SW Norway Research Articles

The Egersund dykes (SW Norway): a robust Early Ediacaran (Vendian) palaeomagnetic pole from Baltica

SUMMARY Palaeomagnetic data for Baltica in the Late Precambrian are highly ambiguous, and have therefore, given rise to different interpretations concerning the need to explain Varangerian glaciations through snowball Earth conditions. We present new palaeomagnetic data from the 616 ± 3 Ma (U-Pb) Egersund dykes in SW Norway, which yield a palaeolatitude of 53 ◦ +16 ◦ /−13 ◦ for the studied location. This would indicate that the Baltica plate spanned latitudes between 50 ◦ and 75 ◦ S in the Early Ediacaran. The pole position (31 ◦ N, 44 ◦ E, dp/dm = 15/17) confirms earlier studies, but the primary nature of the remanence is now supported by two positive contact tests. A new 40 Ar/ 39 Ar biotite age of 600 ± 10 Ma from one of the dykes suggests that remanence acquisition in the dykes took place between 600 and 616 Ma. The Egersund palaeomagnetic data demonstrate that Baltica was located at relatively high latitudes at the time of the Varangerian glaciations. Additional palaeomagnetic sites in the Rogaland Igneous Complex yield a pole position (46 ◦ S, 238 ◦ E, dp/dm = 17/19) that confirms previous studies. The age of this remanence has traditionally been quoted as c. 930 Ma based on U-Pb ages from the complex. However, previous 40 Ar/ 39 Ar hornblende ages of around 870 Ma are now supported by a new 40 Ar/ 39 Ar biotite age of 869 ± 14 Ma obtained from a noritic dyke, and we argue that this represents an uplift/cooling age which better represents the age of the remanence in SW Norway.

N Leaching from Small Upland Headwater Catchments in Southwestern Norway

We investigated nitrogen leaching and input-output budgets from 12 small, upland headwater catchments with differing fractions of peat, heather and bare rock in SW Norway. A close relationship was found between annual inorganic nitrogen loss and the fractional cover bare rock explaining 74% of the variation. It was not possible to distinguish between the N leaching from the peat and heather dominated catchments in this investigation. Nitrate loss was negatively correlated to the C/N ratio of the soil organic horizon. Lowest soil C/N ratios of the soil organic surface horizon occurred in catchments receiving the highest amounts of N deposition. Our data for Norwegian moorland are in agreement with previous investigations relating soil C/N ratio and N leaching in forest soils. Runoff from the catchments showed a seasonal pattern in NO3 − concentrations. The low values during the summer are presumably due to increased biological activity (plant uptake and immobilization by soil microbes). The winter and spring NO3 − concentrations in 2001 in all catchments were much higher than in 2000, perhaps due to soil freezing.

Palaeomagnetism of 935 Ma mafic dykes in southern Sweden and implications for the Sveconorwegian Loop

New palaeomagnetic results for the 935 Ma Göteborg-Slussen mafic dykes in southern Sweden provide a well-dated high-quality palaeomagnetic pole for Early Neoproterozoic Baltica. New U-Pb geochronological data for several palaeomagnetically studied mafic intrusions yield three additional well-dated palaeopoles and one virtual geomagnetic pole. This set of dated poles suggests minimal drift of Baltica in moderate latitudes between ∼965 and 915 Ma. They also support the hypothesis of a post-900 Ma regional remagnetization event in SW Sweden and SW Norway. The positions of three distinct clusters of ∼1100 to 850 Ma palaeopoles suggest a clockwise time progression of the Baltica apparent polar wander path (the Sveconorwegian Loop) during this time interval. New well-dated palaeomagnetic poles for ∼970 to 900 Ma from Laurentia are required to verify the palaeogeographic reconstructions of Baltica and Laurentia.

