AN OCEAN-RIDGE TYPE MAGMA CHAMBER AT A PASSIVE VOLCANIC, CONTINENTAL-MARGIN - THE KAP-EDVARD-HOLM LAYERED GABBRO COMPLEX, EAST GREENLAND

The gabbros of the Tertiary Kap Edvard Holm Layered Series have a stratigraphic thickness of more than 5000 m. Earlier work has shown that the range in cumulus mineral compositions is restricted (plagioclase An81-An51; olivine Fo85-Fo66; pyroxenes Ca43Mg46Fe11 to Ca43Mg37Fe20). Field evidence of magma injections is common, which together with the restricted range in mineral chemistry suggests that the magma chamber was frequently replenished by a less fractionated magma. A detailed study of a 600 m section (900-1500 m) in the Lower Layered Series reveals a period of crystallization when the magma chamber behaved as a closed system (900-1300 m). The rocks formed during this period are well-laminated olivine-gabbros (900-110 m), which evolved to well-laminated oxide-gabbros (1100-1300 m). Compositional trends in the cumulus minerals are towards more evolved compositions (plagioclase An64-An58, pyroxene Mg# from 80 to 76) with stratigraphic height. From 1300 m to 1500 m, granular olivine-gabbros dominate, with more primitive mineral compositions (plagioclase An67-An76, pyroxene Mg# from 78 to 82). The transition olivine-gabbro to oxide-gabbro at 1100 m is a consequence of fractional crystallization, and it is shown how changes in activities of FeO and Fe2O3 in the magma are reflected in the total iron content of plagioclases. The transition from oxide-gabbro to olivine-gabbro at 1300 m results from replenishment by less evolved basaltic magma. On the basis of calcic pyroxene chemistry and the mineral crystallization sequence it is concluded that the Kap Edvard Holm Layered Series crystallized from a tholeiitic magma similar to MORB. Melanogabbroic units occur throughout the intrusion as discordant to subconcordant sill-like bodies 0.2-2.0 m thick. The melanogabbroic units consist of Cr-rich augite-olivine-plagioclase heteradcumulates and contain deformed mica crystals of preemplacement origin. These units crystallized from a wet, MgO-rich magma which was injected into the layered host gabbros after the formation of the cumulus pile, but before the magma was completely solidified. The Kap Edvard Holm Layered Series has several parallels with the plutonic part of ophiolite sequences. These include: cumulus mineral assemblage, compositions of the minerals and the restricted range in compositions with stratigraphic height; field evidence of repeated replenishment of basaltic magma; dyke swarms overlying the roof zone of the magma chamber; and the existence of a late intrusive suite of wet, MgO-rich magma. These parallels suggest that the processes involved in the formation of the Kap Edvard Holm Layered Series were similar to those involved in the formation of the crustal part of many ophiolites and beneath present-day spreading ridges. The Kap Edvard Holm Layered Series is therefore believed to represent a shallow-level magma chamber which acted as a reservoir for basaltic flows at the continental margin during the opening of the North Atlantic Ocean.