The Pan-African Orogenic System is a complex mosaic of different chrono-thermo-barometric domains/terranes in an overall collisional system between the Congo and Kalahari Cratons. The Pan-African Orogenic System in southern Africa, from west to east, consists of the Gariep Belt, Kaoko Belt, Damara Orogen, Lufilian Arc, Zambezi Belt, the Malawi Mosaic, Mozambique Belt and Lurio Belt.
Research in the Zambezi Belt started in 1992 with a two year contract to map the Chewore Inliers for the Zimbabwe Geological Survey. Research in other parts of the Zambezi Belt and the Malawi Mosaic are continuing in collaboration with
Prof. David Gray (Melbourne University),
Prof. David Foster (University of Florida),
Dr. Ben Wade (Adelaide University),
Joe Meert (University of Florida) and technical support from
Paul Hamilton (Perth) and
Angus Netting and John Terlet (Adelaide Microscopy).
Additional ongoing projects are; the Chewore Ultramafic Complex with Simon Johnson (GSWA) and Prof. Tony Crawford (University of Tasmania), age determinations in Grenvillean aged granulite domains with David Foster, Simon Johnson and the tectonic significance of high-P domains in the Zambezi Belt with David Foster, Roland Maas and David Gray. Research on PAOS components in northern Africa are being undertaken in collaboration with Dr Thomas Bekker (BRGM), investigating the metamorphism of the Trans-Saharan Orogen.
Funding has unsuccessfully sought from ARC in 2008. Titled: Secular change in the thermo-mechanical response to orogenesis during the assembly of supercontinents, Goscombe, Clarke, Foster, Meert and Santosh.
Below are links to outcomes on different parts of the Pan-African Orogenic System:
• Malawi granulites
• Chewore Inlier metamorphism
• Chewore Inlier chronology
• Chewore ultra-mafic complex
• Chewore Inlier structure
• Trans-Saharan Orogen
[1] GRANULITES OF THE MALAWI MOSAIC
Ben Goscombe1, David Gray2
1School of Earth and Environmental Sciences, Adelaide University, 5005, South Australia.
2School of Earth Sciences, University of Melbourne, Parkville, 3052, Victoria.
Conference abstract, SGTSG Kalbari conference (2003).
Malawi is centred in a triangle region, here called the Malawi Mosaic, at the junction of 4 major orogenic belts (Zambezi, Ubendian, Mozambique and Irumide orogenic belts) and must surely be a contender for one of the most complex sector of crust preserved. The Malawi Mosaic is composed of a number of juxtaposed terranes, all of granulite grade. P-T loops and metamorphic conditions in all terranes have been constrained by petrologic interpretation in published petrogenetic grids. Peak metamorphic conditions have been calculated using THERMOCALC v3.1 (Powell & Holland, 1998) for homogenized core compositions and assuming aH2O=0.25 and presented as pooled means for each terrane. The peak parageneses are interpretively linked with published metamorphic zircon and monazite U-Pb ages.
These terranes are briefly summarized, from north to south through Malawai. Lulomo and Nyika regions are components of the Ubendian belt and correspondingly preserve the longest history. Main phase orogenesis is two successive high-grade events on an overall clockwise P-T path, through a high-P peak of 867±81 ºC and 10.1 kb at 2002 Ma in the Lulomo region and at 1990 Ma a low-P peak at 673±77 ºC and 4.5±1.1 Ma preserved in the Nyika region. The Kasunga region preserves petrologic and geochronologic evidence for three metamorphic cycles. Similar to the Nyika region, early low-P metamorphism was at 1986 Ma but with an anticlockwise P-T path through peak conditions at approximately 850 ºC and 4.0-6.0 kb. A later high-P clockwise P-T path granulite overprint at 860±67 ºC and 10.5±1.9 kb possibly occurred at 1100 Ma and was followed by amphibolite facies metamorphism anywhere between 990 Ma and 630 Ma. The Chipata region has not been well constrained. Nevertheless, this region has strong similarities to the Zambezi belt and is a granulite terrane of between 1100 and 850 Ma age, and has been reworked by Barrovian metamorphism at 671±51 ºC and 7.0±1.0 kb, anywhere between 800 and 524 Ma. The Lilongwe region experienced an overall clockwise P-T path through two successive granulite metamorphic peaks. Initially a high-P peak at 785±49 ºC and 9.8±1.5 kb at 635 Ma is preserved in most parageneses, followed by a low-P peak at 775±83 ºC and 6.6±1.2 kb at 580 Ma. Further south in the Dedza and Balaka regions the 580 Ma metamorphism is also recorded and similarly record an overall clockwise P-T path to a second low-P granulite peak at 535 Ma. The first peak is at 763±41 ºC and 9.2±1.0 kb and the second peak is at 777±51 ºC and 7.6±1.4 kb. The Blantyre region in the far south of Malawi preserves a clockwise P-T loop through near eclogitic conditions of 930±57 ºC and 14.3±2.1 kb, followed by decompression to high-P granulite conditions of 892±43 ºC and 12.6±1.8 kb at approximately 590-540 Ma.
