The Ediacaran–Fortunian ichnofauna from Central Brittany (NW France) is revised for the first tim... more The Ediacaran–Fortunian ichnofauna from Central Brittany (NW France) is revised for the first time since the pioneering work by Lebesconte at the end of the 19th century. The study is based on fossils from the type-localities of the historical Brioverian taxa Montfortia (traces from Montfort-sur-Meu) and Neantia (wrinkle surfaces from Néant-sur-Yvel), and on two new outcrops from Saint-Gonlay. The ichnofossil assemblage includes Helminthoidichnites tenuis, Helminthopsis tenuis, Gordia marina, Palaeophycus tubularis, ?Neonereites uniserialis, and ?Spirodesmos archimedeus. Locally, the grazing traces are associated with wrinkle surfaces considered as microbially induced sedimentary structures (MISS). The sedimentological characteristics of these deposits correspond to a shelf marine environment. Both U-Pb datings on detrital zircon and ichnostratigraphic criteria tend toward an Ediacaran age of the fossiliferous deposits, but we cannot exclude the possibility of a Fortunian age at the present state of knowledge.
The South Armorican Shear Zone (SASZ), in the French Armorican Variscan belt, is a lithosphericwr... more The South Armorican Shear Zone (SASZ), in the French Armorican Variscan belt, is a lithosphericwrench fault that acted during the Late Carboniferous as a transition zone between two distinct domains: a thickened domain to the south affected by extension and crustalmagmatism, and aweakly thickened domain to the north subjected to dextral wrenching and crust- and mantle-derived magmatism. The Pontivy-Rostrenen complex is a composite intrusion emplaced along the SASZ. To the south, the complex is made of leucogranites whereas, to the north, monzogranites outcrop together with small intrusions of quartz monzodiorite. U-Pb dating of magmatic zircon by LA-ICP-MS reveal that most magmatic rocks were emplaced at ca. 315 Ma (between 316.7 ± 2.5 Ma and 310.3 ± 4.7 Ma), excepted a late leucogranitic intrusion that was emplaced at 304.7± 2.7 Ma. The leucogranites (−4.8 b εNd (T) b 2.1; presence of Archean to Paleozoic inherited zircon) are strongly peraluminous (A/CNK N 1.1) and formed by partial melting ofmetasediments and peraluminous orthogneisses. Themonzogranite (−4.0 b εNd (T) b−3.2; scarce Paleozoic inherited zircon) is moderately peraluminous (1 b A/CNK b 1.3) and formed by partial melting of an orthogneiss with a probable metaluminous composition whereas the quartz monzodiorite (−3.2 b εNd (T) b −2.2; no inherited zircon) is metaluminous (0.7 b A/CNK b 1.1) and formed by partial melting of a metasomatized lithospheric mantle. The evolution of the magmas was controlled by fractional crystallization, magma mixing and/or peritectic mineral entrainment. At the scale of the Armorican Variscan belt, crustal partial melting, to the south of the SASZ, was triggered by lithospheric thinning and adiabatic decompression during extension. Conversely, to the north, asthenosphere upwelling during strike-slip deformation and subsequent slab tearing, as suggested by tomographic data, induced the melting of both the crust and the mantle fertilized during previous subduction events. This process is likely not exclusive to the Armorican Massif and may be applied to other regions in the Variscan belt, such as Iberia.
In the French Armorican Variscan belt, most of the economically significant hydrothermal U deposi... more In the French Armorican Variscan belt, most of the economically significant hydrothermal U deposits are spatially associated with peraluminous leucogranites emplaced along the south Armorican shear zone (SASZ), a dextral lithospheric scale wrench fault that recorded ductile deformation from ca. 315 to 300 Ma. In the Pontivy-Rostrenen complex, a composite intrusion, the U mineralization is spatially associated with brittle structures related to deformation along the SASZ. In contrast to monzogranite and quartz monzodiorite (3 < U < 9 ppm; Th/U > 3), the leucogranite samples are characterized by highly variable U contents (~ 3 to 27 ppm) and Th/U ratios (~ 0.1 to 5) suggesting that the crystallization of magmatic uranium oxide in the more evolved facies was followed by uranium oxide leaching during hydrothermal alteration and/or surface weathering. U-Pb dating of uranium oxides from the deposits reveals that they mostly formed between ca. 300 and 270 Ma. In monzogranite and quartz monzodiorite, apatite grains display magmatic textures and provide U-Pb ages of ca. 315 Ma reflecting the time of emplacement of the intrusions. In contrast, apatite grains from the leucogranite display textural, geochemical, and geochronological evidences for interaction with U-rich oxidized hydrothermal fluids contemporaneously with U mineralizing events. From 300 to 270 Ma, infiltration of surface-derived oxidized fluids leached magmatic uranium oxide from fertile leucogranite and formed U deposits. This phenomenon was sustained by brittle deformation and by the persistence of thermal anomalies associated with U-rich granitic bodies.
The sediments of the Mauges Unit located in the internal zone provide an opportunity of studying ... more The sediments of the Mauges Unit located in the internal zone provide an opportunity of studying the evolution of relief during Palaeozoic time. U–Pb dating on zircon and 39Ar/40Ar on white mica are used to constrain the age and nature of the sources. The first relief identified is marked by an Early Devonian unconformity interpreted as the opening of a northern back-arc basin. Detrital minerals are first reworked from underlying layers indicating a local supply. Magmatic zircons at c. 400 Ma then record the emergence of a magmatic arc. During the Middle Devonian, the gap in the sedimentary record is attributed to an emersion followed by the disappearance of the relief during the Late Devonian. At the Devonian–Carboniferous boundary, the main collision is followed by the onset of a relief. The continental sedimentation in the Ancenis Basin (late Tournaisian–Viséan) is a coarsening-upwards megasequence indicating an increasing and/or approaching relief. The detrital minerals record the progressiv
We report here new LA-ICPMS detrital zircon U-Pb ages of a quartzite from the autochthon of Pelop... more We report here new LA-ICPMS detrital zircon U-Pb ages of a quartzite from the autochthon of Peloponnesus (Feneos locality), southern Greece. The rock classifies as a mature quartz arenite and belongs to an original shale-sandstone succession now metamorphosed into a phyllite-quartzite unit. Zircon age clusters at 0.52-0.75, 0.85, 0.95-1.1, 1.75-2 and 2.4-3 Ga point at the Saharan Metacraton and the Transgondwanan Supermountain as contributing sources; the youngest concordant grain is 522 Ma old. Our data collectively suggest deposition during the Cambro-Ordovician in a collisional setting and are in excellent agreement with those of the virtually intact Cambro-Ordovician sandstone-shale sequences of Libya (Murzuq and Kufrah basins) and the Middle East (Israel and Jordan), interpreted to have been deposited in the Gondwana Super-fan System which draped the northern Gondwanan periphery from similar to 525 to 460 Ma. By contrast, re-evaluating the available zircon age-distribution pattern and depositional setting of an analogous sequence forming the autochthon of north-central Crete (Galinos beds) we demonstrate that it was originally deposited in a completely different setting, i.e. in an accretionary/fore-arc complex outboard of the south Laurussian active margin (Pelagonia) during the Late Carboniferous. Comparing similar Cambro-Ordovician metasiliciclastic rocks from north-eastern Crete (Sfaka paragneiss), north-central continental Greece (Vertiskos terrane), north-western Turkey (central Sakarya terrane) and the Romanian Carpathians we show that their detrital zircon distribution patterns testify to an original depositional setting similar to that of Peloponnesus (Feneos), Libya and the Middle East Using key time-frames from previously published palaeogeographic reconstruction models we are able to trace in space and time the Palaeozoic-Early Mesozoic wondering paths of the aforementioned sequences. Thus, time- and fades-equivalent rocks presently cropping out in the eastern Mediterranean share a common provenance from the Gondwana Super-fan System which was diachronously dispersed between Early Silurian and Early Triassic.
