Magmatic zircon oxygen isotopes of 1.88–1.87?Ga orogenic and 1.65–1.54?Ga anorogenic magmatism in Finland |
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Authors: | B A Elliott W H Peck O T Rämö M Vaasjoki M Nironen |
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Institution: | (1) Department of Physics and Earth Science, University of North Alabama, Florence, AL, USA;(2) Department of Geology, Colgate University, Hamilton, NY, USA;(3) Department of Geology, University of Helsinki, Helsinki, Finland;(4) Geological Survey of Finland, Espoo, Finland |
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Abstract: | Summary Oxygen isotope ratios of igneous zircon from magmatic rocks in Finland provide insights into the evolution and growth of the
Precambrian crust during the Svecofennian orogeny. These data preserve magmatic δ18O values and correlate with major discontinuities in the lower crust. Oxygen isotope ratios of zircon across the 1.88–1.87 Ga
Central Finland granitoid complex (CFGC) range from 5.50‰ to 6.84‰, except for three plutons in contact with the adjacent
greenstone and metasedimentary belts (δ18O(Zrc) = 7.60‰–7.78‰). There is a systematic variation in δ18O(Zrc) with respect to geographic location in the CFGC, ranging from 6.60±0.23‰ (σ) in the northeast to 5.90±0.40‰ in the
west-southwest. These values correlate with a change in crustal thickness and shift in geochemical composition. The oxygen
isotope composition of the 1.65–1.54 Ga rapakivi granites and related rocks in southern Finland show a decreasing trend from
north to south, independent of their emplacement age. The southern anorogenic granite group has an average δ18O in zircon of 6.14±0.07‰ and the northern anorogenic group has an average δ18O in zircon of 8.14±0.59‰. This difference reflects the boundary between island arc terrains accreted during the Paleoproterozoic.
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