2013-2010‎ > ‎

Vol 16/1-2

| 1-17 | Mwabanwa Louis KIPATA, Damien DELVAUX, Mwene Ntabwoba SEBAGENZI, Jacques CAILTEUX & Manuel SINTUBIN

Mwabanwa Louis KIPATA, Damien DELVAUX, Mwene Ntabwoba SEBAGENZI, Jacques CAILTEUX & Manuel SINTUBIN. 2013. Brittle tectonic and stress field evolution in the Pan-African Lufilian arc and its foreland (Katanga, DRC): from orogenic compression to extensional collapse, transpressional inversion and transition to rifting. Geologica Belgica, volume 16 (1-2): 1-17.

Since the first and paroxysmal deformation stages of the Lufilian orogeny at ~ 550 Ma and the late Neogene to Quaternary development of the south-western branch of the East African rift system, the tectonic evolution of the Lufilian arc and Kundelungu foreland in the Katanga region of the Democratic Republic of Congo remains poorly known although it caused important Cu-dominated mineral remobilizations leading to world-class ore deposits. This long period is essentially characterized by brittle tectonic deformations that have been investigated by field studies in open mines spread over the entire arc and foreland. Paleostress tensors were computed for a database of 1889 fault-slip data by interactive stress tensor inversion and data subset separation. They have been assembled and correlated into 8 major brittle events, their relative succession established primarily from field-based criteria and interpreted in function of the regional tectonic context. The first brittle structures observed were formed during the Lufilian compressional climax, after the transition from ductile to brittle deformation (stage 1). They have been re-oriented during the orogenic bending that led to the arcuate shape of the belt (stage 2). Unfolding the stress directions allows to reconstruct a well-defined N-S to NNE-SSW direction of compression, consistent with the stress directions recorded outside the belt. Constrictional deformation occurred in the central part of the arc, probably during orogenic bending. After the bending, the Lufilian arc was affected by a NE-SW transpression of regional significance (stage 3), inducing strike-slip reactivations dominantly sinistral in the Lufilian arc and dextral in the Kundelungu foreland. The next two stages were recorded only in the Lufilian arc. Late-orogenic extension was induced by ó1–ó3 stress axis permutation in a more trans-tensional regime (stages 4). Arc-parallel extension (stage 5) marks the final extensional collapse of the Lufilian orogeny. In early Mesozoic, NW-SE transpressional inversion felt regionally (stage 6) was induced by far-field stresses generated at the southern active margin of Gondwana. Since then, this region was affected by rift-related extension, successively in a NE-SW direction (stage 7, Tanganyika trend) and NW-SE direction (stage 8, Moero-Upemba trend).

Keywords: brittle structural evolution, paleostress tensor, orogenic bending, extension, inversion, rifting.

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| 18-26 | Claire DERYCKE & Gaël Clément

Claire DERYCKE & Gaël Clément. 2013. First assemblage of Acanthodian scales and spines from the Famennian (Upper Devonian) of Durnal (Belgium), palaeobiogeographical and palaeoenvironmental implications. Geologica Belgica, volume 16 (1-2): 18-26.

The vertebrate microremains assemblage from the Famennian (Upper Devonian) of the Belgian locality of Durnal is compared with those of the Famennian of Strud, Latvia and Gondwanan Frasnian localities. These faunal comparisons allow palaeoenvironmental and palaeobiogeographical considerations on the faunal interchanges between Gondwana and Euramerica during the Middle-Late Devonian. Isolated acanthodian spines and scales, usually relatively scarce in the Famennian deposits in Belgium, constitute the major part of the Famennian microremains assemblage from Durnal, together with actinopterygian scales and sarcopterygian teeth and scales. This acanthodian fauna from Durnal is the first known association of spines and scales in Northern France and Belgium.

