Geological Survey of Finland Bulletin 289 - arkisto.gsf.fi

12 Geological Survey oE Finland, Bulletin 289
Fig. 10. Detail frorn Fig. 9; the picture shows quartz crystals in the dyke
where it has become wider and rnelted quartz blobs in the country rock
(no. 225).
A further example of how the diabase magma has penetrated the country rock
can be seen on the island of Sälgören, NE of Kumlinge (Fig. 14). Here a 1-2
cm broad apophysis, forming part of diabase no. 150, has created a grey reaction
rim a few millimetres in thickness in the surrounding gneissose granite (Fig. 15) .
The diabase dyke itself is not marked by a similar rim, only the apophysis.
Multiple intrusions
In some of the dykes the diabase magma has forced irs way into the same
crack on several occasions. This has led to differing results. One is that parallel
diabase dykes have been formed that share the same fissure and have chilled
margins against each other (Figs. 3d, 11 and 12). The chilled margin wirh
neighbouring diabase dykes can be up to 10 cm wide and 1S quite comparable
wirh the chilled margin between diabase dykes and the surrounding rock. In
a few ca ses there is a narrow rim of country rock between the two phases
(Fig. 3d).
A further result of multiple intrusions are different kinds of "composire
dykes" resulting from different intrusion phases and built up concentrically within
each other in the same fissure (Figs. 12 and 14 ). On Rosmunklobb NE of
Kumlinge there is a diabase dyke some 8 m broad which consists of two intrusion
phases. Along the contacts of the diabase there is a medium-grained subophiric
type of diabase that becomes more coarse-grained farther from the contacts. This