Hercynian Belt Research Articles

Sequence stratigraphy and correlation of late Carboniferous and Permian in the CIS, Europe, Tethyan area, North Africa, Arabia, China, Gondwanaland and the USA

Sequence stratigraphy was used to correlate the depositional chronology during Permo–Carboniferous time in various sedimentary basins of Gondwanaland, western Europe, eastern Europe, Tethyan area, North Africa, Arabia, China, and North America. During late Carboniferous and Permian, eleven second-order sequences (SOS) were recognised throughout the whole area. The sequence SI is Serpukhovian pp, Namurian A pp and Chesterian. The sequence SII is Serpukhovian pp–Bashkirian pp, Namurian A pp-B-C–Westphalian A–B pp and Morrowan. The sequence SIII is late Bashkirian–Moscovian, Westphalian B pp-C-D and Atokan–Desmoinesian. The sequence SIV is Kasimovian, early Stephanian and Missourian. The sequence SV is Gzhelian–Orenburgian, late Stephanian and Virgilian. The sequence SVI is Asselian, Autunian and Nealian. The sequences SVII–IX are Saxonian, Sakmarian–Artinskian–Kungurian and Leonardian–Hessian–Cathedralian. The sequences SX–XI are Guadalupian and Lopingian. Depending on the regional setting, different relationships exist between marine transgression and the respective effects of tectonics and of sea-level changes: in western Europe, effects of the glacial processes in Gondwanaland are partly balanced by the late compression in the Hercynian belt during Moscovian. Eastern Europe, Tethyan area, North Africa, China, and North America show a good correlation between glacial (ice melting) processes in Gondwanaland and the intensity of the transgression which is increased by the westward progradation of the orogeny in Urals and Appalachians. In Arabia, Tunisia and Tethyan area, the impact of the Neotethys opening is obvious from the Artinskian and mainly during the late Permian.

Pre-Alpine ophiolites in the basement of Southern Alps: the presence of a bimodal association (LAG- Leptyno-Amphibolitic Group) in the Serie del Laghi (N-Italy, Ticino-CH)

In several portions of the Hercynian belt, a peculiar bimodal lithological association is found. Its name, Leptyno-Amphibolitic Group, reflects its being made of alternating cm-thick, fine-grained leucocratic and melanocratic layers of amphibolite facies metamorphic rocks. In the «Strona Ceneri Border Zone» (SCBZ), a part of the Southalpine basement, the banded amphibolites, contain lenses of metagabbro, retrogressed eclogites and ultramafites. To discriminate among the hypotheses on their origin proposed in the literature, we obtained major and trace elements concentrations and Sr-Nd (Pb) isotopic data. All data concordantly indicate that the SCBZ banded amphibolites derive from arc/back arc bimodal volcanics whose emplacement occurred in an Early Paleozoic convergent zone. The leptynites show geochemical characters,i.e. high HREE and (Tb/Lu)N=1.11 (average), and isotopic composition (εNd from +2.66 to +3.49;87Sr/86Sr466=0.7034−0.7053) that are clearly different from those of the Strona Ceneri metasiliciclastic rocks like the Gneiss Minuti which show an average (Tb/Lu)N=1.52 and εNd466=−6.69. Their isotopic composition is also completely different from that of the Ordovician intrusives of Serie dei Laghi (εNd466<−4.64 and87Sr/86Sr466>0.7078). The fine-grained amphibolitic layers are clearly derived from basaltic tuffites and have εNd466 from +3.21 to +7.73. Their geochemistry is compatible with a derivation from back-arc tholeiites. The SCBZ, including the banded amphibolites, may well represent the reworked products of an early Hercynian suture, in which ophiolitic remnants that underwent HP metamorphism in a subduction zone, have been incorporated in turbiditic sediments derived from back-arc bimodal volcanites prior to the intrusion of the Ordovician granites that cut across all the rocks belonging to the Serie dei Laghi

The Hercynian collision in the Armorican Massif : evidence of different lithospheric domains inferred from seismic tomography and anisotropy

