Videos by Michel Gregoire
Presentation of the "Observatoire Midi-Pyrénées" and of the laboratory Geosciences Environnement ... more Presentation of the "Observatoire Midi-Pyrénées" and of the laboratory Geosciences Environnement Toulouse. Presented at the 2014 Geological Society of America (GSA) Annual Meeting (Vancouver, Canada)
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Papers by Michel Gregoire
Journal of African Earth Sciences
The Pan-African Fe-rich alkaline Bauchi Complex (Central Basement of Nigeria) is located at the j... more The Pan-African Fe-rich alkaline Bauchi Complex (Central Basement of Nigeria) is located at the junction of the N-S Trans-Saharan Belt (TBS) and the E-W Central African Orogenic Belt (CAOB). It is made of a peculiar metaluminous olivine-clinopyroxene-quartz monzonitemonzogabbro suite hosted by migmatitic paragneiss. Previous models for the Bauchi Complex
Frontiers in Earth Sciences, Jan 2026
The Castillon massif, in the northern Pyrenees, features a complex of decimetric
to metric ultram... more The Castillon massif, in the northern Pyrenees, features a complex of decimetric
to metric ultramafic and mafic layers emplaced within metasedimentary
series (from the bottom to the top: garnet, sillimanite, and kyanite-bearing
gneisses and sillimanite + cordierite-bearing gneisses). Ultramafic and mafic
layers and metasediments have been deformed and metamorphosed under
granulitic facies conditions during the Hercynian orogenesis. The mineralogical,
petrologic, and geochemical characteristics of the studied samples allow us
to define two distinct series: 1) a pyroxene-bearing magmatic series (UM-M1)
consisting of ultramafic (UM: dunites, harzburgites, and orthopyroxenites) and
mafic (M1: norites, gabbro–norites, and gabbros) rocks; and 2) a pyroxenefree
and hornblende-bearing series (M2; mela-, meso-, and leucogabbros).
The leucogabbros exhibit some characteristics of anorthosites, including
the high Al2O3 whole-rock content (31 wt%), high An content (An84–96)
in plagioclase, weak rare earth element enrichment, and very positive Eu
anomalies. We propose that the rocks of the ultramafic and pyroxenebearing
rock-series (UM-M1 series) are all associated with the same magmatic
event and that the M2 series rocks are associated with a distinct separate
event. Isotopic data suggest that these formations are Ordovician. The M2
rocks have juvenile Nd isotopic signatures (εNd(460) from +4.59 to +8.11),
suggesting that they are derived from superheated alumina-rich basaltic or
basaltic–andesite melts extracted from a relatively depleted mantle source.
In contrast, most of the M1 rocks derived from parental basaltic melts
show partially crustal contamination, with only a few clearly juvenile samples
(εNd(460) from +0.45 to +6.59). We propose a geodynamic evolution for
the Castillon massif involving a two-stage genesis and the emplacement of
the two series. First, the emplacement of olivine-saturated basaltic melts
in a deep metasedimentary crust resulted in the M1 series. The second
step involves the emplacement of alumina-rich basaltic or basaltic–andesite
melts to produce the M2 hornblende-bearing series (mela-, meso-, and
leucogabbros) devoid of pyroxenes. The leucogabbros show strong similarities with common anorthosites although they have not been previously observed in the Variscan Pyrenees.
to metric ultramafic and mafic layers emplaced within metasedimentary
series (from the bottom to the top: garnet, sillimanite, and kyanite-bearing
gneisses and sillimanite + cordierite-bearing gneisses). Ultramafic and mafic
layers and metasediments have been deformed and metamorphosed under
granulitic facies conditions during the Hercynian orogenesis. The mineralogical,
petrologic, and geochemical characteristics of the studied samples allow us
to define two distinct series: 1) a pyroxene-bearing magmatic series (UM-M1)
consisting of ultramafic (UM: dunites, harzburgites, and orthopyroxenites) and
mafic (M1: norites, gabbro–norites, and gabbros) rocks; and 2) a pyroxenefree
and hornblende-bearing series (M2; mela-, meso-, and leucogabbros).
The leucogabbros exhibit some characteristics of anorthosites, including
the high Al2O3 whole-rock content (31 wt%), high An content (An84–96)
in plagioclase, weak rare earth element enrichment, and very positive Eu
anomalies. We propose that the rocks of the ultramafic and pyroxenebearing
rock-series (UM-M1 series) are all associated with the same magmatic
event and that the M2 series rocks are associated with a distinct separate
event. Isotopic data suggest that these formations are Ordovician. The M2
rocks have juvenile Nd isotopic signatures (εNd(460) from +4.59 to +8.11),
suggesting that they are derived from superheated alumina-rich basaltic or
basaltic–andesite melts extracted from a relatively depleted mantle source.
