraints, inherent errors in geochronological techniques and dif®culty in dating the time of deposition of sedimentary rocks.

Asia is the world's largest but youngest continent, in which Pacific-type (P-type) and collision-type (C-type) orogenic belts coexist with numerous amalgamated continental blocks. P-type orogens represent major sites of continental growth through tonalite-trondhjemite-granodiorite type (TTG-type) juvenile granitoid magmatism and accretion of oceanic crust and intraoceanic arcs. The Asian continent includes several P-type orogenic belts, of which the largest are the Central Asian and Western Pacific. The Central Asian Orogenic Belt is dominated by P-type fossil orogens arranged with a regular northward subduction polarity. The Western Pacific is characterized by ongoing P-type orogeny related to the westward subduction of the Pacific plate. Asia has a multi-cratonic structure and its post-Palaeozoic history has witnessed amalgamation of the Laurasia composite continent and Pangaea supercontinent. Nowadays, Asia is surrounded by double-sided subduction zones, which generate new TTG-type crust and supply oceanic crust and microcontinents to its active margins. The TTGcrust can be tectonically eroded and subducted down to the mantle transition zone to form a 'second' continent, which may generate mantle upwelling, plumes, and extensive intra-plate volcanism. Moreover, recent plate movements around Asia are dominated by northward directions, which resulted in the India-Eurasia and Arabia-Eurasia collisions beginning at 50-45 and 23-20 Ma, respectively, and will result in Africa-Eurasia collision in the near future. Therefore, Asia is the best candidate to serve as the nucleus for a future supercontinent 'Amasia', likely to form 200-250 Ma in the future. In this paper we unravel a puzzle of continental growth in Asia through P-type orogeny by discussing its tectonic history and geological structure, subduction polarity in P-type orogens, tectonic erosion of TTG-type crust and arc subduction at convergent margins, generation of mantle plumes, and prospects of Asia growth and overgrowth.

The principle of uniformitarianism may be applied to Precambrian basin evolution and to the sedimentary record as a whole. The major difference in the Precambrian Eon lay in variability of rates and intensities of processes controlling weathering, erosion, transport, deposition, lithification, and diagenesis. This paper examines Precambrian sedimentation patterns within the larger framework of Earth evolution. Pre-rock record sedimentation probably comprised deep water oceanic realms within which meteoritic and cometary impact events generated very large tsunamis, resulting in very coarse volcaniclastic detritus combined with fine dust settling out of suspension, all reworked by marine current systems and localised turbidites.

Although uniformitarianism applies in a general sense to the controls on relative and global sea level change, some influences thereon were more prominent in the Precambrian. Short-term base level change due to waves and tides may have been enhanced due to possibly more uniform circulation systems on wide, low gradient Precambrian shelves. The lack of evidence for global glacial events in the Precambrian record implies that intraplate stresses and cyclic changes to Earth's geoid were more likely explanations for third-order sea level change than glacio-eustasy. Higher heat flow in the earlier Precambrian may have led to more rapid tectonic plate formation, transport and destruction, along with an increased role for hot spots, aseismic ridges and mantle plumes (superplumes), all of which may have influenced cyclic sedimentation within the ocean basins. A weak cyclicity in the occurrence of plume events has an approximate duration comparable to that of first-order (supercontinental cycle) sea level change. Second-order cyclicity in the Precambrian largely reflects the influences of thermal epeirogeny, changes to mid-ocean ridge volume as well as to ridge growth and decay rates, and cratonic marginal downwarping concomitant with either sediment loading or extensional tectonism. Third-order cycles of sea level change in the Precambrian also reflected cyclic loading/unloading within flexural foreland basin settings, and filling/deflation of magma chambers associated with island arc evolution.

At the transition from the Permian to the Triassic, Eurasia was the site of voluminous flood-basalt extrusion and rifting. Major flood-basalt provinces occur in the Tunguska, Taymyr, Kuznetsk, Verkhoyansk–Vilyuy and Pechora areas, as well as in the South Chinese Emeishen area. ...

