doi: 10.1111/j.1365-3121.2010.00924.x
U–Pb and geochemical evidence for a Cryogenian magmatic arc in
central Novaya Zemlya, Arctic Russia
F. Corfu,1 H. Svensen,2 E.-R. Neumann,2 H. A. Nakrem3 and S. Planke2,4
1
Department of Geosciences, University of Oslo, PB 1047 Blindern, N-0316 Oslo, Norway; 2Physics of Geological Processes (PGP),
University of Oslo, PB 1048 Blindern, N-0316 Oslo, Norway; 3Natural History Museum, University of Oslo, PB 1172 Blindern, N-0318 Oslo,
Norway; 4Volcanic Basin Petroleum Research (VBPR), Oslo Innovation Park, N-0349 Oslo, Norway
ABSTRACT
A unique set of mafic intrusives collected during a 1921
expedition to Novaya Zemlya and reported to cut Devonian
sedimentary rocks, are shown instead to represent a Cryogenian magmatic arc. Zircon and titanite in four samples from
Mashigin Fjord and Matochkin Strait yield U–Pb ages of 716–
704 Ma and the dyke geochemistry shows typical subduction-
2010 Blackwell Publishing Ltd
Novaya Zemlya consists of an elongated and slightly curved, N-trending
80
500 km
couple of islands, which are separated
by a narrow strait (Fig. 1). They are
geologically linked to the Taimyr
thrust and fold belt and the Kara
block to the east and have been
thought to represent the continuation
of the Uralide orogen to the south,
although some consider this to be
incorrect (Torsvik et al., 2008).
Novaya Zemlya is dominated by
sedimentary rocks deposited in a
o
70
Severnaya
Zemlya
90
o
Taimyr
o
SIBERIA
Franz
Josef
Land
Kara
Sea
Svalbard
Fig. 2
Fig. 3
Ur
ali
NT
I
A
Novaya
Zemlya
Barents
Sea
S
Cacan
led din
on avi
ide an
s
N
CaE-G
led ree
on nla
ide nd
s
Correspondence: Fernando Corfu, Department of Geosciences, University of Oslo,
PB 1047 Blindern, N-0316 Oslo, Norway.
Tel.: (+47) 22 85 66 80; fax: (+47) 22
85 42 15; e-mail: fernando.corfu@geo.
uio.no
Geological setting
UR
E
The High Arctic of Scandinavia and
Russia consists of a collage of terranes
joined along orogenic sutures and
disrupted by Mesozoic and Cenozoic
rifting and the development of oceanic
basins. There are still many fundamental questions on the tectonic evolution and mutual relationships of the
different terranes, and potential affinities to either Siberia or Baltica (Cocks
and Torsvik, 2005; Metelkin et al.,
2005; Gee et al., 2006; Pease and
Scott, 2009).
Our study reports geochemical and
U–Pb geochronological data on mafic
dykes from central Novaya Zemlya.
These had been inferred to cut Devonian sedimentary rocks, and consequently to provide a testimony for one
of the post-Devonian Large Igneous
Provinces known in the Arctic. Novaya Zemlya is remote, logistically difficult to reach, and inhospitable after
decades of nuclear test programmes.
For our dating study, however, we
were able to use samples collected in
1921 by a Norwegian expedition led
by Olaf Holtedahl and now stored at
the Natural History Museum in Oslo
(Nakrem and Gradstein, 2007). Thus,
the samples are unique. Our initial
aim was to test whether the dykes
were related to a Large Igneous
Terra Nova, 00, 1–9, 2010
Province, but the results demonstrate
that the original interpretation of the
age of the dykes was incorrect as they
represent Neoproterozoic arc magmatism. The geological implications
of this discovery are discussed.
LA
Introduction
zone features. These rocks have the characteristics of a periGondwanan subduction-related complex, and originally could
have been affiliated to the Avalonian systems before accreting
to Baltica during the Timanian orogeny.
de
s
Timanides
BALTICA
BALTICA
Fig. 1 Palaeocene configuration of the main tectonic elements of the north Atlantic
polar region (modified from Gee, 2005). Coordinates approximate the present day
position.
1
Geochemistry and U–Pb age of mafic dykes, Novaya Zemlya • F. Corfu et al.
Terra Nova, Vol 00, No. 0, 1–9
.............................................................................................................................................................
semi-continuous manner throughout
the Palaeozoic. There is a CambroOrdovician unconformity in the south
(Pease and Scott, 2009), but not in the
north (Gee et al., 2006). Korago et al.
