North-central Norway Research Articles

Geology, geochemistry and emplacement conditions of the Vega intrusive complex: an example of large-scale crustal anatexis in north-central Norway

Abstract The c. 350 km 2 Vega intrusive complex is part of the Bindal Batholith and was emplaced at c. 475 Ma into polydeformed supracrustal rocks of the Helgeland Nappe Complex. The intrusive complex is tilted towards the west, exposing asymmetrical zoning. From east to west, the complex is composed of biotite granite, garnet-biotite granite, garnet-bearing muscovite biotite granodiorite and sillimanite-bearing garnet cordierite muscovite biotite granodiorite. In addition, the complex contains small amounts of intrusive migmatite. Granodiorite and intrusive migmatite contain abundant metasedimentary, mafic and ultramafic enclaves. Granodiorite, granite and migmatite are generally peraluminous to strongly peraluminous, calcic to alkalic and magnesian, with initial 87 Sr/ 86 Sr ratios of 0.7096–0.7469 and ε Nd from −7.0 to −11.0. Emplacement of the Vega intrusive complex was coeval with the intrusion of metaluminous dioritic rocks. The intrusive mafic rocks and enclaves in the complex have MORB-like (mid-ocean ridge basalt-like) to calc-alkaline geochemical characteristics. The lack of an isotopic compositional trend between mafic and granitic rocks indicates that magmas did not mix. Instead granitic magmas formed by unmixing of residual phases from crustally derived magmas. Partial melting of supracrustal source rocks may have been related to intra- and underplating of MORB-like magmas into the lower crust during extension. Supplementary material: Detailed petrographic descriptions, photomicrographs, and field images of selected enclaves are available at http://www.geolsoc.org.uk/SUP18653 .

Magma hybridization in the middle crust: Possible consequences for deep-crustal magma mixing

The 465 Ma Svarthopen pluton in north-central Norway was emplaced under middle-crustal conditions (∼700 MPa) into metasedimentary rocks of the Helgeland Nappe Complex. The pluton is characterized by zones of mingling and mixing of gabbro/diorite with peraluminous, garnet-bearing biotite granite. Variation in bulk-rock Sr and Nd isotope ratios are consistent with simple mixing; however, nonuniform enrichment of Zr and the rare earth elements (REEs) suggests that individual magma batches underwent postmixing fractionation. Hybrid intermediate rocks are characterized by Ca-rich garnet. Such garnet is absent in possible mafic end members, and garnet in felsic end members is Ca poor. Evidently, the ferroan, peraluminous hybrid rocks promoted garnet stability, and we interpret these garnets to be igneous in origin. Garnets in the hybrids have low Zr contents, positive light REE slopes, and flat to negative heavy REE slopes with lower REE abundances than in typical igneous garnet. These trace-element data combined with textural evidence suggest that garnet formed near the solidus, after fractionation of zircon, allanite, and possibly xenotime. The Svarthopen pluton is not unique: similar intermediate rocks with Ca-rich garnets crop out adjacent to three other plutons in the region. Formation of garnet-bearing hybrid rocks in the Svarthopen pluton provides an analog for mixing of peraluminous and ferroan end-member magmas in the deep crust, where such mixing should be widespread, particularly in continental arcs and zones of continental collision. Postmixing fractionation of hybrid magmas could greatly increase the diversity of major- and trace-element abundances yet retain an isotopic signature of mixing. Moreover, formation of garnet-rich hybrids could result in lower-crustal rocks dense enough to delaminate from the arc crust.

Rodingitization and hydration of the oceanic lithosphere as developed in the Leka ophiolite, north–central Norway