The Transvaal is one of three structural basins of the Transvaal Supergroup preserved on the Kaapvaal craton. The evolution of the Neoarchaean±Palaeoproterozoic Transvaal basin is ascribed predominantly to magmatism, palaeoclimate and eustasy, with plate tectonics playing a sporadic role. The supergroup comprises basal`protobasinal' rocks, followed by the Black Reef Formation, Chuniespoort Group and uppermost Pretoria Group. Immature siliciclastic and bimodal volcanic rocks of the protobasinal unit re¯ect a wide zone of rifting related to the c. 2.7 Ga Ventersdorp (Supergroup) mantle plume. Individual protobasinal successions were laid down in separate fault-bounded basins, controlled at least partially by greenstone belt orientations in the Kaapvaal basement.

The Mesozoic and Cenozoic positions of the continents that formed Pangea in the Triassic-Jurassic were derived from paleomagnetic and intraplate volcanic data, paleoclimatic observations, such as reef and fossil flora distribution, and geological observations. Major hot spots helped to determine the longitudinal position of Pangea and to construct a model of plate motion during the Pangean break-up.

Whole rock Sm-Nd isotope dating of the Garhwal and Bhowali mafic volcanics of the Kumaun Lesser Himalayan sector yield an age of 2.51±0.08 Ga and an initial e Nd =5.1±0.5. This age and initial e Nd are the same as that of the Rampur-Mandi volcanics, which occur to the west in the Lesser Himalayas. In addition to confirming the widespread presence of the Late Archaean crustal elements in the Himalayas, these data constrain the age of the oldest rifting event and deposition of supracrustal sediments in the Himalayas and set at rest the long standing controversy about the age and inter-relationship of these mafic volcanics. Despite their present-day disparate occurrence as a result of the post-eruption deformation, the area covered by these mafic volcanics is large enough to classify them as flood basalts, the Rampur flood basalt (RFB) province. Rift-related volcanism of the same age is known from the contiguous Aravalli region of the NW Indian Shield. The orientation of the rifts in the two regions suggests the possibility of a mantle plume setting for the RFB.

Non-arc basalts of Archean and Proterozoic age have model primary magmas that exhibit mantle potential temperatures T P that increase from 1350°C at the present to a maximum of ∼ 1500-1600°C at 2.5-3.0 Ga. The overall trend of these temperatures converges smoothly to that of the present-day MORB source, supporting the interpretation that the non-arc basalts formed by the melting of hot ambient mantle, not mantle plumes, and that they can constrain the thermal history of the Earth. These petrological results are very similar to those predicted by thermal models characterized by a low Urey ratio and more sluggish mantle convection in the past. We infer that the mantle was warming in deep Archean-Hadean time because internal heating exceeded surface heat loss, and it has been cooling from 2.5 to 3.0 Ga to the present. Non-arc Precambrian basalts are likely to be similar to those that formed oceanic crust and erupted on continents. It is estimated that ∼ 25-35 km of oceanic crust formed in the ancient Earth by about 30% melting of hot ambient mantle. In contrast, komatiite parental magmas reveal T P that are higher than those of non-arc basalts, consistent with the hot plume model. However, the associated excess magmatism was minor and oceanic plateaus, if they existed, would have had subtle bathymetric variations, unlike those of Phanerozoic oceanic plateaus. Primary magmas of Precambrian ambient mantle had 18-24% MgO, and they left behind residues of harzburgite that are now found as xenoliths of cratonic mantle. We infer that primary basaltic partial melts having 10-13% MgO are a feature of Phanerozoic magmatism, not of the early Earth, which may be why modern-day analogs of oceanic crust have not been reported in Archean greenstone belts.