(2004) also document occurrences of
Neoproterozoic gabbro, lamprophyre
and granite. The southern parts of
Novaya Zemlya are inferred to have
been part of the Timanian belt, a
succession of metasedimentary and
volcanic rocks evolved at the margins
of Baltica and then accreted to the
craton during the Timanian orogeny
at 600–550 Ma (Puchkov, 1997; Gee,
2005; Pease et al., 2008; Pease and
Scott, 2009). The rocks were folded in
the Late Triassic to Earliest Jurassic
and thrust westward by an amount
originally estimated at 500–700 km
(Otto and Bailey, 1995; Torsvik and
Andersen, 2002), but now considered
to be about 200 km (Buiter and Torsvik, 2007; Cocks and Torsvik, 2007;
Scott et al., in press).
Samples
Three of the samples stem from
Mashigin Fjord and one is from the
western entrance of Matochkin Strait,
which separates the northern island
from the southern island (Figs 1–3).
On the cross-section along the northern shore of Mashigin Fjord (Fig. 2),
Holtedahl (1924) distinguished a western part with shale and limestone,
locally highly fossiliferous and Lower
Devonian to Permian, a central part
with mainly sandstones and phyllites
Fig. 2 Simplified geological map of the Mashigin area. From Tkačenko and
Egiazarov (1970), modified based on descriptions from Holtedahl (1924). Precambrian age of the unfossiliferous sedimentary and volcanic rocks is inferred from our
new data. Approximate sample locations are also shown. The cross-section is taken
from Holtedahl (1924), but the thrust is inferred from the geological descriptions and
our results.
2
intruded by north-trending diabase
dykes and with some lavas, and an
eastern part consisting of a variety of
volcanoclastic rocks, metasandstones
and local fossiliferous Devonian
shales and limestones. The three samples analysed in this study represent
diabase dykes from the central parts
of the section. Samples NZV-288 and
NZV-152 consist of a matrix of
highly altered feldspar with prismatic
and commonly twinned clinopyroxene (1–2 mm) locally replaced and ⁄ or
surrounded by very fine-grained chlorite and ⁄ or hornblende. Sample NZV281 is finer grained (<1 mm), ophitic
and largely altered, plagioclase to
saussurite and the mafic minerals to
amphibole, biotite, chlorite and epidote, with secondary quartz and
carbonate. The petrographic features
of diabase dykes in this area have
been described by Backlund (1930,
pp. 42–47).
According to Backlund (1930,
p. 17), who drew from the field
descriptions of Holtedahl (1924), the
intrusive rocks at Matochkin Strait
(Fig. 3) occur as thick conformable
sheets, locally porphyritic and finegrained. To the west, there are Lower
Devonian Favosites limestone and
dolomite, but the contacts to the
intrusive rocks are of uncertain nature. The bottom of the westernmost
intrusive body displays a schistosity
suggesting faulting or west-directed
thrusting (Holtedahl, 1924; Backlund,
1930). In addition, the inferred hangingwall, studied by Cissarz (1928)
along its projection south of the fjord,
shows the presence of contact metamorphic marbles. For the area north
of the fjord, Backlund (1930) and
Holtedahl (1924) mention the observation by Dietrichson of crosscutting
pegmatites and quartz-veins. These
could be correlative to c. 600 Ma
granitic veins mentioned by Korago
et al. (2004) from Sulmenev Bay farther south. Backlund (1930) states
that the sedimentary rocks to the east
of the intrusive sheets include strata
that are at least Ozarkian (i.e. a period
at the Cambro-Ordovician boundary)
or younger, and he concludes that the
intrusions are therefore likely postEarly Devonian.
More recently, Korago et al. (2004)
define the ÔMitushev kamen Granite
MassifÕ as a north-trending complex
exposed over roughly 40 by 3.8 km,
2010 Blackwell Publishing Ltd
F. Corfu et al. • Geochemistry and U–Pb age of mafic dykes, Novaya Zemlya
Terra Nova, Vol 00, No. 0, 1–9
.............................................................................................................................................................
