Quantitative studies of rock fabrics and textures in layered mafic intrusions of the British tertiary igneous province : implications for magma system emplacement and evolution

Abstract

This thesis examines and considers the processes which led to magmatic layering and associated planar and linear fabrics in mafic and ultramafic rocks of four igneous centres of the British (and Irish) Palaeogene Igneous Province. The intrusions studied are the Ardnamurchan Centre 3 Gabbros, the Rum Layered Suite, the Skye Druim Hain Layered Gabbros (all in NW Scotland) and the Carlingford Later Gabbros (in NE Ireland). The interpretations of layer formation have important implications for magma chamber processes, and for intrusion geometries and emplacement mechanisms. The study utilizes detailed field observations, modem methods of quantitative textural (Crystal Size Distribution; CSD) and rock microfabric (Anisotropy of Magnetic Susceptibility; AMS) analysis together with mineral-chemical data. AMS fabrics are typically carried by magnetite and are generally oriented parallel to visible macroscopic layer-parallel foliations. In the case of the Ardnamurchan and Rum intrusions, deformed structures in the layered rocks and consistently plunging orientations of magnetic lineations are interpreted as strong evidence for significant central subsidence of each body following initial emplacement. For the Ardnamurchan Centre 3 rocks, the combined evidence suggests that the intrusion is a composite lopolith and not a ring-dyke, as previously suggested. The close association of syn- magmatically deformed layering with inward-plunging magnetic lineations in parts of the Carlingford Later Gabbros suggests that this intrusion may also have undergone central subsidence, though less than at Ardnamurchan or Rum.