The Formation of a Flecky Gneiss and Similar Phenomena in Relation to the Migmatite and Vein Gneiss Problem

Abstract

The present investigation has been made to determine the genetic relationships of a few rather special rocks. These have a flecky appearance and have in common aggregations of dark minerals in limited areas surrounded by rims of more or less pure quartzo-feldspathic material. The three-dimensional form of these flecks is approximately ellipsoidal.

Despite the apparent cotectic composition of the leucocratic rims in relation to the system Q-Or-Ab(-H₂O), it could be shown that anatectic processes have not entered into the formation of the rims. The proofs of this are mainly based on the observed microtextural properties. The formation of the flecks has thus been shown to be one of metamorphic differentiation, a process presumed to be due to an anisotropic pressure distribution in material which, prior to fleck formation, had an anisotropic fabric. The consequence of the differentiation is that in certain parts of the rocks femic mineral components have been mobilized and later “precipitated” in more restricted areas (the dark cores of the flecks). Contemporaneously with this centripetal migration of material a migration primarily of alkali took place in the opposite direction to some extent.

The balance of materials calculations carried out to determine the relationship between the composition of the flecks and the host rock showed that the only demonstrable difference in composition concerned K and Si. Thus the flecks have a lower K content than the surrounding rock. This is interpreted as a consequence of fleck formation being associated with “export” of K. The cause of the freeing of K is that the fleck formation is accompanied by some new mineral formation.

The flecky rocks studied below and others of similar nature recorded in the literature show that the flecks of the type in question may occur in rocks with completely different mineralogical and chemical characters.

A consequence of the relationships demonstrated in this investigation is that many rocks regarded as migmatites according to Sederholm's and Scheumann's meaning are not in fact migmatites at all as their leucocratic portions, despite being granitoid in composition, have never been magmatic. In other words, the fact that a migmatite-like rock has a granitoid leucosome is no justification for concluding that it is a true migmatite. To identify a rock as a migmatite several criteria must be satisfied. In this connection the author has also given certain diagnostic criteria for different genetic types of veined gneiss.

In conclusion, the course of events most probably allowing the above-mentioned export of K to take place has been compared with similar processes which, during regional metamorphic and possibly even under ultra-metamorphic conditions, can give rise to a K export and can, therefore, be considered as a source of K in the big process which alkali “economy” in an orogenic zone constitutes.