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Nasser Ennih - One of the best experts on this subject based on the ideXlab platform.
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from Pan African transpression to cadomian transtension at the west African margin new u pb zircon ages from the eastern saghro inlier anti atlas morocco
Geological Society London Special Publications, 2021Co-Authors: Ezzoura Errami, Ulf Linnemann, Mandy Hofmann, Andreas Gartner, Johannes Zieger, Jessica Gartner, Katja Mende, Jamal El Kabouri, Dominique Gasquet, Nasser EnnihAbstract:New U–Pb zircon ages from the Eastern Saghro massif in the Anti-Atlas of Morocco demonstrate alink between Pan-African transpressive collision at c. 600 Ma and transtension caused by the onset of Cadomiansubduction and arc development from c. 570 Ma onwards. We present new U–Pb laser ablation inductivelycoupled plasma mass spectrometry ages of detrital and magmatic zircon from the Saghro, M’Gouna, and Ouar-zazate Groups. The siliciclastic deposits of the Saghro Group were deposited in a back-arc setting developed onstretched continental crust of the West African margin. Collision with the Atlas–Meseta domain led to theclosure of the back-arc basin before 600 Ma. Time of exhumation and surface exposure of the newly formedPan-African basement is bracketed to c. 30 Ma owing to the maximum depositional age of 571 + 4 Ma ofthe overlying M’Gouna Group. The U–Pb age of 567 + 4 Ma for the lowermost ignimbrite of the OuarzazateGroup limits the time for the deposition of the M’Gouna Group to less than 4 Ma. The Pan-African Orogeny wasfinished at c. 600 Ma whereas the onset of transtension related to Cadomian back-arc formation was very muchyounger from c. 570 Ma onwards.
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the tachakoucht iriri tourtit arc complex moroccan anti atlas neoproterozoic records of polyphased subduction accretion dynamics during the Pan African Orogeny
Journal of Geodynamics, 2016Co-Authors: Nasser Ennih, Antoine Triantafyllou, Julien Berger, Jeanmarc Baele, Herve Diot, Gaelle Plissart, Christophe Monnier, Arnaud Watlet, Olivier BruguierAbstract:Abstract We report new mapping, tectonic, metamorphic and U–Pb zircon dating data on the polyphased Tachakoucht–Iriri and Tourtit arc-related units within the Moroccan Pan-African belt (Sirwa window, Anti-Atlas). The studied area contains four different sub-units, from south to north: (1) the Tachakoucht gneisses intruded to its northern part by (2) Iriri intrusions. To the north, the Tachakoucht–Iriri massif is thrusted by (3) the south-verging 760 Ma Khzama ophiolitic sequence intruded by (4) the Tourtit meta-granitic complex. The Tachakoucht gneiss represents former andesitic to dacitic porphyritic rocks crystallized around 740–720 Ma in an intra-oceanic arc setting (IOAS). Subsequently, it has been buried and metamorphosed to 700 °C, 8 kbar in response to early accretion of the arc onto the West African Craton (WAC). This tectono-metamorphic event also led to the dismembering and stacking of back-arc ophiolite onto the arc unit. Subsequently, the Iriri intrusions, a suite of hydrous mafic dykes (hornblende gabbro and fine-grained basalt) and ultramafic (hornblendite) plutons showing subduction zone affinities, intruded the Tachakoucht gneiss under P – T conditions of 750–800 °C and 2–5 kbar. Emplacement of Iriri intrusions led locally to pronounced partial melting of the Tachakoucht gneiss and to the production of leucogranitic melts. These melts crop out into the Iriri–Tachakoucht gneiss contacts as leucogneissic bands (former leucosomes, dated at 651 ± 5 Ma) but also intruded the Khzama ophiolite to form the Tourtit granite (dated at 651 ± 3 Ma). These ages (651–641 Ma) also constrain the timing of Iriri intrusion emplacement. The entire complex has been overprinted by a second deformation event under greenschist to amphibolite facies conditions marked by transposition of primary structures and a development of mylonitic shear zones. These results and those published on the Bou Azzer window show that two phases of subduction-related magmatism occurred in the Anti-Atlas belt and that they were separated by an early accretion of the intra-oceanic arc system (IOAS) onto the West African craton passive margin. Our interpretations also validate thermo-mechanical models predicting an intense perturbation of subduction dynamics during arc-continent collision ( i.e. composite subductions, polarity reversal) which can exPand the production of typical hydrous arc magma and induces a late magmatic phase after partial or total accretion of the IOAS.
Olivier Bruguier - One of the best experts on this subject based on the ideXlab platform.