In this article, we review the significant recent results of geophysical studies and discuss their implications on seismotectonics, magmatism, and mantle dynamics in East Asia. High-resolution geophysical imaging revealed structural heterogeneities in the source areas of large crustal earthquakes, which may reflect magma and fluids that affected the rupture nucleation of large earthquakes. In subduction zone regions, the crustal fluids originate from the dehydration of the subducting slab. Magmatism in arc and back-arc areas is caused by the corner flow in the mantle wedge and dehydration of the subducting slab. The intraplate magmatism has different origins. The continental volcanoes in Northeast Asia (such as Changbai and Wudalianchi) seem to be caused by the corner flow in the big mantle wedge (BMW) above the stagnant slab in the mantle transition zone and the deep dehydration of the stagnant slab as well. The Tengchong volcano in Southwest China is possibly caused by a similar process in BMW above the subducting Burma microplate (or Indian plate). The Hainan volcano in southernmost China seems to be a hotspot fed by a lower-mantle plume associated with the Pacific and Philippine Sea slabs' deep subduction in the east and the Indian slab's deep subduction in the west down to the lower mantle. The occurrence of deep earthquakes under the Japan Sea and the East Asia margin may be related to a metastable olivine wedge in the subducting Pacific slab. The stagnant slab finally collapses down to the bottom of the mantle, which may trigger upwelling of hot mantle materials from the lower mantle to the shallow mantle beneath the subducting slabs and cause the slab-plume interactions. Some of these issues, such as the origin of intraplate magmatism, are still controversial, and so further detailed studies are needed from now.

Continental flood basalts, derived from mantle plumes that rise from the convecting mantle and possibly as deep as the core-mantle boundary, are major hosts for world-class Ni-Cu-PGE ore deposits. Each plume may have a complex history and heterogeneous composition.

Aeromagnetic signatures over the Edward VII Peninsula (E7) provide new insight into the largely ice-covered and unexplored eastern flank of the Ross Sea Rift (RSR). Positive anomalies, 10–40 km in wavelength and with amplitudes ranging from 50 to 500 nT could reveal buried Late Devonian(?)–Early Carboniferous Ford Granodiorite plutons. This is suggested by similar magnetic signature over exposed, coeval Admiralty Intrusives of the Transantarctic Mountains (TAM). Geochemical data from mid-Cretaceous Byrd Coast Granite, contact metamorphic effects on Swanson Formation and hornblende-bearing granitoid dredge samples strengthen this magnetic interpretation, making alternative explanations less probable. These magnetic anomalies over formerly adjacent TAM and western Marie Byrd Land (wMBL) terranes resemble signatures typically observed over magnetite-rich magmatic arc plutons. Shorter wavelength (5 km) 150 nT anomalies could speculatively mark mid-Cretaceous mafic dikes of the E7, similar to those exposed over the adjacent Ford Ranges. Anomalies with amplitudes of 100–360 nT over the Sulzberger Bay and at the margin of the Sulzberger Ice Shelf likely reveal mafic Late Cenozoic(?) volcanic rocks emplaced along linear rift fabric trends. Buried volcanic rock at the margin of the interpreted half-graben-like “Sulzberger Ice Shelf Block” is modelled in the Kizer Island area. The volcanic rock is marked by a coincident positive Bouguer gravity anomaly. Late Cenozoic volcanic rocks over the TAM, in the RSR, and beneath the West Antarctic Ice Sheet exhibit comparable magnetic anomaly signature reflecting regional West Antarctic Rift fabric. Interpreted mafic magmatism of the E7 is likely related to mid-Cretaceous and Late Cenozoic regional crustal extension and possible mantle plume activity over wMBL. Magnetic lineaments of the E7 are enhanced in maximum horizontal gradient of pseudo-gravity, vertical derivative and 3D Euler Deconvolution maps. Apparent vertical offsets in magnetic basement at the location of the lineaments and spatially associated mafic dikes and volcanic rocks result from 2.5D magnetic modelling. A rift-related fault origin for the magnetic lineaments, segmenting the E7 region into horst and graben blocks, is proposed by comparison with offshore seismic reflection, marine gravity, on-land gravity, radio-echo sounding, apatite fission track data and structural geology. The NNW magnetic lineament, which we interpret to mark the eastern RSR shoulder, forms the western margin of the “Alexandra Mountains horst”. This fundamental aeromagnetic feature lies on strike with the Colbeck Trough, a prominent NNW half-graben linked to Late Cretaceous(?) and Cenozoic(?) faulting in the eastern RSR. East–west and north–north–east to NE magnetic trends are also imaged. Magnetic trends, if interpreted as reflecting the signature of rift-related normal faults, would imply N–S to NE crustal extension followed by later northwest–southeast directed extension. NW–SE extension would be compatible with Cenozoic(?) oblique RSR rifting. Previous structural data from the Ford Ranges have, however, been interpreted to indicate that both Cretaceous and Cenozoic extensions were N–S to NE–SW directed.

