Studio petrologico e geochimico delle vulcaniti della porzione centro-orientale del Plateau Nord-Etiopico

Geochemical and petrological data indicate that the 30 Ma Northern Ethiopian continental flood basalts and picrites were generated in connection with the “Afar plume”. Previous studies (Beccaluva et al., 2009) show that: a) Basaltic lavas appear zonally arranged with Low-Ti tholeiites (LT) in the north-west, High-Ti tholeiites (HT1) to the south-east and Very High-Ti transitional basalts and picrites (HT2, TiO2 = 4% – 6,5%) closer to the Afar triple junction; b) the P-T conditions of magma generation increased from 1,3GPa/1200°C to 3GPa/1500°C from West to the East; that is, from the outer zone (LT) to the core of plume head where HT2 ultra-titaniferous picrites were generated. Further sampling has been carried out in the inferred plume axial zone (Lalibela Area) focusing on picritic lavas, as well in the Yemenite counterpart where analogous HT2 products (including picrites) are recorded in Manakhah section. The observed parageneses dominated by olivine and clinopyroxene phenocrysts, within a groundmass containing clinopyroxene, plagioclase, Fe-Ti oxides, alkali feldspar, phlogopite and apatite are consistent with the transitional character of these lavas and their overall enrichment in the most incompatible elements and fractionated REE patterns. New isotopic data carried out at the IGG-CNR Institute of Pisa and at the SUERC Institute of East Kilbride (Glasgow – Scotland UK) integrated with those available in the literature (Pik et al., 1999) show correlations between TiO2 (and other incompatible elements) and Sr-Nd-Pb isotopic signatures, thus defining the isotopic fingerprint of the plume-related metasomatic agents. These metasomatic agents can be envisaged as alkali-silicate melts that integrate various geochemical components (e.g. Titanium and related High Field Strength Elements, Low Field Strength Elements, Light Rare Earth Elements, H2O) scavenged and pooled along the plume axis, and derived from heterogeneous mantle materials mixed during the plume rise. The Ethiopian-Yemenite CFBs are generated from distinct mantle sources, variously metasomatized as a function of depth and position compared to the Afar Plume; so the LT sources (the less metasomatized) are located in marginal areas of the plume where the thermal anomaly is lower, while the HT1 and HT2 sources are the most metasomatized and placed in a position of the plume that is hotter and deeper related to high thermal and geochemical anomalies; in particular the deepest sources of HT2 basalts and picrites are the most metasomatically enriched as they arise in the axial zone of the deep mantle plume and in the core of the plume head which is the most geochemically enriched part where the highest thermal anomaly exceeds 300°C with respect to the ambient mantle (T = 1400-1500°C). The HT2 picritic melts are the most enriched CFBs by the plume component. Their formation is related to complex mechanisms of genesis, including polybaric fractionation processes and flow-differentiation processes during the ascent to the surface. The plume isotopic fingerprint deviates from the “main mantle reservoirs” that characterize typical enriched mantle sources (EMI, EMII and HIMU), though the CFBs isotopic composition shows affinity with the HIMU and EMII end-members, with a significant incidence of EMI component, especially for lead isotopic signature. The peculiar radiogenic signatures of Afar Plume-related CFBs are characterized by 87Sr/86Sr ratio variable by 0,7032 to 0,7047, 143Nd/144Nd ratio among 0,5128 and 0,5131; finally 206Pb/204Pb isotopic ratio is changing by 18,2 and 19,4. The “plume signature” is particularly evidenced by significantly high 3He/4He ratio (low radiogenic signature with R/Ra up to “19”) measured in fluid inclusions of olivine crystals that allow to hypotesize very deep not degassed and volatile-enriched mantle components as “plume-source”. Petrological modeling indicates that the resulting metasomatized mantle sources were characterized by hydrous, iron-titanium-alkali rich exotic parageneses including amphibole, phlogopite, apatite and Ti-bearing minerals such as ilmenite. A debate is still open on the ultimate origin of the metasomatic Fe-Ti rich fluids, in turn related to the depth of the plume convective cell. A direct core contribution is not plausible and most Authors favor long-term mechanism of Oceanic Crust (Fe-Ti basalts/gabbros and their metamorphic equivalent, i.e. eclogites) recycling deep in the mantle via ancient (possibly Proterozoic) subductions (Sobolev et al., 2007). In addition, the oxygen isotopic fingerprint anomalously high for primary magmas derived by deep mantle sources (δ18O up to 6,9), also pointed to consider a recycling process of subducted oceanic lithosphere during ancient subductions (i.e. high radiogenic Sr in HT2 basalts and picrites). Accordingly, the uprising plume could have remobilized domains from the Mantle Transition Zone which may include relicts of older subducted slabs. Within these remobilized domains, characterized by the coexistence of peridotite and eclogite, referred to as a “piclogite” association, the eclogites melt preferentially generating Fe-Ti rich melts that infiltrate and metasomatize the shallower lithospheric mantle (Sub-Continental Lithospheric Mantle – SCLM).