Tectonomagmatic Evolution of the Sveconorwegian Orogen: Insights from Geochemical and Isotopic Data
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The Sveconorwegian orogen has a long, complex geological and tectonic history at the southwestern margin of the Fennoscandian Shield, which took place between 1140 and 920 Ma. For almost 150 million years of that time, between 1070 and 920 Ma, large volumes of felsic magma were emplaced into the Sveconorwegian crust. Magmatic rocks can store information about the source and processes leading up to their formation. This information is stored in the rocks’ geochemical and isotopic compositions and is a powerful tool that can be used to track compositional variations through time and space. This tool provides an indirect view into magma source(s) and petrogenesis, thus providing insight into the underlying tectonic processes. This thesis focusses on the tectonomagmatic evolution of the Sveconorwegian orogen, and how it relates to sulphide mineralisations found in the orogen. A multitude of geochemical and isotopic methods to characterise the felsic magmatic rocks from the Sveconorwegian orogen have been used. The first aim of this thesis is to outline the tectonic evolution of the Sveconorwegian orogen throughout the 150 million years of magmatism. The second aim is to link the sulphide mineralisations to the established tectonic framework. The aims are achieved through two peer-reviewed (published) articles and a manuscript. The compositional data used in this thesis consists of a compilation of published data from literature, the incorporation of unpublished data from the Geological Survey of Norway (NGU) and production of new geochemical and isotopic data on the Sveconorwegian magmatic rocks and four selected sulphide mineralisations. The data include major and trace element data on bulk rock, bulk rock neodymium (Nd) and strontium (Sr) isotopes, hafnium (Hf) isotopes in zircon, lead (Pb) isotopes in Kfeldspar and sulphide, and sulphur (S) isotopes in sulphides. In addition, trace element modelling was used to test a two-stage melting scenario, and Nd and Hf isotope modelling were performed to test binary mixing events and evolution through time. Paper 1 initiates the evolution history of the region by proposing a subdivision of the magmatic rocks into four suites, based on their geological relationship observed in the field, geochronology and distinct geochemical and isotopic features. The four suites are: The 1070–1010 Ma Sirdal magmatic belt (SMB suite); the two 1000–920 Ma hornblende-biotite granite suites (the HBGin and HBGout suites), which comprise hornblende-biotite granites hosted by the SMB suite (HBGin suite) and those who are not hosted by the SMB suite (HBGout suite); and the ca. 925 Ma Flå-Iddefjord-Bohus suite (FIB suite). The paper presents and discusses each suite’s petrogenesis and source(s), as well as the genetic relationship between the suites. Based on the petrogenetic interpretations, the paper proposes a tectonic evolution for the Sveconorwegian orogen. The geochemical data show that the HBGout suite resembles the SMB suite, being magnesian and metaluminous-to-peraluminous. The HBGin suite is ferroan, on average more enriched in incompatible elements, such as the REEs, and is dominantly metaluminous. The Nd isotopic data show that the SMB and both HBG suites can be traced back to a dominant 1.5 Ga source. Trace element modelling shows that the SMB and HBGout suites could have formed by 50% partial melting of the 1.5 Ga crust, whereas 5–10% remelting of the dehydrated and depleted SMB residue accounts for the geochemical composition of the HBGin suite. Zircon saturation temperatures (TZr) show that the melting temperatures increase throughout the Sveconorwegian orogeny. The rise in temperature is associated with the transition from SMB to HBG magmatism at ca. 1000–990 Ma, which is interpreted to reflect an increase of additional heat into the lower crust. The changes in the petrogenetic conditions are ascribed to a continually evolving activemargin setting, rather than an abrupt change in tectonic regime, processes, or sources as would be expected during a change from oceanic subduction and accretionary orogeny to continent–continent collision. In the westernmost parts of the orogen, the SMB and both HBG suites formed as a result of long-lived mafic underplating, driving lower-crustal melting with depletion of lower-crustal sources and activation of new areas