We present the first results of a high-resolution teleseismic traveltime tomography and seismic anisotropy study of the lithosphere–asthenosphere system beneath the western Bohemian Massif. The initial high-resolution tomography down to a depth of 250 km did not image any columnar low-velocity anomaly which could be interpreted as a mantle plume anticipated beneath the Eger Rift, similar to recent findings of small plumes beneath the French Massif Central and the Eifel in Germany. Alternatively, we interpret the broad low-velocity anomaly beneath the Eger Rift by an upwelling of the lithosphere–asthenosphere transition. We also map lateral variations of seismic anisotropy of the mantle lithosphere from spatial variations of P-wave delay times and the shear wave splitting. Three major domains characterised by different orientations of seismic anisotropy correspond to the major tectonic units—Saxothuringian, Moldanubian and the Teplá-Barrandian—and their fabrics fit to those found in our previous studies of mantle anisotropy on large European scales.

40 Ar/ 39 Ar and K-Ar ages of whole rock samples and kaersutite (amphibole) crystals obtained from basanite and tephrite flows, lava lake series and pyroclastic rocks in Makhtesh Ramon (central Negev, southern Israel) define a Late AptianeEarly Albian time span of the Cretaceous magmatic event (Ramon Volcanics) and the order of the local volcanic activity, which had only been partly resolved by stratigraphic relations. The igneous activity started with volcanic flows that originated from several centres between 116.4 G 3.4 Ma and 112.5 G 1 Ma, continued with a lava lake series at 110.3 G 6.1 Ma and terminated by explosive activity at 108.8 G 1.2 Ma.

Evolution of tectono-magmatic processes in the Paleoproterozoic is divisible into four stages each controlled by its own type of endogenic activity. The critical moment in geological history was at the third stage onset 2.05 Ga ago, when the Archean style of evolution gave way to subsequent one. Magmas of siliceous high-Mg (boninite-like) series (SHMS) dominant during the first stage (2.5-2.3 Ga) had the mixed mantle-crust genesis. They originated in highly depleted ultramafic reservoir of asthenospheric mantle being contaminated by the Archean crustal material during ascent to the surface. Typical of the SHMS magmas were the high content of silica, Al, Mg, Cr and elevated concentrations of Ni, Co, Cu, V, PGE and incompatible elements, LREE included, while concentrations of Fe, Ti, Nb and alkalies, especially K, were at a relatively low level. Magmatism of K-granitoid type was of a limited significance. The second stage, especially its second substage (2.3-2.05 Ga), was distinct because of mass eruptions of picritic and basaltic magmas enriched in Fe, Ti, Mn, P and incompatible elements, LREE in particular, which also had elevated Cr, Ni, Co, Cu and Ba concentration being relatively depleted in Mg and Al. This change in geochemistry of magmatism was independent of tectonic processes, which retained the former style. The third stage (2.05-1.8 Ga) was marked by opening of first oceans (Jormua, Purtuniq and other ophiolites) and by formation of orogens comparable with Phanerozoic orogens that was associated with development of subduction zones and back-arc basins with relevant magmatism. High Mg, Fe, Ti, Cr, Ni, Cu, V but low Th concentrations were typical of intense picrite-basaltic volcanism marking onset of the third stage. Magmas of suprasubduction genesis had considerably higher Si, Al, P, Zn, Th concentrations and very low ë ‡é/Al 2 O 3 ratios. Development of orogens came to the end 1.82-1.

