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Permo-Carboniferous magmatism and rifting in Europe: introduction

M. WILSON l, E.-R. NEUMANN 2, G. R. DAVIES 3, M. J. TIMMERMAN 4, M. HEEREMANS 5 & B. T. LARSEN 6 ~School of Earth Sciences, Leeds University, Leeds LS2 9JT, UK (e-mail: M. Wilson @ earth.leeds.ac, uk) 2physics of Geological Processes, University of Oslo, P.O. Box 1048, Blindern, N-0316 Oslo, 3Faculty of Earth and Life Sciences, Vrije University, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands 4Institut fdr Geowissenschaften, Universitdt Potsdam, Postfach 60 15 53, 14415 Potsdam, Germany 5Institute for , University of Oslo, P.O. Box 1047, Blindern, N-0316 Oslo, Norway 6Norsk Hydro ASA/Saga Petroleum ASA, Oslo, Norway

An extensive system developed within the Westphalian series (e.g. McCann 1996) (Fig. 2). northern foreland of the Variscan orogenic belt Regional uplift coincides with the onset of during Late Carboniferous-Early Permian voluminous magmatism across the region, rais- times, post-dating the Devonian-Early Carbo- ing the possibility that uplift could have been niferous accretion of various Neoproterozoic related to the presence of a widespread thermal Gondwana-derived terranes on to the southern anomaly within the upper mantle (i.e. a mantle margin of Laurussia (Laurentia-Baltica; Fig. 1). plume or, possibly, several plumes). In detail, Rifting was associated with widespread magma- however, it is likely that uplift was induced by a tism and with a fundamental change, at the complex combination of wrench-related litho- Westphalian-Stephanian boundary, in the regio- spheric deformation, magmatic inflation of the nal stress field affecting western and central lithosphere and thermal erosion of the base of Europe (Ziegler 1990; Ziegler & Cloetingh 2003). the lithosphere (van Wees et al. 2000). The change in regional stress patterns was Stephanian-Early Permian (Autunian) coincident with the termination of orogenic wrench tectonics affected not only the Variscan activity in the Variscan fold belt, followed by foreland, but also the entire Variscan Orogen major dextral translation between North Africa (Fig. I). Within the orogen wrench faulting and and Europe. associated magmatism were probably accompa- Rifting propagated across a collage of base- nied by detachment of subducted lithospheric ment terranes with different ages and thermal slabs and partial delamination of thickened histories. Whilst most of the Carboniferous- lithospheric roots (Ziegler 1990). Magmatic Permian rift basins of NW Europe developed on underplating of the base of the crust by relatively thin lithosphere, the highly magmatic ascending mafic magmas is likely to have been Oslo Graben in southern Norway initiated widespread (e.g. Downes 1993), providing the within the thick, stable and, presumably, strong heat for partial melting of lower-crustal rocks. (cold) lithosphere of the Fennoscandian craton. Available seismic reflection data provide strong The rift basins in the North Sea, in contrast, support for such underplating of the crust. The developed in younger Caledonian age litho- presence of extensive ignimbrite sequences sphere, which was both thinner and warmer within parts of the North German Basin may than the lithosphere of the craton to the east. be directly attributable to crustal melting A regional hiatus, corresponding to the Early induced in this way (e.g. Breitkreuz & Kennedy Stephanian, is evident in much of the Variscan 1999). foreland, with Stephanian and Early Permian Following the termination of wrench tec- red beds unconformably overlying truncated tonics in the late Autunian two major intra- From: WILSON, M., NEUMANN, E.-R., DAVIES, G.R., TIMMERMAN, M.J., HEEREMANS, M. & LARSEN, B.T. (eds) Permo-CarboniferousMagmatism and Rifting in Europe. Geological Society, London, Special Publications, 223, 1-10. 0305-8719/04/$15 9The Geological Society of London 2004. Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

2 M. WILSON ET AL.

Fig. 1. Plate tectonic reconstructions for the Variscan belt and its foreland between 350 and 300 Ma, indicating the locations of the major basement terranes. Significant northward drift occurred during this period (Torsvik 1998). Plate reconstructions were made using PGIS/Mac (9 PALEOMAP project; http://www.paleomap. com). Terrane boundaries are based on Pharaoh (1999) and Banka et al. (2002). Avalonia was accreted in the latest Ordovician, while the various components of the Armorica-Barrandia terrane were accreted during Devonian and Carboniferous times, climaxing in the Variscan orogeny. Abbreviations: STZ, Sorgenfrei- Tornquist Zone; NGB, North German Basin; MV, Midland Valley of Scotland.