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The 600 Ma-Old Pan-African Magmatism in the In Ouzzal Terrane (Tuareg Shield, Algeria): Witness of the Metacratonisation of a Rigid Block
2019Co-Authors: N. Fezaa, Olivier Bruguier, J. P. Liegeois, N. Abdallah, B. De Waele, A. OuabadiAbstract:The high-level sub-circular North Tihimatine granitic pluton, intrusive in the In Ouzzal terrane, has been dated at 600 ± 5 Ma (LA-ICP-MS U–Pb zircon) and at 602 ± 4 Ma (SHRIMP U–Pb zircon). At this time, while Tihimatine intruded a brittle In Ouzzal without major metamorphism, large high-K calc-alkaline granitoid batholiths emplaced in the adjacent terranes under ductile conditions and regional amphibolite facies metamorphism. Outside In Ouzzal, high-level plutons emplaced under brittle conditions are known only at c. 580 Ma. The In Ouzzal terrane (500 km × 80 to 5 km), made of c. 2 Ga very high-temperature granulitic lithologies with Archean protoliths, is the sole terrane within the Tuareg Shield to have been largely unaffected by the Pan-African Orogeny. The field, petrographic, geochemical and isotopic characteristics of the In Ouzzal granitic plutons studied herein, give keys for the understanding of the atypical behavior of the In Ouzzal terrane. The In Ouzzal Pan-African granitoids present chemical compositions varying from medium-K to high-K calc-alkaline to alkaline compositions. This is recorded by the Sr and Nd radiogenic isotopes (−4
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the tachakoucht iriri tourtit arc complex moroccan anti atlas neoproterozoic records of polyphased subduction accretion dynamics during the Pan African Orogeny
Journal of Geodynamics, 2016Co-Authors: Nasser Ennih, Antoine Triantafyllou, Julien Berger, Jeanmarc Baele, Herve Diot, Gaelle Plissart, Christophe Monnier, Arnaud Watlet, Olivier BruguierAbstract:Abstract We report new mapping, tectonic, metamorphic and U–Pb zircon dating data on the polyphased Tachakoucht–Iriri and Tourtit arc-related units within the Moroccan Pan-African belt (Sirwa window, Anti-Atlas). The studied area contains four different sub-units, from south to north: (1) the Tachakoucht gneisses intruded to its northern part by (2) Iriri intrusions. To the north, the Tachakoucht–Iriri massif is thrusted by (3) the south-verging 760 Ma Khzama ophiolitic sequence intruded by (4) the Tourtit meta-granitic complex. The Tachakoucht gneiss represents former andesitic to dacitic porphyritic rocks crystallized around 740–720 Ma in an intra-oceanic arc setting (IOAS). Subsequently, it has been buried and metamorphosed to 700 °C, 8 kbar in response to early accretion of the arc onto the West African Craton (WAC). This tectono-metamorphic event also led to the dismembering and stacking of back-arc ophiolite onto the arc unit. Subsequently, the Iriri intrusions, a suite of hydrous mafic dykes (hornblende gabbro and fine-grained basalt) and ultramafic (hornblendite) plutons showing subduction zone affinities, intruded the Tachakoucht gneiss under P – T conditions of 750–800 °C and 2–5 kbar. Emplacement of Iriri intrusions led locally to pronounced partial melting of the Tachakoucht gneiss and to the production of leucogranitic melts. These melts crop out into the Iriri–Tachakoucht gneiss contacts as leucogneissic bands (former leucosomes, dated at 651 ± 5 Ma) but also intruded the Khzama ophiolite to form the Tourtit granite (dated at 651 ± 3 Ma). These ages (651–641 Ma) also constrain the timing of Iriri intrusion emplacement. The entire complex has been overprinted by a second deformation event under greenschist to amphibolite facies conditions marked by transposition of primary structures and a development of mylonitic shear zones. These results and those published on the Bou Azzer window show that two phases of subduction-related magmatism occurred in the Anti-Atlas belt and that they were separated by an early accretion of the intra-oceanic arc system (IOAS) onto the West African craton passive margin. Our interpretations also validate thermo-mechanical models predicting an intense perturbation of subduction dynamics during arc-continent collision ( i.e. composite subductions, polarity reversal) which can exPand the production of typical hydrous arc magma and induces a late magmatic phase after partial or total accretion of the IOAS.
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the Pan African kekem gabbro norite west cameroon u pb zircon age geochemistry and sr nd isotopes geodynamical implication for the evolution of the central African fold belt
Journal of African Earth Sciences, 2013Co-Authors: M Kwekam, Pascal Affaton, Olivier Bruguier, Jeanpaul Liegeois, Gerald Hartmann, Emmanuel NjonfangAbstract:The Kekem shoshonitic gabbro-norite association is part of the high-K calc-alkaline (HKCA) post-collisional magmatism, a major feature of the Pan-African Belt in Cameroon. LA-ICP-MS U-Pb zircon analyses provide an age of 576 ± 4 Ma for the Kekem complex. This age is interpreted as dating the emplacement of the massif during the waning stage of the Pan-African Orogeny. The latter is related to dextral movements along the Central Cameroon Shear Zone (CCSZ). The REE patterns display enriched LREE (LaN/YbN = 14.2-23.5) while HREE present a nearly flat profile (DyN/YbN = 1.3-1.7), and the La/Sm and Sm/Yb ratios led to propose that the Kekem gabbro-norites have been derived from the partial melting of a garnet-spinel lherzolite mantle source. The negative Nb-Ta and Ti anomalies and the positive Pb anomalies indicate that this mantle source was modified by contribution of a subduction-related material. The low Ce/Pb (2.6-10.4) and Th/Yb ratios associated to high Ba/La ratios, indicate that source enrichment could be related to slab derived fluids. As a whole, the Kekem geochemical features suggest that primary gabbro-noritic magmas derived from a subduction-modified mantle source (metasomatised lithospheric mantle). Moderately high 86Sr/87Sr initial ratios (0.7068-0.7082), low eNd (−5 to −9) and old Nd TDM model ages (1.6-1.8 Ga) are interpreted to result from contamination of Neoproterozoic mantle by the Paleoproterozoic crust. The ca. 576 Ma movements along the CCSZ are related to a Neoproterozoic metacratonization of the northern margin of the Congo craton during the Pan-African Orogeny. This metacratonization led to vertical planar lithospheric delamination along lithospheric transcurrent faults, asthenospheric uprise and partial melting of the Paleoproterozoic lithospheric mantle.
