Evolution of the Kap Edvard Holm complex: A mafic intrusion at a rifted continental margin

The Kap Edvard Holm Layered Series forms part of the East Greenland Tertiary Province, and was emplaced at shallow crustal level (at depths corresponding to a pressure of 1-2 kbar) during continental break-up. It consists of two suites: a gabbro suite comprising olivine and oxide gabbros, leucocratic olivine gabbros and anorthosites, and a suite of wehrlites that formed from the intrusion of the gabbros during their solidification by a hydrous, high-MgO magma. Ion microprobe analyses of clinopyroxene reveal chemical contrasts between the parental melt of the wehrlite suite and that of the gabbro suite. Thin sills (1-2 m thick) of the wehrlite suite, however, have clinopyroxene compositions similar to the gabbro suite, and were formed by interaction with interstitial melts from the host layered gabbros. All evolved members of the gabbro suite have elevated Nd, Zr and Sr concentrations and Nd/Yb ratios, relative to the melt parental to the gabbro suite. These characteristics are attributed to establishment of a magma chamber at depths corresponding to a pressure of 10 kbar, where melts evolved before injection into the low-pressure magma chamber. Anorthosites of the gabbro suite are believed to have crystallized from such injections. The melts became supersaturated in plagioclase by the pressure release that followed transportation to the low-pressure magma chamber after initial fractionation at ∼10 kbar. The most evolved gabbros formed by subsequent fractionation within the low-pressure magma chamber. Our results indicate that high-pressure fractionation may be important in generating some of the lithological variations in layered intrusions.