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Title: The secondary minerals of the Tertiary lavas of northern and central Skye : zeolite zonation patterns : their origin and formation
Author: King, P. M.
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 1976
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Abstract:
The Lower Eocene lavas of northern and central Skye cover an area of 600 sq miles (1500 km2) and lie in a shallow, faulted oval basin with its long axis running northwest-southeast. The lavas contain an abundant population of secondary minerals including zeolites, clays, calcite and quartz. The amygdale minerals were studied at over 400 localities, the most abundant species, in order of frequency, are thomsonite, analcime, mesolite, chabazite, stilbite, calcite, laumontite, gyrolite, levyne and natrolite. Several zonal assemblages were recognised and mapped, each is named after an index mineral which it contains. With increasing depth in the lava pile the zones are : thomsonite-chabazite; analcime; mesolite; and laumontite. The original thickness of the lava pile has been reconstructed from the distribution and thickness of the amygdale mineral zones. The minerals occur in zones that are flat-lying and nearly parallel with the stratification of the lavas. Zeolitisation of a flow takes place a considerable period after eruption and not until at least a 1000 ft (300 m) of lavas has accumulated. Broadly the intensity of zeolitisation increases with depth, concomitant with increasing temperature. The secondary minerals are shown to have originated from the action of meteoric waters percolating through the basalt, dissolving some of its constituents and redepositing them in vesicles and joint planes. Bulk analyses from various flows show that there is a progressive change outwards from the interior of the flow of the Fe2O3:FeO ratio and the A12O3, CaO, Na2O, SiO2, MgO and H2O contents. These features are thought to reflect a post-consolidational redistribution of material. The overall chemistry moreover suggests that the introduction and removal of components is subordinate to a local redistribution of material. Evidence is presented to show that rock compositions (amounts of CaO, Na2O and SiO2 particularly) and the activity levels of components (aSiO2, aCaO:aNa2O) in solution exerts a strong control over the nature and composition of the secondary minerals. The lavas within the vicinity of the Cuillin intrusive complex show the effects of thermal metamorphism. Three amygdale mineral zones were recognised laumontite, prehnite, and epidote (each named after an index mineral) and appear to constitute an aureole about the complex extending outwards for an average of 4 miles (6.5 km). These concentrically arranged zones cut steeply across the lava stratigraphy and across the regional zeolite zones; and correspond to the area of O18 depletion about the central complex. The aureole probably developed at temperatures above 200 C.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.462088  DOI: Not available
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