Abstract High-MgO lamproite and lamproite-like (i.e. lamprophyric) ultrapotassic rocks are recurrent in the Mediterranean and surrounding regions. They are associated in space and time with ultrapotassic shoshonites and high-K calc-alkaline rocks. This magmatism is linked with the geodynamic evolution of the westernmost sector of the Alpine–Himalayan collisional margin, which followed the closure of the Tethys Ocean. Subduction-related lamproites, lamprophyres, shoshonites and high-K calc-alkaline suites were emplaced in the Mediterranean region in the form of shallow level intrusions (e.g. plugs, dykes and laccoliths) and small volume lava flows, with very subordinate pyroclastic rocks, starting from the Oligocene, in the Western Alps (northern Italy), through the Late Miocene in Corsica (southern France) and in Murcia-Almeria (southeastern Spain), to the Plio-Pleistocene in Southern Tuscany and Northern Latium (central Italy), in the Balkan peninsula (Serbia and Macedonia) and in the Western Anatolia (Turkey). The ultrapotassic rocks are mostly lamprophyric, but olivine latitic lavas with a clear lamproitic affinity are also found, as well as dacitic to trachytic differentiated products. Lamproite-like rocks range from slightly silica under-saturated to silica over-saturated composition, have relatively low Al 2 O 3 , CaO and Na 2 O contents, resulting in plagioclase-free parageneses, and consist of abundant K-feldspar, phlogopite, diopsidic clinopyroxene and highly forsteritic olivine. Leucite is generally absent, and it is rarely found only in the groundmasses of Spanish lamproites. Mediterranean lamproites and associated rocks share an extreme enrichment in many incompatible trace elements and depletion in High Field Strength Elements and high, and positively correlated Th/La and Sm/La ratios. They have radiogenic Sr and unradiogenic Nd isotope compositions, high 207 Pb over 206 Pb and high time-integrated 232 Th/ 238 U. Their composition requires an originally depleted lithospheric mantle source metasomatized by at least two different agents: (1) a high Th/La and Sm/La (i.e. SALATHO) component deriving from lawsonite-bearing, ancient crustal domains likely hosted in mélanges formed during the diachronous collision of the northward drifting continental slivers from Gondwana; (2) a K-rich component derived from a recent subduction and recycling of siliciclastic sediments. These metasomatic melts produced a lithospheric mantle source characterized by network of felsic and phlogopite-rich veins, respectively. Geothermal readjustment during post-collisional events induced progressive melting of the different types of veins and the surrounding peridotite generating the entire compositional spectrum of the observed magmas. In this complex scenario, orogenic Mediterranean lamproites represent rocks that characterize areas that were affected by multiple Wilson cycles, as observed in the Alpine–Himalayan Realm.

ABSTRACT In this paper, we present a comprehensive review of literature data (~2600 analyses), including major and trace elements and Sr-Nd isotopes, on continental flood basalts from the Ethiopia-Yemen, Deccan (India), and Karoo (southern Africa) volcanic provinces in order to evaluate whether they can be attributable to similar tectonomagmatic processes that occurred during the past 200 m.y. in central Gondwana. Results indicate that the three investigated provinces share fundamental features, such as the following: (1) Major and trace element compositions are closely comparable, in terms of parental magmas and fractionation trends, for the various continental flood basalt suites recognized in the provinces, namely, low Ti (LT, TiO 2 0.5–3 wt%), high Ti (HT1, TiO 2 1–4 wt%), and very high Ti (HT2, TiO 2 2.5–7 wt%). (2) There is a clear zonal arrangement of continental flood basalts, with the hottest (potential temperature T p up to ~1600 °C) and deepest (up to 5 GPa) HT picrite-basalt magmas in the central area and cooler and shallower LT basalts (T p down to 1450 °C, pressure [ P ] = 2–3 GPa) at the periphery, corresponding to a maximum thermal difference of 60–110 °C from the inner to the outer zones in each province. This conforms to continental flood basalt generation from a lenticular melting region, plausibly reflecting thermo-compositionally zoned plume heads, with maximum excess temperature T ex = 250–300 °C with respect to the notional mid-ocean-ridge basalt (MORB) ambient mantle. (3) The central area of all provinces is characterized by the nearly exclusive occurrence of superheated HT picrite-basalt (and nearly coeval alkaline-carbonatite complexes) at the intersection of multiple extensional lineaments (faulting, rifting, and dike swarms), reflecting the focus of the tectonomagmatic activities. (4) The common occurrence of rhyolitic differentiates at the top of picrite-basalt lavas (e.g., Lalibela suite, northern Ethiopia; Pavagadh suite, Deccan; Lebombo suite, African Karoo) has to be considered an effect of the inversion of the stress regime, from generalized regional extension (continental flood basalt eruption) to localized continental rifting accompanying magma differentiation to rhyolites; activity at some of these rift and dike systems, e.g., the Western Afar Escarpment, the coastal dikes of Western Deccan, and the Rooi Rand dikes, was protracted until continental breakup and the opening of new oceanic branches of the Red Sea, the central Indian Ocean, and southwestern Indian Ocean, respectively. (5) The Sr-Nd isotope distributions of continental flood basalts show HT picrite-basalt magmas mostly recording mantle values unaffected by continental lithospheric signatures, and LT basalts mainly reflecting either mixed source components located at the lithosphere-asthenosphere transition or continental crust contamination, particularly in the Karoo and Deccan provinces. Overall, results from this review provide compelling evidence that hot mantle plumes impinged diachronously on the central Gondwana lithosphere, causing similar tectonomagmatic events and continental flood basalt zonal arrangements that reflect a common thermocompositional zonation of the plume head in the three investigated provinces.

