Petrology of the Marginal Border Series of the Skaergaard Intrusion

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. The lower MBS and LS crystallized from the same magma, but fractionation occurred at different rates on the walls and floor of the intrusion. The upper margin may have crystallized from a magma of modified composition and fractionated at rates different from that in the lower margin and Upper Border Series (UBS). Crystals on the floor and roof of the intrusion accumulated faster or more efficiently than on the walls. At any given stage of fractionation, crystals also accumulated against all sides of the magma chamber at about the same rate. Either the rates of cooling, crystallization, and crystal retention affected accumulation rates locally as functions of rock type and geometry of the walls, or these rates were largely independent of wall rock owing to buffering of conductive heat loss possibly to an envelope of hydrothermal fluid circulating around the crystallizing magma. The appearance or disappearance of cumulus minerals in the lower MBS occurs at higher structural levels than in the LS and at lower structural levels than in the UBS. These relationships together with cumulus mineral compositions indicate that magma at the margins was always somewhat less fractionated than that at the floor and roof of the chamber. It is proposed that these relationships reflect the combined effects of liquid and crystal fractionation of the magma within largely independent convection systems in the lower and upper parts of the chamber.