Simon Couzinié

@article{Laurent_etal2023,

title = {Timescales of ultra-high temperature metamorphism and crustal differentiation: Zircon petrochronology from granulite xenoliths of the Variscan French Massif Central},

author = {O. Laurent and S. Couzini\'{e} and L. S. Doucet},

doi = {10.1016/j.epsl.2023.118133},

year  = {2023},

date = {2023-01-01},

journal = {Earth and Planetary Science Letters},

volume = {611},

pages = {118133},

abstract = {Lower crustal ultra-high temperature (UHT) metamorphism and the resulting production and transfer of granitic magmas represent key processes of intracrustal differentiation. The timescales of these phenomena are debated, due to the complexity of the granulite zircon U-Pb record and because direct links between granulites and granites are difficult to establish. To address these issues, we present the results of zircon petrochronology (coupled U-Pb dating, trace element and Lu-Hf isotopic analyses) for lower crustal felsic and mafic granulite xenoliths from the Variscan eastern French Massif Central and compare them with data from well-characterized mid-/upper crustal migmatites and granites. The felsic and mafic granulites represent pre-Variscan meta-sedimentary and meta-igneous mafic rocks, respectively; which experienced Variscan UHT peak metamorphism at 940--970\^{a}—¦C and 8 textpm2 kbar. Zircons from the felsic granulites show U-Pb dates spreading over \^{a}`u50 Myr between \^{a}`u315 and \^{a}`u265 Ma, correlated to Ti-in-zircon temperatures decreasing from 940--970\^{a}—¦C to 800\^{a}—¦C and REE contents consistent with growth in equilibrium with garnet at (U)HT conditions. This is best explained by continuous crystallization of zircon upon cooling from the thermal peak owing to decreasing Zr solubility of residual melt, despite significant prograde melt loss. Zircons from the mafic granulite only record the last stage of this time-temperature evolution (295--265 Ma; 800--900\^{a}—¦C) due to later zircon saturation. Upper crustal granite emplacement started at 340 Ma and culminated at the age of the lower crustal thermal peak of 313 textpm3 Ma (defined by the highest-temperature zircons from felsic granulites), reflecting melt extraction along the \^{a}`u27 Myr prograde path of the lower crust. In turn, the crystallization ages of mid-crustal migmatites (315--300 Ma) and the lower crustal granulites (315--265 Ma) are consistent with slow cooling in the presence of melt. These results provide a direct assessment of the timescales of melt production and residence at the crustal scale; and validate the granulite-granite connection in the framework of the melt loss theory in migmatitic systems},

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@article{Condamine_etal2022,

title = {Trace element partitioning during incipient melting of phlogopite-peridotite in the spinel and garnet stability fields},

author = {P. Condamine and S. Couzini\'{e} and A. Fabbrizio and J. L. Devidal and E. M\'{e}dard},

doi = {10.1016/j.gca.2022.04.011},

year  = {2022},

date = {2022-01-01},

journal = {Geochimica et Cosmochimica Acta},

volume = {327},

pages = {53--78},

abstract = {Potassium-rich magmatism represents subordinate magma volumes worldwide but has been observed in many geodynamic settings. Most potassic magmas are thought to derive from very-low degrees of melting of metasomatized mantle lithologies.We performed piston-cylinder experiments to determine trace element partition coefficients between incipient potassic silicate melts and phlogopite textpm pargasite peridotite in the spinel (1 GPa) and garnet stability fields (3 GPa). Most of the rare Earth elements (REEs) are compatible in pargasite but incompatible in phlogopite. Although garnet remains the mineral phase most efficiently fractionating heavy from light REEs (Dgrt=melt Yb =Dgrt=melt La \> 750), orthopyroxene can also significantly fractionate REEs, e.g., achieving Dopx=melt Yb =Dopx=melt La \> 100 at 3 GPa. Mineral-liquid partition coefficients vary by about one order of magnitude between incipient melts derived from the spinel and garnet stability fields. We thus show that trace element partitioning at the onset of melting is controlled more by pressure (through melt composition) than by the extent of melting. With increasing pressure, Rb and Ba exhibit different behaviors in phlogopite, with Dphl=melt Ba \> Dphl=melt Rb at 1 GPa and the opposite at 3 GPa. At 1 GPa, a decrease of melt polymerization (lower NBO/T) with increasing melt fraction translates into a significant decrease of most phlogopite partition coefficients. Finally, we show that resolvable inter-element fractionations do occur when phlogopite- (and pargasite)-bearing peridotite are melted, indicating that trace element ratios are not always faithfully representative of that of their sources but bear the imprint of varied P--T conditions of melting and contrasted pre-metasomatic histories. This self-consistent partition coefficient dataset thus gives a new scope to understand the complex petrogenesis of K-rich magmas in orogenic settings.},