In the Pyrenees, in association with the rotation of the Iberian plate around Europe during the M... more In the Pyrenees, in association with the rotation of the Iberian plate around Europe during the Mid-Cretaceous, a Na-Ca metasomatism is recognized as a complementary record of the hydrothermal activity that led to Na-metasomatism (albitization) and talc-chlorite mineralization. It affected metasedimentary rocks as well as Hercynian granites. In situ laser ablation ICP-MS U-Pb analyses of titanite grains formed in albitites during metasomatism date the Na-Ca metasomatism between 110 and 92 Ma. The temperature of the Na-Ca metasomatism is estimated to be approximately 550 A degrees C. Both the time constraints and temperature estimates suggest that the Na-Ca metasomatism is related to the low-P high-T North Pyrenean metamorphism.
In the homogeneously deformed Murchison Greenstone Belt region,
fluids circulations were focused ... more In the homogeneously deformed Murchison Greenstone Belt region, fluids circulations were focused along a localized deformation brittle-ductile zone, and the so-called Antimony Line. In order to trace the origin of the fluid, its composition, the temperature of the precipitation along the Antimony Line, we investigate the stable isotope (oxygen and carbon) and fluid inclusion compositions of the quartz-carbonate veins, as well as chemical analyses of the carbonates and 40Ar-39Ar dating on the fuchsite associated with the veins. Three types of fluid inclusions are recognized for the H2O-CO2-dominated (plus minor CH4-N2) fluid(s). Microthermometric measurements together with the paragenesis of the veins and host rocks indicate that the pressure-temperature of the precipitation is 350 to 450 °C, 200 to 300 MPa, which is an unusually high temperature for an antimony deposit. The carbonates are Fe-Mg-rich and are characterized by flat MREE and HREE patterns, with slight depletions in LREE. The oxygen isotope compositions of the quartz are rather homogeneous at the scale of the Antimony Line (�18Oquartz � 10.9-14.3‰). In detail, subtle differences in the �18O values for the quartz and carbonate exist depending on the sampling sites. These differences are likely due to minor temperature and/or host rock geochemistry variations along the Antimony Line. Data point to a metamorphic origin for the fluids responsible for the mineralization, which likely escaped from the nearby greenstone lithologies. Finally, we present an overview of the Murchison Greenstone Belt geological evolution by integrating the antimony metallogeny of the Antimony Line, which can be compared to that of an orogenic gold deposit.
The Guérande peraluminous leucogranite was emplaced at the end of the Carboniferous in the southe... more The Guérande peraluminous leucogranite was emplaced at the end of the Carboniferous in the southern part of the Armorican Massif. At the scale of the intrusion, this granite displays structural heterogeneities with a weak deformation in the southwestern part, whereas the northwestern part is marked by the occurrence of S/C and mylonitic extensional fabrics. Quartz veins and pegmatite dykes orientations as well as lineations directions in the granite and its country rocks demonstrate both E–W and N–S stretching. Therefore, during its emplacement in an extensional tectonic regime, the syntectonic Guérande granite has probably experienced some partitioning of the deformation. The southwestern part is characterized by a muscovite biotite assemblage, the presence of restites and migmatitic enclaves, and a low abundance of quartz veins compared to pegmatite dykes. In contrast, the northwestern part is characterized by a muscovite–tourmaline assemblage, evidence of albitization and gresenization and a larger amount of quartz veins. The southwestern part is thus interpreted as the feeding zone of the intrusion whereas the northwestern part corresponds to its apical zone. The granite samples display continuous compositional evolutions in the range of 69.8–75.3 wt.% SiO2. High initial 87Sr/86Sr ratios and low εNd(T) values suggest that the peraluminous Guérande granite (A/CNK N 1.1) was formed by partial melting of metasedimentary formations. Magmatic evolution was controlled primarily by fractional crystallization of Kfeldspar, biotite and plagioclase (An20). The samples from the apical zone show evidence of secondary muscovitization. They are also characterized by a high content in incompatible elements such as Cs and Sn, as well as low Nb/Ta and K/Rb ratios. The apical zone of the Guérande granite underwent a pervasive hydrothermal alteration during or soon after its emplacement. U–Th–Pb dating on zircon and monazite revealed that the Guérande granite was emplaced 309.7± 1.3 Ma ago and that a late magmatic activity synchronous with hydrothermal circulation occurred at ca. 303 Ma. These new structural, petrological and geochronological data presented for the Guérande leucogranite highlight the interplay between the emplacement in an extensional tectonic regime, magmatic differentiation and hydrothermal alteration, and provide a general background for the understanding of the processes controlling some mineralization in the western European Hercynian belt.
In the Western Alps, the Money Complex of the Gran Paradiso Massif, metamorphosed under blueschis... more In the Western Alps, the Money Complex of the Gran Paradiso Massif, metamorphosed under blueschist facies during the Alpine cycle, is considered to be Permo-Carboniferous in age, but no palaeontological or radiometric data constrain this interpretation. A revision of the lithostratigraphy of the Money Complex allows recognizing a polygenic (graphite-rich) and a monogenic (graphite-poor) meta-sedimentary formation. Detrital zircon U–Pb geochronology in both meta-sedimentary formations shows that (i) the main population is Cambrian and Ordovician in age, (ii) the youngest grains are Silurian and Lower Devonian, and (iii) Carboniferous zircon grains are lacking. A careful study of the age distributions in the Alps suggests that potential source for the detrital material in the Money Complex is the Briançonnais basement. Late Carboniferous magmatism is widespread in the Helvetic Zone of the Alps. Permian magmatism is dominant in the Briançonnais, the Austroalpine and the Southalpine basements. The lack of Carboniferous zircons in the Money Complex suggests that the detritus was not shed from the Helvetic zone, which was separated from the Money basin by the Zone Houillère basin, where the main drainage pattern was developed from south to north and where the depocenters migrated northwards from the Upper Missisippian to Upper Pennsylvanian. We suggest that the Money Complex may had been located to the east of the main river drainage inside the Zone Houillère basin or alternatively may represent a small basin, located on the east of the Zone Houillère.
Deciphering Variscan versus Alpine history in the internal Rif system is a key to constraining th... more Deciphering Variscan versus Alpine history in the internal Rif system is a key to constraining the tectonic evolution of the Alboran domain and hence the geodynamics of the western Mediterranean system during the Cenozoic. This study focuses on the evolution of the metamorphic envelope of the Beni Bousera massif and its relation to the underlying peridotites. Combining structural geology, metamorphic petrology, and LA-ICP-MS U-Th-Pb dating of monazite, this study contributes to the understanding of the tectonic history of the western internal Rif. The regional foliation (S2) is characterized by LP/HT mineral assemblages and obliterates a former foliation (S1) developed along a barrovian (MP/MT) metamorphic gradient. The dating of some metamorphic monazite grains from a micaschist and a migmatitic gneiss demonstrates that the crustal envelope of the peridotite recorded two distinct tectonometamorphic episodes. Data from monazite inclusions in S1 garnet suggest that the first event, D1, is older than 250-170 Ma and likely related to the Variscan collision, in agreement with the barrovian type of the metamorphic gradient. The second event, D2, is Alpine in age (at circa 21 Ma) and corresponds to a strong lithosphere thinning allowing subsequent subcontinental mantle exhumation. Such a tectonic context provides an explanation for the LP/HT metamorphic gradient that is recorded in the regional foliation of the western Betic-Rif system. This extension is probably related to a subduction slab roll-back in the western end of the Mediterranean realm during the Oligo-Miocene times. No evidences for a Tertiary HP/LT metamorphism have been identified in the studied area.
