High-MgO lavas associated to CFB as indicators of plume-related thermochemical effects: the case of ultra-titaniferous picrite-basalt from the Northern Ethiopian-Yemeni plateau

A comprehensive petrological and geochemical dataset is reported in order to define the thermo-compositional characteristics of Ti (Fe)-enriched picrite-basalt lavas (HT2, TiO2 3-7 wt%), erupted close to the axial zone of the inferred Afar mantle plume, at the centre of the originally continuous Ethiopian-Yemeni CFB plateau (ca. 30Ma) which is zonally arranged with progressively lower Ti basalts (HT1, TiO2 2-4 wt%; LT, TiO2 1-3 wt%) toward the periphery. Integrated petrogenetic modelling based on major and trace element analyses of bulk rocks, minerals and melt inclusions in olivines, as well as Sr-Nd-Pb-He-O isotope compositional variations enables us to make several conclusions. 1) The phase equilibria constraints indicate that HT2 primary picrites were generated at ca. 1570°C mantle potential temperatures (Tp) in the pressure range 4-5 GPa whereas the HT1 and LT primary melts formed at shallower level (< 2 to 3 GPa, Tp 1530 °C for HT1 and 1430°C for LT). Thus the Afar plume head was a thermally and compositionally zoned melting region with maximum excess temperatures of 300-350°C with respect to the ambient mantle. 2) The HT2 primary melts upwelled nearly adiabatically to the base of the continental crust (ca. 1 GPa) where fractionation of olivine, followed by clinopyroxene, led to variably differentiated picritic and basaltic magmas. 3) Trace element modelling requires that the primary HT2 melts were generated - either by fractional or batch melting (F 9-10%) - from a mixed garnet peridotite source (85%) with 15% eclogite (derived from transitional MORB protoliths included in Panafrican terranes) that has to be considered a specific Ti-Fe and incompatible element enriched component entrained by the Afar plume. 4) The LT, HT1 and HT2 lavas have 143Nd/144Nd = 0.5131-0.5128, whereas Sr-Pb isotopes are positively correlated with TiO2, varying from 87 Sr/86Sr 0.7032 and 206Pb/204Pb 18.2 in LT basalts to 87Sr/86Sr 0.7044 and 206Pb/204Pb 19.4 in HT2 picrite-basalts. High 3He/4He (15-20 RA) ratios are exclusively observed in HT2 lavas, confirming earlier evidence that these magmas require a component of deep mantle in addition to eclogite, while the LT basalts may more effectively reflect the signature of the pre-existing mantle domains. The comparison between high-MgO (13-22%) lavas from several Phanerozoic CFB provinces (Karoo, Paranà-Etendeka, Emeishan, Siberia, Deccan, North Atlantic Province) shows that they share extremely high mantle potential temperatures (Tp 1550-1700°C) supporting the view that hot mantle plumes are favoured candidates for triggering many LIPs. However, the high incompatible element and isotopic variability of these high-MgO lavas (and associated CFB) suggest that plume thermal anomalies are not necessarily accompanied by significant and specific chemical effects, which depend on the nature of mantle materials recycled during the plume rise, as well as by the extent of related mantle enrichments (if any) on the pre-existing lithospheric section

