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International audienceThe late stages of the Variscan orogeny are characterized by middle to lower crustal melting and intrusion of voluminous granitoids throughout the belt, which makes it akin to “hot” orogens. These processes resulted in the development of large granite–migmatite complexes, the largest of which being the 305–300-Ma-old Velay dome in the eastern French Massif Central (FMC). This area also hosts a wide range of late-Variscan plutonic rocks that can be subdivided into four groups: (i) cordierite-bearing peraluminous granites (CPG); (ii) muscovite-bearing peraluminous granites (MPG); (iii) K-feldspar porphyritic, calc-alkaline granitoids (KCG) and (iv) Mg–K-rich (monzo)diorites and lamprophyres (“vaugnerites”). New results of LA-SF-ICP-MS U–Pb zircon and monazite dating on 33 samples from all groups indicate that both granites and mafic rocks emplaced together over a long period of ~40 million years throughout the Carboniferous, as shown by intrusion ages between 337.4 ± 1.0 and 298.9 ± 1.8 Ma for the granitoids, and between 335.7 ± 2.1 and 299.1 ± 1.3 Ma for the vaugnerites. Low zircon saturation temperatures and abundant inherited zircons with predominant late Ediacaran to early Cambrian ages indicate that the CPG and MPG formed through muscovite or biotite dehydration melting of ortho- and paragneisses from the Lower Gneiss Unit. The KCG and vaugnerites contain very few inherited zircons, if any, suggesting higher magma temperatures and consistent with a metasomatized lithospheric mantle source for the vaugnerites. The KCG can be explained by interactions between the CPG/MPG and the vaugnerites, or extensive differentiation of the latter. The new dataset provides clear evidence that the eastern FMC was affected by a long-lived magmatic episode characterized by coeval melting of both crustal and mantle sources. This feature is suggested here to result from a lithospheric-scale thermal anomaly, triggered by the removal of the lithospheric mantle root. The spatial distribution of the dated samples points to a progressive southward delamination of the lithospheric mantle, perhaps in response to rollback following continental subduction, or to “retro-delamination” owing to the retreat of a south-verging subduction zone

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Models of orogens identify subduction of oceanic crust as the key mechanism leading to continental collision. Such models, based inter alia on thermobarometric and geochronological evidence preserved in high-pressure metamorphic rocks and subduction-related magmatism, have been used to explain the convergence of Europe and Adria in the Cretaceous-Cenozoic and the subsequent Alpine orogen. Here, we review the metamorphic, igneous, and sedimentary record of the past 300 Ma of the Alpine orogen to show that there is no evidence of igneous activity during subduction initiation and prograde high-pressure metamorphism, leading to an ~50 Ma hiatus in magmatism, or "arc gap." The closure of rift basins forming the Piemont-Liguria ocean did not follow a classical Wadati-Benioff-type subduction. Instead, subduction initiation at passive margins allowed for the accretion of the hydrated portion of the subducting plate within an orogenic wedge as subduction of dry subcontinental lithosphere inhibited magmatism during subduction initiation and ocean closure.

We present here a tectonic-geodynamic model for the generation and flow of partially molten rocks and 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. These features reflect the horizontal growth of the Variscan belt and the 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 Paleothethys 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 correspond to post-orogenic exhumation during gravitational collapse. All these processes contribute to differentiation and stabilisation of the orogenic crust.

Porphyry deposits are copper-rich orebodies formed by precipitation of metal sulphides from hydrothermal fluids released from magmatic intrusions that cooled at depth within the Earth’s crust. Finding new porphyry deposits is essential because they are our largest source of copper and they also contain other strategic metals including gold and molybdenum. However, the discovery of giant porphyry deposits is hindered by a lack of understanding of the factors governing their size. Here, we use thermal modelling and statistical simulations to quantify the tempo and the chemistry of fluids released from cooling magmatic systems. We confirm that typical arc magmas produce fluids similar in composition to those that form porphyry deposits and conclude that the volume and duration of magmatic activity exert a first order control on the endowment (total mass of deposited copper) of economic porphyry copper deposits. Therefore, initial magma enrichment in copper and sulphur, although adding to the metallogenic potential, is not necessary to form a giant deposit. Our results link the respective durations of magmatic and hydrothermal activity from well-known large to supergiant deposits to their metal endowment. This novel approach can readily be implemented as an additional exploration tool that can help assess the economic potential of magmatic-hydrothermal systems.

a b s t r a c tOver the last few years the Western Alpine Molasse Basin (WAMB) has been attracting large institutional, industrial and scientific interest to evaluate the feasibility of geothermal energy production. However, the thermal state of the basin, which is instrumental to the development of such geothermal projects, has remained to date poorly known. Here, we compile and correct temperature measurements (mostly bottom hole temperature) from 26 existing well data mostly acquired during former hydrocarbon exploration in the basin. These data suggest that the average geothermal gradient of the WAMB is around 25-30 • C/km. We further use these data to build the first well data-driven 3D geostatistical temperature model of the whole basin and generate probabilistic maps of isotherms at 70 and 140 • C. This model highlights a number of positive and negative thermal anomalies that are interpreted in the context of heat advection caused by fluid circulation along faults and/or karst systems. This study confirms that the WAMB has a great potential for low-enthalpy geothermal resources and presents a typology of advection-dominated potential targets.