continental sedimentary basins, generally orogenic belt, extending across the orogenic referred to as the Northern and Southern front in its eastern part. The intensity of the Permian basins, began to subside within western preceding Stephanian-Autunian phase of litho- and central Europe in response to thermal spheric destabilization may have exerted a first- contraction of the lithosphere (e.g. Ziegler order control on the subsequent subsidence 1990; van Wees et al. 2000). The Northern history of the two basins (van Wees et al. 2000). Permian Basin extends over 1000 km in an E-W Whilst on a regional scale the Stephanian- direction from Scotland, across the central early Permian magmatic event appears the most North Sea into northern Denmark; the much significant, locally it was pre-dated by a Visean larger Southern Permian Basin extends over a magmatic phase (e.g. in the Midland Valley of distance of 1700km from England, across the Scotland, Wales, English Midlands, Central southern North Sea and through northern Ireland, SW England and the Baltic Sea; Fig. Germany into Poland and the Baltic States. 2), which may have had a different geodynamic Both the Northern and Southern Permian basins setting. This magmatism was contemporaneous initiated as broad crustal downwarps with little with the later stages of a 45 Ma period of Late associated faulting. They developed within a Devonian-Early Carboniferous (375-330Ma) mosaic of basement terranes of different ages tholeiitic flood basalt volcanism in the Mari- and geodynamic histories, their geometries times Basin of eastern Canada, which has been apparently showing little evidence of control inferred to be plume-related (Dessureau et al. by pre-existing basement structures (van Wees et 2000). Dessereau et al. (2000) have suggested al. 2000). The western parts of the Northern that plume-related magmatic activity may have Permian Basin are located on Caledonian crust, gradually migrated eastwards during Late Dev- while the eastern parts developed on Late onian-Permian times, from the Maritimes Basin Precambrian crust. The Southern Permian Basin through Scotland to Norway and northern subsided in the foreland of the Variscan Germany. Plate tectonic reconstructions Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

PERMO-CARBONIFEROUS MAGMATISM AND RIFTING 3

Fig. 2. Relative timing of Permo-Carboniferous magmatism, extensional tectonics, basin inversion and regional uplift within the Variscan orogenic belt and its northern foreland. Data have been compiled from all of the papers within this volume, and the timescale used is from Menning et al. (2000). Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

4 M. WILSON ET AL.

(Fig. 1), however, suggest that there was rapid Geochemical studies of the magmatic rocks northward plate motion during the Carbonifer- were aimed at addressing the following funda- ous which, when combined with the end- mental questions: Carboniferous phase of wrench tectonics, makes it difficult to constrain potential hot-spot tracks. 9 What was the nature of the mantle source of Consequently, such a hypothesis remains highly the magmas - lithosphere or asthenosphere? speculative. 9 Was there one (or more) mantle plumes beneath Europe during Late Carboniferous- Early Permian times? The PCR Project 9 How did variations in the age and thickness Many of the contributions to this volume of the lithosphere influence the magmatism? represent the results of research conducted as 9 How did the magmas interact with the part of a multi-disciplinary research project lithosphere during migration from source to 'Permo-Carboniferous Rifting in Europe' or surface? 'PCR' funded by the European Commission Although the main emphasis of PCR was on between 1997 and 2001. The aim of the PCR the northern foreland of the Variscan orogenic project was to further our understanding of the belt, petrological-geochemical studies were also geodynamics of Carboniferous-Permian rifting conducted of post-collisional magmatic rocks and associated magmatism within the northern within the orogen (principally in Iberia and foreland of the Variscan orogenic belt. As part central France). The aim of these studies was to of the project we have produced a new Late investigate which parts of the mantle (i.e. litho- Carboniferous-Permian tectono-magmatic map sphere, depleted sublithospheric mantle, of NW Europe, based mainly on seismic and enriched sublithospheric mantle) generated mag- well data. mas in the inner part of the Variscan Orogen The main objectives of the PCR project were during Carboniferous-Permian times and to to: evaluate if there are any similarities with the 9 Determine the relationship between the onset source of magmas of the same age from the of magrnatism, regional uplift and exten- foreland areas in northern Europe. sional tectonics. 9 Constrain, by high-quality Ar-Ar and U-Pb What is the relationship between zircon dating, the chronology of magmatic Carboniferous-Permian magmatism and events within the province. 9 Assess the role of thermally anomalous rifting in Europe? mantle (i.e a mantle plume or superplume) The chapters included in this volume are broadly in the petrogenesis of the magmas and to arranged in a geographical context, focusing on evaluate the excess mantle temperature via different aspects of this fundamental question. geochemical, He-Ne-Ar and Sr-Nd-Pb iso- Emphasis initially is on the orogenic foreland in topic studies of the most primitive mafic the UK, Scandinavia and the North Sea; magmas erupted within the province. subsequent chapters include discussions of the 9 Describe the onset and magnitude of uplift changing style of magmatism as the orogenic and thermal subsidence across the area, in front is crossed in Germany and Poland, and order to evaluate the magnitude of any within more internal parts of the orogen in thermal pulse. France and the Iberian peninsula. 9 Develop rheological models to understand During the Late Carboniferous and Early the thermo-mechanical controls on rift devel- Permian an extensive magmatic province devel- opment, magmatism and subsequent crustal oped within northern Europe, intimately asso- evolution. ciated with extensional tectonics, in an area 9 Integrate petrological-ge0chemical data with stretching from southern Scandinavia, through both onshore and offshore geophysical (seis- the North Sea, into northern Germany. New mic, gravity, magnetic) and geological data to mann et al. demonstrate that magmatism was understand the dynamics of the Carbonifer- unevenly distributed, concentrated mainly in the ous-Permian rifting and its associated mag- Oslo Graben and its offshore continuation in the matism. Skagerrak, Scania in southern Sweden, the 9 Understand the thermal evolution of the island of Bornholm, the North Sea and northern lithosphere in the northern foreland of the Germany. Peak magmatic activity was concen- Variscan Orogen during Carboniferous-Per- trated in a narrow time-span from c. 300 to mian times. c. 280 Ma. The magmatic provinces developed Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

