Subjotnian: Rapakivi granites and related rocks in the surroundings of the Gulf of Finland

The southern Finnish mainland, Aland, the Gulf of Finland and parts of neighbouring Latvia, Estonia and Russia are classical rapakivi areas. A typical rapakivi (adopted Finnish word meaning "rotten stone") is a coarse-grained granite with alkali feldspar ovoids either surrounded by plagioclase mantles (wiborgite) or not (pyterlite). Some other associated granite types (porphyritic with idiomorphic phenocrysts, equigranular and aplitic) are called also rapakivi. The granitic melt of the rapakivi plutons originated from partial melting of lower crust (a mixture of Archaean and Proterozoic lower crust in the Salmi and Ulyalegi plutons and Proterozoic in all the others) whereas the gabbro - anorthosites and diabases represent partial melting of the subcontinental mantle. These conclusions are largely based on isotopic studies. The rapakivi granites of the map area comprise three isotopic age groups: 1620-1650 Ma (SE Finland and Estonia), 1540-1590 Ma (SW Finland and Latvia) and 1540-1560 Ma (Russian Karelia). Gabbros and anorthosites make up the southern part of the Riga rapakivi pluton, the eastern part of the Salmi pluton and the surroundings of the Ahvenisto pluton. There are smaller bodies of mafic plutonic rocks in many other places, e.g., the Abja pluton of southern Estonia and the gem-quality anorthosite (spectrolite) of Ylämaa, Vyborg batholith. There are several swarms of diabase dykes within the rapakivi areas; some swarms are more than 150 km long. Most of these diabases are slightly older than the rapakivi granites and their emplacement was probably connected to the upward movement of rapakivi granite magma in the crust. The trends of the dyke swarms show the directions of maximum stress during rapakivi intrusion.