Central Norwegian Caledonides Research Articles

Deep-water trace fossils in the Ilfjellet rift basin (Middle Ordovician), central Norwegian Caledonides

The late Early to early Middle Ordovician Fjellvollen Formation, Central Norwegian Caledonides, is a part of the Ilfjellet Group volcano-sedimentary succession formed in a rift basin that opened along the Laurentian margin. Depositional facies and trace fossils indicate the Nereites ichnofacies, typically found in distal turbidites and deep marine deposits. The recovered trace fossils include ?Alcyonidiopsis, Chondrites, Dictyodora, Gordia, Helminthoidichnites, Macaronichnus, Monomorphichnus, Nereites, Oikobesalon, Palaeophycus, Protovirgularia and Treptichnus. No body-fossils have been reported from the Fjellvollen Formation, but the trace fossils indicate the presence of varied epi- and infaunas of arthropods, bivalves, gastropods and polychaets. The abundance of meandering and looping trace fossils is comparable to what is found in the inferrably time-equivalent Vuddudalen Group farther north, although lack of graphoglyptids like Megagrapton in the Fjellvollen Formation may indicate more proximal, unstable depositional conditions in fan-fringe and adjacent basin-plain settings.

Revised tectonostratigraphy and structural evolution of the Köli Nappe Complex, Central Caledonides in Nordland, Norway

The nappe stack in the Røssvatnet–Hattfjelldal region in the Central Norwegian Caledonides consists of seven nappes formed at the boundary between tectonostratigraphically upper and uppermost Caledonian levels. The rocks of all nappes share a polyphase tectonometamorphic evolution that is younger than the 491 ± 10 Ma depositional (volcanic) age of parts of the succession. Early stages of deformation characterized by centimetre- to kilometre-scale folding and intense shearing accompanied by greenschist to amphibolite facies peak metamorphism are correlated with the Early Ordovician Taconian accretionary orogeny along the Laurentian margin. The Taconian structures are cut by the Krutfjellet gabbro and diorite, which yield U–Pb zircon ages of 446 ± 5 and 444 ± 4 Ma, respectively. Large-scale nappe stacking and folding post-dating the emplacement of the gabbro is related to the collision of Laurentia with Baltica (Scandian orogeny) and was followed by late- to post-orogenic extension. The revised tectonostratigraphy assigns the structurally higher nappes to the Uppermost Allochthon, whereas the lower nappes are correlated with the Middle Köli Nappe Complex (Upper Allochthon). The boundary between these nappes is marked by an imbricate zone. Taconian deformation was probably much more penetrative and widespread than hitherto thought and therefore parts of the nappe stack were probably assembled before Scandian collision.Supplementary material: Electronic Supplement 1: U–Pb zircon data and Electronic Supplement 2: Methods (laser ablation inductively coupled plasma mass spectrometry U–Pb zircon geochronology) are available at https://doi.org/10.6084/m9.figshare.c.5357255

Early­­­­–Middle Ordovician sedimentation and bimodal volcanism at the margin of Iapetus: the Trollhøtta–Kinna Basin of the central Norwegian Caledonides

Abstract The late Neoproterozoic–Paleozoic Iapetus Ocean developed between Laurentia, Baltica, Siberia and Gondwana. Its Paleozoic closure history is recorded by volcano-sedimentary successions within the Caledonian orogen of Scandinavia, the British Isles and Newfoundland. We present new lithological, geochemical and geochronological data relevant for the Iapetan closure history from the hitherto poorly known Trollhøtta–Kinna Basin (central Norwegian Caledonides). This basin consists of alternating siliciclastic rocks, mid-ocean ridge basalts (MORBs), and felsic volcanic rocks highly enriched in, for example, Th, U and light REEs. Rhyolites from the stratigraphically upper part are dated by zircon U–Pb thermal ionization mass spectrometry to 473.3 ± 1.0 and 472.4 ± 0.7 Ma. Detrital zircon spectra indicate deposition after c. 480 Ma, with sediments derived from composite Cambro-Ordovician and Archean–Neoproterozoic landmass(es), possibly the Laurentian margin or a related microcontinent. The peculiar bimodal volcanic association is interpreted as an intermittent phase of marginal basin rifting, derived from a heterogeneous mantle source previously metasomatized by continental material. The tectonic mechanisms behind rifting could be slab retreat and/or break-off, or far-field tectonic forces within the Iapetan realm. Comparison of this basin with other Iapetus-related, similarly-aged volcano-sedimentary successions along the Caledonian–Appalachian orogen indicate that the bimodal MORBs and highly enriched rocks reflect a palaeotectonic setting hitherto unknown in the orogen.