In contrast, most of the M1 rocks derived from parental basaltic melts
show partially crustal contamination, with only a few clearly juvenile samples
(εNd(460) from +0.45 to +6.59). We propose a geodynamic evolution for
the Castillon massif involving a two-stage genesis and the emplacement of
the two series. First, the emplacement of olivine-saturated basaltic melts
in a deep metasedimentary crust resulted in the M1 series. The second
step involves the emplacement of alumina-rich basaltic or basaltic–andesite
melts to produce the M2 hornblende-bearing series (mela-, meso-, and
leucogabbros) devoid of pyroxenes. The leucogabbros show strong similarities with common anorthosites although they have not been previously observed in the Variscan Pyrenees.
Canadian Journal of Earth Sciences, 2025
The Dolodau syenite–carbonatite complex is one of several syntectonic intrusions of the Abitibi g... more The Dolodau syenite–carbonatite complex is one of several syntectonic intrusions of the Abitibi greenstone belt, Superior craton, Canada. It formed during the syntectonic period, which is the main deformation stage leading to craton stabilization. Such magma can be involved in intrusion-related gold systems (IRGS) and be overprinted by orogenic gold systems (OGS), with IRGS and OGS being the main gold mineralizing systems of the syntectonic period. Determining whether magmas of the syntectonic period can transport enough gold to fertilize the upper crust requires insights into the crystallizing conditions and the volatile content of these magmas. Retrieving such data from the altered rocks generally encountered in mineralized areas is best accomplished by accumulating case studies. This study focuses on the Au–W–Ag mineralization spatially associated with the Dolodau intrusive complex to evaluate its fertility for Au. Using field relationships, petrographic observations, whole rock geochemical analyses, and mineral chemistry (titanite, apatite), we show that Dolodau is a carbonatite–syenite–granodiorite intrusive suite that crystallized rapidly at shallow depth in an active fault zone. The magma was S- and Cl-poor according to apatite chemistry, and it was likely incapable of transporting a significant amount of gold through the crust. It is concluded that the Dolodau Au–W–Ag mineralization is an OGS related to a major northeast–southwest-oriented fault.
BSGF - Earth Sciences Bulletin, 2025
The Djourdé-Sinassi magmatic-migmatitic complex is part of the juvenile Neoproterozoic Garoua Dom... more The Djourdé-Sinassi magmatic-migmatitic complex is part of the juvenile Neoproterozoic Garoua Domain located in Northern Cameroon. It is surrounded by the Zalbi, Poli and Rey-Bouba greenstone belts, limited to the east by the sinistral Tcholliré-Banyo Shear zone and to the south by the Poli Thrust System. The Djourdé-Sinassi magmatic-migmatitic complex is subdivided into a western magmatic domain made of diorite, tonalite, granodiorite, with numerous enclaves of mafic and ultramafic rocks, and an eastern domain dominated by metatexite migmatites enclosing ubiquitous mafic to ultramafic rafts. The plutonic rocks of the western domain display a gently dipping to vertical magmatic fabric with variable trends. Locally, trains of hornblendite enclaves with lobate boundaries and dismembered into a network of felsic veins points to injection of mafic magma that mingled with a host magmatic mush and/or partially molten rocks. The migmatites display a foliation delineated by the alternation of leucosome and mesosome layers, that are mainly subvertical and wrapped around the metric to kilometric size rafts of mafic and ultramafic rocks. A texturally continuous network of leucosome veins concordant to discordant relative to the syn-migmatitic foliation and the mineral assemblage attests for syntectonic melt segregation under granulite facies conditions (Hbl þ Pl ± Bt ± Grt±Opx±Cpx ; peak: ≥800 °C/8-10 kbar). The magmatic fabric of plutonic rocks and the syn-migmatitic foliation are locally transposed into subvertical shear zones with a steeply plunging mineral and stretching lineation. Kinematic criteria are identified in the subhorizontal plane perpendicular to the lineation, which indicates that deformation was not plane strain and occurred in 3D. N-S to NE-SW striking shear zones are sinistral whereas E-W to WNW-ESE trending ones are dextral. These shear zones first developed in the presence of melt but then under solid-state during progressive retrogression from amphibolite to greenschist facies (AbþQtz þ Ep ± Op ± ChlþSerþEp). These data suggest that the Djourdé-Sinassi magmatic-migmatitic complex corresponds to the former root of the Central African Orogenic Belt originating from partial melting of a juvenile mafic crust. This root has been vertically extruded under a pure-shear dominated constrictional regime with vertical stretching in the presence of melt coeval with the injection of mantle-derived mafic magmas and was then progressively exhumed towards the surface. This deformation pattern is consistent with the position of the Sinassi complex surrounded by greenstone belts and on a larger scale, confined in between the West African À Congo cratons and Saharan metacraton.
