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Suomen Geologinen Seura. N: 0 35. Geologiska Sällskapet i Finland. 89 However, it could also be assumed that during the formation of the Karelidic mountain zone or soon after, the Karelidic earth movements gave rise to the formation of the crack zones westward of them, which were <strong>fi</strong>lled with the magmas of the hypersthene rocks. 7) An age determination of two of the hypersthene rocks, pyroxenegranite from Vaarslahti, NE of Pielavesi, and pyroxene granite from Petäjävesi, has recently been published by Wetherill, Kouvo, Tilton and Gast (1962), according to which the age of the hypersthene rocks is 1730-1760 million years. The age of the Svecofennides is given as 1 800 million years. In an earlier publication by Kouvo (1958) the age of the rapakivi has been given as 1 620 ± 20 million years. The age of the Karelides is given by Wetherill, Kouvo, Tilton and Gast (1962) as 1 800 million years. Earlier all Finnish geologists considered the age of the Karelidic movement as less than that of the Svecofennian movement. On account of the age determinations which give ab out the same age for the Svecofennides and the Karelides, some authors have considered both the Svecofennian and the KareIian mountain chains as belonging to the same period of mountain formation; the Kalevian, JatuIian, and Onegian formations (the whole of the KareIian) would then appear to correspond to a Flysch-period of the Svecofennian. This is not the place for a discussion of these important age problems. Here I wish only to point out that the hypersthene rocks are about 100 million years younger than the principal part of the remai~ing rocks of the Svecofennidian movement, and that there are indications that they are older than the Karelidic movement. Also it must be remembered that an uncertainty of 10 to 15 per cent in the age determination of rocks as old as the Svecofennian would me an an uncertainty of about 180-270 million years, which is a longer period of time than that required by some great mountain chains to form. It therefore seems premature to state, considering of present methods of making age determinations, that there had not been aperiod of 200 to 300 million years between the formation of the Svecofennian and the Karelian mountain chains and that the belt of the hypersthene rocks was not formed du ring this interval. Besides the hypersthene rocks dealt with above, there occur in Central FinIand two other series of igneous rocks from the same period, aseries of mnakites» and aseries of apIitic granites, which are described in the following. 14 8996-63
90 Bulletin de la Co=ission geologjque de Finlande N: 0 212. THE OCCURRENCE OF UNAKITE IN CENTRAL FINLAND In his descriptions accompanying the map sheets of Kajaani and Kuopio Wilkman (1931, 1938) has described a great number of occurrences of these peculiar albite-orthoclase and epidote-chlorite rocks, which he called >)Unakites,>) since such rocks had earlier been described by Bradley (1874). Mäkinen had earlier described similar rocks from the Ostrobothnian coastal district, calling them »natron syenites». The name >)Unakite>) was given by Bradley in 1874 to granitic rocks containing orthoclase, epidote, quartz and magnetite, from the Great Smoky Mountains of the Unaka Range of the Blue Ridge between North Carolina and Tennessee. He considered the rocks to be secondary and formed from mica granite or hornblende syenite. Later Phahlen (1904) described similar rocks from Milam's gap on the summit of Blue Ridge, and mentioned as constituents red feldspar and green epidote; quartz, iron oxide, zircon and apatite are mentioned as other constituents. Phahlen (1904) likewise regarded these rocks as secondary after »hypersthene akerite» (probably corresponding to the »hypersthene granodiorites» of this paper). Similar rocks were also described by Laitakari (1918), who called them »helsinkites», as weIl as by Asklund (1925) and by von Eckermann (1925), who also applied the name helsinkite to occurrences in Sweden. Later, however, all the Finnish authors who have written about these rocks have called them unakites, considering them to be primary, deep-seated igneous Tocks. In describing the rocks of the Kajaani sheet, Wilkman(1931) gives the following description of the unakites (original text in Swedish): »The principal constituents of this curious type of rock are alb i t e, iron-rich e p i d 0 t e and green chlor i t e (pennine). Besides these minerals the rock mostly contains microcline, microcline perthite and quartz in varying quantities, and as accessory components magnetite, titanite, ilmenite, apatite and hematite. Common hornblende and biotite do, however, not occur in typical unakites.» THE CONDITIONS OF CRYSTALLIZATION OF THE UNAKITES, AND VARIOUS OBSERVATIONS ON CERTAIN OCCURRENCES Wilkman (1931) writes (original text in Swedish): »On account of the content of albite and the minerals epidote and chlorite, which contain water, it is generally believed that these rocks have crystallized from magmas rich in water at a comparatively low temperature (possibly below 440°) and at a high pressure.»
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