COMMISSION GEOLOGIOUE - Arkisto.gsf.fi

56 Bulletin de la Commission geologique de Finlande N: 0 212.
The information concerning the occurrence of orthoclase in Finnish
Precambrian rocks has also been proved by using X-ray methodb, and such
cases are the following: In the granulites of Lapland the potash feldspar
consists to some extent of orthoclase (Eskola 1952 and 1957); the potash
feldspar contained in the porphyritic granite at Petäjävesi, Central Finland
(Rouhunkoski 1957), and in the porphyritic granodiorites at Nokia,
Kuru and Höytämä, southern Finland (Matisto 1962) is orthoclase.
The instances mentioned are all concerned with either synkinematic
or postkinematic rocks. The only instance of orthoclase so far met with
in the latekinematic granites is that of the granite at Skräddarby in the
parish of Sipoo (Härme 1962).
NEW DISCOVERIES OF ORTHOCLASE IN FINNISH
PRECAMBRIAN ROCKS
Simultaneously with the work of Matisto (1962), who found the orthoclase
in the granodiorites of the areas around Tampere (see above), the
authors of the present paper have also had an active interest in this problem,
and at the Geological Survey of Finland ample material has been available
from different parts of Finland for petrologieal, mineralogieal, and X-ray
investigation. The main results of the authors' work are presented in this
paper, but, considering the preliminary character of the study from the
point of view of the actual regional distribution of orthoclase in Finnish
Precambrian rocks and of the petrological significance of these results, no
far-reaching conclusions have been drawn.
It should also be mentioned that Dr. Olavi Kouvo, formerly the mineralogist
ofthe Outokumpu mining company and now head ofthe age-laboratory
of the Geological Survey of Finland, has performed, in connection with his
work on the radio genie ages of Finnish rocks, a considerable number of
X -ray powder triclinicity determinations for potash feldspars from various
parts of Finland - predominantly from Carelian rocks. It is a pleasure for
us to acknowledge here our gratitude to Dr. Kouvo for his kindness in
placing all his triclinicity data, without any limitations, at our disposal.
The locations of the potash feldspars investigated for the present paper
are marked on the map of Fig. 1.
CENTRAL FINLAND
During the geological mapping of the area of the quadrangle sheet 2232
(Keuruu) of the map of Finland, in the village of Keuruu, an acid rock was
encountered containing well-formed and partly idiomorphic in sets of ortho-