Sustainability, 2025
This study investigates uranium (U) and thorium (Th) levels in surface beach
sediments from the ... more This study investigates uranium (U) and thorium (Th) levels in surface beach
sediments from the Central Gulf of Gabes (SE Tunisia), aiming to identify concentration zones, geochemical behaviors, and enrichment factors. U concentrations ranged from 0.71 to 38.00 mg/kg, exceeding Th levels, which ranged from 1.00 to 10.60 mg/kg. A positive correlation between U and Th indicates a common source, which is most likely phosphogypsum wastes, and similar geochemical behaviors. The central sector near Gabes’ fertilizer factories showed the highest U and Th concentrations, with factors such as,proximity to industrial discharge, port structures’ influence, organic matter enrichment, low seawater pH, and high phosphorus levels affecting the spatial distribution of these elements. Thermochemical analysis suggests that U and Th exhibit parallel chemical behaviors in low-pH, phosphate-rich conditions. This is the first study to document U and Th presence in phosphogypsum-contaminated beach sediments in Gabes, underlining potential risks to the environment and human health. The findings of this work contribute to the international database of U and Th contamination in coastal sediments, providing essential data to support sustainable strategies aimed at safeguarding human health and preserving local environments affected by phosphate fertilizer industry pollution.
sediments from the Central Gulf of Gabes (SE Tunisia), aiming to identify concentration zones, geochemical behaviors, and enrichment factors. U concentrations ranged from 0.71 to 38.00 mg/kg, exceeding Th levels, which ranged from 1.00 to 10.60 mg/kg. A positive correlation between U and Th indicates a common source, which is most likely phosphogypsum wastes, and similar geochemical behaviors. The central sector near Gabes’ fertilizer factories showed the highest U and Th concentrations, with factors such as,proximity to industrial discharge, port structures’ influence, organic matter enrichment, low seawater pH, and high phosphorus levels affecting the spatial distribution of these elements. Thermochemical analysis suggests that U and Th exhibit parallel chemical behaviors in low-pH, phosphate-rich conditions. This is the first study to document U and Th presence in phosphogypsum-contaminated beach sediments in Gabes, underlining potential risks to the environment and human health. The findings of this work contribute to the international database of U and Th contamination in coastal sediments, providing essential data to support sustainable strategies aimed at safeguarding human health and preserving local environments affected by phosphate fertilizer industry pollution.
Journal of Hazardous Materials, 2024
The foamability of dissolved phosphogypsum from the phosphate fertilizer factories of Gabes (SE T... more The foamability of dissolved phosphogypsum from the phosphate fertilizer factories of Gabes (SE Tunisia) is a
spectacular phenomenon that has not yet been thoroughly studied. The main objective of this research was to
investigate the organic properties of phosphogypsum foam (PGF) to understand its formation process, determine
the origin of its enhanced radiochemical contaminants load, and identify its role in pollutants dispersion in
marine environment of the Southern Mediterranean Sea. This study identified PGF as an unnatural, surfactantstabilized,
and ephemeral aqueous foam. PGF-forming process comprises three main steps: (i) formation (through
phosphogypsum dissolution), (ii) stabilization (facilitated by organic surfactants and gypsum crystals), and (iii)
destabilization (geochemical (involving the dissolution of the PGF skeleton gypsum) and/or mechanical (influenced
by wind and wave action)). The amphiphilic nature of PGF organic matter and the presence of specific
organic groups are responsible for its high toxic contaminants load. PGF contributes, through its elevated pollutants content and its ability to migrate far from its source, to the marine dispersion of industrial toxic
radiochemical contaminants. It is therefore recommended to mitigate the environmental and health risks associated
with PGF, including banning the discharge of untreated phosphogypsum and other industrial wastes into
the coastal environment of Gabes.