Aiguillons et écailles d’acanthodiens dans le Famennien (Dévonien supérieur) de Durnal (Belgique), données sur la paléobiogéographie et le paléoenvironnement. Des microrestes de vertébrés ont été extraits de grès fins plus ou moins carbonatés du Famennien (Dévonien supérieur) de Durnal (Province de Namur, Belgique). Les restes d’acanthodiens, relativement rares dans le Famennien de Belgique, ont été trouvés en abondance avec des dents et des écailles de sarcoptérygiens ainsi qu’avec des éléments dermiques d’actinoptérygiens. Cet assemblage, représentant la première association d’aiguillons et d’écailles d’acanthodiens de la Belgique a été comparé aux assemblages du Famennien de Strud, de Lettonie et du Frasnien du Gondwana. Ils renforcent les données paléobiogéographiques concernant les échanges entre l’Est du Gondwana et le Sud de l’Euramerica au cours du Dévonien.

Keywords: Vertebrates, Acanthodians, Actinopterygians, Sarcopterygians, Microremains, Famennian, Belgium.

MOTS-CLES: Vertébrés, Acanthodiens, Actinoptérygiens, Sarcoptérygiens, Microrestes, Famennien, Belgique.

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| 27-34 | Veerle Cnudde, Jan Dewanckele, Tim De Kock, Marijn BOONE, Jean-Marc BAELE, Philippe Crombé & Erick Robinson

Veerle Cnudde, Jan Dewanckele, Tim De Kock, Marijn BOONE, Jean-Marc BAELE, Philippe Crombé & Erick Robinson. 2013. Preliminary structural and chemical study of two quartzite varieties from the same geological formation: a first step in the sourcing of quartzites utilized during the Mesolithic in northwest Europe. Geologica Belgica, volume 16 (1-2): 27-34.

Wommersom and Tienen quartzite are found as varieties of lithified arenite banks within the Cenozoic Tienen Formation. These macroscopically distinct varieties were utilized as raw material for stone tool production during the Mesolithic period in the Rhine-Meuse-Scheldt area of northwest Europe. They were distributed over 80,000 km2 between the Paris and North Sea basins for over four millennia. This distribution has been interpreted by archaeologists as a long-distance exchange network. In this work, samples from both outcrops have been examined by different characterization methods with the aim to develop an operator-independent and objective method to determine the origin of a certain artifact. Petrographical study revealed a distinct difference in grain size of both quartzites. However, considering the results of the petrographical analyses, it is clear that there is a difference between the accessory minerals that are more or less irregularly distributed in the samples. Cathodoluminescence (CL) revealed additionally information on the presence of feldspars and facilitated the recognition of minerals such as apatite, zircons and kyanite. X-ray tomography was used to obtain in a non-destructive way, a structural overview of the samples as well as a detailed 3D grain-size distribution of some accessory minerals. The latter is important given that these nondestructive techniques could be used for sourcing of other geological material as well as archeological artifact. This work is a first step in the sourcing of different quartzite varieties to their different outcrop locations and their specific facies within the same geological formation. However, when characterizing artefacts for sourcing purposes, it will remain crucial to provide attention towards a possible level of heterogeneity inside these samples. In order to do this correctly, besides a characterization of the artefacts themselves, also a detailed analysis of the entire geological formation is needed.

Keywords: Quartzite artefacts, Wommersom, Tienen, microscopy, cold-cathodoluminiscence, X-ray computed tomography.

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| 35-48 | Bernard MAMET & Alain PRÉAT

Bernard MAMET & Alain PREAT. 2013. Essai de description d’algues nouvelles paléozoïques. Geologica Belgica, volume 16 (1-2): 35-48.

Description of a dozen new genera (Cheggatellina, Emsiella, Flexitubulla, Kamaenina, Koninckoporella, Masloviporella, Paraumbellina, Pseudoepiphytella, Sphaerinviella, Vintonella, Wapitella) associated with about forty new species. The microflora, including a few Microproblematica, are derived from many basins on a worldwide scale. The samples range from the Devonian, Carboniferous and Permian.

Description d’une dizaine de nouveaux genres (Chegatellina, Emsiella, Flexitubulla, Kamaenina, Koninckoporella, Masloviporella, Paraumbellina, Pseudoepiphytella, Sphaerinviella, Vintonella, Wapitella) et d’une quarantaine de nouvelles espèces. La microflore, associée à quelques Microproblématiques, provient de bassins à l’échelle mondiale. L’échantillonnage couvre le Dévonien, le Carbonifère et le Permien.

