COMPARAZIONE DEL MANTELLO LITOSFERICO AFRICANO: STUDIO DEGLI XENOLITI DI MANTELLO DEI DISTRETTI VULCANICI DEL MAROCCO E DEL MAIN ETHIOPIAN RIFT

Two distinct mantle xenoliths series has been studied: one from the Azrou-Timahdite volcanic district, in the Middle Atlas (Morocco) and the other one from Mega in the Main Ethiopian Rift (MER), Sidamo region. The aim of the work is the characterization of the lithospheric mantle domains under the above mentioned areas and the comparison of these domains with other African volcanic provinces already studied; in the Saharan belt (Hoggar, Algeria and Gharyan, Lybia), and in the Ethiopian plateau (Injibara, Gojam and Dedessa, Wollega). Mantle xenoliths from the Pliocene-Quaternary volcanic diatremes of Azrou-Timadite area include spinel lherzolites xenoliths, 1 hazburgite and 1 olivin-websterite, with protogranular to porphyroclastic textures with superimposed metasomatic textures involving clinopyroxene, ortopyroxene and spinel as the main reacting phases. Olivine (ol) shows forsterite (Fo) between 88.7 in the lherzolites up to 91.8 in the harzburgite. Ortopyroxene (opx) composition varies with the following ranges: enstatite (En) 88.0 – 90.0, ferrosilite (Fs) 9.1 – 10.7 and wollastonite (Wo) 0.7 – 1.9. Clinopyroxene composition shows the following ranges: En 45.6 – 52.9, Fs 4.4 – 6.0 and Wo 41.3 – 49.0. Spinel is generally characterized by Mg# [Mg/(Mg+Fe)] between 72 – 84 and Cr# [Cr/(Cr+Fe)] between 6 – 32. Bulk rock trace elements, have flat Heavy Rare Earth Element (HREE) patterns (0.6 – 2.0 times chondrite) and are variably enriched in Light REE (LREE, with LaN/YbN between 1.1 and 15.6). The constituent clinopyroxenes are characterized by flat HREE distributions (3 – 10 times chondrite) and variable LREE enrichment with LaN/YbN from 0.1 to 23.7, which generally conform to the bulk rock chemistry. Sr-Nd-Pb isotope compositions of the cpx separates show the following compositional ranges: 87Sr/86Sr from 0.70243 to 0.70335, 143Nd/144Nd from 0.51273 to 0.51325, 206Pb/204Pb from 17.5 to 19.11, 207Pb/204Pb from 14.61 to 15.6 and 208Pb/204Pb from 36.34 to 39.08. Helium isotope determinations have been made on gases trapped in olivine and pyroxene from several xenoliths and show a narrow 3He/4He range between 6.2 – 6.8 Ra. On the whole these data indicate a lithospheric mantle that records a variable depletion by extraction of basic magma and subsequent metasomatic enrichments caused by interaction with alkali-silicate melts with a carbonatitic component. The isotope compositions show a prevalent HIMU metasomatic signature. Moreover, the relatively low 3He/4He isotopic rations suggest an origin within the upper mantle for this metasomatic agent. This situation is also recorded in other Northern African volcanic occurrences such as Hoggar (Algeria, Beccaluva et al., 2007) and Gharyan (Libya, Beccaluva et al., 2008), suggesting a common sublithospheric metasomatic agent across the area of the lithospheric mantle. Therefore, the volcanic district of the Saharan belt could have been related to small, second-order, “hot fingers” triggered by intraplate rifting related to tectonic reactivation of the Pan-African cratonic basement (Azzouni-Sekkal et al., 2007). This, in turn, may be considered a far-field foreland reaction to the Africa-Europe collisional system (Liégeois et al., 2005). Mantle xenolith from southern MER (Mega) consist of spinel lherzolites to harzburites, with 1 olivin-websterite, characterized by prevalent protogranular textures showing various degrees of deformation and re-crystallization with 120° triple boundaries. Microprobe data on the mineral phases show slight compositional difference between lherzolites and harzburgites. Olivine varies in composition between Fo 88.4 – 91.1 in lherzolites, 89.3 – 91.8 in harzurgites and 89.3 – 89.7 in the olivine-websterite. Orthopyroxene varies in the ranges: En 88.1 – 90.5, Fs 8.2 – 10.9, Wo 0.6 – 1.7 in lherzolites; En 89.3 – 91.5, Fs 7.6 – 9.5, Wo 0.9 – 1.4 in harzburgites and En 88.7 – 89.5, Fs 9.5 – 10.1, Wo 0.9 – 1.2 in the olivin-websterite. Clinopyroxene shows the following compositional variation: En 47.0 – 50.6, Fs 3.2 – 5.4, Wo 42.9 – 47.3 in lherzolites; En 48.4 – 52.1, Fs 3.0 – 5.4, Wo 42.9 – 47.2 in harzburgites and En 48.0 – 49.2, Fs 4.3 – 4.9, Wo 46.1 – 47.6 in the olivn-websterite. Spinel compositions in the lherzolites has Mg# between 72 – 83, in harzburgites between 60 – 79 and in the olivine-websterite between 81 – 83. The Cr# ranges between 8 – 58 in lherzolites, between 38 – 79 in harzburgites and between 38 – 40 in the olivin-websterite. The whole rock REE distribution show a flat HREE/Ch decreasing from 2.0 to 0.14 from lherzolites to harzburgites, compatible with a progressive depletion of the mantle material by extraction of basic melts. On the other hand, LREE/Ch shows variable enrichment most probably due to metasomatic processes, with LaN/YbN up to 41.5. Clinopyroxene shows a REE distribution generally parallel to those of the respective bulk rock with a flat HREE/Ch patterns (0.1 – 2.0 chondrite) and variously enrichments in LREE, with LaN/YbN ranging from 0.1 (in the fertile lherzolites) to 64.4 (in the most depleted harzburgites). Sr-Nd isotopic composition of clinopyroxene separated from Mega xenoliths range from 87Sr/86Sr 0.70220 – 0.70311, 143Nd/144Nd 0.51301 – 0.51379 for lherzolites, and 87Sr/86Sr 0.70268 – 0.70326, 143Nd/144Nd 0.51274 – 0.51305 for harzburgites, with the exception of harzburgite MA 29 that displays 87Sr/86Sr 0.70454, 143Nd/144Nd 0.51291. Lead isotopes display: 206Pb/204Pb 18.5 – 19.34, 207Pb/204Pb 15.53 – 15.63, 208Pb/204Pb 38.09 – 38.96 in lherzolites and 206Pb/204Pb 18.38 – 18.52, 207Pb/204Pb 15.21 – 15.67, 208Pb/204Pb 37.58 – 38.42 in harzburgites. The helium isotopic composition varies between R/Ra 7.1 to 8. The petrological and geochemical data on both rock and constituent minerals indicate that mantle xenoliths from Mega record a depletion by melt extraction events with superimposed metasomatic enrichments. The REE and trace elements patterns of MER xenoliths suggest alkali-silicate melt(s) as the main metasomatic agent(s) with carbonatitic component. The isotope data suggest for the metasomatism a fingerprinting approaching the HIMU endmember. The data carried out from this work suggest that the Mega xenoliths display close analogies with those from other areas along the Red Sea (Ottonello et al., 1978) and from the continental margin of the Aden’s Gulf (Baker et al., 1996). By contrast, mantle xenoliths from the northern Ethiopian plateau of Injibara and Dedessa show a distinct metasomatism due to sub-alkaline melts similar to the CFB magmas connected to the Afar Plume.