Variscan Subduction Research Articles

The transition from Variscan collision to continental break-up in the Alps: insights from the comparison between natural data and numerical model predictions

Abstract Records of Variscan structural and metamorphic imprints in the Alps indicate that before Pangaea fragmentation, the continental lithosphere was thermally and mechanically perturbed during Variscan subduction and collision. A diffuse igneous activity associated with high-temperature (HT) metamorphism, accounting for a Permian–Triassic high thermal regime, is peculiar to the Alpine area and has been interpreted as induced either by late-orogenic collapse or by lithospheric extension and thinning leading to continental rifting. Intra-continental basins hosting Permian volcanic products have been interpreted as developed either in a late-collisional strike-slip or in a continental rifting setting. Two-dimensional finite element models have been used to shed light on the transition between the late Variscan orogenic evolution and lithospheric thinning that, since Permian–Triassic time, announced the opening of Tethys. Comparison of model predictions with a broad set of natural metamorphic, structural, sedimentary and igneous data suggests that the late collisional gravitational evolution does not provide a thermo-mechanical outline able to justify mantle partial melting, evidenced by emplacement of huge gabbro bodies and regional-scale high-temperature metamorphism during Permian–Triassic time. An active extension is required to obtain model predictions comparable with natural data inferred from the volumes of the Alpine basement that were poorly reactivated during Mesozoic–Tertiary convergence.

Durbachites–Vaugnerites – a geodynamic marker in the central European Variscan orogen

AbstractDurbachites–Vaugnerites are K–Mg‐rich magmatic rocks derived from an enriched mantle source. Observed throughout the European Variscan basement, their present‐day geographical distribution does not reveal any obvious plate‐tectonic context. Published geochronological data show that most durbachites–vaugnerites formed around 335–340 Ma. Plotted in a Visean plate‐tectonic reconstruction, the occurrences of durbachites–vaugnerites are concentrated in a hotspot like cluster in the Galatian superterrane, featuring a distinctive regional magmatic province. Reviewing the existing local studies on Variscan durbachite–vaugnerite rocks, we interpret their extensive appearance in the Visean in terms of two factors: (i) long‐term mantle enrichment above early Variscan subduction systems; and (ii) melting of this enriched subcontinental mantle source during the Variscan collision stage due to thermal anomalies below the Galatian superterrane, possibly created by slab windows and and/or the sinking of the subducted Rheic slab into the mantle. The tectonic reorganization of Europe in the Late Palaeozoic and during the Alpine orogeny has torn apart and blurred this marked domain of durbachites–vaugnerites.

High-pressure metamorphism in the Early Variscan subduction complex of the SW Iberian Massif

Several units exposed in the boundary area of the Ossa Morena Zone (OMZ) and the South Portuguese Zone (SPZ) preserve petrographic and thermobarometric evidence for an early metamorphic episode (M1), developed under a high-P, low to intermediate-T gradient, related to Early Variscan subduction in the SW Iberian Massif. In the OMZ, these are the Cubito–Moura Unit (Pmin~9.2kbar and T=395±45°C), blueschists from its basal mélange (Pmin~12.4kbar and T=310±11°C), and the underlying Fuenteheridos Group (P=10.9±0.4kbar and T=449±31°C). The equivalent units in the SPZ are the La Minilla Formation (P=8.7±0.4kbar and T=388±16°C) and the lawsonite pseudomorphs-bearing Pulo do Lobo Formation. All these units formed part of an approximately NE verging orogenic wedge (present coordinates) developed by the accretion of subducted slabs of the outermost margin of Gondwana and other elements of the Rheic Ocean realm, from at least the Middle Devonian to the lowermost Tournaisian. High-pressure rocks were subsequently emplaced on more internal zones of the OMZ that only experienced a younger high-T, low to intermediate-P metamorphism (M2). This high-T event was coeval with magmatic activity from the uppermost Devonian to the Middle Mississippian, probably as a consequence of transtensional lithospheric thinning and/or delamination of the lower crust and mantle lithosphere of the Gondwana margin. Pre-Late Devonian synorogenic sedimentation in forearc and back-arc basins of the subduction complex evolved to a Late Devonian–Middle Mississippian foreland basin system in early collisional stages. Finally, a new Middle–Late Mississippian fold-and-thrust belt with opposite (SW) vergence and new foreland basins developed during late collisional stages.

