The Skaergaard Layered Series: I. Structure and Average Compositions

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Abstract
The 660 m thick Basistoppen sill is an Eocene, tholeiitic, layered intrusion emplaced in the upper part of the Skaergaard complex shortly after solidification of the Skaergaard magma. Despite its small size, the Basistoppen sill has one of the most extensive differentiation sequences known. The ranges of the solid solutions in olivine, plagioclase, and pyroxene from the Basistoppen are comparable to those in the Skaergaard and Bushveld intrusions. The rocks of the sill are orthocumulates composed of approximately 35% trapped liquid and 65% cumulus minerals and can be divided into zones based on changes in the cumulus mineral assemblage. From the base upward those zones are: a Gabbro Picrite Zone containing cumulus olivine, Fe-Cr spinel, and minor biotite; a Bronzite Gabbro Zone containing cumulus orthopyroxene, Ca-rich clinopyroxene, plagioclase, and minor Fe-Cr spinel; a Pigeonite Gabbro Zone containing cumulus plagioclase, Ca-rich clinopyroxene, pigeonite, magnetite, and minor ilmenite; and a Fayalite Diorite Zone containing cumulus plagioclase, Ca-rich clinopyroxene, magnetite, ilmenite, apatite, and olivine. The Basistoppen is overlain by a zoned granophyre sill that was most likely derived in part from the Basistoppen magma and in part from melted Precambrian gneiss. The excellent exposure, uncomplicated structure, good chilled margin, and lack of strong modal layering facilitate the calculation of a differentiation trend for the Basistoppen sill. During crystallization the Basistoppen magma became progressively richer in Fe, P, Na, K, Zn, Rb, Zr, La, Sm, and Th, became progressively poorer in Mg, Ca, Al, Cr, and Ni, and remained relatively unchanged in Si, Sc, and Sr through at least the first 90% of crystallization.

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