An increasing number of field examples in mountain belts show that the formation of passive margi... more An increasing number of field examples in mountain belts show that the formation of passive margins during extreme continent thinning may occur under conditions of high to very high thermal gradient beneath a thin cover of syn-rift sediments. Orogenic belts resulting from the tectonic inversion of distal margins and regions of exhumed continental mantle may exhibit high-temperature, low-pressure (HT-LP) metamorphism and coeval syn-extensional, ductile deformation. Recent studies have shown that the northern flank of the Pyrenean belt, especially the North Pyrenean Zone, is one of the best examples of such inverted hot, passive margin. In this study, we provide a map of HT-LP metamorphism based on a data set of more than 100 peak-temperature estimates obtained using Raman spectroscopy of the carbonaceous material (RSCM). This data set is completed by previous PT (pressure and temperature) estimates based on mineral assemblages, and new 40Ar–39Ar (amphibole, micas) and U–Pb (titanite) ages from metamorphic and magmatic rocks of the North Pyrenean Zone. The implications on the geological evolution of the Cretaceous Pyrenean paleomargins are discussed. Ages range mainly from 110 to 90 Ma, and no westward or eastward propagation of the metamorphism and magmatism can be clearly identified. In contrast, the new data reveal a progressive propagation of the thermal anomaly from the base to the surface of the continental crust. Focusing on the key localities of the Mauléon basin, Arguenos–Moncaup, Lherz, Boucheville and the Bas-Agly, we analyze the thermal conditions prevailing during the Cretaceous crustal thinning. The results are synthetized into a series of three regional thematic maps and into two detailed maps of the Arguenos–Moncaup and Lherz areas. The results indicate a first-order control of the thermal gradient by the intensity of crustal thinning. The highest grades of metamorphism are intimately associated with the areas where subcontinental mantle rocks have been unroofed or exhumed.
The Ili-Balkhash Basin in southeastern Kazakhstan is located at the junction of the actively defo... more The Ili-Balkhash Basin in southeastern Kazakhstan is located at the junction of the actively deforming mountain ranges of western Junggar and the Tien Shan, and is therefore part of the southwestern Central Asian Orogenic Belt. The basement of the Ili-Balkhash area consists of an assemblage of mainly Precambrian microcontinental fragments, magmatic arcs and accretionary complexes. Eight magmatic basement samples (granitoids and tuffs) from the Ili-Balkhash area were dated with zircon U-Pb LA-ICP-MS and yield Carboniferous to late Permian (~ 350-260 Ma) crystallization ages. These ages are interpreted as reflecting the transition from subduction to (post-) collisional magmatism, related to the closure of the Junggar-Balkhash Ocean during the Carboniferous – early Permian and hence, to the final late Paleozoic accretion history of the ancestral Central Asian Orogenic Belt. Apatite fission track (AFT) dating of 14 basement samples (gneiss, granitoids and volcanic tuffs) mainly provides Cretaceous cooling ages. Thermal history modeling based on the AFT data reveals that several intracontinental tectonic reactivation episodes affected the studied basement during the late Mesozoic and Cenozoic. Late Mesozoic reactivation and associated basement exhumation is interpreted as distant effects of the Cimmerian collisions at the southern Eurasian margin and possibly of the Mongol-Okhotsk Orogeny in SE Siberia during the Jurassic – Cretaceous. Following tectonic stability during the Palaeogene, inherited basement structures were reactivated during the Neogene (constrained by Miocene AFT ages of ~ 17–10 Ma). This late Cenozoic reactivation is interpreted as the far-field response of the India-Eurasia collision and reflects the onset of modern mountain building and denudation in southeast Kazakhstan, which seems to be at least partially controlled by the inherited basement architecture.
In the Saint-Aubin-des-Châteaux deposit (Massif Armoricain, France), the Ordovician Grès Armorica... more In the Saint-Aubin-des-Châteaux deposit (Massif Armoricain, France), the Ordovician Grès Armoricain sandstones have undergone several fluid-rock interaction events, including diagenetic cementation and orogenic base metal-As-Sb-Au mineralisation. Ironstone layers interbedded in the sandstones contain several generations of spectacular authigenic xenotime overgrowths that formed around detrital zircon grains in response to successive hydrothermal events. Textural and chemical characterisations allow to distinguish three generations of xenotime overgrowths, differing notably in their REE characteristics. In-situ U-Pb data obtained on these xenotime overgrowths show that their U-Pb systematics were largely disturbed by successive hydrothermal events over about 90 Ma between ~ 330 and ~ 420 Ma, a time interval encompassing most phases of the construction of the Variscan orogen in France. The younger dates cluster around ~ 330–340 Ma and likely correspond to the age of the deposition of massive sulphides and base-metals in the Saint-Aubin-des-Châteaux deposits, which is consistent with the structural contexts where they formed. Finally, this study shows that similarly to monazite, another phosphate widely used for U-Pb and Th-Pb dating studies, the U-Pb chronometric system in xenotime appears to be highly sensitive to fluid circulations.
The Chalkidiki block is a major domain in the North Aegean that, contrary to other domains in the... more The Chalkidiki block is a major domain in the North Aegean that, contrary to other domains in the region, largely escaped thermal perturbations during Tertiary extension. As a result, the Chalkidiki block is an ideal candidate to glean information related to the timing of Mesozoic thermal events using appropriate geochronological techniques. We have undertaken a laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) study (U-Th-Pb on monazites and U-Pb on zircons) coupled with 40Ar/39Ar dating on nine samples from various structural levels within the thrust system of the Chalkidiki block. The eastern, and structurally lower part of the system revealed a complete isotopic reset of Carboniferous – Early Triassic monazites coeval with partial monazite destruction, REE-mobilisation and formation of apatite-allanite-epidote coronas at ~ 132 Ma, a reaction that is commonly observed in amphibolite-facies rocks. These coronas formed after crystallisation of garnet (i.e., at T > 580 °C) and, in all probability, either close to the peak-temperature conditions (~ 620 °C) on a prograde path or during retrogression between the peak-temperature and the low-temperature boundary of the amphibolite facies. Cooling of these rocks and arrival at mid-crustal levels occurred at 95–100 Ma. By contrast, the western, and structurally uppermost part of the system went through the same event by 120–125 Ma. Further structural considerations with respect to medium-temperature geochronology data imply that syn-metamorphic thrusting must have ceased by early Late Cretaceous. We emphasize that, with the sole exception of the Chalkidiki block, no pre-45 Ma medium-temperature geochronology data are preserved in other North Aegean domains, a feature that is clearly related to the extension-induced thermal perturbation of the region during the Tertiary.
