Rapakivi is a type of granite. Its type locality is the Vyborg batholith which is located in SE Finland and Karelia, Russia. “Rapakivi” is a term that comes from the Finnish language (rotten or crumbled stone) but this rock type is not restricted to this particular area. The magmatic plutons showing rapakivi texture are found in many places all over the world, including USA, Brazil, China, etc.
Rapakivi is well known for its interesting texture — large ovoidal orthoclase phenocrysts are surrounded by plagioclase mantles. However, not all rocks that are described as rapakivi granites show this texture. To avoid confusion rapakivi is now defined as a granite which comes from a pluton that shows rapakivi texture at least partly. Rapakivi with a rapakivi texture from Vyborg batholith has a special regional name — vyborgite.
Rapakivi from the type locality and its surroundings is approximately 1.6 billion years old. But rapakivis are not necessarily restricted to the Proterozoic eon. Rapakivis from the Archaean and the Phanerozoic eons are known as well. The Vyborg batholith is one of the many rapakivi plutons in Northern Europe but most of them are covered by the younger layers of sedimentary rocks. Rapakivi plutons associated with the Vyborg batholith are located in my home country’s (Estonia) bedrock also but they are nowhere exposed. However, there are millions of rapakivi boulders in Estonia that were brought here by the continental glacier during the last glacial episodes.
Rapakivi is clearly plutonic rock but the volcanic rocks associated with it most likely existed as well. We have even some examples of very old rhyolitic lavas but most of it is long gone. This volcanic episode must have been very violent and explosive but the evidence is very scarce. The chance for a survival is not very high for a supracrustal rocks of that age. The emplacement of rapakivi plutons and associated rocks (diabase, anorthosite, rhyolite) was probably caused by a rifting episode which subsequently failed. These rocks are therefore anorogenic, their emplacement was not caused by the mountain building episode.
Why is this rock “rotten”? Because it is quite susceptible to weathering. Plagioclase feldspar weathers easily and the inequigranular nature of the rock makes it easier to desintegrate by the temperature changes. Hence the rapakivi boulders are often so weathered that it takes only bare hands to crumble it into smaller pieces.
Here are some glacial erratics of this type now on the coast of the Baltic Sea near the Vyborg batholith in Karelia.
Large orthoclase ovoid (5 cm) mantled by the oligoclase (one of plagioclase feldspar minerals) rim. Not all of the orthoclase phenocrysts are mantled by the plagioclase rims though. The large ovoid on the lower right lacks plagioclase rim.
Some orthoclase ovoids are really big (the largest in the middle is 8 cm in diameter). Rapakivi granite on the coast of Karelia, Russia (The Gulf of Finland).
Weathered rapakivi granite on the coast of Karelia, Russia (The Gulf of Finland).
One more crumbled rapakivi granite on the Karelian coastline.
This article is a part of Rocks from Fennoscandia series.
