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

Geological Survey oE Finland, Bulletin 289 13
N
o 2 m
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Fig. 11. Multiple diabase
dyke on Langären.
Two diabase
intrusions with a thin
wall oE gneissose
granite in between.
Typical "stepped
structure" where the
dykes have moved
between different
parallel pre-existing
fissures (nos. 179
and 180).
"marginal diabase" is about 0.5 to 1 m wide at each contact. Within this there
is a finer-grained type of diabase containing feldspat phenocrysts and isolated
quartz-filled irregular amygdales (Fig. 13). The contact between the fine-grained
"central diabase" and the coarser "marginal diabase" is sharp but there is no
chilled margin. The "central diabase" cuts into the coarse seetion of the "marginal
diabase" and is oE a later date. In this complex dyke there are aplitic blebs
(Fig. 13) and curving dyke sections that usually lie at right angles to the direction
oE the diabase and that do not continue into the country rock (Fig. 12).
Another example oE a "composite dyke" is that on Sälgören, Kumlinge (no.
150, Fig. 14). At its edges the dyke consists of a porphyritic diabase which has
also filled fissures round the dyke with apophyses (Fig. 15). The co re of the
dyke is made up of another, rather more coarse-grained non-porphyritic type.
The contact between the two types is sharp.
Intrusion mechanism
On the basis oE stepped dykes that intrude into suitable parallel fissure systems
that lie en echelon and jump in small steps from one fissure to another, Currie
and Ferguson (1970) have proposed an intrusion mechanism characteristic of
volatile and gas-rich magmas. They assurne that there must exist apressure head
along an intruding dyke. If the magma comes into contact with a fissure in the
surrounding bedrock or iE it creates a new fissure, the gases rush into the fis sure