Ophiolite complexes in mountain chains may give supplementary information on the hydration of the oceanic lithosphere to that obtained from dredged and drilled samples from the ocean floor. The ultramafic (mantle) and the layered ultramafic to anorthositic (crustal) sequences of the Cambrian (497 Ma) Leka ophiolite are variably serpentinized and chloritized. Grossular-rodingite (rodingite s.s.) has been found over a c.500 m long and tens of meters wide zone in the layered, crustal section of the complex and is developed in both pyroxenites and gabbro/anorthosite layers. Shear zones and meter wide fracture zones, where the rock has developed a fracture cleavage, are oriented at high angel to the layering and these zones were the main conduits for transport of fluid and solute between the various lithologies. Some 5–15 cm thick layers of anorthosite (or leucogabbro) have been rodingitized around such a fractures zone, with the development of three distinct metasomatic zones along the plagioclase layer. A central grossular-dominated zone with clinopyroxene, clinozoisite, prehnite, chlorite and minor titanite (rodingite zone) extends for up to 3 m along strike and gives way to a clinozoisite-dominated zone (typically 0.5 m wide) with additional grossular, clinopyroxene and chlorite which is followed outward by a LILE-enriched zone (LILE-zone) with clinozoisite, phlogopite, K-feldspar, plagioclase and preiswerkite. The LILE-zone extends more than 3 m out from the clinozoisite-dominated zone (Clz-zone). Assuming constant volume, the rodingite formed from the plagioclase layer by addition of 20 g of CaO per 100 g of rock. All Na 2O (c. 2 g) was removed from both rodingite- and Clz-zones. Ti and V increase almost 10× in the rodingite compared to its protolith. K, Ba, Rb and Cs are strongly enriched in the LILE-zone compared to the protolith and suggest interaction with sea water. The lithologies alternating with the plagioclase layers (clinopyroxenite, wehrlite, websterite and dunite) display textures indicating a number of Ca-releasing (Cpx → Chl, Cpx → Srp, Cpx → Amph) and Ca-consuming (Opx → Cpx2, Ol → Cpx2, Cpx1 → Cpx2) reactions. The replacement textures are distributed around fracture and shear zones, with the Ca -releasing reactions in the core and the Ca -consuming reactions in distal parts, forming a metasomatic column out from the fluid pathways. Serpentinization and chloritization of clinopyroxene was the main Ca-source for the rodingitization process. This first description of rodingite in a layered sequence of an ophiolite complex indicates that the hydration of the oceanic lithosphere occurred at various structural levels and was associated with Ca-metasomatism also in places where rodingite s.s. is lacking. The different lithologies exchanged elements through transport on shear and fracture zones.

Carbonate isotope chemostratigraphy suggests revisions to the geological history of the West Finnmark Caledonides, northern Norway

Strontium and carbon isotopic data from amphibolite-facies marbles of the Falkenes formation, Sørøy succession (structurally upper Kalak Nappe Complex) in West Finnmark, northern Norway, indicate that the marbles were deposited between 760 and 710 Ma. Marbles of similar age have previously been identified only in the Uppermost Allochthon in north–central Norway, where they are considered to have a Laurentian ancestry. A similar origin and tectonostratigraphic position appears likely for the Falkenes formation. In contrast, structurally lower units within the Kalak Nappe Complex appear to have Baltican affinities. This dichotomy requires that commonly held ideas regarding the tectonostratigraphy of the West Finnmark Caledonides be revised. Combined with recent U–Pb dating of zircon and monazite from rocks associated with the marbles, the isotopic data suggest that the Sørøy succession of the Kalak Nappe Complex is not a continuous, depositional sequence as previously thought, but rather consists of a number of disparate and probably unrelated thrust-sheets, assembled during Late Silurian, Scandian orogenesis. This work shows that the West Finnmark Caledonides share a generally similar tectonometamorphic history with certain other parts of the Scandinavian Caledonides, and contributes to already existing data that may allow ‘chemostratigraphic’ correlation and reconstruction of the entire Caledonian belt.

Pervasive assimilation of carbonate and silicate rocks in the Hortavær igneous complex, north-central Norway

The intrusive complex at Hortavær represents a magma transfer zone in which multiple pulses of gabbroic and dioritic magmas evolved along Fe- and alkali-enrichment trends. Extreme alkali enrichment resulted in nepheline-normative and sparse nepheline-bearing monzodioritic and monzonitic rocks. More evolved monzonitic and syenitic rocks are silica saturated and, in some cases, quartz bearing. Previous and current research recognized an abundance of clinopyroxene and other Ca-rich phases, such as scapolite, grossular-rich garnet, and igneous-textured calcite among the mafic and intermediate rocks. Even the most pyroxene-rich samples contain low Sc concentrations, which suggests early, intense fractionation of clinopyroxene. These features and the alkali enrichment are consistent with assimilation of carbonate-rich host rocks. Carbon isotope ratios of the igneous-textured calcite indicate an origin of the carbon from host rocks rich in calcite, consistent with assimilation. However, low ɛ Nd values (−3.4 to −10.2) and moderate initial 87Sr/ 86Sr values (0.7052 to 0.7099) indicate the need for assimilation of quartzofeldspathic rocks as well. Models of combined assimilation and fractional crystallization indicate that assimilation of simple end members, either carbonate or silicate, cannot explain the entire data set. Instead, variable proportions of carbonate and silicate materials were assimilated, with the most pronounced assimilation effects in the mafic rocks. The reasons for variable degrees of assimilation are, as yet, uncertain. It is possible that assimilation of calc-silicate rocks with variable carbonate/silicate proportions resulted in the range of observed compositions. However, the importance of carbonate assimilation in mafic rocks compared to felsic ones suggests that assimilation of carbonates was predominant at high temperature and/or mafic magma compositions and assimilation of silicates was predominant at lower temperature and/or felsic magma compositions. We suggest that the ability of the mafic magma to dissolve higher proportions of carbonate contaminants is the result of the magma's ability to form clinopyroxene as a product of assimilation. In any case, extensive carbonate assimilation was possible because CO 2 escaped from the system.