The terranes comprising the Malawi Mosaic show the following features;
(1) High-grade metamorphic events spanning 2100 to 520 Ma, encompassing some 35% of earths history.
(2) Evidence of reworking by multiple granulite facies events in individual terranes.
(3) Geochronological evidence for at least 9 high-grade metamorphic events, with pooled means of age determinations in the wider region being; 2058±14, 1992±10, 1140-1080, 987±18, 938±11, 840±19, 646±13, 580±9 and 535±6 Ma.
(4) An extraordinary range of peak metamorphic conditions, ranging from 673±77 ºC and 4.5±1.1 kb to 930±57 ºC and 14.3±2.1 kb.
(5) Average thermal gradients range 20 to 50 ºC/km.
(6) Various P-T paths are documented, most are clockwise with multiple thermal peaks.
(7) Each terrane can be uniquely characterized by peak conditions, P-T paths, average thermal gradient and age of metamorphic events.
This data indicates juxtaposed terranes of contrasting tectono-metamorphic style, which infers that a diversity of tectonic environments, over time, caused high-grade metamorphism across the Malawi Mosaic. Progressive continental growth was largely towards the south and out-board towards the east. Nevertheless the youngest granulite metamorphic events (535±6 Ma) are in the core of the complex, are of high-P type (9.0-14.0 kb) and these occur in-board of the 646±13 Ma Mozambique belt to the east. The latest 535±6 Ma period is correlated with main phase deformation during N-S convergence in the Pan-African Orogenic System, which spans the continent from Malawi through the Zambezi Belt to the Inland Branch of the Damara Orogen.
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[2] Tectonometamorphic evolution of the Chewore Inliers:
Partial re-equilibration of high-grade basement during the Pan-African Orogeny.
Ben Goscombe, R. Armstrong1 and J.M. Barton2
Geological Survey of Zimbabwe, P.O. Box CY210, Causeway, Zimbabwe.
(1) Research School of Earth Science, Australian National University, Canberra, Australia.
(2) Geology Dept. Rand Afrikaans University, P.O. Box 524, Johannesburg, Republic of South Africa.
See Journal of Petrology 39, 1347-1384 (1998).
The Chewore Inliers are isolated outcrops of the Zambezi Mobile Belt within the Mesozoic Lower Zambezi Rift Valley in Zimbabwe. Detailed mapping has recognised four terranes the Zambezi, Quartzite and Granulite Terranes and the recently recognised Ophiolite Terrane (Johnson et al., 1996). Apart from the Ophiolite Terrane, all are dominated by supracrustal gneisses with concordant granitic orthogneiss units of 1071±8 and 1083±8 Ma age. These terranes experienced low-P/high-T metamorphism (M1) terminated by isobaric cooling at 945±34 Ma. M1 assemblages of sillimanite-spinel-garnet, garnet-orthopyroxene and two pyroxene mafics are recorded in the Granulite Terrane, and conditions of formation were 4.4±1.7 kb and >800 ºC. M1 mineral parageneses and associated ductile deformation structures dominate the Granulite Terrane, but M1 mineral parageneses are only preserved as sillimanite-spinel inclusions in garnet cores in the other terranes. The Zambezi, Quartzite and Ophiolite Terranes were almost totally recrystallized during reworking in the M2 metamorphic cycle.