Fig. 3 Simplified geological map of the Matochkin Strait area. From Tkačenko and
Egiazarov (1970) modified based on descriptions from Holtedahl (1924) and Korago
et al. (2004). Sample location is also shown.
dominated by alaskite granite, and
generally highly deformed, in tectonic
contact to Upper Silurian and Lower
Devonian sedimentary rocks. Locally,
the granite is overlain unconformably
by Upper Silurian sediments. Korago
et al. (2004) quote Pb–Pb zircon evaporation ages of 680 ± 50 and 730 ±
50 Ma by themselves, and zircon
U–Pb ages of 609 ± 4, 587 ± 7 and
717 ± 4 Ma presented in a conference
report by Kaplan and co-workers.
Our sample (NZV-287) is more mafic
than the typical granite of this
complex and corresponds to the
quartz-diorite described by Backlund
(1930).
2010 Blackwell Publishing Ltd
U–Pb results
The four samples contained comparable zircon populations dominated by
euhedral long prismatic, generally
highly fragmented crystals, a common
type in gabbroic rocks. The U contents range from 170 to over
800 p.p.m. (Table 1) and Th ⁄ U tends
to be high at 0.4 and 1.1, another
common feature of zircon in gabbroic
rocks.
The six analyses from sample
NZV-281 from Mashigin Fjord
(Fig. 2) vary in terms of precision
and are between 1% and 5% discordant and collinear yielding an upper
intercept of 716 ± 8 Ma and a lower
intercept age at 227 ± 170 Ma
(Fig. 4a). The lower intercept corresponds to the time of the main
Mesozoic deformation affecting the
islands and it is reasonable to assume
that the same disturbance also affected the other samples. Those data
are thus regressed through a lower
intercept of 200 Ma using a conservative uncertainty of ±100 Ma. The
analyses from sample NZV-152
are variably discordant yielding an
upper intercept age of 706 ± 14 Ma
(Fig. 4b). The third sample from
Mashigin Fjord, NZV-288, has nominally concordant data and provides
an upper intercept age of 704 ±
5 Ma (Fig. 4a,c), close to the average
207
Pb ⁄ 206Pb age.
Sample NZV-287 from Matochkin
Strait (Fig. 3) yields a cluster of five
analyses that are close to concordant
and when projected from 200 ±
100 Ma intersect the Concordia
curve at 707.1 ± 1.7 Ma (Fig. 4d).
Cores, identified on the basis of their
rounded shape inside some prisms,
yield an older age at around
720 Ma. Titanite is rich in U (360–
440 p.p.m.), has high Th ⁄ U (1.5–
1.9), but also contains 3.4–
4.1 p.p.m. common Pb and hence
the analyses are not very precise.
The two data points overlap the
zircon discordia line and can be
explained either as indicating a
post-crystallization event at 699–
702 Ma or slight resetting by later
Pb loss.
Geochemical results
The Novaya Zemlya dykes show a
restricted range of relatively evolved
major element compositions with 50–
54 wt% SiO2 and 2.8–5.4 wt% MgO
(Table 2) and classify as tholeiitic to
subalkaline basalts to basaltic andesites (Fig. 5). The rocks are relatively
rich in the most incompatible elements (e.g. 90–340 p.p.m. Ba, 1.8–
5.6 p.p.m. Th), but have moderate
concentrations of rare earth elements
(REE; e.g. 7–24 p.p.m. La; Table 2),
and low to moderate concentrations
of compatible elements such as
Ni (5–40 p.p.m.) and Cr (95–120
p.p.m.). The most evolved sample
(NZV-281 with 2.8 wt% MgO) has
the highest concentrations of incompatible, and lowest of compatible
