COMMISSION GEOLOGIOUE - Arkisto.gsf.fi

Suomen Geologinen Seura. N: 0 35. Geologiska Sällskapet i Finland. 11
CONCLUSIONS
The portions of the quartz diorite rich in mafics and devoid of microcline
that are observed in the outcrop and included in the granodioritic rock
most probably represent relicts of a rock that has undergone potash meta.somatism
and from which actual granodiorite and porphyroblastic granite
are derived.
This quartz diorite is by no means, however, the primary rock. The area
we are dealing with is situated in the central part ofthe Svecofennidic Range,
where old sediments and igneous rocks have been strongly metamorphosed,
even up to the point of more or less complete palingenesis.
Pelitic sediments and volcanic rocks were the primary rocks in the area
(more or less metamorphie relicts of them, such as the many amphibolitic
rocks and tuffites, are observed here and there) so that they could have
been the source of the added material necessary for explaining the widespread
occurrence of microcline and alterations like biotitization of amphibole.
During the stage of the regional metamorphism of this area, material
was mobilized, perhaps with formation of a melt, in which differentiation
processes could even have developed. I tentatively attribute to such proces
ses the many smaH intrusions of gabbro and dioritic rocks of the zone,
which are, judging from the manner of their occurrence, most probably
younger than the other rocks of this area.
The gabbro body east of the outcrop is one of such intrusions, and its
presence near the outcrop means that this place has been a zone of tension
or at least of less resistance.
The material the provenance of which was the old sediments (containing
K 2
0, AI 2 0 a , and Si0 2 ) has concentrated in an area of lower free energy,
such as the area in which the outcrop is situated. The result is the granitization
of apart of the rocks present in that area. The porphyroblastic granitic
rock is believed to belong to such a portion of rocks.
Many minerals of the rocks examined show that the outcrop area has
been affected by dynamic actions. The biotite is often deformed (Plate V,
Fig. 9); twinning lamellae in the plagioclase are bent (Plate V, Fig. 10) and
often the plagioclase is fractured, with fragments showing some dislocation,
too; the quartz and microcline are often fractured, the former displaying
a strong undulatory extinction.
Taking into account also the fact that some of the minerals occur as
inclusions in each other, a picture can be drawn of the probable sequence
in the granitization of the rocks examined: because of changed conditions
pyroxene begins to change into amphibole and this further into biotite. These
alteration products (amphibole and biotite) begin to include older minerals
like quartz and primary biotite. Drop quartz in altered amphibole represents