Abstract
The Ilimaussaq alkaline intrusion in South Greenland provides examples of various types of igneous layering. Inch-scale layering occurs in the least differentiated rocks augite syenite and pulaskite and is probably caused by a nucleation-diffusion mechanism. Pegmatitic layering is found in the roof series of the agpaitic rocks (foyaite and naujaite) and is considered to have formed by upward concentration of volatiles under the downward crystallising roof. They indicate that the magma tended to become compositionally zoned. Isomodal layering is found in naujaite and especially in the latest rocks, the lujavrites. The origin of this type of layering is still an unsolved problem. Mineral graded layering is found in the floor series of kakortokite and in some lujavrites. It has mostly been studied in the kakortokite series, which provides one of the best known examples of mineral graded layering and macrorhythmic layering. The mineralogy of the kakortokite series is described, and theories of formation of the layering reviewed. Crystallisation probably took place in a stagnant bottom layer, and the layering is considered to be caused by different nucleation and growth maxima, in relation to the degree of undercooling, of the minerals in a multiply saturated magma. The mineral grading is enhanced by density during settling of the crystals.
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Sørensen, H., Larsen, L.M. (1987). Layering in the Ilimaussaq Alkaline Intrusion, South Greenland. In: Parsons, I. (eds) Origins of Igneous Layering. NATO ASI Series, vol 196. Springer, Dordrecht. https://doi.org/10.1007/978-94-017-2509-5_1
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DOI: https://doi.org/10.1007/978-94-017-2509-5_1
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