over time as the orogen evolved. The change from overall compression to extension at ca. 1000 Ma that drove the additional input of heat played an important role. The FIB suite, which was emplaced in the final stage of the orogeny, is more peraluminous, is rich in inherited zircons and formed at lower temperatures from an isotopically more evolved source than both the HBG suites and the SMB. In the eastern parts of the orogen, deep continental subduction, associated with high-pressure metamorphism, provided an evolved crustal source (e.g., the 1.8–1.6 Ga Transscandinavian Igneous Belt (TIB) rocks) for the FIB suite at the end of the Sveconorwegian orogeny. Paper 2 discusses two contrasting tectonic models that have been suggested for the Sveconorwegian orogen, an active continental margin versus continent-continent collision. The paper uses hafniumstrontium- lead (Hf–Sr–Pb) isotopes on magmatic rocks, zircons, and K-feldspar to assess the tectonic evolution between ca. 1300 and 900 Ma, that is, prior to and during the Sveconorwegian orogeny. This time period was subdivided into three, where each period is characterised by either extensional or compressive tectonics and magmatism: 1300–1130 Ma, 1070–1010 Ma and 1000–920 Ma. The slope of the Hf isotopes reveal whether the source of the magmatism varies during this period. The zircon Hf data showed that there is a cyclical variation in the proportions of mantle- and crust-derived material to the magma source regions. The 1300–1130 and 1000–920 Ma periods, characterised by crustal extension, are marked by increased mantle input, showed by a flatter εHf trend (isotopic pull up). The 1070–1010 Ma SMB magmatic period was characterised by compression and is dominated by increased proportions of evolved crustal material, showed by steeper εHf trend (isotopic pulldown). Hf isotope modelling show that the SMB suite may comprise as much as 30% mantle-derived material, while up to 50% for the HBG suites. The evolved εHf trend of the SMB magmatism, together with the lack of corresponding change in Sr bulk rock and Pb K-feldspar, suggest that SMB magmatism is a result of increased crustal reworking and not an introduction of a new isotopically distinct crustal reservoir. The paper concludes that the observed isotopic variations are best explained by continuous accretionary processes without the involvement of new exotic sources. Manuscript 3 assesses the sulphur (S) and Pb isotopic signature of four selected sulphide mineralisations from the Sveconorwegian orogen: Knaben molybdenum (Mo), Flåt nickel–copper (Ni–Cu), Flottorp Mo and Lastebotn Cu mineralisations. Sulphur isotopes of sulphides may indicate the sulphur source of the four mineralisations, and the Pb isotopes used to assess the source composition of the mineralisations, which is then compared to Pb compositions of the Sveconorwegian granitoid magmatism. This study looked for any isotopic variations and similarities between the mineralisations and answered how the mineralisations relate to the magmatism and tectonic history. The sulphur isotope results show that there are three identified S sources: mantle-derived (Flåt Ni– Cu), lower-crustal/mantle sources (Knaben Mo and Flottorp Mo) and 1.5 Ga Suldal arc source with sedimentary input (Lastebotn Cu). The manuscript then compares the isotopic signature of the sulphides in the mineralisations to Kfeldspar from Sveconorwegian granitoids (the SMB and HBG suites). The Pb data show that the Knaben Mo K-feldspars and Flåt Ni–Cu sulphides record magmatic processes, as they overlap with the Pb signature of the Sveconorwegian magmatic rocks. The Pb compositions of the Flottorp, Lastebotn and the Knaben sulphides are much more radiogenic, and these radiogenic compositions may reflect Pb sources and other processes that are currently unaccounted for. In this study, the mantle-derived and lower-crustal/mantle sources are readily explained by a long-lived and evolving magmatism and tectonism during the Sveconorwegian orogeny. This thesis links the underlying magmatic, tectonic processes and metallogeny to the evolution of the Sveconorwegian orogen. The magmatism along the Sveconorwegian active continental margin is derived from a continuously evolving source, with varying input of lower crustal and mantle-derived material. This thesis demonstrates that the geochemical and isotopic compositions of granitoid rocks may serve as excellent recorders of petrogenetic process and underlying tectonic processes.