Keywords: Calcareous algae, Problematica, Taxonomy.

MOTS-CLES: Algues calcaires, Problématiques, Taxonomie.

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| 49-65 | Alain HERBOSCH & Jacques VERNIERS

Alain HERBOSCH & Jacques VERNIERS. 2013. Stratigraphy of the Lower Palaeozoic of the Brabant Massif, Belgium.Part I: The Cambro-Ordovician from the Halle and Ottignies groups. Geologica Belgica, volume 16 (1-2): 49-65.

Multidisciplinary research in the last 25 years and recent geological mapping in the Brabant Massif, have completely changed our knowledge about one of the poorly known part of Belgian geology. The sedimentary succession is surprisingly complete by comparison with the literature before the 1970s, from the lower Cambrian to the top of the Silurian, and very thick (>13 km), thus highlighting the need to produce an up-to-date stratigraphic nomenclature. In this first paper about the Cambrian and the lowest Ordovician, we describe in detail the formations, which are classified into two new groups, how the description of the units evolved through time, their lithology, sedimentology, boundaries and contacts, thickness, fossil content and type sections or most typical outcrop areas. The lower Halle Group comprises the Blanmont, Tubize and Oisquercq formations that consist of sandstone, siltstone and pale coloured slate. The overlying Ottignies Group comprises the Jodoigne, Mousty and Chevlipont formations formed of a more argillaceous and notably darker lithology. The two groups constitute a very thick (> 9 km) lower Cambrian to lowermost Ordovician siliciclastic succession, mostly pelagic and turbiditic. The cumulative thickness curve is concave-upwards which shows an extensional rift basin. A comparison with the Condroz Inlier shows that only the uppermost part of the sedimentary pile was observed in boreholes (Chevlipont Formation).

Keywords: Brabant Massif, stratigraphy, Cambrian, Avalonia, Condroz Inlier, rift.

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| 66-75 | Stijn DEWAELE, Herman GOETHALS & Tom THYS

Stijn DEWAELE, Herman GOETHALS & Tom THYS. 2013. Mineralogical characterization of cassiterite concentrates from quartz vein and pegmatite mineralization of the Karagwe-Ankole and Kibara Belts, Central Africa. Geologica Belgica, volume 16 (1-2): 66-75.

The Mesoproterozoic Kibara belt (KIB) and the Karagwe-Ankole Belt (KAB) in Central Africa are characterized by the presence of numerous rare metal mineralized Sn-(Nb-Ta) pegmatites and Sn-W mineralised quartz veins that are related to a S-type granite generation formed at 986 ± 10 Ma. Cassiterite concentrates have been studied by different petrographic and mineralogical techniques. The concentrates have been collected from the rock and mineral collection of the Royal Museum for Central Africa (RMCA) and originate from historical exploitations of eluvial and alluvial cassiterite deposits. No quantitative study of the concentrates has been envisaged since no information is available about the history of the samples prior to sampling. Microscopic investigation revealed the presence of cassiterite crystals with metallic and non-metallic luster, of which the latter show growth zoning. The color from the cassiterite crystals can vary from transparent colorless to black non-transparent. The variation in color in a single grain can be as varied as the color variation between grains for an entire concentrate. The mineralogical composition of the cassiterite concentrates contains minerals that are typical gangue and accessory minerals in the primary mineralization and that were liberated during weathering. In addition, minerals can be found that result from the weathering from the metasedimentary and doleritic host-rocks of the primary mineralization. Except the occasional presence of a certain mineral or a special color for a certain cassiterite, no systematic variation can be observed between the concentrates from the different locations. Often, the mineral and color variation in one concentrate can be as large as for concentrates found from different locations.

Keywords: Karagwe-Ankole belt, Kibara belt, cassiterite concentrates, microscopy, XRD, SEM-EDX.

