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Lithos 101 (2008) 480 – 500
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40
Ar– 39 Ar ages of intrusions in East Greenland: Rift-to-drift
transition over the Iceland hotspot
C. Tegner a,⁎, C.K. Brooks b , R.A. Duncan c , L.E. Heister d , S. Bernstein e
a
d
Department of Earth Sciences, University of Aarhus, Høegh-Guldbergs Gade 2, 8000 Århus C, Denmark
b
Institute of Petrology, University of Copenhagen, Øster Voldgade 10, 1350 København K, Denmark
c
College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA
Department of Earth Sciences, University of Arkansas at Little Rock, 2081 S. University Avenue, Little Rock, Arkansas 72204-1009, USA
e
Avannaa Resources Ltd., Geological Museum, Øster Voldgade 5-7, 1350 København K, Denmark
Received 16 October 2006; accepted 18 September 2007
Available online 24 September 2007
Abstract
Sixteen 40Ar–39Ar ages are presented for alkaline intrusions to appraise prolonged post-breakup magmatism of the central East
Greenland rifted margin, the chronology of rift-to-drift transition, and the asymmetry of magmatic activity in the Northeast Atlantic
Igneous Province. The alkaline intrusions mainly crop out in tectonic and magmatic lineaments orthogonal to the rifted margin and
occur up to 100 km inland. The area south of the Kangerlussuaq Fjord includes at least four tectonic lineaments and the intrusions
are confined to three time windows at 56–54 Ma, 50–47 Ma and 37–35 Ma. In the Kangerlussuaq Fjord, which coincides with a
major tectonic lineament possibly the failed arm of a triple junction, the alkaline plutons span from 56 to 40 Ma. To the north and
within the continental flood basalt succession, alkaline intrusions of the north–south trending Wiedemann Fjord–Kronborg
Gletscher lineament range from 52 to 36 Ma.
We show that post-breakup magmatism of the East Greenland rifted margin can be linked to reconfiguration of spreading ridges in
the Northeast Atlantic. Northwards propagation of the proto-Kolbeinsey ridge rifted the Jan Mayen micro-continent away from
central East Greenland and resulted in protracted rift-to-drift transition. The intrusions of the Wiedemann Fjord–Kronborg Gletscher
lineament are interpreted as a failed continental rift system and the intrusions of the Kangerlussuaq Fjord as off-axis magmatism. The
post-breakup intrusions south of Kangerlussuaq Fjord occur landward of the Greenland–Iceland Rise and are explained by mantle
melting caused first by the crossing of the central East Greenland rifted margin over the axis of the Iceland mantle plume (50–47 Ma)
and later by uplift associated with regional plate-tectonic reorganization (37–35 Ma). The Iceland mantle plume was instrumental in
causing protracted rift-to-drift transition and post-breakup tholeiitic and alkaline magmatism on the East Greenland rifted margin, and
asymmetry in the magmatic history of the conjugate margins of the central Northeast Atlantic.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Chronology; Volcanic rifted margin; Intrusions; Iceland mantle plume; Northeast Atlantic; East Greenland
1. Introduction
⁎ Corresponding author.
E-mail address: [email protected] (C. Tegner).
0024-4937/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.lithos.2007.09.001
The Northeast Atlantic igneous province provides a
complete history of rift-to-drift magmatism over 62 m.y.
from volcanic rifted margin formation to present-day mid-
C. Tegner et al. / Lithos 101 (2008) 480–500
ocean ridge spreading (Fig. 1a). Anomalously thick and
compositionally enriched basaltic oceanic crust at Iceland
and the Greenland–Iceland–Faroes rise witness a sustained mantle melting anomaly relative to normal midocean ridge processes and is referred to as the Iceland
mantle plume (White and McKenzie, 1989; Saunders
et al., 1997; Holbrook et al., 2001; Allan et al., 2002). The
geological map suggests symmetry in the distribution of
481
the continental flood volcanics, the volcanics making up
seaward-dipping reflectors at the continent boundaries,
and the Greenland–Iceland–Faroes rise (Fig. 1a). Notwithstanding, the history of magmatism at the conjugate
volcanic rifted margins is highly asymmetrical. In the
British and Faroe Islands, extrusive and intrusive magmatism is confined to a time window from ∼62 to 55 Ma
leading up to continental breakup (e.g. Chambers et al.,
Fig. 1. (a) Geological map of the Northeast Atlantic region showing the distribution of continental flood basalt, volcanics mapped seismically as
seaward-dipping reflectors, and thick oceanic crust of the Greenland–Iceland–Faroes Rise. JMMC = Jan Mayen micro-continent: JMFZ = Jan Mayen
fracture zone. Modified after Larsen and Saunders (1998). (b) Geological map of central East Greenland showing the major rock types and onland
tectonic lineaments (Karson and Brooks, 1999); the location of the anomalously thick oceanic crust of the Greenland–Iceland Rise; location of the
coast-parallel dyke complex (Myers,1980; Klausen and Larsen, 2002); magnetic seafloor anomalies (Larsen, 1990); and location of maps in Figs. 4–6.
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C. Tegner et al. / Lithos 101 (2008) 480–500
Fig. 1 (continued).
2005; Storey et al., 2007a). In contrast, extrusive and
intrusive magmatism in central East Greenland was
prolonged from ∼62 to 13 Ma (e.g. Sinton and Duncan,
1998; Tegner et al., 1998a; Hansen et al., 2002; Brooks
et al., 2004; Storey et al., 2004, 2007a). Likewise, the
location of the present-day spreading ridges is asymmetric. While the Reykjanes and Mohns ridges lie close to the
axis of symmetry, the Kolbeinsey ridge is displaced
towards East Greenland (Fig. 1a). This is linked to a
westward relocation of spreading from the Aegir ridge
(now extinct) and the formation of the Jan Mayen microcontinent (Nunns, 1983; Scott et al., 2005). The magnetic
seafloor spreading anomalies offshore the Blosseville
Kyst demonstrate that the proto-Kolbeinsey ridge propagated northwards between East Greenland and the Jan
Mayen micro-continent from magnetochron 21 (∼47 Ma)
to magnetochron 6 (∼23 Ma) (Fig. 1b). Hence, the rift-todrift transition of the central East Greenland volcanic
rifted margin is expected to be prolonged and combined
from two rifting events.
Here we report 40Ar–39Ar age determinations for
sixteen major alkaline plutons, smaller alkaline intru-
sions, and gabbroic plutons in central East Greenland.
These intrusions were emplaced during and after tholeiitic flood basalt volcanism in major tectonic and
magmatic lineaments perpendicular to the rifted margin
and extending up to 100 km inland, or in the coastal
zone (Fig. 1b). We combine new and published ages to
revise the magmatic history of the central East Greenland margin and discuss the link to asymmetry of rift-todrift transition in the Northeast Atlantic and reconfiguration of oceanic spreading ridges over the Iceland
mantle plume.
2. Geology and chronology background
2.1. Rifted margin structure
South of Kangerlussuaq Fjord at ∼ 68°N (Fig. 1b),
the East Greenland rifted margin is characterised by a
20–40 km wide transition zone between largely undisturbed Archean basement with few Paleogene intrusions and the continent–ocean boundary (Klausen and
Larsen, 2002). In this narrow transition zone, magmatic
C. Tegner et al. / Lithos 101 (2008) 480–500
extension by coast-parallel dyke complexes, layered
gabbro and syenite complexes may occupy 30 to 100%
of outcrops (Myers, 1980; Klausen and Larsen, 2002).
