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Title: The petrology and geochemistry of the Tertiary cone-sheet complex, Island of Mull, Scotland
Author: Thomson, Bonita Ann
ISNI:       0000 0004 2742 396X
Awarding Body: University of London
Current Institution: University of London
Date of Award: 1986
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The Mull Tertiary igneous complex is characterised by large numbers of cone-sheets of a wide range of compositions (45-75% 5i02). Over 200 samples have been analysed for major and trace elements. Some were selected for further detailed chemistry and isotopic analysis. These new data have been combined with recent publ ished and unpublished work of others to propose an evolutionary model for the complex. The petrology and geochemistry of each of the three centres is evaluated in turn. Centre I has a considerable number of the early acid and intermediate suite of conesheets as well as the earliest of the early basic conesheets. It appears that these two suites are chemically related by two main magma chamber processes, crystal fractionation and hybridisation of which the latter is the more important. On REE and isotope evidence the magmas of this centre are shown to have been contaminated by both Lewisian granulites and Moine meta_$ediments. Centre 2 is dominated by the effects of crystal fractionation though hybridisation is also evident. During this period the early magma-type was replaced by one which was moderately more enriched in incompatible elements. The evolution of this late magma-type is characterised by extreme Fe-enrichment (~18%) in the intermediate range, largely due to the late crystallisation of magnetite under conditions of lower f02 than previously. The Fe-rich nature of the late magma-type was firmly established by Centre 3 times. The composition of the cone-sheets from this centre is mainly restricted to ferrodiorite. It is suggested that frequent replenishment by more primitive magma, with crystal fractionation, buffered the magma composition. Mixing between this and any granitic magma was probably inhibited by density differences. Thus the complex resulted from three cycles of activity during which hybridisation was gradually replaced by crystal fractionation as the more dominant magma chamber process, oxygen fugacity levels fell and crustal contamination decreased with time. The basic magmas of the Mull cone-sheet complex are high in calcium and low in alkalis and thus similar to the Preshal Mhor magma-type found in other centres, dykes, and occasional lava flows of the British Tertiary Volcanic Province.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available