Bulletin of the Geological Society of Finland (Jun 1999)
Tectono-metamorphic evolution and timing of the melting processes in the Svecofennian Tonalite-Trondhjemite Migmatite Belt: An example from Luopioinen, Tampere area, southern Finland
Abstract
The Svecofennian Orogen is in southern Finland characterized by two major migmatite belts. These are the so-called Granite Migmatite Belt, in which Kfs-rich leucosomes predominate, and the Tonalite-Trondhjemite Migmatite Belt, which is characterized by Kfs-poor leucosomes and borders the former belt in the north. The present paper deals with selected migmatitic rocks from the latter belt. It is aimed to study the temporal and structural relationships of the different leucosome generations, and to establish the pressure-temperature-time paths of this belt. The Tonalite-Trondhjemite Migmatite Belt consists mainly of migmatitic rocks with various types of synorogenic granitoids and minor mafic and ultramafic crocks. The mesosome of the migmatites consist of garnet-sillimanite-biotite-plagioclase-cordierite-quartz assemblages with rare K-feldspar and late andalusite. The oldest leucosomes are dominated by plagioclase and quartz, and the content of K-feldspar increases in later leucosomes. Microtextural analysis in conjunction with THERMOCALC calculations and geothermometry shows that these rocks were metamorphosed at peak conditions of 700-750°C at 4-5 kbar and aH2O = 0.4-0.7. The formation of cordierite coronas around garnet and the late crystallization of andalusite suggest that the final stage of the P-T history was characterized by decompression and cooling within the andalusite stability field, estimated at 500-650°C and 3-4 kbar. Detailed isotopic dating of mesosome and leucosomes of the migmatites was undertaken by conventional U-Pb analyses on monazite and zircon, Sm-Nd analyses on garnet, and ion probe dating on zircon. The monazites are nearly concordant with an average age of 1878.5±1.5 Ma, and garnet-whole rock analyses show that the concordant leucosomes and the mesosome are coeval within error margins having ages of 1893±40 and 1871±14 Ma, respectively. However, garnet in the discordant vein leucosome provides an age of 1843±11 Ma, which is marginally younger than the age of the adjacent mesosome and the concordant leucosome (1877+18 and 1880±23 Ma, respectively) and the age of monazite. Zircons from the studied migmatites display complex zoning structures using SEM-based CL-imaging. Most grains have distinct cores, clearly remnants of original grains. The cores display various types of zoning but oscillatory zoning dominates. The cores are overgrown by one or two thin outer rims that are of two types: i) unzoned outer rim, considered as overgrowth of new zircon during a metamorphic event, and ii) weakly oscillatory zoned rim, considered as typical of magmatic recrystallization. Ion probe dating of cores yielded slightly discordant 207Pb/206Pb ages of between 2866-2002 Ma, which are interpreted as protolith age. Rims yielded two major age groups: the unzoned rims gave ages of 1872-1886 Ma, whereas the rims with oscillatory zoning yielded ages of 1951—1959 Ma. The youngest age group is consistent with the conventional Sm-Nd dating on garnet and U-Pb dating on monazite and we suggest that the migmatites were metamorphosed at granulite facies conditions at ca. 1880 Ma. The 1951-1959 Ma age group yielded by magmatic zircon rims remains difficult to interpret, but may reflect a magmatic event prior to the metamorphic one. This magmatic event might be related to the rifting of a Svecofennian protocontinent.
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