Solund-Stavfjord Ophiolite Complex Research Articles

Deformation-related alteration of basaltic clasts during deep burial in sedimentary basins

The once deeply buried conglomerates of the Solund basin contain up to 30% basaltic clasts together with up to 20% altered peridotite clasts, both derived from the adjacent Solund-Stavfjord Ophiolite Complex (SSOC). The basaltic clasts range in size from 2 to 30cm across and display centimeter-thick hematite reddened zones. Four types which form an alteration-related evolutionary sequence can be distinguished. Type 1 consists of a green core with a 1–2cm thick red zone along its rim. Type 2 shows, in addition, a perturbation of the red zone into the center of the clasts. In Type 3, numerous compartments with green cores surrounded by red zones build up the clast. Type 4 displays diffuse red zones and patches typically with a circular form that resembles Liesegang's structures. This evolutionary sequence relates to a progressive deformation evident from fractures, faults and shear zones. Hematite reddened zones form along clast boundaries and along fractures that are oriented at high angle to the boundaries of the clasts. The boundary-parallel zones migrate towards the center of the clasts and the fracture-parallel zones migrate away from the fractures. These fracturing and deformation are coupled with chemical and mineralogical changes responsible of the changes in color of the alteration zones. Compared to the basaltic rocks of the SSOC, the basaltic clasts of the Solund basin contain higher contents of K2O, volatiles (LOI) and MgO, lower contents of CaO and are more oxidized with an average Fe3+/(Fe3++Fe2+) ratio of 0.43 (±0.10) as compared to 0.30 (±0.07) in the SSOC basalts. The replacement of magmatic clinopyroxene by chlorite induces the decrease in CaO. The replacement of ilmenite by titanite provides iron to form hematite in the red zones. The red zones progressively replace the green core and are, at their external boundary, in turn replaced by Mg-riched green alteration zones characterized by a high content of zoned amphibole. A decrease in CaO and an increase in MgO towards the rim of the basaltic clasts is contrary to the chemical evolution of the peridotite clasts, where the MgO is depleted and the CaO increases with increasing degree of alteration. This suggests that clasts of different lithologies exchanged components during the alteration process. To explain these observations, we use a model including internal stresses through a diffusion–reaction process and external stresses through sediment loading and tectonic forces. The mineralogy of the altered basaltic clasts (chlorite, epidote, albite, actinolite) corresponds to low-grade metamorphic conditions in accordance with or slightly higher than the peak temperature of 230–320°C previously calculated for this basin. The study shows that the final compaction of multicomponent clast systems takes place through an interaction between mineral reactions, metasomatism and deformation, which differs from the compaction of monocomponent systems that occurs dominantly through pressure solution.

Structure, geochemistry, and tectonic evolution of trench-distal backarc oceanic crust in the western Norwegian Caledonides, Solund-Stavfjord ophiolite (Norway)

