The Archaean sanukitoid series of the Baltic Shield: geological setting, geochemical characteristics and implications for their origin

Archaean high-Mg granitoids (sanukitoids) occur in the Karelian granite–greenstone terrain in the Baltic Shield in two distinct zones. In the west of the Shield sanukitoid intrusions formed between 2700 and 2720 Ma and consist of a single igneous phase that varies in composition from diorite to granite. In the Eastern part of the Shield, sanukitoid intrusions formed between 2730 and 2745 Ma and are strongly differentiated, varying in composition from ultramafic to felsic. All the sanukitoids are enriched in light rare earth elements (LREE: La≤80 ppm, Ce≤150 ppm, LaN/YbN≤30–40), Sr≤2000 ppm, Ba≤2500 ppm, P2O5≤1.5%, alkalis (Na2O+K2O=5–10%), possess high mg# values (0.50–0.65), and show a negative Nb–Ta anomaly. They are spatially and temporally related to syenite intrusions and lamprophyre dykes. Sanukitoid intrusions in the Western and Eastern zones differ in composition. In the west, they have higher SiO2 (mainly>60%) and lower alkalis, Sr, Ba, LREE than in the Eastern zone intrusions. The most differentiated intrusion, the Panozero intrusion in the Eastern zone, was formed in two magmatic cycles separated by ductile deformation. In the first cycle, ultramafic to monzonitic rocks formed, whereas, in the second cycle, the magmas were monzodioritic to quartz monzonite. Ultramafic and mafic rocks make up about 10% of the outcrop and occur as enclaves in monzonites and monzodiorites, and, as dykes, implies a number of discrete magmatic events. All rocks of the Panozero intrusion have high K2O, and the composition of the initial melt, calculated from the weighted average of the first cycle magmas is monzodiorite (SiO2=52%, mg#=0.55, Na2O+K2O∼6%). The presence of magmatic hornblende and biotite, a high carbonate content, widely distributed explosive breccias and evidence of liquid immiscibility are consistent with a high H2O–CO2 content in the sanukitoid melt. The geodynamic model which most satisfactorily explains our geological and geochemical observations is the formation of the Karelian sanukitoids in a cratonic environment adjacent to the Belomorian–Karelian suture.