A variety of xenoliths from the lower crust to mantle transition occur in Quaternary mafic intraplate lavas of the Bayuda volcanic field of northern Sudan. The lower-crust xenoliths include plagioclase- and garnet-bearing mafic granulite. Ultra-mafic garnet-bearing pyroxenite, websterite, hornblendite, and distinct peridotite xenoliths are from the upper lithospheric mantle. Sr, Nd, and Pb isotope signatures distinguish between ultramafic and granulite xenoliths. The latter show a strong compositional affinity to juvenile Neoproterozoic crust. The Pb isotope composition of the ultramafic xenoliths resembles the distinct high-μ signature ( 206 Pb/ 204 Pb >19.5) of their host basanite. These xenoliths may represent cumulates of late Mesozoic to Quaternary mafic intraplate magmatism. The felsic upper crust in a schematic lithospheric profile of the Bayuda area includes predominantly granitoids, migmatites, and metasedimentary rocks that represent reworked old cratonic or juvenile Neoproterozoic rocks. The deep lower crust is represented by mafic granulite, likely cumulate rocks from Neoproterozoic juvenile magmatism. The crust-mantle transition is characterized by ultramafic cumulate rocks possibly from the late Mesozoic to Quaternary magmatism. The peridotites of the same xenolith suites represent typical lithospheric mantle with variable degrees of depletion by melt extraction.

Assessments of the extension directions and their variations are critical for understanding rifting processes. This study provides an overview of the extension directions along the axes of the Main Ethiopian Rift and the Red Sea Rift (or propagator) of Afar, two of the three rifts meeting at the Afar triple junction. This overview is based on new and published field data on the opening direction of significant (width >0.2 m) Holocene extension fractures along the rift axis. The data show that the Red Sea propagator axis opens orthogonally, both in northern and central Afar, even though a significant strike-slip component is recognized at the rift margins in central Afar. The Main Ethiopian Rift axis also opens orthogonal to the trend of the rift, which varies between the different rift segments. Therefore, the axes of two of the three rifts meeting in Afar are characterized by orthogonal extension. However, given the variable orientations of the rift segments, the obtained opening directions are usually not uniform along the rift. Current plate-motion models suggest slightly different divergence directions, especially along the Main Ethiopian Rift, which shows a significant oblique component. The discrepancy between the data along the rift axis and those from plate-motion models suggests an across-rift strain partitioning. The observed orthogonal extension along the rift axis may be magma-induced, provided that a depth-dependent variation in the kinematics exists, at least below the Main Ethiopian Rift axis.

To further advance our understanding of the way in which a portion of the African superswell in eastern Africa formed, and also to draw attention to the importance of eastern Africa for the plume versus plate debate about mantle dynamics, upper-mantle structure beneath eastern Africa is reviewed by synthesizing published results from three types of analyses applied to broadband seismic data recorded in Tanzania, Kenya, and Ethiopia. (1) Joint inversions of receiver functions and surface wave dispersion measurements show that the lithospheric mantle of the Ethiopian Plateau has been significantly perturbed, much more so than the lithospheric mantle of the East African Plateau. (2) Body wave tomography reveals a broad (≥300 km wide) and deep (≥400 km) low-velocity anomaly beneath the Ethiopian Plateau and the eastern branch of the rift system in Kenya and Tanzania. (3) Receiver function stacks showing Ps conversions from the 410 km discontinuity beneath the eastern branch in Kenya and Tanzania reveal that this discontinuity is depressed by 20–40 km in the same location as the low-velocity anomaly. The coincidence of the depressed 410 km discontinuity and the low-velocity anomaly indicates that the low-velocity anomaly is caused primarily by temperatures several hundred degrees higher than ambient mantle temperatures. These findings cannot be explained easily by models invoking a plume head and tail, unless there are a sufficient number of plume tails presently under eastern Africa side-by-side to create a broad and deep thermal structure. These findings also cannot be easily explained by the plate model. In contrast, the breadth and depth of the upper-mantle thermal structure can be explained by the African superplume, which in some tomographic models extends into the upper mantle beneath eastern Africa. Consequently, a superplume origin for the anomalous topography of the African superswell in eastern Africa, in addition to the Cenozoic rifting and volcanism found there, is favored.