Abstract The Hercynian belt is a continental collision orogen extending from south-west Iberia to the Bohemian Massif in Czech Republic. The successive stages of its formation are dated from 400 to 260 Ma.The Armorican Massif is a preserved segment of this orogen. It presents structures oriented NW-SE, parallel to the general Hercynian trend in this region. The massif is divided into three domains (North, Central, and South-Armorican domains) separated by two main shearzones, the North- and South-Armorican shear zones. As the Armorican Massif escaped from any important tectonic or thermal event since the end of Hercynian times, it is particularly suited for the study of an old collision orogen. Thus, in the framework of the GéoFrance3D-ARMOR2 project, two passive seismological experiments were conducted in 1997 and 1999 in the Armorican Massif. The main goals concerned the characterization of the deep geometry of both shear zones, the understanding of their geodynamic bearing on the long term evolution of the Hercynian belt, the study of the lithospheric deformation, and the 3D imaging of the Champtoceaux nappes.The data allow to model seismic anisotropy and to build a 3D P-wave velocity model beneath the Armorican Massif. Crustal images do not evidence any deep rooting of the Champtoceaux nappes in the lower crust. However, the upper mantle images show a clear signal interpreted as the relic of the northward subduction which lasted until Devonian (≈350 Ma). The results also show that the North-Armorican Shear Zone is limited at depth to the crust and topmost mantle, while the South-Armorican Shear Zone can be traced over the whole lithosphere.The strong velocity contrasts are associated to probable relic thermal anomalies but are also significantly related to chemical anomalies.

Pre-metamorphic 40Ar/ 39Ar and U–Pb ages in HP metagranitoids from the Hercynian belt (France)

In most orogenic belts, the age of HP metamorphism and subsequent exhumation events still remain open to debate since geochronology can yield results which appear to conflict with the closure temperature concept [Dodson, M.H., 1973. Contrib. Mineral. Petrol. 40, 259–274], and because the behaviour of daughter radiogenic isotopes under HP to UHP conditions is poorly constrained. To obtain new data on isotope migration under high-pressure conditions, two undeformed HP metagranites with partially preserved magmatic assemblages from the French Variscan belt were investigated using the 40Ar/ 39Ar laser probe and U–Pb ion probe methods. In both cases, 40Ar/ 39Ar biotite and U–Pb zircon ages are consistent and could be related to the emplacement of pre-orogenic granites, despite petrological evidence of a strong metamorphic overprint during the Variscan orogeny. Temperatures experienced by the granites during subduction and exhumation events were more than 400 °C above that normally estimated for argon retention in biotite, but failed to cause significant resetting of the mica 40Ar/ 39Ar chronometer. Only a weak intragrain redistribution of argon is evidenced with the laser probe up to the contact with metamorphic garnet fringing biotite. By contrast, a complete resetting of biotite ages occurs in gneisses enclosing the metagranites. These results suggest that, in these dry undeformed HP metagranites, the thermally activated diffusion was relatively ineffective and that recrystallisation is the main process which controlled isotopic exchanges of Ar and Pb. It is likely that the absence of pervasive deformation and fluid circulation has also exercised some control on the preservation of pre-metamorphic isotopic signature in the studied rocks. The possible influence of several other parameters is also discussed. This example reveals that recovering thermochronological information in high-pressure metamorphic rocks is not straightforward and that great caution must be paid in the use of ages for the reconstruction of P– T– t paths.

Late Paleozoic polyphased tectonics in the SW Belledonne massif (external crystalline massifs, French Alps)

In the southwestern area of the Belledonne massif (external crystalline massifs, French western Alps), the finite strain pattern reflects the geometry and kinematics of Late Devonian-Late Carboniferous deformational events. Previous works suggested that the nappe stacking occurred during Late Devono-Dinantian times. New observations allow us to constrain the subsequent tectono-metamorphic evolution of the various stacked units. Westward thrusting is responsible for the early dismembering of the Rioupéroux-Livet plutono-volcanic units. Micaschist slices have also been inserted within the Rioupéroux-Livet units during this Dinantian tectonics, and have recorded metamorphic conditions of about 7–8 kbar, 600–650 °C. The local NW direction of shortening is similar to the overall shortening direction recorded at this time in the Hercynian belt of western Europe. During the Late Visean, the Early Paleozoic Chamrousse ophiolite was thrust towards the northeast on top of the Rioupéroux-Livet units. The inverted geometry of the Chamrousse ophiolite may be interpreted as a consequence of this tectonics. Northeastward thrusting, also observed in the northeastern part of the French Massif Central is interpreted as the result of lateral escape in a global convergent context. Finally, the Late Carboniferous period is characterized by two extensional events: one leading to southwest dipping low-angle extensional shear zones, during the Westphalian, and a second one during the Stephanian. This latter is characterized by ductile/brittle normal faults, with a northwest-southeast direction of extension and related to the gravitational collapse of the whole Hercynian belt. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Late Paleozoic polyphased tectonics in the SW Belledonne massif (external crystalline massifs, French Alps)

