The Crystallization of Intermediate and Acid Magmas at Ben Nevis, Scotland

The Ben Nevis igneous complex consists of a central mass ofrhyodacites surrounded by four plutonic intrusions. In orderof intrusion, these are: the Outer and Inner Quartz Diorites,which are complex rock types with varying chemical and mineralogicalcompositions; the Porphyritic Outer Granite; and the Inner Granite.The plutonic rocks crystallized under comparatively low pressureand thus contain no primary aluminou shornblende, its placebeing taken in all but the most acid rocks by primary pyroxene,which was commonly replaced by an alumina-poor hornblende atlower temperatures. The ferromagnesian minerals in the acidrocks are more magnesian than those in the more basic rocks.This is considered to be due to the partial pressure of oxygenremaining constant at an unusually high level as the magmasconsolidated, causing the minerals to be increasingly enrichedin magnesium as crystallization proceeded.