Metamorphic mineral assemblages in fossils in the contact metamorphic rocks of the Portrush Sill, Northern Ireland

Abstract

Ammonites and other metafossils have been identified in hornfelsed Jurassic clay from Portrush, Northern Ireland. Thermal metamorphism due to intrusion of a large sill complex produced zoned high temperature mineral assemblages due to reaction between the calcite and calcite/pyrite shells and the clay matrix. Modelling of hornfels compositions using Theriak – Domino gives a minimum temperature for metamorphism of ~600 °C at 200 bars. The metafossils comprise an inner wollastonite-rich core that also contains pyrrhotite and fayalite in the ammonites (zone 1), surrounded by iron-rich wollastonite, hedenbergite, titanite and feldspar (zone 2). The matrix in contact with the metafossil (zone 3) comprises magnesian hedenbergite, plagioclase and potassium feldspar and matrix distant from the fossils (zone 4) comprises augite, plagioclase, potassium feldspar, fayalite, ilmenite and quartz. These assemblages developed by metasomatic exchange of silica, sourced from the matrix with CaO sourced from the calcite shells. The metamorphic transformations, in particular reaction of calcite with silica-bearing fluids to form wollastonite and transformation of pyrite to pyrrhotite result in large volume changes which may have distorted the shape of the fossils, nevertheless original morphological features, particularly of the ammonites can be detected.