Research Article| July 01, 2012 Structure, geochemistry, and tectonic evolution of trench-distal backarc oceanic crust in the western Norwegian Caledonides, Solund-Stavfjord ophiolite (Norway) Harald Furnes; Harald Furnes † 1Department of Earth Science & Centre for Geobiology, University of Bergen, 5007 Bergen, Norway †E-mail: harald.furnes@geo.uib.no Search for other works by this author on: GSW Google Scholar Yildirim Dilek; Yildirim Dilek 2Department of Geology & Environmental Earth Science, Miami University, Oxford, Ohio 45056, USA3School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083, China Search for other works by this author on: GSW Google Scholar Rolf Birger Pedersen Rolf Birger Pedersen 1Department of Earth Science & Centre for Geobiology, University of Bergen, 5007 Bergen, Norway Search for other works by this author on: GSW Google Scholar GSA Bulletin (2012) 124 (7-8): 1027–1047. https://doi.org/10.1130/B30561.1 Article history received: 07 Jun 2011 rev-recd: 10 Feb 2012 accepted: 16 Feb 2012 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Harald Furnes, Yildirim Dilek, Rolf Birger Pedersen; Structure, geochemistry, and tectonic evolution of trench-distal backarc oceanic crust in the western Norwegian Caledonides, Solund-Stavfjord ophiolite (Norway). GSA Bulletin 2012;; 124 (7-8): 1027–1047. doi: https://doi.org/10.1130/B30561.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Late Ordovician (443 Ma) Solund-Stavfjord ophiolite complex in west Norway represents the youngest phase of oceanic crust formation in the western Norwegian Caledonides. It contains three structural domains with different crustal architecture that formed during two episodes of seafloor spreading evolution of a Late Ordovician marginal basin. The fossil oceanic crust of the younger episode contains pillow lavas, massive sheet flows, and hyaloclastites, NE-trending sheeted dikes, and high-level isotropic gabbros. The pillow lava versus massive sheet flow distribution and the occurrence of an extensive sheeted dike complex in the Solund-Stavfjord ophiolite complex are typical of in situ oceanic crust developed at modern intermediate-spreading mid-ocean ridges. The Solund-Stavfjord ophiolite complex lavas and dikes are composed predominantly of normal mid-ocean-ridge basalt (N-MORB) Fe-Ti basalts, and their trace-element patterns indicate a weak subduction influence. The Nd isotope data of these rocks suggest derivation of their magmas from an isotopically homogeneous melt source with no indication of continental crustal contamination. The Solund-Stavfjord ophiolite complex extrusive sequence contains phyllite interlayers and is conformably overlain by a continentally derived, quartz-rich metasandstone that is intercalated with sills of N-MORB basaltic lavas and shallow-level intrusions. The geochemical features of the upper-crustal rocks of the Solund-Stavfjord ophiolite complex indicate their formation from magmas in which the melt evolution involved only minor or no slab-derived fluids. The evolution of the Solund-Stavfjord ophiolite complex oceanic crust occurred in a short-lived (<20 m.y.), trench-distal, continent-proximal backarc basin, adjacent to the eastern margin of Greenland-Laurentia, during the closure of Iapetus. This inferred tectonic setting is reminiscent of the modern Andaman Sea at the eastern periphery of the Indian Ocean. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Tubular textures in pillow lavas from a Caledonian west Norwegian ophiolite: A combined TEM, LA-ICP-MS, and STXM study

Tubular alteration textures, mineralized by titanite, in glassy rims of pillow lavas from a Norwegian ophiolite (Solund-Stavfjord ophiolite complex (SSOC)) are described and characterized by a multimethod approach. Tubular alteration textures, mineralized by titanite, have been previously proposed to result from bioalteration. The microstructure of the titanite and the tubes is investigated using focused ion beam milling in combination with transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM). These indicate an assemblage of submicrometer-sized (about 500 nm) titanite single crystals with no organic film or residue in between the grains. In situ U-Pb radiometric dating of the titanite, using laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA–MC–ICP–MS), yielded a metamorphic age of 442 ± 13 Ma. An isotope dilution–thermal ionization mass spectrometric age obtained previously for zircons from the SSOC plutonic rocks yielded a magmatic age of 443 ± 3 Ma. The overlap in ages indicates that subseafloor metamorphism, responsible for titanite formation, occurred during seafloor or subseafloor formation of the tubular alteration textures. The rare earth element contents of the titanite were determined using LA–ICP–MS and chondrite-normalized patterns are similar to those of the SSOC volcanics; hence these do not reflect hydrothermal or seawater influence. The Y/Ho ratio of ∼20–30 in the titanite is also consistent with an upper mantle–derived origin. The sum of all of the spatial resolved data reported here neither supports nor refutes a biogenic origin for the tubular textures.