Notes on Ordovician graptolites, nautiloids and trace fossils from Lånke, Central Norwegian Caledonides

Notes on Ordovician graptolites, nautiloids and trace fossils from Lånke, Central Norwegian Caledonides

Ordovician shoshonitic to ultrapotassic volcanism in the central Norwegian Caledonides: The result of sediment subduction, mantle metasomatism and mantle partial melting

Shoshonitic to ultrapotassic, mantle-derived volcanic rocks found within certain accretionary and collisional settings have trace element patterns comparable to those of common arc-related rocks, but with extreme enrichments in highly incompatible elements. Such rocks, previously unknown in the Scandinavian Caledonides, have been discovered in the Oppdal area in the Trondheim Nappe Complex, central Norway. The volcanic rocks are part of the Skarvatnet unit, which consists of (1) the Kinna volcanic succession, (2) the Storgruvpiken rhyolite, and (3) the Skaret conglomeratic succession. The Kinna volcanic succession is interpreted as consisting mainly of submarine pyroclastic flows. A trachytic bed from within this succession is dated to 474 ± 1 Ma. The Storgruvpiken rhyolite is interpreted as a shallow, subvolcanic intrusion or volcanic dome, and is dated to 470 ± 1 Ma. The Skaret conglomerates were deposited in a shallow-marine, tectonically active setting post-dating the Storgruvpiken rhyolite. The Kinna volcanic succession is highly enriched in Th, U, Pb and LREE, with trace element signatures remarkably similar to high-K to ultrapotassic rocks of the Alpine-Himalayan and other orogenic belts. By analogy with these recent examples, the Kinna volcanic succession is interpreted as the result of a two-stage process: (1) subduction and partial melting of continent-derived material caused extensive metasomatism of the overlying mantle wedge, (2) partial melting of this heterogeneous mantle source produced the highly enriched mantle melts. The Storgruvpiken rhyolite, particularly enriched in beryllium, is interpreted as the result of partial melting of Kinna-type rocks at depth. The Skarvatnet unit is interpreted as the result of a complex tectonic evolution at the margin of the Iapetus Ocean, involving the following stages: (1) intra-oceanic subduction, producing the Løkken-Vassfjellet-Bymarka ophiolite at 487–480 Ma, followed by (2) the arrival of the Laurentian margin or an associated micro-continent at ca. 480 Ma, leading to the subduction of large amounts of continent-derived material and subsequent metasomatism of the overlying mantle wedge, (3) arc-continent-collision and slab retreat/slab break-off, causing an intermittent phase of mantle-derived, highly enriched magmatism preserved as the Kinna volcanic succession and the Storgruvpiken rhyolite at 475–470 Ma.

The Sognefjell volcanic-subvolcanic complex – A late Sveconorwegian arc imbricated in the central Norwegian Caledonides