Tectonic extension described as a coast-parallel flexure
is manifested in seawards rotation of crustal blocks by
antithetic, landward-dipping faults (Nielsen, 1978;
Myers, 1980; Klausen and Larsen, 2002). The margin
is segmented by tectonic lineaments trending approximately orthogonal to the continent–ocean boundary that
are explained as major accommodation zones (Karson
and Brooks, 1999; Klausen and Larsen, 2002). Of these
lineaments, the Kangerlussuaq Fjord is the most prominent and it has been suggested to represent the failed
arm of a triple junction in which extension and riftrelated magmatism took place mainly in two arms
forming the present coast line, resulting in a ∼120°
angle between the coast-parallel dyke complexes at the
mouth of the Fjord (Fig. 1b) (Brooks, 1973; Myers,
1980; Karson and Brooks, 1999). A similar first-order
triple junction exists south of Greenland (Karson and
Brooks, 1999). Other major, coast-orthogonal lineaments bound segments that are ∼ 100 km wide (Fig. 1).
One of these is located at Agtertia Fjord immediately
north of the Kruuse Fjord gabbro complex and coincides
with an offset in the coastal dyke complex (Myers,
1980; Karson and Brooks, 1999; Klausen and Larsen,
2002). Between the coast-orthogonal lineaments at
Kangerlussuaq and Agtertia Fjords, the coastal transition zone is composed mainly of Archean basement that
is intruded by a coast-parallel dyke complex (Klausen
and Larsen, 2002; Myers, 1980) and layered gabbro
plutons (Bernstein et al., 1998b; Tegner et al., 1998a).
South of the Agtertia lineament, a ∼ 45 km long stretch
of the coast called Kialineq is made up mainly by
intrusive layered gabbro complexes (Imilik and NoeNygaard intrusions) and large, younger syenite and
granite plutons (Brown et al., 1977; Nielsen, 1987;
Tegner et al., 1998a; Bernstein and Bird, 2000).
North of the Kangerlussuaq Fjord, north–south
trending basins with infill of Cretaceous and Paleogene
sediments, the earliest Tertiary volcanics (the Lower
Volcanics) and the Sorgenfrei Gletscher Sill Complex
are exposed (Fig. 1) (Nielsen et al., 1981; Larsen et al.,
1999b; Ukstins Peate et al., 2003). These basins are
overlain by the regional Plateau Lavas in which
lithostratigraphic formations can be correlated from
the Kangerlussuaq area in the south to Scoresby Sund
Fjord in the north, and from the Prinsen af Wales Bjerge
in the west to the Blosseville Kyst in the east (Larsen
et al., 1989; Pedersen et al., 1997; Tegner et al., 1998b;
Hansen et al., 2002). Correlation of certain basalt
formations has further been established across the
483
Atlantic to the Faroe Islands (Larsen et al., 1999a).
Despite the difference in geological setting and the more
easterly trend of the coastal dyke complexes, the
northern rifted margin is consistently rotated seawards
in a fashion similar to the southern segment (Karson and
Brooks, 1999; Klausen and Larsen, 2002). A major
coast-orthogonal tectonic and magmatic north–south
lineament along Wiedemann Fjord–Kronborg Gletscher
separates two structural blocks of the flood basalt
succession (Fig. 1) (Pedersen et al., 1997). This
lineament includes alkaline mafic and ultramafic plutons, nepheline-syenite plutons and north–south trending dyke swarms (Nielsen et al., 2001).
2.2. Chronology of intrusions and lavas
K–Ar, Rb–Sr and fission-track (FT) chronology established a chronological frame for magmatism (Beckinsale
et al., 1970; Pankhurst et al., 1976; Brooks and Gleadow,
1977; Gleadow and Brooks, 1979; Noble et al., 1988;
Holm, 1991). More recent 40Ar–39Ar age determinations
have refined the magmatic history (Fig. 2). Supplementary dataset 1 contains a compilation of available age data
for the alkaline intrusions.
The age of the earliest magmatic event, the Lower
Volcanics remains poorly constrained in the Kangerlussuaq area. In the inland Prinsen af Wales Bjerge, the
lowermost lavas are 61–60 Ma (Hansen et al., 2002).
The main phase of tholeiitic flood volcanism (the
Plateau Lavas) erupted between 56.1 ± 0.5 Ma, the age
Fig. 2. Age summary for intrusions and lavas exposed on land and for
lavas drilled within the Seaward Dipping Reflector Sequences (SDRS)
offshore East Greenland. A compilation of published age data for the
intrusions is given in Supplementary dataset 1. Age data for the lavas
from (Hansen et al., 2002; Heister et al., 2001; Peate et al., 2003;
Storey et al., 2007a) and for the SDRS from (Sinton and Duncan,
1998; Tegner and Duncan, 1999). Also shown are magnetic chrons
from Gradstein et al. (2004).
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C. Tegner et al. / Lithos 101 (2008) 480–500
of a basal lava flow (Storey et al., 2007a), and 55.1 ±
0.1 Ma, the age of an alkaline tephra intercalated in the
top portion of the lava succession (Heister et al., 2001;
Storey et al., 2007b). The Plateau Lavas may have been
emplaced in less than 300,000 years (Larsen and Tegner,
2006) and coincides with continental rupture (Fig. 2)
(Larsen and Saunders, 1998).
The alkaline lavas in the Prinsen af Wales Bjerge
(55.1 ± 1.1 to 52.5 ± 1.1 Ma) are intercalated with the
Plateau Lavas, but eruptions continued about one million years later (Peate et al., 2003). In the northern part
of the Blosseville Kyst (Fig. 1), mildly alkaline basalts
erupted into a graben structure within the Plateau Lavas
at 50–49 Ma (Larsen et al., 1989; Tegner et al., 1998a).
Surprisingly, there is also a report of mildly alkaline
lavas of Miocene age (14–13 Ma) overlying the Plateau
Lavas close to the inland ice at the northern end of
the Wiedemann Fjord–Kronborg Gletscher lineament
(Storey et al., 2004).
Recent 40Ar–39Ar chronology of the intrusions has
focused on the major tholeiitic centres. A precise but
unsurprising age of 55.4 ± 0.1 Ma was obtained for the
Skaergaard intrusion (Hirschmann et al., 1997) and
confirmed by a zircon U–Pb age of 55.59 ± 0.13 Ma
(Hamilton and Brooks, 2004). Slightly older ages of
56.7 ± 0.9 Ma and 56.4 ± 0.4 Ma were reported for the
Sorgenfrei Gletscher Sill Complex (Fig. 1) (Tegner et al.,
1998a). These intrusions thus coincide with the main
flood basalt event. More unexpectedly, the tholeiitic
gabbro complexes at Kap Edvard Holm, Kruuse Fjord,
and Imilik south of Kangerlussuaq Fjord (Fig. 1b) were
emplaced at 50–47 Ma, long after continental breakup
(Fig. 2) (Nevle et al., 1994; Tegner et al., 1998a).
Emplacement of alkaline plutons of the East Greenland
margin span a long period beginning before continental
breakup with 58.3 ± 0.9 Ma silico-carbonatite dykes of
the Sulugsut Complex (Storey et al., 2007a) and
continuing to 36 Ma (Fig. 2). The alkaline intrusions
mainly crop out either in the coast-orthogonal lineaments
(Karson and Brooks, 1999) or closely associated with
the coastal gabbro complexes (Myers, 1980; Brooks and
Nielsen, 1982; Nielsen, 1987; Nevle et al., 1994). Their
geology and age are described in more detail below.