Tracing the influence of the Trans-European Suture Zone into the mantle transition zone

Cratons with their thick lithospheric roots can influence the thermal structure, and thus the convective flow, in the surrounding mantle. As mantle temperatures are hard to measure directly, depth variations in the mantle transition zone (MTZ) discontinuities are often employed as a proxy. Here, we use a large new data set of P-receiver functions to map the 410km and 660km discontinuities beneath the western edge of the East European Craton and adjacent Phanerozoic Europe across the most fundamental lithospheric boundary in Europe, the Trans-European Suture Zone (TESZ). We observe significantly shorter travel times for conversions from both MTZ discontinuities within the craton, caused by the high velocities of the cratonic root. By contrast, the differential travel time across the MTZ is normal to only slightly raised. This implies that any insulating effect of the cratonic keel does not reach the MTZ. In contrast to earlier observations in Siberia, we do not find any trace of a discontinuity at 520km depth, which indicates a rather dry MTZ beneath the western edge of the craton. Within most of covered Phanerozoic Europe, the MTZ differential travel time is remarkably uniform and in agreement with standard Earth models. No widespread thermal effects of the various episodes of Caledonian and Variscan subduction that took place during the amalgamation of the continent remain. Only more recent tectonic events, related to Alpine subduction and Quarternary volcanism in the Eifel area, can be traced. While the East European craton shows no distinct imprint into the MTZ, we discover the signature of the TESZ in the MTZ in the form of a linear region of about 350km width with a 1.5s increase in differential travel time, which could either be caused by high water content or decreased temperature. Taking into account results of recent S-wave tomographies, raised water content in the MTZ cannot be the main cause for this observation. Accordingly, we explain the increase, equivalent to a 15km thicker MTZ, by a temperature decrease of about 80K. We discuss two alternative models for this temperature reduction, either a remnant of subduction or an indication of downwelling due to small-scale, edge-driven convection caused by the contrast in lithospheric thickness across the TESZ. Any subducted lithosphere found in the MTZ at this location is unlikely to be related to Variscan subduction along the TESZ, though, as Eurasia has moved significantly northward since the Variscan orogeny.

Geochemical and Sr–Nd isotopic characteristics and pressure–temperature estimates of mantle xenoliths from the French Massif Central: evidence for melting and multiple metasomatism by silicate-rich carbonatite and asthenospheric melts

Abstract Ultramafic xenoliths from Mont Briançon, Ray Pic and Puy Beaunit in the French Massif Central show variable mineral compositions that indicate a residual origin after various degrees of partial melting of a fertile peridotite. Furthermore, trace element and Sr–Nd isotopic variations of clinopyroxenes indicate mixing processes between depleted mantle and enriched components such as asthenospheric melt and silicate carbonatite melt. Pyroxene geothermometer and CO 2 geobarometer estimates are 860–1060 °C at 0.92–1.10 GPa for Mont Briançon, 930–980 °C at 0.89–1.04 GPa for Ray Pic and 840–940 °C at 0.59–0.71 GPa for Puy Beaunit. From south to north, the xenoliths show the following trends: (1) deeper to shallower origin; (2) more depleted mineral compositions, suggesting higher degrees of partial melting; and (3) more enriched isotopes and trace elements, indicating a mixing process with a silicate-rich carbonatite melt characterized by high H 2 O and K 2 O, possibly during Variscan subduction.

Upper and lower crustal evolution during lithospheric extension: numerical modelling and natural footprints from the European Alps

