European Variscan Belt Research Articles

Kinematics of orocline tightening in the core of an arc: Paleomagnetic analysis of the Ponga Unit, Cantabrian Arc, northern Spain

Paleomagnetic and structural analyses of the Western European Variscan Belt (WEVB) suggest that the most viable kinematic model for Variscan deformation in northern Iberia is oroclinal bending of an originally linear belt in a two‐stage tectonic history. This history represents two regional compression phases (E‐W in the Late Carboniferous and N‐S in the Permian, both in present‐day coordinates), which resulted in the refolding (about steeply plunging axes) of initially north‐south trending thrusts and folds in the hinge zone, and oroclinal tightening due to vertical axis rotation of the belt's limbs. However, the orocline model has yet to be critically tested in the WEVB's core. This study reports new paleomagnetic, rock magnetic, and structural data from the inner core of the WEVB in order to test opposing kinematic models for the well‐documented fault and fold interference structures formed by late stage Variscan deformation and to better understand the overall development of the WEVB arc. Map‐scale structural features within the WEVB core have a highly sinuous geometry characterized by transverse and thrust‐parallel fold systems formed by fault bend folding over footwall ramps. The intersections of these two fold systems produce steeply plunging interference folds, which are best exposed in the Ponga thrust unit. A total of 67 paleomagnetic sites were collected in the Barcaliente Formation of the Ponga Unit, with emphasis placed on detailed spatial coverage of individual structural domains. A multicomponent paleomagnetic remanence was measured, which is interpreted to be composed of two components; a low unblocking temperature recent viscous magnetization and a high unblocking temperature ancient magnetization that was acquired in the latest Stephanian to Early Permian after initial D1 thrusting and folding bur prior to major secondary rotation. Rock magnetic experiments show that the characteristic remanence magnetization is carried by secondary authigenic pseudosingle‐domain magnetite. These paleomagnetic results are used to determine the kinematics and geometry of postmagnetization deformation by restoring in situ magnetic vectors back to a defined reference direction. On the basis of the restored Ponga Unit geometry, the steeply plunging interference folds found in the WEVB core are best described by a combination of secondary buckling of frontal‐ramp‐parallel hanging wall anticlines and modification of D1 lateral/oblique ramps as frontal ramps during late stage north‐south shortening. This combination of structural modification of the inner core of the WEVB was necessary to accommodate oroclinal bending during the final amalgamation of Pangea in the late Paleozoic.

Structural evolution of the footwall of the Indus Suture in Malakand (N Pakistan) during the Himalayan collision

We report new data on the geometry and kinematics of structures in the rocks of the Indian Plate below the Indus Suture in the area of Malakand (N Pakistan). The study area forms part of the exposed northwestern promontory of the Indian Plate indenting Eurasia to form the Himalayan orogenic belt. Earlier widespread structures (i.e. regional foliation and lineation) are consistent with an approximate N–S convergence direction, in coincidence with the movement direction of the Indian Plate. Younger structures, such as NNE trending open folds that laterally become syntaxes (i.e. Besham or Nanga-Parbat), and relatively minor E–W dextral strike-slip shear zones, are related to a regional tectonic shortening direction subparallel to the trend of the suture. These folds are radial to the northwestern corner of the Indian Plate and contemporaneous with the general bulk N–S convergence. It is only very recently in the structural record that the irregular outline of the Indian Plate played a role in determining the orientation of new structures during its indentation. The structural pattern presents similarities to arcuate megastructures in ancient collisional orogens, such as the Ibero-Armorican arc of the Variscan belt in Europe, where indentation tectonics, as well as post-orogenic oroclinal folding, are invoked to explain the present curvature.