The Sognefjell volcanic-subvolcanic complex is a composite unit underlying the Jotun Nappe Complex in the southern Norwegian Caledonides. The identity and origin of the complex have been highly debated in past decades, one interpretation considering the rocks to be highly deformed orthogneisses of the Jotun Nappe Complex and the other extreme considering them to be partly of volcanic origin and defining a Caledonian suture. In this study some critical components of the complex have been dated by U-Pb yielding ages of 961 ± 11 Ma for gabbro and 949 ± 4 Ma for a tonalite. A tuffite includes euhedral zircon crystals with ages of 985 to 955 Ma, suggesting derivation from the same magmatic complex. Zircon in an orthogneiss points instead to an older age close to 1600 Ma suggesting that this is a sliver of the Jotun Nappe Complex. A thin straight pegmatite cutting mafic gneiss is 949 ± 3 Ma. Titanite grains in the various rocks range in age from about 970 to 890 Ma, demonstrating that the dominant metamorphism was Sveconorwegian with minimal resetting in spite of the likely Caledonian deformation. The new observations, combined with published data, support the notion that these rocks are of volcanic-subvolcanic origin, but show that they are late Sveconorwegian, not Caledonian as originally envisaged. They likely developed in an arc complex at the margin of Baltica and possibly within the realm of the Asgard Sea. Arc magmatism was coeval with lower crustal processes in parts of the Baltic craton that would eventually become Caledonian nappes.

Tectonic evolution of syn- to late-orogenic sedimentary–volcanic basins in the central Norwegian Caledonides

We present new structural, geochemical and U–Pb zircon data from syn- to late-orogenic sedimentary–volcanic basins in the southwestern part of the Trondheim Nappe Complex, central Norwegian Caledonides. In this area, a succession of enriched mid-ocean ridge basalt type metabasalt, jasper, ribbon chert with associated sandstone and conglomerate, and green siltstone is interpreted to represent volcanism and sedimentation in a hitherto little-known spreading-dominated tectonic environment. This environment is different from the suprasubduction-zone ophiolite setting dominating the Iapetus rock record elsewhere in the Scandinavian Caledonides. This volcanic and sedimentary succession was overturned and isoclinally folded in a pre-427 Ma orogenic phase. Post-427 Ma cross-bedded sandstones were deposited on the eroded surface of the previously deformed rocks, representing a rare example of a late Silurian or younger sedimentary basin within the Scandinavian Caledonides. The cross-bedded sandstones are intercalated with and/or overlain by post-427 Ma intermediate volcanic or subvolcanic rocks of calc-alkaline composition, representing a hitherto unknown volcanic phase within the Trondheim Nappe Complex and elsewhere within the Scandinavian Caledonides. Their particular geochemical signature could be the result of late-stage subduction-zone volcanism just prior to the onset of continent–continent collision between Baltica and Laurentia, or much younger post-collisional extensional melting with inherited subduction signatures. Supplementary material: A description of analytical methods and a table of geochronological data from LA-ICP-MS analysis of zircons are available at https://doi.org/10.6084/m9.figshare.c.3994605.

Transfer of olivine crystallographic orientation through a cycle of serpentinisation and dehydration

Our ability to decipher the mechanisms behind metamorphic transformation processes depends in a major way on the extent to which crystallographic and microstructural information is transferred from one stage to another. Within the Leka Ophiolite Complex in the Central Norwegian Caledonides, prograde olivine veins that formed by dehydration of serpentinite veins in dunites exhibit a characteristic distribution of microstructures: The outer part of the veins comprises coarse-grained olivine that forms an unusual, brick-like microstructure. The inner part of the veins, surrounding a central fault, is composed of fine-grained olivine. Where the fault movement included a dilational component, optically clear, equant olivine occurs in the centre. Electron backscatter diffraction mapping reveals that the vein olivine has inherited its crystallographic preferred orientation (CPO) from the olivine in the porphyroclastic host rock; however, misorientation is weaker and associated to different rotation axes. We propose that prograde olivine grew epitaxially on relics of mantle olivine and thereby acquired its CPO. Growth towards pre-existing microfractures along which serpentinisation had occurred led to straight grain boundaries and a brick-like microstructure in the veins. When dehydration embrittlement induced slip, a strong strain localisation on discrete fault planes prevented distortion of the CPO due to cataclastic deformation; grain size reduction did not significantly modify the olivine CPO. This illustrates how a CPO can be preserved though an entire metamorphic cycle, including hydration, dehydration, and deformation processes, and that the CPO and the microstructures (e.g. grain shape) of one phase do not necessarily record the same event.