Abstract When continental rifting does not develop on a stable continental lithosphere, geodynamic interpretation of igneous and metamorphic records, as well as structural and sedimentary imprints of rifting-related lithospheric extension, can be highly ambiguous since different mechanisms can be responsible for regional HT–LP metamorphism. This is the case of the European Alps, where the exposure of Variscan structural and metamorphic imprints within the present-day Alpine structural domains indicates that before the Pangaea break-up, the continental lithosphere was thermally and mechanically perturbed by Variscan subduction and collision. To reduce this ambiguity, we use finite-element techniques to implement numerical geodynamic models for analysing the effects of active extension during the Permian–Triassic period (from 300 to 220 Ma), overprinting a previous history of Variscan subduction-collision up to 300 Ma. The lithosphere is compositionally stratified in crust and mantle and its rheological behaviour is that of an incompressible viscous fluid controlled by a power law. Model predictions of lithospheric thermal state and strain localization are compared with metamorphic data, time interval of plutonic and volcanic activity and coeval onset of sedimentary environments. Our analysis confirms that the integrated use of geological data and numerical modelling is a valuable key for inferring the pre-orogenic rifting evolution of a fossil passive margin. In the specific case of the European Alps, we show that a relative high rate of active extension is required, associated for example with a far extensional field, to achieve the fit with the maximal number of tectonic units. Furthermore, in this case only, thermal conditions allowing partial melting of the crust accompanying gabbroic intrusions and HT–LP metamorphism are generated. The concordant set of geological events that took place from Permian to Triassic times in the natural Alpine case is justified by the model and is coherent with the progression of lithospheric thinning, later evolving into the appearance of oceanic crust.

Geochemistry of Fe-rich peridotites and associated pyroxenites from Horní Bory, Bohemian Massif: Insights into subduction-related melt–rock reactions

Variscan, mantle-derived peridotites and associated pyroxenites occur as boudins in Moldanubian granulite near the town of Horní Bory in western Moravia. The peridotites consist of two compositionally distinct suites, one of Mg–lherzolite (Mg-number = 89–91, 87Sr/ 86Sr = 0.7046–0.7068, ε Nd = + 4.1 to + 5.3), and another of Fe–dunite/wehrlite (Mg-number = 83–88, 87Sr/ 86Sr = 0.7079–0.7087, ε Nd = − 2.8 to − 1.3). Modelling of Mg–Fe exchange between peridotite and Fe-rich melts reveals that the modal and chemical composition of the Fe–dunite/wehrlite suite can be produced by melt–rock reactions between lherzolite and SiO 2-undersaturated melts of basaltic composition at melt/rock ratios ranging from 0.3 to 2. In such a model, pyroxenites represent the crystalline product (± trapped liquid) of melts migrating along conduits in peridotite. The Fe–dunite/wehrlite suite and pyroxenites are enriched in the LILE and depleted in the HFSE. The trace element and Sr–Nd isotopic compositions of Horní Bory peridotites and pyroxenites point to a significant component of crustal material in the invasive melts. The melt–rock reactions recorded in the Horní Bory ultramafic boudins are attributed to melt percolation in a mantle wedge above a Variscan subduction zone.

Permian‐Triassic high thermal regime in the Alps: Result of late Variscan collapse or continental rifting? Validation by numerical modeling

The Permian‐Triassic igneous activity, associated with regional scale deformation developed under high‐temperature/low‐pressure (HT‐LP) metamorphic conditions and widely recorded in the pre‐Alpine crust of the European Alps, can result from late orogenic collapse of a collisional belt or from lithospheric thinning leading to a continental rifting process. In order to reduce this ambiguity, we use a two‐dimensional finite element model to give new insights on the sequence of mechanisms operating during active ocean‐continent convergence, followed by continental collision and pure gravitational evolution and on the regional geodynamic interpretation of the Paleozoic‐Mesozoic evolution of the Alpine area. The modeling predictions are compared with the PT climax conditions of Variscan and Permian‐Triassic metamorphism affecting the continental crust of the Helvetic to Southalpine domains. The good agreement between model predictions and natural data realized for the early to Neovariscan evolution indicates that during Paleozoic, the pre‐Alpine crust of the Alps was part of an active ocean‐continent convergence margin and an intracontinental suture zone. Furthermore, modeling results support the interpretation envisaging a Permian‐Triassic lithospheric extension as responsible for the HT metamorphism and related intense igneous activity. This evolution was precursor of the Mesozoic oceanization, during which the tectonic units, coupled and accreted during the Variscan subduction and collision, were separated to form the two passive European and Adriatic continental margins.

