Reactive monazite and robust zircon growth in diatexites and leucogranites from a hot,slowly cooled orogen: implications for the Palaeoproterozoic tectonic evolution of the central Fennoscandian Shield,Sweden |
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| Authors: | Karin Högdahl Jaroslaw Majka Håkan Sjöström Katarina Persson Nilsson Stefan Claesson Patrik Kone?ný |
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| Institution: | 1.Department of Earth Sciences,Uppsala University,Uppsala,Sweden;2.Geological Survey of Sweden,Uppsala,Sweden;3.Laboratory of Isotope Geology,Swedish Museum of Natural History,Stockholm,Sweden;4.Geological Survey of Slovak Republic,Bratislava,Slovak Republic |
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| Abstract: | Monazite in melt-producing, poly-metamorphic terranes can grow, dissolve or reprecipitate at different stages during orogenic
evolution particularly in hot, slowly cooling orogens such as the Svecofennian. Owing to the high heat flow in such orogens,
small variations in pressure, temperature or deformation intensity may promote a mineral reaction. Monazite in diatexites
and leucogranites from two Svecofennian domains yields older, coeval and younger U–Pb SIMS and EMP ages than zircon from the
same rock. As zircon precipitated during the melt-bearing stage, its U–Pb ages reflect the timing of peak metamorphism, which
is associated with partial melting and leucogranite formation. In one of the domains, the Granite and Diatexite Belt, zircon
ages range between 1.87 and 1.86 Ga, whereas monazite yields two distinct double peaks at 1.87–1.86 and 1.82–1.80 Ga. The
younger double peak is related to monazite growth or reprecipitation during subsolidus conditions associated with deformation
along late-orogenic shear zones. Magmatic monazite in leucogranite records systematic variations in composition and age during
growth that can be directly linked to Th/U ratios and preferential growth sites of zircon, reflecting the transition from
melt to melt crystallisation of the magma. In the adjacent Ljusdal Domain, peak metamorphism in amphibolite facies occurred
at 1.83–1.82 Ga as given by both zircon and monazite chronology. Pre-partial melting, 1.85 Ga contact metamorphic monazite
is preserved, in spite of the high-grade overprint. By combining structural analysis, petrography and monazite and zircon
geochronology, a metamorphic terrane boundary has been identified. It is concluded that the boundary formed by crustal shortening
accommodated by major thrusting. |
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