Structural features and emplacement of the late Svecofennian Perniö granite sheet in southern Finland

Abstract

The 1840-1830 Ma old Perniö granite occupies the southern margin of the Sauvo-Perniö granite area located in the western part of the late Svecofennian granite-migmatite zone in southern Finland. The S-type Perniö granite is light to dark red, medium- to coarse-grained with euhedral K-feldspar phenocrysts forming the porphyritic texture of the granite. The high grade supracrustal volcanic rocks and mica gneisses in the Sauvo-Perniö granite area show polyphase deformation. The D1 is characterized by isoclinal and intrafolial Fl folds. During the D2 the supracrustal sequence was thrusted towards the northwest. Combined ductile E-W shear movements and NNW-SSE compressional movements defined a transpressional tectonic regime during the D3 deformation. The Pernio granite intruded along subvertical mid-crustal feeder channels and was emplaced as a sheet or sheets along subhorizontal shear zones during the late stage of the F3 folding. After intruding into the shear zones the viscous granite magma was deformed. The K-feldspar phenocrysts in the granite acted as rigid particles in a viscous matrix and were rotated and imbricated in response to shearing along the subhorizontal shear zones indicating movements of the upper side of the granite sheet towards the west. Strike-slip dilatancy pumping is suggested as a possible mechanism for the emplacement of the Pernio granite.

Keywords

• granites
• phenocrysts
• gneisses
• structural analysis
• deformation
• folds
• shear zones
• emplacement
• dilatancy
• Paleoproterozoic
• Pernio
• Kemiö
• Finland