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The Tertiary Kærven Syenite Complex, Kangerdlugssuaq, East Greenland: Mineral Chemistry and Geochemistry

Published online by Cambridge University Press:  05 July 2018

Paul Martin Holm  and
Niels-Ole Prægel
Paul Martin Holm
Affiliation:
Institute of Petrology, Geologisk Centralinstitut, Øster Voldgade 10, DK-1350 København, Danmark
Niels-Ole Prægel
Affiliation:
Institute of Petrology, Geologisk Centralinstitut, Øster Voldgade 10, DK-1350 København, Danmark
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Abstract

The Kærven syenite complex, which reflects the hitherto earliest recorded stages in the Tertiary of East Greenland, outcrops in the middle reaches of the Kangerdlugssuaq Fjord as a peripheral intrusion to the Kangerdlugssuaq intrusion. The rocks of the Kærven complex range from syenite through alkali feldspar quartz-syenite to alkali feldspar granite. The general sequence of crystallization of the Kærven magmas was: alkali feldspar ± olivine(Fa96−99) ± plagioclase(An41−11), clinopyroxene (augite, ferrosalite, ferrohedenbergite), quartz and amphibole. Whole-rock major and trace-element data show coherent geochemical trends which suggest comagmatism. The data reveal that the Kærven rocks are distinct from the rocks from the adjacent Kangerdlugssuaq intrusion (e.g. higher TiO2, FeOT in low-SiO2 samples, lower Na2O, approx. constant Zr/Nb). The mineral chemistry supports this conclusion, as the Kærven samples typically have calcic amphiboles and clinopyroxenes with a very limited Na-enrichment in contrast to the sodic trends of the Kangerdlugssuaq intrusion. Normative feldspar compositions plot near to the Ab-Or cotectic in the Q-Ab-Or system and a maximum pressure of crystallization of 3–5 kbar with moderate to low PH2O is indicated.

Trace elements preferently incorporated in plagioclase and alkali feldspar, i.e. Sr, Ba and Rb, show systematics which are not compatible with an evolution of the rock suite by crystal fractionation of these phases, though possibly alkali feldspar may be partially accumulated in a few very evolved rocks. Numerical calculations do not suggest a magmatic evolution by fractional crystallization of the observed phases. The variation of Sr, Ba and Rb as well as of the incompatible elements Nb, Zr and Th support a derivation of the rock suite mainly by mixing two components, a syenitic and a granitic end-member. It is concluded that magma mixing was the most significant process in the formation of the Kærven rock suite accompanied by some crystal fractionation. Evidence for crustal contamination is detected in a few samples from the outer part of the intrusion but has not affected the main suite of rocks.

Keywords

Kærven syenite complexKangerdlugssuaqEast Greenlandmineral chemistrygeochemistry
Type
Petrology and Geochemistry
Information
Mineralogical Magazine , Volume 52 , Issue 367 , September 1988 , pp. 435 - 450
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1988

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Footnotes

*

Present address: University Library of Copenhagen, Nørre Alle 49, DK-2200 København, Danmark.

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