New U-Pb dating on zircon yielded ca. 470 Ma ages for the granitoids from the Lévézou massif in t... more New U-Pb dating on zircon yielded ca. 470 Ma ages for the granitoids from the Lévézou massif in the southern French Massif Central. These new ages do not support the previous interpretation of these granitoids as syn-tectonic intrusions emplaced during the late Devonian-early Carboniferous thrusting. The geochemical and isotopic nature of this magmatism is linked to a major magmatic Ordovician event recorded throughout the European Variscan belt and related to extreme thinning of continental margins during a rifting event or a back arc extension. The comparable isotopic signatures of these granitoids on each side of the eclogite-bearing leptyno-amphibolitic complex in the Lévézou massif, together with the fact that they were emplaced at the same time, strongly suggest that these granitoids were originally part of a single unit, tectonically duplicated by either isoclinal folding or thrusting during the Variscan tectonics.
One of the striking features that characterise the late stages of the Variscan orogeny is the dev... more One of the striking features that characterise the late stages of the Variscan orogeny is the development of gneiss and migmatite domes, as well as extensional Late Carboniferous and Permian sedimentary basins. It remains a matter of debate whether the formation of domes was related to the well-documented late orogenic extension or to the contractional tectonics that preceded. Migmatization and magmatism are expected to predate extension if the domes are compression-related regional anticlines, but they must both precede and be contemporaneous with extension if they are extensional core complexes. In the Montagne Noire area (southern French Massif Central), where migmatization, magmatism and the deformation framework are well documented, the age of the extensional event was unequivocally constrained to 300–290 Ma. Therefore, dating migmatization in this area is a key point for discriminating between the two hypotheses and understanding the Late Palaeozoic evolution of this part of the Variscan belt. For this purpose, a migmatite and an associated anatectic granite from the Montagne Noire dome were dated by LA-ICP-MS (U–Th–Pb on zircon and monazite) and laser probe 40Ar-39Ar (K–Ar on muscovite). Although zircon did not record any Variscan age unequivocally related to compression (380-330 Ma), two age groups were identified from the monazite crystals. A first event, at ca. 319 Ma (U–Th–Pb on monazite), is interpreted as a first stage of migmatization and as the emplacement age of the granite, respectively. A second event at ca. 298–295 Ma, recorded by monazite (U–Th–Pb) and by the muscovite 40Ar-39Ar system in the migmatite and in the granite, could be interpreted as a fluid-induced event, probably related to a second melting event identified through the syn-extensional emplacement of the nearby Montalet leucogranite ca. 295 Ma ago. The ages of these two events post-date the Variscan compression and agree with an overall extensional context for the development of the Montagne Noire dome-shaped massif. Comparison of these results with published chemical (EPMA) dating of monazite from the same rocks demonstrates that the type of statistical treatment applied to EPMA data is crucial in order to resolve different monazite age populations.
The Ediacaran–Fortunian ichnofauna from Central Brittany (NW France) is revised for the first tim... more The Ediacaran–Fortunian ichnofauna from Central Brittany (NW France) is revised for the first time since the pioneering work by Lebesconte at the end of the 19th century. The study is based on fossils from the type-localities of the historical Brioverian taxa Montfortia (traces from Montfort-sur-Meu) and Neantia (wrinkle surfaces from Néant-sur-Yvel), and on two new outcrops from Saint-Gonlay. The ichnofossil assemblage includes Helminthoidichnites tenuis, Helminthopsis tenuis, Gordia marina, Palaeophycus tubularis, ?Neonereites uniserialis, and ?Spirodesmos archimedeus. Locally, the grazing traces are associated with wrinkle surfaces considered as microbially induced sedimentary structures (MISS). The sedimentological characteristics of these deposits correspond to a shelf marine environment. Both U-Pb datings on detrital zircon and ichnostratigraphic criteria tend toward an Ediacaran age of the fossiliferous deposits, but we cannot exclude the possibility of a Fortunian age at the present state of knowledge.
The South Armorican Shear Zone (SASZ), in the French Armorican Variscan belt, is a lithosphericwr... more The South Armorican Shear Zone (SASZ), in the French Armorican Variscan belt, is a lithosphericwrench fault that acted during the Late Carboniferous as a transition zone between two distinct domains: a thickened domain to the south affected by extension and crustalmagmatism, and aweakly thickened domain to the north subjected to dextral wrenching and crust- and mantle-derived magmatism. The Pontivy-Rostrenen complex is a composite intrusion emplaced along the SASZ. To the south, the complex is made of leucogranites whereas, to the north, monzogranites outcrop together with small intrusions of quartz monzodiorite. U-Pb dating of magmatic zircon by LA-ICP-MS reveal that most magmatic rocks were emplaced at ca. 315 Ma (between 316.7 ± 2.5 Ma and 310.3 ± 4.7 Ma), excepted a late leucogranitic intrusion that was emplaced at 304.7± 2.7 Ma. The leucogranites (−4.8 b εNd (T) b 2.1; presence of Archean to Paleozoic inherited zircon) are strongly peraluminous (A/CNK N 1.1) and formed by partial melting ofmetasediments and peraluminous orthogneisses. Themonzogranite (−4.0 b εNd (T) b−3.2; scarce Paleozoic inherited zircon) is moderately peraluminous (1 b A/CNK b 1.3) and formed by partial melting of an orthogneiss with a probable metaluminous composition whereas the quartz monzodiorite (−3.2 b εNd (T) b −2.2; no inherited zircon) is metaluminous (0.7 b A/CNK b 1.1) and formed by partial melting of a metasomatized lithospheric mantle. The evolution of the magmas was controlled by fractional crystallization, magma mixing and/or peritectic mineral entrainment. At the scale of the Armorican Variscan belt, crustal partial melting, to the south of the SASZ, was triggered by lithospheric thinning and adiabatic decompression during extension. Conversely, to the north, asthenosphere upwelling during strike-slip deformation and subsequent slab tearing, as suggested by tomographic data, induced the melting of both the crust and the mantle fertilized during previous subduction events. This process is likely not exclusive to the Armorican Massif and may be applied to other regions in the Variscan belt, such as Iberia.
In the French Armorican Variscan belt, most of the economically significant hydrothermal U deposi... more In the French Armorican Variscan belt, most of the economically significant hydrothermal U deposits are spatially associated with peraluminous leucogranites emplaced along the south Armorican shear zone (SASZ), a dextral lithospheric scale wrench fault that recorded ductile deformation from ca. 315 to 300 Ma. In the Pontivy-Rostrenen complex, a composite intrusion, the U mineralization is spatially associated with brittle structures related to deformation along the SASZ. In contrast to monzogranite and quartz monzodiorite (3 < U < 9 ppm; Th/U > 3), the leucogranite samples are characterized by highly variable U contents (~ 3 to 27 ppm) and Th/U ratios (~ 0.1 to 5) suggesting that the crystallization of magmatic uranium oxide in the more evolved facies was followed by uranium oxide leaching during hydrothermal alteration and/or surface weathering. U-Pb dating of uranium oxides from the deposits reveals that they mostly formed between ca. 300 and 270 Ma. In monzogranite and quartz monzodiorite, apatite grains display magmatic textures and provide U-Pb ages of ca. 315 Ma reflecting the time of emplacement of the intrusions. In contrast, apatite grains from the leucogranite display textural, geochemical, and geochronological evidences for interaction with U-rich oxidized hydrothermal fluids contemporaneously with U mineralizing events. From 300 to 270 Ma, infiltration of surface-derived oxidized fluids leached magmatic uranium oxide from fertile leucogranite and formed U deposits. This phenomenon was sustained by brittle deformation and by the persistence of thermal anomalies associated with U-rich granitic bodies.