Assimilation and crystal accumulation in a mid-crustal magma chamber: the Sausfjellet pluton, north-central Norway

The Sausfjellet pluton is made up of two intrusive units emplaced into high-grade metamorphic rocks of the Helgeland Nappe Complex of the Uppermost Allochthon in the Norwegian Caledonides. The eastern part of the pluton intruded marble and less voluminous calc-silicate and pelitic rocks. The western half is hosted predominantly by semi-pelitic migmatite with intercalated marble. Remelting of the migmatite during pluton emplacement occurred in a thermal aureole as much as 1000 m wide. The early gabbroic unit forms the southeastern part of the body; it consists of hornblende-bearing to hornblende-rich gabbro and diorite which is thought to have crystallized from an H 2O-rich andesitic parental magma. The younger dioritic unit underlies the central and western parts of the pluton, as well as a zone as much as 200 m wide that separates the rest of the pluton from its host rocks (herein the “annular zone”). The interior or central zone of the dioritic unit is pyroxene diorite that is locally interlayered with anorthosite. The western and annular zones are, by comparison, mineralogically heterogeneous. They range from diorite to quartz monzonite and from biotite-bearing two- and three-pyroxene assemblages to biotite–hornblende assemblages. Neither rock type nor mafic assemblage is correlated with position in the pluton or proximity to a contact. Stoped blocks of a distinctive coarse-grained diorite, as well as pyroxene-rich calc-silicates, are present in the gabbroic unit and the central zone of the dioritic unit. The few stoped blocks observed in the western zone of the dioritic unit are predominantly quartz-rich gneiss. Chemical variation in the central zone of the dioritic unit is interpreted to result from accumulation of pyroxenes+plagioclase from an H 2O-poor andesitic parent. These rocks have approximately constant δ 18O of +6.6±0.2‰ and lack evidence of in situ assimilation. Heterogeneities in the western and annular zones of the dioritic unit are reflected in variable, anomalously enriched incompatible element contents and in δ 18O, which ranges from +6.7‰ to +8.6‰. Petrologic models indicate that the magma parental to the central zone could also be parental to the western and annular zones. If so, evolution of the western and annular zone magma was by crystal accumulation and assimilation of metapelitic host rocks. As much as 20% of the mass of the western and annular zones can be ascribed to assimilated material, which apparently entered the magma by stoping. Therefore, the asymmetrical zoning of the pluton is due to differences in host rock compositions and the relative ability of the magma to assimilate its host rocks.

The Devonian Nesna shear zone and adjacent gneiss-cored culminations, North–Central Norwegian Caledonides

