Martin J. Timmerman

The Lapland-Kola orogen: Palaeoproterozoic collision and accretion of the northern Fennoscandian lithosphere

Abstract A tectonic model is proposed for the Palaeoproterozoic Lapland-Kola orogen (LKO) in the northern Fennoscandian Shield. Although long regarded as an Archaean craton, integrated geological, geochemical and geophysical observations show that the Lapland-Kola orogen is a Palaeoproterozoic collisional belt containing both Archaean terranes and an important component of juvenile Palaeoproterozoic crust. Rifting, from 2.5 to 2.1 Ga, began under the influence of a mantle plume (> 1000 km diameter), related to the break-up of the Kenorland supercontinent. Two linear suture zones within the orogeriic core mark the sites of continental separation, ocean formation and closure. One of these is identified as a belt of 1.98-1.91 Ga juvenile crust of both arc magmatic and sedimentary origin, marked by the Lapland Granulite, Umba and Tersk terranes. Palaeomagnetic data and ancient sedimentary detritus within these terranes suggest limited oceanic separation. Collision of juvenile terranes with the surrounding Archaean took place mainly between 1.93 and 1.91 Ga, resulting in a Himalayan-scale mountain belt, manifest by a thick-skinned region of high-P granulite-facies metamorphism, including the classical Lapland Granulite Belt and a broad zone of compressional deformation extending southwards into the Belomorian Mobile Belt. Protracted cooling and exhumation, possibly related to the buttressing effect of surrounding lithosphere, culminated in the intrusion of 1.80-1.77 Ga post-tectonic granites.

Ion microprobe UPb zircon geochronology and isotopic evidence for a trans-crustal suture in the Lapland–Kola Orogen, northern Fennoscandian Shield

The Lapland–Kola Orogen (LKO; former Kola craton) in the northern Fennoscandian Shield comprises a collage of partially reworked late Archaean terranes with intervening belts of Palaeoproterozoic juvenile crust including the classic Lapland Granulite Terrane. Rifting of Archaean crust began at c 2.5–2.4 Ga as attested by layered mafic and anorthositic intrusions developed throughout the northernmost Fennoscandian Shield at this time. Oceanic separation was centred on the Lapland Granulite, Umba Granulite (UGT) and Tersk terranes within the core zone of the orogen. Importantly, SmNd data show that Palaeoproterozoic metasedimentary and metaigneous rocks within these terranes contain an important, generally dominant, juvenile component over a strike length of at least 600 km. Evidently, adjacent Archaean terranes, with negative eNd signatures, contributed relatively little detritus, suggesting a basin of considerable extent. Subduction of the resulting Lapland–Kola ocean led to arc magmatism dated by the NORDSIM ion probe at c 1.96 Ga in the Tersk Terrane in the southern Kola Peninsula. Accretion of the Tersk arc took place before c 1.91 Ga as shown by ion probe UPb zircon dating of post-D1, pre-D2 pegmatites cutting the Tersk arc rocks, juvenile metasediments as well as Archaean gneisses in the footwall of the orogen. Deep burial during collision under high-pressure granulite-facies conditions was followed by exhumation and cooling between 1.90 and 1.87 Ga based on SmNd, UPb and ArAr data. Lateral variations in deep crustal velocity and Vp/Vs ratio, together with reflections traversing the entire crust observed in reprocessed seismic data from the Polar Profile, may be interpreted to image a trans-crustal structure — possibly a fossilised subduction zone — supporting an arc origin for the protoliths of the Lapland Granulite, UGT and Tersk terranes and the location of a major lithospheric suture — the Lapland–Kola suture.

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 km3) and in the NE German Basin (ca. 48 000 km3). 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.

