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

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

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

Deformation, Phyllonitization and Associated Element Mobilization of Granitoid Rocks: - A geochemical study of the Fagervika granitoid, Norway.

The Fagervika granitoid is a felsic, peraluminous intrusion that was emplaced in oceanic crust at 481 Ma, now represented as the Bymarka ophiolite in Trondheim, Norway. It belongs to the Støren Group in the Upper Allochthon within the Scandinavian Caledonides. The emplacement onto the Baltican margin during the Caledonian orogeny occurred around 420 Ma, with following collapse in 400 - 380 Ma. Regionally, the rocks have been subjected to greenschist to lower amphibolite facies metamorphic conditions. Detailed mapping at centimetre to decimetre scale of the Gråkallen area reveals the complex distribution of phyllonitic shear zones hosted by a granodiorite, with transport direction interpreted as a top-to-the-west movement. Field observations and petrographic studies were performed for mineral identification and textural descriptions. Comparison of the modal mineralogy with whole-rock data including trace elements is used to show chemical variation between the different lithologies. Mineral chemistry was examined with element mapping by Scanning Electron Microscopy (SEM), Electron Microprobe Analysis (EMPA) and Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS). These methods report the major and trace element distribution along with the Rare Earth Elements (REE) distribution in each mineral phase. Isocon analysis of paired granodiorite-phyllonite samples enabled for quantification of the gains and losses in the system. The rocks at Gråkallen are granodioritic, with only minor variations in modal mineralogy across all samples. The granodiorite is hypidiomorphic, show no preferred mineral orientation with early growth of mica and dynamic recrystallization of quartz. Phyllonitic shear zones have developed locally in the granodiorite and signs of widespread fluid interaction and metasomatism is evident throughout the rocks. The granodioritic assemblage is represented by Qtz + Pl + Kfs + Ms ± Py, with secondary accessories Ep + Aln + Ttn, which has been identified as the main REE-carriers. Allanite is enriched in the LREE, while epidote contains LREE to a lesser degree and titanite holds the HREEs, reflecting each minerals preferential affinity for the REEs. The feldspars are altered, with alkali feldspar neocrystallized to albite, and plagioclase is seritized and saussuritized, with widespread muscovite and epidote. Major elements Na and K have been mobilized and Ca to a lesser degree, while the REEs has only been mobilized at micro scale in the granodiorite. iii The phyllonitic shear zones trend approximately N-S and consist of elongated quartz grains in a muscovite matrix, defining the assemblage of Qtz + Ms ± Py ± feldspar porphyroclasts. The formation of the muscovite indicates the presence of water-rich fluid. The Ca-bearing accessories Ep + Aln + Ttn has been destabilized, along with most of the feldspar due to deformation and high fluid/rock ratio, though some titanite is remnant. In addition, the fluid composition enabled for mobilization of trace elements and REEs. This resulted in depletion of the REE, with a greater loss of LREE compared to HREE. An epidote-rich shear zone shows a high modality of epidote, along with Ms + Py + Qtz + Ttn, with subsequent REE enrichment in comparison to the granodiorite. The REE pattern of the epidote-rich shear zone appeared as an enriched version of the granodiorite REE pattern, suggesting that stabilization of epidote and titanite prevented loss of the REEs. This study shows that metasomatism triggered mineralogical changes and grain size reduction, especially for the feldspars in the undeformed granodiorite, enabling increased fluid flow. The heightened fluid/rock ratio allows muscovite to form, and thus promoting strain weakening and further strain partitioning. Deformation and fluid-rock interaction completely changed the mineralogy by destabilizing feldspar and the REE-bearing phases and by forming muscovite, which promoted mobilization of K, Na, Ca and the REEs during phyllonitization of the granodiorite

Deformation, Phyllonitization and Associated Element Mobilization of Granitoid Rocks: - A geochemical study of the Fagervika granitoid, Norway.