spectacular phenomenon that has not yet been thoroughly studied. The main objective of this research was to
investigate the organic properties of phosphogypsum foam (PGF) to understand its formation process, determine
the origin of its enhanced radiochemical contaminants load, and identify its role in pollutants dispersion in
marine environment of the Southern Mediterranean Sea. This study identified PGF as an unnatural, surfactantstabilized,
and ephemeral aqueous foam. PGF-forming process comprises three main steps: (i) formation (through
phosphogypsum dissolution), (ii) stabilization (facilitated by organic surfactants and gypsum crystals), and (iii)
destabilization (geochemical (involving the dissolution of the PGF skeleton gypsum) and/or mechanical (influenced
by wind and wave action)). The amphiphilic nature of PGF organic matter and the presence of specific
organic groups are responsible for its high toxic contaminants load. PGF contributes, through its elevated pollutants content and its ability to migrate far from its source, to the marine dispersion of industrial toxic
radiochemical contaminants. It is therefore recommended to mitigate the environmental and health risks associated
with PGF, including banning the discharge of untreated phosphogypsum and other industrial wastes into
the coastal environment of Gabes.
Lithos, 2025
The granitoids in St. Martin Island, Lesser Antilles – Caribbean, consist of granodiorites (Type-... more The granitoids in St. Martin Island, Lesser Antilles – Caribbean, consist of granodiorites (Type-I low REE; Type-II high REE), leucotonalites, melatonalites and Qz-monzodiorites. These are I-type calc-alkaline granitoids, although classification of the newly identified melatonalites remains enigmatic, likely reflecting magma mixing between different sources for their formation. Geothermometry applications yield high formation temperatures for the melatonalites and the Type-II granodiorites exceeding by ∼100 °C those calculated for the other granitoids. Pressure conditions were relatively high for the melatonalites and granodiorites (∼4.2 and ∼ 4.0 kbar respectively), with the lowest assigned to the leucotonalites (∼1.8 kbar). Magnesiohornblende crystallized at the final crystallization stages (∼740 °C; ∼2.5 km depth), under hydrous (H2O = ∼3.5 wt%) and highly oxidizing conditions (ΔNNO up to +2.7).
Fractional crystallization significantly contributed to the compositional variability of the evolved granitoid lithotypes, with plagioclase being preferably fractionated in the Type-I granodiorites, relative to the Type-II granodiorites that mostly involved K-feldspar removal. Additionally, fluctuation of the hydrous and slab-derived fluid fluxes further promoted granitoid differentiation. Geochemical and Sr-Nd isotopic data reveal restricted sediment contamination of the mantle wedge. Melatonalites and Type-II granodiorites appear to have been formed during the early evolution stages of subduction initiation, whereas leucotonalites represent the late-stage shallow crystallization granitoid phase.
Fractional crystallization significantly contributed to the compositional variability of the evolved granitoid lithotypes, with plagioclase being preferably fractionated in the Type-I granodiorites, relative to the Type-II granodiorites that mostly involved K-feldspar removal. Additionally, fluctuation of the hydrous and slab-derived fluid fluxes further promoted granitoid differentiation. Geochemical and Sr-Nd isotopic data reveal restricted sediment contamination of the mantle wedge. Melatonalites and Type-II granodiorites appear to have been formed during the early evolution stages of subduction initiation, whereas leucotonalites represent the late-stage shallow crystallization granitoid phase.
Comptes Rendus Géosciences, 2024
The poorly documented volcanic rocks of the Ouaddai massif in Chad are a continuity of the ones o... more The poorly documented volcanic rocks of the Ouaddai massif in Chad are a continuity of the ones of the Cameroon Volcanic Line further to the SE, located within the Central African rift system. New mineralogical and geochemical data from the Iriba basanites (SiO2: 41–45 wt%) show depletion in HREE, slight negative Sm anomaly and high LREE/HREE ratios, which is typical of OIB. The main differentiation process is fractional crystallization with a complete lack of crustal contamination. These features, similar to basanites exposed in southern Cameroon, reflect the partial melting of a metasomatized subcontinental lithospheric root reworked during the formation of the Cenozoic Central Africa Rift System. We propose to define by Cameroon-Chad Volcanic Line this continental scale structure controlling the emplacement of alkaline magmas.
