New Observations of Assimilation-Induced Silicate Liquid Immiscibility in the Portrush Sill, Northern Ireland

The Portrush Sill, located on the north coast of County Antrim, Northern Ireland, is a bowl-shaped Paleogene (c. 58.5 Ma) intrusion emplaced into Jurassic sediments rich in disseminated pyrite and pyritised macrofossils. We have undertaken a detailed remapping of the intrusion, coupled with a study of the stratigraphic variation of microstructure and geochemistry. The central part of the sill is characterised by a striking magmatic texture comprising centimetre–decimetre sized, rounded melanocratic regions (globules) set within a leucocratic matrix. The melanocratic globules vary in size and morphology through the stratigraphy. Globule - matrix pairs from individual samples, along with samples of the homogeneous over- and underlying parts of the sill, were geochemically characterised using XRF (major element oxides), ICP-MS (trace elements), and EPMA (mineral compositions). The globules and matrix are composed of the same mineral phases but in differing proportions: there is no indication of chemical disequilibrium, with the minerals having identical compositions in both globules and matrix. The petrographic and geochemical data, in conjunction with field observations, are consistent with the two components representing conjugate immiscible Fe- and Si-rich liquids produced as the evolving parent magma encountered a binode.

Cooling rates determined from the stratigraphic variation of clinopyroxene-plagioclase-plagioclase dihedral angles show that the sill intruded as a single body, in contrast to previous studies arguing for several separate intrusions separated by screens of sedimentary rock. Field observations show the magma intruding along bedding planes of the host rock, as well as evidence of significant anatexis and contamination of the proximal magma. Analysis of Sr–Nd–Pb isotopes indicates an increase in the extent of contamination towards sedimentary screens in the middle and upper parts of the sill. We infer that the onset of immiscibility, and the unmixing of conjugate Fe- and Si-rich liquids within the Portrush sill, was a consequence of assimilation of pyrite-rich country rock. This represents the first documented example of macro-scale assimilation-induced liquid immiscibility, with major implications for our understanding of magmatic evolution.