The Miocene–Holocene East African Rift in Ethiopia is unique worldwide because it subaerially exposes the transition between continental rifting and seafloor spreading within a young continental flood basalt province. As such, it is an ideal study locale for continental breakup processes and hotspot tectonism. Here, we review the results of a recent multidisciplinary, multi-institutional effort to understand geological processes in the region: the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE). In 2001–2003, dense broadband seismological networks probed the structure of the upper mantle, while controlled-source wide-angle profiles illuminated both along-axis and across-rift crustal structure of the Main Ethiopian Rift. These seismic experiments, complemented by gravity and magnetotelluric surveys, provide important constraints on variations in rift structure, deformation mechanisms, and melt distribution prior to breakup. Quaternary magmatic zones at the surface within the rift are underlain by high-velocity, dense gabbroic intrusions that accommodate extension without marked crustal thinning. A magnetotelluric study illuminated partial melt in the Ethiopian crust, consistent with an overarching hypothesis of magma-assisted rifting. Mantle tomographic images reveal an ~500-km-wide low-velocity zone at ≥75 km depth in the upper mantle that extends from close to the eastern edge of the Main Ethiopian Rift westward beneath the uplifted and flood basalt–capped NW Ethiopian Plateau. The low-velocity zone does not interact simply with the Miocene–Holocene (rifting-related) base of lithosphere topography, but it provides an abundant source of partially molten material that assists extension of the seismically and volcanically active Main Ethiopian Rift to the present day.

A comprehensive petrological study carried out on Ethiopian mantle xenoliths entrained in Neogene–Quaternary alkaline lavas overlying the continental flood basalt area (Dedessa River–Wollega region, Injibara-Gojam region) and from the southern Main Ethiopian Rift (Mega-Sidamo region) provides an ideal means to investigate mantle evolution from plume to rift settings. Mantle xenoliths from the plateau area (Injibara, Dedessa River) range in composition from spinel lherzolite to harzburgite and olivine websterite, showing pressure-temperature ( P-T ) equilibrium conditions in the range 1.3–0.9 GPa and 950–1050 °C. These xenoliths show flat chondrite (ch)–normalized bulk-rock rare earth element (REE) patterns, with only few light (L) REE–enriched samples (La N /Yb N up to 7) in the most refractory lithotypes. Clinopyroxene (cpx) REE patterns are mostly LREE depleted (La N /Yb N down to 0.2) or enriched (La N /Yb N up to 4.4). Sr-Nd isotopes of clinopyroxene mainly show compositions approaching the depleted mantle (DM) end member ( 87 Sr/ 86 Sr < 0.7030; 143 Nd/ 144 Nd > 0.5132), or less depleted values ( 87 Sr/ 86 Sr = 0.7033–0.7034; 143 Nd/ 144 Nd = 0.5129–0.5128) displaced toward the enriched mantle components that characterize the Afar plume signature and the related Ethiopian Oligocene continental flood basalts. The 3 He/ 4 He (R a ) values of olivines range from 6.6 to 8.9 R a , overlapping typical depleted mantle values. These characteristics suggest that most xenoliths reflect complex asthenosphere-lithosphere interactions due to refertilization processes by mafic subalkaline melts that infiltrated and reacted with the pristine peridotite parageneses, ultimately leading to the formation of olivine-websterite domains. On the other hand, mantle xenoliths from the southern Main Ethiopian Rift (Mega-Sidamo region) consist of spinel lherzolite to harzburgites showing various degree of deformation and recrystallization, coupled with a wider range of P-T equilibrium conditions, from 1.6 ± 0.4 GPa and 1040 ± 80 °C to 1.0 ± 0.2 GPa and 930 ± 80 °C. Bulk-rock REE patterns show generally flat heavy (H) REEs, ranging from 0.1 chondritic values in harzburgites up to twice chondritic abundances in fertile lherzolites, and are variably enriched in LREE, with La N /Yb N up to 26 in the most refractory lithologies. The constituent clinopyroxenes have flat HREE distributions and La N /Yb N between 0.1 and 76, i.e., in general agreement with the respective bulk-rock chemistry. Clinopyroxenes from lherzolites have 87 Sr/ 86 Sr = 0.7022–0.7031, 143 Nd/ 144 Nd = 0.5130–0.5138, and 206 Pb/ 204 Pb = 18.38–19.34, and clinopyroxenes from harzburgites have 87 Sr/ 86 Sr = 0.7027–0.7033, 143 Nd/ 144 Nd = 0.5128–0.5130, and 206 Pb/ 204 Pb = 18.46–18.52. These range between the DM and high-μ (HIMU) mantle end members. The helium isotopic composition varies between 7.1 and 8.0 R a , comparable to the xenoliths from the plateau area. Regional comparison shows that HIMU-like alkali-silicate melt(s), variably carbonated, were among the most effective metasomatizing agent(s) in mantle sections beneath the southern Main Ethiopian Rift, as well as along the Arabian rifted continental margins and the whole East African Rift system. The different types of metasomatic agents recorded in Ethiopian mantle xenoliths from the continental flood basalt area and the rift systems clearly reflect distinct tectonomagmatic settings, i.e., plume-related subalkaline magmatism and rift-related alkaline volcanism, with the latter extending far beyond the influence of the Afar plume.