When continents rift to form new ocean basins, the rifting'is sometimes accompanied by massive igneous activity. We show that the production of magmatically active rifted margins and the effusion of flood basalts onto the adjacent continents can be explained by a simple model of rifting above a thermal anomaly in the underlying mantle. The igneous rocks are generated by decompression melting of hot asthenospheric mantle as it rises passively beneath the stretched and thinned lithosphere. Mantle plumes generate regions beneath the lithosphere typically 2000 km in diameter with temperatures raised 100-200øC above normal. These relatively small mantle temperature increases are sufficient to cause the generation of huge quantities of melt by decompression: an increase of 100øC above normal doubles the arnount of melt whilst a 200øC increase can quadruple it. In the first part of this paper we develop our model to predict the effects of melt generation for varying amounts of stretching with a range of mantle temperatures. The melt generated by decompression migrates rapidly upward, until it is either extruded as basalt flows or intruded into or beneath the crust. Addition of large quantities of new igneous rock to the crust considerably modifies the subsidence in rifted regions. Stretching by a factor of 5 above norreal temperature mantle produces immediate subsidence of more than 2 km in order to maintain isostatic equilibrium. If the mantle is 150øC or more hotter than normal, the same amount of stretching results in uplift above sea level. Melt generated from abnormally hot mantle is more magnesian rich than that produced from normal temperature mantle. This causes an increase in seismic velocity of the igneous rocks eraplaced in the crust, from typically 6.8 km/s for normal mantle temperatures to 7.2 km/s or higher. There is a concomitant density increase. In the second part of the paper we review volcmaic continental margins and flood basalt provinces globally and show that they are always related to the thermal anomaly created by a nearby mantle plume. Our model of melt generation in passively upwelling mantle beneath rifting continental lithosphere can explain all the major rift-related igneous provinces. These include the Tertiary igneons provinces of Britain and Greenland and the associated volcanic continental maxgins caused by opening of the North Atlantic in the presence of the Iceland plume; the Paran& and parts of the Karoo flood basalts together with volcanic continental margins generated when the South Atlantic opened; the Deccan flood basalts of India and the Seychelles-Saya da Malha volcanic province created when the Seychelles split off India above the l•union hot spot; the Ethiopian and Yemen Traps created by rifling of the Red Sea and Gulf of Aden region above the Afar hot spot; and the oldest and probably originally the largest flood basalt province of the Karoo produced when Gondwana split apart. New continental splits do not always occur above thermal anomalies in the mantle caused by plumes, but when they do, huge quantities of igneous materiM are added to the continental crust. This is an important method of increasing the volume of the continental crust through geologic time. Massive outbursts of igneous activity sometimes accompany the rifting of continents as they are stretched prior to breaking to form new oceanic basins. Much of the evidence for this is now deeply buried on continental margins under thick piles of sediment and, furthermore, is hidden underwater. In some places, huge areas of flood basalts were extruded onto the surface of the adjacent continental areas at the same time as the continents were breaking apart: examples include the Deccan of India, the Karoo of southern Africa, and the Paran• of South America (Figure 1). Not only were huge quantities of igneous rock generated when some continents rifted, but it appears that the bulk of it in any one area was emplaced in rather short periods of time. For example, when the northern North Atlantic opened during the early Tertiary, up to 10 million krn a of igneous rock was produced on the rifted margins in as little as 2-3 m.y. [White et al., 1987b]. The bulk of the Deccan Volcanic Province of 1-2.5 milhon km 3, to cite a continental flood basalt example, was extruded in probably only 0.5 m.y. [Courtillot and Cisowski, 1987]. These two examples are not insignificant: they are amongst the two largest volcanic events on Earth that have occurred since 200 Ma. They may as a consequence have had considerable effect on the biosphere, and may have been contributory factors to mass extinctions. 7685 7686 WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES NORTH CENTRAL ATLANTIC Ethiopia SOUTH ATLANTIC (•GONDWANA BREAKUP INDIAN OCEAN iil•ea WEST • o •=,• )'=• t Maud Fig. 1. Location of voluminous extrusive volcanic rocks on rifted continental margins (hatched), and extent of associated continental flood basalts (solid) with circled numbers showing section of this paper where each region is discussed. Projection is Airoff equal are• Yet, whereas volcanism was widespread at some continental rifts, at others (such as the Bay of Biscay margin) it was virtually absent. Both volcanically active and nonvolcanic margins occur along the length of individual ocean basins. The Bay of Biscay margin, for example, lies along strike from the volcanically active margins off Rockall Bank and the Norwegian continental shelf. In this paper we develop a simple model to explain the occurrence of volcanic continental margins and flood basalts. We explain the igneous activity as a consequence of relatively small (100-200øC) increases above normal in ,•_A ß ...... ß ..... t,•_ J • ß UI bll• unuenyir, g asthenobll• b•lllp•[i•l,b UI-• spheric mantle across which the rift runs. The igneous rocks are assumed to be produced by partial melting of the asthenosphere due to decompression as it wells up passively to fill the space generated by the stretching and thinning lithosphere. This model has been applied by White et al. [1987b] and White [1988a] to the development of the North Atlantic: in this paper we develop the basic theoretical model and show how it can explain all known occurrences of volcanic continental margins around the world.

