Dominique Laduron

Diffusion processes in resorbed garnets

Mn- (and Ca-) enriched border fringes around resorbed garnets, and Mn-enriched zones along cracks in resorbed garnets and along intergranular boundaries in resorbed sievegarnets, indicate a limited diffusion of Mn (and some Ca) into the garnet structure.

Timing of magmatism, foreland basin development, metamorphism and inversion in the Anglo-Brabant fold belt

New data implying crustal activation of Eastern Avalonia along the Anglo-Brabant fold belt are presented. Late Ordovician subduction-related magmatism in East Anglia and the Brabant Massif, coupled with accelerated subsidence in the Anglia Basin and in the Brabant Massif during Silurian time, indicate a foreland basin development. Final collision resulted in folding, cleavage development and thrusting during the mid-Lochkovian to mid-Eifelian. In the southeast of the Anglo-Brabant fold belt, Acadian deformation produced basin inversion and the regional antiformal structure of the Brabant Massif. The uplift, inferred from the sedimentology, petrography and reworked palynomorphs in the Lower Devonian of the Dinant Synclinorium is confirmed by illite crystallinity studies. The tectonic model discussed implies the presence of two subduction zones in the eastern part of Eastern Avalonia, one along the Anglo-Brabant fold belt and another under the North Sea in the prolongation of the North German-Polish Caledonides.

Cleavage-fold relationships in the lower Cambrian Tubize group, southeast Anglo-Brabant fold belt (Lembeek, Belgium)

Little is known about the cleavage-fold relationships in the Lower Palaeozoic Brabant Massif, primarily because of the scarcity of suitable exposures. Moreover, to date, folds have only been described in the Ordovician and Silurian sequences along the southern extremity of the basement. However, excavation works for the construction of the TGV(railway)-track south of Brussels created an opportunity to study cleavage-fold relationships in the Lower Cambrian terrigenous series (Tubize Group) in a more central part of the Brabant Massif. The structural features observed seemed inconsistent with the suspected regional trends. Primarily, a divergent cleavage fan was observed in a region thought to have a regular cleavage attitude. The symmetrical but divergent disposition of the cleavage with regard to the fold hinges is explained by flexural folding of a pre-existing bedding-parallel compaction fabric. Cleavage development and folding are considered synchronous. The divergent cleavage fan reflects local strain variations. Also the steeply plunging hinge lines of the open, subangular folds are seemingly inconsistent with the regional trend characterized by subhorizontal fold hinges. Taking into account the structural position of the fold assemblage in the subvertical limb of a large-scale, upright, isoclinal fold structure, the fold assemblage is interpreted as an incongruous parasitic feature. The steeply plunging hinge Lines are considered to be caused by fold hinge rotation during progressive coaxial deformation. Although this incongruous fold assemblage with its divergent cleavage fan is localized, it provides important information on both local and regional deformation circumstances in this part of the Anglo-Brabant Fold Belt.

[Pan-African overprint on Eburnian granitoids at the northern boundary of the West African Craton, Zenaga Inlier, central Anti-Atlas, Morocco]

The Zenaga Inlier shows a comprehensive record of the Eburnian and PanAfrican Orogenies. The Eburnian is characterised by high-temperature regional metamorphism and complex magmatism. The early (Azguemerzi) granodiorite has an isotopic mantle signature and was emplaced diapirically during the Eburnian Orogeny causing local thermal metamorphism. The foliation observed in this granitoid is a result of the interference between its primary syn-emplacement foliation and the regional foliation under amphibolite-facies conditions. The northern part of Zenaga has been intruded by the leucocratic granites of Tazenakht. These granites are cut by mylonites and phyllonites, corresponding to the PanAfrican shear zones and accompanied with sub-greenschist-facies metamorphism during the Pan-African Orogeny. The deformation was the result of a regional sinistral transpressive event. This study in the northern part of the West African Craton shows the superposition of the Pan-African on the Eburnian Orogeny and the presence of a major fault in the Anti-Atlas. ((C)) 2002 Elsevier Science Limited. All rights reserved.