The rifted continental margins of Mozambique provide excellent examples of continental passive ma... more The rifted continental margins of Mozambique provide excellent examples of continental passive margins with a significant structural variability associated with magmatism and inheritance. Despite accumulated knowledge, the tectonic structure and nature of the crust beneath the South Mozambique Coastal Plain (SMCP) are still poorly known. This study interprets high-resolution seismic reflection data paired with data from industry-drilled wells and proposes a structural model of the Limpopo transform margin in a magma-rich context. Results indicate that the Limpopo transform margin is characterized by an ocean-continent transition that links the Beira-High and Natal valley margin segments and represents the western limit of the continental crust, separating continental volcano-sedimentary infilled grabens from the oceanic crust domain. These basins result from the emplacement of the Karoo Supergroup during a Permo-Triassic tectonic event, followed by an Early Jurassic tectonic and magmatic event. This latter led to the establishment of steady-state seafloor spreading at ca.156 Ma along the SMCP. A Late Jurassic to Early Cretaceous event corresponds to formation of the Limpopo transform fault zone. Which accommodated the SSE-ward displacement of Antarctica with respect to Africa. We define a new type of margin: the magma-rich transform margin, characterized by the presence of voluminous magmatic extrusion and intrusion coincident with the formation and evolution of the transform margin. The Limpopo transform fault zone consists of several syn-transfer and-transform faults rather than a single transform fault. The intense magmatic activity was associated primarily with mantle dynamics, which controlled the large-scale differential subsidence along the transform margin. ROCHE ET AL.
Dans divers contextes geodynamiques, la fusion de la croute continentale peut etre induite par de... more Dans divers contextes geodynamiques, la fusion de la croute continentale peut etre induite par des intrusions repetees de magmas issus de la fusion du manteau. Pour atteindre les temperatures necessaires a la fusion dans la croute inferieure et aboutir, le processus requiert de perdurer et une frequence soutenue des alimentations mantelliques –frequence d’autant plus elevee que les intrusions individuelles apportant la chaleur sont de volume modere et que l’encaissant, aux contacts, est refractaire. Dans les niveaux plus superficiels de la croute, et potentiellement jusqu’a la surface, des magmas d’origine mantellique –et/ou leurs produits de differenciation– peuvent ainsi venir se mettre en place les premiers et preceder l’intrusion et la cristallisation de magmas d’origine crustale –et/ou hybrides, resultats de melanges. C’est le cas de l’intrusion composite de Ploumanac’h qui se met en place dans la croute superieure du Nord du Massif armoricain a la fin du Carbonifere dans un contexte meconnu. Constituee de 3 unites granitiques principales, elles-memes composites et successivement emboitees vers le centre, l’intrusion est emblematique (Barriere, 1976 –these non publiee, Brest). L’unite externe, la plus ancienne, renferme des facies de granites rouges a hornblende et biotite et des gabbros. Un leucogranite a deux micas constitue l’un des facies les plus jeunes de l’unite centrale. A partir d’echantillons selectionnes parmi ces facies, nous avons entrepris une etude geochronologique U-Th-Pb (zircon, monazite), 40Ar/39Ar (amphibole, muscovite, biotite) et traces de fission sur apatite. Les zircons concordants situent la cristallisation des granites rouges a 308,8 ± 2,5 Ma. Les âges U-Th-Pb sur zircon et monazite du leucogranite, respectivement a 301,3 ± 1,8 Ma et 301,1 ± 1,2 Ma, concordent dans l’erreur analytique. Ces derniers coincident egalement avec la majorite des âges 40Ar/39Ar obtenus dans les deux unites ; les perturbations de spectres etant imputables aux ultimes intrusions. Enfin, les unites atteignent les profondeurs d’effacement partiel des traces de fissions (~3 km) au Trias. Nous concluons (1) que l’exhumation du systeme construit n’a pas ete assistee par une extension regionale due a la soustraction d’un manteau lithospherique dense pouvant engendrer une fusion mantellique, et (2) qu’auparavant, il aura fallu au minimum 4 millions d’annees pour assembler et refroidir des produits issus du manteau et de la croute, au meme niveau structural.