Low-pressure Granulites of the Lišov Massif, Southern Bohemia: Viséan Metamorphism of Late Devonian Plutonic Arc Rocks

The Lisov Granulite Massif differs from neighbouring granulite bodies in the Moldanubian Zone of southern Bohemia (Czech Republic) in including a higher proportion of intermediate–mafic and orthopyroxene-bearing rocks, associated with spinel peridotites but lacking eclogites. In addition to dominantly felsic garnet granulites, other major rock types include quartz dioritic two-pyroxene granulites, tonalitic granulites and charnockites. Minor bodies of high-pressure layered gabbroic garnet granulites and spinel peridotites represent tectonically incorporated foreign elements. The protoliths of the mafic–intermediate granulites (quartz-dioritic and tonalitic) crystallized 360–370 Ma ago, as indicated by laser ablation inductively coupled plasma mass spectrometry U–Pb ages of abundant zircons with well-preserved magmatic zoning. Strongly metamorphically recrystallized zircons give ages of 330–340 Ma, similar to those of other Moldanubian granulites. For the overwhelming majority of the Lisov granulites peak metamorphic conditions probably did not exceed 800–900 C at 4–5 kbar; the equilibration temperature of the pyroxene granulites was 670– 770 C. This is in sharp contrast to conditions of adjacent contemporaneous Moldanubian granulites, which are characterized by a distinct HP–HT signature. The mafic–intermediate Lisov granulites are thought to have originated during Visean metamorphic overprinting of metaluminous, medium-K calc-alkaline plutonic rocks that formed the mid-crustal root of a Late Devonian magmatic arc. The protolith resembled contemporaneous calc-alkaline intrusions in the European Variscan Belt.

Final stages of the Variscan orogeny at the southern Iberian massif: Lateral extrusion and rotation of continental blocks

The southern part of the Iberian Massif contains one of the sutures of the European Variscan belt. Several systems of faults and dikes appear as a result of a period of brittle deformation during the late stages of the Variscan tectonic evolution of this zone. A detailed analysis of these structures has been carried out in this work. Four successive deformation phases have been identified: a first strike‐slip normal faulting stage, a transcurrent stage, a second strike‐slip normal faulting stage, and, finally, a thrusting stage. The age of these phases vary between around 310 Ma and less than 270 Ma. The computed trajectories of the deformation axes show a progressive anticlockwise rotation of around 90°. These data, together with the results of previous geological and geophysical studies of this zone and of the Variscan belt as a whole, lead us to propose a model of tectonic evolution for the last stages of the Variscan collision. Comparison with modern examples of collisional belts and with the results of analog experiments suggests that a promontory of the continental lithosphere of the downgoing plate indented against the active continental margin of the upper plate. The indenter suffered an intense deformation, in accordance with the predictions of some recent analog models simulating the indentation of a four‐layer continental lithosphere. The Variscan and Late Variscan brittle deformation episodes represent the waning stages of the Variscan collision, and not an independent tectonic event.

U-Pb SHRIMP and Nd isotopic data from the western Bohemian Massif (Bayerischer Wald, Germany): Implications for Upper Vendian and Lower Ordovician magmatism

U-Pb SHRIMP dating of zircons of metamagmatites from the Bayerischer Wald (Germany) reveals a complex evolution of this section of the Moldanubian Zone exposed in the western Bohemian Massif of the central European Variscan belt. In the south-western part of the Bayerischer Wald Upper Vendian magmatism is constrained by pooled 206Pb/238U mean ages of 555±12, 549±7 and 549±6 Ma from metarhyolites and a metabasite. Inherited zircon cores were not observed. Zircon overgrowths, yielding pooled 206Pb/238U ages of 316±10 and 319±5 Ma, provide evidence for Variscan metamorphic zircon growth; cathodoluminescence imaging reveals a two-stage metamorphic overprint. In contrast, Lower Ordovician magmatism and anatexis are documented in the north-eastern parts of the Bayerischer Wald by metagranitoids (480±6, 486±7 Ma), an eclogitic metabasite (481±8 Ma) and a leucosome (491 to 457 Ma). Inherited zircon cores are found in Lower Ordovician metagranitoids and the leucosome, indicating a Palaeoproterozoic-Archaean (ca. 2.7, 2.0 Ga) source region, presumably of Gondwana affinity (West African craton), and documenting Cadomian magmatism (ca. 640 Ma). Post-Cadomian metamorphism is inferred from concordant ages of 433±4 and 431±7 Ma. Upper Vendian magmatism is assumed at an active continental margin with ensialic back-arc development (eNd(t) –3.01 to +1.22); the lack of inherited zircon is due to either derivation from juvenile (?volcanic arc) material or complete isotopic resetting of pre-existing zircon. An active continental margin setting, possibly with some lateral variation (accretion/collision) is envisaged for the Lower Ordovician, producing granitoids, rhyolites and leucosomes (eNd(t) -0.5 to -6.27); MORB-type metabasites may be related to ZEV or Marianske Lazně Complex metabasites. A tentative palaeogeographic reconstruction puts the “Bayerischer Wald” in close relationship with the Habach terrane (proto-Alps), as the “eastern” extension of terranes of the northern Gondwana margin.