New and published whole-rock major-element contents of xenolithic peridotites, combined with mineral trace-element, 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, and 3 He/ 4 He isotopic compositions, are used to unravel the metasomatic history of lithospheric mantle sampled by volcanic pipes in the Tanzanian section of the East African Rift. The deepest portion of the mantle beneath Labait (craton margin) exhibits high-μ (HIMU)–like 87 Sr/ 86 Sr (0.7029), 143 Nd/ 144 Nd (0.51286), and 3 He/ 4 He (5.9), which may reflect the plume in this region. Within the Mozambique belt, recent calcio-carbonatite melt metasomatism has overprinted the mantle lithosphere signature beneath Olmani, leading to high whole-rock Ca/Al and low SiO 2 , and remarkably homogeneous 87 Sr/ 86 Sr (0.7034–0.7035) and 143 Nd/ 144 Nd (0.51281–0.51283) of clinopyroxenes. Identical Sr and Nd isotope values are also reported for clinopyroxenes from peridotite xenoliths from the northern portion of the Gregory Rift in Tanzania (Pello Hill and Eledoi), which have a strong rift magma overprint. The silicate and carbonatite metasomatic melts are likely to be related to recent plume-derived magmatism of the East African Rift, and thus 87 Sr/ 86 Sr and 143 Nd/ 144 Nd values of the clinopyroxene from these samples can be used to define the rift isotopic signature beneath northern Tanzania. Some mantle regions beneath Lashaine and Labait escaped the recent rift-related overprint and have highly variable Sr-Nd isotope systematics. Labait clinopyroxenes show a near-vertical array on a 87 Sr/ 86 Sr versus 143 Nd/ 144 Nd plot, indicating highly variable time-integrated rare earth element (REE) patterns and low time-integrated Rb/Sr. Lashaine peridotites range to much higher 87 Sr/ 86 Sr at a given 143 Nd/ 144 Nd, and several plot in the right quadrants in the 87 Sr/ 86 Sr versus 143 Nd/ 144 Nd diagram, suggesting the influence of a (subducted?) Archean upper continental crust component on the lithospheric mantle beneath Lashaine. Their variable whole-rock SiO 2 and high Na 2 O contents, and clinopyroxene with high Sr/Y, low Sm/Nd, and variable Zr/Sm are consistent with this interpretation. Silicate lavas from the eastern branch of the East African Rift show increasingly evolved Sr and Nd isotope composition from north to south and hence increasing input of ancient metasomatized lithosphere (“EM1” and “EM2” components), similar to that beneath Lashaine and Labait, and well outside the suggested range in isotope compositions of the heterogeneous Kenya plume ( 87 Sr/ 86 Sr = 0.7029–0.7036; 143 Nd/ 144 Nd = 0.51275–0.51286). In the western branch, the “anomalous” Sr signature identified in Lashaine peridotites is prominent in silicate lavas and may indicate that the lithospheric mantle beneath that area was similarly enriched during ancient subduction. By contrast, the Sr-Nd isotope systematics of carbonatites reflect EM1 but not EM2 inputs, suggesting that such melts in the East African Rift neither derive from nor have interacted with subduction-modified mantle regions.