Igneous rocks of the Devonian Kola Alkaline Carbonatite Province (KACP) in NW Russia and eastern Finland can be classified into four groups: (a) primitive mantle-derived silicaundersaturated silicate magmas; (b) evolved alkaline and nepheline syenites; (c) cumulate rocks; (d) carbonatites and phoscorites, some of which may also be cumulates. There is no obvious age difference between these various groups, so all of the magma-types were formed at the same time in a relatively restricted area and must therefore be petrogenetically related. Both sodic and potassic varieties of primitive silicate magmas are present. On major element variation diagrams, the cumulate rocks plot as simple mixtures of their constituent minerals (olivine, clinopyroxene, calcite etc). There are complete compositional trends between carbonatites, phoscorites and silicate cumulates, which suggests that many carbonatites and phoscorites are also cumulates. CaO/Al 2 O 3 ratios for ultramafic and mafic silicate rocks in dykes and pipes range up to 5, indicating a very small degree of melting of a carbonated mantle at depth. Damkjernites appear to be transitional to carbonatites. Trace element modelling indicates that all the mafic silicate magmas are related to small degrees of melting of a metasomatised garnet peridotite source. Similarities of the REE patterns and initial Sr and Nd isotope compositions for ultramafic alkaline silicate rocks and carbonatites indicate that there is a strong relationship between the two magma-types. There is also a strong petrogenetic link between carbonatites, kimberlites and alkaline ultramafic lamprophyres. Fractional crystallisation of olivine, diopside, melilite and nepheline gave rise to the evolved nepheline syenites, and formed the ultramafic cumulates. All magmas in the KACP appear to have originated in a single event, possibly triggered by the arrival of hot material (mantle plume?) beneath the Archaean/Proterozoic lithosphere of the northern Baltic Shield that had been recently metasomatised. Melting of the carbonated garnet peridotite mantle formed a spectrum of magmas including carbonatite, damkjernite, melilitite, melanephelinite and ultramafic lamprophyre. Pockets of phlogopite metasomatised lithospheric mantle also melted to form potassic magmas including kimberlite. Depth of melting, degree of melting and presence of metasomatic phases are probably the major factors controlling the precise composition of the primary melts formed.

The Ethiopian continental flood basalt (CFB) province (ϳ30 Ma, Ͼ 3 ϫ 10 5 km 3 ) was formed as the result of the impingement of the Afar mantle plume beneath the Ethiopian lithosphere. This province includes major sequences of rhyolitic ignimbrites generally found on top of the flood basalt sequence. Their volume is estimated to be at least 6 ϫ 10 4 km 3 , which represents 20% of that of the trap basalts. Their phenocryst assemblage (alkali feldspar, quartz, aegyrine-augite, ilmenite Ϯ Ti-magnetite, richterite, and eckermanite) suggests temperatures in the range of 740 to 900°C. Four units were recognized in the field (Wegel Tena, Jima, Lima Limo, and Debre Birhan areas), each with its own geochemical specificity. Zr/Nb ratios remain constant between basalt and rhyolite in each area, and rhyolites associated with high-Ti or low-Ti basalts are, respectively, enriched or depleted in titanium. Their trace element and isotope (Sr, Nd, O) signatures (high 143 Nd/ 144 Nd and low 87 Sr/ 86 Sr ratios, compared to those of rhyolites from other CFB provinces) are clearly different from those of typical crustal melts and indicate that the Ethiopian rhyolites are among the most isotopically primitive rhyolites. Their major and trace element patterns suggest that they are likely to be derived from fractional crystallization of basaltic magmas similar in composition to the exposed flood basalts with only limited crustal contribution. Since Ethiopian high-Ti basalts have been shown to form from melting of a mantle plume, it is likely that Ethiopian ignimbrites, at least those that are Ti-rich, also incorporated material from the deep mantle.

