Migmatites and migmatite-like rocks of Olkiluoto

Bedrock of the Olkiluoto Island in the western end of the Palaeoproterozoic Svecofennian Accretionary Arc Complex, SW Finland is composed of high-grade metamorphic pelites, arenites and intermediate, arc type metavolcanic rocks intruded by granodioritic to tonalitic plutonic rocks. Regional metamorphism culminated with voluminous migmatization in the temperature of 660 - 700 deg C and relatively low pressure of about 3.5 - 4 kbar. The end result of polyphase metamorphism and deformation is a metamorphic rock succession composed of diverse migmatite rocks, metatexites and diatexites. Metatexites are migmatites in which several, discrete components can be detected, and in which the paleosome with some pre-partial-melting textures is identifiable. Diatexites are more advanced migmatites in which the pre-migmatization structures are often totally destroyed and the rock is dominated by different neosome components meaning leucosome, melanosome or mesosome. Based on the migmatite structures the metatexites of Olkiluoto have been classified into six subgroups. Dike-structured metatexites are composed of well preserved paleosome intruded by one single set of narrow, subparallel leucosome dikes which cover ca. 5 - 10 % of the whole rock volume. Net-structure is composed of a network of narrow leucosome dikes which show a reticulated structure in a plane section and cover less than 30 % of the whole rock volume. Breccia-structure is composed of angular or rounded paleosome blocks surrounded by moderate amount of leucosome. Patch-structure is composed of irregular leucosome patches which intruded the well preserved paleosome and compose typically 20 - 70 % of the rock volume. Layer-structure is characterized by more or less regular leucosome dikes sub-parallel to the foliation of the well preserved paleosome. Vein-structured metatexites and also diatexites include a set of pipe-like, longish leucosome veins most probably generated by synchronous melting and deformation, folding or ductile shearing. Diatexites of Olkiluoto fall into four subgroups. Schollen-structure is a combination of roundish paleosome blocks or rafts and abundant mass of neosome which totally envelopes the paleosome blocks. Dark, typically melanocratic schlieren or flame-like, narrow and longish blocks of mafic restites are the crucial elements of schlieren-structured diatexites. Nebulitic structure is typical for highest-grade diatexites in which only diffuse relicts of pre-existing rocks are scarcely distinguishable in the abundant neosome mass. Homophanous structure is typical for migmatites dominated by non-segregated mesosome. It resembles the texture of plutonic rocks but can be identified on the grounds of protolith relicts or other restites. Everyone of the above mentioned migmatite types can be found all over the area of Olkiluoto Island but diatexites are more typical for the diatexite unit in the SE part of Olkiluoto whereas different metatexites dominate the veined gneiss unit in the NW part of the Island. (orig.)