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

• Published in: Geological Magazine
• Main Authors: Bernstein, Stefan, Rosing, Minik T., Brooks, C. Kent, Bird, Dennis K.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 1992
• Subjects: East Greenland; Greenland; Kap Edvard Holm
• Online Access: http://dx.doi.org/10.1017/s001675680001952x; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S001675680001952X

Abstract The gabbros of the Tertiary Kap Edvard Holm Layered Serieshave 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 43 Mg 46 Fe 11 to Ca 43 Mg 37 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 periodare well-laminated olivine–gabbros (900–110 m), which evolved to well-laminated oxide-gabbros (1100–1300 m). Compositional trends in the cumulusminerals 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 moreprimitive mineral compositions (plagioclase An 67 —An 76 , 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 0 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 Cr-rich augite-olivine-plagioclase heteradcumulates and contain deformed mica crystals of pre-emplacement origin. These units crystallized from a wet, MgO-rich magma which was injected into the layered ...