The Early to Mid-Devonian strain distribution in the Norwegian Caledonides was characterized by large-magnitude extension, extension-normal shortening and sinistral strike-slip. In SW Norway, previous work has shown that the angle between the orogen and the Devonian maximum elongation trend decreases northward towards a zone of sinistral strike-slip. We provide evidence that: (1) late- to post-orogenic extension was much more important in North–Central Norway than recognized previously; (2) the Early to Mid-Devonian, roughly orogen-parallel, maximum elongation trend persisted over more than 200 km farther north than hitherto recognized; (3) a number of gneiss-cored culminations in North–Central Norway were affected by, and probably owe their present geometry to, extensional shearing and faulting. The Nesna shear zone is identified here as a major, late Early to Mid-Devonian, low-angle extensional shear zone in the North–Central Norwegian Caledonides. In the study area, the Nesna shear zone now constitutes the lower boundary of the Helgeland Nappe Complex, a terrane exotic to Baltica. The present situation of the Helgeland Nappe Complex is thus that of an extensional allochthon. The transport direction along the medium- to low-grade Nesna shear zone was top-to-the-WSW, and thus roughly orogen-parallel. Extensional shearing in the Nesna shear zone may provide an explanation for a westward-younging, 40 Ar/ 39 Ar cooling pattern previously reported from the Ofoten–Lofoten area. Folding of the Nesna shear zone along axes parallel to the extensional transport direction indicates that extension-normal shortening continued at least into late Early and Mid-Devonian times. The WSW–ENE maximum elongation trend probably persisted in North–Central Norway through most of the Devonian period, and accompanied the transition from ductile to brittle deformation in the rocks of the Caledonian nappe-stack. Deformation in low-angle, initially medium-grade, extensional shear zones such as the Nesna shear zone was succeeded by shearing in steeper, low-grade, ductile-to-brittle shear zones that developed along the western margins of gneiss-cored culminations. Temporal overlap between activity in the Nesna shear zone and in the lower-grade shear zones that bound the culminations can be demonstrated. To the east, however, medium-grade extensional shear zones such as the present boundary between the Seve and Köli nappes were probably back-rotated as the culminations developed in the footwall of the low-grade shear zones. From the Mid–Late Devonian to Early Carboniferous, the low-grade, culmination-bounding shear zones constituted the eastern margin of a regional, transtensional system bounded in the south by the sinistral Møre–Trøndelag Fault Complex.

U–Pb geochronology and geochemistry of early Proterozoic rocks of the tectonic basement windows in central Nordland, Caledonides of north-central Norway

U–Pb dating of Proterozoic rocks of the tectonic basement windows in the Caledonides of north-central Norway has provided new information of the westernmost margin of the Fennoscandian Shield. Four granitic gneisses dated along a 70-km profile in central Nordland have yielded ages in the interval 1800–1795 Ma. These ages are similar to U–Pb ages from the Lofoten–Vesterålen area and the Rombak Window in northern Norway. The granitoid rocks are also coeval with the oldest group of granitoids in the Trans-Scandinavian-Igneous Belt (TIB) in southern Sweden (1.81–1.76 Ga) and the late Svecofennian granitoids in central Sweden (1.80–1.77 Ga), confirming the importance of the 1.80 Ga magmatic period at the western parts of the Fennoscandian Shield. The geochemistry of the dated granitoids is consistent with formation in a subduction-related regime. The granitoid rocks are interpreted to have been generated by mixing of mafic melt and a large proportion of melts derived from early Svecofennian granitoids (1.91–1.87 Ga), probably with minor contribution from Archaean crustal material. The new data support a model where the rocks of the basement windows in central Nordland represent the western continuation of the Fennoscandian Shield under the Caledonian nappes. The Sm–Nd isotope data of the granitoid rocks implies that they were generated to the south and west of the Archaean–Proterozoic boundary.

Conditions of pluton emplacement and anatexis in the Caledonian Bindal Batholith, north-central Norway

The Velfjord plutons are mafic-intermediate plutons in the Bindal Batholith that were emplaced into pelitic and calcareous rocks of the Helgeland Nappe Complex of the Uppermost Allochthon at 448 Ma. Al-in-hornblende barometry suggests emplacement in the range of 6 to 8 kbar, which is consistent with the presence of magmatic epidote. Pelitic rocks in the aureoles underwent partial melting and leucosomes locally accumulated adjacent to the plutons to form ''contact granites''. The results of garnetaluminosilicate-silica-plagioclase (GASP) thermobarometry combined with a recently published petrogenetic grid for dehydration partial melting of pelites suggest a clockwise P-T-t path for the wall rocks. The GASP results, plus the presence of early kyanite, suggest initial melting in the 7 to 8 kbar range. Temperatures close to the plutons reached at least 750°C to 800°C. Nearly isothermal decompression to 4 to 5 kbar resulted in the hottest wall rocks reaching cordierite stability while they were still partly mo...

Timing of Paleozoic orogeny and extension in the continental shelf of north-central Norway as indicated by laser 40Ar/39Ar muscovite dating

Metamorphic rocks of Lofoten, Norway, represent a leading edge of the Baltic craton that was partly subducted beneath Laurentia during Caledonian orogeny. We find that muscovite porphyroblasts in Lofoten record a remarkably complete history of cooling and unroofing and yield data about Lofoten9s tectonic evolution in the interval from 425 to 265 Ma. These data confirm that culminating metamorphism of basement and allochthonous rocks in Lofoten occurred during the Scandian phase of orogeny. Permian ages for muscovite are the youngest obtained for regional metamorphic rocks anywhere in Scandinavia and appear to record significant basement unroofing during extensional evolution of the Norwegian-Greenland seaway. Regional age variations of muscovite in Lofoten can be related to present crustal thickness, Devonian to Permian sedimentation in the Norwegian seaway, and extensional structures, and they indicate the timing of multiple extension events in the evolution of Norway9s passive margin. The results of this study show that 40 Ar closure profiles in porphyroblasts can form even in active tectonic settings characterized by rapid and episodic unroofing.