At Domeyko, 40 km south of Vallenar in northern Chile (28°57'S-70°53'W), the Dos Amigos and Trico... more At Domeyko, 40 km south of Vallenar in northern Chile (28°57'S-70°53'W), the Dos Amigos and Tricolor porphyry copper centers are located within a north-south-elongated hydrothermal alteration zone 6x1.5 km of surface dimensions. The centers are related to tonalite to granodiorite porphyry stocks displaying potassic alteration, which are surrounded by Lower Cretaceous andesitic volcanic rocks with sericitic, kaolinite-illite and propylitic alteration zones. The western boundary of the alteration zone is marked by the post-mineralization Cachiyuyo Batholith of granodioritic to dioritic composition. U-Pb zircon ages for the Dos Amigos porphyry are of 106.1±3.5 and 104.0±3.5 Ma; and 108.5±3.4 for the nearby Tricolor porphyry. The Cachiyuyo Batholith yielded U-Pb zircon ages of 99.6±1.8 and 99.1±1.9 Ma; and 40 Ar/ 39 Ar ages for biotite of 96.9±3.9 and 94.8±0.9 Ma. These dates indicate that batholith emplacement postdated the Dos Amigos and Tricolor porphyries, in agreement with geological relationships. Although copper mineralization is spatially and genetically related to the Lower Cretaceous (Albian) porphyry stocks, most of the dated hydrothermal micas from the Dos Amigos and Tricolor porphyries yielded 40 Ar/ 39 Ar ages between 97.1±2.5 and 96.0±1.4 Ma, which overlap within error with the cooling ages obtained for the neighboring batholith. 40 Ar/ 39 Ar dating of micas revealed significant disturbance of their K-Ar isotopic systematics that complicates accurate determination of the timing of hydrothermal activity at Domeyko. Nevertheless, the 40 Ar/ 39 Ar data establish a minimum Late Cretaceous age for this activity. A fission track age of 59.8±9.8 Ma of apatite from the Dos Amigos porphyry indicates cooling through the temperature range of the apatite partial annealing zone (~125-60°C) during the Paleocene; and an (U-Th)/He age of 44.7±3.7 Ma of apatite from the same porphyry sample shows cooling through the temperature range of the apatite He partial retention zone (~85-40°C) during the Eocene. These ages correspond to the exhumation of the porphyry, and the latter provides a maximum age for the supergene enrichment processes that formed the chalcocite blanket currently mined at Dos Amigos.
As part of a combined geochronologic and palaeomagnetic study of the TrCgor area (Armorican Massi... more As part of a combined geochronologic and palaeomagnetic study of the TrCgor area (Armorican Massif), we have attempted to obtain a reliable age for a Late Proterozoic palaeopole from experiments carried out on the Keralain and Petit Taurel diorites. A heterogeneous population of amphiboles analysed by the 40Ar/39Ar stepheating method gave a flat saddle-shaped age spectrum (characteristic of excess argon) with a minimum age of 604.9f 1.2 Ma, which represents a maximum estimate of the age of the mineral. The isochron calculation gives an initial 40Ar/xAr ratio sensibly higher than the atmospheric ratio. This indicates that the isochron age, 583 f 2 Ma, is a better estimate of the age of the Keralain diorite. Laser single step fusion and step-heating experiments performed on single grains representative of the different components of this heterogeneous population of amphiboles show that the excess argon affects it in its entirety. The minimum age of 590.8 f 2.3 Ma of the flat saddle-shaped age spectrum displayed by the core of a crystal of hornblende is concordant (at the 20 level) with the isochron age. The isotopic closure temperature calculated from the step-heating results is 556 f 43 "C. The palaeomagnetic study has shown, using a thermal demagnetization procedure, that the samples have a multicomponent magnetization. Two characteristic components (A and B) have been recognized respectively at Keralain and the Petit Taurel. A careful rock magnetic study has established that the magnetic carrier of the primary component A, isolated at Keralain, is magnetite with a magnetic blocking temperature of 575°C. The similarity of this temperature with the calculated isotopic closure temperature allows us to assign an age of c. 583 Ma to the corresponding well-defined palaeopole 'DKA', located at 33"N, 256"E. The demagnetization experiments have also clearly shown that component B, well isolated at the Petit Taurel, is a remagnetization probably related to magmatic events at around 550 Ma. Furthermore, its magnetic behaviour, as observed at Keralain, indicates a possible thermochemical origin. The well-dated DKA palaeopole allows us to refine the late Proterozoic polar wander path for Armorica and seems to confirm that the Armorican Massif and southern United Kingdom were linked together at that period.
