Tonalitic Melt Formation from Amphibolite in Hanko Archipelago
Andersson, Emil (2022)
Andersson, Emil
2022
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi-fe2022060643619
https://urn.fi/URN:NBN:fi-fe2022060643619
Tiivistelmä
Water-fluxed and dehydration melting have critical roles in tonalitic melt production in orogenic belts. The melting of amphibolite at middle-lower crustal depths is usually caused by influx of externally derived hydrous fluids or dehydration of amphibole. Water-flux decreases the melting point of amphibolite, whereas dehydration melting requires high PT conditions. Water-fluxed melting of amphibolite produces peritectic hornblende, whereas dehydration melting produces anhydrous peritectic minerals such as pyroxene and garnet. Furthermore, water-fluxed and dehydration melting also have different geochemical characteristics. Hanko archipelago consists of migmatised metavolcanic and metasedimentary rocks. The age of regional metamorphosis and melt formation has been determined by Hopgood et al. (1983) and Bredenberg (2019) to be ca. 1.89 – 1.86 Ga. The earlier metamorphism is thought to have been completely erased from Southern Finland and the well-preserved metamorphic culmination took place ca. 1.83 Ga. Hopgood et al (1983) have argued that an earlier partial melting took place in Southern Finland. The Hanko archipelago is dominated by amphibolite and tonalite. The tonalitic leucosome that occurs in the area is thought to have been part of amphibolite partial melting during the early Svecofennian deformation ca 1.89 Ga. It has been suggested that tonalitic leucosome and tonalite are products of water-fluxed melting and derive from the same source.
In this thesis, field observations, petrographic description, SEM-EDX analysis, and WDXRF-analysis of paleosome, melanosome, leucosome and tonalite were conducted to investigate the melting process and the correlation between leucosome and tonalite. Both leucosome and tonalite contained peritectic hornblende, shared the same microstructures and textures as well as geochemical characteristics that are attributed to water-fluxed melting. However, mineral analysis of biotite, amphibole and plagioclase revealed that they derive from different sources. Furthermore, PT conditions were different for both rocks, indicating that they crystallised in different environments. Therefore, both leucosome and tonalite are products of water-fluxed melting but derive from different sources.
In this thesis, field observations, petrographic description, SEM-EDX analysis, and WDXRF-analysis of paleosome, melanosome, leucosome and tonalite were conducted to investigate the melting process and the correlation between leucosome and tonalite. Both leucosome and tonalite contained peritectic hornblende, shared the same microstructures and textures as well as geochemical characteristics that are attributed to water-fluxed melting. However, mineral analysis of biotite, amphibole and plagioclase revealed that they derive from different sources. Furthermore, PT conditions were different for both rocks, indicating that they crystallised in different environments. Therefore, both leucosome and tonalite are products of water-fluxed melting but derive from different sources.
Kokoelmat
- 1171 Geotieteet [22]