3-D numerical modelling of crustal polydiapirs with volume-of-fluid methods

Gravitational instabilities exert a crucial role on the Earth dynamics and in particular on its differentiation. The Earth’s crust can be considered as a multilayered fluid with different densities and viscosities, which may become unstable in particular with variations in temperature. With the specific aim to quantify crustal scale polydiapiric instabilities, we test here two codes, JADIM and OpenFOAM, which use a volume-of-fluid (VOF) method without interface reconstruction, and compare them with the geodynamics community code ASPECT, which uses a tracking algorithm based on compositional fields. The VOF method is well-known to preserve strongly deforming interfaces. Both JADIM and OpenFOAM are first tested against documented two and three-layer Rayleigh–Taylor instability configurations in 2-D and 3-D. 2-D and 3-D results show diapiric growth rates that fit the analytical theory and are found to be slightly more accurate than those obtained with ASPECT. We subsequently compare the results from VOF simulations with previously published Rayleigh–B´enard analogue and numerical experiments.We show that the VOF method is a robust method adapted to the study of diapirism and convection in the Earth’s crust, although it is not computationally as fast as ASPECT. OpenFOAM is found to run faster than, and conserve mass as well as JADIM. Finally, we provide a preliminary application to the polydiapiric dynamics of the orogenic crust of Naxos Island (Greece) at about 16 Myr, and propose a two-stages scenario of convection and diapirism. The timing and dimensions of the modelled gravitational instabilities not only corroborate previous estimates of timing and dimensions associated to the dynamics of this hot crustal domain, but also bring preliminary insight on its rheological and tectonic contexts

Synmetamorphic Cu remobilization during the Pan-African orogeny: Microstructural, petrological and geochronological data on the kyanite-micaschists hosting the Cu(–U) Lumwana deposit in the Western Zambian Copperbelt of the Lufilian belt

International audienceThe Pan-African Lufilian orogenic belt hosts world-class Cu deposits. In the Congolese Copperbelt (DRC), Cu(-Co) deposits, are mostly hosted within evaporitic and siliciclastic Neoproterozoic metasedimentary rocks (Mines Subgroup) and are interpreted as syn- to latediagenetic deposits. In this paper, we present new data on Cu(-U) deposit hosted in metamorphic rocks of the internal zone of the Lufilian belt known as the Western Zambian Copperbelt in which a primary Cu mineralization is overprinted by a second syn-metamorphic Cu mineralizing event. This mineralizing event is synchronous with the Pan-African metamorphism affecting both the pre-Katanga basement and the Katanga metasedimentary sequence. Cu(-U) occurrences in the Western Zambian Copperbelt are hosted by kyanitemicaschists metamorphosed in the upper amphibolite facies. Mineral inclusions of graphite, micas and sulfides in kyanite porphyroblasts of the Cubearing kyanite-micaschists in the Lumwana Cu deposit point to a sedimentary protolith with relics of an inherited Cu stock. Based on petrologic, microstructural and geochronological evidence, we propose that this initial Cu-stock was remobilized during the Pan-African orogeny. Graphite, micas and sulfides preserved in a first generation of kyanite poikiloblasts (Ky1) define an inherited S0/1 foliation developed during the prograde part of the P-T path (D1 deformation-metamorphic stage) reaching HP-MT metamorphic conditions. Remobilization during the retrograde part of the P-T path is evidenced by chalcopyrite-pyrrhotite and chalcopyrite-bornite delineating a steep-dipping S2 schistosity and by chalcopyrite and bornite delineating a shallow-dipping S3 schistosity associated with top to the south kinematic criteria. This retrograde path is coeval with ductile deformation in the kyanite field as evidenced by a second generation of synkinematic kyanite porphyroblasts (Ky2) transposed in the S3 schistosity (Ky2-3), and is marked by progressive cooling from ca 620°C down to 580°C (rutile geothermometry). Syn-S2-3 metamorphic monazite grains yield U-Th-Pb ages ranging from ca. 540 to 500 Ma. Final retrogression and remobilization of Cu is marked by recrystallization of the sulfides in top to the north C3 shear bands associated with rutile crystals yielding temperatures from ca. 610 to 540°C. This final remobilization is younger than ca. 500 Ma (youngest U-Th- Pb age on syn-S3 recrystallized monazite). These data are consistent with successive Cu remobilization for more than 40 Ma during Pan-African reworking of sediment-hosted deposits either from the basement of the Katanga sedimentary sequence or from the Katanga sequence itself marked by burial (D1), syn-orogenic exhumation (D2), and post-orogenic exhumation during gravitational collapse (D3)

