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| Titel |
How do Kakortokites form? Additional evidence from the Ilimaussaq Complex, S. Greenland |
| VerfasserIn |
E. J. Hunt, A. A. Finch, C. H. Donaldson |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250065941
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| Zusammenfassung |
| The Ilímaussaq Complex, South Greenland, contains some of the most evolved igneous
rocks in the world and is widely considered to represent one of the largest deposits of
rare-earth elements, Ta, Nb and Zr. Our work is focused on the kakortokite layered series at
the base of the complex. The layered series is composed of 29 repetitive 3-layer units (named
-11 to +17, Bohse et al. 1971), successively enriched in arfvedsonite, eudialyte and
nepheline. Despite a large body of work on the development of the kakortokite
series, no consensus on the process/processes that produced the layering has been
forthcoming.
We present the preliminary findings of a combined petrographical, quantitative textural
and geochemical analysis on the kakortokite series, initially focused on layer 0. Although
many of the hypotheses for the formation of these rocks invoke a pressure change, the
enrichment of the series in volatile constituents (CH4 and H; Konnerup-Madsen, 2001)
has led many authors to suggest crystallisation occurred in a closed system, with
processes of gravitational settling formed the layering. Crystal size distribution
(CSD) analysis, performed on hand-digitised photomicrographs, provides insight
into processes of crystal nucleation and growth. The results indicate that simple
cumulate settling is untenable for layer 0. Instead the plot gradients indicate that the
arfvedsonite in the black kakortokite crystallised in situ above a sharp boundary to
the white kakortokite. The CSD plots for the alkali feldspars indicate secondary
nucleation occurred, with the small crystal size fraction forming in situ. The feldspar
phenocrysts also exhibit embayment textures indicating partial resorption. These
graphs are consistent with a model whereby an influx of hotter magma results in the
partial thermal erosion of the underlying white kakortokite, followed by in situ
crystallisation of arfvedsonite above the melt infiltration boundary, followed by in situ
crystallisation of eudialyte. Then nepheline and alkali feldspar crystallised through multiple
modes of nucleation, developing the characteristic layering. Geochemical trends
described by Pfaff et al. (2008) support an open system replenishment model during the
formation of layer 0, and potentially also layers +4 and +8. To further this work we
intend to apply this combined approach to investigate the formation of individual
layers, scaling these processes into a model for the development of the Ilímaussaq
complex.
Bohse et al. (1971). Rapport Grønlands Geologiske Undergesølgelse, 36, 43 pp.
Konnerup-Madsen (2001). Geology Greenland Surv. Bull., 190, 159-166. Pfaff et al. (2008).
Lithos, 106, 280-296. |
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