In the southwestern area of the Belledonne massif (external crystalline massifs, French western Alps), the finite strain pattern reflects the geometry and kinematics of Late Devonian–Late Carboniferous deformational events. Previous works suggested that the nappe stacking occurred during Late Devono-Dinantian times. New observations allow us to constrain the subsequent tectono-metamorphic evolution of the various stacked units. Westward thrusting is responsible for the early dismembering of the Rioupéroux–Livet plutono-volcanic units. Micaschist slices have also been inserted within the Rioupéroux–Livet units during this Dinantian tectonics, and have recorded metamorphic conditions of about 7–8 kbar, 600–650 °C. The local NW direction of shortening is similar to the overall shortening direction recorded at this time in the Hercynian belt of western Europe. During the Late Visean, the Early Paleozoic Chamrousse ophiolite was thrust towards the northeast on top of the Rioupéroux–Livet units. The inverted geometry of the Chamrousse ophiolite may be interpreted as a consequence of this tectonics. Northeastward thrusting, also observed in the northeastern part of the French Massif Central is interpreted as the result of lateral escape in a global convergent context. Finally, the Late Carboniferous period is characterized by two extensional events: one leading to southwest dipping low-angle extensional shear zones, during the Westphalian, and a second one during the Stephanian. This latter is characterized by ductile/brittle normal faults, with a northwest–southeast direction of extension and related to the gravitational collapse of the whole Hercynian belt.

Fluid-assisted large strains in a crustal-scale décollement (Hercynian Belt of South Brittany, France)

Abstract Crustal deformations often occur along gently-dipping shear zones, where the scale of fluid transfers and the coupling between deformations and fluid availability are important questions. We present a structural and geochemical study of felsic rocks deformed at about 350–400°C within a crustal-scale décollement zone from the Hercynian Belt of South Brittany. At regional scale, deformations resulted in large distributed strains accumulated by dissolution-crystallization of quartz and feldspars. Mass transfers produced veins growing parallel to the foliation. Veins were major precipitation zones, while adjacent rocks were the main zones of dissolution. Deformation patterns, vein mineralogy, and oxygen isotope data show that both the amount of fluids and the scale of fluid flow were limited, and that the dominant transfer mechanism was diffusion. Results suggest that the deformation zone has acted as a trap for early fluids rather than as a syn-kinematic fluid channel.

Structure sismique du socle paléozoı̈que du bassin des Doukkala, Môle côtier, Maroc occidental. Indication en faveur de l'existence d'une phase éo-varisque

Abstract Seismic profiles and well data from the Doukkala basin unravel the structure of the Palaeozoic basement and suggest that this coastal zone of western Morocco was affected by a compressive phase during the Frasnian. This resulted in the formation of upright, plurikilometric folds associated with reverse faults (North Doukkala), and of asymmetrical folds associated with mostly west verging ramps (South Doukkala). Folding involved all pre-Upper Frasnian formations and caused partial or total hiatus of Upper Frasnian–Strunian strata. This event can be correlated with the orogenic phase reported from more internal domains of the Morocco Hercynian belt, where it is referred to as the ‘Bretonne’ or ‘Eovariscan’ phase. To cite this article: H. Echarfaoui et al., C. R. Geoscience 334 (2002) 13–20

Les « Porphyroı̈des » de Bretagne méridionale : une unité de HP–BT dans la chaı̂ne hercynienne

Abstract The so-called ‘Porphyroides’ Unit of South Brittany is located below units marked by an early HP–LT event (blueschists from the Ile de Groix) and above units marked by Upper Carboniferous HT metamorphism. P – T estimates in the Porphyroides (at Belle-Ile-en-Mer), using the phengite–chlorite thermobarometer, indicate pressures around 8 kbar and temperatures of 350–400 °C. These new estimates suggest that the early tectonic history of the ‘Porphyroides’ should be linked to that of the overlying Blueschists, and imply a much larger areal extension of the HP–LT domain in this part of the Hercynian Belt. To cite this article: F. Le Hebel et al., C. R. Geoscience 334 (2002) 205–211.