Tectonic control of bioalteration in modern and ancient oceanic crust as evidenced by carbon isotopes

Abstract We review the carbon‐isotope data for finely disseminated carbonates from bioaltered, glassy pillow rims of basaltic lava flows from in situ slow‐ and intermediate‐spreading oceanic crust of the central Atlantic Ocean (CAO) and the Costa Rica Rift (CRR). The δ13C values of the bioaltered glassy samples from the CAO show a large range, between −17 and +3‰ (Vienna Peedee belemnite standard), whereas those from the CRR define a much narrower range, between −17‰ and −7‰. This variation can be interpreted as the product of different microbial metabolisms during microbial alteration of the glass. In the present study, the generally low δ13C values (less than −7‰) are attributed to carbonate precipitated from microbially produced CO2 during oxidation of organic matter. Positive δ13C values &gt;0‰ likely result from lithotrophic utilization of CO2 by methanogenic Archaea that produce CH4 from H2 and CO2. High production of H2 at the slow‐spreading CAO crust may be a consequence of fault‐bounded, high‐level serpentinized peridotites near or on the sea floor, in contrast to the CRR crust, which exhibits a layer‐cake pseudostratigraphy with much less faulting and supposedly less H2 production. A comparison of the δ13C data from glassy pillow margins in two ophiolites interpreted to have formed at different spreading rates supports this interpretation. The Jurassic Mirdita ophiolite complex in Albania shows a structural architecture similar to that of the slow‐spreading CAO crust, with a similar range in δ13C values of biogenic carbonates. The Late Ordvician Solund–Stavfjord ophiolite complex in western Norway exhibits structural and geochemical evidence for evolution at an intermediate‐spreading mid‐ocean ridge and displays δ13C signatures in biogenic carbonates similar to those of the CRR. Based on the results of this comparative study, it is tentatively concluded that the spreading rate‐dependent tectonic evolution of oceanic lithosphere has a significant control on the evolution of microbial life and hence on the δ13C biosignatures preserved in disseminated biogenic carbonates in glassy, bioaltered lavas.

Hydrothermal alteration and tectonic evolution of an intermediate‐ to fast‐spreading back‐arc oceanic crust: Late Ordovician Solund‐Stavfjord ophiolite, western Norway

Abstract The Solund‐Stavfjord ophiolite complex (SSOC) in western Norway represents a remnant of the Late Ordovician oceanic lithosphere, which developed in an intermediate‐ to fast‐spreading Caledonian back‐arc basin. The internal architecture and magmatic features of its crustal component suggest that the SSOC has a complex, multistage sea floor spreading history in a supra‐subduction zone environment. The youngest crustal section associated with the propagating rift tectonics consists of a relatively complete ophiolite pseudostratigraphy, including basaltic volcanic rocks, a transition zone between the sheeted dyke complex and the extrusive sequence, sheeted dykes, and high‐level isotropic gabbros. Large‐scale variations in major and trace element distributions indicate significant remobilization far beyond that which would result from magmatic processes, as a result of the hydrothermal alteration of crustal rocks. Whereas K2O is strongly enriched in volcanic rocks of the extrusive sequence, Cu and Zn show the largest enrichment in the dyke complex near the dyke–volcanic transition zone or within this transition zone. The δ18O values of the whole‐rock samples show a general depletion structurally downwards in the ophiolite, with the largest and smallest variations observed in volcanic rocks and the transition zone, respectively. δ18O values of epidote–quartz mineral pairs indicate 260–290°C for volcanic rocks, 420°C for the transition zone, 280–345°C for the sheeted dyke complex and 290–475°C for the gabbros. The 87Sr/86Sr isotope ratios show the widest range and highest values in the extrusive rocks (0.70316–0.70495), and generally the lowest values and the narrowest range in the sheeted dyke complex (0.70338–0.70377). The minimum water/rock ratios calculated show the largest variations in volcanic rocks and gabbros (approximately 0–14), and generally the lowest values and range in the sheeted dyke complex (approximately 1–3). The δD values of epidote (−1 to −12‰), together with the δ18O calculated for Ordovician seawater, are similar to those of present‐day seawater. Volcanic rocks experienced both cold and warm water circulation, resulting in the observed K2O‐enrichment and the largest scatter in the δ18O values. As a result of metal leaching in the hot reaction zone above a magma chamber, Zn is strongly depleted in the gabbros but enriched in the sheeted dyke complex because of precipitation from upwelling of discharged hydrothermal fluids. The present study demonstrates that the near intact effect of ocean floor hydrothermal activity is preserved in the upper part of the SSOC crust, despite the influence of regional lower greenschist facies metamorphism.