Petrology of the basaltic rocks of the Nankay Trough Basement

Major- and trace-element analyses, mineral chemistry, and Sr-Nd isotopic determinations were obtained on representative igneous rocks drilled from the Nankai accretionary complex (Site 808) during Ocean Drilling Program Leg 131. For the first time, the oceanic basement of the subducting plate below an accretionary prism has been reached. The Nankai Trough basement was encountered at a depth of 1289.9 mbsf and a total of 37.1 m of igneous rocks, middle Miocene (15.6 Ma) in age, was penetrated. Two main lithological units have been distinguished from the top downward; sill-like rocks (Unit I: Cores 105, 106, 107) and pillow lavas (Unit II: Core 108). Basalts are predominantly nonvesicular, hypocrystalline, aphyric to slightly phyric with intersertal to intergranular textures. Alteration is generally slight to moderate. All the basaltic rocks are cut by ramifying veins of varying widths. Secondary mineral assemblages (including vein fillings) are typical of submarine alteration and zeolite to low greenschist facies metamorphism. The order of crystallization of primary minerals is: olivine, Plagioclase, clinopyroxene. This, together with mineral chemistry, characterized by forsteritic olivine (Fo 84-85), highly anorthitic Plagioclase (up to An 90), and in particular the composition of clinopyroxene, are typical of normal mid-ocean ridge basalts (MORB). In terms of Zr/Y (2.9-3.8) and Zr/Nb (21-58), all the analyzed samples plot in the normal MORB field. The chondrite-normalized REE patterns confirm the close affinity with normal MORB type (LaN/SmN: 0.6-0.8). Note that such magmatism does not reveal any evidence of subduction-related geochemical components. The 87Sr/86Sr isotopic ratios range from 0.70339 in pillow lavas to 0.70317 in the least-altered basalts of sill units (ratios reduced to 0.70265-0.70271 by HC1 2.5 N hot leaching), whereas 143Nd/144Nd ratios are 0.51314-0.51326. These values conform with those of normal MORB. Stratigraphy, petrography, and geochemistry of the basaltic rocks recovered at Site 808 appear very similar to those from the Shikoku Basin basement (particularly Sites 442 and 443, DSDP Leg 58), analogously identified as normal MORB

Petrogenesis and tectono-magmatic significance of the Albanide-Hellenide subpelagonian ophiolites

The Mirdita-Subpelagonian ophiolites of the Albanide-Hellenide orogen are parts of a continuous belt extending from the former Yugoslavia to Greece, and share common geological, litho-stratigraphical, geochemical, and metallogenic features. In the Albanian sector, two distinct ophiolitic belts can be clearly identified: the Western Belt, mainly composed of mid-ocean ridge (MORB) ophiolites, and the Eastern Belt characterized by supra-subduction zone (SSZ) ophiolites with prevalent island arc tholeiitic (IAT) and minor boninitic affinity. In the easternmost border of the Western Belt (Central Mirdita), a transitional zone with MORB/IAT intermediate basalts and boninitic dykes also occur. In the Greek sector, a definite distinction into two ophiolitic belts cannot be made, and MORB-type ophiolites (western type) are subordinate, being represented only by the intrusive and lower volcanic sequences of the Pindos Massif. By contrast, SSZ- ophiolites (eastern type) are predominant and well-represented by the IAT and boninitic sequences of the Vourinos Massif, as well as by MORB/IAT intermediate basaltic-andesitic suites and boninites of the upper part of the Pindos volcanic sequence. Petrological and geochemical modelling suggest that the different Albanide -Hellenide ophiolitic sequences originated from distinctly different parental magmas by partial melting of mantle sources progressively depleted by previous melt extractions. MORB may have derived from 10 - 20% partial melting of an undepleted lherzolitic source, while MORB/IAT intermediate basalts may have generated by ca. 10% of H2O-assisted partial melting of a cpx-poor lherzolite that had previously experienced MORB extraction. IAT magmas and boninites may, in turn, have derived from 10 – 20 % and ca. 30% partial melting of the same source, variably enriched by subduction-derived fluids and related incompatible elements. The favoured tectono-magmatic model for the genesis of the Albanide-Hellenide ophiolites implies a low plate-convergence velocity with: 1) intra-oceanic subduction within a pristine MORB lithosphere, resulting in SSZ magmatism with IAT affinity, and generation of a nascent arc by nearly open-system supply of undifferentiated basalts (sheeted dyke complexes); 2) progressive slab sinking and retreat coupled with mantle diapirism and extension from the arc axis to the forearc region, with generation of boninites and/or very low-Ti tholeiites from depleted sub-arc sources, leaving highly depleted harzburgitic residua; 3) contemporaneous generation at the spreading axis of IAT/MORB intermediate basalts resulting from the interference of MORB-source diapirs with suprasubduction mantle sources; 4) convergence processes leading to obduction of large and relatively intact lithospheric sections of SSZ ophiolites onto the Pelagonian continental margin, often with the interposition of metamorphic soles. The latter have prevalent MORB affinity and represent relics of the pristine MORB lithosphere overthrust by the still hot ophiolitic slab

