A microstructural investigation of the Portrush Sill, County Antrim, Northern Ireland: constraints on sill thickness and evidence for crustal assimilation

Abstract

The Portrush Sill of Northern Ireland, intruded into Lower Jurassic mudstones, has geometries of clinopyroxene-plagioclase-plagioclase three-grain junctions consistent with a maximum overall thickness of 170-180 m, although only the central ∼100 m of the stratigraphy is exposed on the mainland. The exposures of mudstones previously thought to represent either the roof of the sill, or a thin screen between two closely-related sills, are argued to be numerous isolated rafts within a single intrusion, in agreement with the profiles of maximum contact metamorphic temperature established by previous workers, and evidence of only localised chill zones. Emplacement of the Portrush Sill therefore occurred along multiple bedding planes in the Jurassic mudstones, resulting in the incorporation of numerous stoped blocks of pyrite-rich mudstone into the intrusion. The sill can be divided into three divisions, with the Lower and Upper Divisions characterised by uniform dolerite (with some olivine-rich layers in the lowest exposures of Lower Division). The Middle Division is defined by the presence of cm-scale mafic blobs set in a felsic matrix. The Middle Division is offset downwards relative to the morphological evolution of clinopyroxene, which changes from ophitic interstitial grains in the upper and lower exposed parts of the sill, to (commonly clustered) elongate primocrysts in the central ∼20 m of the entire body, consistent with similar rates of inwards-propagation of the solidification fronts at the roof and floor of the sill and concomitant fractionation of a magma originally saturated only in olivine and plagioclase. We argue that the bimodal character of Middle Division records cm-scale unmixing of two immiscible silicate liquids, as a consequence of significant assimilation of the pyrite-rich country rock. That it is offset from the centre of the entire body is perhaps a consequence of a greater concentration of (now completely assimilated) blocks of country rock in the lower parts of the remaining magma when Middle Division was solidifying.