Geochemical stratigraphy of the lavas of the Solund-Stavfjord Ophiolite Complex, W. Norway, and magma-chamber dynamics

The stratigraphy and geochemistry of the uppermost 200–300 m of the metabasalt sequence of the Solund-Stavfjord Ophiolite Complex of western Norway has been investigated over a lateral distance of ~6 km. Volcanologically, this sequence was constructed in a cyclic manner. The lowest part of a volcanic cycle is characterised by sheet flows and/or large pillows followed by lavas in which pillows become progressively smaller upwards. In several of the cycles, pillow lavas are differentiated Fe–Ti basalts at the base, and successive flows become gradually more primitive stratigraphically upwards. The compositional differences between the bottom and top of a cycle can be substantial (e.g. TiO2 is 2.98–1.00 wt%, Zr 227–76 ppm, and Cr 125–520 ppm respectively). Nd isotopic and other data suggest that all the basaltic magmas were generated from a uniform source. Concomitant with the upward decrease in the Ti and Fe contents of the metabasalts through a volcanic cycle, estimated magma densities also decrease. We attribute the geochemical stratigraphy of the metabasalts to mainly reflect magma mixing in a frequently replenished magma chamber. Hybrids in the chamber repeatedly mixed with inflowing magmas which were more primitive and less dense. During the time intervals between cycles, the magma chamber was essentially closed and extensive fractional crystallisation took place. In some sequences, however, volcanic cycles and stratigraphical geochemical trends are poorly defined. We tentatively propose that in such cases the volcanics were erupted from more than one magma chamber.

Ophiolites as faithful records of the oxygen isotope ratio of ancient seawater: the Solund-Stavfjord Ophiolite Complex as a Late Ordovician example

AbstractFragments of the Ordovician sea floor preserved in the Solund-Stavfjord Ophiolite Complex in Western Norway serve as proxies for the δ18O of Ordovician seawater. The pillow basalt sections at Oldra and Strand are both enriched in 18O, recording their alteration by seawater at low temperature on the sea floor. In contrast, the sheeted dykes and gabbros generally are depleted of 18O, reflecting the modal proportion of secondary, low-18O chlorite and epidote formed from seawater at high temperature. These isotopic contrasts simply reflect the high water to rock ratio of sea-floor alteration and the temperature dependence of the 18O partitioning between minerals and water. Superposition of high-δ18O pillows on low-δ18O dykes and gabbros is a necessary consequence of alteration at both low and high temperatures by a fluid near 0‰ and is easily recognized in well-preserved ophiolites. Also, the δ18O of seawater can be independently calculated from 18O fractionations among secondary minerals. Older, dismembered and highly metamorphosed segments of the oceanic crust may still retain the original seawater imprint because their subsequent obduction and metamorphism was relatively closed to external fluids. Suites of diamond-bearing nodules from kimberlites still have contrasting high- and low-δ18O eclogites, proving that even subduction into the mantle is not sufficient to erase the seawater fingerprint. Inspection of the sea-floor, ophiolite and eclogite data reveals no secular trend in δ18O, indicating that the δ18O of seawater has not changed with geological age. Because the δ18O of seawater itself is fixed by sea-floor-seawater exchange, the constancy of δ18O of seawater implies that the scale and style of sea-floor-seawater interactions has not changed over time.