3. New
40
parent’ step ages reported in 40Ar–39Ar geochronology
(McDougall and Harrison, 1988). The advantage of
40
Ar–39Ar incremental heating experiments over the K–
Ar method is that regression analysis of colinear multiple step Ar-isotopic compositions may be used to
verify this assumption. Thus, if the initial 40Ar/36Ar of a
sample is within error of atmosphere (295.5), then the
‘apparent’ age is the time since crystallization. If the
sample is contaminated with un-degassed, mantle- or
crustal-derived 40Ar (excess argon), the initial 40Ar/36Ar
is greater than 295.5, and the ‘apparent’ age is calculated
using a false assumption and is thus erroneously high. In
a single two-component system of radiogenic Ar mixed
with excess Ar there are two solutions to calculating
reliable crystallization ages. One is to use the isochron
regression that yields a true crystallization age regardless of the initial 40Ar/36Ar composition. The second is
to recalculate each step age using the initial 40Ar/36Ar
value obtained from the regression analysis (McDougall
and Harrison, 1988). Such step and plateau ages are
denoted “recalculated”. Mixing with a third component
(atmospheric Ar) will displace isotopic compositions
from the binary mixing line, most notably at low temperature steps, and these compositions are not used in
the regression. Excess argon is more likely to be encountered in intrusive rocks than in lavas due to obvious
difficulties of equilibration with atmosphere at depth
and possible contamination from older host rocks.
Another concern with radiometric dating of intrusive
rocks is whether the age reflects igneous crystallization
or tectonic cooling. For the East Greenland intrusions
there is little doubt that 40Ar–39Ar ages represent
Ar–39Ar geochronology
3.1. Background and justification
The conventional K–Ar method assumes that measured 40Ar derives from that accumulated from decay of
40
K (radiogenic) and from initial composition set by
atmospheric Ar. This same assumption applies to ‘ap-
Fig. 3. Age vs. closure temperature for dating methods applied to the
Kangerlussuaq Syenite Intrusion. Abbreviations: MIN Rb–Sr = Rb–Sr
mineral isochron age; WR Rb–Sr = Rb–Sr whole rock isochron age;
FT = fission-track age; BIO = biotite. Data from (Beckinsale et al., 1970;
Gleadow and Brooks, 1979; Pankhurst et al., 1976) and this study.
C. Tegner et al. / Lithos 101 (2008) 480–500
igneous crystallization. The closure temperature of
biotite (∼ 350–300 °C) and amphibole (∼ 550–
500 °C) (McDougall and Harrison, 1988) is higher
than the ambient crustal temperature at the depth of
emplacement. Firstly, all the intrusions were emplaced
into Paleogene volcanics (Brooks and Nielsen, 1982;
Nielsen, 1987) that were no more than 7–8 km thick and
in most cases thinner (Pedersen et al., 1997). This
constrains the country rock temperature to less than
∼ 210 °C for a reasonable geothermal gradient. Low
485
regional temperatures for the flood basalts prior to
emplacement of plutons, sills and dykes are also implied
by the occurrences of zeolite zones in the Plateau Lavas,
which record a thermal gradient of about 40 °C/km
(Neuhoff et al., 1997). Secondly, apatite fission-track
(FT) ages of the basement inland from the thermal
influence of intrusions are consistently over 80 Ma
(Hansen and Brooks, 2002). This suggests that regional
uplift was modest and the present erosion level cooled
below 100 °C before emplacement of the intrusions.
Fig. 4. Geological map of the Kialineq, Søndre Aputitêq and Patulajivit area showing location of major mafic and felsic plutons, and the location of
the dated samples.
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C. Tegner et al. / Lithos 101 (2008) 480–500
Thirdly, age determinations of the Kangerlussuaq Syenite
Intrusion yield concordant ages independent of methods
with a closure temperature above ∼200 °C (Fig. 3). The
discordant age of 36.1 ± 2.0 Ma (Gleadow and Brooks,
1979) obtained by apatite FT is interpreted as a later
cooling event due to exhumation or thermal resetting.
3.2. Samples and methods
Sixteen samples were selected from alkaline intrusions
between 67°N and 69°N. The results for the two samples
from the Gardiner Complex are presented in a companion
paper (Heister et al., in preparation), but are also discussed
Fig. 5. Geological map of the Kangerlussuaq Fjord region, showing major rock types, the location of the major plutons, and the location of the dated
samples. Abbreviations: SSG = Søndre Syenite Gletscher; SS = Snout Series syenite intrusion; AF = Amdrup Fjord. Modified from Gleadow and
Brooks (1979).
C. Tegner et al. / Lithos 101 (2008) 480–500
here. The location of samples is shown in Figs. 4–6 and
Supplementary dataset 2 provides field data and petrographic descriptions. Biotite and hornblende were
separated and analysed by incremental heating experiments in a double-vacuum resistance furnace at Oregon
State University (n = 14), using a MAP 215-50 mass
spectrometer (Sinton and Duncan, 1998; Tegner et al.,
1998a; Storey et al., 2007a) and at Stanford University
(n = 2) using low-blank CO2 laser heating (Heister et al.,
2001). The analytical data calculated using the ArArCalc
487
software (Koppers, 2002) can be found in Supplementary
dataset 3. The Ar release patterns and isotope correlations
are shown in Figs. 7–9 and summarised in Table 1. We
report the isochron age for samples (n = 13) with an
acceptable goodness of fit (MSWD b 2.5). The plateau age
(n = 2) or total fusion age (n = 1) is preferred as the best age
estimate for the remaining samples. All ages are reported
relative to 28.02 Ma for the FCT-3 biotite monitor (Renne
et al., 1998), allowing for direct comparison with the timescale of Gradstein et al. (2004); previously published
Fig. 6. Geological map of the Wiedemann Fjord–Kronborg Gletscher tectonic lineament showing the location of the major plutons, dykes
(schematic), diatremes (general area labelled), and the dated samples.
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C. Tegner et al. / Lithos 101 (2008) 480–500
40
Ar–39Ar ages are recalculated to this monitor age. The
errors reported are 2 standard deviations, and include error
in the J value.
3.3. Excess argon
Excess argon was detected in five samples. In these
cases the “recalculated” plateaus are flatter and better
defined than the apparent release patterns and concordant
with the isochron age (Figs. 8 and 9; Table 1). An example
of significant excess Ar is provided by sample 85659 of
the Kærven gabbro, for which the apparent release pattern
is U-shaped and lacks a plateau while the “recalculated”
release pattern displays a 5-step plateau at 55.1 ± 0.4 Ma for
76% of the gas, within error of the isochron age at 55.1 ±
1.4 Ma (Fig. 8). To be consistent and conservative, we use
the isochron correlation as the estimate of age and of
uncertainty for samples with excess argon.