Lower Permian magmatism of the Iberian Chain, Central Spain, and its relationship to extensional tectonics

Abstract Late Variscan magmatism in the Iberian Chain (Central Spain) is recorded by the presence of both pyroclastic units and high-level intrusions (sills and dykes). This magmatism encompasses a variety of subalkaline igneous rocks, from basalt to rhyolite; andesitic rocks are, however, dominant. The pyroclastic units contain plant fossils and pollen that suggest an Autunian age, which is consistent with the available K-Ar radiometric age data (283–292 Ma) obtained for the hypabyssal intrusions. Earlier amphibole-rich andesite, rich in crustal xenoliths (metapelite, granitoid and quartzite fragments) and xenocrysts (e.g. garnet) suggest the occurrence of significant crustal assimilation in the petrogenesis of the intermediate magmas, whereas later intrusions (pyroxene-rich andesites, basalts and some rhyolites) lack any crustal xenoliths or xenocrysts. Crustal assimilation is also suggested by the radiogenic 87 Sr/ 86 Sr 290 and negative ɛNd 290 values for all the studied rocks. The cross-cutting relationships between the different rock types provide evidence for a multi-stage magmatic evolution, coeval with the post-orogenic transtensive evolution of the southern Variscan belt in Europe. A significant hiatus, covering the Middle Permian and most of the Upper Permian, separates this Lower Permian magmatism from the subsequent Triassic and Jurassic alkaline magmatism that represents different rifting events that affected the Iberian Chain, progressively thinning the Variscan crust as the Alpine cycle began.

Zircon and Garnet Geochronology of Eclogites from the Moldanubian Zone of the Black Forest, Germany

The Moldanubian zone of the Black Forest, Germany, part of the internal zone of the central European Variscan belt, exposes high‐pressure metamorphic rocks as small eclogite lenses in host gneisses. We report zircon U‐Pb and Pb‐Pb ages and garnet Sm‐Nd and Pb‐Pb ages for five eclogites. The less mobile trace element contents of these eclogites are similar to those of mid‐ocean ridge basalts but have moderate to highly fractionated concentrations of the rare earth elements, enrichments in the incompatible elements, and negative anomalies in the high‐field strength elements. Initial ϵNd values range from $$-3$$ to $$+1.8$$ , and initial 87Sr/86Sr ratios range from 0.7040 to 0.7062. These characteristics of the eclogites suggest an origin from mantle‐derived magmas contaminated either by evolved crustal material during intrusion or altered by fluid before or during eclogitization. A combination of zircon U‐Pb and Pb‐Pb evaporation dates of the eclogites constrains a magmatic episode between ∼486 and ∼445 Ma. This episode was contemporary with the early Paleozoic mafic to ultramafic magmatism in the eastern part of the Variscan belt and probably represents an early Paleozoic rift‐stage product. Garnets subjected to the stepwise acid‐leaching method from three eclogites yield Sm‐Nd isochron ages of about 348–340 Ma, similar to the dates previously reported. The same garnets give less precise Devonian to Early Carboniferous Pb‐Pb isochron ages of $$406\pm 31$$ Ma, $$386\pm 21$$ Ma, and $$353\pm 89$$ Ma, which probably date to an earlier stage of the Variscan convergence.

Polymetamorphism of the Variscan basement of the Moldanubian Black Forest (Germany) documented in zircon and garnet minerals from gneisses

High-grade metamorphic Variscan basement is exposed in the Moldanubian zone of the Black Forest (BF), being the internal zone of the European Variscan belt. Zircon grains from K-rich felsic orthogneisses and an anatectic paragneiss in the Moldanubian Black Forest demonstrate a multi-stage crystallization at ∼600 Ma, ∼480 Ma, ∼400–380 Ma, and ∼350 Ma. The last three stages of crystallization probably represent metamorphic overprint during pre-Variscan and Variscan metamorphism.