Iranian Journal of Crystallography and Mineralogy, 2018
Journal of Petrology, 2024
The continental lower crust constitutes a key zone for understanding the mantle-crust magmatic an... more The continental lower crust constitutes a key zone for understanding the mantle-crust magmatic and mechanical transfers, but its study is hampered by the paucity of lower crust samples. Here, we characterise the petrological, geochemical and petrophysical processes structuring the lower crust of the North Patagonian Massif (NPM; Argentina) using a suite of representative mafic granulite and websterite xenoliths. These xenoliths were entrained by alkaline lavas from 5 volcanic centres that erupted between the Oligocene and Pleistocene. Electron microprobe and laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) were used to obtain in-situ geochemical data on the minerals, while microstructural data were obtained by Electron Backscatter Diffraction (EBSD). Both granulites and
The footprints of mafic melts travelling from the depths to the surface are abundant in the mantl... more The footprints of mafic melts travelling from the depths to the surface are abundant in the mantle section of ophiolites. They constitute an important source of information about the melt migration mechanisms and related petrological processes in the shallowest part of the mantle beneath former oceanic spreading centres. In the field, these socalled 'melt migration structures' attract attention when they consist of mineral assemblages contrasting with that of their host peridotite. They therefore record a particular moment in the migration history: when the melt becomes out of equilibrium with the peridotite and causes a reaction impacting its modal composition, and/or when a temperature drop initiates the crystallization of the melt. The existence of cryptic effects of migration revealed by geochemical data shows that melts do not always leave a trail visible in the field. Although incomplete and patchy, the melt migration structures preserved in ophiolites are witnesses of processes that do actually occur in nature, which constitutes an invaluable support to the interpretation of geophysical data and inescapable constraints for numerical simulations and models of chemical geodynamics. Here we show how field observations and related petrological and geochemical studies allow us to propose answers to fundamental questions such as these: At which temperature is porous flow superseded by dyking? What are the factors governing melt trajectories? What is the nature of the 'universal solvent' initiating infiltration melting and making channelized porous flow the most common mode of transport of magmas through a peridotite matrix regardless the tectonic setting? A fundamental message delivered by ophiolites is that the shallow mantle behaves as a particularly efficient reactive filter between the depths and the surface of the Earth. Unexpectedly, the reactions occurring there are enhanced by the hybridization between mafic melts and a hydrous component, whatever its origin (i.e. magmatic vs. hydrothermal). This hybridization triggers out of equilibrium reactions, leading to the formation of exotic lithologies, including metallic ores, and impacting the global geochemical cycle of a whole range of chemical elements.
EMU Notes in Mineralogy, 2024
The present contribution synthesizes the main petrographic, mineralogical and chemical features o... more The present contribution synthesizes the main petrographic, mineralogical and chemical features of mantle xenoliths uplifted by Phanerozoic lavas. The collections of mantle xenoliths consist predominantly of peridotites but minor pyroxenites are commonly associated. Two main petrogenetic processes are responsible for the features of mantle xenoliths: partial melting and circulation of melts/fluids and associated metasomatic and magmatic processes. Partial melting processes lead to the formation of residual pieces of upper mantle while two main types of mantle metasomatism could be recognized such as LILE enrichment, the first referring to asthenosphere upwelling settings (essentially mantle plumes, rifting zones and asthenosphere window zones) and the second to mantle wedge settings. The AUZ (asthenospheric upwelling zones) metasomatism is essentially related to the migration of more or less CO 2-rich alkaline silicate melts and associated fluids while the MWZ (mantle wedge zones) metasomatism is associated with the activity of hydrated liquids (fluids) commonly SiO 2-rich.
Slab melting boosts the mantle wedge contribution to Li-rich magmas
Scientific reports, Jul 2, 2024
The lithium cycling in the supra-subduction mantle wedge is crucial for understanding the generat... more The lithium cycling in the supra-subduction mantle wedge is crucial for understanding the generation of Li-rich magmas that may potentially source ore deposition in continental arcs. Here, we look from the mantle source perspective at the geological processes controlling the Li mobility in convergent margins, by characterizing a set of sub-arc mantle xenoliths from the southern Andes (Coyhaique, western Patagonia). The mineral trace element signatures and oxygen fugacity estimates (FMQ > + 3) in some of these peridotite xenoliths record the interaction with arc magmas enriched in fluid-mobile elements originally scavenged by slab dehydration. This subduction-related metasomatism was poorly effective on enhancing the Li inventory of the sub-arc lithospheric mantle, underpinning the inefficiency of slab-derived fluids on mobilizing Li through the mantle wedge. However, major and trace element compositions of mantle minerals in other xenoliths also record transient thermal and chemical anomalies associated with the percolation of slab window-related magmas, which exhibit an "adakite"-type geochemical fingerprint inherited by slab-derived melts produced during ridge subduction and slab window opening event. As these melts percolated through the shallow (7.2-16.8 kbar) and hot (952-1054 °C) lithospheric mantle wedge, they promoted the crystallization of metasomatic clinopyroxene having exceptionally high Li abundances (6-15 ppm). Numerical modeling shows that low degrees (< 10%) of partial melting of this Li-rich and fertile sub-arc lithospheric mantle generates primitive melts having twofold Li enrichment (~13 ppm) compared with average subduction-zone basalts. Prolonged fractional crystallization of these melts produces extremely Li-enriched silicic rocks, which may stoke the Li inventory of mineralizing fluids in the shallow crust.