3.4. Syenite and diorite of the Kialinêq area
More than ten plutons are exposed over an area
of ∼ 45 × 15 km at the coastal end of a tectonic lineament orthogonal to the coast at ∼67°N (Figs. 1 and 4)
(Karson and Brooks, 1999). The Imilik Gabbro Complex (Tegner et al., 1998a; Bernstein et al., 1998a) and
the Noe-Nygaard gabbro intrusion (Bernstein and Bird,
2000) crop out in the southern part whereas younger
syenite and granite intrusions occur in the northern part
(Brown et al., 1977; Nielsen, 1987). Field relations and
published ages suggest the Imilik Gabbro Complex
consists of 3 units with mutually cross-cutting relations
and varying degrees of deformation, hydrothermal
alteration and intrusive intensity of the coastal dyke
swarm. The oldest unit could not be dated due to lack of
suitable material. The unit of intermediate age is
constrained to be older than 56.6 ± 0.6 Ma by dating a
Fig. 7. Results of 40Ar–39Ar experiments for syenite and diorite intrusions of the Kialineq area and gabbroic complexes at the islands of Søndre
Aputitêq and Patulajivit (Fig. 4). Left column shows apparent step ages as open boxes (height gives ±2σ) plotted against accumulated amount of gas
released (% 39Ar). Plateau ages are indicated by horizontal lines that bracket concordant step ages. The right column shows 36Ar/40Ar vs. 39Ar/40Ar
for the same temperature steps, with isochron age and intercept indicated.
C. Tegner et al. / Lithos 101 (2008) 480–500
489
Fig. 8. Results of 40Ar–39Ar experiments for syenite, granite and gabbroic plutons of the Kangerlussuaq Fjord area. Recalculated step ages with initial
Ar/36Ar equal to the isochron intercept are shown as black boxes.
40
490
C. Tegner et al. / Lithos 101 (2008) 480–500
cross-cutting dyke, and the youngest unit was dated to
49.5 ± 0.2 and 50.1 ±1.2 Ma (Tegner et al., 1998a).
Existing zircon FT, Rb–Sr, and K–Ar ages for the
syenite, granite and diorite intrusions of the kialinêq
range from 49 to 29 Ma (Beckinsale et al., 1970; Brown
et al., 1977; Gleadow and Brooks, 1979; Noble et al.,
1988) but most cluster in the range 39–34 Ma (see
Supplementary dataset 1). For biotite of the Nuuk diorite
we obtained an age of 36.2 ± 0.6 Ma (Fig. 7; Table 1) and
for biotite of the Ikerasangmiit Syenite Intrusion an age
of 37.2 ± 2.9 Ma, confirming previous results.
3.5. Søndre Aputitêq gabbro–granite complex
Sample SA-1 is from a granite vein cutting across
gabbro and basalt xenoliths (Dennis K. Bird, pers.
comm., 2005) from an island located at the mouth of
Agtertia Fjord which coincides with a major coastorthogonal lineament (Fig. 4) (Karson and Brooks,
1999). Amphibole yielded an age of 49.4 ± 1.0 Ma
(Fig. 7; Table 1). This age is within error of a titanite FT
age of 52.5 ± 2.6 Ma from the island (Gleadow and
Brooks, 1979) and an 40Ar–39Ar age of 48.3 ± 1.2 Ma
Fig. 9. Results of 40Ar–39Ar experiments for syenite, gabbro, basanite, nephelinite and lamprophyre intrusions of the Wiedemann Fjord–Kronborg
Gletscher lineament. Recalculated step ages with initial 40Ar/36Ar equal to the isochron intercept are shown as black boxes.
C. Tegner et al. / Lithos 101 (2008) 480–500
491
Table 1
Ar–39Ar incremental heating ages for alkaline intrusions, central East Greenland
40
Location name and rock type
Ar MSWD 40Ar/36Ar Type
(%)
(initial)
Material
Kialineq district
Nuuk Diorite
Ikerasangmiit Syenite Intrusion
67.00
67.04
312001
429360
Biotite
36.2
Amphibole 37.2
0.6
2.9
11
6
98
97
1.7
1.7
301 ± 18
392 ± 91
Isochron
Isochron
Agtertia Fjord lineament and islands
Søndre Aputitêq gabbro–granite
Patulajivit Gabbro Complex
67.30
67.50
SA-1
P-175
Amphibole 49.4
Biotite
47.2
1.0
0.8
3 98
– 100
1.0
–
–
–
Plateau
Total fusion#
Kangerlussuaq Fjord lineament
Kangerlussuaq Biotite Granite
Kræmer Ø Syenite
Kangerlussuaq Syenite Intrusion
Snout Series Syenite
Snout Series Syenite
Kærven Complex Gabbro
Kærven Complex Alkali Granite
68.15
68.22
68.25
68.25
68.30
68.40
68.40
333132
NM27025
EG4583
312139
81-48
85659
40160
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Amphibole
46.6
50.4
50.8
45.6
47.0
55.1
52.8
0.9
1.0
1.1
0.3
0.5
1.4
1.3
5
4
7
12
6
5
4
98
95
96
98
55
76
99
2.4
1.4
0.5
1.0
0.1
0.8
2.3
279 ± 34
417 ± 34
313 ± 69
673 ± 269
283 ± 11
673 ± 93
690 ± 48
Isochron
Isochron
Isochron
Isochron
Isochron
Isochron
Isochron
429285
457127
NM5133
457163
436094
Amphibole
Biotite
Amphibole
Biotite
Biotite
41.5
51.8
46.7
36.6
50.2
0.9
1.1
1.0
0.5
1.2
2
7
2
11
9
92
90
64
99
98
4.9
0.4
6.1
0.7
1.2
–
310 ± 8
–
301 ± 7
371 ± 57
Plateau
Isochron
Plateau
Isochron
Isochron
Wiedemann Fjord–Kronborg Gletscher lineament
Wiedemann Fjord Lamprophyre Dyke
68.55
Ejnar Mikkelsen Fjeld Intrusion Gabbro
68.80
Borgtinderne Intrusion Syenite
68.85
Borgtinderne Basanite Dyke
68.85
Kronborg Gletscher Nephelinite Diatreme 68.90
Age 2σ
N
(Ma) (Ma)
39
Location
Sample
(approximate no.
decimal N)
Ages are reported relative to biotite monitor FCT-3 of 28.02 Ma and 2σ includes error in the J value; #recombined total fusion age.
for the Kruuse Fjord gabbro complex located inland
along the Agtertia lineament (Tegner et al., 1998a).
3.6. Patulajivit gabbro complex
The small islands of Patulajivit and Igtutarajik (Fig. 4)
are composed mainly of layered gabbro with frequent
slump structures similar to coastal gabbros south of
Kangerlussuaq Fjord (Tegner et al., 1998a). Sample P-175
from Patulajivit is taken from a pocket of coarse-grained
leucogabbro formed by partial melting of a metabasalt
xenolith (Dennis K. Bird, pers. comm., 2005). A similar
phenomenon is described in the Kap Edvard Holm complex (Brandriss et al., 1996). Biotite from P-175 did
not produce an age plateau nor an isochron correlation
(Fig. 7). A recombined total fusion age gives 47.2 ±
0.8 Ma. This is identical to the age of 47.3 ± 0.3 Ma
obtained for a gabbro-pegmatite pod in gabbros of the
nearby island of Igtutarajik (Tegner et al., 1998a).
3.7. Kangerlussuaq Alkaline Complex
This huge complex (N 800 km2) crops out west of
Kangerlussuaq Fjord (Fig. 5). Its main constituent is the
Kangerlussuaq Syenite Intrusion, which is known for
concentric zoning from strongly undersaturated foyaite
at the centre, oversaturated pulaskite to an outer zone
composed of oversaturated quartz-syenite (nordmarkite)
(Kempe et al., 1970; Riishuus et al., 2006). Biotite from
sample EG4583 from the pulaskite zone yields an age of
50.8 ± 1.1 Ma (Fig. 8; Table 1) and is within error of
previous age estimates (Fig. 3). If the three syenite zones
are cogenetic, as the lack of intrusive contacts indicate
(Kempe et al., 1970), and are related to one another by
concurrent assimilation and fractional crystallization in
one magma chamber (Brooks and Gill, 1982; Nielsen,
1987; Riishuus et al., 2006), the reported age can be
considered representative of the entire intrusion.