Crustal melting in the lower parts of island arcs: an example from the Bremanger Granitoid Complex, west Norwegian Caledonides

The 440±5 Ma Bremanger Granitoid Complex (BGC), west Norwegian Caledonides, comprises tonalites, granodiorites and minor granites. The BGC is coeval with a nearby subduction-related gabbronoritic/dioritic intrusion (the 443±4 Ma Gasoy Intrusion) and an ophiolite complex generated in a back-arc basin (the 443±3 Ma Solund-Stavfjord Ophiolite Complex). The BGC has a restricted isotopic variation (⊂Nd440Ma=+0.04 to –2.21, 87Sr/86Sr440 Ma=0.7045 to 0.7054) indicating a substantial mantle component, but it is enriched in crustal material relative to the primitive parts of the gabbronoritic rocks of the Gasoy Intrusion (⊂Nd440 Ma=+1.67, 87Sr/86Sr440 Ma=0.7048). The chondrite-normalised REE patterns are fractionated [(La/Yb)N ~8 to ~16], smooth or with negative or positive Eu anomalies. Geochemical modelling suggests that the BGC does not represent residual melts produced by fractional crystallisation of basic magma, but is compatible with partial melting of mafic source rocks with various amounts of H2O. Residual garnet is suggested by HREE depletion, indicating a minimum pressure of 9–10 kbar during melting.

Volcanic rocks in the Devonian Solund Basin, Western Norway: large landslides of Silurian (439 Ma) rhyolites

Abstract: The Devonian Solund Basin in SW Norway is the only Old Red Sandstone basin in the Scandinavian Caledonides from which volcanic rocks have been described. These rhyolitic lavas yield a concordant ID TIMS U–Pb zircon age of 439 ± 1 Ma (Early Silurian) interpreted as dating the crystallization of the rhyolite, and are thus too old to be syndepositional with the Devonian sedimentary rocks. This new age constraint, combined with field observations, suggests that the volcanic rocks form part of a 30 km 2 landslide of rhyolite, granite, gabbro and metasedimentary rocks, rather than extension related volcanism as previously suggested. The landslides show an inverted stratigraphy with respect to the nearby Solund–Stavfjord Ophiolite Complex and its overlying volcanic arc, which was formed immediately prior to obduction of Caledonian exotic terranes onto Baltica. The dated rocks thus appear to record the youngest pre-collisional volcanism in the Scandinavian Caledonides. Tectonic models involving Devonian Old Red volcanic rocks in the Scandinavian Caledonides must thus be revised.

Cyclic volcanic stratigraphy in a late ordovician marginal basin, West Norwegian Caledonides

The well-preserved extrusive sequence of the Solund-Stavfjord Ophiolite Complex (SSOC) in the West Norwegian Caledonides enables reconstruction of the uppermost oceanic crust that developed in a marginal basin. Basaltic sheet flows, pillow lavas and volcanic breccias are the main components of the extrusive sequence and show stratigraphic and structural evidence for a cyclic development. The first stage in a volcanic cycle is characterized by high extrusion rates yielding sheet flows, commonly with the thickest flow units at the base. Sequences of sheet flows can be correlated laterally for at least 6.5 km. Pillow lavas succeed the sheet flows later in a volcanic cycle with progressively smaller pillows forming at decreasing extrusion rates. Volcanic breccias occur towards the end of a volcanic cycle, but may also occur at lower stratigraphie levels. They are made generally of pillow breccias and hyaloclastites. The extrusive sequence of the SSOC oceanic crust was constructed through seven volcanic cycles that resulted in stratigraphic units with thicknesses ranging from 40 to 225 m. This architecture is comparable to sequences in in situ oceanic crust developed along slow- to intermediate-spreading ridges.