The sediments of the Mauges Unit located in the internal zone provide an opportunity of studying ... more The sediments of the Mauges Unit located in the internal zone provide an opportunity of studying the evolution of relief during Palaeozoic time. U–Pb dating on zircon and 39Ar/40Ar on white mica are used to constrain the age and nature of the sources. The first relief identified is marked by an Early Devonian unconformity interpreted as the opening of a northern back-arc basin. Detrital minerals are first reworked from underlying layers indicating a local supply. Magmatic zircons at c. 400 Ma then record the emergence of a magmatic arc. During the Middle Devonian, the gap in the sedimentary record is attributed to an emersion followed by the disappearance of the relief during the Late Devonian. At the Devonian–Carboniferous boundary, the main collision is followed by the onset of a relief. The continental sedimentation in the Ancenis Basin (late Tournaisian–Viséan) is a coarsening-upwards megasequence indicating an increasing and/or approaching relief. The detrital minerals record the progressiv
We report here new LA-ICPMS detrital zircon U-Pb ages of a quartzite from the autochthon of Pelop... more We report here new LA-ICPMS detrital zircon U-Pb ages of a quartzite from the autochthon of Peloponnesus (Feneos locality), southern Greece. The rock classifies as a mature quartz arenite and belongs to an original shale-sandstone succession now metamorphosed into a phyllite-quartzite unit. Zircon age clusters at 0.52-0.75, 0.85, 0.95-1.1, 1.75-2 and 2.4-3 Ga point at the Saharan Metacraton and the Transgondwanan Supermountain as contributing sources; the youngest concordant grain is 522 Ma old. Our data collectively suggest deposition during the Cambro-Ordovician in a collisional setting and are in excellent agreement with those of the virtually intact Cambro-Ordovician sandstone-shale sequences of Libya (Murzuq and Kufrah basins) and the Middle East (Israel and Jordan), interpreted to have been deposited in the Gondwana Super-fan System which draped the northern Gondwanan periphery from similar to 525 to 460 Ma. By contrast, re-evaluating the available zircon age-distribution pattern and depositional setting of an analogous sequence forming the autochthon of north-central Crete (Galinos beds) we demonstrate that it was originally deposited in a completely different setting, i.e. in an accretionary/fore-arc complex outboard of the south Laurussian active margin (Pelagonia) during the Late Carboniferous. Comparing similar Cambro-Ordovician metasiliciclastic rocks from north-eastern Crete (Sfaka paragneiss), north-central continental Greece (Vertiskos terrane), north-western Turkey (central Sakarya terrane) and the Romanian Carpathians we show that their detrital zircon distribution patterns testify to an original depositional setting similar to that of Peloponnesus (Feneos), Libya and the Middle East Using key time-frames from previously published palaeogeographic reconstruction models we are able to trace in space and time the Palaeozoic-Early Mesozoic wondering paths of the aforementioned sequences. Thus, time- and fades-equivalent rocks presently cropping out in the eastern Mediterranean share a common provenance from the Gondwana Super-fan System which was diachronously dispersed between Early Silurian and Early Triassic.
In the Pyrenees, in association with the rotation of the Iberian plate around Europe during the M... more In the Pyrenees, in association with the rotation of the Iberian plate around Europe during the Mid-Cretaceous, a Na-Ca metasomatism is recognized as a complementary record of the hydrothermal activity that led to Na-metasomatism (albitization) and talc-chlorite mineralization. It affected metasedimentary rocks as well as Hercynian granites. In situ laser ablation ICP-MS U-Pb analyses of titanite grains formed in albitites during metasomatism date the Na-Ca metasomatism between 110 and 92 Ma. The temperature of the Na-Ca metasomatism is estimated to be approximately 550 A degrees C. Both the time constraints and temperature estimates suggest that the Na-Ca metasomatism is related to the low-P high-T North Pyrenean metamorphism.
In the homogeneously deformed Murchison Greenstone Belt region,
fluids circulations were focused ... more In the homogeneously deformed Murchison Greenstone Belt region, fluids circulations were focused along a localized deformation brittle-ductile zone, and the so-called Antimony Line. In order to trace the origin of the fluid, its composition, the temperature of the precipitation along the Antimony Line, we investigate the stable isotope (oxygen and carbon) and fluid inclusion compositions of the quartz-carbonate veins, as well as chemical analyses of the carbonates and 40Ar-39Ar dating on the fuchsite associated with the veins. Three types of fluid inclusions are recognized for the H2O-CO2-dominated (plus minor CH4-N2) fluid(s). Microthermometric measurements together with the paragenesis of the veins and host rocks indicate that the pressure-temperature of the precipitation is 350 to 450 °C, 200 to 300 MPa, which is an unusually high temperature for an antimony deposit. The carbonates are Fe-Mg-rich and are characterized by flat MREE and HREE patterns, with slight depletions in LREE. The oxygen isotope compositions of the quartz are rather homogeneous at the scale of the Antimony Line (�18Oquartz � 10.9-14.3‰). In detail, subtle differences in the �18O values for the quartz and carbonate exist depending on the sampling sites. These differences are likely due to minor temperature and/or host rock geochemistry variations along the Antimony Line. Data point to a metamorphic origin for the fluids responsible for the mineralization, which likely escaped from the nearby greenstone lithologies. Finally, we present an overview of the Murchison Greenstone Belt geological evolution by integrating the antimony metallogeny of the Antimony Line, which can be compared to that of an orogenic gold deposit.
The Guérande peraluminous leucogranite was emplaced at the end of the Carboniferous in the southe... more The Guérande peraluminous leucogranite was emplaced at the end of the Carboniferous in the southern part of the Armorican Massif. At the scale of the intrusion, this granite displays structural heterogeneities with a weak deformation in the southwestern part, whereas the northwestern part is marked by the occurrence of S/C and mylonitic extensional fabrics. Quartz veins and pegmatite dykes orientations as well as lineations directions in the granite and its country rocks demonstrate both E–W and N–S stretching. Therefore, during its emplacement in an extensional tectonic regime, the syntectonic Guérande granite has probably experienced some partitioning of the deformation. The southwestern part is characterized by a muscovite biotite assemblage, the presence of restites and migmatitic enclaves, and a low abundance of quartz veins compared to pegmatite dykes. In contrast, the northwestern part is characterized by a muscovite–tourmaline assemblage, evidence of albitization and gresenization and a larger amount of quartz veins. The southwestern part is thus interpreted as the feeding zone of the intrusion whereas the northwestern part corresponds to its apical zone. The granite samples display continuous compositional evolutions in the range of 69.8–75.3 wt.% SiO2. High initial 87Sr/86Sr ratios and low εNd(T) values suggest that the peraluminous Guérande granite (A/CNK N 1.1) was formed by partial melting of metasedimentary formations. Magmatic evolution was controlled primarily by fractional crystallization of Kfeldspar, biotite and plagioclase (An20). The samples from the apical zone show evidence of secondary muscovitization. They are also characterized by a high content in incompatible elements such as Cs and Sn, as well as low Nb/Ta and K/Rb ratios. The apical zone of the Guérande granite underwent a pervasive hydrothermal alteration during or soon after its emplacement. U–Th–Pb dating on zircon and monazite revealed that the Guérande granite was emplaced 309.7± 1.3 Ma ago and that a late magmatic activity synchronous with hydrothermal circulation occurred at ca. 303 Ma. These new structural, petrological and geochronological data presented for the Guérande leucogranite highlight the interplay between the emplacement in an extensional tectonic regime, magmatic differentiation and hydrothermal alteration, and provide a general background for the understanding of the processes controlling some mineralization in the western European Hercynian belt.