Tectonic evolution of the Cevennes para-autochthonous domain of the Hercynian French Massif Central and its bearing on ore deposits formation

Abstract The Cevennes area belongs to the para-autochthonous domain of the Hercynian Belt of the French Massif Central. Three lithological series, namely: sandstone-pelite, black micaschist and gneiss-micaschist, are identified. They form an imbrication of five tectonic units which overthrust the unmetamorphosed Viganais Paleozoic units to the south and the gneissic Mamejean Unit to the north. The structural, metamorphic and magmatic evolution of the Cevennes area is characterized by three events, namely: (1) southward shearing coeval to a MP/MT metamorphism dated around 340 Ma; (2) post nappe anatexis (T&amp;lt;750 degrees C, P&amp;gt;5 kb); (3) Namurian (ca 315 Ma) E-W extensional tectonics and plutonism. The structure of the Mt-Lozere-Borne granitic complex is constrained by new AMS and gravimetric data. The plutons are the driving power of the hydrothermal convective circulations responsible for an early deposition of diffuse arsenopyrite in the thermal aureole. Gold bearing sulfides are afterwards concentrated in quartz veins along brittle normal and wrench faults around the granite. Lastly, ore bearing quartz pebbles are sedimented in the Stephanian Ales coal basin.

Thrust-related, diapiric, and extensional doming in a frontal orogenic wedge: example of the Montagne Noire, Southern French Hercynian Belt

The Montagne Noire, which is situated at the toe of the orogenic wedge of the French Massif Central South European Variscides, appears to be a well-suited area for studying the origin and evolution of middle to upper crustal domes adjacent to foreland basins. The data reported in the present paper show that the Montagne Noire dome is a particular type of basement-involved frontal culmination in an orogenic wedge and foreland basin system. This frontal culmination is characterized by a syn-contractional HT decompression recorded by clockwise PTt paths and widespread strata overturning in thrust and fold structures, which controlled the sedimentation in the adjacent foreland basin. These unusual characteristics are interpreted to be a result of the succession of thrusting, diapirism and extensional collapse. Antiformal stacking of syn-metamorphic thrust sheets controlled the first stages of the foreland basin development. Diapirism was essentially responsible for the HT decompression and widespread strata overturning. Extensional doming was a result of late- to post-metamorphic collapse acting on the pre-existing high-amplitude dome. Diapirism and associated isothermal decompression metamorphism, which constitute the essential difference between the Montagne Noire and ‘ordinary’ frontal ridges in orogenic wedges, were probably enhanced by a local partial melting of the upper to middle crust. It is suggested that the occurrence of these phenomena in front of an orogenic wedge was related to local over-thickening due to the superposition of an upper crustal antiformal stack on top of a lower crustal ramp anticline.

The Ancenis Terrane: an exotic duplex in the Hercynian belt of Armorica, western France

The Ancenis Terrane, bordering the Loire, France, consists of Precambrian Brioverian basement and three Paleozoic sedimentary sequences: marine Ordovician to Devonian shelf sediments; Lower Carboniferous synorogenic intermontane sediments (the ‘Culm’); and coal-bearing Namurian deposits. The terrane is completely bound by faults, and its geology is in marked contrast with that of its neighbours. New data and observations along the terrane's southern boundary show that the Ordovician–Devonian sequence occupies a transpressional shear zone, and can be described as a tectonic mélange. The Culm outcrop, previously interpreted as a sinistral pull-apart structure, is reinterpreted as a Lower Carboniferous dextral pull-apart, and it is shown that Carboniferous transpression and thrusting remodelled the pull-apart basin into its present ‘mica-fish’ shape. With its remarkable concentric structure, and within a plate tectonic setting of mega dextral displacements along the northern boundary of Gondwana, the Ancenis Terrane can be interpreted as an exotic duplex.