U–Th–Pb and Lu–Hf isotopic constraints on the evolution of sub-continental lithospheric mantle, French Massif Central

We have carried out a Pb double-spike and Lu–Hf isotope study of clinopyroxenes from spinel-facies mantle xenoliths entrained in Cenozoic intraplate continental volcanism of the French Massif Central (FMC). U–Th–Pb and Lu–Hf isotope systematics verify the existence of different lithospheric domains beneath the northern and southern FMC. Northern FMC clinopyroxenes have extreme Lu/Hf ratios and ultra-radiogenic Hf ( ε Hf = +39.6 to +2586) that reflect ∼15–25% partial melting in Variscan times (depleted mantle model ages ∼360 Ma). Zr, Hf and Th abundances in these clinopyroxenes are low and unaffected by hydrous/carbonatitic metasomatism that overprinted LILE and light REE abundances and caused decoupling of Lu/Hf–Sm/Nd ratios and Nd–Hf isotopes ( ε Nd = +2.1 to +91.2). Pb isotopes of northern FMC clinopyroxenes are radiogenic ( 206Pb/ 204Pb > 19), and typically more so than the host intraplate volcanic rocks. 238U/ 204Pb ratios range from 17 to 68, and most samples have distinctively low 232Th/ 238U (<1) and 232Th/ 204Pb (3–22). Clinopyroxenes from southern FMC lherzolites are generally marked by overall incompatible trace element enrichment including Zr, Hf and Th abundances, and have Pb isotopes that are similar to or less radiogenic than the host volcanic rocks. Hf isotope ratios are less radiogenic ( ε Hf = +5.4 to +41.5) than northern FMC mantle and have been overprinted by silicate-melt-dominated metasomatism that affected this part of FMC mantle. Major element and Lu concentrations of clinopyroxenes from southern FMC harzburgites are broadly similar to northern FMC clinopyroxenes and suggest they experienced similar degrees of melt extraction as northern FMC mantle. 238U/ 204Pb (53–111) and 232Th/ 204Pb ratios (157–355) of enriched clinopyroxenes from the southern FMC are extreme and significantly higher than the intraplate volcanic rocks. In summary, mantle peridotites from different parts of the FMC record depletion at ∼360 Ma during Variscan subduction, followed by differing styles of enrichment. Northern FMC mantle was overprinted by a fluid/carbonatitic metasomatic agent that carried elements like U, Pb, Sr and light REE. In contrast, much of the southern FMC mantle was metasomatised by a small-degree partial silicate melt resulting in enrichment of all incompatible trace elements. The extreme mantle 238U/ 204Pb (northern and southern FMC), 232Th/ 238U (northern FMC) and 232Th/ 204Pb ratios (southern FMC), coupled with unremarkable present-day Pb isotope ratios, constrain the timing of enrichment. Mantle metasomatism is a young feature related to melting of the upwelling mantle responsible for Cenozoic FMC volcanism, rather than subduction-related metasomatism intimately associated with mantle depletion during the Variscan orogeny. The varying metasomatic styles relate to pre-existing variations in the thickness of the continental lithospheric lid, which controlled the extent to which upwelling mantle could ascend and melt. In the northern FMC, a thicker and more refractory lithospheric lid (⩾80 km) only allowed incipient degrees of melting resulting in fluid/carbonatitic metasomatism of the overlying sub-continental lithospheric mantle. The thinner lithospheric lid of the southern FMC (⩽70 km) allowed larger degrees of melting and resulted in silicate-melt-dominated metasomatism, and also focused the location of the volcanic fields of the FMC above this region.

Comparative review of the Variscan granitoids of Morocco and Iberia: proposal of a broad zonation

The present paper takes the Variscan granitoids of Morocco as the starting point for the discussion of broad correlations of magmatism along the Variscan orogenic belt. Remarkable differences stand out between the Western Moroccan Meseta (peraluminous granites) and the Eastern Moroccan Meseta (potassic to shoshonitic, calc-alkaline granitoids). In Iberia, the Variscan magmatism can be arranged into distinctive zones too, which have been compared with those in Morocco. Although two of these zones mimic the I-type and S-type subduction paired belts, the intrusion of the granitoids was mostly late-collisional. Calc-alkaline, metaluminous (I-type) potassic granitoids of the southern zone may reflect a source of enriched mantle (such an enrichment taking place during Devonian Variscan subduction or latest Precambrian subduction) and different degrees of crustal contamination. Considerable volumes of peraluminous S-type granites in the central magmatic zone reflect a widespread melting of fertile levels in the crust. The third magmatic zone is characterized by the precocity (Middle Devonian to Early Carboniferous) of its magmatism with respect to the two other zones and the abundance of basic rocks. For the latter zone, we suggest that most of the magmas were generated in the context of an intra-orogenic continental rifting, while others may be subduction-related.