Neoproterozoic magmatic evolution of the southern Ouaddaï Massif (Chad)

This paper presents new petrological, geochemical, isotopic (Nd) and geochronological data on magmatic rocks from the poorly known southern Ouaddaï massif, located at the southern edge of the so-called Saharan metacraton. This area is made of greenschist to amphibolite facies metasediments intruded by large pre- to syn-tectonic batholiths of leucogranites and an association of monzonite, granodiorite and biotite granite forming a late tectonic high-K calc-alkaline suite. U-Pb zircon dating yields ages of 635 ± 3 Ma and 613 ± 8 Ma on a peraluminous biotite-leucogranite (containing numerous inherited Archean and Paleoproterozoic zircon cores) and a muscovite-leucogranite, respectively. Geochemical fingerprints are very similar to some evolved Himalayan leucogranites suggesting their parental magmas were formed after muscovite and biotite dehydration melting of metasedimentary rocks. A biotite-granite sample belonging to the late tectonic high-K to shoshonitic suite contains zircon rims that yield an age of 540 ± 5 Ma with concordant inherited cores crystallized around 1050 Ma. Given the high-Mg# (59) andesitic composition of the intermediate pyroxene-monzonite, the very similar trace-element signature between the different rock types and the unradiogenic isotopic signature for Nd, the late-kinematic high-K to shoshonitic rocks formed after melting of the enriched mantle and further differentiation in the crust. These data indicate that the southern Ouaddaï was part of the Pan-African belt. It is proposed that it represents a continental back-arc basin characterized by a high-geothermal gradient during Early Ediacaran leading to anatexis of middle to lower crustal levels. After tectonic inversion during the main Pan-African phase, late kinematic high-K to shoshonitic plutons emplaced during the final post-collisional stage

Crustal-scale convection and diapiric upwelling of a partially molten orogenic root (Naxos dome, Greece)

The goal of this paper is to use the structural, metamorphic and geochronological record from the migmatitic core of the Naxos dome (Greece) and its associated subdomes to address the internal dynamics of a partially molten orogenic root. U-Pb ages from ca. 24 to 16 Ma and textures of zircon in the migmatites suggest successive dissolution and precipitation cycles with a period of 1 to 2 Ma, interpreted as the timescale of convective instabilities in a ca. 20 km thick partially molten layer. Dimensional analysis indicates that convection of this root requires a viscosity lower than 1018 Pa·s, consistent with viscosity values expected for partially molten felsic rocks. Structural analysis and U-Pb geochronology of deformed granitic dikes rooting in the migmatites record the subsequent development of the Naxos dome by diapirism from ca. 16 to 13 Ma. The size of the first order migmatite dome on Naxos (5 × 12 km) requires that the unstable layer at the onset of diapirism was 5 to 10 km thick and presented a moderate viscosity contrast with its envelope. From this analysis we propose that the Naxos migmatite dome documents a two stage dynamic evolution for the partially molten root of the Aegean belt characterized by (1) crustal scale convection for at least 8 Ma and (2) diapirism for about 3 Ma during progressive thinning of the collapsing orogenic crust

A tectonic model for the juxtaposition of granulite- and amphibolite-facies rocks in the Eburnean orogenic belt (Sassandra-Cavally domain, Côte d’Ivoire)

