The felsites, granophyre, explosion breccias and tuffisites of the north-eastern margin of the Tertiary igneous complex of Rhum, Inverness-shire

Abstract

The explosion breccia of the north-eastern margin of the Tertiary complex has a steep northern contact but turns into a sheet to the south. It consists of blocks of either Torridonian or Lewisian age set in a groundmass of the same comminuted material. The tuffisites occur as thin sheets with varying attitude and consist of Torridonian fragments set in a comminuted matrix with blebs of devitrified glass. The felsites are closely associated with the explosion breccia whereas the granophyre cuts unbrecciated country rocks. The mineralogy and chemistry of the felsites and granophyre are shown to be very similar, and are believed to have crystallized from the same magma intruded into different environments. The evolution of the explosion breccias, tuffisites, felsites and granophyres is believed to be closely connected. Firstly it is suggested that the acid magma was generated by partial fusion of the Lewisian country rock; evidence coming from the nature of the thermal metamorphism in the presently exposed Lewisian, the general composition of the gneiss and the presence of a suitable heat source (the basic magma chamber responsible for the accumulative ultrabasic rocks). Secondly, the country rock is believed to have sunk slightly on an underground cauldron subsidence. The escape of volatiles from the acid magma into the breccia zone formed the explosion breccia. The tuffisites may have originated by a fluidization process within cone-sheet-like fractures, the transporting medium being largely water vapour from the acid magma chamber. Finally, the felsites and granophyre are probably different portions of the same magma. The felsite magmas were intruded into the explosion breccias, where they quickly lost their volatiles and chilled, whereas the granophyres were intruded into unbrecciated country rocks and cooled more slowly. Identical oscillatory zoning in plagioclase in both granophyre and felsite, is believed to have formed by relief of pressure, possibly when volatiles were escaping from the magma chamber to form the explosion breccias and tuffisites.