The Kangerlussuaq Alkaline Complex includes several satellite intrusions that either cut or are cut by the
Kangerlussuaq Syenite Intrusion. In the northeast, the
Kangerlussuaq Syenite Intrusion cuts across the Kærven
Complex (Fig. 5). This older complex is composed of an
outer gabbro body along its eastern and northeastern
contact and is, in turn, cut by several ring-shaped syenite
and alkali granite bodies with decreasing ages to the
west (Holm, 1991). A gabbro sample (85659) of the
structurally oldest unit, and an alkali granite sample
(40160) that represents the structurally youngest unit of
the Kærven Complex (Holm, 1991) are dated. Biotite
492
C. Tegner et al. / Lithos 101 (2008) 480–500
from the gabbro yielded an age of 55.1 ± 1.4 Ma (see
Section 3.3) and amphibole of the alkali granite gave
52.8 ±1.3 Ma (Fig. 8, Table 1). A U–Pb zircon age of
53.0 ±0.3 Ma for a gabbro-pegmatite at the northern
contact of the complex (Holm et al., 2006) suggests
that gabbroic activity was prolonged over 1–2 m.y. and
overlapping with syenite and granite intrusions. A
previous investigation of the same alkali granite sample
by 40Ar–39Ar incremental heating experiments yielded
ages of 58.1 ±1.1 Ma for amphibole and 58.5 ± 0.4 Ma
for K-feldspar (Holm, 1991). However, the amphibole
age is the average of only 2 discordant steps that make
up nearly all the gas released while the feldspar age is
the average of 3 concordant steps (c. 45% gas) in a Ushaped release pattern with minimum ages of c. 45 Ma.
Neither of these release patterns qualifies as plateau
ages, and both lack assessment of excess argon. We
therefore conclude that the Kærven Complex was emplaced between 55.1 ± 1.4 and 52.7 ± 0.8 Ma and thus
predates the Kangerlussuaq Syenite Intrusion by at least
1 m.y.
We report the first radiometric ages for satellite
intrusions that cut across the southeastern periphery of
the Kangerlussuaq Syenite Intrusion (Fig. 5). The Snout
Series nordmarkite cuts the eastern margin and basement gneisses north and south of Søndre Syenite
Gletscher (Nielsen, 1987). The northern sample 81-48
yielded an age of 47.0 ± 0.5 Ma and sample 312139 from
south of the glacier gave an age of 45.6 ± 0.3 Ma. This is
close to the low end of zircon FT ages of 50–47 Ma for
the Snout Series (Gleadow and Brooks, 1979) and a
Rb–Sr age of 47.1 ± 0.7 Ma for the nearby Astrophyllite
Bay diorite–syenite complex (Riishuus et al., 2005). To
the south of Amdrup Fjord (Fig. 5), the ∼ 25 km2
Kangerlussuaq Biotite Granite cuts the Kangerlussuaq
Syenite Intrusion and basement (Deer and Kempe,
1976). Judging from the scarcity of cross-cutting dykes
(our observation), this biotite granite appears to represent some of the youngest plutonic activity in the
area. Biotite from granite sample 333132 yielded an age
of 46.6 ± 0.9 Ma, reinforcing this impression. In the
periphery of the Kap Edvard Holm complex (Fig. 5),
syenite intrusions Hutchinson 1 and 2 formed down to
∼ 42 Ma (Nevle et al., 1994). There are even younger
lamprophyre dykes with inclusions of gabbro and
syenite in Amdrup Fjord for which FT and K–Ar ages
range from 37 to 34 Ma (Gleadow and Brooks, 1979),
and minor volcanic and hydrothermal activity of the
Flammefjeld ore deposit dated at 39.7 ±0.1 Ma by Re–
Os of molybdenite (Brooks et al., 2004). Hence, igneous
activity in the Kangerlussuaq Alkaline Complex spans
at least 15 m.y. from ∼ 55 to ∼ 40 Ma.
3.8. Gardiner Complex
The Gardiner Complex forms a circular, strongly
undersaturated alkaline complex between the head of
Kangerlussuaq Fjord and the ice cap (Fig. 5) (Nielsen,
1987). Two 40Ar–39Ar ages are presented in a different
context in a parallel contribution (Heister et al., in
preparation) but are also briefly discussed here in the
present context. One sample of the second oldest magmatic
phase of ijolite dykes (Nielsen, 1994) yielded an age of
56.5 ±0.3 Ma (biotite from sample 29904). A sample from
a melilitolite ring dyke that cuts all other rocks of the
complex gave an age of 54.7 ± 1.2 Ma (biotite from sample
303825). This age is within error of an isotope dilution
Rb–Sr age of 56.2 ± 2.4 Ma (Waight et al., 2002).
3.9. Kræmer Ø Syenite
The Kræmer Ø Syenite at the mouth of Kangerlussuaq Fjord (Fig. 5) is composed of massive nordmarkite
surrounded by a marginal breccia with large basalt
xenoliths (Brooks, 1991). The possible relation to the
Kangerlussuaq Alkaline Complex is obscured by the
Fjord. Biotite separated from massive nordmarkite
in sample NM27025 yielded an age of 50.4 ± 1.0 Ma
(Fig. 8, Table 1). This intrusion is a chronological
marker in that various dyke swarms mapped by Nielsen
(1978) either cut or are truncated by it.
3.10. Alkaline intrusions in the Wiedemann Fjord–
Kronborg Gletscher lineament
Five new ages are reported for alkaline intrusions of
this relatively unknown tectonic and magmatic lineament
(Pedersen et al., 1997; Bernstein et al., 1998a; Nielsen et
al., 2001) (Figs. 1 and 6). The alkaline and ultramafic–
mafic Ejnar Mikkelsen intrusion was discovered in 2000,
and is emplaced into Plateau Lavas in the lower eastfacing slopes of Ejnar Mikkelsen Fjeld, a prominent
mountain located at the Y-fork in the Kronborg Glacier
(Nielsen et al., 2001). Biotite separated from alkali gabbro
sample 457127 yielded an age of 51.8 ± 1.1 Ma. To
the north of Ejnar Mikkelsen Fjeld, several nephelinite
diatremes were discovered in 1995 (A. K. Pedersen, pers.
comm., 2000) cutting the Plateau Lavas (Fig. 6). Biotite
separated from nephelinite sample 436094 yielded an age
of 50.2± 1.2 Ma (Fig. 9; Table 1). The Borgtinderne
Syenite Intrusion is emplaced into the Plateau Lavas east
of Kronborg Gletscher and is dominated by leucocratic
nepheline syenite that intrudes alkali gabbro and
pyroxenite (Brown et al., 1978; Nielsen et al., 2001).
