Gambles, Peter Richard (1984) The geology of the Dragsmark granites, Bohuslan, south west Sweden with emphasis on the nature and origin of their alkali feldspar megacrysts. Doctoral thesis, Keele University.

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The Dragsmark granites comprise a number of sub-concordant sheets within the Stora Le -Marstrand gneiss belt of south-west Sweden. Field evidence, fluid inclusion studies and geochemical restite-melt modelling indicate that they were intruded as wet near-eutectic magma, containing abundant crustally derived xenocrystic material (mainly plagioclase, garnet and sphene), at a depth of c.12 km. Their intrusion probably occurred between 1400 & 1200 Ma ago, a minimum age limit being provided by cross-cutting basic Orust dykes which form a regional chronostratigraphic marker separating the Svecofennian and Sveconorwegian orogenies. The most distinctive feature of the Dragsmark granites is the presence of abundant potassium feldspar megacrysts; such 'augen' granites are widespread in the Sveconorwegian Province.
Populations of megacrysts from the main, lowermost, sheet and the smaller overlying bodies show differences in composition, especially in Ba content. Crystals from the former have roughly constant K/Na, but varying Ba and Rb; samples from the latter show constant K/Ba and K/Sr ratios over a wide range in orthoclase content. Individual megacrysts from the min intrusion show an overall decrease in Ba content from core to rim, on which is superimposed a 100, scale oscillatory zoning of the K/Ba ratio. The primary Al/Si distribution (structural state) of such megacrysts was more disordered than that of the groundmass; the reverse holds true for samples from the upper sheets.
These data favour an interpretation of dominant crystal-magma fractionation processes in the main body and bulk rock fluid interaction in the overlying sheets. The conclusion is drawn that most megacrysts in the former originated as phenocrysts whilst those in the latter are mainly porphyroblasts.

Item Type: Thesis (Doctoral)
Subjects: Q Science > QE Geology
Divisions: Faculty of Natural Sciences > School of Geography, Geology and the Environment
Contributors: Park, RG (Thesis advisor)
Samuelsson, L (Thesis advisor)
Depositing User: Lisa Bailey
Date Deposited: 14 Nov 2019 16:43
Last Modified: 14 Nov 2019 16:43

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