Permo-Carboniferous magmatism and rifting in Europe: introduction

An extensive rift system developed within the northern foreland of the Variscan orogenic belt during Late Carboniferous-Early Permian times, post-dating the Devonian-Early Carboniferous accretion of various Neoproterozoic Gondwana-derived terranes on to the southern margin of Laurussia (Laurentia-Baltica; Fig. 1). Rifting was associated with widespread magmatism and with a fundamental change, at the Westphalian-Stephanian boundary, in the regional stress field affecting western and central Europe (Ziegler 1990; Ziegler & Cloetingh 2003). The change in regional stress patterns was coincident with the termination of orogenic activity in the Variscan fold belt, followed by major dextral translation between North Africa and Europe. Rifting propagated across a collage of basement terranes with different ages and thermal histories. Whilst most of the Carboniferous-Permian rift basins of NW Europe developed on relatively thin lithosphere, the highly magmatic Oslo Graben in southern Norway initiated within the thick, stable and, presumably, strong (cold) lithosphere of the Fennoscandian craton. The rift basins in the North Sea, in contrast, developed in younger Caledonian age lithosphere, which was both thinner and warmer than the lithosphere of the craton to the east. A regional hiatus, corresponding to the Early Stephanian, is evident in much of the Variscan foreland, with Stephanian and Early Permian red beds unconformably overlying truncated Westphalian series (e.g. McCann 1996) (Fig. 2). Regional uplift coincides with the onset of voluminous magmatism across the region, raising the possibility that uplift could have been related to the presence of a widespread thermal anomaly within the upper mantle (i.e. a mantle plume or, possibly, several plumes). In

Post-Variscan (end Carboniferous-Early Permian) basin evolution in Western and Central Europe

Abstract The Variscan orogeny, resulting from the collision of Laurussia with Gondwana to form the supercontinent of Pangaea, was followed by a period of crustal instability and re-equilibration throughout Western and Central Europe. An extensive and significant phase of Permo-Carboniferous magmatism led to the extrusion of thick volcanic successions across the region (e.g. NE German Basin, NW part of the Polish Basin, Oslo Rift, northern Spain). Coeval transtensional activity led to the formation of more than 70 rift basins across the region. The various basins differ in terms of their form and infill according to their position relative to the Variscan orogen (i.e. internide or externide location) and to the controls that acted on basin development (e.g. basement structure configuration). This paper provides an overview of a variety of basin types, to more fully explore the controls upon the tectonomagmatic-sedimentary evolution of these important basins.

SmNd evidence for late Archaean crust formation in the Lapland-Kola Mobile Belt, Kola Peninsula, Russia and Norway

Abstract A SmNd model age traverse across the Kola Peninsula has been undertaken to gain insight into the major episodes of crustal growth and to test the possible presence of early Archaean “Saamian” crust in the region. Granitoid gneisses and subordinate metasediments have been sampled from the main Archaean geological units, i.e. from northeast to southwest: the Murmansk domain (model ages of 2.68–2.96 Ga); Central Kola domain including samples from the Superdeep Borehole (2.74–2.92 Ga); Belomorian domain, including the eastern part of the Tersk area (2.70–2.93 Ga). The narrow range of depleted mantle model ages from 2.68 to 2.96 Ga for Archaean granitoid orthogneisses, close to the oldest crystallization ages from the region, indicate a major episode of late Archaean crustal growth and reveal no indication of ancient “Saamian” material in keeping with other geochronological evidence. Similar results (2.86 to 2.93 Ga) for Archaean metasediments support this conclusion. Proterozoic model ages for supracrustals from the western part of the Tersk area (2.16 to 2.23 Ga) indicate that the crust in this area is much younger than previously recognized. Initial ϵ Nd values for late Archaean samples with reasonable age constraints fall within a range of +2.3 to −2.5. Two well-dated samples from the Belomorian domain have the highest initial ϵ Nd values [ ϵ Nd (2.81 Ga) = + 2.0 and ϵ Nd (2.78 Ga) = + 2.3] and suggest that a depleted mantle source was present in the Kola region in late Archaean times. Dated samples from the Central Kola domain have lower initial ϵ Nd (between − 2.5 and + 1.2) suggesting a possible regional difference in mantle character. In contrast to the depleted signature of the Archaean orthogneisses between 2.7 Ga and ∼ 2.8 Ga, previous studies of earliest Palaeoproterozoic rift-related majic intrusives in the Kola and Karelia regions report negative ϵ Nd values at 2.4–2.5 Ga. Possibly these data reflect a change to more enriched-mantle values.

The multistage exhumation history of the Kaghan Valley UHP series, NW Himalaya, Pakistan from U-Pb and 40Ar/39Ar ages

