The Raudfjellet ophiolite fragment, Central Norwegian Caledonides: principal lithological and structural features (original) (raw)
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Norwegian Journal of Geology, 2021
One of several irregular, plagioclase-phyric felsic veins in the Lillevik ophiolite fragment (Gratangseidet Igneous Complex) in Narvik, northern Norway, yielded a U-Pb zircon age of 494 ± 5 Ma. The veins cut deformed, compositionally layered, light REE-depleted gabbros that arguably constituted part of the now-dismembered ophiolite stratigraphy. The felsic veins were themselves deformed, probably during Silurian Scandian deformation, but the cross-cutting relationships suggest that they postdate initial deformation of the ophiolitic rocks. The felsic veins are strongly depleted in heavy REE and have moderately juvenile Lu-Hf zircon compositions, with εHf494 between 6.2 and 9.9. By analogy with felsic rocks in other Caledonian ophiolites and supported by cross-cutting relationships, the chemical and isotopic data can be interpreted to reflect formation by partial melting of basaltic rocks in the presence of residual garnet. In this case, the felsic veins probably postdate ophiolite formation and obduction onto a continental margin. We therefore interpret the age of 494 Ma to represent the minimum age of formation of the Lillevik ophiolite fragment. Previously published age, isotopic and chemical data from the region document an at least 20 Myr-long complex magmatic evolution following ophiolite obduction. The new data show that Late Cambrian to Early Ordovician ophiolite fragments extend along most of the length of the Scandinavian Caledonides. The tectonic significance of a previously published age of 474 Ma from the Gratangseidet Igneous Complex and 481 to 469 Ma ages from a tonalite sheet in the nearby Lyngen ophiolite, both interpreted to reflect ophiolite formation, needs to be tested by obtaining whole-rock geochemical data from these units.
Geological Magazine, 1990
Metabasalts 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 ro...
Lithos, 1989
Geochemical variation and different extrusion rates of the Late-Ordovician Solund-Stavl]ord Ophiolite Complex (SSOC) suggest both open and closed magma chambers. Closed chambers existed offaxis 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 detritat ultramafics occurring in an ophiolitic m61ange.
Ophicarbonates of the Feragen Ultramafic Body, central Norway
Norwegian Journal of Geology, 2019
The carbonation of ultramafic rocks is a common alteration process in ophiolites and can occur in various settings. We provide the first detailed description of the carbonated peridotites (ophicarbonates) of the Feragen Ultramafic Body, central Norway, which have unusually variable compositions and microstructures. Lithologies range from pervasively carbonated serpentinites through carbonated serpentinite breccias to carbonated ultramafic conglomerates. Carbonate phases are Ca-carbonate, magnesite and dolomite. Some breccias are also cemented by coarsegrained brucite. This variability records strong variations in fluid chemistry and/or pressure and temperature conditions, both spatially and temporally. By analysing these altered ultramafic rocks using field relationships, optical microscopy, electron microprobe analysis and oxygen and carbon isotope compositions, we elucidate the history of the Feragen Ultramafic Body in more detail and emphasise the importance of deformation for the extent and type of alteration.
The Sunnfjord Melange, evidence of Silurian ophiolite accretion in the West Norwegian Caledonides
Journal of the Geological Society, 1990
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 H0yvik Group and the crystalline basement of the Dalsfjord Suite.
American Journal of Science, 2007
The Ottfjället swarm of Late Neoproterozoic mafic dikes, cutting Neoproterozoic sandstones, became a key, 30 years ago, to the tectonics of the Scandian Caledonides. The sandstones were deposited in basins related to opening of Iapetus, and intruded by dikes in distal parts of the Baltoscandian margin close to the developing spreading axis. The sandstones and underlying basement rocks were transported, from west of the present Norwegian coast, to as far east as western Sweden during the Silurian-Devonian Scandian Orogeny. The sandstones with dikes make up the Särv Nappe, up to 2 km thick in Sweden, and the quartzites and amphibolites of the Saetra and equivalent nappes in Norway. These form the upper part of the Middle Allochthon. The lower part of the Middle Allochthon includes Middle Proterozoic basement gneisses and rapakivi granites, containing mafic rocks in some places. The dike-bearing quartzite is a key unit due to contrast with similar rocks lacking dikes at lower tectonic levels derived from inboard parts of Baltica. The Saetra Nappe and equivalents are well constrained on lithotectonic grounds in Norway at Oppdal, Leksdal, and Orkanger. It was also suspected to occur in deep, narrow synclines in the Western Gneiss Region where interlayered feldspathic quartzite and amphibolite are in correct tectonostratigraphic sequence, locally with a total thickness of only 1 to 3 m, and locally with dikes converted to eclogite. To test correlations, 127 samples of mafic rocks were collected from 14 areas west and southwest of Trondheimsfjord into the Western Gneiss region toward Ålesund. Samples include mafic rocks in quartzites and others, some clearly dikes, from the underlying 1190 Ma rapakivi granite/augen gneiss of the