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- 57Fe Mossbauer spectroscopy study of the correlation between the Fe3+ content and the magnetic properties of natural Cr-spinelsPublication . Waerenborgh, João C.; Figueiras, Jorge; A., Mateus; A. Gonçalves, MárioBoth the Fe oxidation degree and the magnetic properties determined by 57Fe Mössbauer spectroscopy were used to characterize the Cr-spinels from the Beja-Acebuches Ophiolite Complex (SE Portugal). Two different types of Cr-spinel were observed. The first one has an average Fe3+/(Fe total) ≈ 25%, and corresponds to unaltered Cr-spinel grains which remain paramagnetic down to 6K. The second one results from the alteration of the first type, has an average Fe3+/(Fe total) ≈40% and order magnetically between 222 K and 78 K. Fe3+/(total Fe) increases with the magnetic ordering temperature. The grain population ordering at the highest temperatures, in the range 202–222K, has the highest Fe3+/(total Fe) ≈ 58%. Quantitative Fe site distributions can be obtained from room-temperature Mössbauer data if the different recoilless factors for tetrahedral Fe2+ and octahedral Fe3+ are considered. The observed second-order Doppler shifts are consistent with Mössbauer temperatures of 330K and 605K, reported in the literature for tetrahedral Fe2+ and octahedral Fe3+ in other oxide spinels. The differences in the magnetic ordering temperatures allowed the characterization of both types of Cr-spinels using a single Mössbauer absorber, thus avoiding the need to separate them physically, which, in this case, would be very difficult. The results are compared to those obtained for other Cr-spinels and, for the first time in the case of natural Cr-spinels, the increase of their magnetic ordering temperatures with their oxidation degree, resulting from natural processes, is discussed in terms of the magnetic exchange interactions between the Fe cations.
- Geodynamic evolution of the South Variscan Iberian Suture as recorded by mineral transformationsPublication . Figueiras, Jorge; A., Mateus; A. Gonçalves, Mário; Waerenborgh, João; Fonseca, PauloNew structural, petrographic, mineralogical and geochemical data from the Beja-Acebuches Ophiolite Complex (BAOC) are presented, and reviewed together with data published elsewhere. The new data obtained shed light on questions such as: 1) the relative importance of the obduction event; 2) its geological record in the deep levels of BAOC; 3) the nature and intensity of the Variscan metamorphism and deformation during subsequent continental (arc) collision; 4) the age relationships between BAOC and the Beja Igneous Complex; and 5) by means of numerical modelling, the thermal metamorphism of the Ossa-Morena autochthonous terranes induced by the ophiolite obduction. The emerging picture is that of a fairly simple overall geological evolution for BAOC, seamlessly integrated within the evolution of the southern branch of the Iberian Variscides. Obduction of BAOC is a relatively minor early event in the general NE–SW convergence that gave rise to the orogen as seen regionally and is recorded by an anisotropic, high-temperature, metamorphic fabric at the gabbro levels and by subtle features of the chemical composition of primary minerals at the underlying peridotite level; it caused chilling of the obducted ophiolitic slab and no significant metamorphism on the autochtonous rocks of the Ossa-Morena Zone. BAOC underwent most of its deformation and (amphibolite facies) metamorphism during a later collisional event, that took place as the most primitive rocks of the Beja Igneous complex were being intruded, and whose waning stages are responsible for extensive serpentinisation of peridotites and for important aquocarbonic fluid discharges along the semibrittle–brittle shear zones meanwhile developed.
- Variscan overthrusting, fluid flow and the genesis of magnetite ore-bodies at Azenhas area (Pedrógão, Ossa-Morena Zone, SE Portugal)Publication . A., Mateus; Araújo, A; A. Gonçalves, Mário; Matos, JIn the Azenhas area (SE Portugal), an important segment of a regional WNW-ESE Variscan thrust can be observed. The hanging wall felsic metavolcanics carried from ENE are evolved rhyolite tuffs. The underlying sequence is mainly composed of variably metasomatised allochthonous amphibolites that are believed to represent an ophiolite slice tectonically overlying the autochthonous Moura-Ficalho Complex. Within this latter sequence, numerous WNW-ESE thrusts with prevailing displacement towards W-SW cut prior sub-parallel structures with northwards thrust shear. According to the observed crosscutting relationships and to the available petrographic data, the metasomatism experienced by the lower amphibolite sequence took place mainly before the emplacement of the upper slices of amphibolites, preceding therefore the installation of the felsic metatuffs. Magnetite ore-bodies are found within strongly metasomatised amphibolites immediately below the major WNW-ESE thrust zone or its subsidiary thrust structures. Their genesis is envisaged as a result of a complex chemical reaction path that involves the ascent of aqueous oxidising fluids under a reverse temperature gradient generated during the tectonic emplacement of amphibolites. In order to test the geological plausibility of the thermal evolution and the time span needed for ore genesis a one-dimensional numerical model was developed. This model enables the tracing of vertical thermal profiles at different times and accounts for the thrust emplacement history, incorporating variable erosion rates of the rock sequence. The results obtained show that an inverted thermal gradient in the thrusted sequence is kept for over 1 Ma and a regular temperature increase (from 400ºC to 500ºC) is observed in the underlying rocks lasting for 4.5 to 5.5 Ma.