Dating the mantle roots of young continental crust

Subcontinental lithospheric mantle xenoliths from beneath the French Massif Central contain clinopyroxene with high Lu/Hf and Hf isotope ratios ( 176 Lu/ 177 Hf = 0.1-10.8; e Hf = +40 to +2600). These Lu-Hf isotope systematics yield model ages of 313-377 Ma and apparently date melt extraction in a mantle wedge during Variscan subduction, with the residual mantle subsequently forming the stabilizing mantle keel of young European continental crust. The extremely depleted Lu-Hf mantle systematics contrast with other isotopic signatures (Sr, Pb, Nd) that have been overprinted by mantle metasomatism. The high Lu/Hf values developed in mantle clinopyroxene by melt extraction, coupled with the robustness of this system to some types of metasomatism, provide a new chronometer for directly dating crust-mantle differentiation in both young and ancient continental regions. The process of metasomatic decoupling of Nd and Hf isotopes, which has been previously recognized in Hawaiian oceanic mantle and in French Massif Central subcontinental mantle, may be responsible for the prominent displacement of terrestrial rocks to lower e Nd at a given e Hf as compared to Bulk Earth reference values obtained from analysis of chondritic meteorites.

Variscan basic dykes in the Pelagonian (Northern Greece and south FYROM): Geodynamic significance based on petrological, geochemical and geochronological studies

Dolerite dykes intruding Variscan plutonites were studied in terms of mineralogy, petrology, geochemistry and geochronology. The main mineral constituents were studied and the sequence of crystallization has been derived. The geochemical characteristic indicate mantle origin of the dolerites and magma sources different from the hosting granitoids. From SHRIMP analyses of five spots on four different zircon crystals, resulted a 292.0±4.1 Ma age that is interpreted as the time of crystallization of the dolerite. The hosting granitoids are probably the result of mixing between two possible end-members: enriched mantle and acid metaigneous or lower crustal metasediments. The Variscan age of the dolerites, in combination with the geochemical characteristics, indicated that the enriched mantle basaltic material should be the source of the dolerite veins. These mantle-derived basaltic melts may represent the underplated material, which probably provided the necessary thermal input to the dehydration melting in the lower crust. The dolerites should have intruded the newly formed batholiths before or at the first stages of their uplift, recording the last events of the Variscan subduction.

4: Transpressional tectonics, lower crust decoupling and intrusion of deep mafic sills: A model for the unusual metallogenesis of SW Iberia

SW Iberia is interpreted as an accretionary magmatic belt resulting from the collision between the South Portuguese Zone and the autochthonous Iberian terrane in Variscan times (350 to 330 Ma). In the South Portuguese Zone, pull-apart basins were filled with a thick sequence of siliciclastic sediments and bimodal volcanic rocks that host the giant massive sulphides of the Iberian Pyrite Belt. Massive sulphides precipitated in highly efficient geochemical traps where metal-rich but sulphur-depleted fluids of dominant basinal derivation mixed with sulphide-rich modified seawater. Massive sulphides formed either in porous/reactive volcanic rocks by sub-seafloor replacement, or in dark shale by replacement of mud or by exhalation within confined basins with high biogenic activity. Crustal thinning and magma intrusion were responsible for thermal maturation and dehydration of sedimentary rocks, while magmatic fluids probably had a minor influence on the observed geochemical signatures. The Ossa Morena Zone was a coeval calc-alkaline magmatic arc. It was the site for unusual mineralization, particularly magmatic Ni–(Cu) and hydrothermal Fe-oxide–Cu–Au ores (IOCG). Most magmatism and mineralization took place at local extensional zones along first-order strike-slip faults and thrusts. The source of magmas and IOCG and Ni–(Cu) deposits probably lay in a large mafic–ultramafic layered complex intruded along a detachment at the boundary between the upper and lower crust. Here, juvenile melts extensively interacted with low-grade metamorphic rocks, inducing widespread anatexis, magma contamination and further exsolution of hydrothermal fluids. Hypersaline fluids ( δ 18O fluid > 5.4‰ to 12‰) were focused upward into thrusts and faults, leading to early magnetite mineralization associated with a high-temperature (> 500 °C) albite–actinolite–salite alteration and subsequent copper–gold-bearing vein mineralization at somewhat lower temperatures. Assimilation of sediments by magmas led in turn to the formation of immiscible sulphide and silicate melts that accumulated in the footwall of the layered igneous complex. Further injection of both basic and sulphide-rich magmas into the upper crust led to the formation of Ni–(Cu)-rich breccia pipes. Younger (330 to 280 Ma?) peraluminous granitoids probably reflect the slow ascent of relatively dry and viscous magmas formed by contact anatexis. These granitoids have W–(Sn)- and Pb–Zn-related mineralization that also shows geochemical evidence of major mantle–crust interaction. Late epithermal Hg–(Cu–Sb) and Pb–Zn–(Ag) mineralization was driven by convective hydrothermal cells resulting from the high geothermal gradients that were set up in the zone by intrusion of the layered igneous complex. In all cases, most of the sulphur seems to have been derived from leaching of the host sedimentary rocks ( δ 34S = 7‰ to 20‰) with only limited mixing with sulphur of magmatic derivation. The metallogenic characteristics of the two terranes are quite different. In the Ossa Morena Zone, juvenile magmatism played a major role as the source of metals, and controlled the styles of mineralization. In the South Portuguese Zone, magmas only acted as heat sources but seem to have had no major influence as sources of metals and fluids, which are dominated by crustal signatures. Most of the magmatic and tectonic features related to the Variscan subduction and collision seem to be masked by those resulting from transpressional deformation and deep mafic intrusion, which led to the development of a metallogenic belt with little resemblance to other accretionary magmatic arcs.

