The radiometric age of the Reposaari granite and its bearing on the extent of the Laitila rapakivi batholith in western Finland

The Reposaari granite crops out in a small area in the northwestern corner of the Satakunta graben (Fig. 1). It is crosscut by Jotnian dolerites, but the relationship to the surrounding Svecofennian formations is obscure, as the contact is under the sea (Fig. 2). The rock is a reddish, coarsegrained potassium granite and resembles in texture the porphyritic granite at Kokemäki. The major constituents of the Reposaari granite are potassium feldspar, quartz and chlorite, with fluorite, zircon, apatite, amphibole and opaque minerals as accessories. Chemically the rock is similar to the biotite rapakivis of the Laitila batholith (Vorma 1976), which contain more S i0 2 and less CaO and N a , 0 than the main type of the Laitila rock suite. The area of Jotnian supergroup in Satakunta (cf. Hämäläinen 1987) forms a distinct minimum on the Bouguer anomaly map of the Geodetic Institute (1979). According to a gravimetric survey by Elo (1976) the maximum thickness of the sandstone-siltstone sequence filling the graben on a profile southwest of Pori is about 1.8 km. The lowest gravimetric Bouguer anomalies, about -55 mgal, occur at Reposaari on the northeastern fringe of the graben. These cannot be attributed to the sandstone alone, as its thickness should be 4—5 km. Moreover, in the fringe area of the graben there are inliers of Svecofennian rocks, dolerite dykes and outcrops of the Reposaari granite. It has been suggested that close to the surface of this area there is a large granite intrusion, possibly belonging to the rapakivi group. To solve the problem of the age of the Reposaari granite, U-Pb analyses on zircon were carried out at the Unit for Isotope Geology of the Geological Survey of Finland. The results indicate that this rock belongs without any doubt to the rapakivi suite of western Finland.