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| Titel |
Development of Peralkaline Rhythmic Layering, Ilimaussaq Complex, S. Greenland |
| VerfasserIn |
Emma J. Hunt, Adrian A. Finch, Colin H. Donaldson |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250086532
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| Publikation (Nr.) |
 EGU/EGU2014-417.pdf |
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| Zusammenfassung |
| The Ilímaussaq alkaline complex, S. Greenland is a peralkaline layered intrusion within the
Gardar (1280-1180 Ma) igneous province. It is considered as a potential deposit of rare-earth
elements, Ta, Nb and Zr. One of the places where these elements are concentrated is in the
kakortokite layered series (KLS), which forms the floor sequence of the complex and is a
spectacular example of macrorhythmic (i.e. typically >5 m in thickness) igneous layering.
The KLS is composed of 29 exposed units numbered -11 to +17 [1], each composed of three
layers distinguished by modal mineralogy: a basal arfvedsonite-rich black kakortokite,
overlain by eudialyte-rich red kakortokite, in turn overlain by alkali feldspar- and
nepheline-rich white kakortokite. Despite much work on the layered series, there is
no consensus on the physico-chemical processes that led to the formation of the
macrorhythmic layering, although most hypotheses suggest that the separation of minerals to
form the tripartite units included the processes of gravitational settling and density
sorting.
Here we use detailed petrographic studies, quantitative textural analysis (crystal size
distributions [CSDs]) and mineral chemistry to interpret processes of nucleation and crystal
growth within the marker horizon unit 0 and the lowest exposed units (-8 to -11) of the KLS.
Sharp boundaries occur between each of the studied units (-11/10, -10/-9, -9/-8 and -1/0)
while the intra-unit boundaries between the black to red and red to white kakortokites grade
over 2-5 cm. The CSD data suggest that multiple modes of crystallisation (in situ,
in suspension and through accumulation) contributed to the crystallisation of the
main modal minerals in each layer, with in situ crystallisation being of greatest
importance in the black and red kakortokites, while suspension and accumulation
processes were of more importance in the white kakortokites. The key control on unit
development was the rate of undercooling, potentially associated with changes in
volatile pressure. This acted to control the order of nucleation, first forming the
arfvedsonite-rich black kakortokite, secondly the eudialyte-rich red kakortokite
and finally the alkali feldspar- and nepheline-rich white kakortokite. Geochemical
discontinuities across some unit boundaries indicate that the FeTOT/Mn ratios of
eudialyte become more primitive (increased ratios) above these unit boundaries,
suggesting that magmatic replenishment events occurred to create the macrorhythmic
layering.
[1] Bohse et al. (1971). Rapport Grønlands Geologiske Undergesølgelse, 36, 43 pp. |
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