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@article{Couzini_etal2022,

title = {Cambro--Ordovician ferrosilicic magmatism along the northern Gondwana margin: constraints from the C\'{e}zarenque--Joyeuse gneiss complex (French Massif Central)},

author = {S. Couzini\'{e} and P. Bouilhol and O. Laurent and T. Grocolas and J. M. Montel},

doi = {10.1051/bsgf/2022010},

year  = {2022},

date = {2022-01-01},

journal = {BSGF- Earth Sciences Bulletin},

volume = {193},

number = {15},

abstract = {It is well-acknowledged that the northern margin of the Gondwana supercontinent was affected by a major magmatic event at late Cambrian (Furongian) to early Ordovician (Tremadocian--Floian) times. However, an accurate assessment of its extent, origin, and significance is partly hampered by the incomplete characterization of the numerous gneiss massifs exposed in the inner part of the Variscan belt, as some of them possibly represent dismembered and deformed Furongian--Lower Ordovician igneous bodies. In this study, we document the case of the ‘‘C\'{e}zarenque--Joyeuse’’ gneiss complex in the C\'{e}vennes parautochthon domain of the French Massif Central. The gneisses form decametre- to kilometre-thick concordant massifs interlayered within a pluri-kilometric sequence of mica- and quartz schists. They encompass two main petrological types: augen gneisses and albite gneisses, both typified by their blue and engulfed quartz grains with the augen facies differing by the presence of centimetre-sized pseudomorphs after K-feldspar and the local preservation of igneous textures. Whole-rock geochemistry highlights that many gneisses have magmatic ferrosilicic (acidic with anomalously high FeOt and low CaO) compositions while others are akin to greywackes. Collectively, it is inferred that the bulk of the C\'{e}zarenque--Joyeuse gneisses represents former rhyodacite lava flows or ignimbrites and associated epiclastic tuffs. Volumetrically subordinate, finer grained, and strongly silicic leucogneisses are interpreted as microgranite dykes originally intrusive within the volcanic edifices. LA--ICP--MS U--Pb dating of magmatic zircon grains extracted from an augen gneiss and a leucogneiss brackets the crystallization age of the silicic magmas between 486.1thinspacetextpmthinspace5.5thinspaceMa and 483.0thinspacetextpmthinspace5.5thinspaceMa which unambiguously ties the C\'{e}zarenque--Joyeuse gneisses to the Furongian--Lower Ordovician volcanic belt of SW Europe. Inherited zircon date distributions, Ti-in-zircon and zircon saturation thermometry demonstrate that they formed by melting at 750--820thinspacetextdegreeC of Ediacaran sediments. Zircon Eu/Eu* and Ce/Ce* systematics indicate that the melts were strongly reduced (fO2 probably close to the values expected for the iron--w\"{u}stite buffer), possibly because they interacted during ascent with Lower Cambrian black shales. This would have enhanced Fe solubility in the melt phase and may explain the peculiar ferrosilicic signature displayed by many Furongian--Lower Ordovician igneous rocks in the northern Gondwana realm. We infer that crustal melting resulted from a combination of mantle-derived magma underplating in an intracontinental rift setting and anomalously elevated radiogenic heat production within the Ediacaran sedimentary sequences.},

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@article{Couzini_etal2021,

title = {When zircon drowns: Elusive geochronological record of water-fluxed orthogneiss melting in the Velay dome (Massif Central, France)},

author = {S. Couzini\'{e} and P. Bouilhol and O. Laurent and L. Marko and J. F. Moyen},

doi = {10.1016/j.lithos.2020.105938},

year  = {2021},

date = {2021-01-01},

journal = {Lithos},

volume = {384-385},

pages = {106938},

abstract = {Zircon U-Pb geochronology is routinely performed to unravel the timing and duration of melting events within the continental crust. A comprehensive understanding of the zircon behavior during anatexis is therefore paramount to accurately assess the impact of crustalmelting on orogenic processes.Wereport on an anatectic systemfrom the southern part of the Velay dome (Variscan French Massif Central) encompassing late Carboniferous migmatites developed at the expense of peraluminous orthogneisses and associated with (leuco)granite bodies intrusive at higher structural levels. Our zircon U-Pb geochronological survey (494 analyzed grains) evidences a very scant record of the Variscan anatectic event. Indeed, oscillatory-zoned melt-precipitated zircon grains extracted from metatexites, leucosomes and (leuco)granites almost systematically yielded the late Neoproterozoic crystallization age of the felsic igneous protolith. Field and petrographic observations, whole-rock geochemical signatures, phase equilibriumand thermodynamically-constrained trace elementmodelling collectively indicatethat partialmelting of the orthogneisses took place at T textasciitilde 700 textdegreeC andwas fluxed by ingress of externalwater. The lack of new zircon growth is ascribed to a range of processes ultimately related to the low melting temperature including limited zircon solubility in the melt phase, occlusion of zircon in non-reacting source biotite and sluggish zircon dissolution kinetics. Peraluminous orthogneisses, yet highly fusible due to their textquotelefttextquotelefteutectictextquoterighttextquoteright subsolidus mineral assemblage, exhibit a very limited zircon record of low temperature water-fluxed melting. Thus, zirconhas little potential to provide reliable chronological constraints on water-fluxed melting episodes affecting such lithologies},

