The Growth of Epidote and Tianite During Upper Amphibolite to Lower Granulite Facies Metamorphism in Stensjoestrand, Southwest Sweden
Dr. Edward Hansen
Titanite and epidote minerals were observed in the Stensjöstrand area of southwest Sweden within a metamorphic reaction zone between two rock types. These minerals are characteristic of rocks with low-grade metamorphism and tend to be consumed when exposed to higher metamorphic conditions. A sequence of lens-like, calcsilicate enclave rocks contain higher calcium and iron concentrations than the titanium-rich, host amphibolite rock in which it occurs. Textures of three traverses between the two rocks were examined by SEM at Hope College and mineral analyses were performed via microprobe at the GeoForschungZentrum in Potsdam, Germany. Iron and calcium-rich minerals such as anorthite and hedenbergite increase in concentration from the host rock to the core of the enclaves. Iron oxides containing titanite occur throughout both rocks, with higher concentrations of titanium found within the oxides of the host rock. Epidote grains contain high concentrations of rare earth elements (REE) in the host rock and outer enclave rims (REE up to 13 wt%), small concentrations in the middle of the enclaves (REE < 0.1 wt%), and a moderate amount in the enclave cores (0.5 < REE <5 wt%). Textural and mineralogical patterns both suggest titanite and REE-poor epidote formed by diffusion from a higher gradient in the enclave to a lower gradient in the host. More recently-crystallized epidote appears to have formed by nucleating on pre-existing REE-rich epidote, while titanite formed by a reaction of the iron titanium oxides.
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