Garnet Sm-Nd and U-Pb systems: A case study of a granulite from the European Variscan belt

This study presents zircon and garnet ages of a mafic granulite from the high-grade Variscan basement of the Black Forest, Germany and discuss isotope closure temperature of garnet Sm-Nd and U-Pb systems. Zircon grains yield 207Pb/206Pb ages between ∼340 and ∼414 Ma by the U-Pb and evaporation methods. In contract, garnet dating gives Sm-Nd and Pb-Pb isochron ages of (398±3) Ma and (411±14) Ma, respectively, which are older than most of zircon ages. These data imply that most of zircons lost radiogenic Pb, probably due to metamictization or recrystallisation during the granulite-facies metamorphism (−800°C) at −340 Ma. Garnet Sm-Nd and U-Pb systems preserve chronological information of pro-grade metamorphism, probably profiting from a fluid-absence metamorphic environment. These results demonstrate that garnet mineral can be a better candidate than zircon mineral to date high-grade metamorphism by the U-Pb and Sm-Nd methods in some cases.

Changes of crack velocities at the transition from the parent joint through the en échelon fringe to a secondary mirror plane

A ‘trefoil’ joint in granite of the Bohemian Massif, in the Variscan belt of Europe, displays intriguing surface morphologies. It consists of ‘primary’ and ‘secondary’ mirrors as well as ‘primary’ and ‘secondary’ fringes, an arrangement not previously described. These fractographic features are seen to record two cycles of increase and decrease in fracture velocities. Fracture velocities increased along the propagating mirrors in association with the increase of released energy. However, beyond a certain velocity energy dissipated along the new multi-surfaces at the fringes, and this involved reduction in fracture velocities.

The rotations of the Variscides during the Carboniferous collision: paleomagnetic constraints from the Vosges and the Massif Central (France)

This study aims at completing and refining results from previous paleomagnetic investigations carried out in various outcrops of the European Variscan belt. Sampling was performed in Visean effusive and intrusive rocks of the southern Vosges basin and the northern Massif Central, all emplaced in the time range 345–340Ma. Stepwise thermal demagnetization of 254 specimens from 13 sites of the Doller Valley and nine sites of the Massif Central has yielded a very similar paleomagnetic record for both areas. With the exception of post-Carboniferous overprints isolated in the southern Vosges, two groups of directions, with both normal and reversed polarity have been evidenced. The first group of directions, labelled C, occurs in both areas and is characterized by scarce pre-tectonic magnetizations (two sites) and by syn-tectonic overprints (five sites) acquired during the Late Visean convergence, i.e. in the time-range 340–328Ma. The mean pole Co is 28°S/45°E (k=139, α95=5°). The main group, labelled B, shows a negative fold test and a positive reversal test, indicating that the magnetizations are post-tectonic overprints, acquired after the main tightening phase, i.e. after 328Ma. At a few sites in both areas were evidenced syn-tectonic B magnetizations that are in favour of an overprinting in the context of post-Visean extension tectonics. The mean post-tectonic B direction of the Doller Valley (southern Vosges) 62°/−25° (k=52, α95=6°, N=11; VGP: 8°N/127°E) is consistent with the mean post-tectonic B direction 72°/−23° (k=126, α95=5°, N=6; VGP: 3°N/114°E) obtained from the tuffs, the grey microgranite and the dyke intrusive into the microgranite of the Massif Central. The occurrence of similar paleomagnetic records in the eastern and central Variscides implies that, from the Massif Central to the Sudetes, the belt has probably moved as one block, since Early Carboniferous.The change in inclination of the C components suggests a southward drift of the Moldanubian and Saxothuringien Variscides in the Late Visean (340–328Ma). According to geological features, in the same time, the Rhenohercynian crust was underplating the Saxothuringian arc and fore-arc zone. Subsequent to 328Ma, the drift turned into a rapid clockwise rotation by about 70° around a pole located on southeastern corner of Baltica, leading to the closure of the Rhenohercynian oceanic basin. In the Namurian, large-scale overprinting (B components), likely related to post-rotation extension, affected the internal belt. The convergence continuing, the Variscides underwent a second clockwise rotation by about 45° in the Late Namurian–Westphalian (320–305Ma). Stacking of the Rhenohercynian sedimentary cover and thrusting over the London–Brabant basement may be related with this motion.