Localized slip controlled by dehydration embrittlement of partly serpentinized dunites, Leka Ophiolite Complex, Norway

Dehydration of partly or completely serpentinized ultramafic rocks can increase the pore fluid pressure and induce brittle failure, a process referred to as dehydration embrittlement. However the extents of strain localization and unstable frictional sliding during deserpentinization are still under debate. In the layered ultramafic sections of the Leka Ophiolite Complex in the Central Norwegian Caledonides, prograde metamorphism of serpentinite veins led to local fluid production and to the growth of Mg-rich and coarse-grained olivine with abundant magnetite inclusions and δ18O values 1.0–1.5‰ below the host rock. Embrittlement associated with the dehydration caused faulting along highly localized (<10 μm-wide) slip planes near the centers of the original serpentinite veins and pulverization of wall rock olivine. These features along with an earthquake-like size distribution of fault offsets suggest unstable frictional sliding rather than slower creep. Structural heterogeneities in the form of serpentinite veins clearly have first-order controls on strain localization and frictional sliding during dehydration. As most of the oceanic lithosphere is incompletely serpentinized, heterogeneities represented by a non-uniform distribution of serpentinite are common and may increase the likelihood that dehydration embrittlement triggers earthquakes.

Textural variation in the pyrite-rich ore deposits of the Røros district, Trondheim Region, Norway: implications for pyrite deformation mechanisms

The Roros district is a pyrite-rich polymetallic sulfide orefield in the southeastern part of the Trondheim region, Central Norwegian Caledonides. All of the ore deposits at Roros are hosted within a Cambrian to Silurian succession that was deformed and metamorphosed at lower greenschist to lower amphibolite facies conditions during the Caledonian orogeny. Samples from five individual deposits across the orefield have been analyzed using a combination of reflected light petrographic observation, orientation contrast imaging, and electron backscatter diffraction. Results indicate that, whereas samples from each ore deposit have a variety of different textures, all of them preserve plastic deformation in pyrite grains that occurred at peak metamorphic conditions characterized by the development of internal lattice misorientation within pyrite grains and low-angle (∼2°) dislocation walls. These observations indicate that the principal deformation mechanisms at peak metamorphic conditions were dislocation glide and creep. The preservation of brittle fracturing represents later overprinting events.

40Ar/39Ar Thermochronology of Late‐ and Postorogenic Extension in the Caledonides of North‐Central Norway

We report 40Ar/39Ar mineral cooling data that elucidate the timing and style of Devonian late‐orogenic extensional collapse and Permian‐Triassic postorogenic continental rifting in the north‐central Norwegian Caledonides. 40Ar/39Ar mineral cooling ages from ductile extensional shear zones that have either excised or reactivated Caledonian thrust faults along the western flanks of two major basement domes indicate that late‐orogenic extensional deformation in this region occurred during the Early (ca. 409–388 Ma) and Late Devonian (ca. 371–355 Ma). These extensional pulses are consistent with events recently identified by workers investigating areas to the south of our study area. The timing and style of the Early Devonian event supports the northern continuation of the Devonian orogen‐parallel, scoop‐shaped extensional deformation documented in the central Norwegian Caledonides. Late Devonian extension, on the other hand, was orogen‐orthogonal. This Late Devonian event is similar in timing and kinematics to the extension documented in the Lofoten Archipelago due west of our study area. 40Ar/39Ar data on K‐feldspar samples from brittle extensional faults of the Vestfjord‐Vanna fault complex, which opened the major NNE‐SSW‐trending Vestfjord basin, document two pulses of E‐W‐directed postorogenic continental rifting. The first pulse of extension occurred during the Early Permian, at ca. 272 Ma, and the other during the Middle Triassic, at ca. 236 Ma. The Early Permian faults are upper‐crustal features found in the hanging wall of the Lofoten metamorphic core complex, the result of rifting the Lofoten block from the Norwegian mainland during the initial opening of the Vestfjord basin. Middle Triassic faulting occurred because of continued movement along the Vestfjord‐Vanna fault complex during further development of the Vestfjord basin. The timing and kinematics of development of the Vestfjord basin have similarities with those of major basins submerged along and exposed on the continental margins of Norway and Greenland, respectively. Permian‐Triassic extension, long recognized throughout eastern Greenland, is evidently more pronounced along the Norwegian continental margin than was previously thought. We interpret this event as marking the transition from Devonian‐Carboniferous extensional collapse to continental rifting sensu stricto, preceding the Eocene opening of the nascent Norwegian‐Greenlandian Sea.

