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Layering in peralkaline magmas, Ilímaussaq Complex, S Greenland

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Hunt_et_al_2017_Ilimaussaq_layering.pdf (14.63Mb)
Date
01/2017
Author
Hunt, Emma J.
Finch, Adrian A.
Donaldson, Colin H.
Keywords
Ilímaussaq Complex
Kakortokite
Agpaitic
Macrorhythmic layering
Crystal size distribution
Eudialyte-group minerals
GE Environmental Sciences
DAS
BDC
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Abstract
The peralkaline to agpaitic Ilímaussaq Complex, S. Greenland, displays spectacular macrorhythmic (> 5 m) layering via the kakortokite (agpaitic nepheline syenite), which outcrops as the lowest exposed rocks in the complex. This study applies crystal size distribution (CSD) analyses and eudialyte-group mineral chemical compositions to study the marker horizon, Unit 0, and the contact to the underlying Unit − 1. Unit 0 is the best-developed unit in the kakortokites and as such is ideal for gaining insight into processes of crystal formation and growth within the layered kakortokite. The findings are consistent with a model whereby the bulk of the black and red layers developed through in situ crystallisation at the crystal mush–magma interface, whereas the white layer developed through a range of processes operating throughout the magma chamber, including density segregation (gravitational settling and flotation). Primary textures were modified through late-stage textural coarsening via grain overgrowth. An open-system model is proposed, where varying concentrations of halogens, in combination with undercooling, controlled crystal nucleation and growth to form Unit 0. Our observations suggest that the model is applicable more widely to the layering throughout the kakortokite series and potentially other layered peralkaline/agpaitic rocks around the world.
Citation
Hunt , E J , Finch , A A & Donaldson , C H 2017 , ' Layering in peralkaline magmas, Ilímaussaq Complex, S Greenland ' , Lithos , vol. 268-271 , pp. 1-15 . https://doi.org/10.1016/j.lithos.2016.10.023
Publication
Lithos
Status
Peer reviewed
DOI
https://doi.org/10.1016/j.lithos.2016.10.023
ISSN
0024-4937
Type
Journal article
Rights
© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Description
EJH acknowledges funding from a NERC PhD studentship and the work was completed at the University of St Andrews, UK.
Collections
  • University of St Andrews Research
URI
http://hdl.handle.net/10023/9817

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