The Sassandra-Cavally (SASCA) domain (SW Côte d’Ivoire) marks the transition between the Archean Kenema-Man craton and the Paleoproterozoic (Rhyacian) Baoule-Mossi domain. It is characterized by the tectonic juxtaposition of granulite-facies and amphibolite-facies rocks. Migmatitic grey gneisses, garnet–cordierite–sillimanite migmatitic paragneisses and garnet–staurolite-bearing micaschists reached peak pressure conditions ranging from ∼ 6.6 kbar at 620 °C to ∼ 10 kbar at 820 °C. These conditions are associated with the first recorded deformation D1 and correspond to a Barrovian geothermal gradient of ∼ 25 °C/km. Subsequent exhumation, associated with a second deformation D2, was marked by decompression followed by cooling along apparent geothermal gradients of ∼ 40 °C/km. A D3 deformation phase is marked by folding and local transposition of the regional S1/S2 foliation into E-W trending shear zones. LA-ICP-MS U–Pb dating of monazite, which displays complex internal structures, reveals four age groups correlated to textural position of monazite grains and analytical points: (1) rare relictual zones yield dates at the Archean-Paleoproterozoic transition (ca. 2400–2600 Ma); (2) a cluster of dates centered at ca. 2037 Ma on grains aligned along the S2 foliation of the migmatitic grey gneiss, attributed to D2; (3) a cluster of dates centered at ca. 2000 Ma, and (4) dates spreading from ca. 1978 to 1913 Ma, documented for the first time in the West African Craton monazites. The ages of the latter two groups are similar to the ones identified in the Guiana Shield, and could be attributed to a disturbance by fluids, to a periodic opening of U–Pb system or to an episodic crystallization of monazite during slow cooling lasting several tens of Myrs. These data allow to propose a model for the tectonic evolution of the SASCA domain at the contact between the Rhyacian Baoule-Mossi domain and the Archean Kenema-Man nucleus whereby crustal thickening is achieved by crustal-scale folding and is followed by and concomitant with lateral flow of the thickened partially molten crust accommodated by regional transcurrent shear zones. This combination of crustal thickening controlled by tectonic forces and gravity-driven flow leads to the juxtaposition of granulite- and amphibolite-facies rocks

Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: 1 the geologic record of the French Massif Central

We present here a tectonic-geodynamic model for the generation and flow of partially molten rocks and for magmatism during the Variscan orogenic evolution from the Silurian to the late Carboniferous based on a synthesis of geological data from the French Massif Central. Eclogite facies metamorphism of mafic and ultramafic rocks records the subduction of the Gondwana hyperextended margin. Part of these eclogites are forming boudins-enclaves in felsic HP granulite facies migmatites partly retrogressed into amphibolite facies attesting for continental subduction followed by thermal relaxation and decompression. We propose that HP partial melting has triggered mechanical decoupling of the partially molten continental rocks from the subducting slab. This would have allowed buoyancy-driven exhumation and entrainment of pieces of oceanic lithosphere and subcontinental mantle. Geochronological data of the eclogite-bearing HP migmatites points to diachronous emplacement of distinct nappes from middle to late Devonian. These nappes were thrusted onto metapelites and orthogneisses affected by MP/MT greenschist to amphibolite facies metamorphism reaching partial melting attributed to the late Devonian to early Carboniferous thickening of the crust. The emplacement of laccoliths rooted into strike-slip transcurrent shear zones capped by low-angle detachments from c. 345 to c. 310 Ma is concomitant with the southward propagation of the Variscan deformation front marked by deposition of clastic sediments in foreland basins. We attribute these features to horizontal growth of the Variscan belt and formation of an orogenic plateau by gravity-driven lateral flow of the partially molten orogenic root. The diversity of the magmatic rocks points to various crustal sources with modest, but systematic mantle-derived input. In the eastern French Massif Central, the southward decrease in age of the mantle- and crustal-derived plutonic rocks from c. 345 Ma to c. 310 Ma suggests southward retreat of a northward subducting slab toward the Paleotethys free boundary. Late Carboniferous destruction of the Variscan belt is dominantly achieved by gravitational collapse accommodated by the activation of low-angle detachments and the exhumation-crystallization of the partially molten orogenic root forming crustal-scale LP migmatite domes from c. 305 Ma to c. 295 Ma, coeval with orogen-parallel flow in the external zone. Laccoliths emplaced along low-angle detachments and intrusive dykes with sharp contacts correspond to the segregation of the last melt fraction leaving behind a thick accumulation of refractory LP felsic and mafic granulites in the lower crust. This model points to the primordial role of partial melting and magmatism in the tectonic-geodynamic evolution of the Variscan orogenic belt. In particular, partial melting and magma transfer (i) triggers mechanical decoupling of subducted units from the downgoing slab and their syn-orogenic exhumation; (ii) the development of an orogenic plateau by lateral flow of the low-viscosity partially molten crust; and, (iii) the formation of metamorphic core complexes and domes that accommodate post-orogenic exhumation during gravitational collapse. All these processes contribute to differentiation and stabilisation of the orogenic crust

