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

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 rocks. 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 aH,0 = 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 1871114 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 zon-32 H. Mouri, K. Korsman and H. Huhma ing 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 207 Pb/ 206 Pb 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 eve...

Pre-1.91 Ga deformation and metamorphism in the Palaeoproterozoic Vammala Migmatite Belt, southern Finland, and implications for Svecofennian tectonics

“…5). Monazites dated by Mouri et al (1999) from mesosomes and leucosomes in VMB migmatites at Luopioinen, about 20 km ESE of Ta mpere, gave a similar mean age of 1878.5±1.5 Ma. These migmatites are characterised by the sillimanite + cordierite ± garnet association, which is largely restricted to the Ta mpere map sheet (Fig.…”

“…1; the Savo Belt in Fig. 2), is identified as older than 1.90 Ga (see also Korsman et al, 1999). The VMB occupies a critical position between the "Accretionary arc complex of southern Finland (1.90-1.82 Ga)" and the "Accretionary arc complex of central and western Finland (1.90-1.87 Ga)"(A and Bi nF ig.1), both of which have been considered to contain exotic terranes, mainly on the basis of geochemical and detrital zircon data (e.g.…”

“…2 & 3) trends E-W and is up to 50 km wide. It is part of a belt extending to east and west that has been given various names in the literature, including Vammala Migmatite Belt , Pori-Vammala-Mikkeli Migmatite Zone (Kilpeläinen, 1998), Mica gneiss-migmatite Belt (Lahtinen, 1994;, Psammitic migmatite zone (Korsman et al, 1999), To nalite Migmatite Zone (Koistinen et al, 1996), and To nalite-Trondhjemite Migmatite Belt (Mouri et al, 1999). The name Pirkanmaa Belt has also recently been recommended (Nironen et al, 2002) for an extensive lithological-geographical area, the limits of which have not been fully defined.…”

“…However, crystallisation of garnet always preceded that of cordierite, and cordierite often clearly post-dates the VD 1 deformation,p robablyr eflecting decompression. Metamorphism and migmatisation in the VMB has recently been dated, using severali sotopic methods, at~1.88 Ga (Mouri et al, 1999). It was inferred (op.…”

Evolution of migmatitic granulite complexes: Implications from Lapland Granulite Belt, Part II: Isotopic dating

“…The exhumation of the LGB took place during the time when granulite-grade metamorphism still took place in the Svecofennian orogen (Korsman et al, 1997Mouri et al, 1999).…”