M2 metamorphism accompanied NE over SW directed transport during Pan-African orogenesis of the Zambezi Belt at 524±16 Ma. Average peak M2 conditions, calculated using THERMOCALC V2.0b (Powell & Holland, 1988), were 7.9-8.6 kb and 590±95 ºC, 630±95 ºC and 717±95 ºC from the south and north Zambezi Terranes and Quartzite Terrane respectively. M2 involved a clockwise P-T path from the chloritoid stability field with matrix assemblages crystallised in the kyanite-staurolite field or at the kyanite/sillimanite transition and near isothermal decompression occurred through the peak of metamorphism into the sillimanite field. In contrast the Granulite Terrane was incorporated within the Zambezi Belt as a thrust-bound slab and experienced only minor structural reworking during M2. Granulite Terrane samples within 2 km of the basal thrust margin, preserve M1 mineral assemblages but these minerals were chemically re-equilibrated without recrystallization during M2 at conditions of 5.6±1.5 kb and 631±100 ºC. Granulite Terrane samples were totally recrystallized in shearzones at the margin of this terrane. These samples equilibrated at conditions identical to the peak of M2, at 7.7±1.9 kb and 590±110 ºC. The re-equilibrated and recrystallized sample sets define two points on the clockwise P-T path experienced by the Granulite Terrane during further burial and reworking in the Pan-African Orogeny, and is consistent with the M2 P-T path documented for the other terranes.
Keywords: Equilibrium thermodynamics, Geochronology, Metamorphism, P-T paths, Reworking, Pan-African Orogeny.
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[3] Geology of the Chewore Inliers, Zimbabwe: Constraining the Mesoproterozoic to Palaeozoic Evolution of the Zambezi Belt.
Ben Goscombe, R. Armstrong1 and J.M. Barton2
Geological Survey of Zimbabwe, P.O. Box CY210, Causeway, Zimbabwe.
(1) Research School of Earth Science, Australian National University, Canberra, Australia.
(2) Geology Dept. Rand Afrikaans University, Auckland Park, 2006, Republic of South Africa.
See Journal of African Earth Sciences 30, 589-627 (2000)
Structural, metamorphic and geochronological studies of the Chewore Inliers of the Zambezi Belt within the Karoo age Zambezi Rift, allow recognition of a protracted multi-stage evolution, from the Mesoproterozoic to culminating in the early Palaeozoic Pan-African Orogeny. Tectono-metamorphic events recognised in the Chewore Inliers occur throughout the Zambezi Belt and alternative models for the history of the Zambezi Belt are presented. Four terranes are recognised in the Chewore Inliers, and contacts between them are observed or inferred to be ductile thrusts, along which juxtaposition of the terranes occurred late in the Pan-African metamorphic cycle (M2, at 526 Ma). The oldest portion of the inliers is a metamorphosed sequence of mafic and ultramafic gneisses with an age of 1393 Ma (Johnson et al., 1996). These constitute what is tentatively called the Ophiolite Terrane which is closely associated with high-P/moderate-T schists and, together with this terrane possibly represents a suture. The other three terranes (Granulite, Zambezi and Quartzite Terranes) experienced a common history of tectonothermal events but show variable degrees of reworking during the latest tectono-metamorphic event (M2). Concordant granitic orthogneisses were emplaced at 1087 Ma into supracrustal sequences. No Pan-African supracrustals are recognised in the Chewore Inliers, which are wholly basement gneisses and quartzites that have been reworked during successive orogenies including the Pan-African Orogeny.
A high-T/low-P metamorphic event (M1) of possibly 1068-1071 Ma age, with a minimum age of 943 Ma, was responsible for totally recrystallizing the Granulite Terrane during south to north tectonic transport. M1 mineral parageneses are only preserved as inclusion phases and overgrown fabrics in the other terranes. These other terranes were pervasively recrystallized at high-P/moderate-T conditions accompanying a clockwise P-T path related to NE over SW tectonic transport and crustal over-thickening during the Pan-African metamorphic cycle (M2) at approximately 526 Ma. Reworking of the Granulite Terrane during M2 was minor, leaving M1 fabrics and mineral assemblages preserved with little recrystallization. M2 orogenesis culminated in the juxtaposition of the terranes, rapid uplift through the thermal peak and eventual slow cooling accompanying a multitude of post-tectonic intrusions; pegmatites at 480 Ma, the Chewore Ultramafic Complex and dolerite dykes. The 830 Ma tectonothermal event involving pervasive syn-tectonic granitic orthogneisses in the south Zambezi Belt is not recognised in the Chewore Inliers, suggesting a localized, possibly extensional, regime restricted to the southern part of the Zambezi Belt at 830 Ma.