Volatile phase fluxed anatexis of metasediments during late Caledonian ophiolite obduction: evidence from the Sogneskollen Granitic Complex, west Norway

In the west Norwegian Caledonides minor granitic sheets were intruded during obduction of the c. 443 Ma Solund–Stavfjord Ophiolite Complex. The Sogneskollen Granitic Complex is the largest intrusive complex and consists of several minor lenses and one larger lens of granodiorite. The Sogneskollen Granitic Complex samples generally have high Ba, Sr and Na 2 O, very low Y values (&lt;7 ppm), negative ϵ Nd 420 Ma values ( c. −7 to −10.1) and 87 Sr/ 86 Sr 420 Ma ratios of 0.70437–0.70705. The REE patterns are strongly HREE depleted and La/Yb n varies from 0.3 to 36. Most samples display small positive Eu anomalies, but those that are most fractionated have small negative anomalies. The main part of the Sogneskollen Granitic Complex is a white composite granodiorite which is intruded by a suite of grey biotite-granite dykes. It is suggested that this, and other similar granitic sheets in the area, most probably formed by melting of sediments in the presence of a H 2 O-bearing volatile phase during obduction of the ophiolite. The quartzofeldspathic source sediments most probably contained detritus from long-term Rb-depleted lower continental crust and from an island arc. The sediments were deposited in large amounts in the marginal basin now represented by the Solund–Stavfjord Ophiolite Complex. During obduction the sediments were buried beneath the hot overriding Ophiolite Complex and the associated island-arc, and yielded granitic magmas during partial melting.

Geochemistry and petrogenesis of extrusive rocks, dykes and high-level plutonic rocks on the island of Oldra, Solund-Stavfjord Ophiolite Complex, western Norway

The island of Oldra, part of the Late Ordovician Solund-Stavfjord Ophiolite Complex of the western Norwegian Caledonides, comprises extrusive rocks, a sheeted dyke complex and high-level gabbros. The metabasalts are of N-MORB affinity, and their Nd isotopic composition ( k Nd = +7.8 to +8.4) indicates generation from a relatively homogeneous, but strongly depleted mantle. Eighty percent of the metabasalts classify as FeTi-basalts (FeO t /MgO >1.75, TiO 2 >2 wt%), but pseudostratigraphically their distribution varies significantly (51% of the extrusive rocks and 92% of the sheeted dykes). The contents of FeO t , MgO, Al 2 O 3 , Cr, Ni and Zr indicate that differences in depth of melting (30 to 12 kbar), crystal fractionation of olivine, plagioclase and clinopyroxene at different levels in the mantle and crust (<8 kbar) and magma mixing affected the composition of the rocks. The MgO-Zr relations of the established chemostratigraphy of the extrusive rocks indicate that magma mixing caused significant scatter among magma compositions.

The gabbro–dyke transition zone demonstrated on Tviberg, Solund–Stavfjord Ophiolite Complex

AbstractThe transition zone between 100 % dykes and high-level plutonic rocks of the Solund-Stavfjord Ophiolite Complex is complex due to the existence of many lithologies with different and variable contact relationships. The rocks of the plutonic complex vary in composition from FeTi basaltic to quartz dioritic, and the grain sizes vary from fine to pegmatitic. Felsic varieties are produced by fractional crystallization of basaltic magma as demonstrated by geochemical evolution and by gradual transition from gabbro to quartz diorite. Patches of fractionated dioritic rocks may show both gradual and intrusive relationships with the surrounding host gabbro. This demonstrates that late-stage liquids commonly left the source region and locally intruded the surrounding parent rocks. The high-level plutonic rocks are thoroughly epidotized and are cut by dykes consisting of granoblastic epidote and quartz. The high-level plutonic complex is associated with irregular bodies of fine- to medium-grained plagioclase-porphyritic diabase of high MgO content. These diabase bodies are intruded by dykes that become progressively more regular in shape. The plutonic complex locally shows intrusive relationships with the overlying 100% dyke complex, but is itself cut by two dyke swarms. The dykes of the first swarm formed while the plutonic complex experienced sinistral shear strain, and the dykes are generally less regular and thinner than the dykes of the second swarm. This indicates that the dykes of the first swarm intruded while the rocks of the plutonic complex were still hot, while the next dyke swarm intruded later when the rock complex was colder. Dykes of both swarms range in composition from slightly to strongly fractionated, suggesting that the magma chambers they were expelled from underwent significant fractionation in between magma replenishment. Numerous dykes of both swarms carry large quantities of glomeroporphyritic aggregates of plagioclase and altered clinopyroxene, indicating that the source area to the dykes very often was a crystal mush.

