Petrology of the upper border series of the skaergaard intrusion

The Upper Border Series of the Skaergaard intrusion consists of a 960 m thick sequence of rocks that crystallized against the roof of the magma chamber. The texture and composition of the unit vary systematically from top to bottom as a result of changes that occurred in the magma during the solidification of the intrusion. The order of crystallization of primocryst minerals in the Upper Border Series was: olivine; + plagioclase; + apatite; + ilmenite; + magnetite; + Ca-rich pyroxene;-olivine; + olivine; + ferrobustamite. The major silicate phases vary from high-temperature compositions to low-temperature compositions with increasing distance from the upper contact. Post-crystallization re-equilibration has affected the compositions of the oxide minerals and to a lesser extent the compositions of olivine and Ca-rich pyroxene. The Upper Border Series differentiation sequence differs from the Layered Series sequence, in that: (1) apatite appears much earlier; (2) magnetite precipitated before Ca-rich pyroxene rather than after it; (3) orthopyroxene is much less common; (4) the plagioclase is systematically poorer in K₂O; and (5) the rocks are systematically richer in K₂O and SiO₂. The upper part of the Skaergaard magma appears to have been enriched in H₂O, K₂O, SiO₂, and P₂O₅ relative to the part that was parental to the Layered Series.