GOHMISSION GEOLOGIQVE - Arkisto.gsf.fi

Suomen Geologinen Seura. N:o 12. Geologiska Sallskapet i Finland. 39
SiO, ..............................
Al,Os .............................
Fe,O, .............................
FeO ...............................
MnO ..............................
MgO ..............................
CaO ...............................
Na,O .............................
K,O ..............................
TiO, ..............................
H,O+ ............................
On the other hand, it seems quite clear that the present low-temperature
mineral facies must be due to a retrogressive or diaphtoretic
metamorphism. The texture of the rock is characteristically granoblastic
and equigranular, like that commonly observed in the Archaean
amphibolites, with the difference that it is albite and not labradorite
that forms the honey-combed pavement. What has escaped epidotization
of the hornblende is exactly the common green variety found
in ordinary amphibolites. Thus it is the amphibolite metamorphism,
and not the greenschist metamorphism that has obliterated all the
primary features of the rock, and it may well have been a spili*, as
its bulk composition seems to indicate, before ih became an amphibolite.
QRANITE.
Granite occurs west and south of the greenschist ärea. In the
southern outcrops (16-22) the rock is reddish grey, medium-grained
and slightly gneissose microcline-biotite-granite. The amount of
microcline decreases towards the quartzite. The rock of the western
outcrops (1 and 2) is light grey in colour and rather poor in microcline.
The plagioclase in both groups of outcrops is almost the same, An,,.
The biotite is dark brown as usual in granites. This granite area continues
farther southwards.
The wider surroundings of the quartzite area are largely underlain
by migrnatites in which the older portions consist of various micaceous
gneisses and the granitic veins are pegmatitic. As mentioned above,
pegmatitic dikes intrude from all sides into the mantle-rocks around
the quartzite, and on Rautakankara they have also succeeded in pen-