PERMO-CARBONIFEROUS MAGMATISM AND RIFTING 5 within basement terranes of different ages and mian-earliest Triassic ages (c. 240-245 Ma) for lithospheric characteristics (including thickness) syenitic-granitic intrusions in the northwestern brought together during the preceding Variscan and eastern parts of the Oslo Rift probably orogeny. This suggests that the magmatism may reflect a separate magmatic event of limited represent the local expression of a common extent and duration. tectono-magmatic event with a common causal One of the goals of the PCR project was to mechanism. produce a new map of northern Europe Timmerman reviews the timing, geodynamic (Heeremans et aL) showing the distribution of setting and characteristics of Early Carbonif- Late Carboniferous-Early Permian (Lower eous-Permian magmatism in the foreland of the Rotliegend) volcanics, dykes and sills, and the Variscan Orogen in NW Europe. During the pattern of faulting associated with the Southern Early Carboniferous, final closure of the Rhe- and Northern Permian basins. Production of nohercynian Ocean, accretion of a magmatic arc this map required an overview of all the and docking of microcontinents caused fault available seismic and borehole data, including reactivation, extension and fault-controlled unpublished data provided by our industrial basin ~subsidence in the foreland. Lithospheric partners. stretching locally resulted in eruption of mildly Praeg investigates models of orogenic col- alkaline basaltic volcanism that peaked in the lapse involving diachronous tectonism in which Visean (Fig. 2). In the internal Variscides, rapid crustal uplift and extension are compensated by uplift and granitoid ptutonism shortly followed peripheral compression. He tests first-order continental collision and was probably due to predictions against published data on late- slab detachment(s) or removal of orogenic root orogenic extensional and compressive structures material. along a 1500km-transect from the Variscan A regional-scale, E-W-oriented stress field central internides in France to the foreland in was superimposed on the collapsing orogen and the Irish Sea area. The collapse of the Variscan its foreland from the Westphalian onwards. In Orogen is shown to have expanded northward the Stephanian-Early Permian, a combination over time, via three main stages: of outward propagating collapse, mantle or slab detachment, and the regional stress field resulted 9 collapse of the central internides (late Visean- in widespread formation of fault-controlled mid-Westphalian, c. 335-310 Ma); basins and extensive magmatism dated at 290- 9 reorientation and expansion of collapse (mid- 305 Ma. In the foreland, large amounts of felsic Westphalian-late Stephanian, c. 310- volcanic rocks erupted in northern Germany, 300 Ma); accompanied by mafic-felsic volcanics and 9 collapse of the foreland (late Stephanian- intrusions in the Oslo Rift, and dolerite sills Early Permian, c. 300-290 Ma). and dyke swarms in Britain and Sweden. In the internal Variscides, mafic rocks are rare and These three stages are argued to support a model felsic-intermediate compositions predominate; of Variscan late-orogenic collapse in response to typically, these have a distinctive subduction- three successive detachments of negatively buoy- related trace-element geochemical signature that ant lithospheric material: a collisionally thick- may have been inherited from partial melting of ened orogenic root and two (Rheic) oceanic subduction-modified (metasomatized) litho- slabs, subducted, respectively, southward spheric-asthenospheric mantle sources and/or (beneath the orogen) and northward (beneath caused by extensive assimilation of continental the foreland). Multiple detachments are a crust by mafic mantle-derived magmas. predictable consequence of ocean closure and Latest Carboniferous-earliest Permian continental collision, so that episodic collapse doleritic dykes, sills and flows from Britain to may be a common process in the rise and fall of Scandinavia yield ages in the range 295-305 Ma orogenic belts, and the tectonic evolution of (Fig. 2), confirming that the magmatism was their forelands. broadly coeval with voluminous rhyolitic-ande- Pascal et aL use finite-element modelling to sitic volcanism in the North German Basin investigate the role of changing lithospheric (c. 302-297Ma based on U-Pb zircon ages; thickness as a first-order control on the localiza- Breitkreutz & Kennedy 1999). Younger, 270- tion of major such as the Oslo Graben and 285 Ma, crystallization ages obtained for post- the focusing of magmatic activity. Compared to tectonic dykes and sills in Scotland and Norway other Permo-Carboniferous rift basins within indicate that magmatic activity, although small NW Europe, the Oslo Graben has two distinct in volume and different in composition (alka- characteristics. First, it initiated inside cold and line), continued into the Permian. Latest Per- stable Precambrian lithosphere, whereas most of Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

