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Scandian metamorphism of metapelites and serpentinites in the pre- Caledonian magma-poor hyperextended margin of Baltica

Abstract : Basins formed at magma-poor hyperextended margins that are partly floored by transitional crust exhibit a characteristic lithological assemblage including exhumed subcontinental mantle, basement slivers, syn-rift deep- marine basin sediments, coarse-grained sediments derived from pre-rift supracrustal rocks or basement, and minor mafic intrusives/volcanics. Such a lithological assemblage is preserved below the large crystalline nappes in South Norway, including the Jotun, Upper Bergsdalen, and Lindås nappe complexes and comprises metapelites, metasandstones, metaconglomerates, solitary serpentinite bodies, detrital serpentinite-soapstone conglomerates, minor late Scandian intrusives, and thin discontinuous sheets of Proterozoic and Ordovician gneisses. This rock association, non-genetically referred to as “mélange”, can be traced almost seamlessly from Bergen to Otta and is interpreted to represent the vestiges of the pre-Caledonian magma-poor hyperextended margin of Baltica. This study focusses on the metamorphism and metasomatism of the metapelites and the serpentinites. The metamorphic mineral assemblage of the metapelites includes white mica, chlorite, biotite, plagioclase, and quartz as well as local garnet and amphibole. The ultramafics are to various degrees serpentinised, talcified, and carbonated. The metamorphic mineral assemblage of the serpentinites includes serpentine, olivine, talc, carbonates (magnesite, dolomite), magnetite, diopside. The majority of the olivine grains are markedly rich in Mg (Fo0.9-0.98) and the diopside grains are rich in Ca and poor in Al, which suggests that both olivine and diopside are secondary (Enger 2016, Master’s thesis, UiO). Locally, olivine, diopside, and carbonate are replaced by serpentine. Talc-carbonate, actinolite-chlorite, and talc-chlorite schists were locally developed during blackwall alteration of the ultramafics and the country rocks. Peak metamorphic temperatures for the metapelites were estimated at approximately 500 ±50 °C by the means of Raman spectroscopy of carbon-rich material (RSCM) and are remarkably consistent along strike of the mélange zone. Comparisons of the metamorphic mineral assemblage and the RSCM results with thermodynamic calculations that are based on realistic whole rock compositions of the metapelites indicate pressures of 5.25-7.25 kbar during Scandian thrusting. The lateral consistency of the metamorphism is furthermore indicated by the isotope composition of carbonates in the mélange. The δ18OCarb values of the partly carbonated serpentinites and the metasediments fall within a narrow range between +11 and +15.5 ‰ (SMOW) whereas the δ13CCarb values fall into three groups: (1) +1.6 to +0.3 ‰ (PDB), (2) −1.8 to −3.9 ‰, and (3) −6 to −8.6 ‰, irrespective of their textural setting, i.e. whether the carbonate is biogenic, vein material, or replacing serpentine minerals. The unit-wide homogenization of the δ18O values in a narrow range and the only locally occurring equilibration of the δ13C values suggest fluid-rock alteration with an externally derived, mostly aqueous fluid. During prograde metamorphism of metapelitic rocks aqueous metamorphic fluids are released by the formation of, e.g., biotite and garnet on the expense of chlorite and feldspar. Aqueous fluids are also released during the dehydration of serpentinite, e.g. by the breakdown of talc and serpentine minerals to form forsterite and orthopyroxene. Thermodynamic calculations indicate that the metapelites may have released up to 4 wt% and the metaperidotites up to 9 wt% of aqueous fluids during prograde metamorphism. Minor amounts of carbonic fluids may have been produced by the decarbonation of the metapelites and the partially carbonated serpentinites. We suggest that the ultramafics were already mostly or fully serpentinised prior to Scandian thrusting and underwent prograde metamorphism and dehydration together with the metapelites. Fluid-rock interactions with metamorphic aqueous fluids homogenized unit-wide the δ18OCarb values and locally the δ13CCarb values. The shared Scandian metamorphic history demonstrates that the rocks of the mélange represent an almost seamless tectonic unit in the Caledonides of South Norway and were juxtaposed prior to the onset of Scandian thrusting most likely in the pre-Caledonian magma-poor hyperextended margin of Baltica.
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https://hal-insu.archives-ouvertes.fr/insu-01442039
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Submitted on : Friday, January 20, 2017 - 1:31:21 PM
Last modification on : Friday, March 27, 2020 - 3:00:25 AM

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  • HAL Id : insu-01442039, version 1

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Johannes Jakob, Torgeir B. Andersen, Philippe Boulvais, Olivier Beyssac. Scandian metamorphism of metapelites and serpentinites in the pre- Caledonian magma-poor hyperextended margin of Baltica. NGF Winter Conference 2017, Norsk Geologisk Forening, Jan 2017, Oslo, Norway. ⟨insu-01442039⟩

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