THE FORMATION OF GHOST STRUCTURE AND STRATIGRAPHY IN A S-TYPE INTRUSION: THE VEGA GRANODIORITE UNIT OF THE VEGA INTRUSIVE COMPLEX, NORTH-CENTRAL, NORWAY

The Vega granodiorite is a medium-grained enclave-bearing biotite ± sillimanite ± garnet ± cordierite unit and contains magmatic layering that imitates the orientation of the surrounding host rock foliation. The granodiorite is part of the westward-tilted 475 Ma Vega intrusive complex and is a crustally-derived “S-type” intrusion that contains residual garnet and biotite as well as residual enclaves and xenoliths of calc-silicate, marble, quartzite and calcareous schist, all of which have host rock equivalents.

Granodioritic magmas intruded a N–NE-trending structural boundary which separates eastern and western structural domains. The mean metasedimentary host rock foliation strike and dip in the eastern domain is 21°/67°SE and the mean magmatic layering strike and dip in the eastern domain is 39°/49°SE. Poles to host rock foliation in the western domain lie along a cylindrical best fit with a strike and dip of 188°/63°NW and the orientation of the cylindrical best fit to poles to magmatic layering in this domain is 170°/55°SW.

Layering is defined by the modal abundance of biotite. Discontinuous, intensely layered zones extend along strike for more than eight km in sheet-like bodies less than one km wide. Retro-deformation of the tilted intrusion indicates that sheets formed in a sub-vertical orientation. Layering is interpreted to have formed by crystal-melt segregation in response to shear-related velocity gradients in residuum-rich magma.

Host rock xenoliths wider than 1 m and up to 1.5 km long are sparsely distributed and have long axes and internal foliation that are sub-parallel to layering. The mean xenolith foliation strike and dip is 66°/40°SE. Quartzite xenoliths were negatively buoyant with respect to granodioritic magma and indicate that most xenoliths could have foundered, but xenolith foliation orientations are similar to host rock foliation orientations, suggesting that they are nearly in place.

Ghost structure in Vega granodiorite was created as incremental emplacement of heterogeneous dikes and sheets formed layering along pre-existing host rock foliation. Ghost stratigraphy formed as host rock xenoliths where isolated by magma batches and frozen in place. Additionally, the paucity of xenoliths is consistent with the interpretation that most xenoliths likely sank as magma ascended.