6 M. WILSON ET AL. the Permo-Carboniferous basins developed in The Carboniferous-Permian evolution of the weaker Phanerozoic lithosphere, and, second, it central North Sea is characterized by three main is characterized by large volumes of magmatic geological events: (1) the development of the rocks despite relatively little extension. The Oslo West European Carboniferous Basin; (2) a Graben appears to have evolved at the transition period of basaltic volcanism during the Lower between two lithospheric domains with contrast- Rotliegend (latest Carboniferous-early Per- ing thickness. Seismic reflection surveys show mian); and (3) the development of the Northern that the crust thickens from southern Norway to and Southern Permian basins in Late Permian southern Sweden, the most significant Moho times. The timing of the Late Carboniferous- deepening occurring from the Oslo region east- Permian basaltic volcanism in the North Sea is wards. Deep seismic studies also suggest that the poorly constrained, as is the timing of exten- base of the lithosphere deepens markedly east- sional tectonic activity following the main phase wards from the Oslo region. Such a long- of inversion during the Westphalian (Fig. 2) due wavelength lithospheric geometry cannot be to the propagation of the Variscan deformation explained by the Permian or post-Permian front. Results of high-precision Ar-Ar dating on evolution of the area. Numerical thermo- basalt samples taken from a core from explora- mechanical modelling is applied to test if this tion well 39/2-4 (Amerada Hess), in the UK transitional position can influence the dynamics sector of the central North Sea, suggests that of rifting and facilitate melting of the mantle. basaltic volcanism was active in the Late Different models with varying lithosphere thick- Carboniferous, at c. 299 Ma. The presence of ness contrast are considered; those with a crust volcanics below the dated horizon suggests that and lithosphere thickness contrast comparable the onset of Permo-Carboniferous volcanism in to that of the Oslo region predict rifting and the central North Sea commenced earlier, focusing of magmatism in a narrow zone with probably at c. 310 Ma (Westphalian C). This is minor thinning of the crust. contemporaneous with observations of tholeiitic Heeremans & Faleide and Heeremans et aL volcanism elsewhere in NW Europe, including discuss the relationship between magmatism and the Oslo Graben, the NE German Basin, south- rifting in the Skagerrak, Kattegat and North Sea. ern Sweden and Scotland. Interpretations of Special attention is paid to the distribution of available seismic data show that the main intrusive and extrusive magmatic rocks in rela- extensional faulting occurred after the volcanic tion to fault geometries, based upon an extensive activity, but the exact age of the fault activity is database of industrial seismic and well data. Rift difficult to constrain with the data available. structures (with characteristic half-graben geo- Extensional tectonics to the north of the metries) and the distribution of magmatic rocks Variscan front during the Early Carboniferous (intrusives and extrusives) were mapped using generated fault-controlled basins across the seismic and potential field data. In the Sorgen- British Isles, with accompanying basaltic mag- frei-Tornquist Zone and the North Sea addi- matism (Fig. 2). Upton et al. demonstrate that in tional constraints are provided by well data. The Scotland Dinantian magmatism was dominantly rift structures in the Skagerrak, which represents of mildly alkaline-transitional basaltic composi- the offshore continuation of the Oslo Graben, tion. Tournaisian activity was followed by can be linked with extensional structures in the widespread Visean eruptions, largely concen- Sorgenfrei-Tornquist Zone in which similar fault trated within the Scottish Midland Valley where geometries are observed. Both in the Skagerrak the lava successions, dominantly of basaltic- and the Kattegat, lava sequences were deposited hawaiitic composition, attained thicknesses of that generally parallel the underlying Lower up to 1000 m. Changing stress fields in the late Palaeozoic strata. This volcanic episode, there- Visean coincided with a change in the nature of fore, pre-dates the main fault movements and the the igneous activity; subsequently, wholly basic development of half-grabens filled with Permian magmatism persisted into the Silesian. As volcaniclastic material. Upper Carboniferous- sedimentary basin fills increased, sill intrusion Lower Permian extrusives and intrusives have tended to dominate over lava extrusion. In the been penetrated in wells in the Kattegat, Jutland Late Carboniferous (Stephanian) a major melt- and the North Sea (Horn and Central grabens). ing episode, producing large volumes of tholei- Particularly in the latter area, the dense seismic itic magma, gave rise to a major dyke swarm and and well coverage permits detailed mapping of sills across northern England and Scotland. the distribution of Upper Palaeozoic magmatic Alkali basaltic magmatism persisted into the rocks, although the presence of salt often Permian, possibly until as late as 250Ma in conceals the seismic image of the underlying Orkney. Geochemical data suggest that the strata and structures. Carboniferous-Permian magmas were domi- Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

