The field relationships and mineral chemistry of mafic and ultramafic intrusions in the Loch Uisg area, Isle of Mull

• Main Author: Westland, James Alexander
• Format: Thesis
• Language: English
• Published: 2014
• Subjects: QE Geology; Baffin Island; Mull; Baffin
• Online Access: http://theses.gla.ac.uk/5482/; http://theses.gla.ac.uk/5482/1/2014WestlandMSc.pdf

The Loch Uisg Granite-Gabbro Intrusion, previously known as the Loch Uisg Granophyre and Gabbro, is a 2-component intrusion, which crops out in the south-east of the Island of Mull. It was first described in the Mull Memoir of the BGS (Bailey et al, 1924). It comprises an extensive granitic component and two smaller gabbroic units, intruded into folded basaltic lava country rocks. The latter gabbroic units are slightly different to one another in terms of their field characteristics and their mineral chemistry, but are thought to comprise one gabbroic intrusion. The granite and the gabbro were re-mapped and samples taken for optical and SEM analysis. As a result of this work, a new model for emplacement is proposed based on field relationships of the units and the mineral chemistry. Interaction between the felsic and the mafic components is considered in detail. By detailed examination of the field relationships and the mineral chemistry it is proposed that the granite was intruded first and that the gabbro was intruded soon after. Intruded into the Loch Uisg Granite-Gabbro Intrusion and the surrounding basaltic country rocks are two very different suites of mafic and ultramafic rocks. These are described in this work as the “Loch Uisg Picrite Suite” and the “Miscellaneous Minor Intrusions”. The Loch Uisg Picrite Suite comprises three intrusions: an irregular dyke-like intrusion and two sill-like intrusions of very fresh picrite, rich in magnesian olivine (up to Fo91.5). These figures place the LUPS rocks in the same category as the Baffin Island Picrites, the Horingbaai dykes from Namibia, the Gorgona komatiites, and the M9 dyke of Rum. Due to the highly magnesian nature of these rocks, these intrusions are unusual within the British Palaeogene Igneous Province. The dyke-like body, known as the Gleann Beag Intrusion after its location, displays very pronounced igneous layering. Layering is also present in the sill-like bodies. The mineral chemistry is described in detail and a petrogenetic model is proposed for the origin of these unusual rocks. Because of the unusual nature of these rocks, the igneous history of SE Mull has been re-assessed. It is proposed that ultramafic activity in SE Mull recommenced at a late stage in the overall history of the Palaeogene magmatism. Due to the freshness of the LUPS rocks, it is proposed that the emplacement of the intrusions of this particular suite may represent some of the very last igneous events in Mull. The third suite of mafic rocks is composed of several altered medium to coarse-grained dykes and dyke-like bodies which cross the area, broadly following the regional NW-SE trend where they are found cutting the Loch Uisg Granite-Gabbro Intrusion and the basalt country rocks towards the eastern end of Loch Uisg. Their geographical extent is limited to a small area of approximately 1 km2. They are distinct from the dykes of the Mull Dyke Swarm and display a wide variety of textures, mineralogy and grain size. The field relationships of these dykes are described and optical and SEM analysis provides an insight into the makeup of these rocks. All the members of this dyke suite have been extensively altered by hydrothermal action. It is proposed that they were intruded in a similar way to the Mull Dyke Swarm, from a similar basic to ultrabasic source magma. An explanation is given for the difference in grain size and mineral chemistry between these “Miscellaneous Minor Intrusions” and the dykes of the Mull Dyke Swarm.