The Ślęża Ophiolite is one of several thrust-bounded crustal slices dominated by metabasites in the western Sudetes. The apparent field association of serpentinites, gabbros and amphibolitic components led previous workers to consider that this lithological assemblage represented an ophiolite sequence. Fieldwork suggests that the ophiolite is now highly inclined, partly overturned, so that an ophiolitic pseudostratigraphy can be deduced, grading from serpentinites and gabbros in the south to metabasite lavas in the north. The recent discovery of pillow lava structures (at Gozdnica Hill, to the west of Sobótka town) confirms that the volcanic top of the ophiolite lies in the northern section, as might be expected from the ophiolite model.The gabbros have undergone greenschist facies metamorphism with the random development of low-grade amphibole. The volcanic portion of the sequence comprise metamorphosed dolerites and basalts partly within the contact aureole of the Variscan Strzegom-Sobótka granite. Previous work dated plagiogranites associated with the gabbros at about 400–420 Ma (U-Pb zircon ages).Geochemical data suggest that the gabbros are distinct and apparently not comagmatic with the volcanic section of sheeted dykes and lavas. The gabbros, in particular, although very depleted in incompatible elements are dissimilar to supra-subduction zone ophiolites, exhibiting instead N-MORB-like light REE depleted patterns. Depletion is both a feature of the cumulate character of many of the gabbros, as well as a source effect (especially the uniformly low Nb content). The metabasalts and metadolerites, on the other hand, are a well-evolved single comagmatic suite with high incompatible element contents, Zr/Y approximately 3–4, and generally flat to light REE-depleted patterns. The geochemical dichotomy of the plutonic and volcanic segments calls into question a simple interpretation of the body as a single-stage coherent stratiform ophiolite.Chemical comparison with Sudetic metabasites from within the nearby Rudawy-Janowickie and Kacazawa Complexes shows that the Ślęża metabasites have a number of features in common, including the presence of both low-Ti (gabbros) and high-Ti (dykes and lavas) chemical groups. The correlation of the gabbros, dykes and lavas with the low-Ti and high-Ti (Main Series) metatholeiites respectively, seen throughout the Bohemian Massif, as well as the Sudetes, places them within the regional collage of Palaeozoic crustal blocks separated by the Saxothuringian Seaway.Comparison with Bohemian Massif metabasites also indicates that sediment contamination of the Ślęża Ophiolite sources was not an important process and that an enriched plume source played no part in the generation of the ophiolitic melts. The two Ślęża chemical groups were derived from variably depleted asthenospheric mantle sources. Simple modelling suggests that the volcanic segment could have been derived by 10–15% partial melting of a depleted N-MORB source, whereas the plutonic segment represents around 30% partial melting of a more depleted source. To develop varying degrees of depletion in an oceanic environment, the two sources could be related via incremental partial melting of a shallow MORB-type source.
Abstract. The Umm Nar BIF was formed in a sedimentary environment. It is confined to an upper stratigraphic zone of pre-Pan-African metamorphosed shelf deposits. During the Pan-African deformational history, the BIF and the host metasediments were tectonically overlain by ophiolitic m61ange succession. The metasediments and the m61ange were subjected to a major folding phase and then thrust over the "Shaitian" sheared granite, prior to the intrusion of syn-to late-orogenic granitoids. The BIF is divisible into two main types: oxide-bands including magnetite and hematite, and oxide-silicate bands including magnetite, hematite and stilpnomelane. The associated gangues are quartz, calcite, epidote, garnet, plagioclase, graphite and muscovite. Rhythmic banding and lamination, cross-lamination and flaser structure are the most prominent primary features in the IF bands. The iron minerals and the associated gangue show a variety of textural aspects and microscopic interrelationships which indicate successive episodes of mineral accumulation and formation, involving deposition, recrystallization, blastic growths, overgrowths, replacement and deformations, during continuous burial and subsequent tectonic deformations.Banded iron ores (BIF) are known to occur at several localities in the Central Eastern Desert, Egypt. It is generally accepted that BIFs are intimately associated with geosynclinal metavolcanics and metasediments ( = island arc volcanics and volcaniclastics), and pertain to one genetic type. The BIFs in Egypt are assumed to be of Late Proterozoic (Pan-African) age and considered either as (a) chemical marine sediments in a geosynclinal basin (e.g. E1 Shazly 1957; Rasmy 1968) or as (b) subaqueous volcanogenic deposits in an island arc environment (e.g. Ivanov et al. 1973;Garson and Shalaby 1976;Sims and James 1984; E1 Gaby et al. 1988 and El Sharkawi 1990). None of these models provides a comprehensive and convincing view on the sequence of crystallization of the iron minerals and the factors that controlled their formation.The study of the BIF at Umm Nar (Fig. 1) revealed that it differs in many aspects from the other Egyptian BIF occurrences described from other localities. The present work summarizes the results of megascopic and microscopic investigations carried out on the Umm Nar BIF, Eastern Desert, Egypt (Fig. 1). Attention was concentrated on the paragenesis of the BIF intervals, morphology of the iron minerals and associated gangues, and modifications resulting from metamorphism and deformation. The chronologic sequence of the mineral constituents and their modes of formation are discussed. A large number of oriented samples were collected, consideration being given to the setting of these samples in respect to the original bedding habitat and deformational features of the country rocks. The samples were petrographically examined under polarizing and reflecting microscopes. The identity of the iron silicates was confirmed by X-Ray diffractions (XRD).In order to clarify the chemical composition of th...
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