A geochemical model for the transformation of gabbro into vesuvianite-bearing rodingite
Chemical geology, Jun 1, 2024
Rodingite is a Ca-rich and Si-poor metasomatic rock commonly occurring in association with serpen... more Rodingite is a Ca-rich and Si-poor metasomatic rock commonly occurring in association with serpentinites. This rock is characterized by specific mineral assemblages consisting of hydrated garnet, diopside, vesuvianite, epidote-zoisite, chlorite, or prehnite. However, natural rodingites are significantly heterogeneous in mineral composition and vesuvianite occurs only in some extensively rodingitized rocks. Major factors controlling the mineral diversity as well as details on fluid-rock interactions leading to the evolution of mineral and chemical composition during rodingitization have not yet been fully constrained. In this work, we use PHREEQC software to present a geochemical model for the transformation of a mafic rock into vesuvianite-bearing rodingite at a temperature of 300 °C. Through these simulations, we investigate the effect of fluid composition and progress of the metasomatic process on rodingite formation. Our results show that rodingitization requires an open system with a high input of hydrothermal fluid. Additionally, a decrease in the Si/Ca ratio in the metasomatized rock is correlated to an increase in the volume of incoming fluid. Whole rock chemical and mineral composition in natural rodingites are well reproduced by the model. Furthermore, the diversity of mineral parageneses results mainly from different degrees of transformation and only to a lesser extent to the chemical composition of hydrothermal fluid or protolith. The hydrothermal fluid doesn't need to be especially rich in calcium to transform a mafic rock into rodingite, but it must be low in magnesium, silicon, and have a high pH, which is naturally controlled by serpentinization of surrounding ultramafic rocks.
Journal of Geosciences, May 20, 2024
The ultramafic Kukes Massif is located in the eastern part of the Mirdita ophiolite (N Albania), ... more The ultramafic Kukes Massif is located in the eastern part of the Mirdita ophiolite (N Albania), which is recognised as representing Supra-Subduction lithosphere. It comprises a thick (0.8-2.5 km) dunite zone containing abundant occurrences of chromite ores and is cut by orthopyroxenitic and clinopyroxenitic veins. In this paper we focus on the genesis and evolution of olivine and chromite forming dunite in the northern part of the Kukes Massif. The chemical composition of minerals in dunites is highly variabile and apparent at outcrop scale. The most significant changes are recorded by olivine, which contains over 90 Fo in the host dunite but decreases to 87.5 in proximity of clinopyroxenitic veins. The composition of spinel is also sensitive to the presence of veins: in host dunite its Cr# is over 80 (chromite type I), whereas the presence of veins causes its decrease to 68 (type II). Clinopyroxene in vein-forming clinopyroxenite has Mg# from 86 to 92 and is Al-rich (Al 2 O 3 0.8-2.6 wt. %). Orthopyroxene forms orthopyroxenites (Mg# 90-93, Al 2 O 3 0.2-1.6 wt. %), but also screens (Mg# 83-91, Al 2 O 3 0.8-2.4 wt. %) at the contact between clinopyroxenite veins and the host dunite. The thick dunitic sequence at Kukes must have been formed as a result of intensive percolation of possibly boninitic melt through parental harzburgite. Another step in the evolution of the Kukes massif was related to intrusion of the pyroxenitic veins. These melts were not equilibrated with the host dunite and led to metasomatic modification of chromite and olivine, increasing Al 2 O 3 content in former (from 6-8 up to 18 wt. %) and decreasing Fo (extremely from 92 to 87.5) in the latter. The process is evident proximal to clinopyroxenite veins, but a subtle effect is also recorded in the chemical composition of dunite contacting orthopyroxenite, leading to increase in Fe 2 O 3 content. Metasomatism modified the composition of dunites in a zone of 0.5 m around pyroxenites. Our studies indicate a multistage evolution of the SSZ peridotites and show that its deciphering requires careful mineralogical examination.
