The Metasomatic Rocks Associated with the Criffell-Dalbeattie Granodiorite

Abstract

The field relations and petrography of a number of small rock bodies occurring near Rockcliffe at the southern margin of the Criffell-Dalbeattie granodiorite are described. The evidence suggests that they were formed by the metasomatic replacement of greywacke and shale hornfelses by Na-bearing and later K-bearing solutions which preceded the ascending granodiorite magma. This metasomatic transformation brought about the development of two distinctive rock types. Equigranular quartz-dioritic rocks which generally grade into the hornfelses are regarded as the result of static metasomatic replacement of the hornfelses with no appreciable movement of the permeated rock. In contrast, rocks of similar mineralogy but occurring as sharply bounded bodies with a conspicuous porphyroblastic texture, are thought to have attained their characteristic features by a certain amount of metasomatic mobilization of the permeated material. Because of the intrusive appearance of these porphyroblastic rocks and their occurrence near the margin of the granodiorite pluton they are here referred to as “marginal porphyrites”.