Magmatic zircon oxygen isotopes of 1.88–1.87 Ga orogenic and 1.65–1.54 Ga anorogenic magmatism in Finland

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 (symbol omitted)18O values and correlate with major discontinuities in the lower crust. Oxygen isotope ratios of zircon across the 1.88-1.87Ga Central Finland granitoid complex (CFGC) range from 5.50(symbol omitted) to 6.84(symbol omitted), except for three plutons in contact with the adjacent greenstone and metasedimentary belts ((symbol omitted)18O(Zrc) = 7.60(symbol omitted)-7.78(symbol omitted)). There is a systematic variation in (symbol omitted)18O(Zrc) with respect to geographic location in the CFGC, ranging from 6.60 +/- 0.23(symbol omitted) ((symbol omitted)) in the northeast to 5.90 +/-0.40(symbol omitted) in the west-southwest. These values correlate with a change in crustal thickness and shift in geochemical composition. The oxygen isotope composition of the 1.65-1.54Ga 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 (symbol omitted)18O in zircon of 6.14 +/- 0.07(symbol omitted) and the northern anorogenic group has an average (symbol omitted)18O in zircon of 8.14 +/- 0.59(symbol omitted). This difference reflects the boundary between island arc terrains accreted during the Paleoproterozoic. Deceased [PUBLICATION ABSTRACT]