Seismic properties of mantle metasomatism from mantle xenoliths beneath the North Tanzania Divergence, East African Rift
Gondwana research, Jul 1, 2024
We use mantle xenoliths brought to the surface by alkaline lavas to determine the chemical and ph... more We use mantle xenoliths brought to the surface by alkaline lavas to determine the chemical and physical properties of the metasomatized lithospheric mantle that contribute to the earliest rifting stage in East Africa. Our results help to interpret the seismic tomographic images in terms of vein and inclusions proportions in the lithospheric mantle. We focus on mantle xenoliths from the in-rift Pello Hill volcano in the North Tanzanian Divergence (NTD). These xenoliths reveal the presence of refractory mantle harzburgites and dunites with coarse granular to porphyroclastic textures and 6-80 % of diopside, phlogopite and amphibole-bearing veins and phlogopite-rich hornblendite xenolith. The presence of calc-potassic and FeO, TiO 2-rich veins, and mineral equilibria of olivine and pyroxenes indicate that fluid/melt-rock interactions occurred at depth from 40 km to 80-90 km, and indicate the presence of a high-temperature isotherm beneath the NTD (T = 1040-1200°C). We computed the seismic properties of the mantle xenoliths with different proportions, compositions, and geometric distributions of crystallized and fluid-filled veins. Compared to vein-free peridotites, for crystallized vein-bearing xenoliths, the velocity is lowered by 2-4 % to 28-37 % for Vp and by 2-3 % to 25-29 % for Vs for 6 % to 60 % veins, respectively. For fluid-filled inclusions, hydrous melt lens-shape inclusions are the most effective parameter to reduce P velocity, compared to dry or 2.5 %-CO 2 peridotitic melt. A comparison with seismic tomography velocities allows us to discuss the current state of the lithospheric mantle. The best agreement obtained between P teleseismic tomography (Vp anomalies between −9 % and −15 %) and vein-bearing peridotites (depth 40-90 km) corresponds to 12-25 % of crystallized veins or 8-15 % for fluid filled-veins for a vertical foliation and transtensional strain regime in the mantle lithosphere beneath the NTD. © 20XX
Bulletin de la Société géologique de France, Feb 15, 2024
This paper presents and discusses new geochronological and petrological data on a suite of calc-a... more This paper presents and discusses new geochronological and petrological data on a suite of calc-alkaline plutons composed predominantly of diorites and tonalites from the West Massif Central. Their petrochemical fingerprints are compatible with partial melting of a hydrous mantle wedge followed by fractional crystallization of amphibole and plagioclase before final emplacement between 5 and 8 kbar within the continental upper plate of a subduction system. In situ U-Pb zircon dating on tonalites yields a fairly narrow age range of 365À354 Ma (including uncertainties) for igneous crystallization. These calcalkaline plutons imply active margin magmatism near the Devonian-Carboniferous boundary and are contemporaneous with the back-arc magmatism and HP metamorphism as dated by recent studies. However, such isolated igneous bodies do not form a transcrustal magmatic arc but rather represent dispersed plutons emplaced within less than 30 Myr when all data from the Variscan belt of France are considered. In Limousin, they intrude migmatitic paragneisses and retrogressed eclogites from the Upper Gneiss Unit (UGU), suggesting that the high pressure rocks were already exhumed at 19À30 km depth before 365 Ma. Moreover, the diorites and tonalites are never found within units below the UGU. It therefore suggests that these tectono-metamorphic units of the Western French Massif Central were piled up after 354 Ma. Altogether these results support the monocyclic model for Variscan geodynamics in the French Massif Central, with the transition between oceanic subduction and continental collision taking place between Upper Devonian and Lower Carboniferous.
Arabian Journal of Geosciences, Dec 7, 2023
Plutonic xenoliths have been found within a pipe and a related phreatomagmatic leucitite deposit ... more Plutonic xenoliths have been found within a pipe and a related phreatomagmatic leucitite deposit in the Eğirdir lake area, belonging to the Potassic-Ultrapotassic Afyon volcanic Province, West Anatolia. They consist of kamafugite-type, feldsparbearing syenite, pyroxenite, leucitolite, some small-sized melilitolite and garnet-rich xenoliths, and a carbonatite. A new occurrence of kalsilite is described as either homogeneous acicular crystals or tabular two phases-exsolved crystals in the kamafugite-type and melilitolite xenoliths. Rock textures and compositions indicate cumulates and near-liquid composition rocks corresponding to relatively evolved magmas. All the rocks are strongly silica-undersaturated, Ca-, Mg-, and K-rich, and Al-poor. The fractional crystallization model includes clinopyroxene, apatite, phlogopite, melilite and leucite. Fe-Ti oxides and garnet may be also concerned. The P H2O during crystallization and differentiation is not more than 0.8 GPa. Major elements, trace elements, and REE patterns for xenoliths, which indicate near-liquid compositions, are typical of ultrapotassic series in a post-collisional geodynamic context, as it is the case for the Roman and Central ultrapotassic Italian provinces. The stable isotope 13 C and 18 O values of the calcio-carbonatite plot close to the primary carbonatite field, whereas the carbonates of the feldspar-bearing syenite and the peperite matrix suggest a low-T extensive contamination process. The origin of the carbonatite from kamafugite-type magmas by immiscibility or by fractional crystallization remains questionable; an origin by fractionation-melting of a metasomatized mantle source should be tested in the future.