Mise en évidence d'une tectonique tangentielle antérieure au plissenent majeur dans la chaîne hercynienne de l'Anti-Atlas occidental, Maroc

Variscan deformation in the Palæozoic foreland of the Moroccan Anti-Atlas is characterised by its heterogeneous character and its dampening effect towards the Tindouf Basin. Structural analyses shows three sequential episodes of deformation, D1, D2 and D3, displayed at different levels of observation. The first episode D1, which was observed for the first time in the western Anti-Atlas Hercynian Belt provides clear evidence for tangential tectonic deformation operating before the major phase of folding. It corresponds to an early event of ‘thin-skin tectonics’, which develops a composite schistosity related to ductile horizontal pure shearing. It is considered that D1 was responsible for the first folding of the sedimentary sequences associated with several imbricated thrust sheets with vergence to the east. The second episode, D2, represents the major phase of north-northeast-southwest Variscan folding. The D3 episode was more discrete, of brittle type and marked by subequatorial sinistral kink-bands. This D1 tangential event, in the western Anti-Atlas, Morocco, represents a typical structural domain as part of the tectonic evolution during Variscan-Appalachian collision.

The Hercynian orogeny in the South Armorican Massif (Saint‐Georges‐sur‐Loire Unit, Ligerian Domain, France): rifting and welding of continental stripes

A new approach to explain the Western Hercynian Belt is given by a lithological and structural study of the St‐Georges‐sur‐Loire Unit (southern part of the French Armorican Massif). This unit is interpreted as a Devonian olistostrome sheared to the North. It is formed in an accretionary prism linked with a south‐dipping subduction zone, called the Layon subduction zone. It results from a chaotic sedimentation including exotic blocks of various lithologies and ages in a Middle Devonian pebbly mudstone matrix. An inovative geodynamic model involving microblock rifting and collision is discussed to explain the presence of the St‐Georges‐sur‐Loire Unit in the Armorican Massif. This model involves two Palaeozoic subductions and emphasizes the role of the Nort‐sur‐Erdre fault, considered as a major polyphase suture zone separating South and Central Armorica.

The metamorphic evolution of the high pressure mafic granulites of the Bacariza Formation (Cabo Ortegal Complex, Hercynian belt, NW Spain) ☆

The high pressure mafic granulites of the Bacariza Formation outcrop in the two uppermost structural units of the Cabo Ortegal Complex (La Capelada unit and Cedeira unit) were separated by a Variscan thrust. In both cases, they appear as heterogeneous metabasites in normal contact between ultramafic rocks and other more homogeneous and less differentiated metabasic rocks, also affected by catazonal metamorphism. The main difference between the mafic granulites in the two units is the degree of deformation, which is higher in the underlying Cedeira unit. Petrologie and mineralogical data indicate that the high-pressure (HP) granulites (Gt-Cpx ± Amp-Pl ± Qtz ± Scp-Rt ± Ilm-Czo) are already retrograde (M2 Stage), post-dating an earlier eclogite facies metamorphism (Ml Stage) characterised by the mineral associations: Gt-Cpx ± Amp ± Ky ± Qtz-Rt and Gt-Cpx ± Amp ± Qtz ± Zo-Rt. The main structure relatted to the exhumation processes is the development of a general mylonitic foliation that, although initiated in granulite facies conditions, was mainly equilibrated in amphibolite facies (M3 Stage). This foliation was affected by isoclinal folds, which led to the formation of the Variscan thrusts responsible for the present stacking position. Thrust conditions were transitional between amphibolite and greenschist facies (M4 Stage). Thermobarometric data point to different P—T exhumation paths in the two units. Estimated P—T conditions were higher in La Capelada unit during Ml (P ≥ 13 kbar; 860 °C) and M2 (15 kbar; 800 °C) than in the Cedeira unit (Ml: P ≥ 11 kbar, 770 °C; M2: 12 kbar; 750 °C). Temperatures for the M3 stage were comparable (720 °C) in both units but rocks from the Cedeira unit show a much bigger drop in pressure. This resulted in an isothermal decompression type path for the Cedeira unit, while both P and T decreased more steadily in La Capelada rocks. These were always located at deeper level than the Cedeira rocks before the Variscan stacking. The difference in the two paths is related to different exhumation rates; higher in rocks from the Cedeira unit than in those from La Capelada. Exhumation processes coeval with underthrusting, and a different location of the rocks with respect to the main shear zone responsible for the exhumation would account for the distinct paths.