PERMO-CARBONIFEROUS MAGMATISM AND RIFTING 7

nantly of asthenospheric origin, derived from enced time-integrated U-Th enrichment. This variable degrees of partial melting of a hetero- enriched mantle was preferentially melted fol- geneous mantle source; varying degrees of lowing the transition from early Palaeozoic interaction with the lithosphere are indicated. compression to late Palaeozoic extensional Peridotite, pyroxenite and granulites-facies tectonics. The helium isotope data provide no metabasic rocks entrained as xenoliths within evidence for the presence of primordial plume- the most primitive magmas provide evidence for type mantle beneath this part of Scotland during metasomatism of the lithospheric mantle and the late Carboniferous-early Permian. high-pressure crystal fractionation. Permo-Carboniferous rifting in Europe was Monaghan & Pringle report new 4~ accompanied by the widespread emplacement of step-heating age determinations on mineral mantle-derived magmas in regional dyke swarms separates from intrusive and extrusive Carboni- and sills in northern England, Scotland, Norway ferous and Permian igneous rocks in the Mid- and southern Sweden during the late Stephanian land Valley of Scotland. These ages resolve and early Permian (Autunian). Obst et aL note inconsistencies between existing K-Ar dates on that the regional trends of the dyke swarms the same samples and stratigraphical constraints intersect at a focal point in the Kattegat south of correlated to recently published timescales. the Oslo Graben, and suggest that the dykes Twenty-one precise 4~ dates are strati- could all emanate from a single magmatic graphically constrained to stage level and con- centre. They show that the WNW- to NW- tribute important new Carboniferous timescale trending dyke swarm at the SW margin of the tie points at the Tournaisian-Visean boundary, Fennoscandian Shield in southern Sweden is within the Visean and at the Carboniferous- composed mainly of tholeiitic dolerites, with Permian boundary. Two distinct phases of lesser amounts of alkaline mafic rocks (campto- extension-related transitional to alkaline volcan- nites, alkali basalts and spessartites) and tra- ism are recognized in the Dinantian: the chytes. The alkaline mafic rocks are enriched in Garleton Hills Volcanic Formation in the east- Ba, Sr, Nb, P and CO2, implying a metasomatic ern Midland Valley near the Tournaisian- enrichment of their upper mantle source prior to Visean boundary at c. 342Ma and the Clyde melting. After generation of alkaline melts by Plateau Volcanic Formation in the western relatively small degrees of partial melting, Midland Valley during the mid Visean (335- increased extension was accompanied by the 329Ma). Alkaline basic sills near Edinburgh, formation of subalkaline tholeiitic magmas. Two previously thought to be Namurian, appear to groups of tholeiitic dolerites can be distinguished be coeval with the Clyde Plateau Volcanic that exhibit slight differences in their mantle- Formation at c. 332-329Ma. The new ages normalized trace-element patterns and Nb/La allow correlation between these short-lived ratios, suggesting that they were generated from Dinantian magmatic pulses and extensional different mantle sources. Group I dolerites and magmatic phases in the Northumberland- appear to have been formed from a sublitho- Solway and Tweed basins further to the south. spheric garnet-bearing mantle source, whereas After a phase of late Westphalian compression group II dolerites were formed by mixing of (Fig. 2) and the regionally important tholeiitic asthenosphere-derived magmas with lithospheric intrusive phase at c. 301-295Ma, alkaline partial melts. magmatism related to post-Variscan extension Ziegler et aL consider the post-Variscan occurred in the central and western Midland evolution of the lithosphere in the Rhine Graben Valley until at least 292 Ma. area of central Germany based on subsidence Kirstein et aL present the results of a noble modelling. The Cenozoic Rhine Graben rift gas isotope study of well-characterized spinel system transects the deeply truncated Variscan peridotite facies lithospheric mantle xenoliths Orogen with its superimposed system of Permo- and garnet megacrysts entrained within Scottish Carboniferous wrench-induced troughs, and Permo-Carboniferous dykes, sills and vents in Late Permian and Mesozoic thermal sag basins. an attempt to evaluate whether a high 3He/4He Ziegler et al. propose that at the time of its lower-mantle plume could have been involved in Westphalian consolidation, the Variscan Orogen the petrogenesis of the host magmas. The was probably characterized by 45-60km-deep samples were collected from the Northern High- crustal roots that were associated with the main land and the Midland Valley basement terranes, Rheno-Hercynian-Saxo-Thuringian, Saxo- which vary from Archaean-Proterozoic to Pro- Thuringian-Bohemian and Bohemian-Molda- terozoic-Palaeozoic in age. The He isotope data nubian tectonic sutures. During Stephanian- suggest that the mantle lithosphere beneath Early Permian wrench-induced disruption of the Scotland during the late Palaeozoic had experi- Variscan Orogen, they propose that subducted Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

