Metamorphic Evolution Of The Ilomantsi Greenstone Belt In The Archaean Karelia Province, Eastern Finland

The Ilomantsi greenstone belt is a Neoarchaean, c. 2.75-2.70 Ga volcanic-sedimentary complex in which metamorphic grade increases from staurolite grade in the SW of the belt to sillimanite grade in the NE. In the staurolite zone, prograde garnet zoning indicates pressure and temperature increases from 480-500 degrees C at 2-4 kbar to 560-570 degrees C at 6-7 kbar. Within the sillimanite zone temperatures peaked at 660-670 degrees C at pressures of around 6 kbar. The U-Pb age determinations on monazite from the sillimanite zone yielded both Archaean and Proterozoic ages. One sample contains an exclusively Archaean monazite population of 2620 +/- 24 Ma, while another sample has two generations of monazite, with ages of 2664 +/- 33 Ma and 1837 +/- 13 Ma. The monazite data confirm that the Ilomantsi greenstone belt was metamorphosed simultaneously with the surrounding Neoarchaean migmatite complexes. The apparent clockwise PT path and medium P/T-type metamorphism are consistent with collisional tectonic settings, but the two distinct metamorphic events recorded by monazite indicate that a second, Palaeoproterozoic thermal event caused recrystallization and new mineral growth, in line with previous evidence from other isotopic systems. Accordingly, great care is necessary in defining metamorphic evolutionary P-Tt paths in rocks with complex mineral assemblages, to ensure correct identification of truly coeval mineral assemblages.