ARCHEAN GEOLOGY OF THE TURKU DISTRICT IN SOUTHWESTERN FINLAND

Abstract

The Turku area is situated in the islands and along the sea coast in southwestern Finland. On the northwest it joins the Kalanti area which has been mapped and described earlier (Hietanen, 1943). The oldest rocks are kinzigites (cordierite-garnet gneisses) and amphibolites, both highly metamorphosed and migmatized. Kinzigites are of sedimentary origin and show bedding locally. Amphibolites are volcanic or sedimentary as are also the finer-grained leptites which occur with them. The intrusive rocks are grouped as follows: (1) intrusives of the first cycle, (2) intrusives of the second cycle, and (3) the post-tectonic intrusives. (1) The oldest intrusives occur as concordant oblong bodies in the oldest rocks. The acidic members are poor in potash, belonging to the trondhjemite series. Intermediate and basic rocks are members of either trondhjemite or charnockite series. Texture is granoblastic. Remnants of older rocks and basic inclusions are common. In small bodies lineation and cleavage parallel the same structures in the country rocks. The cores of the larger plutons are structureless or show a vertical lineation. Many occurrences among charnockitic and trondhjemitic rocks are clearly formed metasomatically or metamorphically. The bulk of these rocks, however, show features common in igneous rocks. (2) The intrusives of the second cycle (the granite series) are rich in potash and penetrate the older rocks concordantly and also disconcordantly. The structures of the narrow bodies parallel the walls; the centers of the larger ones are structureless. (3) The youngest intrusives in the area are diabase dikes, anorthosite, and rapakivis. Migmatization and granitization of the oldest rocks are a result of joint action of long duration of (1) introduction of feldspars and quartz, and (2) metamorphic differentiation. The processes started contemporaneously with the folding and emplacement of the oldest intrusives and continued after the folding until emplacement of pegmatites of the red microcline granite series.