The structural, metamorphic and magmatic evolution of Mesoproterozoic orogens

The Mesoproterozoic (1600–1000 Ma) is an Era of Earth history that has been defined in the literature as being quiescent in terms of both tectonics and the evolution of the biosphere and atmosphere (Holland, 2006, Piper, 2013b and Young, 2013). The ‘boring billion’ is an informal term that is given to a time period overlapping the Mesoproterozoic period, extending from 1.85 to 0.85 Ga (Holland, 2006). Orogenesis was not absent from this period however, with various continents featuring active accretionary orogenesis along their margins for the entire Mesoproterozoic (see Condie, 2013 and Roberts, 2013), and others featuring major continental collisional orogenesis that relates to the formation of the supercontinent Rodinia towards the end of the Mesoproterozoic. Looking at it another way, this period followed the formation of perhaps the first long-lived supercontinent, Columbia (a.k.a. Nuna), and then it prepared the ground for the momentous geological and biological events in the Neoproterozoic that paved the way for the Cambrian explosion of life. As such it is a very important period of Earth history to understand better. Do orogens formed in the Mesoproterozoic differ from those formed in the recent past, or those formed in early Earth history, and if so in what way? Do orogens in the Mesoproterozoic have distinct structural, metamorphic or magmatic characteristics? How are Mesoproterozoic orogens related geodynamically and kinematically? These are overarching questions that this collection of sixteen research papers aims to address. This introduction presents a brief discussion of the contribution of these papers to these questions and topics

Breaking the Grenville–Sveconorwegian link in Rodinia reconstructions

The Grenville, Sveconorwegian, and Sunsas orogens are typically inferred to reflect collision between Laurentia, Baltica, and Amazonia at ca. 1.0 Ga, forming a central portion of the Rodinia supercontinent. This triple‐junction configuration is often nearly identical in otherwise diverse Rodinia reconstructions. However, available geological data suggest that although the Grenville and Sveconorwegian provinces shared a similar tectonic evolution from pre‐1.8 to ca. 1.5 Ga, they record distinctly different tectonic histories leading up to, during, and possibly following Grenville–Sveconorwegian orogenesis. Moreover, palaeomagnetic data suggest the two continents were separated at peak orogenesis, further invalidating any direct correlation. A number of possible interpretations are permissible with available geological and palaeomagnetic data, of which a “classic” triple‐junction configuration appears least likely. In contrast to the commonly inferred intertwined Proterozoic evolution of Baltica and Laurentia, the possibility remains that they were unrelated for a billion years between 1.5 and 0.45 Ga

Linking orogenesis across a supercontinent: the Grenvillian and Sveconorwegian margins on Rodinia

The Sveconorwegian orogeny in SW Baltica comprised a series of geographically and tectonically discrete events between 1140 and 920 Ma. Thrusting and high-grade metamorphism at 1140–1080 Ma in central parts of the orogen were followed by arc magmatism and ultra-high-temperature metamorphism at 1060–920 Ma in the westernmost part of the orogen. In the eastern part of the orogen, crustal thickening and high-pressure metamorphism took place at 1050 in one terrane and at 980 Ma in another. These discrete tectonothermal events are incompatible with an evolution resulting from collision with another major, continental landmass, and better explained as accretion and re-amalgamation of fragmented and attenuated crustal blocks of the SW Baltica margin behind an evolving continental-margin arc. In contrast, the coeval, along-strike Grenvillian orogeny is typically ascribed to long-lived collision with Amazonia. Here we argue that coeval, but tectonically different events in the Sveconorwegian and Grenville orogens may be linked through the behavior of the Amazonia plate. Subduction of Amazonian oceanic crust, and consequent slab pull, beneath the Sveconorwegian may have driven long-lived collision in the Grenville. Conversely, the development of a major orogenic plateau in the Grenville may have slowed convergence, thereby affecting the rate of oceanic subduction and thus orogenic evolution in the Sveconorwegian. Convergence ceased in the Grenville at ca. 980 Ma, in contrast to the Sveconorwegian where convergence continued until ca. 920 Ma, and must have been accommodated elsewhere along the Grenville–Amazonia segment of the margin, for example in the Goiás Magmatic Arc which had been established along the eastern Amazonian margin by 930 Ma. Our model shows how contrasting but coeval orogenic behavior can be linked through geodynamic coupling along and across tectonic plates