3
4
U
(p.p.m.)*
4.8
7.4
0.7
1.0
0.5
0.7
0.9
1.1
0.5
0.6
1.2
2.1
2.8
35.3
3.3
2.3
1.9
4.6
14.4
15.9
0.93
0.76
0.75
0.81
0.90
1.10
0.83
0.61
0.40
1.05
0.77
1.12
0.96
0.99
0.96
0.85
0.80
0.79
1.86
1.50
Th ⁄ U Pbc
(pg)
879
7254
14 850
6770
2899
567
820
4951
1555
1020
1542
236
4058
2202
1484
219
4414
2769
7568
4551
Pb z
204 Pb
206
Pb x
U
(abs)
1.0321
0.9979
0.9996
1.0001
0.9991
0.9907
0.9956
0.9973
0.9944
0.9951
1.0009
0.9623
0.9524
0.9991
1.0068
0.9961
0.9944
0.9548
0.9961
0.9993
235
207
0.0039
0.0027
0.0023
0.0025
0.0027
0.0055
0.0040
0.0045
0.0062
0.0072
0.0055
0.0295
0.0079
0.0046
0.0033
0.0133
0.0041
0.0041
0.0232
0.0179
±2r
Pb x
U
0.11801
0.11508
0.11517
0.11529
0.11510
0.11457
0.11505
0.11511
0.11457
0.11476
0.11529
0.11349
0.11044
0.11492
0.11568
0.11462
0.11437
0.11016
0.11411
0.11463
238
206
0.00024
0.00027
0.00023
0.00024
0.00026
0.00029
0.00024
0.00046
0.00053
0.00061
0.00042
0.00076
0.00090
0.00037
0.00026
0.00050
0.00042
0.00039
0.00277
0.00217
±2r
0.60
0.85
0.94
0.89
0.84
0.47
0.58
0.89
0.73
0.63
0.62
0.43
0.94
0.74
0.71
0.36
0.83
0.76
0.95
0.93
q
Pb x
Pb
(abs)
0.06343
0.06289
0.06295
0.06292
0.06296
0.06271
0.06277
0.06284
0.06295
0.06289
0.06296
0.06149
0.06255
0.06306
0.06313
0.06303
0.06306
0.06286
0.06331
0.06323
206
207
0.00019
0.00009
0.00005
0.00007
0.00009
0.00031
0.00021
0.00013
0.00027
0.00036
0.00027
0.00175
0.00017
0.00020
0.00015
0.00079
0.00015
0.00018
0.00047
0.00045
±2r
Pb x
U
(Ma)
719.1
702.2
702.7
703.4
702.3
699.2
702.0
702.3
699.2
700.3
703.4
693.0
675.3
701.2
705.6
699.5
698.1
673.7
696.6
699.6
238
206
Pb x
U
720.0
702.7
703.6
703.9
703.4
699.1
701.6
702.4
700.9
701.3
704.3
684.5
679.3
703.4
707.3
701.8
700.9
680.6
701.8
703.5
235
207
Pb x
Pb
722.7
704.5
706.5
705.4
706.7
699
700.3
702.7
706.5
704
707.0
656
692.8
710.2
712.5
709
710.2
703.5
719
716
206
207
6.4
3.0
1.6
2.4
3.0
10
7.0
4.4
9.0
12
9.1
60
5.9
6.6
4.9
26
4.9
6.0
16
15
±2r
0.5
0.3
0.6
0.3
0.7
)0.1
)0.2
0.1
1.1
0.6
0.5
)5.9
2.7
1.3
1.0
1.5
1.8
4.5
3.3
2.4
Disc
(%)
Geochemistry and U–Pb age of mafic dykes, Novaya Zemlya • F. Corfu et al.
Z, zircon (unless otherwise specified all clear and transparent); T, titanite; eu, euhedral; an, anhedral; lp, long prismatic (l ⁄ w ‡ 4); fr, fragment; co, cores; pb, pale-brown; b, brown; r, red; pnk, pink; [1], number of grains in
fraction; Pbc, total common Pb in sample (initial + blank); Disc, degree of discordancy.
*Weight and concentrations are known to better than 10%, except for those near and below the 1 lg limit of resolution of the balance.
Th ⁄ U model ratio inferred from 208 ⁄ 206 ratio and age of sample.
àRaw data corrected for fractionation.
§Corrected for fractionation, spike, blank and initial common Pb; error calculated by propagating the main sources of uncertainty.
NZV-281, diabase, Mashigin Fjord
Z lp-fr (eu) [5]
6
163
Z lp-fr [1]
1
208
Z fr [5]
1
442
Z fr pb [3]
1
385
Z fr [10]
1
573
Z fr [2]
1
440
NZV-152, diabase, Mashigin Fjord
Z fr (eu) [1]
1
183
Z fr (eu) [1]
1
34
Z fr pnk [1]
1
310
Z fr [4]
1
191
NZV-288, diabase, Mashigin Fjord
Z fr (eu) [8]
1
844
Z fr (eu) [1]
1
455
Z fr (eu) [1]
1
384
NZV-287, quartz-diorite, Matotchkin Strait
Z an co [6]
17
242
Z lp-fr [30]
13
252
Z lp-fr [15]
28
172
Z lp-fr [15]
10
179
Z lp-fr [13]
10
185
T fr b-r [2]
3
367
T fr b-r [3]
4
443
Sample
Weight
(lg)*
Table 1 U–Pb data.
