Discussions of certain phases of the genesis of pegmatites
1, 2, 3, 4

Pegmatites have received considerable attention from petrologists and mineralogists during recent years, and many papers onv arious subjects pertaining to these multifarious bodies have been contributed to the geological literature. These contributions have brought out a wealth of new details regarding the minerals and mineral associations of pegmatites; they have widened the field of study by including new types and thus made possible a comprehensive system of classification of pegmatites; they have also to some extent modified the genetic theories, especially by emphasizing the late-magma tie phases of the genetic processes; but they have not ,so far, produced any radical change in the generally accepted ideas of pegmatite genesis.

The majority of writers still agree on the magmatic origin of pegmatites and all seem to realize that the "life history" of pegmatites is one of evolution through several stages, varying conspicuously even within the same paragenetic group.

These leading ideas were established more than fourty years ago largely through the writings of W. C. Brögger.

Brögger's conclusions were based on studies of the syenite pegmatitesand granite pegmatites of Southern Norway. On simple evidence of geologic, petrographic, and mineralogic observations, Brögger succeeded in proving the magmatic origin of pegmatites and in establishing definite stages of evolution. Thus the magmatic, the pneumatolytic, and the hydrothermal stages were expressively mentioned as representing crystallizations at successively lower temperatures and with increasing influence of volatile substances. Recent studies by other investigators have elaborated, without actually changing, the genetic theory of pegmatites thus first clearly expressed by Brögger.

In the recent discussions of genetic processes the problems are dealt with chiefly from a geochemical point of view, the discussions being based on petrogenetic theories supported by mineralogic and petrographic evidence. With all these contributions, however, the subject has not been exhausted; even in the field of geochemical study of pegmatites there are numerous unsolved problems, and when considered from other points of view the pegmatites offer a richfield for study.

In the present paper some of these other problems will be briefly discussed. Special attention will be given certain genetic problems that can be most profitably discussed by laying stress on the structural features and on various aspects of the relationship between the pegmatites and their containing rocks: the mode of occurrence of the pegmatites. I will base the discussion on examples from the pre-Cambrian pegmatites of Southern Norway, more especiallythe area southwest of the Oslo region (largely "Sörlandet"), but the results may hold true for other pegmatites as well.

The more important points to be considered are the following:
1) Form and dimensions of pegmatite bodies and the attitude of these bodies especially in relation to structural features of the containing rocks or to geological features of the area in which they occur. 2) Distribution of pegmatites in various types of rocks with emphasis on the selective distribution of certain types of pegmatitein certain types of rocks. 3) Certain features of the structure of pegmatites. 4) A few types of paragenesis of pegmatites in comparisonwith the mineral association of the containing rocks. These points will not be treated in the sequence mentioned, but will be discussed at proper places in the general presentation of the subject.