The Cadomian active margin (North Armorican Massif, France): a segment of the North Atlantic Panafrican belt

Within the European Variscan belt, the Armorican Massif (northwestern France) contains an isolated fragment of the North Atlantic Panafrican belt — the Cadomian block. Mostly unaffected by Variscan deformation, this block is composed of Neoproterozoic rocks, ranging in age from 620 to 540Ma (Brioverian), locally overlying a composite basement of Icartian (2000Ma) or Pentevrian (750Ma) age.The Cadomian tectonics, the last magmatic manifestations of which are dated at around 540Ma, were sealed by a cover of Early Cambrian rocks in Normandy and Early Ordovician rocks in Brittany.Five units are distinguished in this Cadomian block — Trégor, Saint-Brieuc, Guingamp, Saint-Malo and Fougères. Although the units are bound by major faults, the overall coherence of their different protolithic characteristics suggests that they can be fitted into a simple geodynamic model.The Trégor Unit, represented by the North Trégor Batholith of crustal affinity and its Icartian host rock, probably reflects an early Cadomian evolution.The Saint-Brieuc Unit, essentially made up of magmatic rocks of juvenile affinity, includes two assemblages juxtaposed by the Cadomian tectonics: (1) a volcanosedimentary succession formed in a typical island-arc setting reflecting subduction around 610Ma; and (2) a volcanic–plutonic complex emplaced around 590Ma, in an extensional active continental margin of Pentevrian age.The Guingamp Unit, predominantly migmatitic, includes a tectonic slice of deep origin, the extrusion of which accompanied the exhumation of the migmatites; it may represent the trace of a possible suture.The Saint-Malo Unit is composed of Brioverian detrital rocks, the significance of which is uncertain — passive continental margin or accretionary prism; it is structured around a late migmatitic dome (540Ma).The Fougères Unit corresponds to the outer domain of the Cadomian orogeny; it was only weakly deformed by Cadomian tectonics, and is intruded by the large Mancellian Batholith of crustal origin, dated at 540Ma.Together the structural units are interpreted in the framework of an evolving active margin juxtaposing continental margin, marginal basin and volcanic arc, before being shortened during the Cadomian convergent tectonics.

Ammonium concentration and nitrogen isotope composition in metasedimentary rocks from different tectonometamorphic units of the European Variscan Belt

The content and isotopic composition of ammonium ( NH 4 +) were measured by using Kjeldahl distillation for 63 samples of predominantly metasedimentary rocks from two different geological settings of the European Variscides, i.e., the Erzgebirge and the Zone of Erbendorf-Vohenstrauss in Germany. The studied Erzgebirge schists represent an early Paleozoic passive margin sequence that reappears in different deep subducted metamorphic units. This allows an efficient examination of nitrogen loss and isotope fractionation during prograde metamorphism. Ammonium is found to be progressively depleted, accompanied by a shift in δ 15N from the Low-grade Units (≈2 kbar/300°C) with 638 ± 124 ppm and δ 15N = +2.2 ± 0.6‰, to the Garnet-Phyllite Unit (≈9 kbar/470°C) with 621 ± 190 ppm and δ 15N = +3.5 ± 0.9‰, the Mica Schist/Eclogite Unit (≈12 kbar/550°C) with 394 ± 113 ppm and δ 15N = +3.9 ± 0.8‰ and to the Gneiss/Eclogite Unit (>12 kbar/730°C) with 99 ± 32 ppm and δ 15N = +7.7 ± 2.0‰. Using equilibrium models for Rayleigh distillation and batch volatilization suggest that the nitrogen depletion took place by ammonia release. Only for the mica schists, this isotope fractionation can be explained by the loss of molecular nitrogen. In comparison with the early Paleozoic schists, the Proterozoic ortho- and paragneisses of the Erzgebirge Gneiss Unit (≈6–8 kbar/650°C) contain significantly lower amounts of ammonium (≈70 ppm) and reach relatively low δ 15N values (+2.5‰ to +3.6‰). The Paragneisses from the Zone of Erbendorf-Vohenstrauss (ZEV) sampled from the KTB pilot hole represents a separate tectonometamorphic unit that has undergone polyphase metamorphic conditions with the major imprint being amphibolite facies (7 kbar/650–700°C). The metagraywackes have NH 4 + contents of ≈80 ppm and δ 15N of approximately +6‰. Gneisses in the vicinity of a late cataclastic shear zone at 2000 m depth are characterized by a shift in NH 4 + (>250 ppm) and δ 15N up to +12‰, which suggest late fluid-rock interactions with a nitrogen- and 15N-enriched fluid.