Amphibole from sample NM5133 of leucocratic
C. Tegner et al. / Lithos 101 (2008) 480–500
nepheline syenite yielded an age of 46.7 ± 1.0 Ma. This
is within the range of earlier titanite FT (49–46 Ma;
Gleadow and Brooks, 1979) and K–Ar ages (49–40 Ma;
Beckinsale et al., 1970; Noble et al., 1988). Swarms of
lamprophyre and basanite dykes trending approximately
N–S cut the Plateau Lavas, the above-mentioned plutons,
and the Lilloise Complex (50.3 ± 0.4 Ma; Tegner et al.,
1998a) (Fig. 6) (Pedersen et al., 1997; Bernstein et al.,
1998a; Nielsen et al., 2001). An age of 41.5 ± 0.9 Ma is
obtained for amphibole of sample 429285 from one of
many N–S trending lamprophyre dykes with mantle
xenoliths at Wiedemann Fjord (Bernstein et al., 1998a).
Biotite from a basanite dyke (also carrying mantle
xenoliths; sample 457163) cutting leucocratic syenite
of the Borgtinderne Syenite Intrusion, yielded an age of
36.5 ± 0.5 Ma. There is no apparent systematic relation
between age and location of the intrusions in this
lineament.
4. A revised chronology for magmatism in central
East Greenland
4.1. Chronology of plutons
Fig. 10 summarises precise 40Ar–39Ar, U–Pb, Rb–Sr
and Re–Os ages for intrusions of central East Greenland. A striking feature is the dominance of post-break-
493
up plutons of both alkaline and tholeiitic compositions.
In the coastal zone south of the Kangerlussuaq Fjord
lineament, the plutons are emplaced in three time
windows at 56–54 Ma, 50–47 Ma and 37–35 Ma. In
contrast, within the Kangerlussuaq Fjord lineament, the
age of intrusions spreads from 56 to 40 Ma. North of the
Kangerlussuaq Fjord lineament, the tholeiitic Sorgenfrei
Gletscher Sill Complex was emplaced at ∼ 56–55 Ma.
In the Wiedemann Fjord–Kronborg Gletscher lineament, the plutons are alkaline and span the period from
52 to 46 Ma.
4.2. Chronology of dyke swarms
Several generations of dyke complexes have been
described in central East Greenland. Here we use crosscutting relations and available radiometric ages (Nielsen, 1978; Klausen and Larsen, 2002; Hanghøj et al.,
2003) to reevaluate their chronology. In the coastal
zone, the dominant coast-parallel dykes termed either
THOL-1 (Nielsen, 1978), Tholeiitic Series (Hanghøj
et al., 2003) or Late Type 1 (Klausen and Larsen, 2002)
are dark dolerites that were rotated seawards during
breakup. They are interpreted as feeders to the overlying
Plateau Lavas and the Sorgenfrei Gletscher Sill
Complex (Nielsen, 1978; Hanghøj et al., 2003). One
of these dykes was dated at 56.6 ± 0.6 Ma (Tegner et al.,
Fig. 10. Summary of precise 40Ar–39Ar, U–Pb, Rb–Sr and Re–Os ages for intrusions in central East Greenland plotted vs. distance from Ammassalik
(sample locations projected to a line between Ammassalik and Scoresby Sund, Fig. 1). The rift-to-drift transition corresponds to the approximate age
consistent with magnetic anomalies at the continent–ocean boundary (Larsen, 1990). 40Ar–39Ar intrusion data from: (Holm, 1991) (n = 1); (Nevle
et al., 1994) (n = 3); (Hirschmann et al., 1997) (n = 1); (Tegner et al., 1998a) (n = 10); Heister et al. (in preparation) (n = 2), and this study (n = 16). U–Pb
intrusion data from Holm et al. (2006) (n = 2). Rb–Sr intrusion data from Waight et al. (2002) (n = 1) and Riishuus et al. (2005) (n = 1). Re–Os
intrusion data from Brooks et al. (2004) (n = 1). Also shown are 40Ar–39Ar age ranges of tholeiitic and alkaline lavas (Heister et al., 2001; Hansen
et al., 2002; Peate et al., 2003; Storey et al., 2007a). Magnetic chrons from Gradstein et al. (2004).
494
C. Tegner et al. / Lithos 101 (2008) 480–500
1998a). At Kap Gustav Holm located south of the
present study area (Fig. 4), a seawardly-rotated dyke has
been dated by 40Ar–39Ar to 54.7 ± 1.5 Ma (Lenoir et al.,
2003). Hence, the dominant, seawardly-rotated part of
the dolerite dyke complex is coeval with the Sorgenfrei
Gletscher Sill Complex and the Plateau Lavas (Tegner
et al., 1998a; Hansen et al., 2002; Storey et al., 2007a).
In the coastal zone, the coast-parallel, post-breakup
dykes are light-grey to grey or chocolate-brown and
termed TRANS (Hanghøj et al., 2003; Nielsen, 1978) or
Type 2 (Klausen and Larsen, 2002). Their compositions
are transitional between tholeiitic and alkali basalt and
comparable flood basalts are not known in East Greenland (Hanghøj et al., 2003). These dykes are often close
to vertical and therefore post-date coastal rotation
(Lenoir et al., 2003). They are typically cut by the
coastal layered gabbros and syenites (Nielsen, 1978)
and form a pattern that appears to emanate from the
large post-breakup gabbro complexes (Bernstein et al.,
1992; Klausen and Larsen, 2002; Callot and Geoffrey,
2004). One of these vertical dykes at Kap Gustav Holm
was dated to 48.0 ± 2.8 Ma (Lenoir et al., 2003), corroborating to the close spatial and petrogenetic relationship with the gabbro complexes dated at 50–47 Ma
(Tegner et al., 1998a) (Fig. 10). Coast-parallel, alkaline
basaltic dykes (ALK-1, Nielsen, 1978) cut across the
47.6 ± 0.3 Ma middle series of the Kap Edvard Holm
gabbro complex (Tegner et al., 1998a) and therefore
extend to younger ages than the transitional dykes.
A highly variable group of alkaline to strongly alkaline dykes including lamprophyre and basanite (ALK2) have been described in the Kangerlussuaq Fjord
(Nielsen, 1978) and the Wiedemann Fjord–Kronborg
Gletscher lineaments (Bernstein et al., 1998a). Two
ALK-2 dykes in the Wiedemann Fjord–Kronborg
Gletscher lineament are dated at 41.5 ± 0.9 Ma and
36.6 ± 0.5 Ma (Table 1), which indicates the likely time
span of these dykes. In the Kangerlussuaq Fjord, most
dykes are older than 46.6 ± 0.4 Ma, the age of the crosscutting Biotite Granite. The quartz porphyry dykes of
Flammefjeld and the late lamprophyre dykes north of
Amdrup Fjord, however, extend down to ages of 39.7 ±
0.1 Ma (Brooks et al., 2004) and 37–34 Ma (Gleadow
and Brooks, 1979), respectively.
5. Petrogenesis of post-breakup alkaline magmatism
Nielsen (1987) reviewed the petrology of alkaline
plutons, dyke complexes and lavas between 66° and
75°N. The large plutons range from alkaline ultramafic
and mafic cumulates over foyaite and silica-undersaturated syenite to quartz-syenite (Brooks and Nielsen,
1982; Deer, 1976). Despite this rich compositional
variety, Nielsen (1987) demonstrated that there were
essentially two kindreds of alkaline magmas; one was
strongly undersaturated nephelinite and includes trachybasalt and phonolite derived by fractional crystallization. The other kindred is mildly alkaline and appears to
represent the waning stages of tholeiitic magmatism.
