Abstract
Geological, petrographic and geochemical accounts of two of the basic units of the Insch Igneous Mass are provided. One, the Insch Differentiated Suite, consists of iron-enriched olivine-gabbro with up to 24% total iron-oxides, that passes upwards through syeno-gabbro into syenite, with over 7% potash. This suite is inter-preted as resulting from the seeping of low-temperature constituents towards the roof of the crystallising olivine-gabbro magma which, itself, may have been enriched in iron by the abstraction of refractory magnesium-rich phases from the regional gabbro magma. The second basic unit dealt with is the Bourtie Series of dunites, peridotites and troctolitic gabbros, all rich in magnesia, and interpreted as early cumulates. The possibility of the geological connection of the two units to make a wider differentiated series ranging from dunite to syenite is discussed. Whilst this interpretation is favoured, it is agreed that special geochemical and mineralogical investigations are necessary to test its validity. Thirty-seven new analyses are used in the petrogenetic discussions. A revised geological map of the whole Insch Igneous Mass is presented.
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