.............................................................................................................................................................
Terra Nova, Vol 00, No. 0, 1–9
2010 Blackwell Publishing Ltd
F. Corfu et al. • Geochemistry and U–Pb age of mafic dykes, Novaya Zemlya
Terra Nova, Vol 00, No. 0, 1–9
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(a)
NZV-281 6 pts
227 ± 170 & 716 ± 8 Ma
MSWD = 0.33
(b)
710
710
NZV-152 4 pts,
from 200 ± 100 Ma
706 ± 14 Ma
0.116
MSWD = 2.7
700
Zircon
700
0.114
690
Zircon
690
0.112
680
680
206Pb/238U
0.110
0.95
(c)
Diabase
Maschigin Fjord
0.99
0.93
NZV-288 3 pts
0.97
1.01
706
NZV-287 4 pts
(d)
706
from 200 ± 100 Ma
0.1156
670
Diabase
Maschigin Fjord
from 200 ± 100 Ma
707.1 ± 1.7 Ma
704 ± 5 Ma
MSWD = 0.42
704
MSWD = 0.65
704
0.1152
Zircon
702
702
Xenocrystic zircon
Titanite
0.1148
700
722
700
718
698
Zircon
698
0.1144
0.1140
Diabase
Maschigin Fjord
0.988
0.996
Quartz-diorite
Matochkin Strait
696
0.984
1.004
0.992
1.000
207Pb/235U
Fig. 4 (a–d) Concordia diagram with U–Pb data for zircon and titanite. The analyses were carried out by ID-TIMS (Krogh, 1973)
on hand-picked and abraded (Krogh, 1982) single- or multigrain fractions of zircon and titanite. Details of the procedure are given
in Corfu (2004). Decay constants are those of Jaffey et al. (1971). The initial common Pb correction, when needed, was based on
Stacey and Kramers (1975). Plotting and age calculations employed the program Isoplot by Ludwig (2003). Ellipses are drawn at 2r.
elements. Strontium shows a shift
from positive anomalies in the most
Mg-rich samples to a strong negative
anomaly in the most evolved sample
(Fig. 6a). The essentially parallel patterns imply that, although altered,
the dykes have retained their original
trace element characteristics.
Normalized to primordial mantle
(PM; McDonough and Sun, 1995;
Fig. 6a), the data show a strong
enrichment in the most incompatible
elements (Cs, Rb, Ba, Th, U) relative
to REE, marked negative anomalies
for Nb–Ta and positive anomalies for
Pb. Two of the samples also show
positive K-anomalies.
2010 Blackwell Publishing Ltd
Discussion
Geological significance of the ages
The new ages clearly demonstrate that
the hypothesis of a Mesozoic (or
younger) age of the dykes was incorrect. The four samples yield coherent
data indicating intrusion between 716
and 704 Ma, with essentially coeval
titanite, but also some older cores, in
NZV-287.
A Neoproterozoic age of the dykes
is also consistent with the evidence
reported in the literature. The geological descriptions of Mashigin Fjord
(Holtedahl, 1924; Backlund, 1930)
indicate that the dykes intrude unfossiliferous sedimentary rocks, separated
from
the
fossil-bearing
Devonian strata by a zone of shearing
(Fig. 2). Hence, the contact is probably tectonic and the sequence hosting
the dykes probably represents Precambrian strata.
At Matochkin Strait (Fig. 3), the
fossiliferous Devonian strata in the
west are bordered by sheared diorite
sheets with no evidence of contact
metamorphism (Backlund, 1930).
There is a contact metamorphic aureole
in sedimentary rocks at the eastern edge
of the main granitoid pluton, but those
have no fossils. These relationships are
5
Geochemistry and U–Pb age of mafic dykes, Novaya Zemlya • F. Corfu et al.
Terra Nova, Vol 00, No. 0, 1–9
.............................................................................................................................................................
Table 2 Major (wt%) and trace element
(p.p.m.) concentrations for dykes from
Novaya Zemlya. The data were acquired
using ICP-AES ⁄ MS at the Department
of Earth Sciences, Royal Holloway University of London, UK.
