The Glas Bheinn Appinitic Complex, Glen Roy: a model for foliation development during emplacement

Abstract

Synopsis The Glas Bheinn Appinitic Complex forms part of a suite of Silurian calc-alkaline igneous intrusions exposed in the Glen Roy area. The complex comprises three main components: (1) horn-felsed schist and schist breccia; (2) quartzite and quartzite breccia; and (3) a suite of mafic and ultra-mafic, variably foliated and/or lineated appinitic intrusive igneous rocks. Emplacement of the complex post-dated the main tectonothermal history of the country rock, which is entirely of Leven Schist (Appin Group), and predated the intrusion of the Ben Nevis Dyke Swarm. A four stage model for the emplacement of the Glas Bheinn Complex has been erected: (1) development of a breccia pipe with minor amounts of ultramafic material; (2) compaction and consolidation of the breccia; (3) intrusion of the dioritic (appinitic) magma; and (4) crystallization and foliation development within the appinitic diorites. A detailed petrological study indicates that fabric development within the appinitic igneous rocks occurred in response to the heterogeneous collapse of a primocryst framework prior to full crystallization, probably in response to deformation associated with continued forceful intrusion. The residual melt expelled during this process migrated into regions of low strain and crystallized as leucocratic domains within the appinitic diorite. Initial results suggest that final equilibration of the plagioclase–K-feldspar–amphibole assemblage occurred at temperatures of c. 500°C (±60°C).