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A lower crustal mafic source for the ca. 2550 Ma Qorqut Granite Complex in southern West Greenland

Naeraa, Tomas LU ; Kemp, A. I. S.; Scherstén, Anders LU ; Rehnstrom, E. F.; Rosing, M. T. and Whitehouse, K. J. (2014) In Lithos 192. p.291-304
Abstract
The late Neoarchaean Qorqut Granite Complex is the youngest large igneous intrusion in the Nuuk region in southern West Greenland, where basement is primarily of Eoarchaean and Mesoarchaean age with a tonalite-trondhjemite-granodiorite (TTG) composition. The Qorqut granite is generally undeformed and it intruded during a prolonged period, starting at ca. 2730 Ma, characterised by crustal reworking, possibly related to syn- or post accretion tectonics or continental collision. We present major and trace element whole rock chemistry and combined U/Pb, Hf and O isotope data from zircon. We obtained a mean zircon U/Pb age of 2547 +/- 4 Ma (MSWD = 0.63). Initial sHf values range from - 12 to -18 requiring a long residence time and a rather... (More)
The late Neoarchaean Qorqut Granite Complex is the youngest large igneous intrusion in the Nuuk region in southern West Greenland, where basement is primarily of Eoarchaean and Mesoarchaean age with a tonalite-trondhjemite-granodiorite (TTG) composition. The Qorqut granite is generally undeformed and it intruded during a prolonged period, starting at ca. 2730 Ma, characterised by crustal reworking, possibly related to syn- or post accretion tectonics or continental collision. We present major and trace element whole rock chemistry and combined U/Pb, Hf and O isotope data from zircon. We obtained a mean zircon U/Pb age of 2547 +/- 4 Ma (MSWD = 0.63). Initial sHf values range from - 12 to -18 requiring a long residence time and a rather homogeneous source. Sample averaged zircon delta O-18 values range from 6.1 +/- 0.2%. to 6.5 +/- 0.3/0.7%o best interpreted with a source region of mainly unweathered mantle derived igneous rocks. Compared to the regional TTG basement, the QGC is characterised by low CaO and Na2O and high K2O, LREE and Rb contents, and a stronger fractionated REE pattern with a negative Eu anomaly. We show that the homogeneous Hf isotope signature of the granite together with its low epsilon value and its pristine oxygen isotope composition are best explained with an Eoarchaean mafic source with a Lu-176/Hf-176 around 0.015-0.019. Trace element modelling confirms that a mafic source in REE and with an eclogitic residue and with plagioclase as a fractionating phase would generate appropriate melt compositions. Modelling requires residual rutile in the source which constrain the pressures to > ca. 13-18 kbar. Zirconium saturation temperatures suggest magma temperatures in the range 750-850 degrees C. The obtained P-T conditions suggest a lower crustal source region in a thickened crustal unit consistent with a post or late continental collisional setting. (C) 2014 Elsevier B.V. All rights reserved. (Less)
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organization
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Contribution to journal
publication status
published
subject
keywords
Neoarchaean granite, Greenland, Zircon U/Pb, oxygen and Hf isotopes, trace element modelling
in
Lithos
volume
192
pages
291 - 304
publisher
Elsevier
external identifiers
  • wos:000335102100019
  • scopus:84896045043
ISSN
0024-4937
DOI
10.1016/j.lithos.2014.02.013
language
English
LU publication?
yes
id
5e8ff67c-c81b-4119-9a10-0112387b0637 (old id 4487324)
date added to LUP
2014-06-26 09:27:43
date last changed
2017-02-19 03:23:25
@article{5e8ff67c-c81b-4119-9a10-0112387b0637,
  abstract     = {The late Neoarchaean Qorqut Granite Complex is the youngest large igneous intrusion in the Nuuk region in southern West Greenland, where basement is primarily of Eoarchaean and Mesoarchaean age with a tonalite-trondhjemite-granodiorite (TTG) composition. The Qorqut granite is generally undeformed and it intruded during a prolonged period, starting at ca. 2730 Ma, characterised by crustal reworking, possibly related to syn- or post accretion tectonics or continental collision. We present major and trace element whole rock chemistry and combined U/Pb, Hf and O isotope data from zircon. We obtained a mean zircon U/Pb age of 2547 +/- 4 Ma (MSWD = 0.63). Initial sHf values range from - 12 to -18 requiring a long residence time and a rather homogeneous source. Sample averaged zircon delta O-18 values range from 6.1 +/- 0.2%. to 6.5 +/- 0.3/0.7%o best interpreted with a source region of mainly unweathered mantle derived igneous rocks. Compared to the regional TTG basement, the QGC is characterised by low CaO and Na2O and high K2O, LREE and Rb contents, and a stronger fractionated REE pattern with a negative Eu anomaly. We show that the homogeneous Hf isotope signature of the granite together with its low epsilon value and its pristine oxygen isotope composition are best explained with an Eoarchaean mafic source with a Lu-176/Hf-176 around 0.015-0.019. Trace element modelling confirms that a mafic source in REE and with an eclogitic residue and with plagioclase as a fractionating phase would generate appropriate melt compositions. Modelling requires residual rutile in the source which constrain the pressures to > ca. 13-18 kbar. Zirconium saturation temperatures suggest magma temperatures in the range 750-850 degrees C. The obtained P-T conditions suggest a lower crustal source region in a thickened crustal unit consistent with a post or late continental collisional setting. (C) 2014 Elsevier B.V. All rights reserved.},
  author       = {Naeraa, Tomas and Kemp, A. I. S. and Scherstén, Anders and Rehnstrom, E. F. and Rosing, M. T. and Whitehouse, K. J.},
  issn         = {0024-4937},
  keyword      = {Neoarchaean granite,Greenland,Zircon U/Pb,oxygen and Hf isotopes,trace element modelling},
  language     = {eng},
  pages        = {291--304},
  publisher    = {Elsevier},
  series       = {Lithos},
  title        = {A lower crustal mafic source for the ca. 2550 Ma Qorqut Granite Complex in southern West Greenland},
  url          = {http://dx.doi.org/10.1016/j.lithos.2014.02.013},
  volume       = {192},
  year         = {2014},
}