The sources of metals in sulfide deposits in the Helgeland nappe complex, north-central Norway; Pb isotope evidence

The Helgeland Nappe Complex of the uppermost Caledonian allochthons of Norway is host to a number of different sulfide deposits. These occur in epicontinental metasedimentary rocks close to, or at, the contact with the granitoid Bindal batholith. Sixty-eight common Pb analyses for six Zn-Pb sulfide deposits and one As-Au deposit make it possible to recognize three distinct episodes of sulfide ore formation, which can be related to successive stages of the tectonic and petrogenetic evolution of the Helgeland Nappe Complex.Prior to peak Caledonian deformation, Zn-Pb sulfides were deposited from ore-forming solutions that derived their lead from a probable Late Proterozoic continental source, as indicated by low 206 Pb/ 204 Pb and 208 Pb/ 204 pb ratios and high 207 Pb/ 204 Pb ratios. Shortly after peak metamorphism and emplacement of the Bindal batholith, skarn related base metal and As-Au vein deposits were formed (lead isotope ratios of the sulfides are indistinguishable from the initial lead isotope ratios of the Bindal batholith), and the metals and ore-forming fluids were probably derived from the granitoid magmas. During the final uplift of the Caledonian nappes after orogenic collapse, structurally controlled base and precious metal sulfide deposits and occurrences were formed. The radiogenic 206 Pb/ 204 Pb ratios and low 208 Pb/ 204 Pb ratios indicate an ore-forming process involving convection of meteoric water and leaching of metals from the rocks in the brittle zone.This lead isotope survey of the sulfide deposits in the Helgeland Nappe Complex has revealed repeated ore-forming activity at previously mineralized sites which involved introduction of new lead when new metal reservoirs became available as a result of changing tectonic settings.

The Caledonian Heilhornet Pluton, north-central Norway: geological setting, radiometric age and implications for the Scandinavian Caledonides

The Heilhornet Pluton is a medium-sized ( c . 50 km 2 ), sub-circular, discordant intrusion which penetrates the thrust zone separating the Helgeland Nappe Complex from paragneisses in the Vestranden region, north-central Norway. The intrusion consists of biotite ± hornblende granite and ranges in composition from 61.5 to 72.5 wt.% Si0 2 The initial Sr-ratio ( c . 0.7070) shows that crustal sources have been involved in the petrogenesis of the granite. A U-Pb zircon age of 444 ± 11 Ma for the pluton indicates that juxtaposition of the nappe complex with metasedimentary rocks in Vestranden took place prior to or during the late Ordovician. A consequence of this is that the general correlation of the metasedimentary rocks in Vestranden with the Seve Nappes must be revised. Final eastward thrusting of the complex, together with a yet unknown part of the Vestranden supracrustal sequence, probably took place during the middle Silurian to early Devonian, Scandian, orogenic event.

Tectonostratigraphy and structure of the Kjukkelen area, Børgefjellet, north-central Norway

Abstract The Kjukkelen area in the B⊘rgefjellet National Park occupies a key position concerning the tectonostratigraphical relationship between the K⊘li Nappes north and south of the B⊘rgefjellet Window (BW). The thickness of the K⊘li Nappes between the rock units of the BW and the overlying Helgeland Nappe Complex (HNC) is locally reduced to 75 m in the Kjukkelen area. Several K⊘li Nappes (Stikke, Gelvenakko and Leipikvattnet N.) consisting of R⊘yrvik Group lithologies wedge out along the southwestern margin of the BW. Only the Gjersvik Nappe consisting of Limingen Group lithologies enters the narrow passage between the BW and the HNC. North of the passage, K⊘li lithologies belong to the R⊘yrvik Group. It is uncertain, which nappe unit they can be correlated with. Rocks belonging to the Limingen Group reappear further north in the Jofjallet Nappe which attenuates southward beneath the HNC. Investigations in the Kjukkelen area show that the Jofjallet Nappe appears at the tectonostratigraphic level of the...