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.

A relict blueschist in meta-ophiolite from the central Norwegian Caledonides—discovery and consequences

Blueschists in ophiolite sequences are meaningful indicators of the kinematics and thermal evolution of ancient subduction zones. Throughout the Caledonian–Appalachian orogen, blueschist occurrences in ophiolites of Early–Middle Ordovician age have been used to decipher the development and destruction of arc-subduction settings around the active Paleozoic Iapetus Ocean margin. We describe herein the first occurrence of glaucophane relics within an Early Ordovician meta-ophiolite sequence from the mainland Scandinavian Caledonides within the Trondheim Nappe Complex. The rock contains a relict blueschist assemblage of glaucophane, epidote, and rutile with pervasive retrograde overprint, first under amphibolite-, and subsequently, greenschist-facies conditions. The existence of blueschist in meta-ophiolite from mainland Scandinavia is consistent with the Ordovician intra-oceanic subduction and arc-continent interactions proposed for the Iapetus oceanic realm, generally, and outboard Baltica, in particular.

Devonian, orogen-parallel opposed extension in the Central Norwegian Caledonides: Comment and Reply

[Braathen et al. (2000)][1] described Early to Middle Devonian extensional faults around the central Norway basement window, one of many tectonic windows exposing basement in the Scandinavian Caledonides (here these basement rocks are called tectonic window basement). Extension was orogen parallel,

Devonian, orogen-parallel, opposed extension in the Central Norwegian Caledonides

Devonian, orogen-parallel, opposed extension in the Central Norwegian Caledonides

Devonian, orogen-parallel, opposed extension in the Central Norwegian Caledonides

Late orogenic, Early to Middle Devonian extension in the Scandinavian Caledonides was unidirectional in western Norway. New data from two detachment zones (Hoybakken and Kollstraumen) north of the More-Trondelag Fault Complex show that bidirectional, opposed, orogen-parallel extension dominated in this region. At this time, the fault complex acted as a transfer zone for the Hoybakken detachment. Extension and uplift in central Norway triggered significant magmatic activity, in contrast to the lack of granite intrusions during exhumation of western Norway.

U-Pb zircon age and geochemistry of the Bl fjellhatten granite, Grong-Olden Culmination, Central Norway

U-Pb zircon dating of the Blåfjellhatten granite in the Olden Nappe, Grong-Olden Culmination, Central Norwegian Caledonides, has yielded a discordia upper intercept of 1633.2 - 2.9 Ma, interpreted as the crystallisation age of the pluton. This precise age is comparable to an approximate age of emplacement, 1650 Ma, reported for the neighbouring Olden granite but contrasts with an earlier, Rb-Sr whole-rock, isochron age of 1356 - 29 Ma for the Blåfjellhatten body. Geochemical data indicate that the Blåfjellhatten granite was derived from a fairly evolved crustal source in a continental, extensional, and probably anorogenic setting. Both the Blåfjellhatten and the Olden granite are representative of I-type granitoids (though with some A-type features) of the youngest magmatic episode of the Palaeoproterozoic, Transscandinavian Igneous Belt (TIB). The Blåfjellhatten granite thus appears to be one of the youngest plutons in the TIB domain of the Fennoscandian Shield, extending the terminal stages of TIB magmatism by some 15-20 million years.