Fluid circulations during collapse of an accretionary prism (Example of the Naxos Island Metamorphic Core Complex (Cyclades, Greece))

Cette thèse a pour objectif de caractériser les circulations de fluides en contexte d effondrement d un prisme d accrétion crustal. Le Metamorphic Core Complex (MCC) de Naxos comprend un système de détachement/décollement caractérisé par mylonites, ultramylonites, cataclasites et failles normales dont les relations géométriques témoignent du litage rhéologique de la croûte continentale. La chimie des inclusions fluides déterminée par l analyse microthermométrique, la spectroscopie RAMAN, l ablation laser couplée à l analyse spectroscopique (LA-ICP-MS), le crush-leach , et les signatures isotopiques C et H des inclusions fluides permettent d identifier trois grands types de fluides (1) des fluides salés riche en métaux, ii) des fluides aquo-carboniques en équilibre avec les encaissants métamorphiques, et iii) des fluides aqueux, probablement d origine météorique. Ces données indiquent que la croûte est subdivisée en deux réservoirs séparés par la transition fragile-ductile. Les fluides météoriques circulent en association avec la déformation fragile de la croûte supérieure alors que les fluides salés et les fluides aquo-carboniques circulent en relation avec la déformation ductile. La géométrie de ces réservoirs évolue lors de la formation du MCC, conjointement avec l exhumation et le refroidissement des roches métamorphiques. Le passage des roches du réservoir ductile au réservoir fragile est associée à un changement depuis un gradient géothermique élevé (60-100C/km) vers un gradient géothermique plus faible (35-60C/km). La transition fragile-ductile correspond ainsi à la fois à une limite rhéologique corrélée à une limite thermique et une limite de perméabilité.The aim of this thesis is to characterize fluid circulations in the context of the collapse of a crustal accretionary belt. The Naxos Metamorphic Core Complex comprises a detachment/decollement system characterized by mylonites, ultramylonites, cataclasites and normal faults with structural relationships reflecting the rheological layering at the crustal scale. Fluid inclusion chemistry is determined by microthermometry, Raman spectroscopy; laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), crush-leach and stable isotopes (C and H) analyses. These data characterize three different types of fluids: (1) high salinity fluids with a high metal content and high Th, (2) aqueous-carbonic fluids in equilibrium with the wall rocks and (3) aqueous probably surface-derived fluids. These data indicate that the crust is subdivided into two crustal reservoirs separated by the brittle/ductile transition. Surface-derived aqueous fluids circulate in association with the brittle deformation within the upper crust whereas aqueous-carbonic and high salinity fluids circulate in relation with ductile deformation. The characteristics of the trapped fluids indicate that as rocks have passed through the ductile/brittle transition they undergo a drastic change in geothermal gradient from 60 to 100C/km within a lithostatic pressure regime to 35-60C/km within a hydrostatic pressure regime. This implies that the fluid circulations are closely related to the rheological layering within the crust and its evolution during crustal extension. The ductile/brittle transition corresponds to a rheological boundary correlated to a thermal boundary and impermeable cap.NANCY1-Bib. numérique (543959902) / SudocSudocFranceF