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
Sum
Ba
Co
Cr
Cu
Li
Ni
Sc
Sr
V
Zn
Zr
Pb
U
Th
Rb
Nb
Cs
Hf
Ta
Tl
Y
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Mo
Sn
NZV152
NZV281
NZV287
NZV288
49.91
1.62
12.41
15.67
0.236
4.96
9.69
2.13
0.37
0.14
97.14
53.52
2.72
12.24
14.01
0.234
2.81
5.72
3.51
1.75
0.64
97.15
53.70
1.38
15.59
11.97
0.159
3.35
7.10
3.25
1.21
0.12
97.83
50.69
1.59
12.51
14.45
0.238
5.41
9.40
2.68
0.44
0.13
97.54
89
44
124
212
37
36
45
471
397
119
112
10
0.46
2.05
14
7.3
3.01
2.97
0.43
0.14
36
8.2
21.3
2.7
12.3
4.31
1.36
4.45
0.90
5.53
1.12
3.04
0.47
3.21
0.48
1.1
1.1
340
34
95
33
32
7
36
168
279
115
200
11
1.28
5.89
45
16.0
1.11
5.32
1.10
0.43
44
24.3
56.6
6.7
29.4
7.57
2.34
6.66
1.22
6.97
1.36
3.65
0.53
3.55
0.52
0.8
1.4
200
32
102
41
25
5
26
236
366
99
123
6
0.32
2.05
35
6.5
1.73
3.25
0.41
0.36
30
11.6
29.0
3.7
15.4
4.23
1.10
3.91
0.78
4.61
0.95
2.64
0.39
2.70
0.44
0.8
1.3
125
42
100
185
25
40
45
267
361
112
99
7
0.41
1.80
15
5.5
0.78
2.58
0.36
0.14
30
7.2
18.0
2.5
10.6
3.71
1.15
3.82
0.81
4.62
1.00
2.60
0.39
2.56
0.42
0.3
0.9
consistent with thrusting of a Neoproterozoic basement complex on the
Devonian sedimentary assemblage, a
relationship supported by the c. 730–
590 Ma granite dated further north
(Korago et al., 2004).
6
Fig. 5 Total alkali–SiO2 classification diagram of Le Bas et al. (1986) and Kuno
(1968). Also plotted for comparison is the field for amphibolites of the Yakorninsky
Suite from the same location from data reported by Korago et al. (2004). The data for
granites of the Sulmenesvsky and Mitushevsky Suite plot to the right of the diagram
at SiO2 concentrations of 65–78% and total alkalis of 4.2–8.6%, probably affected
mostly by fractionation processes.
(a)
(b)
Fig. 6 Spidergrams comparing PM-normalized trace element abundances of the
dykes in central Novaya Zemlya (a) to those of Volcán Ollagüe lavas from the Andes
(b). The similarity supports an origin of the Novaya Zemlya dykes in a subduction
setting.
In detail, the available maps and
descriptions of the region are not
always clear and, in part, contradic-
tory. The nature of some important
contacts is speculative with evidence
of intrusive contacts, unconformities
2010 Blackwell Publishing Ltd
F. Corfu et al. • Geochemistry and U–Pb age of mafic dykes, Novaya Zemlya
Terra Nova, Vol 00, No. 0, 1–9
.............................................................................................................................................................
Melt origin
The Novaya Zemlya dykes show compositional characteristics that are typical of arc magmatism with the
characteristic high Rb–Ba, strongly
negative Nb–Ta-anomalies, positive
K-, Pb- and Sr-anomalies and relatively flat REE patterns (e.g. Sun,
1980; Wilson, 1989). The similarity to
arc lavas is emphasized in Fig. 6b
where the Novaya Zemlya rocks are
compared to calcalkaline lavas with
similar MgO contents in one of the
Central Andean stratavolcanoes, Volcán Ollagüe (Feeley and Davidson,
1994). We note the similar concentrations of incompatible elements and
trace element patterns, both showing a
shift from positive to negative Sranomalies accompanied by a general
increase in the concentrations in other
incompatible elements with decreasing
MgO. This pattern is the typical result
of fractional crystallization dominated
by removal of feldspar. The chemical
characteristics of these dykes thus
support emplacement during arc magmatism rather than LIP volcanism.
Regional implications: Cryogenian
subduction and a peri-Gondwanan
connection?
Geological relationships, such as unconformities, suggest that southern
Novaya Zemlya was affected by the
Timanian orogeny by accreting to
Baltica at about 600–550 Ma (Gee,
2005; Pease and Scott, 2009). In the
investigated region, there are reports
of granitic activity at 620–600 Ma
(Korago et al., 2004). We notice,
however, that titanite of sample NZ287 has not been affected in any
strong way at 600–550 Ma, in spite
of the susceptibility of this mineral to
metamorphic resetting.