The Problem of Dating High-pressure Metamorphism: a U-Pb Isotope and Geochemical Study on Eclogites and Related Rocks of the Marianske Lazne Complex, Czech Republic

This study presents new geochemical (major and trace element, Nd–Sr isotope) and U–Pb zircon, monazite, titanite and rutile data for various rock types (eclogite, high-pressure granulite, amphibolite, orthogneiss, leucosome) of the high-grade metamorphic Marianske Lazně Complex in the western Bohemian Massif. Concordant U–Pb zircon analyses from the mafic to intermediate samples disclose the Marianske Lazně Complex as a mixture of c. 540 Ma oceanic rocks juxtaposed at depth with lower-crustal rocks of the structurally overlying Tepla–Barrandian Unit. This interpretation refutes earlier models in which the Marianske Lazně Complex was interpreted as a dismembered Cambro-Ordovician ophiolite complex affected by Variscan subduction. Metamorphic zircon in mafic rocks of the Marianske Lazně Complex, as well as monazite from orthogneiss, and titanite in leucosome yield ages around 380 Ma. On the basis of detailed petrography and cathodoluminescence imaging we conclude that the c. 380 Ma age reflects the timing of Variscan exhumation and associated decompression melting under upper amphibolite-facies to granulite-facies conditions. Thereby, the data derived from metamorphic zircon of eclogites and high-pressure granulite, unexpectedly, do not date the timing of eclogitization, which could have happened just before Variscan exhumation, or even shortly after Late Cadomian protolith formation.

Carboniferous-Permian rifting and magmatism in southern Scandinavia, the North Sea and northern Germany: a review

