P-T-t modelling of Proterozoic terranes in Lithuania: geodynamic implications for accretion of southwestern Fennoscandia

The East European Craton (EEC) consists of Archaean nucleii and accreted Proterozoic terranes. Pressure-Temperature-time (P-T-t) paths for the West Lithuanian Granulite Domain (WLG), the East Lithuanian Domain (ELD) and the Mid-Lithuanian Suture Zone (MLSZ) in the southwestern part of the EEC constrain the sequence of tectonic events and their deformational and metamorphic conditions. A kinematic and thermal model has been developed for the evolution of these terranes, given the accretion of the ELD to the Sarmatian and the WLG to the Fennoscandian continents, respectively, based on their structural and compositional differences. For initial surface heat flow values ranging from 80-90 mWm(-2), the numerical model predicts a thermal relaxation phase of at least 30-50 m.y., following very slow accretion (0.54 cm/yr) of the WLG in a subduction-type environment, prior to granulite facies metamorphism at 1.80 Ga. Only the use of very high initial surface heat flow values (c. 120 mWm(-2)) allows the direct link of accretion to the observed granulite facies metamorphism. Exhumation of rocks to shallower crustal levels occurs still within granulite facies conditions, being accompanied by partial melting. An observed cooling-reheating event can be explained by underthrusting of the ELD beneath the WLG at c. 1.70-1.65 Ga in the MLSZ, which is a boundary between accretionary belts related to Fennoscandia and another set of such belts formed at the margin of Sarmatia. Subsequent amphibolite facies metamorphism within the MLSZ is followed by what is interpreted as a collapse related exhumation phase. The predicted temperature field indicates that the collapse might have led to a substantial elevation of isotherms, possibly triggering Rapakivi magmatism at 1.60-1.50 Ga.