The Significance of the Hydrothermal Stage in the Formation of Igneous Rocks

Abstract

A group of eruptive rocks (weilburgites), widespread in the Lahn-region, is discussed in respect to its genesis. Their close relation to the keratophyres suggests a later mobilization of the keratophyric magma through the volatiles originating from the ascending diabasic magma (allopegmagenesis). In accordance with the character of the weilburgitic magma its ascent was accomplished by adaptation to the orogenic process (gradational ascent). Conditions of this kind might influence the magmatic evolution in several respects. They are intimately related to geological conditions. To them are referred the favouring of allopegmagenetic or even allomagmagenic rocks in Palaeozoic time, the possible difference in age between the beginning of ascent and the emplacement, the lack of correspondence in composition of some rocks with the composition of the originating magma. Thus, the question arises as to how far correlations may exist generally between the mode of magmatic evolution in space and time, the composition of the rocks and the geological age.