Abstract During the Late Carboniferous and Early Permian an extensive magmatic province developed within northern Europe, intimately associated with extensional tectonics, in an area stretching from southern Scandinavia, through the North Sea, into northern Germany. Within this area magmatism was unevenly distributed, concentrated mainly in the Oslo Graben and its offshore continuation in the Skagerrak, Scania in southern Sweden, the island of Bornholm, the North Sea and northern Germany. Available geochemical (major- and trace-element, and Sr-Nd isotope, data) and geophysical data are reviewed to provide a basis for understanding the geodynamic setting of the magmatism in these areas. Peak magmatic activity was concentrated in a narrow time-span from c. 300 to 280 Ma. The magmatic provinces developed within a collage of basement terranes of different ages and lithospheric characteristics (including thicknesses), brought together during the preceding Variscan orogeny. This suggests that the magmatism in this area may represent the local expression of a common tectono-magmatic event with a common causal mechanism. Available geochemical (major and trace element and Sr-Nd isotope data) and geophysical data are reviewed to provide a basis for understanding the geodynamic setting of the magmatism in these areas. The magmatism covers a wide range in rock types both on a regional and a local scale (from highly alkaline to tholeiitic basalts, to trachytes and rhyolites). The most intensive magmatism took place in the Oslo Graben (ca. 120 000 km 3 ) and in the NE German Basin (ca. 48 000 km 3 ). In both these areas a large proportion of the magmatic rocks are highly evolved (trachytes-rhyolites). The dominant mantle source component for the mildly alkali basalts to subalkaline magmatism in the Oslo Graben and Scania (probably also Bornholm and the North Sea) is geochemically similar to the Prevalent Mantle (PREMA) component. Rifting and magmatism in the area is likely to be due to local decompression and thinning of highly asymmetric lithosphere in responses to regional stretching north of the Variscan Front, implying that the PREMA source is located in the lithospheric mantle. However, as PREMA sources are widely accepted to be plume-related, the possibility of a plume located beneath the area cannot be disregarded. Locally, there is also evidence of other sources. The oldest, highly alkaline basaltic lavas in the southernmost part of the Oslo Graben show HIMU trace element affinity, and initial Sr-Nd isotopic compositions different from that of the PREMA-type magmatism. These magmas are interpreted as the results of partial melting of enriched, metasomatised domains within the mantle lithosphere beneath the southern Olso Graben; this source enrichment can be linked to migration of carbonatite magmas in the earliest Paleozoic (ca. 580 Ma). Within northern Germany, mantle lithosphere modified by subduction-related fluids from Variscan subduction systems have provided an important magma source components.

Permian magmatism and basin dynamics in the southern Pyrenees: a record of the transition from late Variscan transtension to early Alpine extension

Abstract Permian magmatism in the Pyrenees is characterized by two compositionally different and temporally consecutive magmatic episodes: a calc-alkaline-transitional phase (andesites) and a midly alkaline phase (basalts and dolerites). These two magmatic episodes were related to the attenuation of late Variscan transtensional tectonics and the onset of extension related to regional rifting. The strike-slip fault systems that affected the Pyrenees in late Variscan times initially controlled the development and morphology of the sedimentary basins. These were periodically affected by phases of extension, which controlled the subsidence of the basins, and, in addition, the emplacement of magmas. The whole-rock trace-element and isotopic signature of the andesites suggests that they were derived from the upper mantle and variably hybridized with late orogenic crustal melts, whereas the alkali basalts could have been derived from a lithospheric mantle source, enriched as a consequence of Variscan subduction processes with the contribution, in some areas, of an enriched (asthenospheric) component.

A Permian underplating event in late- to post-orogenic tectonic setting. Evidence from the mafic–ultramafic layered xenoliths from Beaunit (French Massif Central)

The Puy Beaunit volcano vent, French Massif Central, displays a population of plutonic mafic to ultramafic xenoliths, commonly showing asymmetric, millimetre to centimetre thick, layering. Layers are pyroxenitic to gabbroic, and less commonly peridotitic (lherzolite, dunite, websterite) and anorthositic. These xenoliths are interpreted as samples of a layered intrusion, located at the crust–mantle boundary. Primary cumulate phases are olivine and orthopyroxene, followed by clinopyroxene and plagioclase; rare intercumulus accessory phases (apatite, rutile and zircon) are observed in the most differentiated layers. Homogeneous xenoliths, interpreted as single cumulate layers, have a calc-alkaline geochemistry with LREE and large ion lithophile elements (LILE) enrichments relative to Nb, Ta and Ti. The negative Eu anomaly of pyroxenite can be related to earlier plagioclase fractionation, as observed in the gabbroic layers. Trace element laser ablation inductively coupled plasma emission mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS) analyses of plagioclase, orthopyroxene and zircon from layered rocks suggest equilibrium and cogenetic relations between the silicate phases. U–Pb SIMS dating of a 1.5 mm zircon crystal gives a magmatic or sub-solidus equilibration age of 257±6 Ma.The Beaunit layered intrusion belongs to the large Permian within-plate magmatic episode commonly of calc-alkaline geochemical signature observed over Europe and North Africa. It probably corresponds to a mafic underplating event spatially controlled by post-Variscan trans-tensional to trans-pressional basin tectonics in an intracontinental setting. The subduction-related geochemical signature of the magmatic suite is interpreted as resulting from the passive remobilisation of a mantle source, which was previously metasomatised during the Variscan subduction.

