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Journal of Petrology | Volume 40 | Number 6 | Pages 1037-1064 | 1999
© Oxford University Press 1999

Effects of H2O on Phase Relations during Crystallization of Gabbros in the Kap Edvard Holm Complex, East Greenland

Mark E. Brandriss1,* and Dennis K. Bird2

1 Department of Geosciences, Williams College 947 Main Street, Williamstown, MA 01267, USA
2 Department of Geological and Environmental Sciences, Stanford University Stanford, CA 94305–2115, USA

Received July 15, 1998; Revised typescript accepted January 8, 1999


  Abstract

In the Kap Edvard Holm Complex of East Greenland, layered olivine gabbros host numerous small, discordant bodies of ultramafic rock and magnetite gabbro that formed by late-magmatic metasomatic replacement of partially solidified gabbroic cumulates. Similar replacive rocks are also present as large semi-conformable sheet-like bodies within the layered gabbro sequence. Replacive bodies of all sizes are spatially associated with xenoliths of metabasaltic lava: the small bodies cluster around xenoliths, and the large bodies host swarms of xenoliths. The replacive bodies are inferred to have formed by reaction of gabbroic cumulates with pore liquids that were enriched in H2O by degassing or dehydration melting of the metabasalts. As these liquids migrated through the cumulus pile, they resorbed plagioclase and produced ultramafic bodies. Where the liquids came to rest, they reacted with gabbroic cumulates to produce bodies of magnetite-rich gabbro. The most evolved pore liquids segregated to form hornblende-rich pegmatitic dikes of roughly dacitic composition, suggesting that hydration and possibly oxidation of the magma shifted the liquid line of descent from a tholeiitic trend toward a calc-alkaline trend. These inferences imply that contamination of magmas by dehydration of hydrothermally altered crustal rocks may significantly influence the evolution of mafic magma systems.

KEY WORDS: gabbro; Kap Edvard Holm; metasomatism; postcumulus; xenolith

* Corresponding author. Telephone: (413) 458–2221. Fax: (413) 597–4116. E-mail: Mark.Brandriss{at}williams.edu


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