New major-element, trace-element, and isotopic (Nd, Sr) analyses of undersaturated alkaline lavas from the Kilimanjaro volcano (north Tanzania) are presented. These data concern 54 samples, ranging from basanites to phonolites, collected during a 1 mo field trip in March 2005. The three main cones of Kilimanjaro were sampled, Shira, Mawenzi, and Kibo, together with numerous parasitic cones located on a SE lineament on the main edifice. On the basis of both spatial distribution and major- and trace-element characteristics of analyzed samples, the previous classification of Kilimanjaro lavas is simplified into five groups: Shira, Mawenzi, Kibo 1, Kibo 2, and parasitic activity, each of which has distinct petrological and geochemical features. The major- and trace-element characteristics of the rare primitive lavas erupted on the volcano yield the ubiquitous signature of amphibole within the magma source. We propose that Kilimanjaro melts originated from the partial melting of lithospheric mantle. Combined modeling of trace-element behavior during partial melting + fractional crystallization and isotopic constraints allow us to propose a schematic model of melt genesis under the Kilimanjaro area. Thermal heating of the ancient continental lithosphere by an upwelling plume triggered partial melting in parts of the lithosphere where amphibole was present and led to the Shira volcanic episode. Then, during a time span of ~1 m.y., the depleted lithosphere was progressively infiltrated by plume melts that resulted in crystallization of a new generation of metasomatic amphibole. Finally, this rejuvenated lithosphere underwent partial melting, leading to the magmas that formed the main edifice (Mawenzi and Kibo), and leading to a progressive depletion of the source with time. An active contribution of true asthenospheric melts during the last magmatic events of the volcano cannot be excluded, but further detailed isotopic investigations are needed to test the model.

Fieldwork was conducted in the active north crater of Oldoinyo Lengai volcano, Tanzania. Gases, aqueous fumarole condensates, and freshly erupted natrocarbonatite lavas were collected from several hornitoes associated with the same eruptive center and are considered to represent genetically related products of the same shallow magma chamber. Apparent trace-metal mineral-mineral partition coefficients were derived for the major carbonate phases, gregoryite and nyerereite, and several accessory phases within the fresh lava samples. Trace metals display an affinity for the accessory minerals. Textural information suggests that fluorite and coexisting sylvite are also present interstitially as quenched immiscible salt melts, and that any trace metals present may be scavenged from the carbonatite by the immiscible separation of these salt phases. Gas condensate analyses from the fumaroles associated with the eruption reveal further partitioning of trace elements into the vapor phase. Chalcophile elements show particularly high volatility, and this implies either gas release prior to sulfide formation or the decomposition of sulfides prior to eruption. The strong partitioning of metals into the halogenide and vapor phases has broad implications for the mobility of trace elements in the mantle source, the genesis of exotic mineralization associated with other carbonatites, and the ability of fumarole condensates to carry a direct chemical signature from their parent magma.

Alkaline magmatism along the Cameroon Line has been active for at least 67 m.y. and is currently defined by an almost SW-NE geological lineament (mean value: N30°E). Available petrological, geochemical, and structural data obtained over the last 20 yr lead us to reappraise its mechanism of emplacement. Known as the second most important geological curiosity in Africa, after the East African Rift system, it displays a continental part and an oceanic part, a unique feature in Africa and even in the world. The continental part contains both plutonic and volcanic massifs, and the oceanic part consists only of volcanic massifs. Plutonic rocks as a whole define a complete series of gabbro-diorite-monzonite-syenite-granite type, whereas volcanic rocks display abundant basic (basalt-hawaiite) and felsic (trachyte-phonolite-rhyolite) lavas with very few intermediate ones (mugearite-benmoreite). The formerly entire alkaline nature of these rocks is here ruled out by the discovery of volcanoes with geochemically transitional affinities in some areas of the continental sector. On the other hand, new K-Ar and 40 Ar/ 39 Ar dates confirm the absence of any age migration associated with the SW-NE linear trend. This lack of steady time-space migration and the SW-NE trend have also been observed in the magmatic provinces of Nigeria and Benue Trough, which share similar geochemical features with the Cameroon Line, and along the NE-SW major igneous lineaments in South Africa. The mechanism of such episodic emplacement of alkaline magmatism can be better explained in terms of complex interactions between hotspots and lithospheric fractures during African plate motion.