Thermobarometric evolution and metasomatic processes of upper mantle in different tectonic settings: evidence from spinel peridotite xenoliths.

none2---noneF. SIENA; COLTORTI M.Siena, Franca; Coltorti, Massim

Lithospheric mantle evolution beneath the Iberian plate: evidence from mantle xenoliths and peridotite massifs.

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Tertiary-Quaternary volcanism and tectono-magmatic evolution in Italy.

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The alkaline-carbonatite complex of Jacupiranga (Brazil) revisited: magma genesis, mode of emplacement and tectono-magmatic significance.

Field, petrological and geochemical data is reported on the Jacupiranga alkaline-carbonatite complex (133-131 Ma) which, together with other alkaline complexes of southern Brazil and south-western Africa, occurs in the central part of - and is coeval with - the Paranà-Etendeka CFB province. It consists of a shallow intrusion in the Precambrian crystalline basement, and can be subdivided in two main diachronous plutonic bodies: an older dunite-gabbro-syenite in the NW and a younger clinopyroxenite – ijolite (s.l.) in the SE, later injected by a carbonatitic core (< 1% volume). A petrogenetic model based on bulk rock major and trace element, mineral chemistry and Sr-Nd-Pb-C isotopes indicate that the two silicate intrusions generated from different parental magmas that rose from distinct mantle sources and evolved at shallow level in two zoned cup-shaped plutonic bodies. The first intrusion was generated by OIB-like alkaline to mildly alkaline parental basalts that initially led to the formation of a dunitic adcumulate core, surrounded by gabbroic cumulates, in turn injected by subanular syenite intrusive and phonolite dikes. Mela-nephelinitic (± melilite) melts likely generated deep (≥ 3 GPa) in the lithosphere were the parental magmas of the second intrusion and gave rise to large coarse-grained clinopyroxenite ad- to meso-cumulates, in turn surrounded and partially cut, by semi-annular fine-layered melteigite-ijolite-urtite ortho-cumulates. Isotopically, carbonatites do not evidence genetic links with the associated silicate intrusions, thus suggesting either a direct mantle origin or shallow liquid immiscibility from hypothetical silicate magmas currently not observed in the complex. An important result arising from the new Sr-Nd-Pb isotopes on Jacupiranga rocks is the clear correspondence of the clinopyroxenite-ijolite (s.l.) intrusion with the “Gough component” recently identified as the initial plume signature that characterizes the magmatic activity since 132 Ma, encompassing the oldest part of the Walvis Ridge volcanism and the Etendeka picrite-basalt association. Therefore, a model is proposed to account for the Early Cretaceous tectonomagmatic evolution of western Gondwana, where the impinging proto-Tristan (Walvis) mantle plume caused lithospheric arching, extension and radial fracturing of the south American-African plate triggering widespread small volume alkaline-carbonatite episodes mostly coeval with the eruption of the majority of Paranà-Etendeka CFB; the model could explain the coexistence of 1) small degree alkaline melts mostly generated from lithospheric mantle sources that, for the deepest magmas such as Jacupiranga parental melanephelinites, may also record the signature of sublithospheric plume-related geochemical components; and 2) higher degree melting CFB picrites generated from the hottest and deepest sublithospheric mantle sources at the core of the plume head

Mantle xenoliths from Bir Ali (Yemen)