8 M. WILSON ET A L. lithospheric slabs were detached, causing upwel- reflect derivation of the parental magmas from ling of hot mantle material. As a consequence of a mantle source that was metasomatized during the resulting thermal surge, partial delamination an earlier (Devonian?) subduction event. and/or thermal thinning of the continental Magma generation may have been triggered by mantle-lithosphere induced regional uplift. Con- decompression partial melting of the mantle due temporaneously, the Variscan orogenic roots to delamination of the base of the lithosphere were destroyed and crustal thicknesses reduced and its replacement by upwelling hotter astheno- to 28-35 km in response to the combined effects spheric mantle. Lithospheric mantle detachment of mantle-derived melts interacting with the may have caused post-collisional, Namurian lower crust, regional erosional unroofing of the uplift and cooling of the crust, and facilitated crust and, on a more local scale, by its emplacement of lamprophyre dykes along fault mechanical stretching. Towards the end of the zones at high crustal levels. Early Permian, the temperature of the astheno- Von Seckendorff et al., in their second sphere is considered to have returned to ambient chapter, review the magmatism of the Saar- levels. Subsequently, regional, long-term sub- Nahe Basin in Germany. This is a late Variscan sidence of the lithosphere commenced, control- intermontane basin that developed on the site of ling the development of a new system of Late an earlier island arc, the Mid-German Crystal- Permian and Mesozoic thermal sag basins. The line Rise. A wide variety of igneous rocks was evolution of these basins was repeatedly over- emplaced into the thick continental sedimentary printed by minor short-term subsidence accel- fill of the basin over a period of c. 4Ma, from erations related to the build-up of far-field 296 to 293 Ma, as high-level intrusions and lava stresses linked to rifting in the Tethyan and flows, extrusive domes, diatremes and pyroclas- Atlantic domains. tic deposits, ranging in composition from basalt Zeh & Brfitz discuss the relationship between and basaltic andesite to rhyodacite, rhyolite and Late Carboniferous-Early Permian magmatism trachyte. Composite intrusive-extrusive com- and transtensional tectonics in the Thuringian plexes consist of andesite, rhyodacite and Forest region of Germany. Here, dextral trans- trachyte. The geochemical characteristics of the tensional movements along a NW-trending fault most primitive mafic magmas indicate derivation system caused complex block faulting accom- from a slightly enriched upper-mantle source panied by intense magmatism. The age of this modified by subduction-related fluids. Nd-Sr-O tectono-magmatic activity is well constrained by isotope data indicate that crustal contamination geochronological data (2~176 single zir- was important in the petrogenesis of the more con, SHRIMP, 40 Ar/ 39 Ar mica, zircon fission- differentiated magmas. track ages) and field relationships. NE-trending Dubifiska et al. focus on the problems structures appear to have formed between 300 associated with geochemical studies of Permian and 294 Ma, with formation or re-activation of volcanic rocks in NW Poland where intense low- W- to NW-trending structures between 290 and grade metamorphism, of probable Upper Jur- 275 Ma. assic age, has partially obliterated the primary Von Seekendorff et aL report the results of a magmatic signature. Despite the degree of programme of 4~ step-heating dating of alteration, they consider that the geochemical mineral separates from lamprophyre dykes characteristics of the most mafic rocks are (spessartites, minettes and kersantites) from the consistent with derivation from an enriched Saxothuringian Zone of the Variscan Orogen. mantle source. These dykes give Visean-Namurian (334- Perini et al. consider the petrogenesis of the 323Ma) and Stephanian-early Permian (295- most primitive mafic magmas within the Var- 297 Ma) crystallization ages indicating magma iscan belt of Spain and France, in particular the generation over a period of some 30Ma. In nature of the mantle source of the magmas. many cases, dyke emplacement was controlled Carboniferous-Permian magmatism in the by faults. Many of the dykes are composite or Spanish Central System, Iberian Ranges, Can- show evidence for mingling of primitive mantle- tabrian Chain, Pyrenees and the French Massif derived and more evolved magmas, and, in some Central includes a range of mafic calc-alkaline cases, contamination with crustal melts. Ker- and shoshonitic rock types, as well as amphi- santites and minettes have similar incompatible bole-bearing lamprophyres (spessartites) and trace-element characteristics and appear to have minor alkaline lamprophyres (camptonites). originated from partial melting of deeper mantle Subalkaline basalts, with intermediate charac- sources than the associated spessartites; negative teristics between enriched mid-ocean ridge Ta, Nb and Ti anomalies are common in mantle- basalts (E-MORB) and the mafic calc-alkaline normalized trace-element patterns and may rocks, also occur in the Pyrenees. The incom- Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