Conditions et ages 40 Ar/ 39 Ar de mise en place des granitoides de la zone externe sud du Massif central francais; exemple des granodiorites de St-Guiral et du Liron (Cevennes, France)

Abstract The St-Guiral and the Liron laccolithic granodiorites outcropping in the southeastern area of the French Massif Central intrude the Cambro-Ordovician low grade series of the western Cevennes. The petrological study of their contact aureoles shows that these plutons emplaced at low depth (1-2 kb; 690-700 degrees C). The laser probe 40 Ar/ 39 Ar allows us to suggest a minimum age of 324 Ma for the emplacement of the St-Guiral granodiorite. This emplacement seems synchronous with the LP-HT regional metamorphism in the western Cevennes for which the cooling ages are identical. The Liron granodiorite (310 Ma) is younger and crosscuts the thermal structures of the LP-HT regional metamorphism. Accordingly these plutons emplaced at the Namurian-early Westphalian during the extensional phases related to the generalised gravitational collapse of the previously thickened Hercynian belt. The detachment of the subducted lithosphere (slab detachment) could be related to these phenomena and could explain the granitization of the south external area of the Hercynian belt as well as the migration of the magmatism towards the south of the belt.

Late Visean thermal event in the northern part of the French Massif Central: new 40Ar/39Ar and Rb–Sr isotopic constraints on the Hercynian syn-orogenic extension

he French Massif Central belongs to the central metamorphic domain of the European Hercynian Belt. 40Ar/39Ar mica and amphibole dates have been obtained on magmatic and metamorphic rocks from the northern part of the French Massif Central (Sioule and Brevenne-Violay series). The 336- to 337-Ma dates recorded by a granodiorite (the St. Gervais pluton) and its mylonitic border along the Ste Christine fault and the 337-5-Ma muscovite ages from the Brevenne series are consistent with the age of wrenching related to the late stage of the Hercynian compression. The remaining age spectra from the Sioule area provide ages ranging from 331 to 335 Ma that are interpreted as cooling ages related to rock exhumation after the main stages of metamorphism and deformation in the studied area. Late Visean dates widespread in North Massif Central correspond to a huge magmatic event characterized by the emplacement of red granites, granophyres and sub-aerial acidic volcanics known as the Tufs anthraciferes formation. Rb-Sr isotopic data from volcanic and hypovolcanic rocks from the Sioule and Montagne Bourbonnaise areas place constraints on the source of the magmas. Simple metapelite crustal melting or a major contribution from depleted mantle sources are ruled out, and instead, mantle-derived magmas were probably involved, at least as a heat source, during the Late Visean magmatism. Exhumation and cooling of metamorphic rocks below 300°C occurred contemporaneously with this Late Visean magmatism. In the geodynamic evolution of the Hercynian Belt, the Late Visean (ca. 335-330 Ma) magmatism corresponds to a major break marking the onset of post-collisional extension in the inner zone which had been thickened by collision in Latest Devonian-Earliest Carboniferous. This Late Visean extension in the inner zone which is synchronous to compression in the northern and southern forelands of the belt appears as a syn-orogenic process.

La tectonique cisaillante polyphasee du Sud Limousin (Massif central francais) et son interpretation dans un modele d'evolution polycyclique de la chaine hercynienne

Abstract Structural, kinematics and thermo-barometric analyses of the ductile deformation of the south-Limousin metamorphic formations show a polyphase shear tectonics corresponding to two different thrusting events. The older one, is a to the top-to-the-SW thrusting during middle Devonian. This deformation occurs under minimum PT conditions of 7 Kbar/700 degrees C simultaneously to anatexis. The second event is a top-to-the-NW shearing which occurred in late Devonian-early Carboniferous under Barrovian conditions (5 kbar/600 degrees C). Diorites bodies and non-eclogitized mafic rocks allow us to argue for an extensional phase between the two thrusting events. These two ductile and syn-metamorphic deformations take place in a polycyclic evolution model of the Hercynian belt of the French Massif Central.