Magma chamber processes in the Tellnes ilmenite deposit (Rogaland Anorthosite Province, SW Norway) and the formation of Fe–Ti ores in massif-type anorthosites

The origin of igneous Fe–Ti oxide ores associated with massif-type anorthosites is investigated through a detailed study of the world-class Tellnes ilmenite deposit, part of the late-Proterozoic (930–920 Ma) AMC series of the Rogaland Anorthosite Province (SW Norway). More than 100 samples from drill cores reveal significant petrographical and compositional variations within the ore body. Four zones are defined, based on variations in modal proportions and cumulus mineral assemblages: the Lower and Upper Central Zones and the Lower and Upper Marginal Zones. Plagioclase and whole-rock compositions discriminate the zones and display patterns interpreted as a result of mixing of either plagioclase–ilmenite or plagioclase–ilmenite–orthopyroxene–olivine cumulates with a melt of ferrodioritic (jotunitic) composition with a content decreasing from 80 to 20% from the margins to the central part of the ore body. Phase diagrams for a jotunitic parental magma reproduce the crystallization sequence at 5 kb. The orthopyroxene–olivine liquidus boundary is a peritectic in the Bjerkreim-Sokndal layered intrusion and a cotectic in Tellnes and this explains the differences in the sequence of crystallization of the two intrusions. The high concentration of ilmenite, well above cotectic proportions, resulted from gravity-sorting in the Tellnes ore body, which represents the lower part of a larger magma chamber. Uniform Sr isotope ratios do not support magma mixing. The cryptic layering of the ore body precludes injection as a crystal mush but favours in situ crystallization from an evolving magma in a sill-like magma chamber. The present trough-shape and mineral orientations result from deformation during gravity-induced subsidence and by up-doming of the anorthosite. Fractional crystallization of a TiO 2-rich magma with ilmenite as an early liquidus mineral and plagioclase buoyancy are the principal mechanisms responsible for the formation of Fe–Ti deposits in Proterozoic massif-type anorthosites.

Ecological process indicators used for nature protection scenarios in agricultural landscapes of SW Norway

Abstract Conflicts between changing landscapes and static nature protection concepts were addressed as an example of the agricultural landscape of SW Norway. We aimed to deduce indicators for spatio-temporal landscape changes to draw scenarios for future protection perspectives of a RAMSAR and nature reserve. To estimate the variability of bird diversity, changes in vegetation patterns were analysed to predict bird occurrence. We obtained a differentiated analysis of present landscape dynamics by measuring landscape structure, vegetation, hydrology and nutrient concentration. Multivariate statistics were used to extract the main driving forces for changes in vegetation patterns out of a complex landscape ecological data set. Subsequently, we compared the measured data with those of past landscape stages to determine landscape changes and their mechanisms at different spatio-temporal scales. Ecological process indicators (EPI) were derived, and three different indicator constellations were used for scenario descriptions. These scenarios were chosen as to the current assumptions of typical contrasting nature protection strategies. Concluding, we used EPIs to evaluate nature protection aims and to assess scenarios of changing landscapes. This approach will be transferable to other examples of nature protection conflicts in the agricultural landscape in general.

Hummocky moraine landscape on Jæren, SW Norway—implications for glacier dynamics during the last deglaciation

The Jæren area, SW Norway, can topographically be divided in two regions, Lågjæren and Høgjæren. Høgjæren is a plateau > 120 m a.s.l. covered by Quaternary sediments. The stratigraphically youngest deposit is a sandy diamicton, which accumulated in ridges and hummocks during the last deglaciation. This hummocky moraine landscape has previously been interpreted as the record of a stagnant glacier. Detailed morphological mapping and sedimentological investigations have revealed that various glacigenic elements exist and dominate different areas of the Høgjæren plateau. Glacigenic elements formed by an active wet-based glacier characterize the southern central hummocky area and include marginal moraine ridges and drumlins, while circular moraine ridges and glaciofluvial features formed in a stagnant glacial regime dominate the northern central hummocky area. The distribution of glacigenic elements within the hummocky moraine landscape shows that active and stagnant ice coexisted on Høgjæren during the last deglaciation, and that actively flowing ice was focused through eastward extending valleys. It is not possible to determine whether ice flow was kept active in the southern area throughout the deglaciation or if the glacier was reactivated. A possible readvance is suggested to have happened simultaneous to a readvance previously suggested based on the morphology on Lågjæren.

