Search

International audience; This paper presents a reappraisal of the main structural and metamorphic features of the Southern Svecofennian Arc Complex (southern Finland) and provides new data that allow us to reconsider its tectonic evolution. Results emphasize that (1) regional shortening was accommodated by distributed thickening with no clear evidence for large-scale thrusting, (2) syn-convergence lateral flow of ductile crust most probably competed with crustal thickening since early stages of deformation, (3) no evidence exists for thickening-driven extensional collapse associated with normal shear zones, the latest structures developed being compressive, (4) exhumation appears basically driven by distributed erosion that competed with crustal thickening. Observations are interpreted in terms of a peculiar mechanical behaviour of a hot and weak juvenile lithosphere undergoing compression.

The granitoids of Marajoara in the Rio Maria terrain (Carajas Mineral Province, Brazil) consist of: (i) a broad unit of 2.96 Ga syntectonic tonalites (Arco Verde Tonalites) displaying a trondhjemitic differentiation trend; (ii) 2.93 Ga syntectonic monzogranites (Guaranta); and (iii) 2.87 Ga post-tectonic monzogranites (Mata Surrao) and granodiorites (Rio Maria), displaying a calc-alkaline differentiation trend. Deformation of the Arco Verde tonalites is heterogeneous with low strain domains (well preserved magmatic banding and textures) and orthogneissic domains displaying an E–W trending mainly subvertical foliation, associated with horizontal lineations, upright folds and subvertical shear zones. Microstructures and phase assemblages suggest that deformation occurred within a large temperature range (i.e. during magma emplacement and cooling), from high-T conditions (synmagmatic shear zones and subsolidus ductile deformation with intense quartz and feldspar recrystallization; Pl+Qtz+Hbl+Bt assemblages) to medium- and low-T conditions (ductile to brittle deformation with weakly recrystallized quartz and undulose extinction in feldspars; Qtz+Pl+Bt+Mu or Chl+Ep+Ab+Qtz assemblages). These data, finite strain analysis and structures reported from the surrounding greenstone belts suggest that deformation did not result from a post-emplacement prograde tectono-metamorphic event as considered previously, but that the Marajoara granitoids are synkinernatic intrusions which were deformed together with the supracrustal rocks during a regional NS horizontal shortening. Although the Rio Maria terrain presents similarities with Archaean domains controlled by diapiric processes (lithologies dominated by thick greenstone sequences and TTG plutons, and forming a dome-and-keel structure), its structural evolution is controlled dominantly by a transpressional event which shaped the granite–greenstone terrains. The Rio Maria area, probably as many Archaean ‘grey gneisses’ domains, represents an intermediate case between terrains controlled by Raleigh–Taylor instabilities in a thermally softened crust with insignificant external forces related to plate convergence (e.g. east Pilbara craton) and those controlled by thrust tectonics related to convergence of rigid plates (e.g. Superior Province). The closest analog to the Rio Maria terrain seems to be the Chilimanzi area in the Zimbabwe craton.

The Eseka migmatites provide a good example of high-P—high-T anatexis of Archaean quartz-plagioclase-biotite grey gneisses. Partial melting resulted in the development of neosomes comprising a quartzo-feldspathic fraction of granodioritic to trondhjemitic composition and ferromagnesian minerals represented by hornblende mainly (15–45 vol.%), garnet in lesser amounts (4–8 vol.%) and scarce clinopyroxene. Thermobarometry suggests that anatexis occurred close to the amphibolite-granulite facies transition, under conditions estimated at around 9 kbar and 750°C. A late retrogression is documented by the growth of epidote and chlorite and by plagioclase albitization. Textural relations, mineralogical data and recalculation of the primary mineral assemblages suggest that the biotite dehydration melting conditions were reached leading to the generation of granodioritic melts in equilibrium with K-feldspar, hornblende and garnet (±clinopyroxene). In some cases, this was followed, during melt crystallization, by a retrograde reaction involving the granodioritic melt and hornblende to produce biotite-epidote assemblages and trondhjemitic neosomes. Compared to similar migmatites, the Eseka neosomes are characterized by higher Al and Ti contents of hornblende, the presence of garnet and the lack of sphene which can be related to higher P, T conditions. Geochemical data point to the segregation and removal of some melt in the Eseka migmatites resulting in limited concentration of hornblende and garnet at the site of melting. This together with previous field observations and experimental data, could suggest that some hornblende-rich gneisses or amphibolites found in the lower middle crust may represent a residue left after extraction of anatectic melts. Geochronological and petrological data further suggest that large portions of the Archaean TTG rocks from the Congo craton have been reworked under high-grade conditions during the Paleoproterozoic.

The newly discovered Adam Talha epizonal sequence extends to the west the epizonal sequences defined by Sougy (1960) in the Reguibat Dorsale. In the Algerian part of the Reguibat Dorsale, Sabate and Lameyre (1973) have shown the existence of two Eburnean orogenic cycles. These two cycles: the Yetti cycle (older than 2,000 m.y.) and the Eglab cycle (near 2,000 m.y.) have been observed in the Mauritanian part of the Reguibat Dorsale. In the Ivory Coast, the two structural stages defined by Tagini (1971) seem to be the equivalents of the Yetti and Eglab cycles. However, some differences exist in the chemical compositions of the volcanic rocks (spilitic in the Ivory Coast, acid in the Reguibat Dorsale) and in the absence of volcanic emissions in the upper stage of the Ivory Coast. In Mali, according to Reichelt (1972), the Gourma Basin formations (Hombori and Ydouban groups) belong to the pan-African and not to the Eburnean orogeny. The greater part of West Africa seems to have been affected by the two cycles of the Eburnean orogeny.

The Yaounde series is composed of low to high-grade garnet-bearing schists and gneisses belonging to the Pan-African North-equatorial fold belt. The high-grade rocks consist of kyanite—garnet gneisses and garnet—plagioclase gneisses containing layers of calcsilicate rocks, marbles, quartzites and magnetite-rich orthopyroxenites. The chemical patterns of these rocks are those of a sedimentary sequence of shales and greywackes and minor dolomite-rocks, dolomitic marls, evaporitic beds, quartzites and iron-rich sediments. Some volcanogenic greywackes may have been of alkaline affinity. This sequence was probably deposited in shallow-water, near-shore, semi-barred conditions which can be related either to an intracontinental distensive environment or to a passive margin at the northern edge of the Congo craton. The Yaounde gneisses were deformed during two main tectonic phases: the first phase (D1) is marked by a layering (S0-S1) resulting from tectonic transposition; the second one (D2) corresponds to tangential tectonics with isoclinal folds and flat-lying shear zones associated with a S2 schistosity and a L2 lineation; a third phase (late-D2) is marked by the development of low temperature mylonitic shears. Scarce wrench faults indicate later E—W compression. The D1-D2 transition is characterized by the emplacement of mafic and ultramafic rocks and by the development of high-pressure granulite facies conditions (T=750–800°C, P=10–12 kbar). These peak metamorphic conditions triggered a widespread migmatization dated at 565±22 Ma and coeval with the D2 event. The gradual decrease of the metamorphic conditions toward the south and the existence of a syn-D2, retrograde, inverse metamorphic gradient, suggest that the overall structure of the Yaounde series is a large tectonic nappe thrust onto the Congo craton. A geotectonic scheme for the North-equatorial fold belt is discussed.