Use of porphyry indicator zircons (PIZs) in the sedimentary record as an exploration tool for covered porphyry copper deposits in the Atacama Desert, Chile
Journal of Geochemical Exploration, Dec 31, 2023
This work explores the potential of geochemical and petrographic characteristics of detrital zirc... more This work explores the potential of geochemical and petrographic characteristics of detrital zircons coming from the sedimentary record of the Centinela District in Northern Chile to identify the presence of buried porphyry copper deposits under a transported gravel cover. The sampled sedimentary section was recovered from the pit of the exotic copper deposit of El Tesoro, located approximately 2 and 4 km west of the Esperanza and Mirador porphyries, respectively. The sedimentary cover comprises four units, Tesoro II, Tesoro III, Arrieros and Recent gravels, deposited since the late Cenozoic in an arid continental environment dominated by alluvial fans. Except for the Tesoro III gravels, all other units contain exotic-Cu mineralisation. In order to interpret the geochemical footprint of the investigated zircons, the Porphyry Indicator Zircon (PIZ) concept (Pizarro et al., 2020) is used. A PIZ need to comply with each of the following geochemical values: Hf >8,750 (ppm), Ce/Nd >1, Eu/Eu* >0.4, 10,000×(Eu/Eu*)/Y >1, (Ce/Nd)/Y >0.01, Dy/Yb <0.3 and 0.1 < Th/U < 1. These zircons also have Ti <9 ppm and Ce/Ce* <100 and usually show euhedral morphologies characterised by prismatic forms of type {110}. The geochemical andpetrographic characteristics of the PIZs collected in the gravels are similar to zircons from the nearby Mirador and Esperanza porphyries. The highest PIZ concentration coincides with the gravel horizons with exotic-Cu mineralisation. Therefore, the PIZs found in the sedimentary record are a potential tracer of adjacent copper porphyries and represent a promising exploration tool for this type of hidden ore deposits in challenging sediment-covered areas.
Scientific Reports
The lithium cycling in the supra-subduction mantle wedge is crucial for understanding the generat... more The lithium cycling in the supra-subduction mantle wedge is crucial for understanding the generation of Li-rich magmas that may potentially source ore deposition in continental arcs. Here, we look from the mantle source perspective at the geological processes controlling the Li mobility in convergent margins, by characterizing a set of sub-arc mantle xenoliths from the southern Andes (Coyhaique, western Patagonia). The mineral trace element signatures and oxygen fugacity estimates (FMQ > + 3) in some of these peridotite xenoliths record the interaction with arc magmas enriched in fluid-mobile elements originally scavenged by slab dehydration. This subduction-related metasomatism was poorly effective on enhancing the Li inventory of the sub-arc lithospheric mantle, underpinning the inefficiency of slab-derived fluids on mobilizing Li through the mantle wedge. However, major and trace element compositions of mantle minerals in other xenoliths also record transient thermal and chemical anomalies associated with the percolation of slab window-related magmas, which exhibit an "adakite"-type geochemical fingerprint inherited by slab-derived melts produced during ridge subduction and slab window opening event. As these melts percolated through the shallow (7.2-16.8 kbar) and hot (952-1054 °C) lithospheric mantle wedge, they promoted the crystallization of metasomatic clinopyroxene having exceptionally high Li abundances (6-15 ppm). Numerical modeling shows that low degrees (< 10%) of partial melting of this Li-rich and fertile sub-arc lithospheric mantle generates primitive melts having twofold Li enrichment (~13 ppm) compared with average subduction-zone basalts. Prolonged fractional crystallization of these melts produces extremely Li-enriched silicic rocks, which may stoke the Li inventory of mineralizing fluids in the shallow crust.