Isotope compositions (Sr, Nd, Hf, Pb, H and O) of the
ultramafic and foyaitic plutonic rocks of the Gardiner
Complex, the Kangerlussuaq intrusion, and the Lilloise
intrusion are similar to those estimated for Iceland
plume mantle (Pankhurst et al., 1976; Nielsen and
Holm, 1993; Chambers and Brown, 1995; Bernstein
et al., 1998b; Riishuus et al., 2006). The quartz-syenites
and granites also relate to a nephelinite parent but often
with significant crustal assimilation (Brooks and Gill,
1982; Nielsen, 1987; Riishuus et al., 2005, 2006). The
mildly alkaline, post-breakup magmas, likewise, have
Sr–Nd–Hf–Pb–Os isotopic and trace element compositions consistent with mixing between enriched Iceland
plume mantle and local Archean crust (Brown et al.,
1996; Peate et al., 2003; Storey et al., 2004). The
formation of alkaline magmas with Icelandic isotopic
compositions has been explained by preferential melting
of enriched mantle components beneath thick, cold
lithosphere that suppresses melting of ambient upper
mantle peridotite (Bernstein et al., 2001; Peate et al.,
2003). This enriched plume signature carries evidence
of either recycled oceanic lithosphere (Bernstein et al.,
2001; Peate et al., 2003) or mantle metasomatised
during the crossing of the rifted margin over the axis of
the ancestral Iceland mantle plume (Storey et al., 2004;
Riishuus, 2005). In the following section, we discuss the
possible mechanisms that triggered melting of enriched
mantle components in the coast-orthogonal lineaments
up to 100 km inland from the continent–ocean boundary
and up to 20 m.y. after continental breakup.
6. Linking intrusions and rift-to-drift
6.1. Review of rift-to-drift models
A prolonged history of overlapping spreading ridges
and rift jumps in the central Northeast Atlantic adjacent
to the study area demonstrates that the rift-to-drift transition was complicated. Lundin (2002) argued that the
Atlantic spreading system propagated northwards in the
proto-Reykjanes and proto-Kolbeinsey ridges, close to
present-day East Greenland coast. At the same time the
Arctic spreading system propagated southwards by way
of the proto-Mohns and proto-Aegir ridges, creating
overlapping spreading ridges on the Greenland–Iceland
C. Tegner et al. / Lithos 101 (2008) 480–500
Rise that persisted to chron 6 (∼23 Ma) (Fig. 11c). In this
model, the Jan Mayen micro-continent was rifted away
from East Greenland by anti-clock rotation in response to
495
northwards propagation of the proto-Kolbeinsey ridge
(Nunns, 1983). According to others (Nunns, 1983; Scott
et al., 2005), the proto-Aegir ridge was displaced to the
east at the time of breakup and connected to the protoReykjanes and proto-Mohns ridges via two transform
faults (Fig. 11a). In the model of Larsen (1988), a
strongly curved “initial line of opening” (not shown in
Fig. 11) included the proto-ridges shown in Fig. 11a but
did not involve major transform faults. In these
scenarios, the proto-Kolbeinsey ridge was subsequently
initiated on the Greenland–Iceland Rise close to the
present-day Blosseville Kyst and propagated northwards
(Nunns, 1983; Larsen, 1988; Scott et al., 2005). In contrast to Nunns (1983) and Lundin (2002), Scott et al.
(2005) argued that the propagation of the protoKolbeinsey ridge and the retreat of the Aegir ridge
were linked by left-lateral, east–west trending transform
zones resulting in differential displacement of blocks of
the micro-continent (Fig. 11b). This was caused by a
counter-clockwise shift in the direction of plate movement (plate vectors in Fig. 11a and b).
We note that the proposed models unanimously
involve northward propagation of the proto-Kolbeinsey
ridge close to the East Greenland margin long after
continental breakup and flood basalt volcanism. The
leading tip of this system must have been a continental
rift that developed into oceanic spreading as it propagated northwards (Fig. 11b). The northwards younging
of magnetic seafloor anomalies along the Blosseville
Kyst (Fig. 1b) constrain the history of ridge propagation.
Identification of magnetochron 21 offshore the southern
Blosseville Kyst shows that the inception of the protoKolbeinsey ridge took place at ∼ 47 Ma or slightly older.
The presence of magnetochron 6 offshore from the
Scoresby Sund area (Fig. 1b) pinpoints the connection
Fig. 11. Schematic maps showing the physiography of proposed early
rift systems of the Northeast Atlantic. a) The initial spreading
configuration (∼ 50 Ma) of the proto versions of the Reykjanes ridge
(RR), the Aegir ridge (AR) and Mohns ridge (MR) linked via major
transform zones (Scott et al., 2005). Also shown are the tentative
locations of the Kangerlussuaq Fjord tectonic lineament (KF) (Karson
and Brooks, 1999) and post-breakup alkaline plutons at the inland
extension of the southern transform zone. b) Illustration at ∼40 Ma of
the separation of the Jan Mayen micro-continent (JM) and the
Blosseville Kyst (BK) by northwards propagation of the protoKolbeinsey ridge (KR) linked to retreat of the Aegir ridge via leftlateral transform zones (Scott et al., 2005). Tentative placing of the
Wiedemann Fjord–Kronborg Gletscher (WK) tectonic lineament
(Pedersen et al., 1997) and alkaline intrusions are also shown.
c) Illustrates the overlapping spreading model of Kolbeinsey and Aegir
ridges (∼ 30 Ma) of Lundin (2002). Abbreviations: IP = Iceland plume;
WJMFS and EJMFS = Western and Eastern Jan Mayen Fracture Zone,
respectively.
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C. Tegner et al. / Lithos 101 (2008) 480–500
of the proto-Kolbeinsey ridge, via the Jan Mayen
Fracture Zone, to the Mohns Ridge at 23 Ma, establishing the present-day spreading configuration of the
Northeast Atlantic (Fig. 1a) (Nunns, 1983).
6.2. Model for the formation of post-breakup intrusions
The age of seafloor basalts given by the magnetochrons along the continent–ocean boundary is compared
to the age of intrusions in Fig. 10. In the northern part of
the study area, the oldest seafloor basalts formed roughly
contemporaneously with the ∼ 52–36 Ma plutons and
dykes in the Wiedemann Fjord–Kronborg Gletscher
lineament. The north–south trend of the Wiedemann
Fjord–Kronborg Gletscher lineament (Figs. 1 and 6)
leads us to suggest that this magmatism represents a
continental rift zone related to the northward propagation
of the proto-Kolbeinsey ridge (Fig. 10c), perhaps into a
pre-existing tectonic lineament (Pedersen et al., 1997;
Karson and Brooks, 1999). This rifting attempt failed,
probably because it propagated into thick, cold continental lithosphere.
At more or less the same time (∼ 55–40 Ma) abundant and voluminous syenite, granite and gabbro
plutons were emplaced into the Kangerlussuaq Fjord
lineament (Figs. 1 and 5). This lineament has been
interpreted as a failed arm of a major triple junction
(Brooks, 1973; Karson and Brooks, 1999) and coincides
with the continent-ward extension of the transform fault
separating the proto-Reykjanes and proto-Aegir ridges
(Fig. 11a). We therefore explain the plutons of the
Kangerlussuaq Fjord as a result of prolonged off-axis
mantle melting beneath a pre-existing tectonic lineament (Fig. 11a). In contrast to the coastal area south of
Kangerlussuaq Fjord where magmatic activity is confined to three time windows at 56–54 Ma, 50–47 Ma
and 37–35 Ma (Tegner et al., 1998a), magmatism in the
Kangerlussuaq Fjord lineament appears to be continuous but with most activity in these time windows
(Fig. 10) (Holm et al., 2006). This reinforces the view of
an origin by off-axis magmatism focused in a major
tectonic lineament. We note that the Snaefellsnes
Volcanic Zone in Iceland (Fig. 1b) bears many similarities to the Kangerlussuaq Fjord lineament. Here it
has been suggested that contraction of the lithosphere
during cooling of the thick hotspot-generated crust
created a zone of weakness triggering renewed magmatism (Steinthorsson et al., 1985). Such a mechanism
could contribute to prolonged alkaline activity of the
Kangerlussuaq Fjord lineament. Below we discuss the
role of the Iceland mantle plume in driving post-breakup
magmatism in East Greenland.