Diachronous cooling on both sides of a major strike slip fault in the Variscan Maures Massif (south-east France), as deduced from a detailed 40Ar/ 39Ar study

A detailed 40Ar/ 39Ar geochronological study was performed in the Maures Massif (SE France) to define the thermal history of this geochronologically poorly constrained segment of the Variscan belt. A total of 22 plateau ages were obtained on single grains and bulk samples of amphibole, muscovite and biotite extracted from 21 metamorphic and magmatic rocks sampled on both sides of the Grimaud Fault. Migmatites, micaschists, gneisses, pegmatites and granites displayed muscovite and biotite plateau ages ranging from 317.2±1.0 to 322.9±1.7 Ma on the western side and from 300.2±0.6 to 306.0±2.4 Ma to the east of the Grimaud fault. In the western block, amphiboles from amphibolites yielded plateau ages of 328.1±2.8 and 329.9±2.1 Ma, whereas in the eastern block, amphiboles from amphibolite and magmatic bodies yielded variable ages ranging from 307.9±1.2 to 317.4±2.4 Ma. These data clearly demonstrate distinct cooling histories between at least 330 and 300 Ma, on both sides of the Grimaud Fault, which consequently appears as a major crustal fault during the late Variscan orogeny. As shown by the concordant (for each type of mineral) data obtained on biotite, muscovite, and amphibole from magmatic and metamorphic rocks, the two crustal blocks cooled down independently, each one homogeneously, in the range of temperatures 550–300°C. Two distinct periods of fast cooling appear, at 320 Ma to the west and 305–300 Ma to the east. These data are consistent with those previously obtained on other regions of the European Variscan belt.

Late-Variscan antimony mineralisation in the Rheinisches Schiefergebirge, NW Germany: evidence for stibnite precipitation by drastic cooling of high-temperature fluid systems

Antimony-rich vein mineralisation is widespread in the German part of the Variscan orogenic belt. Mineralogical investigation of a representative suite of these deposits, coupled with fluid inclusion characterisation and microthermometry, permits a reconstruction of their genetic evolution. Two structural settings host antimony mineralisation: the cores or flanks of anticlinal zones and major lithological contrasts. Channelled migration of geothermal fluids through permeable rock sequences and later stagnation of fluids in cap-rock situations inside the anticlinal zones led to mineral deposition. The mineralising event is interpreted as relating to input of deep-sourced fluids during late-orogenic exhumation at the transitional stage between collision tectonics and the late-Variscan extensional regime. Fluid inclusion data, chlorite geothermometry and the presence of meneghinite as a characteristic Pb-Sb-sulfosalt mineral in a number of vein systems allows constraints on model P-T conditions at the onset of mineralisation to be made. These are as high as 390 to 440 °C at 0.6–1.0 kbar for the Saarsegen, Apollo and Schone Freundschaft deposits, with lower temperatures of 320–340 °C being obtained for the Spes deposit. The fluid inclusion data indicate drastic fluid cooling during the mineralising event; minimum temperatures of approximately 150–220 °C are obtained for all deposits at the end of vein quartz formation, which coincided with deposition of stibnite and most of the Pb-Sb sulfosalts. Besides the formation of extensional quartz-stibnite-Pb-sulfosalt veins, the mineralising, low-salinity NaCl-KCl-rich high-temperature tectonic brines have overprinted sulfide assemblages within earlier siderite-(Cu)-Pb-Zn veins. This has led to replacement reaction textures and remobilisation of sulfide components within the vein systems. In contrast with the earlier siderite-(Cu)-Pb-Zn veins, neither the quartz-stibnite-sulfosalt nor the (Cu)-Pb-Sb sulfosalt assemblages were affected by Variscan deformation. Rather, they display characteristic extensional features crosscutting all earlier structures and can thus be assigned to a later phase of mineralisation. Fluid composition characteristics and structural criteria indicate formation in the latest part of the Variscan mineralisation cycle; a post-Variscan genesis being rejected on grounds of conspicuously diverging fluid characteristics. A comparison of antimony deposits in the Rheinisches Schiefergebirge with other late-orogenic deposits elsewhere in the European Variscan belt indicates a significant number of shared features, enabling them to be placed into a common model related to the onset of late-Variscan brittle extensional tectonics.