A test of the englacial thrusting hypothesis of ‘hummocky’ moraine formation: case studies from the northwest Highlands, Scotland

The melt‐out of material contained within englacial thrust planes has been proposed to result in the formation of stacked moraine sequences with characteristic proximal rectilinear slopes. This model has been applied to explain the formation of Scottish Younger Dryas ice‐marginal (‘hummocky’) moraines on the basis of these morphological characteristics. However, no sedimentological data exist to support this proposal. This article reviews hitherto proposed models of ‘hummocky’ moraine formation and presents detailed geomorphological and sedimentological results from the NW Scottish Highlands with the aims of reconstructing the dynamics of Younger Dryas glaciers and of testing the applicability of the englacial thrusting model. Exposures demonstrate that moraines represent terrestrial ice‐contact fans throughout, with a variety of postdepositional deformation structures being identified in most cases, indicating that glacier retreat was incremental and oscillatory; proximal rectilinear slopes are interpreted as ice‐contact faces formed after ice support was withdrawn during retreat. This evidence strongly suggests a temperate glacier regime and short glacier response times similar to those in present‐day SW Norway or Iceland. It contradicts the thrusting model and the proposal that Svalbard might form a suitable analogue for Younger Dryas moraines in Scotland.

The Hardangerfjord Shear Zone in SW Norway and the North Sea: a large-scale low-angle shear zone in the Caledonian crust

The Hardangerfjord Shear Zone is a more than 600 km long low-angle extensional structure that affects the South Norway and North Sea Caledonides. The ductile shear zone, which shows total maximum onshore displacement of the order of 10–15 km, is primarily a basement structure with an associated passive, monoclinal fold structure of the overlying Caledonian nappes. Deep seismic data indicate that the shear zone continues down to the lower crust (20–25 km) at a dip of 22–23°, where it appears to flatten and merge with the general lower-crustal deformation fabric. Onshore, the Hardangerfjord Shear Zone consists of a system of hard-linked ductile shear-zone segments. Brittle faults (the Lœrdal–Gjende fault system) occur in the folded Caledonian allochthons in the NE part of the Hardangerfjord Shear Zone, and reappear in the North Sea. These may represent a high-level brittle response to the Devonian development of the Hardangerfjord Shear Zone, but were reactivated during Permo-Triassic and late Jurassic extensional events. A c . 5 km thick package of seismic reflectors along the Hardangerfjord Shear Zone is presumed to represent a mylonite zone, which is too thick to be formed entirely by 10–15 km of Devonian displacement. Hence the Hardangerfjord Shear Zone is likely to be a Proterozoic shear zone, reactivated during Devonian extension.

In situ U–Pb dating of zircon formed from retrograde garnet breakdown during decompression in Rogaland, SW Norway

AbstractRegionally metamorphosed metapelites from Rogaland, SW Norway, contain zircon formed during the decompression reaction garnet + sillimanite + quartz → cordierite. The zircon, which occurs as inclusions in cordierite coronas around garnet, is texturally, chemically and isotopically distinct from older zircon in other textural settings in the matrix. A SHRIMP U–Pb age of 955 ± 8 Ma based on analyses in thin section on the decompression zircon from the cordierite coronas, therefore dates a point on the retrograde path, estimated from garnet–cordierite equilibria to be 5.6 kbar, 710 °C. This population was under‐represented in conventional SHRIMP analyses of individual zircon in a mono‐mineralic grain mount and, in the absence of a textural context, its significance unknown. The dominant age identified from SHRIMP analyses of the grain mount, in combination with analyses from matrix zircon in thin section, was 1035 ± 9 Ma. Based on the lack of consistent textural relationships with any specific minerals in thin section, as well as rare earth element chemistry, the 1035‐Ma population is interpreted to represent zircon growth during incipient migmatization of the rocks at 6–8 kbar and c. 700 °C. This is consistent with previous estimates for the age of regional M1 metamorphism during the Sveconorwegian Orogeny. The most important outcome of this study is the successful analysis of zircon grains in a specific, well‐constrained reaction texture. Not only does this allow a precise point on the regional P–T path to be dated, but it also emphasizes the possibility of zircon formation during the retrograde component of a typical metamorphic cycle.

Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway). Part I: Constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent

Whole-rock major element compositions are investigated in 99 cumulates from the Proterozoic Bjerkreim–Sokndal layered intrusion (Rogaland Anorthosite Province, SW Norway), which results from the crystallization of a jotunite (Fe–Ti–P-rich hypersthene monzodiorite) parental magma. The scattering of cumulate compositions covers three types of cumulates: (1) ilmenite–leuconorite with plagioclase, ilmenite and Ca-poor pyroxene as cumulus minerals, (2) magnetite–leuconorite with the same minerals plus magnetite, and (3) gabbronorite made up of plagioclase, Ca-poor and Ca-rich pyroxenes, ilmenite, Ti-magnetite and apatite. Each type of cumulate displays a linear trend in variation diagrams. One pole of the linear trends is represented by plagioclase, and the other by a mixture of the mafic minerals in constant proportion. The mafic minerals were not sorted during cumulate formation though they display large density differences. This suggests that crystal settling did not operate during cumulate formation, and that in situ crystallization with variable nucleation rate for plagioclase was the dominant formation mechanism. The trapped liquid fraction of the cumulate plays a negligible role for the cumulate major element composition. Each linear trend is a locus for the cotectic composition of the cumulates. This property permits reconstruction by graphical mass balance calculation of the first two stages of the liquid line of descent, starting from a primitive jotunite, the Tjörn parental magma. Another type of cumulate, called jotunite cumulate and defined by the mineral association from the Transition Zone of the intrusion, has to be subtracted to simulate the most evolved part of the liquid line of descent. The proposed model demonstrates that average cumulate compositions represent cotectic compositions when the number of samples is large (> 40). The model, however, does not account for the K 2O evolution, suggesting that the system was open to contamination by roof melts. The liquid line of descent corresponding to the Bjerkreim–Sokndal cumulates differs slightly from that obtained for jotunitic dykes in that the most Ti-, P- and Fe-rich melts (evolved jotunite) are lacking. The constant composition of the mafic poles during intervals where cryptic layering is conspicuous is explained by a compositional balance between the Fe–Ti oxide minerals, which decrease in Fe content in favour of Ti, and the pyroxenes which increase in Fe.

Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway): Part II. REE and the trapped liquid fraction

Rare earth elements in bulk cumulates and in separated minerals (plagioclase, apatite, Ca-poor and Ca-rich pyroxenes, ilmenite and magnetite) from the Bjerkreim–Sokndal layered intrusion (Rogaland Anorthosite Province, SW Norway) are investigated to better define the proportion of trapped liquid and its influence on bulk cumulate composition. In leuconoritic rocks (made up of plagioclase, Ca-poor pyroxene, ilmenite, ±magnetite, ±olivine), where apatite is an intercumulus phase, even a small fraction of trapped liquid significantly affects the REE pattern of the bulk cumulate, together with cumulus minerals proportion and composition. Contrastingly, in gabbronoritic cumulates characterized by the presence of cumulus Ca-rich pyroxene and apatite, cumulus apatite buffers the REE content. La/Sm and Eu/Eu* vs. P 2O 5 variations in leuconorites display mixing trends between a pure adcumulate and the composition of the trapped liquid, assumed to be similar to the parental magma. Assessment of the trapped liquid fraction in leuconorites ranges from 2 to 25% and is systematically higher in the north-eastern part of the intrusion. The likely reason for this wide range of TLF is different cooling rates in different parts of the intrusion depending on the distance to the gneissic margins. The REE patterns of liquids in equilibrium with primitive cumulates are calculated with mass balance equations. Major elements modelling ( Duchesne, J.C., Charlier, B., 2005. Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway): Part I. Constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent. Lithos. 83, 299–254) permits calculation of the REE content of melt in equilibrium with gabbronorites. Partition coefficients for REE between cumulus minerals and a jotunitic liquid are then calculated. Calculated liquids from the most primitive cumulates are similar to a primitive jotunite representing the parental magma of the intrusion, taking into account the trapped liquid fraction calculated from the P 2O 5 content. Consistent results demonstrate the reliability of liquid compositions calculated from bulk cumulates and confirm the hypothesis that the trapped liquid has crystallized as a closed-system without subsequent mobility of REE in a migrating interstitial liquid.