Many islands and headlands south of Kangerlussuaq
Fjord are composed of layered gabbro plutons cut by
syenite and granite intrusions (Fig. 4) (Myers, 1980;
Brooks and Nielsen, 1982). The plutons are most abundant in the vicinity of the tectonic lineaments outlined by
Karson and Brooks (1999). The intrusive activity is
confined to three time windows at 56–54 Ma, 50–47 Ma
and 37–35 Ma (Fig. 10). The activity at 56–54 Ma, that
includes the oldest part of the Imilik Gabbro Complex
(Tegner et al., 1998a) and the seawardly-rotated part of
the dyke complex (Lenoir et al., 2003) relates to the
breakup event. Most of these intrusions are 50–47 Ma
(Fig. 10) and have been interpreted as renewed magmatism caused by the crossing of the East Greenland
margin over the axis of the Iceland plume (Tegner et al.,
1998a; Bernstein et al., 1998b). The origin of the 37–
35 Ma syenite, granite, and diorite intrusions of the
Kialineq area (Fig. 4) is more difficult to explain.
Passage of the rifted margin over the plume axis, for
example, cannot explain peaks in magmatic activity both
at 50–47 Ma and 37–35 Ma. The magmatic hiatus
between 47 and 37 Ma (Fig. 10) and the distance to the
proto-Reykjanes spreading ridge at the time suggest that
off-axis magmatism is not a viable explanation either.
Apatite FT thermochronology demonstrates a pronounced regional uplift of the rifted margin at ∼36 Ma
(Brooks, 1973; Gleadow and Brooks, 1979; Hansen and
Brooks, 2002). This uplift has been correlated to magmatism and offers the best explanation for the generation
of these intrusions, and possibly relate to plate-tectonic
reorganization associated with cessation of spreading in
the Labrador Sea (Nunns, 1983; Brooks et al., 2004).
6.3. Role of the Iceland plume
Brooks (1973) and Morgan (1981) explained the
anomalously thick oceanic crust of the ∼300 km wide
Greenland–Iceland–Faroes rise as the trace of voluminous plume magmatism centred on the spreading ridge
of the Northeast Atlantic (Fig. 1a). This view is adopted
by many subsequent workers (Larsen and Jacobsdottir,
1988; White and McKenzie, 1989; Saunders et al.,
1997). Holbrook et al. (2001) showed that the thickness
of igneous crust of the Greenland–Iceland Rise is nearconstant at ∼ 30 km and requires continuous melting of
upwelling mantle plume material. Plate-kinematic
reconstructions, however, show that the Iceland plume
was located beneath Greenland at the time of breakup
and the rifted margin crossed over the plume axis after
breakup (Lawver and Müller, 1994; Torsvik et al.,
2001). Bernstein et al. (1998b) and Tegner et al. (1998a)
argued that the abundance of 50–47 Ma layered gabbro
C. Tegner et al. / Lithos 101 (2008) 480–500
complexes in East Greenland (Fig. 10) recorded this
crossing. As discussed above, the magnetic seafloor
anomalies offshore the Blosseville Kyst demonstrate a
time gap between the main flood basalt succession in
East Greenland erupted at 56–55 Ma (Storey et al.,
2007a) and the oldest oceanic crust at the continent–
ocean boundary (∼47–23 Ma) (Fig. 1b). Larsen (1988)
explained this by initiation of spreading at the protoKolbeinsey ridge close to the East Greenland continent–
ocean boundary caused by a westward relocation of
spreading towards the axis of plume upwelling. Likewise, Müller et al. (2001) argued that thermal weakening
due to the crossing of a young (b 25 m.y. old), volcanic
rifted margin over a plume stem resulted in relocation of
the spreading ridge over the plume. The rifting of the Jan
Mayen micro-continent away from East Greenland was
suggested as an example of this process (Müller et al.,
2001). The isotopic similarities between the uncontaminated rocks of alkaline plutons in East Greenland (see
above) and Iceland corroborated to the causal role of the
Iceland mantle plume in post-breakup magmatism and
prolonged rift-to-drift transition.
7. Conclusions
Precise 40Ar–39Ar age determinations for syenite,
granite, gabbro, diorite, nephelinite, basanite, lamprohyre, and strongly undersaturated alkaline intrusions
revise the chronology of breakup and post-breakup
magmatism of the central East Greenland volcanic rifted
margin. South of Kangerlussuaq Fjord the plutons are
mainly emplaced in the coastal zone close to coastorthogonal tectonic lineaments and are confined to three
time windows at 56–54 Ma, 50–47 Ma and 37–35 Ma.
In the Kangerlussuaq Fjord, which coincide with a major
tectonic lineament possibly representing the failed arm
of a triple junction, the ages of plutons span from 56 to
40 Ma. A north–south trending tectonic and magmatic
lineament at Wiedemann Fjord–Kronborg Gletscher
cuts the continental flood basalts of the Blosseville
Kyst and includes alkaline plutons and dykes ranging
from 52 to 36 Ma.
Such prolonged post-breakup magmatism cannot be
explained by a simple rift-to-drift transition of a
volcanic rifted margin. We show that post-breakup
magmatism of the East Greenland rifted margin was
linked to reconfiguration of spreading ridges in the
central Northeast Atlantic Ocean basin. The protoKolbeinsey ridge was established at ∼ 47 Ma close to
the continent–ocean boundary of central East Greenland due to a westward relocation of spreading and
propagated northwards splitting the Jan Mayen micro-
497
continent away from the Blosseville Kyst. We explain
the intrusions of the Wiedemann Fjord–Kronborg
Gletscher lineament as a failed continental rift in the
early spreading history of the proto-Kolbeinsey ridge.
The alkaline post-breakup intrusions of the Kangerlussuaq Fjord lineament are explained as off-axis
magmatism to the proto-Kolbeinsey ridge. The crossing of the central East Greenland rifted margin, which
was moving in a westerly direction, over the axis of
the Iceland mantle plume was instrumental in causing a
prolonged rift-to-drift transition and widespread and
prolonged post-breakup tholeiitic and alkaline magmatism. This explains the asymmetry in the magmatic
history of conjugate rifted margins of the central
Northeast Atlantic.
Acknowledgements
We thank Dennis K. Bird, Kresten Breddam, Paul
Martin Holm and Asger Ken Pedersen for providing
samples SA-1, P-175, 85659, 40160, and 436094. Lew
Hogan and John Huard (Oregon State University) are
thanked for assistance with analytical work. We appreciate reviews by Ingrid Ukstins Peate, Anthony A. P.
Koppers and an anonymous colleague, and editorial
handling by Nelson Eby. This research was supported
by the Danish Lithosphere Centre and the Danish
Natural Science Research Council.
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.
lithos.2007.09.001.
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