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@article{Couzini_etal2020,

title = {Crystalline inliers near Lake Iro (SE Chad) : Post-collisional Ediacaran A2-type granitic magmatism at the southern margin of the Saharan Metacraton},

author = {S. Couzini\'{e} and R. P. M\'{e}not and J. C. Doumnang and J. L. Paquette and P. Rochette and Y. Quesnel and P. Deschamps and G. M\'{e}not},

doi = {10.1016/j.jafrearsci.2020.103960},

year  = {2020},

date = {2020-01-01},

journal = {Journal of African Earth Sciences},

volume = {172},

pages = {103960},

abstract = {The structure and evolution of the continental crust in the southern part of the Saharan Metacraton (central Africa) is poorly known due to extensive laterite and sediment cover as well as geographic isolation. We report on a series of five crystalline inliers newly discovered near Lake Iro (south-eastern Chad) that offered the opportunity to unravel the nature of the local basement in this area. Two spatially determined petrographic types are exposed in the inliers: a medium-grained porphyritic amphibole--biotite granite and a biotite microgranite with typical embayed (rhyolitic) quartz phenocrysts, both containing fine-grained melanocratic igneous enclaves. Some porphyritic granites display evidence for low temperature deformation along N60 shear zones. The Lake Iro igneous rocks are typified by their ferroan mineralogy, define a ferroan alkali-calcic metaluminous to weakly peraluminous potassic association, and exhibit elevated HFSE (Zr, Nb, Y) and REE contents. Therefore, they are best classified as A2-type granites. They are variably ferromagnetic with total magnetization lower than 1 A/m. The Lake Iro granites most likely correspond to a single subvolcanic--plutonic silicic complex because: (i) first-order geochemical modelling indicates that the microgranites can be generated by 30% fractional crystallization of a two feldspar--amphibole assemblage from a porphyritic granite melt and (ii) both granite types yield zircon U--Pb emplacement ages consistent within a 575.3 textpm 5.6 to 581.3 textpm 3.8 Ma time frame. The Lake Iro granites thus represent the oldest of a series of post-collisional igneous associations exposed in southern Chad and nearby countries which, collectively, show that the amalgamation of the constituent blocks of the southern Saharan Metacraton is older than 580 Ma.},

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@article{Couzini_etal2019,

title = {Detrital zircon U--Pb--Hf systematics of Ediacaran metasediments from the French Massif Central:Consequence Detrital zircon U--Pb--Hf systematics of Ediacaran metasediments from the French Massif Central: Consequences for the crustal evolution of the north Gondwana margin},

author = {S. Couzini\'{e} and O. Laurent and C. Chelle-Michou and P. Bouilhol and J. L. Paquette and Gannoun A.M. and J. F. Moyen},

doi = {10.1016/j.precamres.2019.01.016},

year  = {2019},

date = {2019-01-01},

journal = {Precambrian Research},

volume = {324},

pages = {269--284},

abstract = {Combining U--Pb and Lu-Hf isotopic data of detrital zircon grains has proven a powerful tool to unravel the provenance of sediments and address continental crust evolution. In this study, we explore the origin of thick siliciclastic metasedimentary units from the high-grade internal domains of the Variscan belt of Europe and examine their significance for Neoproterozoic crust formation and evolution along the North Gondwana margin. We present data and U--Pb/Lu--Hf systematics of detrital zircons from five amphibolite-facies metasediments sampled in the eastern French Massif Central, measured in situ by LA--(MC)--ICP--MS. The sedimentary protoliths were deposited in the Ediacaran as evidenced by field relationships and maximum depositional ages ranging between 592.4 textpm 5.5 and 556.8 textpm 5.1 Ma. All samples contain three main zircon populations in terms of age distribution and Hf isotopes: (i) abundant 0.55--0.65 Ga zircons with considerably scattered $epsilon$Hf(t) from −19 to +14; (ii) varied amounts of 0.65--1.0 Ga zircons showing dominantly positive $epsilon$Hf(t) with the exception of the c. 1.0 Ga zircons; and (iii) �W1.8 Ga zircons (mostly between 1.9--2.1 and 2.5--2.8, up to 3.2 Ga) with $epsilon$Hf(t) ranging between +5 and −7. Multidimensional scaling of our U--Pb dataset and a data compilation of Ediacaran to Lower Cambrian (meta)sediments from the North Gondwana margin reveals that the relative proportions of the three age components carry discriminant provenance information. Geological data indicate that the Ediacaran basins of the eastern French Massif Central collected the erosion products of two main source regions: (i) the Neoproterozoic Cadomian magmatic arc; and (ii) the cratonic hinterland, mostly from the Saharan Metacraton and the Arabian-Nubian Shield. Our dataset attests to the reworking of: (i) juvenile Neoproterozoic crust and, (ii) old Paleoproterozoic to Neoarchean crustal components, either as part of the detritus or incorporated in Neoproterozoic arc magmas. First-order estimates derived from the isotopic signature of S-type granitic magmas sourced in the Ediacaran metasediments suggest that 60--75% of the detritus would correspond to young Neoproterozoic crust and thus represent net additions to the continental crust volume.},

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