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

Geological Survey of Finland, Bulletin 289 19
pressure the limes tone has been deformed and the diabase dyke broken into small
seetions along aseries of left-hand shear zones (totaloffset 17.5 m) Another
diabase dyke cuts through the same limes tone zone without being affected by
deformation. The deformation is, therefore, of the same age as the diabases while
some of the diabases are younger than this deformation. It is difficult to say
whether there is any connection between the deformation described by Suominen
and the general right-hand shearing in the swarms. However, it is conceivable
that the same forces that caused cataclastic shearing of certain of the diabase
dykes may also have given rise to shearing in the relatively easily deformed limestone.
The direction and type of shear fit in weIl as a conjugate direction of shear
to the predominant right-hand shearing of the diabases.
The fact that not all the diabases have been subjected to shearing may be
linked with the fact that they were presumably intruded at different times. Some
of the dykes are perhaps of younger age than the shearing, or perhaps much
older and more firmly "glued" to the country rock. Others were more recently
intruded and perhaps more easily deformed. If the pressure is released through
shearing along one dyke, then the strain on dykes in the vicinity is reduced so
that they may escape shearing. The multiple diabases with chilled margins against
each other also show that the diabases were intruded at different periods. This
was Hausen's (1964) assumption, too.
The vertical shear component shows that the country rock on the NW side
of the sheared dykes has been raised compared with the SE side. Shearing along
the diabase dykes is obviously of later date than the intrusion of the dykes.
Consequently, it is unlikely that there is any connection between the shearing and
the en echelon position of the dykes.
DESCRIPTION OF TYPES OF ROCK
Mineralogy
Macroscopically the diabases are fine to medium-grained, black-grey or blacgreen
dykes that cut sharply through the older types of rock. The characteristic
subophitic texture can often be distinguished with the naked eye on a weathered
surface. In the chilIed margins the plagioclase phenocrysts lie parallel to the
contacts and form a flow structure (Fig. 19). A few isolated dykes are rich in
phenocrysts of plagioclase (nos. 150, Fig. 14, 155 Fig. 12, and 158). A rather
higher proportion (approximately 10 per cent) have quartz and/or calcite-filled
amygdales about 1 mm in cross seetion. The subophitic texture is formed by
plagioclase laths of andesine-oligoclase composition some mm or less in length.
Between these there is a fibrous or fine-grained mass of green hornblende.