Basaltic lavas of Nyos volcano (Cameroon) mostly contain mantle peridotite xenoliths consisting of spinel-bearing lherzolites and harzburgites. Based on the trace-element patterns, especially rare earth element (REE) patterns, two groups of samples have been distinguished: group 1 samples are characterized by spoon-shaped REE patterns, and group 2 samples show light (L) REE–enriched patterns. Mineralogical characteristics together with major- and trace-element compositions point to a low degree of partial melting (less than 5%) and metasomatic processes. The latter mechanism explains in particular the LREE content of bulk rocks and clinopyroxenes and the occurrence of hydrous minerals in some samples. All the metasomatic features observed in both groups of samples are related to more or less alkaline—and carbonated—mafic silicate melts. These melts are related to the magmatic activity of the Cameroon volcanic line, leading in particular to the eruption of the host lava xenoliths.

Volcaniclastic carbonatites in southeast Zambia contain dolomitic melt lapilli, in which high-Cr chromite phenocrysts indicate direct eruption from the mantle. There are no lava flows. Vent tuffisite, with dolomite lapilli in a matrix of dolomite + iron oxides, provides the least contaminated samples of primary erupted material. This tuffisite has high trace-element levels typical of carbonatite, but its high Cr and Ni make it exceptional. Fragmental phlogopite, sanidine, and orthoclase are indicators of potassium (K) mineralization in deeper parts of the conduit. This was the only known case of dolomite eruption directly from source until 2005, when dolomite volcanism in Spain and France provided the first opportunity for comparing these characteristics. Eruptions are fragmental: typically with rounded lapilli of low-Fe, high-Mn dolomite. Outstanding common features are euhedral chrome spinels, and a high-temperature, platy habit of the dolomite crystals. In all cases, the sparse interstitial residuum between the dolomite plates is potassic, and in Spain and France, the Cr spinels are zoned to high-Ti rims, analogous to those in high-temperature kimberlites. Experiments have long predicted that dolomite should be the initial melt from carbonated mantle below 70 km: In Spain and France, the eruptions carry mantle xenoliths, and in all three provinces, the dolomite volcanic rocks have Mg# >0.65, indicative of primary magmas. Thus, for the first time, fresh constraints are emerging from a multicomponent natural system. Bearing in mind the differences between the European (Cenozoic) and Zambian (Cretaceous) provinces (including lithosphere structures, history, and tectonics), their common features indicate that dolomite melts in the mantle are in equilibrium with chromite, and contain K-Al-Si melts (without Na). The Zambian vents are part of the large, classic Chilwa carbonatite province, which was first described in Malawi and Mozambique, where, around the intrusions, K-metasomites typically exceed carbonatite in amount. When this K is included in the eruptive budget, the total introduced material invites comparison with high-K carbonate melts formed at high pressure, e.g., carbonatitic fluid inclusions in diamonds that were trapped in the diamond stability field are dolomitic and characterized by high-K contents. Intrusive and volcanic carbonatites show a bimodal distribution in Sr-Nd isotopes, similar to group 1 and 2 kimberlites, respectively. Diamond inclusions span the same range. Together with the indications of a deep mantle source for the volcanic facies at Rufunsa, this raises the possibility that the Chilwa complexes may be highlighting a key aspect of carbonatite activity in which potassium has a special role.

Post-Paleozoic magmatism in Angola and Namibia (SW Africa) is widespread along the continental margin (flood tholeiites of the Paraná-Etendeka system), and along transverse lineaments (alkaline and alkaline-carbonatitic complexes; sodic and potassic suites). These different magmatic suites are strictly associated in space and/or time. Variable melting degrees of a veined lithospheric mantle are proposed for the most “primitive” magmas from geochemical modeling and Sr-Nd isotope systematics. A complex evolution emerges for some ultramafic rocks (cumulus processes) and for differentiated rock compositions (assimilation and fractional crystallization, AFC, magma mixing), which may also involve anatexis of the crystalline basement and emplacement of S-type granites and rhyolites. Melting of a lithospheric mantle, without an appreciable contribution of the asthenosphere (thermal input excepted), is consistent with regional thermal anomalies in the deep mantle, mapped by gravity of the geoid, seismic tomography, and paleomagnetic analysis. The Walvis Ridge and Rio Grande “hotspot tracks” are interpreted as stress response in the lithosphere during rifting. A plume-related heat source is not favored by our results.

Thick continental lithosphere (tectosphere) beneath African cratons has been stable for ~2.5 b.y. despite its mechanical interaction with sublithospheric mantle. Water is known to have significant influence on mechanical stiffness, and the depletion of water is often considered to be a key to preserving the thick lithosphere. Although water-rich environments indicated by the present water content of cratonic xenoliths appear to contradict this hypothesis, these water contents might have been modified at later stages due to the high diffusivity of hydrogen in minerals. Deformation microstructures such as lattice-preferred orientation indicate water-poor conditions (<200 ppm H/Si) during long-term plastic deformation in the continental lithosphere. Analysis of convective instability further constrains the water content to be less than 100 ppm H/Si. We suggest that the continental tectosphere beneath southern Africa must have a low water content, at least one order of magnitude less than oceanic upper mantle, and that the present-day water content of cratonic xenoliths most likely reflects localized metasomatism before eruption.