Geochemistry of the Sunnfjord Melange: sediment mixing from different sources during obduction of the Solund–Stavfjord Ophiolite Complex, Norwegian Caledonides

AbstractThe Sunnfjord Melange is a strongly tectonized sedimentary assemblage occurring tectonostratigraphically below the Solund–Stavfjord Ophiolite Complex of Ashgill age, and upon continental margin deposits of Wenlock or older age. The Sunnfjord Melange, which probably started to form within an oceanic transform fault, developed further during obduction of the Solund–Stavfjord Ophiolite Complex. The Sunnfjord Melange is divided into two units, the lower Øyravatn Unit of predominantly fine grained calcareous quartz-bearing chlorite–muscovite schists, and the upper Markavatn Unit, mainly metagreywackes. Both units contain blocks of metabasalts, marble and meta-arkose. Serpentinite blocks are restricted to the Markavatn Unit. The appearance of serpentinite detritus is marked by much higher contents of Cr and Ni in the metasediments of the Markavatn Unit than in those of the Øyravatn Unit. The geochemistry of the metasediments from the Øyravatn Unit and the Markavatn Unit is compatible with mixing of detritus from continental and ophiolitic sources. Despite the strong and pervasive deformation of the Sunnfjord Melange, its geochemical composition from the lowest to the highest tectonostratigraphic levels reflects that of the inverse pseudostratigraphy of the Solund–Stavfjord Ophiolite Complex. Thus, only metabasalts of the Solund–Stavfjord Complex and continental meta-arkoses yielded detritus to the schists of the Øyravatn Unit. The compositions of the schists and metagreywackes of the Markavatn Unit, on the other hand, show that a multicomponent detritus derived from metabasalt and serpentinite of the Solund–Stavfjord Ophiolite Complex, mixed with the detritus from continental meta-arkoses.

Petrogenesis and significance of late Caledonian granitoid magmatism in western Norway

Obduction of the late Ordovician Solund-Stavfjord Ophiolite Complex (443±3 Ma), west Norwegian Caledonides, involved generation and high-level emplacement of granitic and granodioritic dikes and plutons. Initial 87Sr/86Sr ratios in the granites are low (0.7042–0.7059), suggesting either a mantle component or a Rb-poor crustal source. Initial eNd (eNd(t)) ranges from-0.8 to-8.8, indicating that the granites represent recycling of old crustal rocks, which is supported by Precambrian inheritance in zircons from two of the studied granites. I argue that the Rb-Sr and the Sm-Nd isotope systems are decoupled in the sense that the Sr-and the Nd-isotopes derive their dominant signals from two different sources, a mantle source and a crustal source respectively. The granites are metaluminous to peraluminous and typically have high Sr, Ba and Na2O/K2O ratios. SiO2 contents range from 66 to 74 wt%. REE abundances are highly variable; the La contents range from ∼80 to ∼200 times chondrite, and are inversely correlated with the contents of SiO2. The concentration of Nd in the granites decreases asymptotically with decreasing eNd(t) suggesting fractional crystallization of accessory phases and assimilation of continental crust. This argument is supported by the presence of partly dismembered xenoliths in the granites with eNd(t)-values that are significantly lower than eNd(t)-values in the host granite. The following models are suggested for the granites. When the ophiolite complex obducted, an outboard subduction zone approached the continental margin, and subduction-related magmas accumulated beneath the continental margin, and probably intruded the overlying eugeosynclinal deposits. The mantle-derived magmas most likely evolved to granitoid composition by assimilation of these eugeosynclinal sediments and by fractional crystallization of amphibole, feldspar, sphene, and allanite. Alternatively, but less likely, the heat content of the mantle-derived magmas caused extensive melting of immature graywackes and calc-alkaline volcaniclastic rocks in the deepest portions of the eugeosyncline. Either way, during ascent, the compositions of the granitic melts were modified by fractional crystallization of LREE-rich phases and by assimilation of continental metasediments.