PERMO-CARBONIFEROUS MAGMATISM AND RIFTING 9 patible trace-element characteristics of the least Chain of Central Spain, which includes both differentiated subalkaline rocks and lampro- pyroclastic units and high-level intrusions (sills phyres indicate that variably enriched mantle and dykes) emplaced in a transtensional, post- sources were involved in their petrogenesis; the orogenic tectonic setting. A variety of subalka- trace-element signatures of the calc-alkaline and line igneous rocks occur in this region, ranging shoshonitic rocks require either assimilation of from basalt to rhyolite; andesitic rocks are, crustal rocks plus fractional crystallization however, dominant. The pyroclastic units con- (AFC) of the parental mafic magmas or melting tain plant fossils and pollen that suggest an of a previously subduction-modified mantle Autunian age, consistent with available K-Ar source. In the Cantabrian Chain and the Massif radiometric age data (283-292Ma) for the Central melting of a subduction-modified mantle hypabyssal intrusions. Significant crustal assim- source seems the most likely process. In the ilation appears to have been involved in the Central System, Iberian Ranges and Maladeta petrogenesis of the intermediate magmas. A area of the Pyreneees, the lack of any evidence significant hiatus, spanning the Middle Permian for a contemporaneous subduction system sug- and most of the Upper Permian, separates this gests that AFC processes were probably respons- Lower Permian magmatism from subsequent ible for the crustal signature of the magmas. The episodes of Triassic and Jurassic alkaline mag- alkaline camptonites from the Central System matism that represent later rifting events which appear to have been generated from an enriched affected the Iberian Chain, progressively thin- mantle source that was distinct from the source ning the Variscan crust as the Alpine cycle of the older calc-alkaline magmas from the same began. area. The incompatible trace-element patterns and ratios (e.g. Y/Nb, Zr/Nb) of the subalkaline basalts from Panticosa, Cinco Villas and La Summary Rhune suggest that they were generated from The results of the PCR project have provided similar parent magmas, formed by mixing of important new constraints on the relative roles partial melts of an asthenospheric mantle source of far-field stresses v. dynamic upwelling of the and a crustal component. upper mantle (i.e. mantle plumes) in the initia- Lago et aL in their first chapter, discuss the tion of the Permo-Carboniferous rift system of relationship between Permian magmatism and northern Europe and in the petrogenesis of the basin dynamics in the Pyrenees during the resulting magmas. Magmatism and regional transition from late Variscan transtension to tectonics are intimately linked, and heterogene- early Alpine extension. They provide evidence ities in lithospheric thickness clearly play an for two compositionally distinct, consecutive, important role in localizing sites of decompres- magmatic episodes: a calc-alkaline-transitional sion partial melting. The magmatic rocks phase (andesites) and a mildly alkaline phase emplaced within the orogen and its northern (basalts and dolerites). These two magmatic foreland exhibit a wide spectrum of geochemical events are related to the attenuation of late characteristics, consistent with melt generation Variscan transtensional tectonics and the onset in a number of distinct geodynamic settings. of extension related to regional rifting. The The majority of the most primitive Permo- strike-slip fault systems that affected the Pyr- Carboniferous lavas in the Oslo Rift, southern enees in late Variscan times initially controlled Scotland, northern England and parts of Ireland the development and morphology of the sedi- appear to be small-degree partial melts of one or mentary basins. These were periodically affected more sublithospheric mantle sources. The rising by phases of extension, which controlled basin mantle-derived magmas, however, have often subsidence and the emplacement of magmas. been modified subsequently by shallow-level The whole-rock trace-element and isotopic processes, including magmatic differentiation signature of the andesites suggests that their and lithospheric (both crust and mantle) con- parent magmas were derived from the upper tamination, that mask the geochemical signature mantle but were subsequently hybridized with of the mantle source. The enriched astheno- late-orogenic crustal melts, whereas the alkaline spheric mantle source of the most primitive basalts could have been derived from a litho- magmas in many areas bears some similarity to spheric mantle source, enriched as a conse- the source of modern-day Hawaiian-Icelandic quence of Variscan subduction processes, with basalts, which are generally assumed to be a contribution in some areas of an enriched plume-related. Whilst the limited He isotope (asthenospheric) mantle source component. data available from Scotland could be consid- The second chapter by Lago et aL focuses on ered to be consistent with the involvement of a the Late Variscan magmatism within the Iberian low 3He/4He mantle plume (or several plumes), Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