Sub-Local Differences in Late Holocene Land Use at Orstad, Jæren in SW Norway, revealed by Soil Pollen Stratigraphy

Four soil profiles along an 84 m transect through a clearance cairn field on podsol soils at Orstad, Jæren, SW-Norway, were investigated. By relating pollen-analytical results to soil stratigraphical and morphological features, it was possible to reconstruct the landuse history of the site, and to reveal differences along the transect, although pollen preservation was poor. Human activity at Orstad began about 4500 uncalibrated 14C years BP. The reason for an intermediate abandonment of the site between about 4000 and 3600 uncalibrated 14C years BP was presumably a higher ground-water level, caused by local deforestation and/or by a climatic change. After that period, people seem to have grown cereals (Triticum and Hordeum) on at least two different field patches on the site. This land use was presumably occasional, and related to extraordinary needs. Fire-clearance seems to have been practised to prepare the fields for cultivation after long fallow periods. Between 2900 and 3200 uncalibrated 14C years BP, Orstad was possibly permanently inhabited. The agricultural fields were moved to higher levels, as the initial fields had become nutrient-depleted and too moist. Thin black layers in the profile may be remains of manure.

Sub-Local Differences in Late Holocene Land Use at Orstad, Jæren in SW Norway, revealed by Soil Pollen Stratigraphy

Four soil profiles along an 84 m transect through a clearance cairn field on podsol soils at Orstad, Jæren, SW-Norway, were investigated. By relating pollen-analytical results to soil stratigraphical and morphological features, it was possible to reconstruct the landuse history of the site, and to reveal differences along the transect, although pollen preservation was poor. Human activity at Orstad began about 4500 uncalibrated 14C years BP. The reason for an intermediate abandonment of the site between about 4000 and 3600 uncalibrated 14C years BP was presumably a higher ground-water level, caused by local deforestation and/or by a climatic change. After that period, people seem to have grown cereals (Triticum and Hordeum) on at least two different field patches on the site. This land use was presumably occasional, and related to extraordinary needs. Fire-clearance seems to have been practised to prepare the fields for cultivation after long fallow periods. Between 2900 and 3200 uncalibrated 14C years BP, Orstad was possibly permanently inhabited. The agricultural fields were moved to higher levels, as the initial fields had become nutrient-depleted and too moist. Thin black layers in the profile may be remains of manure.

Mesoproterozoic bimodal volcanism in SW Norway, evidence for recurring pre-Sveconorwegian continental margin tectonism

Between ca. 1.3 Ga and the onset of the Sveconorwegian orogeny (ca. 1.0 Ga) in the Baltic Shield, the margin of Baltica was subjected to repeated episodes of bimodal magmatism. A new TIMS U–Pb date of 1259 +/− 2 Ma for magmatic zircon from metarhyolite of the Trossovdal Formation is presented. At least two distinct episodes of bimodal volcanism in southern Norway: the 1.26 Ga Valldal Group and 1.16 Ga Oftefjell and Høydalsmo groups are now recognized. The Dal Group in south-western Sweden is a more distal equivalent of the latter. In the Valldal region, volcanism developed in an extensional setting, following the initial deposition of clastic sediments. The earliest volcanics are A-type peraluminous rhyolites, with Nd and Hf isotopic compositions supporting formation by remelting of 1.5–1.9 Ga granitic basement, initiated by the intrusion of mafic magmas. Subsequent volcanism comprised subaerial basaltic lava flows, locally intercalated with clastic sediments. The earliest basaltic magmas (Hagastøl Formation) are chemically the more evolved and contain the lavas that suffered crustal contamination, whereas, the later basalts (Austmannali Formation) are consistent with melting of a depleted mantle source. However, all of the basalts have elevated La/Nb values relative to MORB. This feature is consistent with asthenospheric melts variably contaminated by a lithospheric mantle component, which had been previously enriched by an earlier subduction event. The combination of early clastic sedimentation, bimodal volcanism and igneous geochemistry suggests that the Valldal volcanics formed in a continental back-arc setting. The new data suggest that at least from ca. 1.3 Ga the proto-margin of Baltica was possibly a convergent continental margin, with back-arc development occurring diachronously along its length from ca. 1.26 to 1.16 Ga. Magmas produced during these two volcanic episodes have distinct geochemical and isotopic signatures and their geographic distribution suggests that each is restricted to a discrete crustal terrain. This further supports the view that during the Sveconorwegian orogeny large-scale strike slip tectonics juxtaposed the different terranes to give the present crustal geometry.

