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Department of Mineralogy and Petrology, Institute of Geology, Lund University, Sölvegatan 13, S-223 62 LUND, Sweden
In the Late Archaean Belomorian Belt of the Baltic Shield, shearing and high-grade metamorphism occurred in association with bimodal gabbroic-granitic igneous activity between 2.45 and 2.40 Ga. That period is generally regarded as one of anorogenic rifting and extension. In the studied case of the Tolstik intrusion, the time of metamorphism is constrained by the 2434 ± 7 Ma age of a metamorphosed mafic intrusion and the 2405 ± 20 Ma age of a subsequently intruded potassic granite. Several generations of mafic dykes allow a detailed study of the metamorphic evolution during that time span.
The conditions of post-intrusion peak metamorphism in the mafic intrusion were 700–800°C and 11–12 kb which was followed first by near-isothermal decompression and, after the loss of c. 5 km crustal thickness, by near-isobaric cooling to c. 600° C at 8–9 kb.
These results suggest metamorphism possibly triggered by igneous intrusions into previously thickened crust, but the P/T-ratios of metamorphism were moderately high. Later followed crustal extension concomitantly with continuous igneous input and the P/T- ratios decreased. Finally, a post-extensional period of cooling ensued as igneous activity ceased.
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