COMMISSION GEOLOGIOUE - Arkisto.gsf.fi

14 Bulletin de la Co=ission geologique de Finlande N: 0 212.
SUMMARY
In the rocks examined, which have been affected more or less by granitization,
many interesting features are 0 bserved. Plagioclase occurs in two
generations, the porphyroblastic being the younger; its abundance shows
maxima in the intermediate members of the series (zone of gradual transition),
whereas the microcline shows minima. There are variations in the
relative and absolute abundance of some twinning laws in the plagioclase,
notably in the Albite-Carlsbad and Albite-Pericline ones, the former being
more abundant in granitic types where the latter disappears. This seems
somehow to be associated with magmatic-like conditions. Continuous alteration
processes (pyroxene-amphibole-biotite) lead to the assumption that the
granitic portion of the rocks should be regarded as the result rather than
the agent of the granitization. In this instance material has been added, as
can be deduced from the considerations and calculations regarding the
alteration processes, the occurrence and abundance of microcline porphyroblasts,
the amount of normal and normative feldspar, the number of cation
per standard cell and data from the Svecofennidic silicic synkinematic rocks.
Added material is believed to have concentrated in an area of lower free
energy like that in which the outcrop is situated and its provenance is
supposed to be metamorphosed old sediments and volcanic rocks (actually
granitic gneis ses and amphibolites) probably similar to the rocks of these
groups near the outcrop. Elements added are Na, K, Si and FeH , while
Mg, Ca and Al have been removed.
ACKNOWLEDGMENTS
The author wishes heartly to thank Dr. V. Marmo, Director of the
Geological Survey of Finland, for his most valuable criticism and counsel.
Thanks are due to Mr. Paul Sjöblom who corrected the English of the
manuscript.