First results of an ongoing study of mantle xenoliths from Bayuda, Sudan, are presented. The Bayuda Volcanics carrying the xenoliths belong to a group of intracontinental alkaline basalts at the rim of the Arabian-Nubian Shield, NE Africa, which have an OIB-like geochemical signature, and some of which (Darfur Dome) appear to indicate the existence of mantle plumes based on their isotopic compositions. However, no direct evidence on the nature and composition of the SCUM in this part of NE Africa has been available so far, contrary to the Arabian Shield. The xenoliths reported here constitute the first mantle sample from this region. The suite consists of anhydrous and amphibole-bearing spinel lherzolites, and of a complex magmatic suite of olivine-, garnet-, spinel-, and plagioclase clinopyroxenites and
hornblendites.
The lherzolites, presumed to be host rocks to the magmatic suite, are unusually fertile based on a very high content of pyroxenes, mineral compositions that are comparatively Fe-rich (Ol: Mg-# 0.89-0.90) and high jadeite cntents in clinopyroxene (> 2wt% Na2O). A metasomatic event intro-duced amphibole (titanian pargasite) replacing spinel + clinopyroxene, and some lherzolites are veined by amphibole clinopyroxenites. However the samples show a common chemical reequilibration at about 900-950°C including the veins. Pressure is difficult to estimate but very low Ca contents in olivine (ª 200 ppmw) may indicate a deep origin. The fertile character of the peridotites is similar to samples from the Arabian Shield, and is surprising as the mantle in this area should be depleted by the Panafrican event of crustal growth.
The magmatic suite is interpreted to represent a fairly complete tracking of melts evolving by fractional crystallization within the mantle. Initial (sequence based on Mg-#) cumulates are olivine+chromite-rich, followed by garnet-spinel-bearing clinopyroxenites eventually evolving to plagioclase-garnet-bearing clinopyroxenites and hornblendites. Presence of garnet and exsolution of garnet from cpx constrains crystallization to have occurred at least partially within the ariegite facies, i. e. ª 12 kbar (Brumm et al. 1996). One of the outstanding features of this suite is its high ferric iron content. Occurrence of primary (?) carbonate is restricted to the evolved (mostly amphibole-bearing) cumulates. Occurrence of garnet kelyphites, garnet-spinel relations, possible garnet reaction rims around plagioclase argue for a pre-eruption metamorphic history and against an interpretation as high-P cumulates of the xenolith-bearing Bayuda Basalts.
The relation between the magmatic suite and the lherzolites is as yet not clear. The cumulate-forming melt(s) must have risen from greater depth, as olivine was the liquidus phase, but later fractions were too rich in silica to be in equilibrium with host lherzolites. Some of these show evidence of reheating and reactions from olivine + Cpx to Opx which may represent back-infiltration of silica-rich liquids. However, the main significance of the suite as a whole lies in the fact that it proves the presence of garnet in favourable compositions within the spinel field within the upper lithosphere below NE Africa. Spidergrams of the Bayuda Volcanics indicate a significant role of amphibole + garnet melting for their trace element budget, and are dominated by a positive Nb anomaly. The mineralogy of the cumulate suite documented here makes it most prone to yield melts on reheating, and initial melts from it will show HFSE-enriched trace element characteristics similar to those of the Bayuda Volcanics. It is therefore possible that spidergrams of the latter reflect admixture of a remelted cumulate component and not simply the composition of their inevitably deep source. Isotopic data are not yet abotained, but first trace element data on the cumulate phases will be presented and discussed in this respect.
Brumm, R., Zinngrebe, E., Abs-Wurmbach, I., Pudlo, D. & Meyer, H.P., J. Conf. Abs. 1, 84 (1996).