Summary The structural setting of a refoliated belt of sapphirine granulites in northern Uganda and petrography of two selected rocks are described. Electron-probe analyses of the following minerals are given: ilmenite, titanian hematite, rutile, magnetite, sapphirine, hyperstheue, brown and green biotite, garnet, and cordierite. Field and experimental data suggest the following paragenesis: deposition of ferruginous shales with siliceous bands, followed by burial and regional metamorphism under granulite facies conditions, and finally rapid unloading associated with refoliation and shearing and crystallization of sapphirine and cordierite.
Summary A Miocene porphyry belonging to the sheshonite association contains 7 cm sanidine megacrysts in a groundmass of microphenocrysts of labradoritebytownite, augite, sporadic hastingsite, magnetite, sphene, K feldspar, apatite, Ca zeolite and calcite. The megacrysts (Or 78–94 ) are enriched in BaO (≃ 2.0 wt%) and SrO relative to the groundmass. Although mineralogr and texture suggest that the megacrysts were suspended in a liquid, now represented by the groundmass, calculated liquid densities except for dry melts are less than those observed for the megacrysts and it is concluded that the original magma contained very little water. It possibly originated in a subduction zone with the main hycration taking place at shallow levels of intrusion during contact with groundwater. This produced zeolitization of the feldspars and may have played a part in the K enrichment at the margins of the sanidine megacrysts.
The petrogenesis of pyroxenite layers within the Beni Bousera peridotite massif is investigated by means of elemental and Nd-Sr-Pb-O-S isotope analyses. The light rare earth element (LREE) depleted nature of many of the pyroxenites, their wide variation in composition, and lack of correlation between incompatible elements and fractionation indices preclude them from representing crystallized melts from a peridotitic source. The physical characteristics of the pyroxenites and their large (greater than a factor of 20) range in Ni rule out partial melting as the cause of their petrological and geochemical diversity. Major and compatible trace element geochemistry is consistent with formation of most of the pyroxenite suite via high-pressure crystal segregation in magma conduits intruding the peridotites. These magmas crystallized clinopyroxene, orthopyroxene, and garnet. The pressure of crystallization is constrained to be above ˜45 kbar from the presence of graphitized diamonds in pyroxenite layers. Lack of correlation between fractionation indices and highly incompatible elements and the wide variation in incompatible element abundances suggest that the suite did not form from genetically related magmas. The presence of positive and negative Eu anomalies (Eu/Eu* = 0⋅54–2⋅0) in pyroxenites which crystallized at pressures much greater than the plagioclase stability field (˜ 45 kbar) suggests that the parental magmas originated from precursors which formed in the crust. Oxygen isotope compositions of coexisting minerals in the pyroxenites indicate high-temperature equilibration but δ18O values vary from +4⋅9 to + 9⋅3‰, ruling out their derivation from the host peridotites or other normal mantle sources. The extreme O-isotope variation, together with δ34S values of up to + 13‰ in sulphides included within CPX strongly suggests that the melts from which the pyroxenites crystallized were derived from hydrothermally altered, subducted oceanic lithosphere. Extreme initial radiogenic isotope variation in the pyroxenites (εNd + 26 to –9 , 87Sr/86Sr 0⋅7025–0⋅7110, 206Pb/204Pb 18⋅21–19⋅90) support such an origin but also require a component with ancient, high U/Pb and Th/Pb in their source to explain the high Δ7/4 and Δ8/4 values of some pyroxenites. This component may be subducted hemi-pelagic sediment. Further evidence for a sediment component in the pyroxenites is provided by isotopically light carbon in the graphite pyroxenites (δ13C–16 to – 28‰). Parentdaughter isotopes in the pyroxenites are strongly decoupled, making estimation of formation ages speculative. The decoupling occurred recently (<200 Ma), probably as a result of partial melting associated with diapiric upwelling and emplacement of the massif into the crust from the diamond stability field. This late partial melting event further depleted the pyroxenites in incompatible elements. The variably altered nature of the subducted protolith and complex history of trace element fractionation of the pyroxenites has largely obscured geochemical mixing trends. However, Nd–Pb isotope systematics indicate that incorporation of the component with high U/Pb–Th/Pb occurred relatively recently (<200 Ma) for some pyroxenites. Other pyroxenites do not show evidence for incorporation of such a component and may be substantially older. Tectonic, geophysical, and isotopic constraints indicate formation of the pyroxenites in the mantle wedge above a subducting slab during the Cretaceous. Physical and chemical evidence for high-pressure fractionation seen in most of the pyroxenites precludes them from simply representing ancient subducted oceanic lithosphere, thinned by diffusion. However, the petrological and isotopic diversity of the massif support the concept of a 'marble cake' mantle capable of producing the observed geochemical diversity seen in oceanic magmas.
ABSTRACT An account is given of Yelele, a Miocene volcano, situated on the Turkana escarpment, north-east Uganda. It is composed of lavas and dykes of nephelinite, phonolite, tephrite, and trachyte together with olivine-bearing varieties, and has a central plug of ijolite and nepheline syenite. The lavas are described and compared with those of other eastern Uganda centres and the petrogenesis discussed in the light of experimental data from the Geophysical Laboratory of the Carnegie Institution of Washington. A northern (Yelele) plagioclase-bearing suite of lavas and a southern (Elgon) melilite-bearing suite are derived by different fractionation trends from an olivine melanephelinite parent which itself is the dense portion of a nephelinitic parent magma.
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