Marjorie Wilson

Imaging a mantle plume beneath the French Massif Central

Abstract 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.

Magmatism and rifting in Western and Central Africa, from Late Jurassic to Recent times

Abstract Mesozoic-Cenozoic magmatic activity in West and Central Africa is reviewed, with particular emphasis on the relationship between Mesozoic magmatism, major phases of continental rifting and the opening of the Equatorial Atlantic. It is suggested that during the initial stages of rifting, the activity of a mantle plume, the St. Helena hotspot, may have been important in weakening the lithosphere across the region. Evidence for magmatism concurrent with the onset of rifting in some basins supports such an active rifting model. Magma compositions range from alkali to tholeiitic basalts and their differentiates. Transitional to tholeiitic basalts are comparatively rare and are generated by greater degrees of partial melting, at probably shallower mantle depths, than associated alkali basalts. In some instances their occurrence may be correlated with higher amounts of lithospheric extension. However, in other instances they appear early in the rift sequence when overall amounts of extension were small. These tholeiitic basalts often have geochemical characteristics dominated by an ancient sub-continental lithosphere isotopic signature, which may have been introduced by crustal contamination. Cenozoic magmatism of alkaline affinity is widespread in West and Central Africa. In many instances, sites of activity appear to be structurally controlled by pre-existing basement fractures/lineaments of Mesozoic-Precambrian age. Most of the volcanic fields lie outside the boundaries of the Cretaceous rifts and many are associated with broad basement uplifts. However, there has also been a rejuvenation of tectono-magmatic activity within several of the Cretaceous rift basins during the Neogene. The parental magmas are considered to be generated mainly by partial melting of a zone at the base of the sub-continental lithosphere, which was variably metasomatised by the activity of mantle plumes beneath the African plate during the Mesozoic.

Thermal evolution of the Central Atlantic passive margins: continental break-up above a Mesozoic super-plume

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.

Melilitites: partial melts of the thermal boundary layer?

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 (<0.1%) partial melts from an underlying, convecting, depleted (MORB-source) mantle reservoir. On the basis of this observation, the regional geodynamic setting and the melilitite data, we propose that an isotopically distinct mantle plume, impinging on the base of the European lithosphere during the Early Cenozoic, is involved in the petrogenesis of the melilitite magmas.

Tertiary-Quaternary magmatism within the Mediterranean and surrounding regions

Abstract Tertiary-Quaternary magmatism within the Mediterranean and surrounding regions, including the European foreland of the Alps, the North African margin and the Eastern Mediterranean, occurs in three distinct associations: • anorogenic, extension-related intra-plate magmas, typically Na-rich alkali basalts, basanites and their differentiates, but locally including subalkaline basalts and potassic partial melts (e.g. leucitites) of the mantle lithosphere; • orogenic, subduction-related/post-collisional magma series related to plate convergence; typically with potassium-rich geochemical characteristics which may be related to subduction of continental crustal materials; • subalkaline basalts, similar to mid-ocean ridge basalts, formed at localized oceanic spreading centres. The magmatism is spatially and temporally associated with the Late Cretaceous-Cenozoic convergence of Africa-Arabia with Eurasia which resulted in the progressive closure (by subduction) of oceanic basins in the Mediterranean domain and ultimately the collision of the Alpine orogen with the southern passive continental margin of Europe. Break-off of subducted lithospheric slabs may have provided an important trigger for magmatism in several localities. The timing and geographical distribution of magmatism of orogenic and anorogenic affinity is summarized. Detailed discussion of the major and trace element and Sr-Nd-Pb isotope characteristics of the magmatism in the Central Mediterranean region provides important insights into the petrogenesis of the magmas and the nature of the main mantle source components.

Magmatism and the geodynamics of rifting of the Pripyat-Dnieper-Donets rift, East European Platform

Abstract 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.

Tertiary volcanism of the Galatia province, north-west Central Anatolia, Turkey

Abstract Large volumes of trachyandesitic-dacitic lava flows and pyroclastics of Miocene age are associated with small volumes of alkali basalt lava flows in the Galatia volcanic province, northwest Central Anatolia, Turkey. The volcanism postdates continental collision, occurring in a transtensional tectonic setting associated with movement along the North Anatolian Fault zone. Major and trace element (including REE) and Sr-Nd isotope data and K-Ar ages for representative samples of mafic-intermediate volcanic rocks have been obtained from a series of localities within the province. The K-Ar age data indicate that alkali basalts were erupted during two distinct time periods in the Early Miocene (17–19 Ma) and Late Miocene (

Mafic alkaline magmatism associated with the European Cenozoic rift system

Abstract Primitive mafic alkaline volcanic rocks from the Cenozoic rift system of western and central Europe have major and trace element and Sr-Nd-Pb isotopic characteristics which suggest the involvement of both lithospheric and asthenospheric mantle source components in their petrogenesis. The geochemical characteristics of the lithospheric component are in part constrained by those of spinel lherzolite and mafic granulite xenoliths entrained within the magmas. This component appears to be the product of partial melting of phlogopite/amphibole bearing mantle which was metasomatised as a consequence of magmatic activity during and preceeding the Hercynian orogeny and during subsequent phases of Permo-Carboniferous and Cenozoic extension. Partial melting of phlogopite is required to account for the generation of potassic magmas (leucitites and leucite nephelinites) with K 2 O Na 2 O ratios > 1. This component appears to differ between the individual Hercynian terrane blocks of Europe, reflecting, in part, their previous magmatic histories. The asthenospheric component has affinities with the source of HIMU OIB. This could represent a zone of enriched mantle in the thermal boundary layer at the base of the subcontinental lithosphere which is preferentially partially melted during extension. Alternatively it could, in part, be 500-400 Ma oceanic lithosphere subducted during the Hercynian orogeny. Although the data do not preclude the existence of deep mantle plumes, there is no need to invoke them in order to explain the HIMU characteristics.

Chronology and geodynamic setting of Cretaceous-Cenozoic rifting in West and Central Africa

Abstract The development of the Early Cretaceous to Palaeogene West and Central African rift system, which extends from Nigeria (Benue trough) to Kenya (Anza trough), can be related to the build-up of intraplate tensional stresses during the break up of Gondwana, which caused reactivation of pre-existing zones of lithospheric weakness. Repeated changes in the intraplate stress regime of Africa are reflected by phases of crustal extension alternating with episodes of compression. Many of these events can be correlated with changes in rates of seafloor spreading in the Central and South Atlantic oceans, as reflected in flowline patterns. The West and Central African rifts can be considered as typical ‘passive’ rifts which evolved in response to the build-up of intraplate stresses. However, the St. Helena hot spot appears to have been located beneath the Equatorial plate boundary at approximately 120 Ma and may have played an important role in weakening the lithosphere during extension.

Tertiary-Quaternary intra-plate magmatism in Europe and its relationship to mantle dynamics

Abstract 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