The temporal evolution of sanukitoids in the Karelian Craton, Baltic Shield: an ion microprobe U–Th–Pb isotopic study of zircons

The U–Th–Pb isotopic systematics of zircons from six sanukitoid intrusions in the Karelian Craton has been investigated, using the secondary ion mass spectrometer (SIMS). Our results show that Karelian sanukitoids formed between 2745 and 2705 Ma ago and that there is an age difference of about 30 Ma between sanukitoids in the eastern and western Karelia. Results for intrusions made up of a number of magmatic phases show no internal age variation. In the east of the study region, zircons from a monzodiorite and two quartz monzonites from the Panozero pluton were dated at 2737±11, 2739±11, and 2741±8 Ma; in the Hautavaara pluton, a monzodiorite and a subalkaline granite were dated at 2743±8 and 2742±23 Ma, and a quartz diorite from the Elmus pluton gave an age of 2741±7 Ma. In addition, a lamprophyre dyke within the Sjargozero pluton gave an age of 2742±14 Ma. In the west, zircons from a quartz monzonite and a granodiorite in the Njuk intrusion gave ages of 2709±10 and 2716±11 Ma. In the Kurgelampi pluton, zircons from a diorite and two granodiorites were dated at 2707±9, 2719±6, and 2712±9 Ma, respectively. Zircons from the granodiorites and the alkaline granites contain older cores with 207Pb/206Pb ages greater than 2800 Ma. The absence of inherited zircons in the more mafic sanukitoids is attributed to their higher saturation levels for zirconium. Sanukitoid zircons have Th/U ratios above 0.5 and often higher, whereas nearby tonalite–tondhjemite–granodiorites (TTGs) have Th/U ratios below 0.5. Typically, tonalites are older than sanukitoids by more than 100 Ma and it is possible that tectonic processes associated with the collision of the Belomorian Mobile Belt provided the heat necessary to initiate the generation of the sanukitoid melts.