It occurred whilst I was sweating away in a cocoa plantation on the hunt for outcrop, in the hope of furthering our understanding how the Earth was changing around two billion years ago, during its teenage years. My research focuses on the geology of central SW Ghana, with the aim of furthering the understanding of crustal evolution and accretion of the West African Craton during the Palaeoproterozoic. The Palaeoproterozoic domain is unique in that it is dominated by vast, alternating granite-greenstone provinces bound by craton-scale shear zones, reminiscent of Archean domains. The craton, therefore, provides us with an opportunity to study how Earth processes evolved at the Archean-Palaeoproterozoic boundary, relative to the well-constrained processes occurring today. Furthermore, it represents the first orogenic event during a period of global orogenesis between 2. Figure 1.
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Pierre Debat. Didier Beziat. A new metamorphic constraint for the Eburnean orogeny from Paleoproterozoic formations of the Man shield Aribinda and Tampelga countries, Burkina Faso. Biotite, chloritoid, staurolite—kyanite zones are distributed concentrically around the Aribinda pluton on its northern margin but are displaced by later thrust faults in the south of the Aribinda area and in the nearly Tampelga area.
They show the leading part of the granitoid emplacement in the thermic structuration of the greenstone belts and the post-metamorphic character of the tangential tectonic. In Burkina Faso, the Eburnean orogeny underwent: 1 initial greenschist-facies metamorphism; 2 emplacement of granitoids at mid- deep-crustal levels leading to unusual medium pressure metamorphism; 3 later thrusting. All rights reserved. Introduction imentary basins and linear and arcuate volcanic belts intruded by several generations of granitoids Leube The Paleoproterozoic Birimian Group Junner, et al.
They correspond to a period the West African Craton Bessoles, , at the east- of accretion during the 2. The Birimian terranes, which make et al. Debat et al. The Birimian crust levels; ii a thick sequence of basalts, locally pillowed, s.
We put forward ended by a coarse clastic sedimentary sequence be- the hypothesis that the staurolite—kyanite assemblage longing to the Tarkwaian Group Hirdes et al. Locally, however, ev- idence of medium amphibolite-facies metamorphism The characteristics of the amphibolite-facies assem- can be observed in the vicinity of granitoid plutons, blages, particularly of the staurolite—kyanite assem- and is marked by the presence of the kyanite—staurolite blage, have been determined in the Aribinda area, assemblage.
The occurrence of this mineral pair was which is part of the Djibo greenstone belt, located in first described by Junner , in Ghana and northern Burkina Faso near the Mali border Fig.
Lithology and structure domain, with the exception of the Kedougou-Kenieba inlier Junner, , ; Woodfield, ; Moon The Djibo greenstone belt Fig. Junner batholiths Ducellier, The these minerals are localized around granitic batholiths. Aribinda pluton maps out as a roughly EW-trending Pons et al.
On the other hand, Ledru et al. Table 1, analysis 1 to 0. It belongs to the alkaline resulted from crustal thickening associated with thrust granitoid group described by Ducellier , which tectonism. Thus, it appears evident that understanding consist of relatively small intrusive bodies emplaced the significance of this relatively high pressure assem- within Paleoproterozoic sediments across the entire blage is fundamental in the definition of the charac- Man shield Lacroix, ; Bessoles, ; Hirdes teristics of the Eburnean orogeny.
Locations of Figs. However, Bessoles , Leube the margin of the pluton — m away from the et al. Near to post-orogenic, thus, emplaced after the basin- and the contact, mineral orientation is stronger. Quartz belt-type granitoids. In its northern part Fig. In this contact zone the and -conglomerate and -volcano sediments. This enclaves are strained and display oblate ellipsoidal contact is well exposed north of the village of Pem shape. The shape of the enclaves suggests a low vis- Fig.
Except on the peripheral zone, the Aribinda granite A significant and progressive change in intensity has an apparent overall homogeneous and isotropic and direction of deformation can be observed in the magmatic structure. All minerals including quartz are country rock when approaching the pluton. For in- undeformed and the biotite and amphibole-rich en- stance Fig.
MgO 0. CaO 1. Na2 O 4. TiO2 0. Geological map a and cross-section b of the northern aureole of the Aribinda granite. Near the pluton, this foliation becomes a pene- man, ; Ramsay, ; Pons et al. This deformation is gen- to the limit of the granite and its planar fabric. The erally interpreted to be related to the final expansion schistosity, in the metasediments near the pluton, is ballooning of the pluton Bateman, ; Ramsay, defined by a strong orientation of muscovite lamellae ; Pons et al.
However, the abundant kyanite and staurolite porphyroblasts remain undeformed and 2. Petrographic features of the metamorphic zones unoriented. Similarly garnet and chloritoid have eu- hedral textures and contain inclusion trails of schisto- On the basis of index-mineral distribution in the sity.