In the Western Alps, the Money Complex of the Gran Paradiso Massif, metamorphosed under blueschis... more In the Western Alps, the Money Complex of the Gran Paradiso Massif, metamorphosed under blueschist facies during the Alpine cycle, is considered to be Permo-Carboniferous in age, but no palaeontological or radiometric data constrain this interpretation. A revision of the lithostratigraphy of the Money Complex allows recognizing a polygenic (graphite-rich) and a monogenic (graphite-poor) meta-sedimentary formation. Detrital zircon U–Pb geochronology in both meta-sedimentary formations shows that (i) the main population is Cambrian and Ordovician in age, (ii) the youngest grains are Silurian and Lower Devonian, and (iii) Carboniferous zircon grains are lacking. A careful study of the age distributions in the Alps suggests that potential source for the detrital material in the Money Complex is the Briançonnais basement. Late Carboniferous magmatism is widespread in the Helvetic Zone of the Alps. Permian magmatism is dominant in the Briançonnais, the Austroalpine and the Southalpine basements. The lack of Carboniferous zircons in the Money Complex suggests that the detritus was not shed from the Helvetic zone, which was separated from the Money basin by the Zone Houillère basin, where the main drainage pattern was developed from south to north and where the depocenters migrated northwards from the Upper Missisippian to Upper Pennsylvanian. We suggest that the Money Complex may had been located to the east of the main river drainage inside the Zone Houillère basin or alternatively may represent a small basin, located on the east of the Zone Houillère.
Deciphering Variscan versus Alpine history in the internal Rif system is a key to constraining th... more Deciphering Variscan versus Alpine history in the internal Rif system is a key to constraining the tectonic evolution of the Alboran domain and hence the geodynamics of the western Mediterranean system during the Cenozoic. This study focuses on the evolution of the metamorphic envelope of the Beni Bousera massif and its relation to the underlying peridotites. Combining structural geology, metamorphic petrology, and LA-ICP-MS U-Th-Pb dating of monazite, this study contributes to the understanding of the tectonic history of the western internal Rif. The regional foliation (S2) is characterized by LP/HT mineral assemblages and obliterates a former foliation (S1) developed along a barrovian (MP/MT) metamorphic gradient. The dating of some metamorphic monazite grains from a micaschist and a migmatitic gneiss demonstrates that the crustal envelope of the peridotite recorded two distinct tectonometamorphic episodes. Data from monazite inclusions in S1 garnet suggest that the first event, D1, is older than 250-170 Ma and likely related to the Variscan collision, in agreement with the barrovian type of the metamorphic gradient. The second event, D2, is Alpine in age (at circa 21 Ma) and corresponds to a strong lithosphere thinning allowing subsequent subcontinental mantle exhumation. Such a tectonic context provides an explanation for the LP/HT metamorphic gradient that is recorded in the regional foliation of the western Betic-Rif system. This extension is probably related to a subduction slab roll-back in the western end of the Mediterranean realm during the Oligo-Miocene times. No evidences for a Tertiary HP/LT metamorphism have been identified in the studied area.
An increasing number of field examples in mountain belts show that the formation of passive margi... more An increasing number of field examples in mountain belts show that the formation of passive margins during extreme continent thinning may occur under conditions of high to very high thermal gradient beneath a thin cover of syn-rift sediments. Orogenic belts resulting from the tectonic inversion of distal margins and regions of exhumed continental mantle may exhibit high-temperature, low-pressure (HT-LP) metamorphism and coeval syn-extensional, ductile deformation. Recent studies have shown that the northern flank of the Pyrenean belt, especially the North Pyrenean Zone, is one of the best examples of such inverted hot, passive margin. In this study, we provide a map of HT-LP metamorphism based on a data set of more than 100 peak-temperature estimates obtained using Raman spectroscopy of the carbonaceous material (RSCM). This data set is completed by previous PT (pressure and temperature) estimates based on mineral assemblages, and new 40Ar–39Ar (amphibole, micas) and U–Pb (titanite) ages from metamorphic and magmatic rocks of the North Pyrenean Zone. The implications on the geological evolution of the Cretaceous Pyrenean paleomargins are discussed. Ages range mainly from 110 to 90 Ma, and no westward or eastward propagation of the metamorphism and magmatism can be clearly identified. In contrast, the new data reveal a progressive propagation of the thermal anomaly from the base to the surface of the continental crust. Focusing on the key localities of the Mauléon basin, Arguenos–Moncaup, Lherz, Boucheville and the Bas-Agly, we analyze the thermal conditions prevailing during the Cretaceous crustal thinning. The results are synthetized into a series of three regional thematic maps and into two detailed maps of the Arguenos–Moncaup and Lherz areas. The results indicate a first-order control of the thermal gradient by the intensity of crustal thinning. The highest grades of metamorphism are intimately associated with the areas where subcontinental mantle rocks have been unroofed or exhumed.
The Ili-Balkhash Basin in southeastern Kazakhstan is located at the junction of the actively defo... more The Ili-Balkhash Basin in southeastern Kazakhstan is located at the junction of the actively deforming mountain ranges of western Junggar and the Tien Shan, and is therefore part of the southwestern Central Asian Orogenic Belt. The basement of the Ili-Balkhash area consists of an assemblage of mainly Precambrian microcontinental fragments, magmatic arcs and accretionary complexes. Eight magmatic basement samples (granitoids and tuffs) from the Ili-Balkhash area were dated with zircon U-Pb LA-ICP-MS and yield Carboniferous to late Permian (~ 350-260 Ma) crystallization ages. These ages are interpreted as reflecting the transition from subduction to (post-) collisional magmatism, related to the closure of the Junggar-Balkhash Ocean during the Carboniferous – early Permian and hence, to the final late Paleozoic accretion history of the ancestral Central Asian Orogenic Belt. Apatite fission track (AFT) dating of 14 basement samples (gneiss, granitoids and volcanic tuffs) mainly provides Cretaceous cooling ages. Thermal history modeling based on the AFT data reveals that several intracontinental tectonic reactivation episodes affected the studied basement during the late Mesozoic and Cenozoic. Late Mesozoic reactivation and associated basement exhumation is interpreted as distant effects of the Cimmerian collisions at the southern Eurasian margin and possibly of the Mongol-Okhotsk Orogeny in SE Siberia during the Jurassic – Cretaceous. Following tectonic stability during the Palaeogene, inherited basement structures were reactivated during the Neogene (constrained by Miocene AFT ages of ~ 17–10 Ma). This late Cenozoic reactivation is interpreted as the far-field response of the India-Eurasia collision and reflects the onset of modern mountain building and denudation in southeast Kazakhstan, which seems to be at least partially controlled by the inherited basement architecture.
In the Saint-Aubin-des-Châteaux deposit (Massif Armoricain, France), the Ordovician Grès Armorica... more In the Saint-Aubin-des-Châteaux deposit (Massif Armoricain, France), the Ordovician Grès Armoricain sandstones have undergone several fluid-rock interaction events, including diagenetic cementation and orogenic base metal-As-Sb-Au mineralisation. Ironstone layers interbedded in the sandstones contain several generations of spectacular authigenic xenotime overgrowths that formed around detrital zircon grains in response to successive hydrothermal events. Textural and chemical characterisations allow to distinguish three generations of xenotime overgrowths, differing notably in their REE characteristics. In-situ U-Pb data obtained on these xenotime overgrowths show that their U-Pb systematics were largely disturbed by successive hydrothermal events over about 90 Ma between ~ 330 and ~ 420 Ma, a time interval encompassing most phases of the construction of the Variscan orogen in France. The younger dates cluster around ~ 330–340 Ma and likely correspond to the age of the deposition of massive sulphides and base-metals in the Saint-Aubin-des-Châteaux deposits, which is consistent with the structural contexts where they formed. Finally, this study shows that similarly to monazite, another phosphate widely used for U-Pb and Th-Pb dating studies, the U-Pb chronometric system in xenotime appears to be highly sensitive to fluid circulations.
