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

Synopsis 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. REE profiles and chondrite normalised ‘spidergrams’ exhibit a marked parallelism for the dolerites and teschenite, with ratios of incompatible trace elements being almost constant throughout differentiation. Both petrographic and geochemical data demonstrate that differentiation was controlled by early crystallisation of olivine and plagioclase, with clinopyroxene having little influence on the chemical evolution of the boss. The syenites represent residual liquids which were segregated from the partially crystallised dolerites and teschenite by dynamic processes. Differentiation occurred in situ at low pressures, with P H 2 O increasing with increasing degree of differentiation. This boss is a type example of the low pressure crystallisation of an alkali olivine-basalt magma.