High-strain zones from mesoto macro-scale at different structural levels, Central Norwegian Caledonides

High-strain shear zones of diverse character, metamorphic grade and scale occur in different tectonic settings in the Caledonides of Central Norway. Three specific examples are described. Strain products range from high-PT granulite to amphibolite-facies, thrust-related, Scandian mylonites at deep structural levels, to ultracataclasites and mylonites generated in the ductile to ductile-brittle transition regimes along the multiply reactivated, orogen-oblique Møre-Trøndelag Fault Complex. In another example, geochemical data from a transformation of granite to mylonite at an intermediate structural level indicate that the high-strain change-over probably occurred as an open, volume-gain system involving silicification. In the case of the Møre-Trøndelag Fault Complex, a sinistral strike-slip regime during Devonian time was coeval with regionally extensive top-to-the-SW shear in the country rocks outside the fault zone. Later components of movement along the multiphase fault zone, from latest Palaeozoic to Cenozoic time, were mainly of brittle, dip-slip to dextral strike- or oblique-slip character.

Contact relationships in the southeastern Trondheim Nappe complex, central-southern Norway: Implications for early Paleozoic tectonism in the Scandinavian Caledonides

The Upper Allochthon, a composite thrust complex containing elements of an accreted terrane, has long been a controversial feature in the Scandinavian Caledonides. Although final emplacement of the Upper Allochthon occurred during the Siluro-Devonian Scandian orogeny, the complex experienced an intricate and still debatable early Paleozoic history. In the central Norwegian Caledonides, the Upper Allochthon comprises the Trondheim Nappe Complex, which contains an enigmatic axial unit of metasedimentary and meta-igneous rocks flanked by oceanic crustal sequences to the east and west. Determination of the nature of internal contacts and timing of assembly of the Trondheim Nappe Complex has been hampered by wide variations in lithology, strain and metamorphic grade, and scanty age constraints in some rock units. In the Lake Savalen area, southeastern Trondheim Nappe Complex, a polydeformed continental margin quartzite/schist sequence occurs in the core of the Einunnfjellet dome. These rocks, which can be linked to the Baltoscandian miogeocline, were overthrust by oceanic crust and pelagic sediment, then eroded and unconformably succeeded by polymict conglomerates and fine-grained clastic sediments. Early thrust and unconformable contacts were later overprinted by Scandian deformation and metamorphism. The garnet isograd cuts across lithologie contacts, and does not appear to be folded or interrupted by faulting within the 250-km 2 map area. Regional correlations indicate the widespread occurrence of tectonostratigraphic relationships similar to the Einunnfjellet-Savalen area, and imply that prior to the Scandian orogeny, portions of the Upper Allochthon were assembled into a composite mass consisting of: 1. (1) a lower Baltoscandian miogeoclinal sequence 2. (2) a thrust complex of fragmented oceanic terranes 3. (3) an unconformable succession that overstepped both (1) and (2). This mass was subsequently thrust eastward, imbricated, and metamorphosed during Scandian orogenesis. As a result of the pervasive Scandian overprint, pre-Scandian faults and unconformable contacts in the Upper Allochthon are commonly cryptic.

Post-recrystallisation mobilisation phenomena in metamorphosed stratabound sulphide ores

AbstractMetamorphosed stratabound iron- and base-metal sulphide deposits often exhibit microtextures in which fractures in cataclastically-deformed pyrite porphyroblasts are filled with matrix sulphides; chalcopyrite, sphalerite, pyrrhotite or galena. Discussions of such textures have mostly centred on whether solid-phase or fluid-phase mechanisms were responsible for the movement of the matrix sulphides.The small Zn-Cu sulphide body at Gressli, in the central Norwegian Caledonides, shows these textural features to an extreme degree. Both chalcopyrite and sphalerite show heavy replacive relations to the cataclastically deformed metablastic pyrite, along fracture walls and grain boundaries. They also occur injected along the opened-up triple junctions of foam-textured pyrite. In addition, parts of the ore show patchy quartz with clear replacive relationship to all three sulphides, a feature not often reported from such ores. Such textures can be interpreted to support a mobilisation sequence chalcopyrite-sphalerite-quartz within the Gressli ore. Their extent and degree of development indicate that fluid-phase mobilisation of the three minerals must have played a dominant role. Chalcopyrite and sphalerite are most likely derived from within the ore-mass itself; an external source for the SiO2 seems most probable, in the form of metahydrothermal solutions moving along retrograde shear zones at or near ore-walls.