Olivier Féménias’s research while affiliated with Université Libre de Bruxelles and other places

The 2·7 Ga Iguilid mafic intrusion is a post-metamorphic plutonic body made of cumulate gabbronorite and norite characterized by limited variations in modal proportions but significant disparities in trace element contents. The cumulates display microtextural evidence for syn-magmatic deformation, leading to reorientation of cumulus minerals, indented contacts at plagioclase grain boundaries, local bending and fracturing of euhedral cumulus plagioclase and interstitial melt migration/segregation structures. All these features suggest that the Iguilid intrusion formed after in situ crystallization and syn-magmatic deformation of an unconsolidated mush within solidification fronts. Quantitative investigations of syn-magmatic fabrics together with geochemical estimations of the trapped melt fraction reveal a strong inverse correlation between the anisotropy degree P and the volume of trapped liquid retained in the cumulates. We interpret the dominantly vertical planar and linear fabrics in the Iguilid rocks as a result of horizontal shortening of the cumulates induced by filter pressing of a partly solidified mush against the vertical walls of the intrusion. Flattening of the cumulates occurred in response either to a syn-magmatic rotation of the Iguilid body relative to the tectonic main stress or to stages of over- and under-pressurization owing to cycles of magma recharge or discharge. Deformation of the unconsolidated cumulates led to migration of the silicic interstitial melt and controlled their bulk content in incompatible elements. This migrating liquid had similar trace element and isotopic compositions to synchronous 2·7 Ga dioritic to granodioritic plutons, forming a bimodal, post-metamorphic, magmatic suite with the Iguilid body. The interstitial melt extracted from the Iguilid mafic intrusion could have fed synchronous metaluminous intermediate to felsic plutons.

As rock textures reflect the physical conditions and the mechanisms of formation of the rocks, new approaches are used for improving texture analyses, both qualitatively and quantitatively. Pioneer work has recently boosted interest in fractal analysis for quantifying and correlating patterns. Fractal-like patterns relate to a degree of multiscale organization, and fractal dimensions (FD) and their potential variations can be used to infer the physical conditions of rock formation at various scales of observation. Here, we characterize quantitatively the shape and distribution of orthopyroxene grains in ultramafic xenoliths in terms of FD and their relation with temperature of equilibration. Fractal analysis has been applied to several populations of mantle xenoliths: 7 xenoliths collected in the vicinity of Pico Santa Isabel on Bioko Island, an alkaline basaltic volcano in oceanic domain (Gulf of Guinea, Equatorial Atlantic), 9 samples from Sangilen, in the Agardag alkaline lamprophyre dyke (Russia), and 11 samples form Śnieżnik (Lutynia, Poland), in the continental domain. Fractal analysis has been conducted to characterize the degree of complexity of the petrographic textures: it is indeed known that large values of FD are associated to more complex textures. The correlation here observed between the orthopyroxene fractal dimension and the temperature of equilibration suggests that FD captures a significant textural feature directly related to the temperature (i.e. generated by a temperature-controlled process). The significant difference in the FD–T correlation observed for the continental and oceanic mantle domains suggests that the mechanical and rheological behaviour is distinct in the oceanic and continental lithospheres. These first promising results should be confirmed by analysing other mantle suites of rocks in different geodynamic settings. Copyright © 2015 John Wiley & Sons, Ltd.

This work proposes a new geodynamic model for the development of the Cameroon Hot Line (CHL) in Equatorial Africa. It is based on the analysis of the distribution of lineaments and of magmatic bodies (Paleogene anorogenic ring-complexes and Neogene volcanic centres). Two successive geodynamic models are proposed to explain the distribution of the Cainozoic to recent magmatic activity. They are both sinistral. The first one, during the Paleogene, developed around the N ∼ 70°E direction while the second one (Neogene) is oriented around the N ∼ 130°E direction. The two periods are separated by a short transition. The emplacement follows the local reactivation of pre-existing (Pan-African) faults in relation to the collision between the Afro-Arabian and Eurasian plates, during the Alpine history.

In this paper, the complexity in the spatial distribution of the lineaments was investigated from on remote sensing to-pographic (SRTM DEM) and multispectral (Landsat) data. Lineaments in equatorial Africa were chosen to apply the fractal analysis methodology. The good correlations of the obtained data with some geophysical data from the same area allow that the complexity in the spatial distribution of the lineaments can give qualitative information on the inte-rior of the earth (or on other planets). This method can provide a bridge between classical geology and geophysics, and particularly powerful for studying large and inaccessible regions.

The Archean Guelb el Azib layered complex (GAC) in the West African craton of Mauritania is composed of an association of serpentinites, chromitites, amphibolites and anorthosites with few fine-grained amphibolite dykes. The complex forms tectonic slices in 2.9-3.5 Ga TTG gneiss terrains in close association with supracrustal rocks (BIFs, impure marbles, amphibolites). It was affected by a main granulite-facies grade metamorphism (up to 900 degrees C at 5-6 kbar) with subsequent retrogression in amphibolite and greenschist facies conditions. The preserved igneous macrostructures, the mineral compositions and the nature of relic magmatic assemblages have been used to constrain the composition of the parental melts and the conditions of crystallization. According to petrological observations and to comparison with experimental data, the formation of the complex can be explained by fractionation of a slightly hydrous high-alumina basaltic melt at low pressure. The early fractionation of olivine and the absence of massive clinopyroxene fractionation before plagioclase saturation led to crystallization of highly calcic plagioclase with Fe-, Al-rich but Cr-poor chromite from a hydrous tholeiitic parental magma, similar to worldwide Archean tholeiites. The complex shares many similarities with Archean anorthosite layered complexes, possibly formed in a supra-subduction zone environment according to results obtained on similar 2.9-3.0 Ga complexes from Greenland and India (namely Fiskenaesset and Sittampundi). Three phases of PGE mineralization affected the GAC chromitites: (i) igneous crystallization of laurite; (ii) formation of late magmatic IPGE sulpho-arsenides (irarsite-hollingworthite) and (iii) hydrothermal Pt-Pd mineralization represented by sperrylite and rustenburgite. (c) 2012 Elsevier B.V. All rights reserved.