THE NORTH ATLANTIC IGNEOUS PROVINCE
Crystallization processes in a Tertiary gabbro-norite intrusion, East Greenland, and implications for calculated equilibrium melt compositions

Stefan Bernstein

The quantitative modelling of fractional crystallization in gabbroic intrusions is pivotal to the understanding of precious metal mineralizations, such as the Au–Pd–Pt reefs in the Skaergaard intrusion, East Greenland.
To address crystallization processes in mafic plutons in more detail, a suite of rocks from a small plug at Courtauld Fjord in East Greenland has been studied. The plug consists of coarse-grained gabbro, lined by a chilled margin towards the host gneisses. Ion-microprobe analyses of clinopyroxene from the coarse gabbro are used to compare calculated equilibrium melts with the composition of the chilled margin in terms of REE, Zr, Sr, Y and a few major elements.
The result is a very good fit between the composition of the computed melts and that of the chilled margin. Closed system fractional crystallization accounts well for the observed major- and trace-element distribution in strongly zoned clinopyroxene and orthopyroxene crystals from the gabbro-norite.

Figure text
1. Photomicrograph of textures in gabbro-norite. a (XPL) group of zoned clinopyroxene crystals, radiating from a fine-grained cluster of clinopyroxene, olivine and plagioclase grains. Stippled lines show the typical zoning structure, and arrow gives direction of zoning from core (c) to rim (r). b (XPL) with analysed clinopyroxene crystal cpx#1, and showing euhedral orthopyroxene and subhedral olivine. c (PPL) chilled margin sample #9010. d (XPL) interstitial material between plagioclase laths in gabbro-norite #9011, with fine-grained intergrowth of clinopyroxene, plagioclase, Fe-Ti oxide, biotite, and apatite.

2. Variation in clinopyroxene chemistry (here Mg-Fe) with zonation in crystal cpx1 (see photo b).