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 (LaN/YbN up to 7) in the most refractory lithotypes. Clinopyroxene (cpx) REE patterns are mostly LREE depleted (LaN/YbN down to 0.2) or enriched (LaN/YbN up to 4.4). Sr-Nd isotopes of clinopyroxene mainly show compositions approaching the depleted mantle (DM) end member (87Sr/86Sr < 0.7030; 143Nd/144Nd > 0.5132), or less depleted values (87Sr/86Sr = 0.7033–0.7034; 143Nd/144Nd = 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 3He/4He (Ra) values of olivines range from 6.6 to 8.9 Ra, 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 LaN/YbN up to 26 in the most refractory lithologies. The constituent clinopyroxenes have flat HREE distributions and LaN/YbN between 0.1 and 76, i.e., in general agreement with the respective bulk-rock chemistry. Clinopyroxenes from lherzolites have 87Sr/86Sr = 0.7022–0.7031, 143Nd/144Nd = 0.5130–0.5138, and 206Pb/204Pb = 18.38–19.34, and clinopyroxenes from harzburgites have 87Sr/86Sr = 0.7027–0.7033, 143Nd/144Nd = 0.5128–0.5130, and 206Pb/204Pb = 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 Ra, 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 Vermion Massif (northern Greece) is located across the boundary between the Pelagonian and Vardar Zones and includes several tectonic units bearing ophiolitic rocks, which represent remnants of the oceanic lithosphere formed in the Neotethyan Vardar Ocean, between the Pelagonian and the Serbo- Macedonian continental realms. This massif consists of tectonic units belonging to the Pelagonian Domain, which are tectonically overlain by units associated with the Almopias sub-Zone (Vardar Zone). Ophiolitic rocks consist of mantle harzburgites and ophiolitic melanges. The ophiolitic melanges incorporate rocks exhibiting a wide range of composition, including various intrusive rocks and volcanic rocks ranging from basalts, basaltic andesites, andesites, dacites, to rhyolites.
Incompatible elements and rare earth elements analyses indicate that a number of different rock-types formed in distinct tectonic settings can be identified. Mantle harzburgites have a very depleted nature and represent portions of the supra-subduction (SSZ) mantle developed in an intra-oceanic arc setting. The melanges units include six rock types variably distributed in the Pelagonian and Almopias Units. They are: (1) calc-alkaline rocks with marked depletion in Nb, Ta Ti and enrichment in LREE, Th; (2) LREE-depleted N-MORB; (3) LREE-enriched E-MORB; (4) alkaline within-plate basalts showing marked enrichments in Th, Ta, Nb, LREE; (5) low-Ti island arc tholeiites featuring depletion in HFSE; and (6) very-low-Ti boninites characterized by strong depletion in HSFE and REE. Previous interpretations have referred the Vardar ophiolites to a MORB-type oceanic setting and to a MORB-type backarc setting; however, the widespread occurrence of SSZ ophiolites has never been documented in the Vardar Zone before and is particularly important as it testifies for the existence of an intra-oceanic arc basin in the Vardar oceanic domain.
The results presented in this paper compared with literature data on other magmatic rocks within the Vardar Zone suggest that the opening and closure of the Vardar Ocean record several distinct accretion events in this basin, that is oceanic crust generation at mid-ocean ridge, alkaline seamounts in the oceanic domain and SSZ setting, as well as two accretion events in the western realm of the Serbo-Macedonian continent, that is volcanic arc and backarc settings.
Based on the comparison between the modern west Pacific subduction system and the results from this study a new model for a multistage tectono-magmatic evolution of the Vardar Ocean is proposed.
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