Palaeomagnetic data are used to study the configurations of continents during the Proterozoic. Applying stringent reliability criteria, the positions of the continents at 12 times in the 2.45- to 1.00-Ga period have been constructed. The continents lie predominantly in low to intermediate latitudes. The sedimentological indicators of palaeoclimate are generally consistent with the palaeomagnetic latitudes, with the exception of the Early Proterozoic, when low latitude glaciations took place on several continents.The Proterozoic continental configurations are generally in agreement with current geological models of the evolution of the continents. The data suggest that three large continental landmasses existed during the Proterozoic. The oldest one is the Neoarchaean Kenorland, which comprised at least Laurentia, Baltica, Australia and the Kalahari craton. The protracted breakup of Kenorland during the 2.45- to 2.10-Ga interval is manifested by mafic dykes and sedimentary rift-basins on many continents. The second ‘supercontinental’ landmass is Hudsonland (also known as Columbia). On the basis of purely palaeomagnetic data, this supercontinent consisted of Laurentia, Baltica, Ukraine, Amazonia and Australia and perhaps also Siberia, North China and Kalahari. Hudsonland existed from 1.83 to ca. 1.50–1.25 Ga. The youngest assembly is the Neoproterozoic supercontinent of Rodinia, which was formed by continent–continent collisions during ∼1.10–1.00 Ga and which involved most of the continents. A new model for its assembly and configuration is presented, which suggests that multiple Grenvillian age collisions took place during 1.10–1.00 Ga. The configurations of Kenorland, Hudsonland and Rodinia depart from each other and also from the Pangaea assembly. The tectonic styles of their amalgamation are also different reflecting probable changes in sizes and thicknesses of the cratonic blocks as well as changes in the thermal conditions of the mantle through time.

The early Cretaceous (Albian–Aptian) Sung Valley ultramafic–alkaline–carbonatite complex is one of several alkaline intrusions that occur in the Shillong Plateau, India. This complex comprises calcite carbonatite and closely associated ultramafic (serpentinized peridotite, pyroxenite and melilitolite) and alkaline rocks (ijolite and nepheline syenite). Field relationship and geochemical characteristics of these rocks do not support a genetic link between carbonatite and associated

Two dimensional crustal models derived from four different ocean bottom seismographic (OBS) surveys have been compiled into a 1,580 km long transect across the North Atlantic, from the Norwegian Møre coast, across the extinct Aegir Ridge, the continental Jan Mayen Ridge, the presently active Kolbeinsey Ridge north of Iceland, into Scoresby Sund in East Greenland. Backstripping of the transect suggests that the continental break-up at ca. 55 Ma occurred along a west-dipping detachment localized near the western end of a ca. 300 km wide basin thinned to less than 20 km crustal thickness. It is likely that an east-dipping detachment near the present day Liverpool Land Escarpment was active during the late stages of continental rifting. A lower crustal high-velocity layer (7.2-7.4 km/s) interpreted as mafic intrusions/underplating, was present beneath the entire basin. The observations are consistent with the plume hypothesis, involving the Early Tertiary arrival of a mantle plume beneath central Greenland and focused decompression melting beneath the thinnest portions of the lithosphere. The mid-Eocene to Oligocene continental extension in East Greenland is interpreted as fairly symmetric and strongly concentrated in the lower crustal layer. Continental break-up which rifted off the Jan Mayen Ridge, occurred at ca. 25 Ma, when the Aegir Ridge became extinct. The first ca. 2 m.y. of oceanic accretion along the Kolbeinsey Ridge was characterized by thin magmatic crust (ca. 5.5 km), whereas the oceanic crustal formation since ca. 23 Ma documents ca. 8 km thick crust and high magma budget.