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| 76-83 | David DEBRUYNE, Lieve BALCAEN, Frank VANHAECKE & Philippe MUCHEZ

David DEBRUYNE, Lieve BALCAEN, Frank VANHAECKE & Philippe MUCHEZ. 2013. Rare earth element and yttrium characteristics of carbonates within the sediment-hosted Luiswishi and Kamoto Cu-Co deposits, Katanga Copperbelt (Democratic Republic of Congo - DRC). Geologica Belgica, volume 16 (1-2): 76-83.

The Neoproterozoic Central African Copperbelt is a world-class metallogenic province characterized by sedimenthosted stratiform Cu-Co ore deposits and polymetallic vein-type deposits. Two main mineralization phases have been recognized in the Katanga Copperbelt (Democratic Republic of Congo). The first phase occurred during early to intermediate diagenesis and the second during deep burial and the Lufilian orogeny. The Rare Earth Element and Yttrium (REY) concentrations of gangue dolomites associated with the second mineralization phase at the Kamoto and Luiswishi ore deposits were determined and compared to those of their respective host rocks. Kamoto samples exhibit convex Upper Continental Crust-normalized patterns with a pronounced light REE depletion without significant Eu anomalies and with positive La anomalies. The Luiswishi samples exhibit either progressively increasing REY patterns or ramp-shaped patterns with minor mid REE-heavy REE fractionation, accompanied by pronounced negative Ce and Eu anomalies. These differences suggest that a more intense metamorphic overprint at Luiswishi contributed to the differences with the Kamoto deposit.

Keywords: dolomite, hydrothermal, fluid-rock interaction, remobilization, Eu anomaly, Ce anomaly.

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| 84-94 | Kenneth T. HIGGS, Cyrille PRESTIANNI, Maurice STREEL & Jacques THOREZ

Kenneth T. HIGGS, Cyrille PRESTIANNI, Maurice STREEL & Jacques THOREZ. 2013. High resolution miospore stratigraphy of the Upper Famennian of eastern Belgium, and correlation with the conodont zonation. Geologica Belgica, volume 16 (1-2): 84-94.

The Ourthe valley in eastern Belgium provides the most complete exposures of the upper Famennian Condroz Group. A detailed miospore stratigraphy has been established for this succession. The lower three miospore biozones of the Upper Famennian (GF, VCo and VH Oppel Zones) are re-defined and subdivided into five new interval zones. The GF Oppel Zone which spans the Middle and Upper Famennian boundary was previously subdivided into three interval zones is now reduced to two. The highest Retispora macroreticulata (Mac) Interval Zone is now removed (due to its very sporadic occurrence) which results in the Grandispora microseta (Mic) Interval Zone of the GF Oppel Zone now being the lowest miospore interval zone of the Upper Famennian. The overlying VCo Oppel zone is subdivided into two new interval zones, the Rugospora radiata (Rad) and overlying Grandispora cornuta (Cor) interval Zones. A detailed study based on new and published material of the two nominal zonal taxa of the VCo Zone has clarified their morphology and resolved identification problems with other closely related Upper Devonian taxa. Furthermore, a biometric comparison of Grandispora cornuta with Grandispora tamarae has resulted in the specific diagnosis of G. cornuta being emended to increase its minimal spine length to 8 ìm. The biostratigraphic implications for intercontinental correlations with the Upper Famennian of North America are discussed. The overlying VH Oppel Zone is subdivided into two new interval zones, the Apiculiretusispora verrucosa (Ver) Interval Zone and the overlying Vallatisporites hystricosus (Hys) Interval Zone. All of the new miospore interval zones are closely correlated with the well established conodont zones occurring in five sections of the Ourthe Valley. The new integrated data sets provide a high resolution biostratigraphy for the Upper Famennian stratigraphic interval in eastern Belgium.

Keywords: lithostratigraphy, biostratigraphy, guide fossils, Upper Devonian.

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| 91-104 | Sander VAN LANGENDONCK, Philippe MUCHEZ, Stijn DEWAELE, Alphonse KAPUTO KALUBI & Jacques CAILTEUX

Sander VAN LANGENDONCK, Philippe MUCHEZ, Stijn DEWAELE, Alphonse KAPUTO KALUBI & Jacques CAILTEUX. 2013. Petrographic and mineralogical study of the sediment-hosted Cu-Co ore deposit at Kambove West in the central part of the Katanga Copperbelt (DRC). Geologica Belgica, volume 16 (1-2): 91-104.