The Massif d'Ambre is the largest stratovolcano (~2500 km 2 ) in the Cenozoic igneous province of northern Madagascar. It is broadly elongated in a N-S direction and is formed by hundreds of lava flows, plugs, spatter cones, tuff rings, pyroclastic flows, and pyroclastic fall deposits. New 40 Ar- 39 Ar age determinations for lavas of Massif d'Ambre and Bobaomby Peninsula (the northernmost tip of Madagascar) yield ages of 12.1 ± 0.2 Ma and 10.56 ± 0.09 Ma. These ages indicate that at least part of the volcanic activity of the Bobaomby Peninsula occurred later than the beginning of the activity of the Massif d'Ambre. The volcanic products of Massif d'Ambre are mildly to strongly alkaline (with sodic affinity) to tholeiitic with very limited amounts of evolved magmas. The mafic rocks have compositions similar to those of primitive mantle–derived magmas (MgO >10 wt%, Cr and Ni >400 and >200 ppm, respectively). The strongly alkaline suite shows a liquid line of descent from basanite to phonolite, dominated by fractional crystallization of clinopyroxene and olivine. The mafic rocks (basanites, alkali basalts, transitional and tholeiitic basalts) have Zr/Nb (2.4–5.8), Ba/Nb (7–24) and La/Nb (0.7–1.1) ratios typical of incompatible element–rich within-plate basalts. The primitive mantle–normalized incompatible element patterns of the Massif d'Ambre mafic rocks are characterized by peaks at Nb and troughs at K, and are identical in shape and absolute abundances to those of the Nosy Be and Bobaomby (Cap d'Ambre) basanites. The range of (La/Yb) n ratios (9–24) indicates that the Massif d'Ambre primitive compositions are the product of variable degrees of partial melting (4%–12%) of a broadly similar and slightly incompatible element–enriched mantle source. Initial 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios of alkali basalts and basanites vary from 0.70326 to 0.70359 and 0.51279 to 0.51286, respectively. Alkali basalts and basanites have little variation in 206 Pb/ 204 Pb (19.073–19.369), 207 Pb/ 204 Pb (15.613–15.616), and 208 Pb/ 204 Pb (39.046–39.257). This range is well within that of Sr-Nd-Pb isotope values of the basanites of the Nosy Be Archipelago, thus again confirming substantially similar source compositions throughout northern Madagascar.