The Solund–Stavfjord Ophiolite Complex and associated rocks, west Norwegian Caledonides: geology, geochemistry and tectonic environment

AbstractMetabasalts of the Upper Ordovician Solund-Stavfjord Ophiolite Complex of the westernmost Norwegian Caledonides, show N-to E-MORB affinity, with high Th/Ta (or Nb) ratios giving evidence of subduction influence. The Solund–Stavfjord Ophiolite Complex is overlain by a heterogeneous assemblage of sedimentary and volcanic rocks, the Stavenes Group, of which the Heggøy Formation of metasandstones and phyllites conformably overlies the metabasalts of the Solund–Stavfjord Ophiolite Complex. The Heggøy Formation contains, in places, abundant metabasalt pillow lavas and minor intrusions, geochemically similar to those of the Solund–Stavfjord Ophiolite Complex, and basic metavolcaniclastites of island arc tholeiite (IAT) composition. This indicates that the Solund–Stavfjord Ophiolite Complex and Heggøy Formation developed in a marginal basin between a continental margin and an active subduction system, for which the present-day Andaman Sea may provide a realistic model. The other magmatic rocks of the Stavenes Group, showing both calc-alkaline and alkaline affinities, are less well time-constrained, but they are thought to represent an advanced stage of the island arc development, and ocean island build-up, respectively.

The Sunnfjord Melange, evidence of Silurian ophiolite accretion in the West Norwegian Caledonides

A major composite terrane, the Sunnfjord Melange, has been identified in the West Norwegian Caledonides. The rocks of the melange provide a terrane-link between the allochthonous continental rocks of the Dalsfjord Suite with its cover of continental margin deposits and the oceanic terrane of the Solund-Stavfjord Ophiolite Complex. The melange was formed as the ophiolite was emplaced on the the fossiliferous Lower to Middle Silurian continental margin deposits of the Herland Group. This group unconformably overlies older metasedimentary rocks of the Hoyvik Group and the crystalline basement of the Dalsfjord Suite. A structural style characteristic of thin-skinned thrust-foldbelts is locally preserved within the Herland Group on Atloy, and the thrust-foldbelt was developed in the foreland of the continental margin during the ophiolite accretion. A U-Pb zircon age of 443 ± 3 Ma from the ophiolite, the Silurian fossils in the Herland Group and the identification of the Sunnfjord Melange as an obduction melange provide the basis for a well constrained model of ophiolite accretion in the Scandinavian Caledonides.

Magmatic development and tectonomagmatic models for the Solund-Stavfjord Ophiolite Complex, West Norwegian Caledonides

Geochemical variation and different extrusion rates of the Late-Ordovician Solund-Stavfjord Ophiolite Complex (SSOC) suggest both open and closed magma chambers. Closed chambers existed off axis and within a transform fault and are indicated by dykes and pillow lavas that evolved from high-MgO basalt to ferrobasalt by fractional crystallization of olivine, plagioclase and clinopyroxene. Hybrid magmas with ferrobasaltic composition represent open magmatic systems and are characterized by frequent eruptions of sheet flows. The SSOC is interpreted as representing either a normal or a propagating ridge-transform intersection (RTI). The propagating RTI model is favoured as it explains both the high frequency of ferrobasalt and uplift of ultramafic rocks to above the sea-level which is suggested by detrital ultramafics occurring in an ophiolitic mélange.