The Tibetan Plateau was formed by intense Cenozoic shortening (up to 1100 km) of a composite “pro... more The Tibetan Plateau was formed by intense Cenozoic shortening (up to 1100 km) of a composite “proto-Tibet”, itself the product of a long Paleozoic and Mesozoic history of accretion of Gondwana-derived continental fragments and volcanic arcs against the Asian continental margin. The difficult access and the scarcity of outcrops have long limited the possibilities of studying these Mesozoic suture zones in the heart of the Plateau. In this work, we present new U-Pb and 40Ar/39Ar ages from the highly deformed units of the Yushu mélange, along the Jinsha Suture in the northeastern Qiangtang terrane. Early Triassic (c. 253 Ma) to Middle Jurassic ages (c. 165 Ma) complement the existing dataset and help to refine the chronology of the Paleo-Tethyan oceanic subductions which have structured the northeastern part of the Qiangtang terrane. The Yushu mélange records at least three successive tectono-magmatic events. The opening of a back-arc basin during the northward Paleo-Tethyan subduction along the Longmu Co-Shuanghu Suture during Early to Middle Triassic; then its closure during the southward subduction of the Songpan-Ganze Ocean along the Jinsha Suture in Late Triassic. Finally, a shortening phase related to the continental collision of the Songpan-Ganze and Qiangtang blocks from Late Triassic to Early-Middle Jurassic. No evidence for any high- or mid-temperature Cenozoic reactivation of the Jinsha suture in our study area is recorded.
The Imini mining district (Morocco) hosts the largest manganese (Mn) ore deposits of North Africa... more The Imini mining district (Morocco) hosts the largest manganese (Mn) ore deposits of North Africa (~120.000 T in 2016). The area is world recognized for hosting several epigenetic karst-type high-grade manganese deposits in a 10-15 meters thick Cenomanian-Turonian dolostone unit. These unconventional Mn oxide deposits occur along a belt of ~25-30 km in the southern foreland of the intraplate Atlasic belt of Morocco. This is due to two laterally extensive ore bodies of nearly pure pyrolusite-rich manganese ores (72-88 wt.% MnO2) and a third discontinuous medium-grade coronadite-rich Mn ore (40–48 wt.% MnO2)(Dekoninck et al., 2016a, b; Gutzmer et al., 2006). Our recent works allowed precise dating and geodynamic reconstructions of the ore deposit genesis. The ore depositions occurred during (i) late Cretaceous to late Paleocene (~ 92 Ma, ~ 78–82 Ma, ~ 65–67 Ma and ~58 Ma), (ii) late Eocene (c. 36 Ma), and (iii) early Burdigalian to early Serravalian probably in two pulses at c. 19–20 Ma and c. 13 Ma (Dekoninck et al., 2021, 2023). This multistage deposition coincides with three geodynamic events linked to the uplift of the Atlas. The late Cretaceous uplift of the Atlas created the required hydraulic head to sustain (1) fluid-rock interactions between O2-poor acidic ground waters and the Triassic series source, (2) migration of the metal-rich low-temperature hydrothermal fluid from the rock source and (3) overpressure fluid in the Imini depositional site. The vanishing of Triassic series above the Imini anticline forced these hydrothermal fluids to mix with oxygenated ground and alkaline waters resident in the karst system and precipitated the Mn oxides. The N70°-oriented Atlasic tectonic structure controls the orientation of these epigenetic karst-hosted Mn deposits. The late Eocene – Early/Middle Miocene uplifts generated additional supplies and/or in-situ remobilizations of the primary late Cretaceous medium-grade ore to form the high-grade pyrolusite-rich ore.References Dekoninck et al. 2023 (accepted). Mineralium Deposita. Dekoninck et al. 2016. Mineralium Deposita 51, 13-23. Dekoninck et al. 2016. in: Mineral Deposits of North Africa. Springer International Publishing, Cham, pp. 575–594. Dekoninck et al. 2021. Mineralium Deposita 59, 935–956. Gutzmer et al. 2006. Economic Geology 101, 385–405.
Two phases of penetrative deformation are documented in the Taconian hinterland of the Appalachia... more Two phases of penetrative deformation are documented in the Taconian hinterland of the Appalachian orogen in the Gaspé Peninsula. D1 is associated with the obduction of the Mont-Albert ophiolite onto the Paleozoic Laurentian margin, whereas D2 corresponds to later transport of allochthons across the margin. In the metamorphic sole, S1 is a SE-dipping mylonitic fabric with a downdip lineation. In underlying metabasalts, D1 is characterized by NW-overturned and recumbent folds, and a subhorizontal S1 schistosity with an ENE-trending orogen-parallel lineation. D2 is characterized by a S2 steeply dipping penetrative axial-planar crenulation cleavage and NE-trending F2 folds. The intraoceanic thrusting of ophiolite is dated at 465 Ma (early D1) whereas emplacement of ophiolite and subsequent deformation of the margin was recorded by isotopic signatures between 459 and 456 Ma (late D1). D2 is dated at 448 Ma throughout the hinterland. Taconian transpressive deformation is related to an oblique collision within the Quebec reentrant of the Canadian Appalachians during the Ordovician.