Sm-Nd Isotope Tracer Study of UHP Metamorphic Rocks: Implications for Continental Subduction and Collisional Tectonics

Sm-Nd isotope tracer techniques are powerful tools in identification of the protolith nature of UHP and HP rocks and can be used to constrain modeling of tectonic processes of continental collision. UHP rocks may have diverse origins, and not all of them carry the same significance for subduction of continental blocks. In this paper, Sm-Nd isotopic data are compiled for UHP and HP rocks, mostly represented by eclogites and garnet peridotites, from the Alpine, Hercynian (Variscan), and Caledonian belts of western Europe; the Pan-African belts of northern Africa; and the Ross belt of Antarctica. These data then are compared with the isotopic characteristics of the UHP rocks from the Dabie orogen of central China. Except for the coesite-bearing quartzitic metasediments of Dora-Maira (Western Alps), which are clearly of continental origin, all HP and UHP rocks (eclogites and ultramafic rocks) from the Alpine, Hercynian, and Pan-African belts have oceanic affinities with the characteristic positive ϵNd(T) values (= metamorphic initial 143Nd/144Nd ratios). They represent segments of oceanic lithosphere that were subducted, underwent eclogite-facies metamorphism, and later were tectonically transported into orogenic zones during continental collisions. By contrast, the majority of UHP rocks from the European Caledonide and the Dabie orogen have negative ϵND(T) values, indicating continental affinity. This suggests that these mafic and ultramafic rocks have had a long crustal residence time and that their UHP metamorphism is indicative of subduction of ancient and cold continental blocks, as represented by some Precambrian gneiss terranes containing mafic components including greenschists, amphibolites, or basic granulites. In the Dabie orogen, none of the UHP eclogites analyzed thus far have shown oceanic affinity; thus they do not represent subducted Tethys Ocean crust. The preservation of ultrahigh ϵND(0) values (+170 to +260) in eclogites of very low Nd concentrations (average 0.5 ppm) from the Weihai region and of the extraordinarily low δ18O in many eclogites and gneisses, the general absence of syntectonic granites in the Dabie Shan, and the available age data obtained by different techniques all point to a rapid rate of exhumation and the absence of a pervasive aqueous fluid phase during the entire process of subduction and exhumation of the Dabie UHP terrane.

Intrusion level of granitic massifs along the Hercynian belt: balancing the eroded crust

Hercynian granitoid intrusions form a long (3200 km) belt comparable in size to other batholiths in the world. Six massifs have been selected which encompass Cabeza de Araya (Extremadura, Spain), Guitiriz (Galicia, Spain), Pontivy and Mortagne (Brittany, France), La Marche (Massif Central, France) and Fichtelgebirge (Bavaria, Germany). Detailed gravity surveys over these massifs and subsequent inversion provide their shape at depth. Correlation of the deeper zones with internal structures determine the place of the root zones. The shape of the massifs is examined along the strike of the chain. The emplacement of individual massifs is controlled by local tectonics. Most granites are not deeply rooted, but one massif (Cabeza de Araya, Spain) shows a root zone presently as deep as 14 km. Most have about half of their volume in the first 3 km below the present surface. Estimates of the magma volume transferred result in 1500 km 3 issued from one specific feeder, yielding a total of 70,000 km 3 of magma intruded all along the chain. The depth of emplacement of the granitic massifs does not show any significant trend along the strike of the chain. The shallower massifs in the French Massif Central correspond to more deeply eroded areas in the center of the chain. Their root zone, as well as the change in the dip of the walls, are presently observed at depths ranging between 4 and 6 km in Hercynian granites. Both variations are interpreted as being related to the brittle/ductile transition at the time of emplacement. Gross thermal considerations place the transition at its former place during magma emplacement, indicating that the upper crust has not been eroded by more than 6–8 km. This estimate severely contrasts with models involving a doubled crust.