Mantle–crust interactions during Variscan subduction in the Eastern Alps (Nonsberg–Ulten zone): geochronology and new petrological constraints

We have analyzed the Sm–Nd and Rb–Sr whole-rock and mineral isotope systematics of garnet peridotites and associated eclogites and migmatitic gneisses from the Nonsberg–Ulten zone of the Eastern Alps. The garnet peridotites include coarse-grained varieties, characterized by well-preserved to slightly modified mantle geochemical signatures, and finer-grained varieties enriched in amphibole and LILE. Hydration of some of the most strongly deformed, fine-grained peridotites by crustal fluids caused isotopic disequilibrium between the peridotite minerals, preventing accurate age determinations. The coarse-grained peridotites, the eclogites and the country migmatitic gneisses yield garnet–whole-rock and garnet–clinopyroxene Sm–Nd ages that indicate for all rock types an isotopic homogenization event at ca. 330 Ma. The similar ages suggest that all rock types shared a common history since the incorporation of the peridotites in the crust, and constrain the garnet-facies metamorphism of the peridotites, as well as partial melting of the crust, to an episode of crustal subduction at the end of the Variscan orogenic cycle.

The amphibolites from the Ossa–Morena/Central Iberian Variscan suture (Southwestern Iberian Massif): geochemistry and tectonic interpretation

Basic rocks included in the Ossa–Morena/Central Iberian Variscan suture of SW Iberia have been studied in order to decipher the pre-orogenic evolution along this major boundary. These rocks appear as garnet-free amphibolites and garnet-bearing amphibolites. Previous geochronological data on these basic rocks suggest the existence of two different protolith ages: Upper Precambrian and Lower Palaeozoic (Ordovician). Upper Precambrian amphibolites form small-size (up to several metres thick), lense-, dike- or elongated-shaped bodies, while Lower Palaeozoic ones form elongated bodies with thicknesses up to several hectometres. The chemical compositions of the amphibolites enable us to separate cumulate rocks from those representing more basaltic magmas. As for the originally basaltic rocks, their geochemical characteristics indicate a shallow melting with different proportions of at least three end-members, namely (i) a MORB-like asthenospheric source, (ii) a more enriched (plume-derived or subcontinental lithosphere) mantle source, and (iii) a continental crust component. Moreover, Upper Precambrian and Lower Palaeozoic amphibolites have different geochemical affinities. In the former group, we envisage an advanced intra-continental rifting situation or an intra-continental back-arc setting related to the Cadomian orogeny, while for the latter, we propose an environment of oceanic crust formation predating Variscan subduction and collision. The recognition of these remains of oceanic-affinity basic rocks along the OMZ/CIZ boundary allows us to propose that some sort of ocean may well have existed between the two zones during Early Palaeozoic times, and that this ocean was presumably closed at a later stage, probably during the Devonian, by oceanic subduction under the CIZ crust.

The nature of the Variscan basement in southeast England: evidence from integrated potential field modelling

The Variscides of southeast England are buried beneath post-Carboniferous cover. Interpretations of the basement are based mainly on deep boreholes. Geophysical signatures from the basement are contained within the regional gravity and magnetic data. A gravity stripping exercise has been undertaken to remove the gravitational effect of the post-Variscan cover to generate a residual gravity map. This map is interpreted along with integrated potential field modelling along four long interconnected profiles and compared with a revised pre-Permian subcrop map. The magnetic evidence suggests that Precambrian magnetic basement of the Midlands Microcraton has been buried southwards by north-vergent Variscan thusting over the foreland. North of the Variscan Front, short-wavelength anomalies superimposed upon this deep Precambrian source are due to shallower Silurian and Carboniferous volcanic rocks. Many residual gravity lows within the Rhenohercynian zone may be related to thick, low-density Devonian basins. In the English Channel a change in geophysical signature occurs north of the Portland–Wight Fault, coinciding with phyllites in the basement. Models are presented in which the English Channel magnetic anomalies originate within the pre-Permian basement. Comparisons with anomalies in the Southwestern Approaches suggest that the Portland–Wight Thrust is a terrane boundary, possibly a subduction-related suture, implying southerly directed Variscan subduction.