COMMISSION GEOLOGIOUE - Arkisto.gsf.fi

8 Bulletin de la Co=ission geologique de Finlande N: 0 212.
Moreover, plagioclase porphyroblasts retain the same composition throughout
the whole series (An abQut 30 %) and their abundance in series 1, 2, 3 has a
variation diagram similar to the total plagioclase amount, i.e., they are more
abundant in intermediate members.
Another feature of the plagioclase porphyroblasts is a clear albitic rirn
against microcline, as weil as myrmekitic structures occurring in some cases
abundantly and in weil-developed form.
Plagioclase porphyroblasts often include smaller plagioclase grains. which
are often included by many other minerals.
In both plagioclase generations a variation is observed in the occurrence
of certain twinning laws. While Albite twinning is common in every section
of each series, Albite-Carlsbad twinning is more abundant in the granitic
types, and Albite-Pericline twinning always becomes less and less frequent
until it disappears in the acid members. This was observed in each series.
As reported by Gorai (1950) and Turner (1951), the presence and relative
abundance of A-twinning (Albite, Pericline, Acline) in plagioclase is indicative
in some way of a metamorphic origin.
It may be supposed that the disappearance of the Albite-Pericline
ttwinning is somehow related to certain magmatic-like conditions. The concomitant
high abundance of Carlsbad twinning denotes, too, according to
Köhler (1949), high crystallization temperatures.
MICROCLINE
It occurs mostly as large porphyroblasts, more frequent in acid members
of the series; it shows perthitic structures, sometimes quite conspicuosly.
Perthitic structures present are: stringlet, rod, string and patch perthite, the
string type being the most frequent (Plates land H, Figs. 1-3). The microcline
is likely to include nearly all the other minerals and this probably
me ans that the microcline (and likewise potash) developed last in the rock.
The microcline shows a percentage minimum in the intermediate members,
where plagioclase has a maximum.
It must be no ted that a certain amount of the microcline is not porphyroblastic
and this would suggest the possibility of the existence of a
primary microcline, a view supported by the fact that this mineral can
occur as inclusions in plagioclase porphyroblasts (which are believed to have
preceded the introduction of potassium) and even in microcline porphyroblasts.
The abundance of microcline (and quartz) is shown in Fig. 8. The
shape of the diagram lines for the microcline roughly displays a concavity
in all the three series, a notable feature considering that the percentage of
microcline in the thin sections has been affected by a random and irregular