Kambove West is a sediment-hosted Cu-Co deposit in the Neoproterozoic Mines Subgroup, which is mainly composed of dolomitic shales and dolostones. Key factors and processes responsible for high-grade mineralization were identified by studying multiple cores. The stratigraphic position of the mineralized zones adjacent to the Roches Siliceuses Cellulaires and brecciated zones are the most important factor controlling the formation of a high-grade mineralization as they most likely acted as conduits for the mineralizing fluids. The presence of organic matter, stromatolite fragments and anhydrite pseudomorphs promoted mineralization. The organic matter caused a reducing environment required for the precipitation of the sulfides. Sulfate-reducing microorganisms, which were main components of Precambrian stromatolite communities, could also have caused the necessary reduction reactions. In addition to sulfate from the pores of the sediment, anhydrite formed a sulfate source. A paragenesis with six stages was established based on microscopic observations and cold cathodoluminescence petrography. Two major hypogene mineralization phases, which consist of pyrite, chalcopyrite, bornite, chalcocite and carrollite, formed during diagenesis and low-grade metamorphism, and were succeeded by supergene remobilization, of which chalcocite forms the dominant Cu-phase in the cementation zone. The distinction between hypo- and supergene chalcocite is based on the identification of multiple microtextures, chalcocite polymorphism, and the association with iron and other (hydro)oxides or late generations of dolomite. However, none of the features are conclusive. The combination of them is highly suggestive for a supergene remobilization.

Keywords: Neoproterozoic mineralization, stratiform Cu-Co ore deposits, Central African Copperbelt, Kambove West, ore distribution and grade, supergene enrichment, chalcocite.

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| 105-117 | Niels HULSBOSCH, Jan HERTOGEN, Stijn DEWAELE, Luc ANDRÉ & Philippe MUCHEZ

Niels HULSBOSCH, Jan HERTOGEN, Stijn DEWAELE, Luc ANDRÉ & Philippe MUCHEZ. 2013. Petrographic and mineralogical characterisation of fractionated pegmatites culminating in the Nb-Ta-Sn pegmatites of the Gatumba area (western Rwanda). Geologica Belgica, volume 16 (1-2): 105-117.

The Karagwe-Ankole belt in Central Africa hosts numerous rare-metal pegmatites and Sn–W mineralised quartz veins, which are related to the granite generation that formed at 986 ± 10 Ma, i.e. the G4-granites in Rwanda. This early Neoproterozoic granite generation features in the Gatumba area (western Rwanda) a linkage with a zoned cluster of barren and Nb-Ta-Sn mineralised pegmatites. The most distal pegmatite bodies in this area suffered from intense alkali metasomatism, i.e. widespread growth of albite and white mica. A petrographic and geochemical study has been carried out on the pegmatite bodies in order to determine the petrogenetic evolution of the pegmatite zonation. The compositional variation of schorl and elbaite, sampled along the regional zonation sequence, implies that the zonation can be expressed by 4 zones: a biotite, a two-mica, a muscovite and a mineralised pegmatite zone. Alkali element variations and enrichments in muscovite and K-feldspar along the zonation sequence indicate that the origin and the formation of the different pegmatite zones can be explained by a single path of fractional crystallisation. This substantial chemical differentiation accompanies in the Gatumba pegmatite field the evolution from the G4-granite generation to common pegmatites and eventually rareelement pegmatites.

Keywords: Karagwe–Ankole belt, G4-granites, regional pegmatite zonation, LCT-family, mineral geochemistry, fractional crystallisation, alkali metals.

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Pieter R. Gurdebeke & Bert Van Bocxlaer. 2013. Conchological differentiation in an ongoing radiation of Lanistes gastropods from ancient Lake Malawi: how adaptive is shell morphology?. Geologica Belgica, volume 16 (1-2): 118-119.



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