Sill propagation and climbing in layered crystalline host-rocks: Examples from saucer-shaped sills of the Faroe Islands

Mafic sills/dolerites, commonly occurring in layered sedimentary and crystalline settings worldwide, may occur as sub-lateral sheets or as saucer-shaped bodies. Values and distributions of Young’s Modulus within their ambient host-rocks determine their mode of emplacement. Current models on development of saucer-shaped sills depict either melt propagation from single sources along sub-lateral relatively weak layers, from which they abruptly climb/transgress through stronger layers at intervals, or they may evolve by radial melt propagation/intrusion from one or more sources, while gradually/continuously ascending/climbing through strong and weak layers alike. The first model invokes involvement of sill overburdens and overlying free surfaces, while the latter envisages closed igneous systems where host-rocks both above and below the advancing magmas are affected without involvement of overlying free surfaces. Margins of saucer-shaped sills, cropping out in the Faroe-Islands, offer new insight of sill evolvement in layered crystalline host-rocks. This study strongly suggests that the slightly upward-curving geometries of these sills stem from radial propagation/intrusion of thin magma fronts. Their melts likely propagated as lobes or thin magma-fingers in a mole-like fashion, without noticeably affecting the overlying free surfaces prior to the main inflation phases.