International audienceNumerous Sb-Au deposits are known in the European Variscan belt and are oft... more International audienceNumerous Sb-Au deposits are known in the European Variscan belt and are often associated with the late Variscan hydrothermal events linked to postorogenic extension and strike-slip faults, through the whole belt. Central Brittany hosts several Sb-Au occurrences, such as the Le Semnon Sb-Au deposit or the Saint-Aubin-des-Châteaux base metal-Sb-Au occurrence. In order to provide new constraints on the Sb-Au mineralization framework of the Central Brittany, structural, geochronological and chemical analyses have been performed on the Le Semnon deposit and the Saint-Aubin-des-Châteaux occurrence. Main results show that Sb-Au trapping of these two occurrences occurred at ca. 360 Ma. This indicates an Early Carboniferous economic Sb-Au mineralizing peak in the eastern part of the Central Brittany. Moreover, a mafic magmatism is widespread in the region and can acted as the plumbing system for Sb-Au mineralizing fluids, such as the dolerite dyke of the Semnon Sb-Au deposit. Absolute dating of this mafic magmatism coupled with the early character of hydrothermal alteration associated with Sb-Au mineralizing fluids strongly suggest that the emplacement of Sb-Au mineralization and mafic magmatism was coeval. It appears that the emplacement of mafic magmatism represent a major trigger for this mineralizing system at shallow depths (less than 3 km). These new data and the ca. 360 Ma massive sulfides of south Iberia emphasize that the Early Carboniferous mafic magmatism has to be taken into account for the understanding of the genesis of mineralizing systems and the redistribution of metals in the crust, at the scale of the whole Variscan belt
Le domaine sud-armoricain est constitue d’un ensemble de roches typiques de la croute inferieure,... more Le domaine sud-armoricain est constitue d’un ensemble de roches typiques de la croute inferieure, dont des migmatites, exhumees au Carbonifere superieur sous l’action conjointe du Cisaillement Sud Armoricain et des zones de detachements majeures (e.g. Quiberon, Guerande). Des granites syn-tectoniques se mettent en place dans ce contexte. Regionalement, les granites syntectoniques et plus generalement les roches deformees ductilement portent la trace d’alterations hydrothermales, datees par des perturbations des systemes geochronologiques K-Ar sur muscovite et U-Th-Pb sur monazite entre 320 et 300 Ma. La caracterisation des fluides impliques (isotopes stables de l’oxygene, inclusions fluides sur veines de quartz geantes associees aux structures regionales) indique une contribution significative de fluides d’origine meteorique, specifiquement dans la region ou les roches de la croute inferieure ont ete exhumees. Circulation de fluides, deformation regionale, exhumation des roches de la croute inferieure et magmatisme granitique sont donc synchrones. La penetration des fluides meteoriques jusqu’a la transition fragile-ductile ainsi que le maintien de l’activite hydrothermale en profondeur sont entretenues par l’effet synchrone de l’advection de chaleur dans la croute moyenne (mise en place des granites et exhumation des migmatites) et du fonctionnement de failles en extension dans la croute superieure qui augmentent la permeabilite et canalisent les fluides de la surface vers les zones profondes. L’interaction entre ces fluides et les granites syn-tectoniques s’est traduite par une alteration de ces derniers, avec en particulier, pour les facies petrologiques les plus differencies, la dissolution d’uraninite magmatique. L’uranium ainsi mobilise s’est localement reconcentre, formant des mineralisations, comme par exemple dans la region de Piriac-sur-mer. Le domaine sud-armoricain permet ainsi de reconstituer un paysage metallogenique tardi-hercynien dont l’evolution des zones profondes (exhumation des migmatites, deformation, magmatisme) et l’infiltration d’eaux meteoriques constituent les conditions aux limites. La penetration des fluides de surface en profondeur reste a mieux comprendre. Des donnees a venir sur la topographie de la Chaine Varisque (via la composition isotopique en hydrogene des mineraux des zones de deformation ; these Camille Dusseaux, Plymouth University) apporteront des informations precieuses sur l’impact de la tete hydraulique dans les circulations de fluides en profondeur.