De la marge Ouest du craton du Yangtze à la bordure Est du plateau Tibétain (évolution géodynamique à partir de l'étude structurale, pétrologique, géochimique et géochronologique de roches magmatiques et métamorphiques)

La géométrie actuelle de l'est du plateau Tibétain, constitué par le Songpan Ganze et les Longmen Shan, est probablement dictée par une différence de rhéologie entre le craton du Yangtze à l'est, et le Songpan Ganze à l'ouest, qui vient buter contre la marge du craton en réponse à la collision Inde-Asie. L'histoire géodynamique de la région a été reconstituée depuis le Néoprotérozoïque, afin d'évaluer l'influence des différents cycles orogéniques sur la structure thermique, minéralogique et chimique actuelle. L'étude des massifs cristallins bordant le craton du Yangtze et de l'ophiolite de San Dao Qiao a montré que durant le Néoprotérozoïque, le craton était bordé par une marge active, avec ouverture de bassins marginaux sous l'influence de panaches mantelliques. L'étude pétrologique et structurale du complexe métamorphique de Danba permet de déterminer que lors de l'orogénèse Indosinienne (200-180Ma), l'exhumation des niveaux structuraux profonds se produit par extrusion le long d'un grand chevauchement ductile. La position des granites du Songpan Ganze par rapport aux sutures au sein du plateau Tibétain permet de proposer un double retrait de slab sous le Songpan Ganze à la fin de l'orogénèse Indosinienne qui explique la diversité des granitoïdes. Ce double retrait de slab et la différence de réponse à la contrainte entre le Songpan Ganze et le craton du Yangtze a conduit à un déchirement de slab. La délamination complète de la partie mantellique de la lithosphère du Songpan Ganze à la fin de l'orogénèse Indosinienne peut expliquer l'accolement d'une lithosphère à croûte épaisse et manteau fin contre une lithosphère cratonisée à la bordure est du plateau TibétainThe present geometry of the Eastern Tibetan plateau, made of the Songpan Ganze terrane and the Longmen Shan range, is probably due to a rheological contrast between the Yangtze craton to the East, and the Songpan Ganze, which bump into the craton margin in response to the India-Asia collision. We reconstituted the geodynamic evolution of the area from Neoproterozoic times to present to evaluate how the different orogenic cycles influence the thermal, mineralogical and chemical structures observed today. Studies of the crystalline massifs of the western margin of the Yangtze craton and of the San Dao Qiao ophiolite show that during the Neoproterozoïc times, the craton margin was a subduction zone with opening of marginal basin due to mantle plumes. Petrologic and structural studies of the Danba metamorphic complex allow determining that during the Indosinian orogeny (200-180 Ma), exhumation mechanism of the deep structural level of the complex is an extrusion along a ductile thrust. Plutons position relative to sutures zones in the Tibetan plateau lead to a model of double slab roll-back under the Songpan Ganze at the end of the Indosinian orogeny which explain the diversity of the granites. The double slab roll-back and the difference in deformation between the Songpan Ganze and the Yangtze craton lead to a slab tear along the craton margin. The complete delamination of the lithospheric mantle under the Songpan Ganze terrane can explain the present juxtaposition of a thick crust and thin mantle lithosphere against a cratonized one.NANCY1-Bib. numérique (543959902) / SudocSudocFranceF

Pressure-temperature-time evolution of metamorphic rocks from Naxos (cyclades, Greece) : constraints from thermobarometry and Rb/Sr dating.

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