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geochronology and metamorphic p t x evolution of the eburnean granulite facies metapelites of tidjenouine central hoggar algeria witness of the latea metacratonic evolution
Geological Society London Special Publications, 2008Co-Authors: Abderrahmane Bendaoud, Jean-robert Kienast, Olivier Bruguier, Jeanpaul Liegeois, Khadidja Ouzegane, Gaston Godard, Amar DrareniAbstract:Central Hoggar, within the Tuareg shield to the east of the West African craton, is known for its complexity owing to the interplay of the Eburnean and Pan-African orogenies. The Tidjenouine area in the Laouni terrane belongs to the LATEA metacraton and displays spectacular examples of granulite-facies migmatitic metapelites. Here, we present a detailed petrological study coupled with in situ U–Pb zircon dating by laser-ablation inductively coupled plasma mass spectrometry (ICP-MS) that allows us to constrain the relative role of the Eburnean and Pan-African orogenies and hence to constrain how the LATEA Eburnean microcontinent has been partly destabilized during the Pan-African Orogeny; that is, its metacratonic evolution. These metapelites have recorded different metamorphic stages. A clockwise P–T evolution is demonstrated on the basis of textural relationships, modelling in KFMASH and FMASH systems and thermobarometry. The prograde evolution implies several melting reactions involving the breakdown of biotite and gedrite. Peak metamorphic P–T conditions of 860 + 50 8C and 7–8 kbar (M1) were followed by a decrease of pressure to 4.3 + 1 kbar and of temperature to around 700 8C, associated with the development of migmatites (M2). After cooling, a third thermal phase at c. 650 8C and 3–4 kbar (M3) occurred. U–Pb zircon laser ablation ICP-MS analysis allows us to date the protolith of the migmatites at 2151 + 8 Ma, the granulite-facies and migmatitic metamorphisms (M1–M2) at 2062 + 39 Ma and the medium-grade metamorphic assemblage (M3) at 614 + 11 Ma. This last event is coeval with the emplacement of large Pan-African granitic batholiths. These data show that the main metamorphic events are Eburnean in age. The PanAfrican Orogeny, in contrast, is associated mainly with medium-grade metamorphism but also mega-shear zones and granitic batholiths, characterized by a high temperature gradient. This can be considered as typical of a metacratonic evolution. The Tidjenouine metapelites (Central Hoggar, Fig. 1) show a great diversity of minerals (garnet, biotite, quartz, sillimanite, gedrite, corundum, orthopyroxene, cordierite, spinel, feldspar, plagioclase, ilmenite, rutile) forming different assemblages depending on whole-rock composition and extent of metamorphic transformation. The rocks were involved in a prograde metamorphic evolution followed by decompression. Granulite-facies metamorphism was accomPanied by melting favoured by biotite or gedrite dehydration. The successive stages of melting, with a progressively increasing amount of melt escape, produced metapelites with a restitic composition. In these rocks, corundum, spinel and sillimanite crystallized in the most All rich microdomains and orthopyroxene in the From: ENNIH, N. & LIEGEOIS, J.-P. (eds) The Boundaries of the West African Craton. Geological Society, London, Special Publications, 297, 111–146. DOI: 10.1144/SP297.6 0305-8719/08/$15.00 # The Geological Society of London 2008. most Mg-rich zones. In Central Hoggar, this prograde metamorphism in granulite facies has never been described and the large variability of the metapelite compositions allows us to constrain the P– T–aH2O evolution. On the other hand, the Tuareg shield is characterized by the interplay of the Eburnean (c. 2 Ga) and the Pan-African (c. 0.6 Ga) orogenies. Several terranes of this shield were mostly generated during the Pan-African Orogeny (Black et al. 1994) whereas others have been only slightly affected, such as the In Ouzzal terrane (Ouzegane et al. 2003, and references therein), perfectly preserving ultrahigh-temperature parageneses (Ouzegane & Boumaza 1996; Adjerid et al. 2008). The situation of Central Hoggar is much more debated: for some researchers (e.g. Caby 2003), its granulite-facies metamorphism is Pan-African in age (protoliths being mostly Palaeoproterozoic or Archaean); for others, this metamorphism is Eburnean in age, the Pan-African Orogeny having generated only high-T greenschistor amphibolite-facies metamorphism, with high-pressure metamorphism being present only in Neoproterozoic oceanic material thrust on the granulitic basement constituting the LATEA metacraton (Liegeois et al. 2003; Peucat et al. 2003). This debate sharply emphasizes the question of how a cratonic basement behaves during an Orogeny and how it can be remobilized and what are the consequences of such behaviour. This questions also the nature of the LATEA microcontinent: craton, metacraton or mobile belt? To tackle this problem, this paper focuses on the well-preserved granulites of the Tidjenouine area. It aims at (1) reconstructing the thermotectonic evolution of Fig. 1. Geological sketch maps of the Hoggar (a, Bertrand et al. 1986), of the Tuareg shield (b, Black et al. 1994) and geological map of the study area (c, Liegeois et al. 2003). Eg-Al, Egere–Aleksod; Te, Tefedest; Az, Azrou-n-Fad; Se, Serouenout; Is, Issalane; La, Laouni; Isk, Iskel; It, In Teidini; Tz, Tazat; As-Is, Assode-Issalane. A. BENDAOUD ET AL. 112
Jeanpaul Liegeois - One of the best experts on this subject based on the ideXlab platform.