P– T evolution of ‘crustal’ garnet peridotites and included pyroxenites from Nonsberg area (upper Austroalpine), NE Italy: from the wedge to the slab

Up-to-date thermobarometers have been applied to garnet-bearing peridotites and pyroxenites from Nonsberg area (upper Austroalpine, NE Italy), a remnant of the lower crust of the Central European Variscan belt. The Nonsberg ultramafic rocks record an unusual metamorphic history, from high-temperature, moderate-pressure, spinel-facies conditions to low-temperature, high-pressure, garnet-facies conditions. This peculiar P– T evolution is interpreted as a result of subduction processes, which involved mantle-wedge peridotites overlying a subducting continental slab. The following scenario is proposed based on our P– T estimates and available physical models for subduction zones. Relatively high-temperature (∼1200°C, 1.31–1.58 GPa) mantle-wedge peridotites (spinel lherzolites) were intruded by hot (>1400°C) hydrous melts rising from deeper, inner portions of the wedge. Coarse-grained, garnet-free pyroxenites were segregated from these melts and formed veins in the host peridotites. Convection within the wedge induced by the downgoing slab caused the peridotites to flow towards the slab while cooling at essentially constant pressure. The peridotites were then dragged to greater depths at slightly decreasing, or nearly constant, temperature by the downward mantle flow near the wedge corner and near the wedge-slab interface and eventually emplaced into the underlying slab. Entrainment in the continental crust, either as tectonic slices or as sinking mantle blobs, was probably favored by density contrast between overlying mantle and underlying crust. Mantle flow and entrainment in the cold continental crust caused the peridotites to cool down to ∼850°C before, or while, being subducted together with the slab to depths of about 90 km. During this T-decrease, P-increase path, garnet formed at the expense of spinel and pyroxenes, giving rise to garnet ± spinel-bearing lherzolites and pyroxenites.

The geometric evolution of structures in granite during continuous deformation from magmatic to solid-state conditions; an example from the central European Variscan Belt

Intrusive and in-situ granites of the late-Variscan South Bohemian batholith (Austria) were deformed continuously during crystallization and after solidification by horizontal crustal shortening. When the proportion of melt was relatively high, the granitic magmas accomodated deformation in (low-angle) thrust zones. With decreasing melt content shearing became more localized and the dip angle of shear planes increased during the formation of submagmatic reverse shear zones. During this submagmatic stage the orientation of shear planes was variable. They probably rotated from flat in the magmatic stage to a steeper orientation perpendicular to the shortening direction and/or parallel to post-magmatic conjugated strike-slip zones, that became active at high-T, solid state conditions. Thermobarometry and the compositions of plagioclase phenocrysts, subgrains, and recrystallized grains indicate that the magmatic, submagmatic, and high-T, solid state strike-slip deformation occurred at similar P-T conditions. Conjugate strike-slip zones are well known structures of solid-state crustal shortening. In syn-tectonic plutons, they may have magmatic or submagmatic precursors, and because they preferably crosscut the plutons rather than the country rock, this could be the result of earlier magmatic or submagmatic deformation, that evolves to solid state shear zones. Hot (migmatitic) crust during magma emplacement, coupled with low cooling rates and a constant direction of crustal shortening probably aids the generation and preservation of related magmatic, submagmatic, and postmagmatic structures.

Ore deposits of the French Massif Central: insight into the metallogenesis of the Variscan collision belt

The French Massif Central (FMC) represents the whole West European Variscan (WEV) belt, in terms of both the geodynamic evolution and the metallic content. Thus, a study of the metallogenic evolution of the FMC may elucidate the conditions that allow the mineralisation of a collision belt, since recent collision belts, e.g. the Himalayas or the Alps show that mineralisation does not necessarily result from the collision process. The Palaeozoic history of the FMC is divided into three geodynamic stages unevenly involved from the metallogenic view point. The Eo-Variscan stage (Cambrian to Silurian) was not important; the Meso-Variscan stage (Devonian-Early Carboniferous) was of limited importance; and most of the mineralisations formed during the Neo-Variscan stage (Late Carboniferous-Early Permian). In addition, some more mineralisation was produced during the Mesozoic because of the thermal reactivation linked with the Alpine orogenies.