Keywords: Zambezi Belt, Geochronology, Metamorphism, Pan-African, Reworking, Mesoproterozoic, Neoproterozoic, P-T paths.
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[4] Chewore Ultramafic Complex and associated Karoo Intrusives in the Chewore Inliers; Zambezi/Luangwa Rift Junction, Zimbabwe.
Ben Goscombe, Simon Johnson, Tony Crawford, Peter Fey
NOTES on the Chewore Ultra-Mafic Complex (CUMC):
• CUMC is sited in centre of a failed Cretaceous triple junction rift.
• Region (Zambia, Zimbabwe, Mozambique) contains suite of post-kinematic, Karoo-aged? intrusives and extrusives.
• This magmatism includes; carbonatite extrusives, basaltic flood basalts, dunite-chromitite-harzburgite-pyroxenite-wherlite-norite layered ultramafic complexes (Chewores, Atchiza Complex), dolerite bodies, dolerite and olivine-gabbro dykes, dolerite-gabbro plugs, dacite plugs, wherlite-harzburgite plugs and carbonatite intrusives?.
• This magmatic region also sited over both Irumide-aged? (1100-1000 Ma) and Pan-African-aged (575-520 Ma) sutured crustal lithosphere between the Congo and Kalahari Cratons.
• Karoo aged extensional tectonics, rifting, faulting, differential block uplift, block tilting, sedimentation, intrusives and extrusives.
• Anticipate a Karoo age for the CUMC and associated late-stage intrusives.
• Region is focus for mantle derived magmas because tectonic setting central to triple junction of failed rifts. The resultant vertical sigma1 tectonics and crustal extension has resulted in magmatism (intrusive and extrusive), dilational-normal faulting, differential block uplift, tilting, segmentation of the CUMC, deep sedimentary basins.
• CUMC was originally correlated with Archaean Great Dyke (Orpen et al. 1992, Wiles, 1955, etc).
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[5] Structural Evolution of the Chewore Inliers, Zambezi Mobile Belt, Zimbabwe.
Ben Goscombe, P. Fey and F. Both.
Geological Survey of Zimbabwe, P.O. Box 210, Causeway, Harare.
See Journal of African Earth Sciences 19, 199-224 (1994).
Detailed mapping of the Chewore Inliers, Zambezi Valley, Zimbabwe has recognised three distinct geological terranes. The areally most significant "Gneissic Terrane" is dominated by amphibolite facies quartzo-feldspathic gneisses with minor migmatitic, sillimanite/kyanite-garnet-biotite metapelites, kyanite-staurolite-garnet schists, calc-silicate rocks, amphibolite and quartzite. The "Quartzite Terrane" is comprised almost entirely of quartzite with minor kyanite-bearing metapelitic gneisses and the "Granulite Terrane" of anhydrous quartzo-feldspathic gneisses, intermediate two-pyroxene gneisses and sillimanite-bearing metapelitic gneisses. The boundary between the Quartzite and Granulite Terranes is a steep mylonite zone that involved oblique over-thrusting of the granulites to the SW. The boundary between the Quartzite and Gneissic Terranes is also discordant and possibly also a thrust surface, though it is nowhere exposed.
The Granulite Terrane involved two phases of isoclinal folding (Fg2 and Fg3) by S to N transport and with accompanying high grade metamorphism. The granulites have well annealed granoblastic textures, no strong planar foliation is developed but two mineral lineations are recognised. The Quartzite and Gneissic Terranes display a strong layer-parallel S2-L2 fabric that immediately post-dates the peak of the major metamorphic event (M1). The orientation of D2 structural elements are different in the two terranes. Furthermore, these terranes display different orientations of the major tight to isoclinal folding (D3) and the Gneissic Terrane alone, displays a second tight folding event (D4).