We report major and trace element abundances and Sr, Nd and Pb isotopic data for Miocene (16Á5^11 Ma) calc-alkaline volcanic rocks from the western segment of the Carpathian arc. This volcanic suite consists mostly of andesites and dacites; basalts and basaltic andesites as well as rhyolites are rare and occur only at a late stage. Amphibole fractionation both at high and low pressure played a significant role in magmatic differentiation, accompanied by highpressure garnet fractionation during the early stages. Sr^Nd^Pb isotopic data indicate a major role for crustal materials in the petrogenesis of the magmas.The parental mafic magmas could have been generated from an enriched mid-ocean ridge basalt (E-MORB)type mantle source, previously metasomatized by fluids derived from subducted sediment. Initially, the mafic magmas ponded beneath the thick continental crust and initiated melting in the lower crust. Mixing of mafic magmas with silicic melts from metasedimentary lower crust resulted in relatively Al-rich hybrid dacitic magmas, from which almandine could crystallize at high pressure. The amount of crustal involvement in the petrogenesis of the magmas decreased with time as the continental crust thinned. A striking change of mantle source occurred at about 13 Ma. The basaltic magmas generated during the later stages of the calc-alkaline magmatism were derived from a more enriched mantle source, akin to FOZO. An upwelling mantle plume is unlikely to be present in this area; therefore this mantle component probably resides in the heterogeneous upper mantle. Following the calc-alkaline magmatism, alkaline mafic magmas erupted that were also generated from an enriched asthenospheric source. We propose that both types of magmatism were related in some way to lithospheric extension of the Pannonian Basin and that subduction played only an indirect role in generation of the calc-alkaline magmatism.The calc-alkaline magmas were formed during the peak phase of extension by melting of metasomatized, enriched lithospheric mantle and were contaminated by various crustal materials, whereas the alkaline mafic magmas were generated during the post-extensional stage by low-degree melting of the shallow asthenosphere. The western Carpathian volcanic areas provide an example of long-lasting magmatism in which magma compositions changed continuously in response to changing geodynamic setting.

Amongst all the perceptible igneous manifestations (volcanic tuffs and agglomerates, minor rhyolitic flows and andesites, dolerite dykes and sills near the basin margins, etc.) in the Vindhyan basin, the two Mesoproterozoic diamondiferous ultramafic pipes intruding the Kaimur Group of sediments at Majhgawan and Hinota in the Panna area are not only the most conspicuous but also well-known and have relatively deeper mantle origin. Hence, these pipes constitute the only yet available 'direct' mantle samples from this region and their petrology, geochemistry and isotope systematics are of profound significance in understanding the nature of the sub-continental lithospheric mantle beneath the Vindhyan basin. Their emplacement age (∼ 1100 Ma) also constitutes the only reliable minimum age constrain on the Lower Vindhyan Group of rocks. The Majhgawan and Hinota pipes share the petrological, geochemical and isotope characteristics of kimberlite, orangeite (Group II kimberlite) and lamproite and hence are recognised as belonging to a 'transitional kimberlite-orangeite-lamproite' rock type. The name majhagwanite has been proposed by this author to distinguish them from other primary diamond source rocks. The parent magma of the Majhgawan and Hinota pipes is envisaged to have been derived by very small (< 1%) degrees of partial melting of a phlogopite-garnet lherzolite source (rich in titanium and barium) that has been previously subjected to an episode of initial depletion (extensive melting during continent formation) and subsequent metasomatism (enrichment). There is absence of any subduction-related characteristics, such as large negative anomalies at Ta and Nb, and therefore, the source enrichment (metasomatism) of both these pipes is attributed to the volatile-and K-rich, extremely low-viscosity melts that leak continuously to semi-continuously from the asthenosphere and accumulate in the overlying lithosphere. Lithospheric/crustal extension, rather than decompression melting induced by a mantle plume, is favoured as the cause of melting of the source regions of Majhgawan and Hinota pipes. This paper is a review of the critical evaluation of the published work on these pipes based on contemporary knowledge derived from similar occurrences elsewhere.