Aluminosilicate Polymorphs as Indicators of Metamorphic Conditions in the Helags Area of the Scandinavian Caledonides

Sammanfattning

Aluminosilicate polymorphs are powerful metamorphic index minerals as they share the same chemical composition (Al₂SiO₅) but form at different pressure (P) and temperature (T) conditions largely unaffected by surrounding elements. The tectonometamorphic history of the Scandinavian Caledonides, which formed when Baltica and Laurentia collided during the Silurian and Devonian, has been of great research interest in recent years. However, much remains to be established. This thesis aimed to propose a P-T path for the Helags Mountain area and compare it with the results of Johansson (2016), who studied garnet-bearing gneisses from the same area. Optical microscopy, microprobe analysis, Raman spectroscopy and cathodoluminescence imaging were utilized. The results of this thesis do not allow a robust reconstruction of the P-T path. While all Al₂SiO₅ polymorphs are found within the same thin section, the absence of well-constrained formation textural relationships precludes the determination of their relative timing of formation. It is, however, noteworthy that the findings in this report are broadly consistent with those of Johansson (2016). Two relative formation sequences are more strongly supported by the observations in this study: in sample IK25-110, sillimanite and andalusite appear to have formed before muscovite, followed by later kyanite growth; whereas in sample IK25-118, sillimanite and andalusite appear to predate later muscovite and corundum formation. Cathodoluminescence imaging proved to be a valuable tool for analysing Al₂SiO₅ polymorphs and is recommended for broader application in similar studies.