The Chalkidiki block is a major domain in the North Aegean that, contrary to other domains in the... more The Chalkidiki block is a major domain in the North Aegean that, contrary to other domains in the region, largely escaped thermal perturbations during Tertiary extension. As a result, the Chalkidiki block is an ideal candidate to glean information related to the timing of Mesozoic thermal events using appropriate geochronological techniques. We have undertaken a laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) study (U-Th-Pb on monazites and U-Pb on zircons) coupled with 40Ar/39Ar dating on nine samples from various structural levels within the thrust system of the Chalkidiki block. The eastern, and structurally lower part of the system revealed a complete isotopic reset of Carboniferous – Early Triassic monazites coeval with partial monazite destruction, REE-mobilisation and formation of apatite-allanite-epidote coronas at ~ 132 Ma, a reaction that is commonly observed in amphibolite-facies rocks. These coronas formed after crystallisation of garnet (i.e., at T > 580 °C) and, in all probability, either close to the peak-temperature conditions (~ 620 °C) on a prograde path or during retrogression between the peak-temperature and the low-temperature boundary of the amphibolite facies. Cooling of these rocks and arrival at mid-crustal levels occurred at 95–100 Ma. By contrast, the western, and structurally uppermost part of the system went through the same event by 120–125 Ma. Further structural considerations with respect to medium-temperature geochronology data imply that syn-metamorphic thrusting must have ceased by early Late Cretaceous. We emphasize that, with the sole exception of the Chalkidiki block, no pre-45 Ma medium-temperature geochronology data are preserved in other North Aegean domains, a feature that is clearly related to the extension-induced thermal perturbation of the region during the Tertiary.
New U-Pb dating on zircon yielded ca. 470 Ma ages for the granitoids from the Lévézou massif in t... more New U-Pb dating on zircon yielded ca. 470 Ma ages for the granitoids from the Lévézou massif in the southern French Massif Central. These new ages do not support the previous interpretation of these granitoids as syn-tectonic intrusions emplaced during the late Devonian-early Carboniferous thrusting. The geochemical and isotopic nature of this magmatism is linked to a major magmatic Ordovician event recorded throughout the European Variscan belt and related to extreme thinning of continental margins during a rifting event or a back arc extension. The comparable isotopic signatures of these granitoids on each side of the eclogite-bearing leptyno-amphibolitic complex in the Lévézou massif, together with the fact that they were emplaced at the same time, strongly suggest that these granitoids were originally part of a single unit, tectonically duplicated by either isoclinal folding or thrusting during the Variscan tectonics.
One of the striking features that characterise the late stages of the Variscan orogeny is the dev... more One of the striking features that characterise the late stages of the Variscan orogeny is the development of gneiss and migmatite domes, as well as extensional Late Carboniferous and Permian sedimentary basins. It remains a matter of debate whether the formation of domes was related to the well-documented late orogenic extension or to the contractional tectonics that preceded. Migmatization and magmatism are expected to predate extension if the domes are compression-related regional anticlines, but they must both precede and be contemporaneous with extension if they are extensional core complexes. In the Montagne Noire area (southern French Massif Central), where migmatization, magmatism and the deformation framework are well documented, the age of the extensional event was unequivocally constrained to 300–290 Ma. Therefore, dating migmatization in this area is a key point for discriminating between the two hypotheses and understanding the Late Palaeozoic evolution of this part of the Variscan belt. For this purpose, a migmatite and an associated anatectic granite from the Montagne Noire dome were dated by LA-ICP-MS (U–Th–Pb on zircon and monazite) and laser probe 40Ar-39Ar (K–Ar on muscovite). Although zircon did not record any Variscan age unequivocally related to compression (380-330 Ma), two age groups were identified from the monazite crystals. A first event, at ca. 319 Ma (U–Th–Pb on monazite), is interpreted as a first stage of migmatization and as the emplacement age of the granite, respectively. A second event at ca. 298–295 Ma, recorded by monazite (U–Th–Pb) and by the muscovite 40Ar-39Ar system in the migmatite and in the granite, could be interpreted as a fluid-induced event, probably related to a second melting event identified through the syn-extensional emplacement of the nearby Montalet leucogranite ca. 295 Ma ago. The ages of these two events post-date the Variscan compression and agree with an overall extensional context for the development of the Montagne Noire dome-shaped massif. Comparison of these results with published chemical (EPMA) dating of monazite from the same rocks demonstrates that the type of statistical treatment applied to EPMA data is crucial in order to resolve different monazite age populations.
The origin of lead in the atmosphere of Johannesburg, South Africa was investigated on the basis ... more The origin of lead in the atmosphere of Johannesburg, South Africa was investigated on the basis of elemental and lead isotopic analyses of coals, mine dumps, gasoline, and about 30 epiphytic lichen samples. Lead predominantly comes from automotive exhausts in urban and suburban areas, as leaded antiknock additives were still in use in South Africa at the time of the study. Although dust emissions from the numerous mine-tailing dumps were expected to contribute significantly to the heavy metal budget, the southern townships that are surrounded by the dumps (such as Soweto and other historically Black residential areas) do not appear to be more than partially influenced by them, and this influence seems to be geographically limited. Domestic coal burning, suspected to account for the total lead content in the air, is also recognised, but only acts as a minor source of lead, even in townships.
Uploads
acted during the Late Carboniferous as a transition zone between two distinct domains: a thickened domain to the
south affected by extension and crustalmagmatism, and aweakly thickened domain to the north subjected to dextral
wrenching and crust- and mantle-derived magmatism. The Pontivy-Rostrenen complex is a composite intrusion
emplaced along the SASZ. To the south, the complex is made of leucogranites whereas, to the north,
monzogranites outcrop together with small intrusions of quartz monzodiorite. U-Pb dating of magmatic zircon
by LA-ICP-MS reveal that most magmatic rocks were emplaced at ca. 315 Ma (between 316.7 ± 2.5 Ma and
310.3 ± 4.7 Ma), excepted a late leucogranitic intrusion that was emplaced at 304.7± 2.7 Ma. The leucogranites
(−4.8 b εNd (T) b 2.1; presence of Archean to Paleozoic inherited zircon) are strongly peraluminous (A/CNK N 1.1)
and formed by partial melting ofmetasediments and peraluminous orthogneisses. Themonzogranite (−4.0 b εNd
(T) b−3.2; scarce Paleozoic inherited zircon) is moderately peraluminous (1 b A/CNK b 1.3) and formed by partial
melting of an orthogneiss with a probable metaluminous composition whereas the quartz monzodiorite (−3.2 b
εNd (T) b −2.2; no inherited zircon) is metaluminous (0.7 b A/CNK b 1.1) and formed by partial melting of a
metasomatized lithospheric mantle. The evolution of the magmas was controlled by fractional crystallization,
magma mixing and/or peritectic mineral entrainment. At the scale of the Armorican Variscan belt, crustal partial
melting, to the south of the SASZ, was triggered by lithospheric thinning and adiabatic decompression during extension.