The rifted continental margins of Mozambique provide excellent examples of continental passive ma... more The rifted continental margins of Mozambique provide excellent examples of continental passive margins with a significant structural variability associated with magmatism and inheritance. Despite accumulated knowledge, the tectonic structure and nature of the crust beneath the South Mozambique Coastal Plain (SMCP) are still poorly known. This study interprets high-resolution seismic reflection data paired with data from industry-drilled wells and proposes a structural model of the Limpopo transform margin in a magma-rich context. Results indicate that the Limpopo transform margin is characterized by an ocean-continent transition that links the Beira-High and Natal valley margin segments and represents the western limit of the continental crust, separating continental volcano-sedimentary infilled grabens from the oceanic crust domain. These basins result from the emplacement of the Karoo Supergroup during a Permo-Triassic tectonic event, followed by an Early Jurassic tectonic and magmatic event. This latter led to the establishment of steady-state seafloor spreading at ca.156 Ma along the SMCP. A Late Jurassic to Early Cretaceous event corresponds to formation of the Limpopo transform fault zone. Which accommodated the SSE-ward displacement of Antarctica with respect to Africa. We define a new type of margin: the magma-rich transform margin, characterized by the presence of voluminous magmatic extrusion and intrusion coincident with the formation and evolution of the transform margin. The Limpopo transform fault zone consists of several syn-transfer and-transform faults rather than a single transform fault. The intense magmatic activity was associated primarily with mantle dynamics, which controlled the large-scale differential subsidence along the transform margin. ROCHE ET AL.
Dans divers contextes geodynamiques, la fusion de la croute continentale peut etre induite par de... more Dans divers contextes geodynamiques, la fusion de la croute continentale peut etre induite par des intrusions repetees de magmas issus de la fusion du manteau. Pour atteindre les temperatures necessaires a la fusion dans la croute inferieure et aboutir, le processus requiert de perdurer et une frequence soutenue des alimentations mantelliques –frequence d’autant plus elevee que les intrusions individuelles apportant la chaleur sont de volume modere et que l’encaissant, aux contacts, est refractaire. Dans les niveaux plus superficiels de la croute, et potentiellement jusqu’a la surface, des magmas d’origine mantellique –et/ou leurs produits de differenciation– peuvent ainsi venir se mettre en place les premiers et preceder l’intrusion et la cristallisation de magmas d’origine crustale –et/ou hybrides, resultats de melanges. C’est le cas de l’intrusion composite de Ploumanac’h qui se met en place dans la croute superieure du Nord du Massif armoricain a la fin du Carbonifere dans un contexte meconnu. Constituee de 3 unites granitiques principales, elles-memes composites et successivement emboitees vers le centre, l’intrusion est emblematique (Barriere, 1976 –these non publiee, Brest). L’unite externe, la plus ancienne, renferme des facies de granites rouges a hornblende et biotite et des gabbros. Un leucogranite a deux micas constitue l’un des facies les plus jeunes de l’unite centrale. A partir d’echantillons selectionnes parmi ces facies, nous avons entrepris une etude geochronologique U-Th-Pb (zircon, monazite), 40Ar/39Ar (amphibole, muscovite, biotite) et traces de fission sur apatite. Les zircons concordants situent la cristallisation des granites rouges a 308,8 ± 2,5 Ma. Les âges U-Th-Pb sur zircon et monazite du leucogranite, respectivement a 301,3 ± 1,8 Ma et 301,1 ± 1,2 Ma, concordent dans l’erreur analytique. Ces derniers coincident egalement avec la majorite des âges 40Ar/39Ar obtenus dans les deux unites ; les perturbations de spectres etant imputables aux ultimes intrusions. Enfin, les unites atteignent les profondeurs d’effacement partiel des traces de fissions (~3 km) au Trias. Nous concluons (1) que l’exhumation du systeme construit n’a pas ete assistee par une extension regionale due a la soustraction d’un manteau lithospherique dense pouvant engendrer une fusion mantellique, et (2) qu’auparavant, il aura fallu au minimum 4 millions d’annees pour assembler et refroidir des produits issus du manteau et de la croute, au meme niveau structural.