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the Pan African kekem gabbro norite west cameroon u pb zircon age geochemistry and sr nd isotopes geodynamical implication for the evolution of the central African fold belt
Journal of African Earth Sciences, 2013Co-Authors: M Kwekam, Pascal Affaton, Olivier Bruguier, Jeanpaul Liegeois, Gerald Hartmann, Emmanuel NjonfangAbstract:The Kekem shoshonitic gabbro-norite association is part of the high-K calc-alkaline (HKCA) post-collisional magmatism, a major feature of the Pan-African Belt in Cameroon. LA-ICP-MS U-Pb zircon analyses provide an age of 576 ± 4 Ma for the Kekem complex. This age is interpreted as dating the emplacement of the massif during the waning stage of the Pan-African Orogeny. The latter is related to dextral movements along the Central Cameroon Shear Zone (CCSZ). The REE patterns display enriched LREE (LaN/YbN = 14.2-23.5) while HREE present a nearly flat profile (DyN/YbN = 1.3-1.7), and the La/Sm and Sm/Yb ratios led to propose that the Kekem gabbro-norites have been derived from the partial melting of a garnet-spinel lherzolite mantle source. The negative Nb-Ta and Ti anomalies and the positive Pb anomalies indicate that this mantle source was modified by contribution of a subduction-related material. The low Ce/Pb (2.6-10.4) and Th/Yb ratios associated to high Ba/La ratios, indicate that source enrichment could be related to slab derived fluids. As a whole, the Kekem geochemical features suggest that primary gabbro-noritic magmas derived from a subduction-modified mantle source (metasomatised lithospheric mantle). Moderately high 86Sr/87Sr initial ratios (0.7068-0.7082), low eNd (−5 to −9) and old Nd TDM model ages (1.6-1.8 Ga) are interpreted to result from contamination of Neoproterozoic mantle by the Paleoproterozoic crust. The ca. 576 Ma movements along the CCSZ are related to a Neoproterozoic metacratonization of the northern margin of the Congo craton during the Pan-African Orogeny. This metacratonization led to vertical planar lithospheric delamination along lithospheric transcurrent faults, asthenospheric uprise and partial melting of the Paleoproterozoic lithospheric mantle.