Conversely, to the north, asthenosphere upwelling during strike-slip deformation and subsequent slab
tearing, as suggested by tomographic data, induced the melting of both the crust and the mantle fertilized during
previous subduction events. This process is likely not exclusive to the Armorican Massif and may be applied to
other regions in the Variscan belt, such as Iberia.
fluids circulations were focused along a localized deformation brittle-ductile zone, and
the so-called Antimony Line. In order to trace the origin of the fluid, its composition,
the temperature of the precipitation along the Antimony Line, we investigate the stable
isotope (oxygen and carbon) and fluid inclusion compositions of the quartz-carbonate
veins, as well as chemical analyses of the carbonates and 40Ar-39Ar dating on the
fuchsite associated with the veins.
Three types of fluid inclusions are recognized for the H2O-CO2-dominated (plus
minor CH4-N2) fluid(s). Microthermometric measurements together with the paragenesis
of the veins and host rocks indicate that the pressure-temperature of the precipitation
is 350 to 450 °C, 200 to 300 MPa, which is an unusually high temperature for an
antimony deposit. The carbonates are Fe-Mg-rich and are characterized by flat MREE
and HREE patterns, with slight depletions in LREE. The oxygen isotope compositions
of the quartz are rather homogeneous at the scale of the Antimony Line (�18Oquartz �
10.9-14.3‰). In detail, subtle differences in the �18O values for the quartz and
carbonate exist depending on the sampling sites. These differences are likely due to
minor temperature and/or host rock geochemistry variations along the Antimony Line.
Data point to a metamorphic origin for the fluids responsible for the mineralization,
which likely escaped from the nearby greenstone lithologies. Finally, we present an
overview of the Murchison Greenstone Belt geological evolution by integrating the
antimony metallogeny of the Antimony Line, which can be compared to that of an
orogenic gold deposit.
biotite and plagioclase (An20). The samples from the apical zone show evidence of secondary muscovitization. They are also characterized by a high content in incompatible elements such as Cs and Sn, as well as low Nb/Ta and K/Rb ratios. The apical zone of the Guérande granite underwent a pervasive hydrothermal alteration during or soon after its emplacement. U–Th–Pb dating on zircon and monazite revealed that the Guérande granite was emplaced 309.7± 1.3 Ma ago and that a late magmatic activity synchronous with hydrothermal circulation occurred at ca. 303 Ma. These new structural, petrological and geochronological data presented for the Guérande leucogranite highlight the interplay between the emplacement in an extensional tectonic regime, magmatic differentiation and hydrothermal alteration, and provide a general background for the understanding of the processes controlling some mineralization in the western European Hercynian belt.
acted during the Late Carboniferous as a transition zone between two distinct domains: a thickened domain to the
south affected by extension and crustalmagmatism, and aweakly thickened domain to the north subjected to dextral
wrenching and crust- and mantle-derived magmatism. The Pontivy-Rostrenen complex is a composite intrusion
emplaced along the SASZ. To the south, the complex is made of leucogranites whereas, to the north,
monzogranites outcrop together with small intrusions of quartz monzodiorite. U-Pb dating of magmatic zircon
by LA-ICP-MS reveal that most magmatic rocks were emplaced at ca. 315 Ma (between 316.7 ± 2.5 Ma and
310.3 ± 4.7 Ma), excepted a late leucogranitic intrusion that was emplaced at 304.7± 2.7 Ma. The leucogranites
(−4.8 b εNd (T) b 2.1; presence of Archean to Paleozoic inherited zircon) are strongly peraluminous (A/CNK N 1.1)
and formed by partial melting ofmetasediments and peraluminous orthogneisses. Themonzogranite (−4.0 b εNd
(T) b−3.2; scarce Paleozoic inherited zircon) is moderately peraluminous (1 b A/CNK b 1.3) and formed by partial
melting of an orthogneiss with a probable metaluminous composition whereas the quartz monzodiorite (−3.2 b
εNd (T) b −2.2; no inherited zircon) is metaluminous (0.7 b A/CNK b 1.1) and formed by partial melting of a
metasomatized lithospheric mantle. The evolution of the magmas was controlled by fractional crystallization,
magma mixing and/or peritectic mineral entrainment. At the scale of the Armorican Variscan belt, crustal partial
melting, to the south of the SASZ, was triggered by lithospheric thinning and adiabatic decompression during extension.
Conversely, to the north, asthenosphere upwelling during strike-slip deformation and subsequent slab
tearing, as suggested by tomographic data, induced the melting of both the crust and the mantle fertilized during
previous subduction events. This process is likely not exclusive to the Armorican Massif and may be applied to
other regions in the Variscan belt, such as Iberia.
fluids circulations were focused along a localized deformation brittle-ductile zone, and
the so-called Antimony Line. In order to trace the origin of the fluid, its composition,
the temperature of the precipitation along the Antimony Line, we investigate the stable
isotope (oxygen and carbon) and fluid inclusion compositions of the quartz-carbonate
veins, as well as chemical analyses of the carbonates and 40Ar-39Ar dating on the
fuchsite associated with the veins.
Three types of fluid inclusions are recognized for the H2O-CO2-dominated (plus
minor CH4-N2) fluid(s). Microthermometric measurements together with the paragenesis
of the veins and host rocks indicate that the pressure-temperature of the precipitation
is 350 to 450 °C, 200 to 300 MPa, which is an unusually high temperature for an
antimony deposit. The carbonates are Fe-Mg-rich and are characterized by flat MREE
and HREE patterns, with slight depletions in LREE. The oxygen isotope compositions
of the quartz are rather homogeneous at the scale of the Antimony Line (�18Oquartz �
10.9-14.3‰). In detail, subtle differences in the �18O values for the quartz and
carbonate exist depending on the sampling sites. These differences are likely due to
minor temperature and/or host rock geochemistry variations along the Antimony Line.
Data point to a metamorphic origin for the fluids responsible for the mineralization,
which likely escaped from the nearby greenstone lithologies. Finally, we present an
overview of the Murchison Greenstone Belt geological evolution by integrating the
antimony metallogeny of the Antimony Line, which can be compared to that of an
orogenic gold deposit.
biotite and plagioclase (An20). The samples from the apical zone show evidence of secondary muscovitization. They are also characterized by a high content in incompatible elements such as Cs and Sn, as well as low Nb/Ta and K/Rb ratios. The apical zone of the Guérande granite underwent a pervasive hydrothermal alteration during or soon after its emplacement. U–Th–Pb dating on zircon and monazite revealed that the Guérande granite was emplaced 309.7± 1.3 Ma ago and that a late magmatic activity synchronous with hydrothermal circulation occurred at ca. 303 Ma. These new structural, petrological and geochronological data presented for the Guérande leucogranite highlight the interplay between the emplacement in an extensional tectonic regime, magmatic differentiation and hydrothermal alteration, and provide a general background for the understanding of the processes controlling some mineralization in the western European Hercynian belt.
isotopic analyses of coals, mine dumps, gasoline, and about 30 epiphytic lichen samples. Lead predominantly comes from
automotive exhausts in urban and suburban areas, as leaded antiknock additives were still in use in South Africa at the
time of the study. Although dust emissions from the numerous mine-tailing dumps were expected to contribute
significantly to the heavy metal budget, the southern townships that are surrounded by the dumps (such as Soweto and
other historically Black residential areas) do not appear to be more than partially influenced by them, and this influence
seems to be geographically limited. Domestic coal burning, suspected to account for the total lead content in the air, is also
recognised, but only acts as a minor source of lead, even in townships.