LOCH-DOON GRANITE - EXAMPLE OF GRANITE PETROGENESIS IN THE BRITISH CALEDONIDES

The Loch Doon igneous complex lies to the N of the Solway-Shannon Iapetus Ocean suture line through the British Isles, but in a region lacking the Proterozoic continental basement which is known to exist further N. It comprises a genetically related magma suite ranging from hypersthene diorite margins through quartz monzonite to granite at the centre. Petrochemical data, studies of xenolith and autolith densities and field radiometric mapping indicate that the suite lacks sharp internal boundaries but is gradational and based on a monzodiorite parental magma which evolved through a two-stage crystal fractionation process. The early crystallization of dioritic margins was dominated by pyroxene-plagioclase precipitation and these minerals were joined later by biotite as acidic residual liquids developed. Biotite formation and fractionation at the expense of pyroxenes is related to contamination of the magma by water from the roof zone. Partial assimilation of roof material, particularly in the form of xenoliths, and mixing between cumulates and residual liquids are also recognized but are of relatively minor importance. These petrochemical, geophysical and isotopic studies of the Southern Uplands granites indicate the generation of parental magmas by the partial fusion of metabasaltic lower crust or crustal underplate as a late event in the evolution of the Iapetus Ocean. © 1979 The Geological Society.