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geochronology and metamorphic p t x evolution of the eburnean granulite facies metapelites of tidjenouine central hoggar algeria witness of the latea metacratonic evolution
Geological Society London Special Publications, 2008Co-Authors: Abderrahmane Bendaoud, Jean-robert Kienast, Olivier Bruguier, Jeanpaul Liegeois, Khadidja Ouzegane, Gaston Godard, Amar DrareniAbstract:Central Hoggar, within the Tuareg shield to the east of the West African craton, is known for its complexity owing to the interplay of the Eburnean and Pan-African orogenies. The Tidjenouine area in the Laouni terrane belongs to the LATEA metacraton and displays spectacular examples of granulite-facies migmatitic metapelites. Here, we present a detailed petrological study coupled with in situ U–Pb zircon dating by laser-ablation inductively coupled plasma mass spectrometry (ICP-MS) that allows us to constrain the relative role of the Eburnean and Pan-African orogenies and hence to constrain how the LATEA Eburnean microcontinent has been partly destabilized during the Pan-African Orogeny; that is, its metacratonic evolution. These metapelites have recorded different metamorphic stages. A clockwise P–T evolution is demonstrated on the basis of textural relationships, modelling in KFMASH and FMASH systems and thermobarometry. The prograde evolution implies several melting reactions involving the breakdown of biotite and gedrite. Peak metamorphic P–T conditions of 860 + 50 8C and 7–8 kbar (M1) were followed by a decrease of pressure to 4.3 + 1 kbar and of temperature to around 700 8C, associated with the development of migmatites (M2). After cooling, a third thermal phase at c. 650 8C and 3–4 kbar (M3) occurred. U–Pb zircon laser ablation ICP-MS analysis allows us to date the protolith of the migmatites at 2151 + 8 Ma, the granulite-facies and migmatitic metamorphisms (M1–M2) at 2062 + 39 Ma and the medium-grade metamorphic assemblage (M3) at 614 + 11 Ma. This last event is coeval with the emplacement of large Pan-African granitic batholiths. These data show that the main metamorphic events are Eburnean in age. The PanAfrican Orogeny, in contrast, is associated mainly with medium-grade metamorphism but also mega-shear zones and granitic batholiths, characterized by a high temperature gradient. This can be considered as typical of a metacratonic evolution. The Tidjenouine metapelites (Central Hoggar, Fig. 1) show a great diversity of minerals (garnet, biotite, quartz, sillimanite, gedrite, corundum, orthopyroxene, cordierite, spinel, feldspar, plagioclase, ilmenite, rutile) forming different assemblages depending on whole-rock composition and extent of metamorphic transformation. The rocks were involved in a prograde metamorphic evolution followed by decompression. Granulite-facies metamorphism was accomPanied by melting favoured by biotite or gedrite dehydration. The successive stages of melting, with a progressively increasing amount of melt escape, produced metapelites with a restitic composition. In these rocks, corundum, spinel and sillimanite crystallized in the most All rich microdomains and orthopyroxene in the From: ENNIH, N. & LIEGEOIS, J.-P. (eds) The Boundaries of the West African Craton. Geological Society, London, Special Publications, 297, 111–146. DOI: 10.1144/SP297.6 0305-8719/08/$15.00 # The Geological Society of London 2008. most Mg-rich zones. In Central Hoggar, this prograde metamorphism in granulite facies has never been described and the large variability of the metapelite compositions allows us to constrain the P– T–aH2O evolution. On the other hand, the Tuareg shield is characterized by the interplay of the Eburnean (c. 2 Ga) and the Pan-African (c. 0.6 Ga) orogenies. Several terranes of this shield were mostly generated during the Pan-African Orogeny (Black et al. 1994) whereas others have been only slightly affected, such as the In Ouzzal terrane (Ouzegane et al. 2003, and references therein), perfectly preserving ultrahigh-temperature parageneses (Ouzegane & Boumaza 1996; Adjerid et al. 2008). The situation of Central Hoggar is much more debated: for some researchers (e.g. Caby 2003), its granulite-facies metamorphism is Pan-African in age (protoliths being mostly Palaeoproterozoic or Archaean); for others, this metamorphism is Eburnean in age, the Pan-African Orogeny having generated only high-T greenschistor amphibolite-facies metamorphism, with high-pressure metamorphism being present only in Neoproterozoic oceanic material thrust on the granulitic basement constituting the LATEA metacraton (Liegeois et al. 2003; Peucat et al. 2003). This debate sharply emphasizes the question of how a cratonic basement behaves during an Orogeny and how it can be remobilized and what are the consequences of such behaviour. This questions also the nature of the LATEA microcontinent: craton, metacraton or mobile belt? To tackle this problem, this paper focuses on the well-preserved granulites of the Tidjenouine area. It aims at (1) reconstructing the thermotectonic evolution of Fig. 1. Geological sketch maps of the Hoggar (a, Bertrand et al. 1986), of the Tuareg shield (b, Black et al. 1994) and geological map of the study area (c, Liegeois et al. 2003). Eg-Al, Egere–Aleksod; Te, Tefedest; Az, Azrou-n-Fad; Se, Serouenout; Is, Issalane; La, Laouni; Isk, Iskel; It, In Teidini; Tz, Tazat; As-Is, Assode-Issalane. A. BENDAOUD ET AL. 112
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the latea metacraton central hoggar tuareg shield algeria behaviour of an old passive margin during the Pan African Orogeny
