Anorthosite formation and emplacement coupled with differential tectonic exhumation of ultrahigh-temperature rocks in a Sveconorwegian continental back-arc setting

The tectonic setting and mechanisms and duration of emplacement of Proterozoic massif-type anorthosites and the significance of typically associated ultrahigh-temperature (UHT) host rocks have been debated for decades. This is particularly true of the Rogaland Anorthosite Province (RAP) in the SW Sveconorwegian Orogen. Earlier studies suggest that the RAP was emplaced over 1–3 Myr around 930 Ma towards the end of orogenesis, resulting in an up to 15–20 km-wide contact metamorphic aureole. However, our structural observations show that the RAP is located in the footwall of a 15 km-wide extensional detachment (Rogaland Extensional Detachment, RED), separating the intrusions and their UHT host rocks from weakly metamorphosed rocks in the hanging wall. U–Pb zircon dating of leucosome in extensional pull-aparts associated with the RED yields ages of 950–935 Ma, consistent with Re–Os molybdenite ages from brittle extensional structures in the hanging-wall block that range between 980 and 930 Ma. A metapelite in the immediate vicinity of the RAP yields a 950 Ma U–Pb age of matrix-hosted monazite, and part of the RAP was intruded by the Storgangen norite dike at ca. 950 Ma, providing a minimum age of emplacement. These ages are consistent with Ar–Ar hornblende and biotite ages that show rapid cooling of the footwall before 930 Ma, but slow cooling of the hanging wall. Field and geochronologic data suggest that the RAP formed and was emplaced over a long period of time, up to 100 Myr, with different emplacement mechanisms reflecting an evolving regional stress regime. The distribution of UHT rocks around the RAP reflects differential extensional exhumation between 980 and 930 Ma, not contact metamorphism. The duration and style of orogenic activity and externally (as opposed to gravitationally) driven extension suggest that the RAP formed in a continental back-arc setting

The Early to Late Ordovician rock record of the Oppdal area, Scandinavian Caledonides: Explosive volcanism, element recycling and basin infill during closure of the Iapetus Ocean

This thesis describes the Ordovician rocks of the Oppdal area, central Norwegian Caledonides, which record the evolution of a section across the Laurentian margin. By fieldwork, geochronology and geochemistry a revised geological map has been made, and the processes active during rock formation have been examined. A ca 475-470 Ma succession of sandstone and MORB-like basalt coexist with pyroclastic rocks with a conflicting geochemical signature; extreme enrichment in elements associated with evolved continental crust combined with a high content of elements which point to a mantle source and limited fractionation. We suggest that subducted continental material partially melted and reacted with the mantle to form anomalous mantle domains which sourced the enriched rocks, whereas normal mantle sourced the MORB, all in an extensional setting possibly related to the death of a subduction zone close to Laurentia. These rocks are unconformably overlain by a volcanosedimentary unit formed in a continental arc setting >435 Ma, before the collision between Laurentia and Baltica when units described herein were thrusted onto Baltica along with the rest of the western Trondheim Nappe Complex

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

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 Skuggliberga unit of the Oppdal area, central Scandinavian Caledonides: Calc-alkaline pyroclastic volcanism in a fluvial to shallow-marine basin following a mid-ordovician orogenic event

We present field observations, geochemical data and detrital zircon U–Pb ages from the newly defined Skuggliberga unit of the Oppdal area, central Norwegian Caledonides. The unit occurs in two separate areas, where it unconformably overlies the c. 475–470 Ma Trollhøtta and Skarvatnet units. The Skuggliberga unit consists of a local basal conglomerate and a cross-stratified to massive sandstone, overlain by bedded, fragment-bearing volcanic rocks, probably representing pyroclastic units erupted in a terrestrial or shallow-marine setting. Geochemical data from nine samples of volcanic rocks show calc-alkaline basaltic andesitic and andesitic compositions, with negative Nb–Ta anomalies indicating a continental arc-related signature, which is distinctly different from the underlying Trollhøtta and Kinna volcanic rocks. One sandstone sample from the Skaret succession, below the unconformity, is dominated by Palaeozoic detrital zircons, with a major peak at c. 490 Ma. Two sandstone samples from the Skuggliberga unit contain Archaean, Palaeo- to Neoproterozoic and Palaeozoic detrital zircons. Different methods to calculate the maximum depositional age for these samples vary by up to 50 Myr (from c. 427 to c. 475 Ma), some estimates being younger than the c. 435 Ma plutonic Innset massif that intrudes the Skuggliberga unit, indicating that the youngest detrital zircons must have experienced concealed lead loss. This lead loss could have occurred during regional metamorphism of the unit; an Ar–Ar plateau age of 416 ± 3 Ma was derived for biotite in one Skuggliberga volcanic sample representing cooling after regional greenschist-facies metamorphism. The Skuggliberga unit has no known direct correlative in the Trondheim Nappe Complex but overlaps in age and geochemical characteristics with arc-related plutonic rocks along the central Norwegian coast, indicating that it represents the surface expression of the latter. The orogenic phase predating Skuggliberga deposition is probably related to Taconian accretionary events along the Laurentian margin

The Skuggliberga unit of the Oppdal area, central Scandinavian Caledonides: Calc-alkaline pyroclastic volcanism in a fluvial to shallow-marine basin following a mid-ordovician orogenic event

We present field observations, geochemical data and detrital zircon U–Pb ages from the newly defined Skuggliberga unit of the Oppdal area, central Norwegian Caledonides. The unit occurs in two separate areas, where it unconformably overlies the c. 475–470 Ma Trollhøtta and Skarvatnet units. The Skuggliberga unit consists of a local basal conglomerate and a cross-stratified to massive sandstone, overlain by bedded, fragment-bearing volcanic rocks, probably representing pyroclastic units erupted in a terrestrial or shallow-marine setting. Geochemical data from nine samples of volcanic rocks show calc-alkaline basaltic andesitic and andesitic compositions, with negative Nb–Ta anomalies indicating a continental arc-related signature, which is distinctly different from the underlying Trollhøtta and Kinna volcanic rocks. One sandstone sample from the Skaret succession, below the unconformity, is dominated by Palaeozoic detrital zircons, with a major peak at c. 490 Ma. Two sandstone samples from the Skuggliberga unit contain Archaean, Palaeo- to Neoproterozoic and Palaeozoic detrital zircons. Different methods to calculate the maximum depositional age for these samples vary by up to 50 Myr (from c. 427 to c. 475 Ma), some estimates being younger than the c. 435 Ma plutonic Innset massif that intrudes the Skuggliberga unit, indicating that the youngest detrital zircons must have experienced concealed lead loss. This lead loss could have occurred during regional metamorphism of the unit; an Ar–Ar plateau age of 416 ± 3 Ma was derived for biotite in one Skuggliberga volcanic sample representing cooling after regional greenschist-facies metamorphism. The Skuggliberga unit has no known direct correlative in the Trondheim Nappe Complex but overlaps in age and geochemical characteristics with arc-related plutonic rocks along the central Norwegian coast, indicating that it represents the surface expression of the latter. The orogenic phase predating Skuggliberga deposition is probably related to Taconian accretionary events along the Laurentian margin

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

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

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

Geochronological data from LA–ICP–MS analysis of zircons

Boxplots showing till samples grouped by main lithologies. Fifty-one chemical elements in till from the Oppdal region, Mid-Norway: relation to mineralization, Quaternary and bedrock geology

Geochemical maps for all elements, boxplots showing till samples grouped in tectonic units, boxplots showing till samples grouped by main lithologie