Influence of deformation partitioning and metamorphic re‐equilibration onP–Tpath reconstruction in the pre‐Alpine basement of central Southern Alps (Northern Italy)

The dominant foliation (S2) in the metapelites of the Southalpine basement, near the western side of the Tertiary Adamello intrusive stock, is a Variscan greenschist facies planar fabric, slightly reworked during thick‐skin Alpine tectonics. S2 is defined by muscovite and chlorite and was formed by decrenulation of pre‐existing foliations, which are confined to metre‐size, less‐deformed domains and defined by biotite and white mica. The pre‐S2 fabric is composite (D1a & D1b) and defined by contrasting amphibolite facies metamorphic assemblages in different residual sites. Cld+BtI+Grt+MsI+Pl+Qtz and St+BtII+Grt+MsII+Pl+Qtz assemblages mark D1a and D1b fabrics respectively; these developed during successive steps of a single, temperature‐prograde polyphase event, rather than during separate tectonometamorphic imprints affecting different tectonic units, later coupled during a D2 greenschist facies stage. Thermobarometric estimates of assemblages formed during D1a, D1b and D2 show a transition fromT =480–540 °C (during D1a) toT =570–660 °C (during D1b), corresponding to a slight pressure‐increase from 0.75–0.95 GPa to 0.85–1.15 GPa. D2 greenschist retrogression corresponds to a pressure and temperature decrease (T <400–550 °C andP<0.3–0.4 GPa). ThisP–T–deformation–time path is inferred to be the result of uplift from a depth ofc.35 km, after Palaeozoic subduction and continental collision; it is consistent with models postulated for other metamorphic units of the Variscan Belt in Europe. This is the first documented example in the Southern Alps of temperature‐prograde metamorphism before Palaeozoic collision.

Low-Pressure Metamorphism in the Sierra Albarrana Area (Variscan Belt,Iberian Massif)

A low-pressure metamorphic zonation ranging from biotite to migorogenic belts, including the Variscan Belt of Southwest matite zones occurs in the Sierra Albarrana area (Variscan Belt of Europe. De Yoreo et al. (1991) presented a review of some southwestern Iberian Peninsula) in uppermost Precambrian to Lower possible tectonic settings of low-pressure metamorphism. Palaeozoic metasedimentary rocks. The principal deformation in Those workers considered low-pressure metamorphism this area is related to a major ductile shear zone whose central part to be that in which peak temperature is above 500°C is localized immediately to the southwest of the Sierra Albarrana and pressure does not exceed that of the aluminium Quartzites. The metamorphism is synchronous with respect to this silicate (Als) triple point. The following four tectonic deformation. The metamorphic zones are symmetrically distributed situations were envisaged for the low-pressure metawith respect to the Sierra Albarrana Quartzites. Pressure–temperature morphism (De Yoreo et al., 1991): (1) magmatic arcs; (P–T) conditions are ~3·5–4 kbar and range from ~400°C (2) regions of crustal extension; (3) continent–continent (biotite zone) to 500°C (staurolite–garnet zone) up to 650–700°C collision zones where thermal relaxation occurs after (migmatite zone). We have not detected pressure variations along crustal thickening; (4) regions with significant fluxes of the different metamorphic zones. Relic kyanite is observed in the aqueous fluids, especially those located above subduction form of inclusions in andalusite within veins in the lower-grade part zones. of the staurolite–andalusite zone. The low-pressure metamorphism of This paper is concerned with the description of a highthe Sierra Albarrana area arises from a two-stage history including temperature–low-pressure facies series terrane (the Sierra moderate crustal thickening followed by subsequent localization of Albarrana area, Variscan Belt, southwestern Iberian Masdeformation in a transcurrent shear zone during peak P–T conditions. sif ) (Figs 1 and 2). In this area, a metamorphic zonation Channelized fluid flow within the major ductile shear zone may from the biotite zone up to the migmatite zone developed have contributed to the heat budget of the low-pressure metamorphism. at low pressure (Garrote, 1976; Gonzalez del Tanago & Peinado, 1990). As will be shown here, the metamorphic zonation is not related to post-thickening thermal relaxation but rather exhibits a roughly symmetric pattern