At Domeyko, 40 km south of Vallenar in northern Chile (28°57'S-70°53'W), the Dos Amigos and Trico... more At Domeyko, 40 km south of Vallenar in northern Chile (28°57'S-70°53'W), the Dos Amigos and Tricolor porphyry copper centers are located within a north-south-elongated hydrothermal alteration zone 6x1.5 km of surface dimensions. The centers are related to tonalite to granodiorite porphyry stocks displaying potassic alteration, which are surrounded by Lower Cretaceous andesitic volcanic rocks with sericitic, kaolinite-illite and propylitic alteration zones. The western boundary of the alteration zone is marked by the post-mineralization Cachiyuyo Batholith of granodioritic to dioritic composition. U-Pb zircon ages for the Dos Amigos porphyry are of 106.1±3.5 and 104.0±3.5 Ma; and 108.5±3.4 for the nearby Tricolor porphyry. The Cachiyuyo Batholith yielded U-Pb zircon ages of 99.6±1.8 and 99.1±1.9 Ma; and 40 Ar/ 39 Ar ages for biotite of 96.9±3.9 and 94.8±0.9 Ma. These dates indicate that batholith emplacement postdated the Dos Amigos and Tricolor porphyries, in agreement with geological relationships. Although copper mineralization is spatially and genetically related to the Lower Cretaceous (Albian) porphyry stocks, most of the dated hydrothermal micas from the Dos Amigos and Tricolor porphyries yielded 40 Ar/ 39 Ar ages between 97.1±2.5 and 96.0±1.4 Ma, which overlap within error with the cooling ages obtained for the neighboring batholith. 40 Ar/ 39 Ar dating of micas revealed significant disturbance of their K-Ar isotopic systematics that complicates accurate determination of the timing of hydrothermal activity at Domeyko. Nevertheless, the 40 Ar/ 39 Ar data establish a minimum Late Cretaceous age for this activity. A fission track age of 59.8±9.8 Ma of apatite from the Dos Amigos porphyry indicates cooling through the temperature range of the apatite partial annealing zone (~125-60°C) during the Paleocene; and an (U-Th)/He age of 44.7±3.7 Ma of apatite from the same porphyry sample shows cooling through the temperature range of the apatite He partial retention zone (~85-40°C) during the Eocene. These ages correspond to the exhumation of the porphyry, and the latter provides a maximum age for the supergene enrichment processes that formed the chalcocite blanket currently mined at Dos Amigos.
As part of a combined geochronologic and palaeomagnetic study of the TrCgor area (Armorican Massi... more As part of a combined geochronologic and palaeomagnetic study of the TrCgor area (Armorican Massif), we have attempted to obtain a reliable age for a Late Proterozoic palaeopole from experiments carried out on the Keralain and Petit Taurel diorites. A heterogeneous population of amphiboles analysed by the 40Ar/39Ar stepheating method gave a flat saddle-shaped age spectrum (characteristic of excess argon) with a minimum age of 604.9f 1.2 Ma, which represents a maximum estimate of the age of the mineral. The isochron calculation gives an initial 40Ar/xAr ratio sensibly higher than the atmospheric ratio. This indicates that the isochron age, 583 f 2 Ma, is a better estimate of the age of the Keralain diorite. Laser single step fusion and step-heating experiments performed on single grains representative of the different components of this heterogeneous population of amphiboles show that the excess argon affects it in its entirety. The minimum age of 590.8 f 2.3 Ma of the flat saddle-shaped age spectrum displayed by the core of a crystal of hornblende is concordant (at the 20 level) with the isochron age. The isotopic closure temperature calculated from the step-heating results is 556 f 43 "C. The palaeomagnetic study has shown, using a thermal demagnetization procedure, that the samples have a multicomponent magnetization. Two characteristic components (A and B) have been recognized respectively at Keralain and the Petit Taurel. A careful rock magnetic study has established that the magnetic carrier of the primary component A, isolated at Keralain, is magnetite with a magnetic blocking temperature of 575°C. The similarity of this temperature with the calculated isotopic closure temperature allows us to assign an age of c. 583 Ma to the corresponding well-defined palaeopole 'DKA', located at 33"N, 256"E. The demagnetization experiments have also clearly shown that component B, well isolated at the Petit Taurel, is a remagnetization probably related to magmatic events at around 550 Ma. Furthermore, its magnetic behaviour, as observed at Keralain, indicates a possible thermochemical origin. The well-dated DKA palaeopole allows us to refine the late Proterozoic polar wander path for Armorica and seems to confirm that the Armorican Massif and southern United Kingdom were linked together at that period.