Tectonomagmatic evolution of the Sveconorwegian orogen recorded in the chemical and isotopic compositions of 1070–920 Ma granitoids

The Sveconorwegian Province in Southern Norway and Sweden hosts at least four granitoid suites, representing apparently continuous magmatism at the SW margin of the Fennoscandian Shield between 1070 and 920 Ma. This study presents a compilation of published and new zircon LA-ICP-MS U-Pb geochronology, whole-rock and zircon geochemistry and Sm-Nd isotope data for the granitoid suites and demonstrates the granitoids’ ability to record changes in the tectonomagmatic evolution of this orogenic Province. The Sirdal Magmatic Belt (SMB, ca. 1070–1010 Ma) represents the earliest magmatism, west in the Province, followed by two hornblende-biotite granitoid suites (HBG, ca. 1000–920 Ma) and the Flå–Iddefjord–Bohus suite (FIB, ca. 925 Ma), in central and eastern parts of the Province, respectively. The SMB and the HBG bodies located outside of the SMB (referred to as HBGout) are chemically similar, whereas the HBG bodies located in the same region as the SMB (referred to as HBGin) are more ferroan, enriched in incompatible elements and have higher zircon saturation temperatures. Isotopically, the SMB and both HBG suites fall on an evolutionary trend from widespread 1.5 Ga crust in the region, suggesting this was the dominant crustal contribution to magmatism. The FIB suite is more peraluminous, rich in inherited zircon, and has isotopic compositions suggesting a more evolved source than both the HBG suites and the SMB. Trace element modelling shows that the SMB and HBGout suites could have formed by 50% partial melting of 1.5 Ga crust, whereas 5–10% remelting of the dehydrated and depleted SMB residue accounts for the geochemical composition of the HBGin suite. The available data suggest a scenario where the 1.5 Ga lower crust underwent melting due to long-lived mafic underplating giving rise to the SMB suite. After ca. 1000 Ma, regional-scale extension may have led to more widespread mafic underplating causing remelting of the residue following SMB melt extraction, forming the HBGin suite, with lower-crustal melting farther east forming the HBGout suite. Changes in melt composition over this 150 Myr time interval may thus be ascribed to an evolving melt source rather than fundamental changes in tectonic regime. Deep continental subduction at ca. 990 Ma, east in the orogen, provided an isotopically evolved crustal source for the FIB suite. The data underline the difference in tectonic processes across the orogen, with long-lived, high temperatures in the western and central parts and colder, high-pressure events in the eastern parts of the orogen

Evolution of the Gállojávri ultramafic intrusion from U-Pb zircon ages and Rb-Sr, Sm-Nd and Lu-Hf isotope systematics

The Karasjok–Central Lapland Greenstone Belt is one of the largest Palaeoproterozoic greenstone belts in the Fennoscandian Shield and includes multiple (ultra)mafic intrusions, some with notable ore reserves, formed during three episodes at 2.44, 2.22 and 2.05 Ga. This study presents new mineralogical, geochronological and isotopic data for the Gállojávri ultramafic intrusion, in the Karasjok Greenstone Belt, northern Norway. Previous petrogenetic modelling suggests that the intrusion was emplaced as a conduit system open for influx of melt with signs of polybaric fractionation and assimilation. Zircon U-Pb geochronology yields an age of 2051 ± 8 Ma, interpreted to reflect magmatic crystallisation. Large variations in isotopic signature over decimetres to metres indicate incomplete magma mixing. In bulk samples, εNd(t) ranges from −15 to 4. Zircon εHf(t) ranges from −14 to −1. Bulk 87Sr/86Sr(t) shows an apparent range from 0.5041 to 0.7072: the anomalously low values and general alteration indicates that 87Sr/86Sr is non-primary, whereas the less mobile Sm-Nd/ Lu-Hf systems are interpreted to represent primary magmatic signatures. We ascribe the large variations in the Nd and Hf isotopic signatures to local melting or dissolution of xenoliths and influx of variably contaminated melt into the semi-consolidated Gállojávri magma chamber, consistent with a conduit model involving variable replenishment and crustal interaction. The most evolved isotopic signatures cannot be accounted for by interaction with the local Archaean basement, indicating the presence of unidentified crustal components at depth. The Gállojávri intrusion shows many petrogenetic similarities to other c. 2.05 Ga (ultra)mafic intrusions in the Central Lapland Greenstone Belt