Mantle xenoliths from the Bir Ali diatrema (southern Yemen) consist of spinel peridotites and clinopyroxenites. In this contribution we present bulk rock (XRF, ICP-MS) and mineral (EMPA, LA-ICP-MS) major and trace element analyses carried out on a collection of 62 samples in order to characterize the lithospheric mantle of the area and constrain its relative petrological evolution. Peridotites are mainly equilibrated at 950-1000 °C and include fertile lherzolites and more refractory harzburgites and dunites, suggesting that partial melting processes variably affected the pristine mantle composition. Subsequent metasomatic reactions are evidenced by: a) glassy fims and/or patches; b) interstitial plagioclase; c) rare pargasite amphibole (observed only in one sample). Further evidence of measomatic interactions is given by anomalously low Fo content (<0.87) of olivine and low Mg# of pyroxene (< 0.88). These metasomatic events are confirmed by the bulk rock trace element budget that reveal enrichments in the most incompatible elements, especially in the most refractory dunites (LaN/YbN up to 4.1). Coherently, enrichments in the most incompatible elements are also observed in diopsidic clinopyroxene (LaN/YbN up to 3.9) and glasses (LaN/YbN up to 9.9). Trace element discrimination ratios suggest that the causative metasomatic agents were mainly represented by alkali-silicate melts. Noteworthy, samples characterized by plagioclase impregnation show unfractionated (flat) bulk-rock and clinopyroxene REE patterns suggesting refertilization by subalkaline metasomatic melts. Therefore, the considered lithospheric section appears to be characterized by remarkable heterogeneity, in contrast with what observed in mantle xenoliths collected from the Ethiopian plateau area that display clear evidence of pervasive refertilization by CFB melts. This suggests that the lithospheric mantle of southern Yemen wasn’t affected by the thermochemical effects of the Afar plume

Coexistence of alkaline-carbonatite complexes and high-MgO CFB in the Paranà-Etendeka province: Insights on plume-lithosphere interactions in the Gondwana realm

A careful review of petrological and geochemical data on the Paranà-Etendeka igneous province is reported, with particular attention being devoted to the relationships between high-MgO CFB (tholeiitic basalts-picrites) and nearly coeval alkaline-carbonatite complexes linked to the same extensional tectonics on a regional scale. At 135–130 Ma, the tectonomagmatic activity was focused in Etendeka, the centre of the restored province, and characterised by an exclusive occurrence of the hottest and deepest high-MgO CFB (potential temperature Tp up to 1590 °C and pressure up to 5 GPa) possessing the same Sr-Nd-Pb isotopic composition of the “Gough” geochemical component, a marker of the initial Tristan plume activity. Etendeka high-MgO CFB thus represent the most genuine proxies of sublithospheric melts generated at the plume axis and are relatively unaffected by lithospheric contamination. Nearly coeval (133–128 Ma) alkaline‑carbonatite complexes cluster around the extensional structures of the Ponta Grossa Arch (e.g., Jacupiranga and Juquia in Brazil) and the Damara Belt (e.g., Erongo, Okurusu, Okenyenya and Paresis in Namibia), both of which intersect the early track of the south Atlantic opening. Compared to high-MgO CFB, alkaline magmas display distinctive isotopic signatures and an incompatible element distribution consistent with their generation from lithospheric mantle sources, which were variably metasomatised (veined?) by amphibole and phlogopite. Metasomes of alkaline mantle sources have a HIMU affinity and are dominated by amphibole in Namibia, whereas they display EM1 tendency and a more relevant role of phlogopite in Brazil, which implies important lithospheric differences at a regional scale. The tectonomagmatic features of Paranà-Etendeka –also shared by other Gondwana LIPs, such as Deccan and Karoo– can be reconciled by a generalized model where a hot plume impinging on a relatively thick lithosphere caused, in the axial zone, the contemporaneous generation and rise of high-MgO CFB and alkaline magmas from distinct asthenospheric and lithospheric mantle sources, respectively. In the asthenosphere, the volatile-poor solidus was crossed under an adiabatic thermal regime, mostly in the range of 4 to 5 GPa and Tp 1500 to 1600 °C with the development of high-MgO CFB. In the overlying metasomatised lithosphere, the plume effects caused a perturbation of the conductive thermal regime and a crossing of volatile-rich solidus (mostly P 2–3 GPa, Tp 1300–1400 °C) with the generation of alkaline melts from the most fusible (hydrated and carbonated) mantle domains