Modally-Graded Rhythmic Layering in the Skaergaard Intrusion

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Abstract
The Marginal Border Series (MBS) of the Skaergaard intrusion consists of rocks formed by in situ crystallization against the walls of the intrusion. Most of these rocks are products of fractional crystallization, though samples believed to represent chilled liquid occur locally at the intrusive contact. The MBS comprises only 5% of the exposed volume of the intrusion, but within its thickness, the order of crystallization and the compositions of fractionated rocks and minerals vary systematically with distance inward from the intrusive contact in largely the same manner as rocks and minerals upward through the Layered Series (LS). Earliest differentiates are cumulates of olivine and plagioclase. The most basic compositions of cumulus plagioclase (An72) and olivine (Fo84) in these rocks indicate that the amount of fractionation preceding formation of the exposed LS was substantially less that previously believed. Field and compositional data indicate that picritic blocks are xenoliths rather than cumulates of the Skaergaard magma. Xenoliths of gneiss in all stages of reaction are locally abundant; however, there is no evidence that upper crustal material contaminated the magma from which the MBS cumulates formed. Compositions of cumulus minerals in the MBS differ from those in comparable LS rocks. Cumulates in the lower margins contain more calcic plagioclase, more magnesian augite in all but the late differentiates, and more iron-rich olivine. The compositions of cumulus olivine and to a lesser degree those of other mafic silicates, were modified to more iron-rich compositions by re-equilibration with relatively large amounts of interstitial liquid.

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