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 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.

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.

Abstract Supplementary material: An extended dataset for calatrava xenoliths is available at: http://geolsoc.org.uk/sup18410 . Mantle xenoliths from the Calatrava Volcanic District (CLV), central Spain, are characterized by a wide compositional range that includes lherzolites (prevalent), as well as minor amounts of wehrlite, olivine (ol)-websterite and rare dunites. They generally have a bulk-rock Mg# of less than 89, lower than any primordial mantle estimates. Intra-suite variations in modal proportions are inconsistent with those predicted by melting models irrespective of the starting composition; mineral and bulk-rock variation diagrams show inconsistencies between the CLV compositions (anomalously enriched in Fe–Ti) and those predicted from the partial melting of primordial mantle material. Processes other than pure melt extraction are confirmed by the whole-rock REE (rare earth element) budget, typically characterized by LREE enrichments, with La N /Yb N (up to 6.7), probably related to pervasive metasomatism. CLV mantle clinopyroxenes (cpx) generally display fractionated REE patterns with upwards-convex shapes, characterized by low HREE (Tm–Lu) concentrations (typically <6× chondrite) and enrichments in middle–light REE (MREE–LREE) (Nd N /Yb N up to 7, La N /Yb N up to 5). These ‘enriched’ cpx compositions either result from re-equilibration of primary mantle cpx with an incoming melt, or represent cpx crystallization directly from the metasomatic agent. The latter was plausibly generated at greater depths in the presence of residual garnet (from peridotite or eclogite starting materials). Separated cpx have homogeneous 87 Sr/ 86 Sr compositions between 0.7031 and 0.7032; 143 Nd/ 144 Nd ranges from 0.51288 to 0.51295 (ɛNd 4.74–6.07) and 176 Hf/ 177 Hf is in the range 0.28302–0.28265 (ɛHf −3.6 to 9.0). Unlike mantle xenoliths and alpine-type peridotites from other Iberian occurrences, which range in composition from the depleted mantle (DM) to the enriched mantle (EM), the CLV mantle cpx approach the composition of the HIMU mantle end member, the genesis of which is generally interpreted as the result of long-term recycling of oceanic basalts/gabbros (or their eclogitic equivalent) via ancient subduction. A model is proposed for the mantle evolution under central Iberia, where sublithospheric convective instabilities – possibly triggered by the neighbouring subduction along the Betic collisional belt – could have remobilized deep domains from the mantle ‘transition zone’ (410–660 km), which may include relicts of older subducted slabs. Within these remobilized domains, characterized by the coexistence of peridotite and eclogite and referred to as a ‘piclogite’ association, the eclogites melt preferentially generating Fe–Ti rich melts characterized by a HIMU isotopic signature that infiltrates and metasomatizes the shallower lithospheric mantle.

We investigated the petrogenetic characteristics of the Paleogene Veneto volcanic province and compared them with other intraplate magmatic occurrences of the Adria–North Africa plate since Late Cretaceous time. Veneto volcanic province magmas were erupted through a transtensional rift system that resulted from intra-plate reactions to the Alpine collisional events. The lavas, mostly basic in composition, encompass a wide range of serial affinities from (mela)-nephelinites to quartz-normative tholeiites. Nephelinites and basanites often carry spinel-peridotite mantle xenoliths that have rheologic and thermobarometric characteristics that indicate an origin from the mechanical boundary layer at depths not exceeding 50–60 km. Incompatible element patterns of the most primitive Veneto magmas, together with their isotopic signature ( 87 Sr/ 86 Sr 0.70315–0.70386; 143 Nd/ 144 Nd 0.51279–0.51298; 206 Pb/ 204 Pb 18.8–19.8), share geochemical characteristics with other magmatic occurrences of Adria–North Africa domains, and they show a clear affinity with intraplate sodic lavas, particularly HIMU (high U/Pb = high µ) and, to a lesser extent, enriched-mantle–ocean-island basalt (EM2-OIB) magmas. An integrated petrogenetic model, generally applicable for Adria–North Africa domains, suggests that most of the magmas were generated within the spinel-peridotite lithospheric mantle, from progressively deeper sources (30–100 km) and with a concomitant decrease in the degrees of partial melting (25%–3%) from quartz-normative tholeiites to nephelinites. The modeled magma sources invariably require enrichments in incompatible elements and metasomatic phases comparable (or equivalent) to those observed in some mantle xenoliths associated with the Veneto volcanic province lavas. Two kinds of mantle sources were identified: lherzolites bearing amphibole ± phlogopite for tholeiites to basanites, and lherzolites bearing amphibole ± phlogopite plus carbonatitic components for nephelinites. The elemental and isotopic characteristics of these mantle sources correspond to variable mixing of HIMU and, to a lesser extent, EM2 metasomatic components with a pristine depleted-mantle (DM) lithosphere. The HIMU metasomatizing agents may possibly be related to the mantle plume that is thought to extend from the eastern Atlantic to Europe and the Mediterranean, including Adria–North Africa domains, since the Late Cretaceous. These components more effectively accumulated in the lower lithospheric portion, i.e., the thermal boundary layer, whereas older metasomatic EM2 components may have been better preserved in the upper, more rigid, mechanical boundary layer.