Modelling impacts of climate and deposition changes on nitrogen fluxes in northern catchments of Norway and Finland

Abstract. The Integrated Nitrogen model for Catchments (INCA) was applied to three upland catchments in Norway and Finland to assess the possible impacts of climate change and nitrogen (N) deposition on concentrations and fluxes of N in streamwater in cold regions of Europe. The study sites cover gradients in climate and N deposition from the southern boreal Øygard Brook (2.6 km2) in SW Norway, via the southern/middle boreal Simojoki River (3610 km2) in northern Finland to the sub-arctic Dalelva Brook (3.2 km2) in northern Norway. The INCA scenario simulations included future N deposition scenarios (current legislation and maximum feasible reduction) and climate scenarios for 2050 (ECHAM4/OPYC3; HadCM3) treated separately and in combination. As a result of climate change, the INCA model predicted markedly reduced duration and amounts of snow cover in all catchments. The occurrence of winter rainfall and melting periods was predicted to become more frequent so that more frequent floods in winter will to a large extent replace the regular snowmelt flood in spring. At the northernmost catchment, Dalelva, the predicted temperature increase might result in a doubling of the net mineralisation rate, thereby greatly increasing the amount of available inorganic N. At all catchments, the increased N supply was predicted to be largely balanced by a corresponding increase in N retention, and relatively small increases in NO3- leaching rates were predicted. This dynamic relationship is, however, strongly dependent on the temperature responses of the key N transformation processes modelled. A future reduction in N emissions and deposition, as agreed under current legislation, would have pronounced effects on concentrations of NO3- in streamwater at the southernmost catchment, Øygard, even following a climate change around 2050. At the more remote Dalelva and Simojoki catchments, the N emission reductions will be small compared to the internal N recycling processes, and climate change will to a large extent offset the effects of reduced N deposition. Keywords: catchments, surface water, scenarios, climate, hydrology, nitrogen deposition, nitrate leaching

Lu–Hf and U–Pb isotope systematics of zircons from the Storgangen intrusion, Rogaland Intrusive Complex, SW Norway: implications for the composition and evolution of Precambrian lower crust in the Baltic Shield

The Storgangen orebody is a concordantly layered, sill-like body of ilmenite-rich norite, intruding anorthosites of the Rogaland Intrusive Complex (RIC), SW Norway. 17 zircon grains were separated from ca. 5 kg of sand-size flotation waste collected from the on-site repository from ilmenite mining. These zircons were analysed for major and trace elements by electron microprobe, and for U–Pb and Lu–Hf isotopes by laser ablation microprobe plasma source mass spectrometry. Eight of the zircons define a well-constrained (MSWD=0.37) concordant population with an age of 949±7 Ma, which is significantly older than the 920–930 Ma ages previously reported for zircon inclusions in orthopyroxene megacrysts from the RIC. The remaining zircons, interpreted as inherited grains, show a range of 207Pb/ 206Pb ages up to 1407±14 Ma, with an upper intercept age at ca. 1520 Ma. The concordant zircons have similar trace element patterns, and a mean initial Hf isotope composition of 176Hf/ 177Hf 949 Ma=0.28223±5 ( ε Hf=+2±2). This is similar to the Hf-isotope composition of zircons in a range of post-tectonic Sveconorwegian granites from South Norway, and slightly more radiogenic than expected for mid-Proterozoic juvenile crust. The older, inherited zircons show Lu–Hf crustal residence ages in the range 1.85–2.04 Ga. One (undated) zircon plots well within the field of Hf isotope evolution of Paleoproterozoic rocks of the Baltic Shield. These findings indicate the presence of Paleoproterozoic components in the deep crust of the Rogaland area, but do not demonstrate that such rocks, or a Sveconorwegian mantle-derived component, contributed significantly to the petrogenesis of the RIC. If the parent magma was derived from a homogeneous, lower crustal mafic granulite source, the lower crustal protolith must be at least 1.5 Ga old, and it must have an elevated Rb/Sr ratio. This component would be indistinguishable in Sr, Nd and Hf isotopes from some intermediate mixtures between Sveconorwegian mantle and Paleoprotoerzoic felsic crust, but it cannot account for the initial 143Nd/ 144Nd of the most primitive, late Sveconorwegian granite in the region, without the addition of mantle-derived material.

