An ocean-ridge type magma chamber at a passive volcanic, continental margin: the Kap Edvard Holm layered gabbro complex, East Greenland

Abstract

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 An 81 -An 51 ; olivine Fo 85 -Fo 66 ; pyroxenes Ca 4 3Mg 4e Fe n to Ca 43 Mg 3 ,Fe 20 ). 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 An 64 -An 58 , 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 An 67 -An 7(i , pyroxene Mg# from 78 to 82). The transition olivine-gabbro to oxide-gabbro at 1100m is a consequence of fractional crystallization, and it is shown how changes in activities of FeO and Fe 2 O 3 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 Crrich 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. t Present address: Department of Earth Sc...

Post-breakup basaltic magmatism along the East Greenland Tertiary rifted margin

“…The clinopyroxenes from this study are all Ca-rich (20.41-23.86 wt% CaO). Mg# (100 ð Mg=(Mg C Fe) atomic) is in the range of 70-90 (Table 1; see also [12,14]). …”

“…The layered mafic intrusive rocks of the Kap Edvard Holm (KEH), Nordre Aputiteq, Patulajivit, Kruuse Fjord and Imilik complexes consist mainly of olivine and oxide gabbros, with restricted ranges in mineral compositions [12][13][14][15]17,18]. During the evolution of the magma chambers, all were intruded by a primitive melt that crystallized to form wehrlite sills and plugs [12,14,15,26].…”

“…During the evolution of the magma chambers, all were intruded by a primitive melt that crystallized to form wehrlite sills and plugs [12,14,15,26]. The wehrlite intrusions of the KEH are distinct from the host layered gabbros in that the wehrlites are enriched in incompatible elements at a given Mg=Fe of olivine.…”

“…1). Many of the mafic intrusions are gabbro complexes that experienced multiple replenishment episodes, and acted as low-pressure fractionation sites for basaltic melts [12][13][14][15]. Most were emplaced into the unconformity between the Precambrian basement and the flood basalts [16][17][18], commonly in the hinge zone of the coastal flexure [1,19].…”

Basaltic-volcano systems

“…Some, but not necessarily all, gabbro intrusions seen in eroded volcanoes are solidified magma chambers, but it is generally unlikely that the whole volume of the intrusion was fluid at one time. An example of a large intrusion that apparently is a solidified chamber is the Kap Edvard Holm gabbro in East Greenland, the minerals of which show that it behaved as an open system (Bernstein et al 1992), whereas nearby Skaergaard intrusion that shows extreme fractionation behaved as a closed system (McBirney & Noyes 1979). Fedotov (1982) investigated the growth and extinction conditions for magma chambers.…”