EVOLUTION OF THE SKAERGAARD MAGMA - EVIDENCE FROM CRYSTALLIZED MELT INCLUSIONS

Polymineralic aggregates composed of clinopyroxene, Fe-Ti oxide minerals, apatite and accessory K-feldspar, biotite, and amphibole are enclosed in cumulus plagioclase grains in the Middle Zone of the Skaergaard intrusion. The chemistry of the minerals in the aggregates, and the textural relations between the aggregates and the host plagioclase grains indicate that they represent inclusions of the contemporaneous melt of the Skaergaard intrusion. Through mass balance calculations a quantitative estimate of the melt composition for this level in the intrusion can be obtained, and this estimate confirms that the silica content in the Middle Zone melt was similar to, or possibly even lower than, the silica content in the initial Skaergaard melt, and relatively enriched in iron.