Journal of African Earth Sciences, 2003Co-Authors: Jeanpaul Liegeois, Louis Latouche, Mustapha Boughrara, Jacques Navez, Michel GuiraudAbstract:Historically, the Tuareg shield is divided into three parts bordered by mega-shear zones with the centre, the Central Polycyclic Hoggar, characterized byArchaean and Palaeoproterozoic lithologies. Nearly10 y ears ago, the Tuareg shield was shown to be composed of 23 displaced terranes (Geology22 (1994) 641) whose relationships were deciphered in A € i to the SE (Precambr. Res. 67 (1994) 59). The Polycyclic Central Hoggar terranes were characterized by the presence of well preserved Archaean/Palaeoproterozoic and Neoproterozoic lithologies. We show here that the terranes from Central Hoggar (Laouni, Azrou-n-Fad, Tefedest, Eg��e belonged to a single old passive margin, to which we gave the acronym name LATEA, which behaved as a craton during the Mesoproterozoic and the Early- Middle Neoproterozoic but was partlydestabilized and dissected during the Late Neoproterozoic as a consequence of its involvement as a passive margin in the Pan-African orogen. An earlyPan-African phase consisted of thrust sheets including garnet-bearing lithologies (eclogite, amphibolite, gneiss) that can be mapped and correlated in three LATEA terranes. In the Tin Begane area, P -T -t paths have been established from >15 kbar--790 � C (eclogite) to 4 kbar--500 � C (greenschist retrogression) through 12 kbar--830 � C (garnet amphibolite) and 8 kbar--700 � C (garnet gneiss), corresponding to the retrograde path of a Franciscan-type loop. Sm-Nd geochronology on minerals and laser ablation ICP-MS on garnet show the mobilityof REE, particularlyLREE, during the retrograde greenschist facies that affects, although slightly, some of these rocks. The amphibolite-facies metamorphism has been dated at 685 ± 19 Ma and the greenschist facies at 522 ± 27 Ma. During the thrust phase, the Archaean-Palaeoproterozoic basement was onlylocallyaffected bythe Pan- African tectonics. LATEA behaved as a craton. Other juvenile terranes were also thrust earlyonto LATEA: the Iskel island arc at � 850 Ma to the west of LATEA, the Serouenout terrane in the 700-620 Ma age range to the east. No subduction-related magmas have intruded LATEA during this epoch, which behaved as a passive margin. During the main Pan-African phase (625-580 Ma), LATEA was dissected bymega-shear zones that induced several hundreds km of relative displacement and allowed the emplacement of high-K calc-alkaline batholiths. Smaller movements continued till 525 Ma, accomPanied bythe emplacement of subcircular plutons with alkaline affinity. Here is dated the Ounane granodiorite (624 ± 15 Ma; 87 Sr/ 86 Sri ¼ 0.70839 ± 0.00016; 6WR, MSWD ¼ 0.87) and the Tisselliline granite (552 ± 15 Ma; 87 Sr/ 86 Sri ¼ 0.7074 ± 0.0001; 5WR, MSWD ¼ 1.4). Nd isotopes indicate a preponderant Palaeoproterozoic crustal source for these two plutons: eNd ¼� 14 to )21 at 624 Ma and TDM ¼ 1650-2320 Ma for Ounane and eNd ¼� 13 to )15 at 555 Ma and TDM ¼ 1550-1720 Ma for Tisselliline. Our model links these intrusions to a linear lithospheric delamination along mega-shear zones, allowing the hot asthenosphere to rise, melt by adiabatic pressure release and inducing the melting of the Palaeoproterozoic and Archaean lower crust.
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the latea metacraton central hoggar tuareg shield algeria behaviour of an old passive margin during the Pan African Orogeny
Journal of African Earth Sciences, 2003Co-Authors: Jeanpaul Liegeois, Louis Latouche, Mustapha Boughrara, Jacques Navez, Michel GuiraudAbstract:Historically, the Tuareg shield is divided into three parts bordered by mega-shear zones with the centre, the Central Polycyclic Hoggar, characterized byArchaean and Palaeoproterozoic lithologies. Nearly10 y ears ago, the Tuareg shield was shown to be composed of 23 displaced terranes (Geology22 (1994) 641) whose relationships were deciphered in A € i to the SE (Precambr. Res. 67 (1994) 59). The Polycyclic Central Hoggar terranes were characterized by the presence of well preserved Archaean/Palaeoproterozoic and Neoproterozoic lithologies. We show here that the terranes from Central Hoggar (Laouni, Azrou-n-Fad, Tefedest, Eg��e belonged to a single old passive margin, to which we gave the acronym name LATEA, which behaved as a craton during the Mesoproterozoic and the Early- Middle Neoproterozoic but was partlydestabilized and dissected during the Late Neoproterozoic as a consequence of its involvement as a passive margin in the Pan-African orogen. An earlyPan-African phase consisted of thrust sheets including garnet-bearing lithologies (eclogite, amphibolite, gneiss) that can be mapped and correlated in three LATEA terranes. In the Tin Begane area, P -T -t paths have been established from >15 kbar--790 � C (eclogite) to 4 kbar--500 � C (greenschist retrogression) through 12 kbar--830 � C (garnet amphibolite) and 8 kbar--700 � C (garnet gneiss), corresponding to the retrograde path of a Franciscan-type loop. Sm-Nd geochronology on minerals and laser ablation ICP-MS on garnet show the mobilityof REE, particularlyLREE, during the retrograde greenschist facies that affects, although slightly, some of these rocks. The amphibolite-facies metamorphism has been dated at 685 ± 19 Ma and the greenschist facies at 522 ± 27 Ma. During the thrust phase, the Archaean-Palaeoproterozoic basement was onlylocallyaffected bythe Pan- African tectonics. LATEA behaved as a craton. Other juvenile terranes were also thrust earlyonto LATEA: the Iskel island arc at � 850 Ma to the west of LATEA, the Serouenout terrane in the 700-620 Ma age range to the east. No subduction-related magmas have intruded LATEA during this epoch, which behaved as a passive margin. During the main Pan-African phase (625-580 Ma), LATEA was dissected bymega-shear zones that induced several hundreds km of relative displacement and allowed the emplacement of high-K calc-alkaline batholiths. Smaller movements continued till 525 Ma, accomPanied bythe emplacement of subcircular plutons with alkaline affinity. Here is dated the Ounane granodiorite (624 ± 15 Ma; 87 Sr/ 86 Sri ¼ 0.70839 ± 0.00016; 6WR, MSWD ¼ 0.87) and the Tisselliline granite (552 ± 15 Ma; 87 Sr/ 86 Sri ¼ 0.7074 ± 0.0001; 5WR, MSWD ¼ 1.4). Nd isotopes indicate a preponderant Palaeoproterozoic crustal source for these two plutons: eNd ¼� 14 to )21 at 624 Ma and TDM ¼ 1650-2320 Ma for Ounane and eNd ¼� 13 to )15 at 555 Ma and TDM ¼ 1550-1720 Ma for Tisselliline. Our model links these intrusions to a linear lithospheric delamination along mega-shear zones, allowing the hot asthenosphere to rise, melt by adiabatic pressure release and inducing the melting of the Palaeoproterozoic and Archaean lower crust.