10 M. WILSON ET AL. similar to the source of present-day HIMU DESSUREAU, G., PIPER, D.J.W & PE-PIPER, G. 2000. (High-#) ocean island basalts (Hofmann 1997), Geochemical evolution of earliest Carboniferous in the petrogenesis of the magmas, it seems continental tholeiitic basalts along a crustal-scale much more likely that magma generation was a shear zone, southwestern Martimes basin, eastern Canada. Lithos, 50, 27-50. complex response to far-field stresses and the DOWNES, H. 1993. The nature of the lower continental tectonic collapse of the Variscan Orogen. crust of Europe; petrological and geochemical Geochemical and structural studies of regio- evidence from xenoliths. Physics of the Earth and nal dyke swarms in Scotland, Norway and Planetary Interiors, 76, 195-218. Sweden suggest that there were at least five HOFMANN, A.W. 1997. Mantle geochemistry: the different intrusive events. One of the most message from oceanic volcanism. Nature, 385, extensive is the widespread emplacement of 219-229. tholeiitic dykes and sills over a short time MCCANN, T. 1996. Pre-Permian of the Northeast interval close to the Carboniferous-Permian German Basin. Geological Journal, 31, 159- 177. boundary (c. 300Ma). These intrusions appear MENNING, M., WEYER, D., DROZDZEWSKI, G., to reflect a short-lived, high-degree melting event AMEROM, H.W. & WENDT, I. 2000. A Carbonifer- over a wide region that may have been ous Time Scale 2000: Discussion and use of tectonically induced by wrench faulting. geological parameters as time indicators from Within the Variscan Orogen, the earlier Central and Western Europe. Geologisches Jahr- subalkaline post-orogenic magmas appear to buch, 156, 3-44. be derived from partial melting of subduction- PHARAOH, T.C. 1999. Palaeozoic terranes and their modified mantle lithosphere. Later, alkaline lithospheric boundaries within the Trans-European mafic magmas appear to be derived from an Suture Zone (TESZ): a review. Tectonophysics, 314, 17-41. asthenospheric mantle source, similar to that TORSVIK, T.H. 1998. Palaeozoic palaeogeography: a beneath the northern foreland. North Atlantic viewpoint. Geologiska F6"reninger, i Stockholm F6"rhandlingar, 120, 109-118. VAN WEES, J.-D., STEPHENSON, R.A., ZIEGLER, P.A., BAYER, U., MCCANN, T., DADLEZ, R., GAUPP, References R., NARKIEWICZ, M., BITZER, F. & SCHECK, M. BANKA, D., PHARAOH, T.C., WILLIAMSON, J.P. & 2000. On the origin of the Southern Permian Basin, TEZC POTENTIAL FIELD CORE GROUP. 2002. Central Europe. Marine and Petroleum Geology, Potential field imaging of Palaeozoic orogenic 17, 43-59. structure in northern and central Europe. Tecto- ZIEGLER, P.A. 1990. Geological Atlas of Western and nophysics, 360, 23-45. Central Europe (2nd edn). Shell Internationale BREITKREUZ, C. & KENNEDY, A. 1999. Magmatic Petroleum Maatschappij, The Hague. flare-up at the Carboniferous-Permian boundary ZIEGLER, P.A. & CLOETINGH, S. 2003. Dynamic in the NE German basin revealed by SHRIMP processes controlling evolution of rifted basins. zircon ages. Tectonophysics, 302, 307-326. Earth Science Reviews, 64, 1-50.