The origin of these 716–704 Ma
basement units remains uncertain.
2010 Blackwell Publishing Ltd
30o N
(a)
NORTH
CHINA
BALTICA
LAURENTIA
Equator
S-Taimyr
RIO
PLATA
SIBERIA
Tim
an
ian
AMAZONIA
Avalon
ian
and thrusts. The various lines of
evidence suggest that this is a zone of
imbrication formed by juxtaposition
of slivers of Palaeozoic and Precambrian rocks, the latter locally with an
unconformable Palaeozoic sedimentary cover. These units were folded
and tectonically interleaved during
westward thrusting, presumably in
the Late Triassic (Otto and Bailey,
1995).
Baikalian
30o S
750 Ma
(b)
SIBERIA
30o N
NORTH
CHINA
KARA
LAURENTIA
Equator
RIO
PLATA
AMAZONIA
BALTICA
30o S
Fig. 7 Hypothetical continental configurations at 750 Ma: (a) Baltica in an
upside-down position as proposed by
Hartz and Torsvik (2002); from Cocks
and Torsvik (2005). (b) Baltica in the
conventional orientation, here in the
position given by Li et al. (2008).
There is no record of magmatism at
this time in the present northern
regions of the Baltic Shield, except
for the Caledonian Kalak Nappe of
Finnmark that, however, has an affinity to Laurentian and western
Gondwana elements and probably
originated outside of Baltica (Kirkland et al., 2006, 2007, 2008; Corfu
et al., 2007). The Kalak Nappes were
thrust on Baltic basement during the
Caledonian orogeny (e.g. Corfu et al.,
2006), while Novaya Zemlya had
already joined Baltica at 600–
550 Ma (Roberts and Siedlecka,
2002). Neoproterozoic elements are
also present in the central Taimyr
terranes, now welded to northern
Siberia (e.g. Pease et al., 2001; Metelkin et al., 2005; Pease and Scott,
2009). Pease and Persson (2006) document c. 700 Ma island arc volcanism
in northern Taimyr. The Enganepe
ophiolite, now part of the Uralian
orogen, developed at 670 Ma in an
oceanic island arc setting and was
then thrust on Baltica during the
Timanian convergence (Scarrow et
al., 2001). The events at 716–704 Ma
also correlate with arc magmatism
recorded on different margins of the
Siberian craton (Amelin et al., 1996;
Vernikovsky et al., 2003, 2004). In
more general terms, it also fits with
the protracted activity recorded in the
peri-Gondwanan realm.
There is still much uncertainty on
the Neoproterozoic configuration of
the continents, depending on variable
interpretations of palaeomagnetic evidence and geological constraints. The
model proposed by Hartz and Torsvik (2002) (Fig. 7a) considers that
Baltica was in an upside-down position in the Cryogenian. In this configuration, the arc magmatism in
Novaya Zemlya and Timanian convergence can easily be explained
within the framework of the magmatic arcs developed at the periphery
of Gondwana (e.g. Roberts and Siedlecka, 2002). A class of alternative
models, such as that discussed by Li
et al. (2008), place Baltica at the edge
of Rodinia and arc magmatism is
inferred to have been active along
this margin, potentially linking the
development of Novaya Zemlya to
coeval successions in Siberia, China
and elsewhere (Fig. 7b).
Conclusions
A study of the geochemistry and age
of dykes collected during the 1921
Norwegian Arctic expedition has
shown that intrusions of mafic rocks
in central Novaya Zemlya were related to arc magmatism in the Cryogenian at 716–704 Ma. The hosting
sedimentary succession is therefore of
Precambrian age, rather than Devonian. The major and trace element
geochemistry shows Andean-type signatures, suggesting emplacement in a
subduction setting. A connection with
the peri-Gondwanan realm most easily explains the tectonic relationships,
the alternative requiring the existence
of an independent arc system or
extensive translation of terranes.
Acknowledgements
We thank Morten Schjoldager for preparing the mineral separates and acknowledge
constructive reviews by V. Pease, B. Bingen
and D. Gee.
7
Geochemistry and U–Pb age of mafic dykes, Novaya Zemlya • F. Corfu et al.
Terra Nova, Vol 00, No. 0, 1–9
.............................................................................................................................................................
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9