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the taourirt magmatic province a marker of the closing stage of the Pan African Orogeny in the tuareg shield review of available data and sr nd isotope evidence
Journal of African Earth Sciences, 2003Co-Authors: Abla Azzounisekkal, Jeanpaul Liegeois, Faten Bechiribenmerzoug, Safia Belaidizinet, Bernard BoninAbstract:The Tuareg Shield, located between the Archaean to Palaeoproterozoic Saharan metacraton and the West African craton, is composed of 23 recognized terranes that welded together during the Neoproterozoic Pan-African Orogeny (750–520 Ma). Final convergence occurred mainly during the 620–580 Ma period with the emplacement of high-K calc-alkaline batholiths, but continued until 520 Ma with the emplacement of alkali-calcic and alkaline high-level complexes. The last plutons emplaced in central Hoggar at 539–523 Ma are known as the ‘‘Taourirt’’ province. This expression is redefined and three geographical groups are identified: the Silet-, Laouni- and Tamanrasset-Taourirts. The Silet-Taourirts are cross-cutting Pan-African island arc assemblages while the two others intrude the Archaean–Palaeoproterozoic LATEA metacraton. The Taourirts are high-level subcircular often nested alkalicalcic, sometimes alkaline, complexes. They are aligned along mega-shear zones often delimiting terranes. Mainly granitic, they comprise highly differentiated varieties such as alaskite (Silet-Taourirts) and topaz–albite leucogranite (Tamanrasset-Taourirts). Different subgroups were defined on the basis of REE patterns and major and other trace elements. The Taourirt province displays a wide transition from dominant alkali-calcic to minor alkaline granite varieties. Sr isotopes indicate that these complexes were affected by fluid circulation during the Ordovician along shear zones probably contemporaneous to the beginning of the Tassilis sandstone deposition. Nd isotope systematic indicates a major interaction with the upper crust during the emplacement of highly differentiated melts, particularly in samples showing seagull wing-shaped REE patterns. On the other hand, all Taourirt plutons are strongly contaminated by the lower crust: eNd vary from ) 2t o)8 and TDM from 1200 to 1700 Ma. This implies the presence of an old crust at depth, also below the Silet-Taourirts, which are emplaced within Pan-African island arc assemblages. A model is proposed for the genesis of the Taourirt province where reworking of the mega-shear zones, which dissected the LATEA metacraton, provoked a linear delamination of the lithospheric mantle, asthenosphere uprise and partial melting of the lower crust (or strong interaction with), giving rise to a mixed source. � 2003 Elsevier Ltd. All rights reserved.
Beatriz Paschoal Duarte - One of the best experts on this subject based on the ideXlab platform.
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petrogenesis of the orthogneisses of the mantiqueira complex central ribeira belt se brazil an archaean to palaeoproterozoic basement unit reworked during the Pan African Orogeny
Gondwana Research, 2004Co-Authors: Beatriz Paschoal Duarte, Monica Heilbron, Sergio De Castro Valente, M Campos C NetoAbstract:Abstract The Occidental terrane of the central segment of the Brasiliano-Pan-African Ribeira belt comprises two crustal scale thrust sheets (Andrelândia and Juiz de Fora domains) taken as reworked Neoproterozoic products of the Sao Francisco cratonic margins. Pre-1.8 Ga orthogneisses and associated rocks of the Mantiqueira Complex comprise the basement for rocks of the Andrelândia Depositional Cycle within the Andrelândia tectonic domain. Geochemical data indicate that the Mantiqueira Complex comprises rocks that can be grouped as follows: intermediate to acid calc-alkaline rocks and a transitional basaltic series. On the basis of quantitative analysis of the lithogeochemical data, these lithotypes cannot be related. Statistical and/or petrological criteria made it possible to define suites and/or groups within each one of those units and to constrain petrogenetic models based mostly on their REE data. Simple least-square regression analysis indicates that the basic rocks are unlikely to constitute a single suite themselves. The results of the geochemical modelling presented in this work suggest that crustal partial melting rather than fractional crystallisation is the most likely petrogenetic process associated with the rocks of the Mantiqueira Complex. The partial melting processes might have taken place under oxidising conditions, typical of tectonic settings associated with the generation of calc-alkaline rocks.