Both the Gneissic and Quartzite Terranes developed conjugate crenulation cleavages (S5) and all terranes have N-S trending open warps (F4b and Fg4). SW over NE tectonic transport during D2 and S over N transport during Dg3 cannot be correlated with any known orogenic periods in Central Eastern Africa. However, SE over NW transport during D3 is tentatively correlated with the Irumide orogeny (1100-1300 Ma) and the NE over SW transport during D4-D5 with the Zambezi/Mozambique orogeny (830±30 Ma). Post-D5 events involved over-thrusting of the Granulite Terrane and possibly also the Quartzite Terrane to the SW, followed by the intrusion of the Chewore Ultramafic Complex, dolerite dykes and pegmatites. Late-stage pegmatites may have occurred at Pan-African times (450-650 Ma). The igneous and ductile structural evolution of the Chewore Inliers ceased before Karoo times (150-285 Ma).Metamorphic parageneses indicate that a total of 21 km of crust has been removed at some stage subsequent to M1 metamorphism and prior to deposition of Karoo Supergroup sediments on the Chewore gneisses. A protracted period of faulting and uplift of the inliers during and after Karoo times resulted in the Chewore Inliers being a significant topographic high within the Zambezi Rift Valley.
[6] High-P metamorphism in the Trans-Saharan Orogen, Gourma, Mali.
Ben Goscombe1, Thomas Bekker2, Monique Tegyey2
1Integrated Terrane Analysis, 18 Cambridge Rd, Aldgate, 5154, Australia.
2CDG, BRGM, BP 6009, Avenue Claude Guillemin, 45060, Orleans Cedex, France.
P-T calculations by THERMOCALC and phase stability relationships within appropriate published PT pseudosections, document three distinct thermal regimes (G=ºC/km) in different parts of the Trans-Saharan Orogen. The central Ansongo Internal Nappe samples experienced extremely low average thermal gradients of 9.0-9.5 ºC/km and extremely high pressures of 16.6-17.4 kb at 545-550 ºC. Peak-temperature conditions in the central Ansongo Nappe are only 610 ºC at 14 kb, corresponding to a typical subduction zone average thermal gradient of 12.4 ºC/km. Samples from southeast Ansongo Internal Nappe also experienced low average thermal gradients of 12-13 ºC/km at pressures of 13-15 kb and temperatures of 560-630 ºC. All parts of the Ansongo Nappe experiences clockwise P-T paths with near isothermal decompression from peak-P through peak-T to approximately 3-4 kb pressures before significant cooling.
Ansongo Nappe conditions and P-T paths of evolution are typical of a subduction zone setting. The variation in thermal regime from 9 to 13 ºC/km in different parts of the Ansongo Internal Nappe may correspond to local variation in down advection rate in the subduction setting, local variation in exhumation rates or different locations within the nappe. The absence of an obvious and significant Barrovian overprint obliterating the high-P history in all Ansongo samples, suggests that this Internal Nappe was transported rapidly into the upper crust, without pervasive reworking during the ongoing collision and crustal thickening of the Trans-Sahara Orogen. Consequently, transport of the Ansongo Internal Nappe into the upper crust presumably happened early in this orogenic evolution, before it could be re-equilibrated. Furthermore, by being emplaced early into an upper-plate setting above the External Nappes, the Ansongo Nappe would have been significantly less affected by later Barrovian metamorphism experienced at depth within the External Nappes during the collisional phase.
Samples in the Adouf External Nappe and Soudeheni-Fafa Parautochthonous Domain all experienced metamorphism in a very similar thermal regime. Metamorphism was characterized by average thermal gradients of 17-23 ºC/km, typical of Barrovian metamorphism. These moderate average thermal gradients and moderate depths of burial of 24-31 km (7-9 kb) are typical of crustal over-thickening in a collisional orogen. Temperature ranges from 550-570 ºC to 450-480 ºC in different parts of the Adouf Nappe and is 535 ºC in the Soudeheni-Fafa domain. These conditions are consistent with the lower-plate setting of the External Nappes and being buried during a collisional orogenesis phase in the Trans-Saharan Orogen.
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[7] Evolution of the Trans-Saharan Orogen, Mali.
Fullgraf, T., Savanier, D., Affaton, P., Caby, R., Cocherie, A., Goscombe, B., Lahondere, D., Le Metour, J., Tegyey, M., Wemmer, K.