Risberget Nappe and adjacent basement gneisses. Typical Saetra dikes in the Oppdal quarries, and mafic rocks from other quartzites and related rocks, have La n /Sm n ratios of 1.0 to 1.8, and Nb/La ratios of 0.8 to 1.4 (Oppdal group). Most REE patterns are moderately LREE-enriched with no or very small, mostly negative, Eu anomalies. All have similar multi-element patterns typically with small positive P anomalies, negative Zr-Hf anomalies and an absence of Nb-Ta anomalies, showing that the dikes are unrelated to arcs and have no notable continental crust component. A subset of Saetra, Risberget, and basement dikes is distinguished by higher La n /Sm n ratios of 1.8 to 2.5, but otherwise has very similar characteristics (Ystland Group). These data support correlation of the Saetra Nappe quartzite and dikes into highly deformed parts of the Western Gneiss Region and correlation of nearby dike-rich parts of basement gneiss with the Middle Allochthon. One sample in quartzite at Ura, at an unusual tectonostratigraphic position, has La n /Sm n ؍ 0.7, Nb/La ؍ 0.6, and a multi-element pattern different from Saetra dikes, suggesting it is unrelated to the Ottfjället dikes. Non-Saetra-like amphibolites also occur in the Risberget Nappe, and have La n /Sm n ratios of 1.4 to 3, all Nb/La ratios <0.6, and multi-element patterns with sharp negative anomalies for Nb-Ta, P, Zr-Hf and Ti. These are probably Mesoproterozoic magmas and cumulates emplaced into the rapakivi granite protolith of the augen gneiss. Data from this and earlier studies of correlative nappes in adjacent regions can be geographically partitioned into transitional MORB-like compositions with relatively
Geological Society of America Bulletin, 2012
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 seafl oor spreading evolution of a Late Ordovician marginal basin. The fossil oceanic crust of the younger episode contains pillow lavas, massive sheet fl ows, and hyaloclastites, NEtrending sheeted dikes, and high-level isotropic gabbros. The pillow lava versus massive sheet fl ow 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 traceelement patterns indicate a weak subduction infl uence. 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 fl uids . 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.
2005
Introduction During the 1980s and early 1990s, several investigations focused on the complex tectonostratigraphy in the western part of the Helgeland Nappe Complex (HNC). Important discoveries were made, such as the recognition of a number of ophiolite fragments and a primary, angular unconformity between the ophiolites and metasedimentary cover sequences. The ophiolite fragments were correlated with other Late Cambrian-Early Ordovician ophiolites in the Caledonides, including the Leka Ophiolite Complex (Fig. 1). Several of these ophiolites have yielded ages of 500-485 Ma (Dunning & Pedersen 1988). Other metasedimentary rocks in the area include migmatite (mica) gneiss, calc-silicate gneiss and marble interpreted to represent remnants of a Neoproterozoic continental margin sequence. Both sedimentary successions have been affected by pervasive deformation and medium-grade metamorphism, and primary features are, in most areas, totally obliterated. However, on some small islands to the...
In the gneiss area of northern Vestranden , kilometre-scale upright domes and basins refo ld tight and isoclinal folds of banded amphibolite facies orthogne isses and supracrustal rocks. In the Roan area , a major culmination in its core exposes a window , the Roan Igneous Complex (RIC). The RIC is made up of quartz-rnonzonitic to qu artz-rnonzonioritic gneisses, charnock ttlc and basic rocks , which are cross-cut by granit es, and still younger dolerite dykes. Maf ic rocks of Caledonian age form large layered complexes. The RIC is partly remarkably well preserve d from the intense Silurian to Devonian deformation and amphibolitiza tion, and retains both pri mary intrusive relationsh ips and granulite facies paragenese s. The RIC was ove rthrust by the granitic and inter mediate orthogneisses , banded amphibolites and suprac rustal rocks of the Banded Gneiss Complex (BGC), which now occur interleaved and folded above the RIC. The supracr ustal sequences of the BGC are made up of parag neisses, rnatic rocks , marbles and calc-s lucate rocks. The rocks within the RIC and the BGC experienced similar tecto nothermal evolutions. During the Palaeozoic, Scandian, co ntinent-con tinent collision, the rock s were depressed to depths of c. 50 km where they equilibrated at peak temperatur es of BOO-B70°C. Locally preserved high-pressure granulite facies tecton ites indicate that the thrust zone between the RIC and the BGC w as active during this event , in response to telescopi ng and depression of the western edge of Baltica. The initial stage of uplift involved a near-isothermal decompre ssion into medium-pres sure granulite and upper amphibo lite facies cond itions. Subsequent deformation caused exte nsive retro gres sion at amphibolite facies conditions, in part coeval with migmatitization. The rocks then coole d through the andalusite stabi lity field. The preservation of the high-pressure granulite facies assemblages and the symplectite reaction textures. combined with the near-isoth ermal uplift suggest a rapid tectonometamorphic evolution. The thermal evolu tion during uplift, and the intense deformation and thr ust reactivatio n at amphibolite facies conditions support a tecto nic model where the uplift of the WGR is aided by tectonic unroofing .