The Sveconorwegian orogeny: reamalgamation of the fragmented southwestern margin of Fennoscandia

The Sveconorwegian orogeny encompasses magmatic, metamorphic and deformational events between ca. 1140 and 920 Ma at the southwestern margin of Fennoscandia. In recent years, the tectonic setting of this nearly 200 Myr-long evolution has been debated, with some workers arguing for collision with an unknown continent off the present-day southwest coast of Norway, and others advocating accretionary processes inboard of an active margin. Recently, it has been suggested that orogeny may have been gravity-driven by delamination and foundering of heavy subcontinental lithospheric mantle in an intraplate setting, in some ways similar to proposed sagduction processes in the Archaean. Resolving the tectonic setting of the Sveconorwegian orogen has implications for correlation with other orogens and Rodinia supercontinent reconstructions and for assessments of the evolution of plate tectonics on Earth, from the Archaean to the present. Here, we present new mapping and geochronological data from the Bamble and Telemark lithotectonic units in the central and western Sveconorwegian orogen – the former representing a critical region separating western parts of the orogen that underwent long-lived high- to ultrahigh-temperature metamorphism and magmatism from parts closer to the orogenic foreland that underwent episodic high-pressure events. The data show that the units constituting the Sveconorwegian orogen most likely formed at the southwestern margin of Fennoscandia between ca. 1800 and 1480 Ma, followed by fragmentation during widespread extension between ca. 1340 and 1100 Ma marked by bimodal magmatism and sedimentation. A summary of Sveconorwegian magmatic, metamorphic and depositional events in the different units shows disparate histories prior to their assembly with adjacent units. The most likely interpretation of this record seems to be that episodic, Sveconorwegian metamorphic and deformational events in the central and eastern parts of the orogen represent accretion and assembly of these units. This process most likely took place behind an active margin to the southwest that sustained mafic underplating in the proximal back-arc, resulting in high- to ultrahigh-temperature metamorphism in the western parts. In this interpretation, all features of the Sveconorwegian orogen are readily explained by modern-style plate tectonic processes and hypotheses involving some form of vertical, intraplate tectonics are not supported

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

Subduction and loss of continental crust during the Mesoproterozoic Sveconorwegian Orogeny

The late Mesoproterozoic Sveconorwegian Orogeny in SW Fennoscandia is characterized by tectonically bound units that record different metamorphic, magmatic, and deformation histories, interpreted to indicate separation by some unknown distance prior to orogeny. New zircon U–Pb and Lu–Hf isotope data from a 1200 km-long NE–SW transect including Archean to 1450 Ma rocks constrain the likely age and isotopic architecture of western Fennoscandia prior to the late Mesoproterozoic Sveconorwegian Orogeny. Zircon age and Hf-isotope patterns indicate that the units comprising the Sveconorwegian Province are both younger and isotopically more juvenile than the surrounding autochthonous Fennoscandian crust, and thus most likely derived from west of the present-day Norwegian coastline. The Mylonite Zone defines a major tectonic structure separating allochthonous Sveconorwegian units in its hanging wall from autochthonous Fennoscandian crust in its footwall. New and compiled metamorphic age data demonstrate that the Mylonite Zone can be traced westward through the Western Gneiss Region, aligning with Nordfjord in western Norway, where it was reused during Caledonian deformation. The proposed westward continuation of the Mylonite Zone accommodated several hundred kilometers of sinistral strike-slip movement. Eastward translation of crust probably took place sometime between 1020 and 990 Ma, coinciding with a magmatic lull, followed by a shift to more evolved isotopic compositions in the hanging wall (Telemark) and high-pressure eclogite-facies metamorphism in the footwall (Eastern Segment) to the Mylonite Zone. Following this relatively short period of compression, the entire orogen and its foreland underwent extension lasting until at least 930 Ma. The nature and fate of the ca. 500 km of crust originally separating the autochthonous and allochthonous units remain elusive. There is no evidence of arc magmatism related to Benioff-style subduction of oceanic crust, and thus we propose an amagmatic Ampferer-style subduction comprising spontaneous subduction of thinned continental crust, as proposed for the Western Alps. Subduction of continental crust and associated radioactive heat-producing elements could also account for the anomalously high temperatures in the lithospheric mantle under the Sveconorwegian Province, which cannot easily be accounted for by other mechanisms. The Sveconorwegian Province may be an anomalous feature in an otherwise larger-scale orogen, the nature of which remains obscure

