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

10 Geological Survey of Finland, Bulletin 289
N
Fig. 8. Diabase dyke
on Snäckö, SW of
Kumlinge, which has
intruded parallel to
the schistosity of the
surrounding gneisses.
Il conlains numerous
fragmnets of coun try
rock (no. 107). The
symbols show direclion
of strike and
dip (vertical) in the
country rock.
consistent tendency in relation to the bedrock (e.g. the Kumlinge-Enklinge area,
and the circular structure on Äva) _ In spite of the fact that many of the dykes
lie almost at right angles to the schistosity of the bedrock, this fact does not
seem to have affected the direction of the dykes to any great degree_ In some
ca ses the strike of the surrounding rock has influenced the appearance of the
diabases insoEar as the diabase magma has penetrated into the plane oE the
schistosity (Figs. 8 and 9) .
Contact conditions
As a rule the diabase dykes cut sharply through the country rock. Most oE
them are straight and have sharp parallel contacts. Many of the dykes have
a stepped structure with narrower parts in a different direction (Figs. 11 and 18).
This is c1early the result oE the diabase magma opening up neighbouring, en
echelon lying fissures that already existed and breaking up the rock lying between
the fissures. The linking crack has consequently been split open less when the
walls oE the main cracks have been forced apart. In many places that the diabase
magma has broken out of a parallel crack and into another before the end of the
crack. The result is then so-called "horned offsets" as in Fig. 4, for example
(see Currie & Ferguson 1970).
Approximately 11 per cent oE the diabase dykes contain fragments of the
surrounding rock. In all cases these xenoliths lie near the contact and consist
of local country rock. The xenoliths are sharp-edged and generally have undergone
litde change macroscopicall y.
In the majority of cases the diabases have a fine-grained chilled margin 3-10
cm broad wirh the country rock. It is not possible to prove any relation between
the width oE the dykes and that oE the chilled contact. In the fine-grained chilled