Based on phase geochemistry and Re-Os isotopic ratios, an exotic (in an oceanic setting) K-rich silicate melt, named kimberlite-type, has been claimed to be the metasomatizing agent interacting with subcontinental lithospheric mantle fragments beneath the Cape Verde Archipelago. On the basis of textural features and major- and trace-element chemistry, we constrain key geochemical indicators able to discriminate percolation at depth of this exotic melt from infiltration of the host magma in Cape Verde mantle xenoliths. Cape Verde type A lherzolites and harzburgites show evidence of dissolution of the primary phases (mainly pyroxenes) and the presence of large patches of secondary mineral (and glass) assemblages, and they do not show textural evidence of host basalt infiltration. Cape Verde type A mantle xenoliths frequently contain almost pure K-feldspar (An 3.8–8.8 , Ab 6–24 , Or 72–89 ) in the secondary mineral assemblage. They have an anomalously high K content (up to 0.49 wt%), and K/Na ratios generally >1, with respect to Cape Verde peridotites clearly affected by host basalt infiltration (type B samples). The dichotomy between Na and K observed in the two textural types suggests that the Na-alkaline host basalt (K/Na <1), which infiltrated at low pressure, was able to modify the whole-rock Na content of the xenoliths (type B samples). In turn, a completely different K-rich alkaline melt, which interacted at depth with the peridotite, imposed its alkali ratio (K/Na >1) on the bulk composition and formed the type A xenoliths. The kimberlite-type metasomatic agent, which reacted with the Cape Verde peridotite assemblage (mainly orthopyroxenes) in those regions where the mantle xenoliths are entrapped in the host basalt ( P = 17 kbar; T = 1092 °C), reasonably tends toward SiO 2 -saturated, K-rich basic magmas (lamproite-type?) with K-feldspars as the “liquidus” phase. Isotopic data on separate clinopyroxenes do not contribute to discrimination between metasomatism and infiltration processes but certainly concur to reinforce the hypothesis that a fragment of subcontinental lithospheric mantle domain was preserved during the opening of the Atlantic Ocean, forming K-rich undersaturated silicate melts that percolated through the peridotite matrix. Whole-rock major- and trace-element and isotopic geochemistry alone would not contribute to the interpretation of the processes occurring in the mantle xenolith. The most reliable tool would be an in situ mineral (and glass) study, which would be valid for Cape Verde mantle xenoliths and others. Small-melting-degree undersaturated silicate melts percolating at depth are olivine-saturated and may form, by reaction and dissolution of pyroxene, type A olivine without substantially modifying the original Fe/Mg peridotite ratio. By contrast, under low-pressure (<1.5 GPa), high-temperature regimes, olivine silicate melts infiltrating the mantle xenoliths form type B olivine, in which Fe/Mg ratios will be controlled by fractionation. Mantle diopsides interact (at depth) with undersaturated silicate melts, rearranging the most fusible elements into a new diopside composition: type A clinopyroxene. By contrast, diopsides that interact with melts at progressively lower pressure react and are locally rearranged in a new chemical structure that is able to accommodate the high diffusive elements (i.e., Fe and Ti): type B aegirine-augites. Fe 3+ in spinel is a key element in the investigation of the processes acting on Cape Verde mantle xenoliths. As a metasomatic product, secondary chromian spinel tends toward a Fe 3+ -buffered composition, mainly depending on pressure and chemistry of the magma. A decompression system is able to change the percolation regime from porous flow to open conduit. At this stage, the chromian spinel would be the low-pressure phase able to accommodate larger amounts of Fe 3+ . Type A glasses have exceptionally high K 2 O content, and, when associated with K-feldspars, they are buffered at ~9 K 2 O wt%, in a silica range of 55.7–66.8 wt%. By contrast, type B glasses follow a hypothetical major-element trend toward the host basanites. In conclusion, the compositional features (in particular major elements) of minerals and glasses in relation to their chemical behavior in mantle systems are the most efficient tools to distinguish metasomatism-related (type A) from infiltration-related (type B) samples and consequently to place the mantle xenoliths in a correct genetic framework.

Major- and trace-element and Sr-Nd-Pb isotope compositions are presented for an extensive data set of samples (44) recovered along the active ridge axis of the southern Tyrrhenian backarc basin represented by the Marsili volcano (<0.7 Ma). In addition, major and trace elements are presented for the few lavas sampled from the summit area (active at 0.4–0.1 Ma) of Vavilov volcano, located in the oceanic subbasin (i.e., Vavilov Basin) formed during the first stage of the southern Tyrrhenian backarc basin formation. Overall, these data confirm that southern Tyrrhenian backarc basin magmatism is chemically heterogeneous, ranging from an island-arc basalt (IAB) type, prevalent throughout the backarc evolution, to an ocean-island basalt (OIB)–like magmatism that occurred later in the development of the basin. Since the lavas sampled from Vavilov volcano have been strongly altered, their isotope composition was not acquired in this study. Thus, our attempt to identify the mantle domains beneath the southern Tyrrhenian system was restricted to its youngest and active part, i.e., the Marsili backarc basin and the Aeolian arc. These new data together with previously published trace-element and Sr-Nd-Pb isotope data for Marsili volcano lavas reveal that two mantle domains are involved in their petrogenesis: One represents the southern Tyrrhenian ambient mantle from which the mid-ocean-ridge basalts (MORB) flooring the Vavilov backarc basin are derived, and the other has HIMU (high U/Pb) OIB-like character, resembling the mantle source of the nearby Ustica Na-alkaline lavas. Subduction-related signatures characterize both, although to a lesser extent in the OIB-like domain with respect to the MORB-like mantle. The new data provide a much needed insight into the evolution of the southern Tyrrhenian backarc basin system, confirming that the HIMU OIB-like component results from the propagation of deep northern African mantle inflow around the southern tear of the subducting Ionian slab, rather than being a component that was present in the mantle wedge prior to the Ionian subduction process. Furthermore, a comparison between the Marsili backarc and Aeolian arc lavas permits interpretation of the trajectories followed by the African OIB-like mantle inflow within the southern Tyrrhenian mantle wedge. In particular, mantle trajectories involve upward-directed flow from the slab edge beneath Ustica Island and Prometeo submarine lava field, slab-parallel flow beneath the Marsili backarc volcano, and arcward-directed flow affecting the western margin of the Aeolian arc, thus compositionally influencing the Alicudi basic lavas.