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Analysis of element behavior in mylonites of the Seve Nappe of the Scandinavian Caledonides using different core scanning methods


Hierold,  Johannes
Scientific Technical Report STR, Deutsches GeoForschungsZentrum;
6.4 Centre for Scientific Drilling, 6.0 Geotechnologies, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;

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Hierold, J. (2016): Analysis of element behavior in mylonites of the Seve Nappe of the Scandinavian Caledonides using different core scanning methods, Master Thesis, (Scientific Technical Report STR ; 16/07), Potsdam : Deutsches GeoForschungsZentrum GFZ, 116 p.

Cite as: https://gfzpublic.gfz-potsdam.de/pubman/item/item_1649893
The International Continental Scientific Drilling Program (ICDP) performed a dual-phase scientific drilling project to investigate mountain-building processes called Collisional Orogeny in the Scandinavian Caledonides (COSC). The borehole COSC-1 was drilled through the Lower Seve Nappe, as the first of two 2.5 km deep drill holes close to Åre, central Sweden. The recovered rocks comprise a 1650 m thick suite of high grade gneisses and amphibolites with clear Seve Nappe affinities, while the lower 850 m comprise rather homogenous mylonitic gneisses with interfingered K-rich phyllonite bands of cm to several m size and some intercalated amphibolites. The different lithologies all crosscut the core in a subhorizontal direction with foliation of gneisses and phyllonites in the same direction. Albite and garnet porphyroblasts with pressure shadows show syn-deformational growth and the same sub-horizontal alignment. The focus of this thesis is to detect chemical and mineralogical differences in mylonitic and host rocks and to relate these differences to either metasomatism and deformation or inherited source rock variance. Another goal of this work is to compare chemical core scanning instruments. For this purpose two different μ-Energy-Dispersive X-Ray Fluorescence (μ-EDXRF), Laser Induced Breakdown Spectroscopy (LIBS) and hyperspectral imaging techniques served to measure seven samples from the lower 850 m of the COSC-1 core. The measurements reveal sharp borders between different rock types without indication of metasomatic changes, pointing to a heterogeneous protolith such as greywacke. Element and mineral maps show strong pervasive ductile deformation with mylonite recrystallization. The comparison of the scanning devices shows that the μ-EDXRF scanner with 50 μm resolution can be used perfectly for microstructural investigations and heavy element analysis. The XRF core scanner from AVAATECH is very useful and sufficiently precise for element profiles of line scans. The LIBS scanner is great to create distribution maps of elements from H to U with a resolution of 200 μm. The hyperspectral cameras are extremely fast in acquiring spectral mineral maps and structural information. However, several rock forming minerals in gneisses can currently not be identified and a calibration for metamorphic rocks is still needed.