Precambrian Research 105 2001 165 – 181
Geochemical comparison between Archaean and Proterozoic orthogneisses from the Nagssugtoqidian orogen, West
Greenland
Feiko Kalsbeek
a,b,
a
Geological Sur6ey of Denmark and Greenland, Thora6ej
8
, DK-
2400
, Copenhagen NV, Denmark
b
Danish Lithosphere Centre, DK-
1350
, Copenhagen K, Denmark Received 29 January 1999; accepted 8 June 1999
Abstract
In the Palaeoproterozoic Nagssugtoqidian orogen of West Greenland reworked Archaean and juvenile Proterozoic orthogneisses occur side by side and are difficult to differentiate in the field. Archaean gneisses have tonalitic to
trondhjemitic compositions with relatively low Al
2
O
3
and Sr, and may have been derived from magmas formed by melting of basaltic or amphibolitic rocks at moderate pressures. The Proterozoic rocks are on average more mafic,
and it is likely that they crystallised from mantle-derived magmas. Felsic varieties of the Proterozoic igneous suite probably formed from the original magma by fractional crystallisation, in which hornblende played an important
role, and at SiO
2
\ 65 Archaean and Proterozoic rocks have very similar major and trace element compositions
including REE, illustrating that different modes of origin may lead to very similar results. © 2001 Elsevier Science B.V. All rights reserved.
Keywords
:
Archaean orthogneisses; Geochemistry; Greenland; Nagssugtoqidian orogen; Palaeoproterozoic orthogneisses www.elsevier.comlocateprecamres
1. Introduction
The investigation dealt with in this study was inspired by the work of H. Martin and co-workers
on the petrogenesis of Archaean granitoid rocks e.g. Martin, 1986, 1987, 1993. Martin’s main
conclusions can be summarised as follows: most Archaean granitoid magmas are formed by melt-
ing of hot oceanic crust hydrated tholeiitic basalts during subduction, in contrast to most
younger granitoid rocks which form from mag- mas generated in the mantle wedge above the
subducted slab. This contrast in origin can be deduced from differences in geochemistry between
Archaean trondhjemitic and younger calc-alka- line granitoids, and is the result of steeper
geothermal gradients during the Archaean.
In the central part of the Palaeoproterozoic Nagssugtoqidian orogen, West Greenland Fig.
1, reworked Archaean and juvenile Proterozoic meta-granitoid rocks occur side by side. Because
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PII: S 0 3 0 1 - 9 2 6 8 0 0 0 0 1 1 0 - 8
Fig. 1. Geological sketch map of the Nagssugtoqidian orogen, West Greenland modified after van Gool et al., 1996, with outline of the area investigated in this paper. SNF = southern Nagssugtoqidian front. The Kangaˆmiut dykes 2040 Ma, Nutman et al.,
1999 predate high-grade Nagssugtoqidian metamorphism and deformation in the southern Nagssugtoqidian orogen.
of lithological similarity and strong deformation it was not realised until the late 1980’s that they
belong to two age groups; previously all or- thogneisses were thought to represent reworked
Archaean rocks e.g. Hickman and Glassley, 1984. The presence of juvenile Palaeoproterozoic
rocks was documented by Kalsbeek et al. 1984, 1987 with the help of isotope data, but even then
it was not possible to distinguish Archaean and Palaeoproterozoic rocks in the field, and chemi-
cally the dated Archaean and Proterozoic rocks appeared to be very similar Kalsbeek et al.,
1987.
Recently, parts of the Nagssugtoqidian orogen have been re-investigated by the Danish Litho-
sphere Centre Marker et al., 1995; van Gool et al., 1996; Mengel et al., 1998. To get an insight
into the regional distribution of Archaean and Palaeoproterozoic rocks, zircons from a large
number of samples, scattered over the whole oro- gen, were analysed in a geochronological recon-
naissance programme Kalsbeek and Nutman, 1996, and in nearly all cases a distinction could
be made between reworked Archaean and Palaeoproterozoic rocks. The objective of this
work is in more detail to compare the geochem- istry of dated Archaean and Palaeoproterozoic
meta-igneous rocks from a part of the orogen where they occur in close spatial association, and
to see to what extent the chemical distinctions described by Martin 1986 can be recognised for
these rocks.
2. Nagssugtoqidian orogen and regional setting of the investigated rocks