Continental growth and reworking on the edge of the Columbia and Rodinia supercontinents; 1.86–0.9 Ga accretionary orogeny in southwest Fennoscandia

Geological history from the late Palaeoproterozoic to early Neoproterozoic is dominated by the formation of the supercontinent Columbia, and its break-up and re-amalgamation into the next supercontinent, Rodinia. On a global scale, major orogenic events have been tied to the formation of either of these supercontinents, and records of extension are commonly linked to break-up events. Presented here is a synopsis of the geological evolution of southwest Fennoscandia during the ca. 1.9–0.9 Ga period. This region records a protracted history of continental growth and reworking in a long-lived accretionary orogen. Three major periods of continental growth are defined by the Transscandinavian Igneous Belt (1.86–1.66 Ga), Gothian (1.66–1.52 Ga), and Telemarkian (1.52–1.48 Ga) domains. The 1.47–1.38 Ga Hallandian–Danopolonian period featured reorganization of the subduction zone and over-riding plates, with limited evidence for continental collision. During the subsequent 1.38–1.15 Ga interval, the region is interpreted as being located inboard of a convergent margin that is not preserved today and hosted magmatism and sedimentation related to inboard extensional events. The 1.15–0.9 Ga period is host to Sveconorwegian orogenesis that marks the end of this long-lived accretionary orogen and features significant crustal deformation, metamorphism, and magmatism. Collision of an indenter, typically Amazonia, is commonly inferred for the cause of widespread Sveconorwegian orogenesis, but this remains inconclusive. An alternative is that orogenesis merely represents subduction, terrane accretion, crustal thickening, and burial and exhumation of continental crust, along an accretionary margin. During the Mesoproterozoic, southwest Fennoscandia was part of a much larger accretionary orogen that grew on the edge of the Columbia supercontinent and included Laurentia and Amazonia amongst other cratons. The chain of convergent margins along the western Pacific is the best analogue for this setting of Proterozoic crustal growth and tectonism

The importance of trace element analyses in detrital Cr-spinel provenance studies: An example from the Upper Triassic of the Barents Shelf

Investigations of sandstone provenance often involve U–Pb dating and chemical/mineralogical investigations of detrital minerals that are stable in sediments. As most stable detrital minerals are from felsic–intermediate rocks, investigations of the only mafic–ultramafic mineral considered stable in sediments, chromian spinel (Cr-spinel), can reveal contributions from mafic–ultramafic sources. Cr-spinel chemical compositions are tied to petrogenesis, making it possible to identify the nature of, and differentiate between, potential sources. Earlier detrital Cr-spinel studies have focused on major and minor element compositions, however, the advent of laser-ablation analytical techniques now allow routine mineral trace element analyses. Here, we integrate major, minor and trace element compositions of detrital Cr-spinel from sandstones with a well-characterised provenance from the Triassic (Anisian to Early Norian) Snadd and De Geerdalen formations of the Barents Shelf. The analysed Cr-spinel compositions are depleted in the major element cations Fe3+, Al and Mg and enriched in Cr and Fe2+. Relative to MORB chromite, the minor and trace element data show high concentrations of Zn, Co and Mn, low concentrations of Ni and Ga and variable concentrations of Ti, V and Sc. The major element compositions of the detrital Cr-spinel are similar to ophiolite-associated Cr-spinel, while the trace element compositions indicate a more complex petrogenesis influenced by metamorphic alteration. The compositional variations between sample locations are small, suggesting similar source rocks for the detrital Cr-spinel throughout the study area. The most likely sources of the Cr-spinel grains are metamorphosed ophiolite complexes in the Uralian Orogen, in accordance with earlier provenance studies. The novel addition of trace element compositions to detrital Cr-spinel studies adds significant source-sensitive information