COMMISSION GEOLOGIOUE - Arkisto.gsf.fi

96 Bulletin de la Commission geologique de Finlande N: 0 212.
intersection, which all are here interpreted as a system of craks. These craks
probably opened under the pressure of great masses of magma not containing
much water-vapour, from which the hypersthene-bearing rocks were
crystallized. The unakites seem to have erupted from magma very rich in
water and volatiles. The representatives of these two magmas, which we
now find exposed at the surface, never reached the surface of the earth at
the time in question, i . e. about 1 700 million years ago as shall be explained
later on, but crystallized as deep-seated eugranitic )}hypidiomorphic granular
rocks)} at such places along the crack lines and near by in larger cavities
which they had managed to open for themselves. The magma of the
hypersthene granites and related hypersthene diorites erupting along the
crack system has produced some sixty fairly considerable occurrences of
hypersthene granites and associated rock types (granodiorites, quartz diorites).
This cracking of the rock ground did not, however, cease with the
crystallization of the principal part of the hypersthene granites, but continued
in some parts of the vicinity of the belt, causing some of the hypersthene
granite occurrences to become strongly brecciated. This is especially
the case where the important crack-line II crosses the present site of the
lake district of Haukivesi (Hackman 1931).
The eruption of the unakites seems, however, to have taken place in
quite a different way. They occur mostly a.t single points. Only in a few
places are they described as now forming short, broad dykes or as smaller,
narrow, often porphyritic dykes penetrating the already consolidated
hypidiomorphic unakite bodies (Hackman 1931). It is probable that the
unakites forced their way upwards in the earth's crust by some kind of
»stooping)} and ultimately also they crystallized when the pressure had
increased sufficiently to cause a complete crystallization and, at that time,
the formation of deep-seated rocks. Wetherill, Kouvo, Tilton and Gast (1962)
recently determined by the potassium-argon method the time at which
the crystallization of two of the unakite occurrences took place and found
a value of 1 700 million years. This is in accordance with the field observations,
according to which the hypersthene granites and also the unakites
evolved later than the post-Bothnian granites (about 1 800 million years
or more). There is further field evidence that the Wiborg (Viipuri) rapakivi
at Taipalsaari cuts the hypersthene granite. Further, it would seem that
the hypersthene granites are older than the Karelidian orogenesis since no
hypersthene granites or unakites have been observed inside the domains
of the remnants of the Karelides. They occur, however, in the old Archean
east of the main region of Karelidian rocks in Finland (Wilkman 1928).