The Tibetan Plateau was formed by intense Cenozoic shortening (up to 1100 km) of a composite “pro... more The Tibetan Plateau was formed by intense Cenozoic shortening (up to 1100 km) of a composite “proto-Tibet”, itself the product of a long Paleozoic and Mesozoic history of accretion of Gondwana-derived continental fragments and volcanic arcs against the Asian continental margin. The difficult access and the scarcity of outcrops have long limited the possibilities of studying these Mesozoic suture zones in the heart of the Plateau. In this work, we present new U-Pb and 40Ar/39Ar ages from the highly deformed units of the Yushu mélange, along the Jinsha Suture in the northeastern Qiangtang terrane. Early Triassic (c. 253 Ma) to Middle Jurassic ages (c. 165 Ma) complement the existing dataset and help to refine the chronology of the Paleo-Tethyan oceanic subductions which have structured the northeastern part of the Qiangtang terrane. The Yushu mélange records at least three successive tectono-magmatic events. The opening of a back-arc basin during the northward Paleo-Tethyan subduction along the Longmu Co-Shuanghu Suture during Early to Middle Triassic; then its closure during the southward subduction of the Songpan-Ganze Ocean along the Jinsha Suture in Late Triassic. Finally, a shortening phase related to the continental collision of the Songpan-Ganze and Qiangtang blocks from Late Triassic to Early-Middle Jurassic. No evidence for any high- or mid-temperature Cenozoic reactivation of the Jinsha suture in our study area is recorded.
The Imini mining district (Morocco) hosts the largest manganese (Mn) ore deposits of North Africa... more The Imini mining district (Morocco) hosts the largest manganese (Mn) ore deposits of North Africa (~120.000 T in 2016). The area is world recognized for hosting several epigenetic karst-type high-grade manganese deposits in a 10-15 meters thick Cenomanian-Turonian dolostone unit. These unconventional Mn oxide deposits occur along a belt of ~25-30 km in the southern foreland of the intraplate Atlasic belt of Morocco. This is due to two laterally extensive ore bodies of nearly pure pyrolusite-rich manganese ores (72-88 wt.% MnO2) and a third discontinuous medium-grade coronadite-rich Mn ore (40–48 wt.% MnO2)(Dekoninck et al., 2016a, b; Gutzmer et al., 2006). Our recent works allowed precise dating and geodynamic reconstructions of the ore deposit genesis. The ore depositions occurred during (i) late Cretaceous to late Paleocene (~ 92 Ma, ~ 78–82 Ma, ~ 65–67 Ma and ~58 Ma), (ii) late Eocene (c. 36 Ma), and (iii) early Burdigalian to early Serravalian probably in two pulses at c. 19–20 Ma and c. 13 Ma (Dekoninck et al., 2021, 2023). This multistage deposition coincides with three geodynamic events linked to the uplift of the Atlas. The late Cretaceous uplift of the Atlas created the required hydraulic head to sustain (1) fluid-rock interactions between O2-poor acidic ground waters and the Triassic series source, (2) migration of the metal-rich low-temperature hydrothermal fluid from the rock source and (3) overpressure fluid in the Imini depositional site. The vanishing of Triassic series above the Imini anticline forced these hydrothermal fluids to mix with oxygenated ground and alkaline waters resident in the karst system and precipitated the Mn oxides. The N70°-oriented Atlasic tectonic structure controls the orientation of these epigenetic karst-hosted Mn deposits. The late Eocene – Early/Middle Miocene uplifts generated additional supplies and/or in-situ remobilizations of the primary late Cretaceous medium-grade ore to form the high-grade pyrolusite-rich ore.References Dekoninck et al. 2023 (accepted). Mineralium Deposita. Dekoninck et al. 2016. Mineralium Deposita 51, 13-23. Dekoninck et al. 2016. in: Mineral Deposits of North Africa. Springer International Publishing, Cham, pp. 575–594. Dekoninck et al. 2021. Mineralium Deposita 59, 935–956. Gutzmer et al. 2006. Economic Geology 101, 385–405.
Two phases of penetrative deformation are documented in the Taconian hinterland of the Appalachia... more Two phases of penetrative deformation are documented in the Taconian hinterland of the Appalachian orogen in the Gaspé Peninsula. D1 is associated with the obduction of the Mont-Albert ophiolite onto the Paleozoic Laurentian margin, whereas D2 corresponds to later transport of allochthons across the margin. In the metamorphic sole, S1 is a SE-dipping mylonitic fabric with a downdip lineation. In underlying metabasalts, D1 is characterized by NW-overturned and recumbent folds, and a subhorizontal S1 schistosity with an ENE-trending orogen-parallel lineation. D2 is characterized by a S2 steeply dipping penetrative axial-planar crenulation cleavage and NE-trending F2 folds. The intraoceanic thrusting of ophiolite is dated at 465 Ma (early D1) whereas emplacement of ophiolite and subsequent deformation of the margin was recorded by isotopic signatures between 459 and 456 Ma (late D1). D2 is dated at 448 Ma throughout the hinterland. Taconian transpressive deformation is related to an oblique collision within the Quebec reentrant of the Canadian Appalachians during the Ordovician.