Lateglacial landform associations at Jæren (SW Norway) and their glaci-dynamic implications

Lateglacial landform associations at Jæren (SW Norway) and their glaci-dynamic implications

Lateglacial landform associations at Jæren (SW Norway) and their glaci-dynamic implications

Lateglacial landform associations at Jæren (SW Norway) and their glaci-dynamic implications

Lateglacial landform associations at Jæren (SW Norway) and their glaci‐dynamic implications

The Jæren lowland is located on the southwestern coast of Norway between a mountainous region in the east and the offshore Norwegian Channel in the west. During the Last Glacial Maximum, Jæren was in an intermediate position between an ice stream following the Norwegian Channel northwards, and westward flowing inland ice. The dynamic behaviour of the inland ice and the interaction with the ice stream are examined by means of geomorphological analysis of digital terrain models and sedimentological investigations. SW‐trending drumlins were formed at Jæren below tributary ice from the inland, feeding into the Norwegian Channel Ice Stream. The presence of Rogen moraine in the central part of Jæren indicates a frozen substratum prior to their formation, and this suggests a transition to cold‐based ice between the tributaries. The deglaciation of the Norwegian Channel at about 15 ka BP resulted in an unstable ice front for the inland ice sheet. The formation of Rogen moraine may be explained by a dynamic advance resulting in extensional flow and fracturing of the frozen substratum between the tributaries. The dynamic advance was followed by an early deglaciation of the coastal areas as evidenced by shallow marine sediments. Deformation of the shallow marine sand indicates a glacial readvance through the valleys formerly acting as tributaries to the ice stream.

Petrogenesis of jotunitic and acidic members of an AMC suite (Rogaland anorthosite province, SW Norway): a Sr and Nd isotopic assessment

Sr and Nd isotopic data from the anorthosite-mangerite-charnockite (AMC) suite of the Late-Sveconorwegian (ca. 0.93 Ga) Rogaland anorthosite province of SW Norway are discussed. The study focuses on new data obtained from three distinct rock occurrences: (1) primitive (=MgO-rich and K 2 O-poor) jotunites that represent the parental magmas of leuconorite and mafic plutons ( 87 Sr / 86 Sr 0.93 Ga = ca . 0.704–0.707 , ε Nd (0.93 Ga )=+3.6 down to +1.2); (2) evolved jotunites that differentiated from the primitive jotunites and constitute the starting components of dyke-scale fractionation trends ( 87 Sr / 86 Sr 0.93 Ga = ca . 0.705–0.713 , ε Nd (0.93 Ga )=+0.4 down to −2.0); and (3) felsic plutons ( 87 Sr / 86 Sr 0.93 Ga = ca . 0.707–0.723 , ε Nd (0.93 Ga )=+1.4 down to −1.7). A comparison of the Sr and Nd isotopic compositions with available geochemical data (major and trace elements) is also made. Fractionation of the primitive to the evolved jotunites, below the intrusion level of the anorthosite province, is shown to have involved crustal contamination. A case of fractional crystallization and simultaneous contamination/assimilation is substantiated in the felsic cap of a layered intrusion (the Bjerkreim-Sokndal intrusion). It is further proposed that the Sr and Nd isotopic compositions of the various felsic occurrences result from fractional crystallization of primitive jotunitic melts, with or without contamination/assimilation; this strengthens the possibility for large volumes of silica-rich magmas to have been produced by fractionation, through evolved jotunites, of primitive jotunites. The entire range in Sr and Nd isotopic compositions of the Rogaland anorthosite province is interpreted as reflecting a mixing of crustal contaminants, having variable 87 Sr / 86 Sr 0.93 Ga ratios and (negative) ε Nd (0.93 Ga) values, with a more isotopically primitive source. The crustal end members of that mixing array are represented by moderately to strongly LILE-enriched high-grade gneisses from the Pre-Sveconorwegian basement of southernmost Norway, that have a crustal history extending back to 1.5–1.9 Ga in the surroundings of the anorthosite province. The primitive end member is either an unusual LREE-depleted and Rb-enriched component, possibly corresponding to an originally depleted source subsequently modified by metasomatic fluids, or a more classical depleted component; it corresponds to a gabbronoritic rock of the deep crust, as shown by recent experimental data, with a quite short crustal residence time (<0.4–0.2 Ga). If that mafic end member is the lower granulitic crust itself, this would imply the presence in the deep continental crust of southwesternmost Norway of a crustal material strongly different from the 1.7- to 1.9-Gyr-old, moderately LILE-enriched component that is supposed to constitute the largest volume of the present deep continental crust in southern Norway.