Migmatitic rock outcrop, Kaivopuisto Park, Helsinki. The grey gneiss and intruding granitic dykes were created by a continental collision nearly 2 billion years ago. There is also an erratic rapakivi granite boulder deposited during the last ice age.
"The Helsinki Region is located in southern Finland on Fennoscandian shield area. The shield is situated in the north- western part of the East European Craton and is the largest exposed area of very old Precambrian rocks in Europe. In southern Finland the ancient Precambrian bedrock is about 1.9 - 1.8 billion years old and consists of gneisses and granitic rocks. Old shields are typically very stable areas where earthquakes, tectonic movements and natural movements of the ground are unusual. The structure of the bedrock reflects the metamorphic character. The rocks were originally sedimentary and volcanic, and the deeply buried, mixed and partly melted nature of the rocks is seen nowadays in form of banded gneisses and migmatites. Granitic rocks are also typical in the Helsinki area. All the gneissic zones in southern Finland have a strong , nearly E – W trend."
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"The bedrock of Finland belongs to the Fennoscandian Shield and was formed by a succession of orogenies during the Precambrian. The oldest rocks of Finland, those of Archean age, are found in the east and north. These rocks are chiefly granitoids and migmatitic gneiss. Rocks in central and western Finland originated or were emplaced during the Svecokarelian orogeny. Following this last orogeny rapakivi granites intruded various locations of Finland during the Mesoproterozoic and Neoproterozoic, especially in the Åland Islands and in the southeast. Jotnian sediments occur usually together with rapakivi granites.
Mountains that existed in Precambrian time were eroded into a level terrain already during the Late Mesoproterozoic. With Proterozoic erosion amounting to tens of kilometers, many of the Precambrian rocks seen today in Finland are the "roots" of ancient massifs.
As Finland is in the older part of the Fennoscandian Shield, its basement rocks are within three of the shield's older subdivisions known as domains: the Kola, Karelian and Svecofennian domains. This subdivision, established by Gaál and Gorbachev in 1987, is based on the different geological histories of the domains prior to their final amalgamation 1,800 million years ago.
Svecofennian Domain -
Migmatitic rock outcrop in {Kaivopuisto Park, Helsinki}. The lighter parts of the outcrop are granite and the darker parts are mica schist.
"Migmatite is a rock that is a mixture of metamorphic rock and igneous rock. It is created when a metamorphic rock such as gneiss partially melts, and then that melt recrystallizes into an igneous rock, creating a mixture of the unmelted metamorphic part with the recrystallized igneous part. They can also be known as diatexite.
Migmatites form under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks. Migmatites are not crystallized from a totally molten material, and are not generally the result of solid-state reactions. Commonly, migmatites occur within extremely deformed rocks that represent the base of eroded mountain chains, typically within Precambrian cratonic blocks.
Migmatites often appear as tightly, incoherently folded (ptygmatic folds) dikelets, veins and segregations of light-colored granitic composition called leucosome, within dark-colored amphibole and biotite rich material called the melanosome. If present, the mesosome, intermediate in color between a leucosome and melanosome, is mostly a more or less unmodified remnant of the original parent rock (protolith). The light-colored material has the appearance of having been mobilized or molten."
SOURCE - (visit link) The southwestern part of Finland is mainly made up of rocks of the Svecofennian Domain or Svecofennian orogen. These rocks are invariably of Proterozoic age. Its boundary with the Karelian Domain (of mixed Archean and Paleoproterozoic rocks) is a northwest-southeast diagonal. Plutonic rocks that formed during accretion of volcanic arcs or continental collisions of the Svecofennian orogeny are common in Svecofennian Domain. Among these rocks the largest grouping is the Central Finland granitoid complex covering up much of Central Finland, Southern Ostrobothnia and Pirkanmaa. Granitoids that intruded in the aftermath of the Svercofennian orogeny are common in southern Finland occurring mostly within ca. 100 km of the Gulf of Finland or Lake Ladoga. These so-called Lateorogenic granites are distinguished by usually containing garnet and cordierite and being accompanied by rather few rocks of mafic and intermediate composition. Scattered small granitoids crop out within the same zone. Formed 1810–1770 million years ago, these are the youngest granitoids in southern Finland associated with the Svecofennian orogeny."
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The orange/brown granite boulder is an 'erratic' of Rapakivi granite.
"Rapakivi granite is a hornblende-biotite granite containing large rounded crystals of orthoclase each with a rim of oligoclase (a variety of plagioclase). The name has come to be used most frequently as a textural term where it implies plagioclase rims around orthoclase in plutonic rocks. Rapakivi is Finnish for "crumbly rock", because the different heat expansion coefficients of the component minerals make exposed rapakivi crumbly.
Rapakivi was first described by Finnish petrologist Jakob Sederholm in 1891. Since then, southern Finland's rapakivi granite intrusions have been the type locality of this variety of granite."
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The origin of this erratic boulder is in eastern Finland, in the Wiborg rapakivi granite batholith east from here.