The differentiated dolerite boss, Cnoc Rhaonastil, Islay: a natural experiment in the low pressure differentiation of an alkali olivine- basalt magma

The Cnoc Rhaonastil dolerite boss is a differentiated minor intrusion elongated parallel to one of the four component sub-swarms of the Islay-Jura dyke swarm. Differentiation is from alkali olivine-dolerite to teschenite with minor nepheline-syenite locally. On the basis of field, petrographic, mineralogical and geochemical data, three distinct varieties of dolerite are identified, the most primitive of which represents the chilled margin of the intrusion, with a compositional continuum being evident for the boss as a whole. Differentiation occurred in situ at low pressures, with P_H2O increasing with increasing degree of differentiation. This boss is a type of example of the low pressure crystallisation of an alkali olivine-basalt magma. -from Authors