The amphibole has a hastingsite composition according to the clas- sification of Leake et al. Poikiloblastic garnet encloses quartz, biotite and hastingsite Fig. The conglomerate is composed of elon- gated quartzite pebbles 10—30 cm for the long axis included in a matrix consisting of biotite, garnet, plagioclase An 35—50 , epidote and quartz Table 3, Fig.
Chemical composition of the metasediments plotted on a sample A Garnet forms small euhedral grains, par- Herron scheme. Rock types defining the biotite zone include The metapelites are typ- erate. The metapelite Table 1, analysis 2 , shale in the ified by two assemblages Table 3, samples 9. The metavolcano sediments in isolated ovoid spots Fig.
They are typified by the assemblage quartz, pla- quartz-rich layers Fig. Table 3 Petrography of investigated rocks Minerals A16 9. The staurolite—kyanite zone is about m wide.
The metaba- euhedral porphyroblasts of staurolite, kyanite and, sic rocks correspond to metabasalt and metagabbro locally, chloritoid. Kyanite and chloritoid occur as showing the characteristic calk-alkaline affinities of elongated prisms while staurolite grains exhibit a the upper Birrimian sequence Table 1, analysis 6. Kyanite is intergrown with Metabasic rocks show a well-developed metamor- chloritoid, although more commonly with staurolite phic assemblage consisting of randomly-oriented in a manner similar to that described by Wenk Leake et al.
Metaarkose Fig. Presence of the and K2 O contents It is in contact metamorphic aureoles. This mineral as- made up of quartz, muscovite and kyanite but lacks semblage has been observed in regional metamor- staurolite and chloritoid Table 3, sample S1. A stable relation- quartz, muscovite, kyanite, staurolite, ilmenite and ship among these phases has also been described tourmaline, plus chloritoid in the upper level of the from the chloritoid—staurolite quartzite of the Clough zone Fig.
In this area New Hampshire , position Fig. Garnet in hastingsite-bearing aureole. Representative analyses are listed in Table 2; iron is expressed as FeO for the calculation of mineral formulae except for garnet and amphibole, for which it is listed Fe2 O3. This inference is also supported bearing S4 and in chloritoid-absent A13 assem- by the chemical evolution of the ubiquitous mus- blages, and is characterized by the same exceptionally- covite, i.
Diagnostic features for high-pressure con- 9, and Fig. Overall P—T conditions can be higher grade zone S4 , is characterized by a high Fe estimated from diagnostic assemblage. No mineral reaction evidence has been observed in Biotite in equilibrium with muscovite in the any rock types throughout the different zones.
Biotite in equilibrium with garnet common mineral of pelitic associations, is poorly A21 is richer in Fe than that belonging to the asso- represented as a reactant or a product in metapelites ciation hastingsite—garnet 9.
Plagioclase in the conglomerate matrix A21 and 2. Existing pelitic P—T grids usually depict rocks with in metavolcano sediment 9. Mather, ; Miyashiro, ; Miyashiro and Shido, Petrogenetic studies, integrating the effects of Hastingsite, specific to the metavolcano sediments minor components, especially MnO, have recently re- of the biotite zone 9.
Mahar et al. Nevertheless, these studies can- not be applied to the unusual sequence of assem- 2. P—T conditions blages found in the Aribinda metapelites which, in the high grade rocks, contain biotite and are commonly The metapelites are nearly homogeneous in com- chlorite- and garnet-free. It is well known that the Al position across the different metamorphic zones, i.
Table 1 in addition to high Al contents promoted Fig. Therefore, in our petrogenetic presence of kyanite are of 4 and 5 kbar for the upper grid we can only consider garnet-free assemblages and the lower parts of the same zone, respectively see observed in metapelite characterized by low MnO above. Ferry, zone are representative of the highest grade condi- ; Yardley, , we can reasonably estimate that tions experienced by the metamorphic rocks around the whole range of temperatures experienced by the the Aribinda granite.
Aluminium silicate triple point e after Holdaway The arrow illustrates the inferred metamorphic evolution within the Aribinda aureole, going towards the pluton. Metamorphism and thrusting aureole Table 1, analysis 10 , notably, high SiO2 It is es- The relationships between the amphibolite-facies sentially composed Table 3, sample Y52 of quartz, metamorphism and thrusting have been observed in muscovite, randomly distributed kyanite porphyrob- rocks surrounding the southern part of the Aribinda lasts and magnetite.
The metaconglomerate occurs as granite and in the Tampelga area.
Box , Koforidua, Ghana. This faulting tectonics analysis concerns the southernmost segment of the Dahomeyide Orogen and the West-African craton eastern margin in southeast Ghana. These paleostress systems define a clockwise rotation of the compressional axis during the structuring of the Dahomeyide Orogen — Ma. An extensional episode expressed as late normal faulting is also recorded in this study.