See Abstracts for 22th Coll. Afr. Geol., 2008
The Gourma region of eastern Mali is situated west of the Pan-African Dahomeydian and Pharusian belts. In the past, the Neoproterozoic sediments of that area have been interpreted as basin fill of an aulacogen (Reichelt, 1972). More recently, the central to eastern parts of the Gourma have been reinterpreted as a tectonic pile (Caby, 1979, Sacko, 1985) that was stacked from the east during collision of the West African craton (WAC) with the Touareg shield (TS) about 623 Ma ago (Jahn et al. 2001). The nappe complex has been subdivided into three external Barrovian type nappes and one internal HP to UHP/LT nappe. All units were believed to have been transported over the Paleo-proterozoic “Horst de Bourré” which remained in a parautochthonous to autochthonous position (Caby, 1993).
New regional mapping of the Gourma region was carried out in 2007, under the SYSMIN project “Cartographie géologique du Gourma malien”. Mapping was combined with structural analysis and petrography complemented by metamorphic petrology, isotope analysis (K-Ar illite, U-Pb detrital zircons, 13C/18O) and illite crystallinity determinations. The new data allow definition of five tectono-metamorphic domains, labled 1 to 5and indicate a prolonged polyphase Pan-African orogenic history.
The first metamorphic event (M1) at around 700 Ma, is recorded by 0.2 micron fraction K-Ar illite ages determined in the low to very low-grade parautochthonous to allochthonous domains 1 and 2in the southwest part of the study area. Island arc collision with the WAC from the east is a possible mechanism to explain M1 and the early tectonic transport of the low-grade domains 1 and 2to the west and southwest. An early M1 event is also supported by the lack of Neoproterozoic detrital zircons in the the Gourma sediments that in the Neoproterozoic Volta basin of Ghana occur in the upper part of the stratigraphy (Affaton, pers. comm.). Therefore the Gourma basin may have never existed but represents an allochthonous passive margin sequence derived from the eastern fringe of the WAC.
In the south, the low-grade domains 1 and 2 overlie the autochthonous Firgoun formation which in Burkina Faso transgresses the Birrimian basement. Presently included into the Firgoun formation is a sequence of possibly glaciogenic sediments (diamictite?), carbonates and chert that resemble the Triade elsewhere in West Africa. Negative values of both 13C and 18O (normalised to PDA) determined on the carbonate rocks support their interpretation as Marinoan cap carbonate. This constrains the sedimentation of the Firgoun series between 650-600 Ma which is younger than M1. Therefore decoupling of low-grade domains and all higher units from the basal sequence took place during a second tectono-metamorphic stage (M2) post-dating the Marinoan.
This event was initiated by the collision of the WAC with the TS shield about 630 Ma ago resulting in the stacking of the tectono-metamorphic domains 3 to 5. Modelling of the P/T path yields peak conditions of 550-570°C and 7-9 kb for both the domain 3 (e.g. the former external nappes) and the Paleoproterozoic Bourré complex (domain 4). They document the Barrovian type metamorphism of these units and show that the Bourré complex is a nappe and not a horst.
In contrast, average thermal gradients in the HP to UHP metamorphic complex of Ansongo (domain 5) are extremely low (9-13°C/km). They can occur only in a subduction zone setting where the rate of down advection of material far out ways the rate of thermal conduction, giving rise to anomalously cold conditions at extreme depths. P/T estimates vary in different parts of the domain 5 from 16.6-17.4 kb at 545-550°C to 13-15 kb at 560-630°C. This may correspond to local variation in down advection rate in the subduction setting. Or, alternatively, due to local variation in exhumation rates due to ad hoc heterogeneities in the evolving orogen, isostatic response, nappe transport and location within the nappe, or a combination of these prograde and retrograde processes. Low-angle thrusting led to the tectonic juxtaposition of the domains 4 to 5 but subsequently was largely obscured by both reverse and subhorizontal sinistral movements along moderately to steeply dipping shear zones. An important phase of back-thrusting and back-folding resulted in the upper-plate structural position of domain 2 over domains 3 and 5. The final uplift and cooling of the orogen is constrained by K-Ar ages in the interval [514-547] Ma that were determined on the < 0.2 micron fraction of samples taken in the north-eastern corner of domain 2.
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