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the juiz de fora granulite complex of the central ribeira belt se brazil a paleoproterozoic crustal segment thrust during the Pan African Orogeny
Gondwana Research, 1998Co-Authors: Monica Heilbron, Beatriz Paschoal Duarte, Jose Renato NogueiraAbstract:Abstract The granulites of the Juiz de Fora complex occur within thick basement thrust slices associated with the Pan-African shortening process in the central segment of the Ribeira belt. Five lithological units of the Intermediate tectonic domain of the belt can be identified on the basis of detailed geological mapping: a) orthogranulites, b) orthogneisses; c) kinzigite; d) intrusive garnet charnockite and e) amphibolite facies metasediments of probable Meso to Neoproterozoic age, correlated to the cover of the belt. Petrological data indicate high temperatures and intermediate to low lithostatic pressure conditions for the Paleoproterozoic granulite facies metamorphism. Textures and CO 2 -rich fluid inclusions are probably related to an IBC path. Geochemical data do not show relevant compositional change as a result of the granulite metamorphism. Two calc-alkaline suites and tholeiitic to alkaline basic rocks can be related to compressional and extensional settings, respectively. The overall composition of the granulites, the lack of substantial LILE depletion as well as the composition of the fluid inclusion points to granulitization process driven by CO 2 -rich fluids. Orthogranulites gave rise to banded gneisses as a result of the Pan-African retrograde metamorphism and intense deformation. The U and Th depletion detected in few rocks is possibly related with the hydrated conditions of the retrograde reactions.
Monica Heilbron - One of the best experts on this subject based on the ideXlab platform.
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petrogenesis of the orthogneisses of the mantiqueira complex central ribeira belt se brazil an archaean to palaeoproterozoic basement unit reworked during the Pan African Orogeny
Gondwana Research, 2004Co-Authors: Beatriz Paschoal Duarte, Monica Heilbron, Sergio De Castro Valente, M Campos C NetoAbstract:Abstract The Occidental terrane of the central segment of the Brasiliano-Pan-African Ribeira belt comprises two crustal scale thrust sheets (Andrelândia and Juiz de Fora domains) taken as reworked Neoproterozoic products of the Sao Francisco cratonic margins. Pre-1.8 Ga orthogneisses and associated rocks of the Mantiqueira Complex comprise the basement for rocks of the Andrelândia Depositional Cycle within the Andrelândia tectonic domain. Geochemical data indicate that the Mantiqueira Complex comprises rocks that can be grouped as follows: intermediate to acid calc-alkaline rocks and a transitional basaltic series. On the basis of quantitative analysis of the lithogeochemical data, these lithotypes cannot be related. Statistical and/or petrological criteria made it possible to define suites and/or groups within each one of those units and to constrain petrogenetic models based mostly on their REE data. Simple least-square regression analysis indicates that the basic rocks are unlikely to constitute a single suite themselves. The results of the geochemical modelling presented in this work suggest that crustal partial melting rather than fractional crystallisation is the most likely petrogenetic process associated with the rocks of the Mantiqueira Complex. The partial melting processes might have taken place under oxidising conditions, typical of tectonic settings associated with the generation of calc-alkaline rocks.
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the juiz de fora granulite complex of the central ribeira belt se brazil a paleoproterozoic crustal segment thrust during the Pan African Orogeny
Gondwana Research, 1998Co-Authors: Monica Heilbron, Beatriz Paschoal Duarte, Jose Renato NogueiraAbstract:Abstract The granulites of the Juiz de Fora complex occur within thick basement thrust slices associated with the Pan-African shortening process in the central segment of the Ribeira belt. Five lithological units of the Intermediate tectonic domain of the belt can be identified on the basis of detailed geological mapping: a) orthogranulites, b) orthogneisses; c) kinzigite; d) intrusive garnet charnockite and e) amphibolite facies metasediments of probable Meso to Neoproterozoic age, correlated to the cover of the belt. Petrological data indicate high temperatures and intermediate to low lithostatic pressure conditions for the Paleoproterozoic granulite facies metamorphism. Textures and CO 2 -rich fluid inclusions are probably related to an IBC path. Geochemical data do not show relevant compositional change as a result of the granulite metamorphism. Two calc-alkaline suites and tholeiitic to alkaline basic rocks can be related to compressional and extensional settings, respectively. The overall composition of the granulites, the lack of substantial LILE depletion as well as the composition of the fluid inclusion points to granulitization process driven by CO 2 -rich fluids. Orthogranulites gave rise to banded gneisses as a result of the Pan-African retrograde metamorphism and intense deformation. The U and Th depletion detected in few rocks is possibly related with the hydrated conditions of the retrograde reactions.
