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

Ehlers, C. and Ehlers, M. 1977: Shearing and multiple intrusion in
the diabases of Aland arehipelago, SW Finland. Geological Survey
0/ Finland, Bulletin 289, 31 pages, 23 figures , one table and one
appendix.
A set of narrow diabase dykes runs in a NE·SW direetion out·
side the eastern eontaet of the Aland rapakivi massif. Chilled margins
between adjaeent dykes, eomposite dykes eonsisting of several
intrusion phases, and areal differenees in mineral eomposition show
that the dykes were intruded in pulses of slightly different ages .
Many dykes exhibit eataclastic right-hand shearing. A eonnection
between this shearing and the intrusion of the rapakivi granites is
suggested.
Chemieally the dykes are rieh in iron. Mineralogieally they
eonsist of hornblende and plagioclase with a subophitie texture.
It is suggested that the diabases intruded as a very liquid, waterrieh
magma whieh rushed into fissures, resultning in rapid quenehing
at the eontaets and slower erystallisation, possibly under rising
water-pressure, in the central parts of the dykes. The pressure builds
up until new fissures are formed and a new intrusion pulse is
initiated.
It is coneeivable that the diabase dykes are eonneeted to the
rapakivi granites but are a li ttle earlier in their intrusion beeause
of their lower viscosity.
The authors' address
Carl Ehlers
Mary Ehlers
Geologisk-Mineralogiska Institutionen
Abo Akademi
20500 Abo 50
ISBN 951-690-071-2
Helsinki 1977. Valtion painatuskeskus