Amphibole and mica 40Ar/39Ar ages as well as zircon, rutile and titanite U-Pb geochronology of eclogites and associated host rocks from the Higher Himalayan Crystalline Nappes (Indian Plate) in the Upper Kaghan Valley, Pakistan allow distinction of a multistage exhumation history. An Eocene age for peak-pressure metamorphism has been obtained by phengite 40Ar/39Ar (47.3 ± 0.3 Ma) and zircon U-Pb (47.3 ± 0.4 and 47.4 ± 0.3 Ma) ages from cover and basement gneisses. A very short-lived metamorphic peak and rapid cooling is documented by an amphibole 40Ar/39Ar age of 46.6 ± 0.5 Ma and a rutile U-Pb age of 44.1 ± 1.3 Ma from eclogites. Phengite and biotite ages from cover and basement sequences metamorphosed during the Himalayan orogeny are 34.5 ± 0.2 to 28.1 ± 0.2 Ma whereas youngest biotites, yielding 23.6 ± 0.1 and 21.7 ± 0.2 Ma, probably reflect argon partial resetting. The amphibole age, together with those derived from phengite and zircon demonstrate a rate of initial exhumation of 86–143 mm/a i.e . an extremely rapid transport of the Indian Plate continental crust from ultra-high pressure (UH P ) conditions back to crustal levels (47–46 Ma for transport from 140 to 40 km depth). Subsequent exhumation (46–41 Ma, 40–35 km) slowed to about 1 mm/a at the base of the continental crust but increased again later towards slightly higher exhumation rates of ca . 2 mm/a (41–34 Ma, 35–20 km). This indicates a change from buoyancy-driven exhumation at mantle depths to compression forces related to continent-continent collision and accompanied crustal folding, thrusting and stacking that finally exposed the former deeply-buried rocks.

Timing, geodynamic setting and character of Permo-Carboniferous magmatism in the foreland of the Variscan Orogen, NW Europe

Abstract In the early Carboniferous, final subduction of the Rhenohercynian Ocean, accretion of a magmatic arc and docking of microcontinents caused fault reactivation, extension and fault-controlled basin formation in the foreland of the Variscan Orogen. Lithospheric stretching resulted in generally mildly alkaline basaltic volcanism that peaked in the Visean. In the internal Variscides, rapid uplift and granitoid plutonism shortly followed collision and was probably due to slab detachment(s) or removal of orogenic root material. A regional-scale, E-W-oriented stress field was superimposed on a collapsing orogen and its foreland from the Westphalian onwards. In the Stephanian-Early Permian, a combination of outward-propagating collapse, mantle or slab detachment and the regional stress field resulted in widespread formation of fault-controlled basins and extensive magmatism dated at 290-305 Ma. In the foreland, large amounts of felsic volcanic rocks erupted in northern Germany, accompanied by mafic-felsic volcanics and intrusions in the Oslo Rift, and dolerite sills and dyke swarms in Britain and Sweden. In the internal Variscides, mafic rocks are rare and felsic-intermediate compositions predominate. Their apparent subduction-related signature may have been inherited from metasomatized mantle sources or caused by extensive assimilation of continental crust.

Evidence of the proto-Iceland plume in northwestern Ireland at 42 Ma from helium isotopes

Combined K–Ar and Ar–Ar dating of a xenolith-bearing alkaline dyke, previously considered to be of late Palaeozoic age, suggest a mid-Eocene age of c. 43 Ma, indicating that it is part of extensive rift-related Tertiary intrusive activity. High 3He/4He ratios of 11–16Ra, potentially up to 23Ra (Ra=atmospheric ratio of 1.39×10−6) have been measured from mantle xenoliths in the dyke and are consistent with previous determinations of the helium isotope composition of Tertiary lavas in Greenland and Scotland linking the magmatism with the proto-Iceland mantle plume.

New constraints on the timing of late Carboniferous-early Permian volcanism in the central North Sea

Abstract The Permo-Carboniferous evolution of the central North Sea is characterized by three main geological events: (1) the development of the West European Carboniferous Basin; (2) a period of basaltic volcanism during the Lower Rotliegend (latest Carboniferous-early Permian); and (3) the development of the Northern and Southern Permian Basins in late Permian times. The timing of the late Carboniferous-Permian basaltic volcanism in the North Sea is poorly constrained, as is the timing of extensional tectonic activity following the main phase of inversion during the Westphalian, due to the diachronous propagation of the Variscan deformation front. Results of high precision Ar-Ar dating on basalt samples taken from a core from exploration well 39/2-4 (Amerada Hess) in the UK sector of the central North Sea suggests that basaltic volcanism was active in the late Carboniferous, at c. 299 Ma. The presence of volcanics below the dated horizon suggests that the onset of Permo-Carboniferous volcanism in the central North Sea commenced earlier, probably at c. 310 Ma (Westphalian C). This is contemporaneous with other observations of tholeiitic volcanism in other parts of NW Europe, including the Oslo Graben, the NE German Basin, southern Sweden and Scotland. Interpretations of available seismic data show that main extensional faulting occurred after the volcanic activity, but the exact age of the fault activity is difficult to constrain with the data available.