International audienceNumerous Sb-Au deposits are known in the European Variscan belt and are oft... more International audienceNumerous Sb-Au deposits are known in the European Variscan belt and are often associated with the late Variscan hydrothermal events linked to postorogenic extension and strike-slip faults, through the whole belt. Central Brittany hosts several Sb-Au occurrences, such as the Le Semnon Sb-Au deposit or the Saint-Aubin-des-Châteaux base metal-Sb-Au occurrence. In order to provide new constraints on the Sb-Au mineralization framework of the Central Brittany, structural, geochronological and chemical analyses have been performed on the Le Semnon deposit and the Saint-Aubin-des-Châteaux occurrence. Main results show that Sb-Au trapping of these two occurrences occurred at ca. 360 Ma. This indicates an Early Carboniferous economic Sb-Au mineralizing peak in the eastern part of the Central Brittany. Moreover, a mafic magmatism is widespread in the region and can acted as the plumbing system for Sb-Au mineralizing fluids, such as the dolerite dyke of the Semnon Sb-Au deposit. Absolute dating of this mafic magmatism coupled with the early character of hydrothermal alteration associated with Sb-Au mineralizing fluids strongly suggest that the emplacement of Sb-Au mineralization and mafic magmatism was coeval. It appears that the emplacement of mafic magmatism represent a major trigger for this mineralizing system at shallow depths (less than 3 km). These new data and the ca. 360 Ma massive sulfides of south Iberia emphasize that the Early Carboniferous mafic magmatism has to be taken into account for the understanding of the genesis of mineralizing systems and the redistribution of metals in the crust, at the scale of the whole Variscan belt
Le domaine sud-armoricain est constitue d’un ensemble de roches typiques de la croute inferieure,... more Le domaine sud-armoricain est constitue d’un ensemble de roches typiques de la croute inferieure, dont des migmatites, exhumees au Carbonifere superieur sous l’action conjointe du Cisaillement Sud Armoricain et des zones de detachements majeures (e.g. Quiberon, Guerande). Des granites syn-tectoniques se mettent en place dans ce contexte. Regionalement, les granites syntectoniques et plus generalement les roches deformees ductilement portent la trace d’alterations hydrothermales, datees par des perturbations des systemes geochronologiques K-Ar sur muscovite et U-Th-Pb sur monazite entre 320 et 300 Ma. La caracterisation des fluides impliques (isotopes stables de l’oxygene, inclusions fluides sur veines de quartz geantes associees aux structures regionales) indique une contribution significative de fluides d’origine meteorique, specifiquement dans la region ou les roches de la croute inferieure ont ete exhumees. Circulation de fluides, deformation regionale, exhumation des roches de la croute inferieure et magmatisme granitique sont donc synchrones. La penetration des fluides meteoriques jusqu’a la transition fragile-ductile ainsi que le maintien de l’activite hydrothermale en profondeur sont entretenues par l’effet synchrone de l’advection de chaleur dans la croute moyenne (mise en place des granites et exhumation des migmatites) et du fonctionnement de failles en extension dans la croute superieure qui augmentent la permeabilite et canalisent les fluides de la surface vers les zones profondes. L’interaction entre ces fluides et les granites syn-tectoniques s’est traduite par une alteration de ces derniers, avec en particulier, pour les facies petrologiques les plus differencies, la dissolution d’uraninite magmatique. L’uranium ainsi mobilise s’est localement reconcentre, formant des mineralisations, comme par exemple dans la region de Piriac-sur-mer. Le domaine sud-armoricain permet ainsi de reconstituer un paysage metallogenique tardi-hercynien dont l’evolution des zones profondes (exhumation des migmatites, deformation, magmatisme) et l’infiltration d’eaux meteoriques constituent les conditions aux limites. La penetration des fluides de surface en profondeur reste a mieux comprendre. Des donnees a venir sur la topographie de la Chaine Varisque (via la composition isotopique en hydrogene des mineraux des zones de deformation ; these Camille Dusseaux, Plymouth University) apporteront des informations precieuses sur l’impact de la tete hydraulique dans les circulations de fluides en profondeur.
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Papers by Gilles Ruffet