The Fagervika granitoid is a felsic, peraluminous intrusion that was emplaced in oceanic crust at 481 Ma, now represented as the Bymarka ophiolite in Trondheim, Norway. It belongs to the Støren Group in the Upper Allochthon within the Scandinavian Caledonides. The emplacement onto the Baltican margin during the Caledonian orogeny occurred around 420 Ma, with following collapse in 400 - 380 Ma. Regionally, the rocks have been subjected to greenschist to lower amphibolite facies metamorphic conditions. Detailed mapping at centimetre to decimetre scale of the Gråkallen area reveals the complex distribution of phyllonitic shear zones hosted by a granodiorite, with transport direction interpreted as a top-to-the-west movement. Field observations and petrographic studies were performed for mineral identification and textural descriptions. Comparison of the modal mineralogy with whole-rock data including trace elements is used to show chemical variation between the different lithologies. Mineral chemistry was examined with element mapping by Scanning Electron Microscopy (SEM), Electron Microprobe Analysis (EMPA) and Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS). These methods report the major and trace element distribution along with the Rare Earth Elements (REE) distribution in each mineral phase. Isocon analysis of paired granodiorite-phyllonite samples enabled for quantification of the gains and losses in the system. The rocks at Gråkallen are granodioritic, with only minor variations in modal mineralogy across all samples. The granodiorite is hypidiomorphic, show no preferred mineral orientation with early growth of mica and dynamic recrystallization of quartz. Phyllonitic shear zones have developed locally in the granodiorite and signs of widespread fluid interaction and metasomatism is evident throughout the rocks. The granodioritic assemblage is represented by Qtz + Pl + Kfs + Ms ± Py, with secondary accessories Ep + Aln + Ttn, which has been identified as the main REE-carriers. Allanite is enriched in the LREE, while epidote contains LREE to a lesser degree and titanite holds the HREEs, reflecting each minerals preferential affinity for the REEs. The feldspars are altered, with alkali feldspar neocrystallized to albite, and plagioclase is seritized and saussuritized, with widespread muscovite and epidote. Major elements Na and K have been mobilized and Ca to a lesser degree, while the REEs has only been mobilized at micro scale in the granodiorite. iii The phyllonitic shear zones trend approximately N-S and consist of elongated quartz grains in a muscovite matrix, defining the assemblage of Qtz + Ms ± Py ± feldspar porphyroclasts. The formation of the muscovite indicates the presence of water-rich fluid. The Ca-bearing accessories Ep + Aln + Ttn has been destabilized, along with most of the feldspar due to deformation and high fluid/rock ratio, though some titanite is remnant. In addition, the fluid composition enabled for mobilization of trace elements and REEs. This resulted in depletion of the REE, with a greater loss of LREE compared to HREE. An epidote-rich shear zone shows a high modality of epidote, along with Ms + Py + Qtz + Ttn, with subsequent REE enrichment in comparison to the granodiorite. The REE pattern of the epidote-rich shear zone appeared as an enriched version of the granodiorite REE pattern, suggesting that stabilization of epidote and titanite prevented loss of the REEs. This study shows that metasomatism triggered mineralogical changes and grain size reduction, especially for the feldspars in the undeformed granodiorite, enabling increased fluid flow. The heightened fluid/rock ratio allows muscovite to form, and thus promoting strain weakening and further strain partitioning. Deformation and fluid-rock interaction completely changed the mineralogy by destabilizing feldspar and the REE-bearing phases and by forming muscovite, which promoted mobilization of K, Na, Ca and the REEs during phyllonitization of the granodiorite

Multi-isotope tracing of the 1.3–0.9 Ga evolution of Fennoscandia; crustal growth during the Sveconorwegian orogeny

Magmatism between 1.3 and 0.9 Ga at the southwestern margin of Fennoscandia, comprising mainly granitic batholiths and subordinate bimodal volcanic rocks, provides a nearly continuous magmatic record of the Fennoscandian tectonic evolution. Here, we present new and published zircon Hf, K-feldspar Pb and whole-rock Sr isotopic data from the granitic rocks. The εHf isotopic evolution since 1300 Ma starts out as relatively juvenile, with a flat superchondritic trend at 1300–1130 Ma followed by a steeper trend towards lower, but still superchondritic values at 1070–1010 Ma. During the 1000–920 Ma period, the trend flattens out at near-chondritic values. The variations between flat and steep εHf trends correspond to previously documented extensional and compressional periods, respectively. Although the change to a steeper εHf trend at ca. 1100 Ma may indicate the emergence of a new isotopic reservoir (i.e. a colliding continent), there is no corresponding change in the K-feldspar Pb or whole-rock Sr isotopic composition. We argue that the trends are better explained by varying proportions of isotopically evolved crust and juvenile mantle in the magma source regions, similar to Nd and Hf isotopic pull-downs and pull-ups observed in many accretionary orogenic systems. We therefore conclude that continuous accretionary processes without involvement of exotic sources is the best explanation for the isotopic evolution before and during the Sveconorwegian orogeny, and that the orogeny involved generation of significant volumes of new crust to the SW margin of the Fennoscandia