Petrology of Anorthite-bearing Gabbroic Anorthosite Dykes in northwest Skye |
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| Authors: | DONALDSON COLIN H |
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| Institution: | Department of Geology, University of Manchester Manchester M13 9PL, England |
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| Abstract: | A group of gabbroic anorthosite dykes in northwest Skye containsmegacrysts of calcic plagioclase (An93–87) and olivine(FO87–78.5and plagioclase-rich xenoliths set in a matrixof low-alkali, high Ca olivine tholeiite. Field, mineralogicand petrochemical evidence indicates (1) that the dykes arefilled with crystals concentrated from magmas feeding lava flowsand (2) that, although the megacrysts crystallized from low-alkali,high Ca olivine tholeiite magmas at depths <4 km, the megacrystsand the matrix in each dyke are not cognate. The emplacementtemperature was less than 1230 °C. Low-alkali, high Ca olivinetholeiite is an abundant Hebridean magma type, resembling unfractionatedoceanic tholeiites from the Atlantic. The temporal change fromalkali olivine basalt to low-alkali, high Ca olivine tholeiitein the Hebridean proto-spreading centre may mimic compositionalchanges during initial development of the mid-Atlantic ridge. |
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