Research Note 

An Early Proterozoic U-Pb zircon age from an 
Eskolabreen Formation gneiss in southern Ny 
Friesland, Spitsbergen 
YU. A. BALASHOV. A. N. LARIONOV. L. F. GANNIBAL. A. N .  SIROTKIN, A. M. TEBENKOV. G .  1 .  
RYUNGENEN and Y. OHTA 

.. 
Balashov, Yu. A . ,  Larionov, A. N.. Gannibal. L. F.. Sirotkin. A .  N., Tebenkov, A. M.. Ryiingenen, G .  
I .  & Ohta. Y. 1993: An Early Proterozoic U-Pb zircon age from an Eskolabreen Formation gneiss in 
southern N y  Friesland. Spitsbergen. Polar Research 12(2), 147-152. 

A preliminary U/Pb zircon age determination has been carried out on a grey gneiss of the Eskolabreen 
Formation, the lowest observable lithostratigraphic unit of Precambrian metamorphic rocks in southern 
Ny Friesland. NE Spitsbergen. The obtained age, ca. 2,400Ma, is considered to be a metamorphic age 
and suggests an Early Proterozoic tectonothermal event. 

Yu. A .  Balashou, A .  N .  Larionov, L .  F. Cannibal, and C .  I .  Ryungenen. Laboratory of Isotope Geochem- 
istry, Geological Institute, Kola Science Centre. Russian Academv of Sciences, 14 Fersman Str., 184200 
Apatity, Russia; A .  M .  Tebenkov and A .  N .  Sirorkin, Polar Marine Geological Exploration Expedition. 24 
Pobedu Sir., I89510 Lomonosou-St. Petershurg. Russia: Y. Ohta. Norsk Polarinstitutt. P. 0. Box 5072 
Majorstua, N-0301 Oslo. Norway. 

Introduction 
Rcccnt gcochronological works on the pre-Caledonian base- 
ment of Svalbard have revealed not only a late Middle Pro- 
terozoic tectonothermal event, the Grenvillian (Peucat et al. 
1989: Tebcnkov ct al. 1991: Balashov et al. 1992; Gee et al. in 
press), hut also a late Early Proterozoic event (Gee et al. 1992; 
Gavrilenko & Kamensky 1992) and some Early Proterozoic and 
older U-Pb zircon upper interccpt ages (Peucat et al. 1989; 
Balashov ct at. 1992) and Sm-Nd model ages (Bernard-Griffiths 
et al. 1993). 

Since the N y  Friesland area, northeastern Spitsbergen. has 
complex structures (Manby 1990: Gee et at. 1992) and the 
largest distribution of the pre-Devonian crystalline basement. 
Early Proterozoic rocks can naturally be expected. This note 
reports an Early Proterozoic U-Pb zircon age obtained from a 
gneiss of the Eskolabreen Formation, the lowest litho- 
stratigraphic division of the basement rock succession as defined 
by Harland et at. (1966). 

Geological setting 
N y  Friesland occupies the middle part of the basement areas of 
northern Svalbard (Fig. I ) .  The structure in the southern part 
of Ny Friesland is dominated by an asymmetric antiform, the 
Atomfjella antiform (Harland 1959). The axis plunges gently 
to the north. The western limb of the antiform is cut by a steep 
fault zone, dipping 5C-80" to the west, which is a part of the N-S 

trending Billefjorden Fault Zone (Harland et al. 1974), the 
latter down throws to the west. Some Carboniferous cover has 
been preserved on the basement block in the east side of 
Austfjordcn. The castern limb of the antiform is cut by a steep 
E dipping, partly thrust. fault within the antiform and the 
Finlandvcggcn Group is in contact with the Harkerbrcen Group 
(Table 1 )  by this fault. Along the eastern margin of the antiform 
the Harkerbreen Group is in contact with the Mossel series 
(Krasil'BEikov 1973) or the Planetfjella Group of the upper 
Stubendorffbreen Supergroup (Harland et at. 1966) by a W- 
dipping thrust fault. 

The core of the antiform is composed of the Smutsbreen 
and the Eskolabreen Formations of the Finlandveggen Group 
(Harland et at. 1966; Krasil'Sfikov 1973; Harland 1985, 1992). 
The Smutsbreen Formation consists of grey biotite gneisses with 
or without garnet and distinctive marble layers. The latter are 
in both the lower and upper parts, and small amounts of calc- 
silicate-gneisses and amphibolites also present. The Esko- 
labreen Formation is the lowest observable lithostratigraphic 
unit and is composed of garnet-hornblende-biotite- and horn- 
blende-biotite gneisses. amphibolites and granitic gneisses. 
These rocks have upper amphibolite facies mineral assemblages 
of high temperature scries with andalusite-sillimanite, while the 
rocks of this formation in the limbs have kyanite-garnet-mica 
assemblages without sillimanite. indicating a lower temperature 
condition than in the core and an intermediate temperature- 
pressure series. 

The FlPtan granite in the northern part of Ny Friesland (Fig. 
1) has a 1,788 + 53/-45 Ma U-Pb zircon isochron age. It also 



148 Yu. A. Balashou et al. 

Fig. 1. Generalized geological map of southern Ny Friesland. with the sample locality. lnsertcd map: Solid square = map area, 
dashed area = prc-Devonian basement, NF = Ny Friesland, F = Flatan granite. Legend: Cb = Carboniferous. PI = Planetfjella 
Group or Mosscl series. Rt = Ritlervatnet Formation, Hk = Polhem o r  Harkerbreen Formation. Sm = Smutsbreen Formation. 
Es = Eskolabrecn Formation, Ft = fault and thrust, Ant = Atomfjella antiform. S with a solid circle = sample locality. 

yielded zircon single grain ages of ca. 1.70UMa (Gee ct al. 
1992). This granite possibly intruded in the gneisses and 
amphibolitcs of the Polhem Formation in the lower part of 
the Harkerbreen Group (Table 1). Thus the protoliths of the 
gneisses and amphibolites are of Early Proterozoic. Ultramafic 
rocks separating the Polhem Formation from the Planetfjella 
Group have yielded a ca. 1.8 G a  age and a biotite concentrated 
rock adjacent to them gives an age of 500Ma. both by the 
K-Ar method (Gavrilenko & Kamensky 1992). though the 
reliability of these K-Ar data is a matter of debate. 

This note presents a preliminary U-Pb zircon age obtained 
from an Eskolabreen Formation gneiss in southern Ny Fries- 
land. This age was orally presented by Balashov in August 1993 
at Turku, Finland. 

Materials and methods 
Sample description 
A grey gneiss (ca. 30 kg) from the Eskolabreen Formation in 
the middle-southern side of Stubendorffbreen has becn used 
for this isotopic study. The locality is shown in Fig. I .  The rock is 
a coarse-grained, feldspar porphyroblastic, garnet-hornblende- 
biotite gneiss with a modal composition of 2.%30% quartz, 2.5- 
30% plagioclase (An,&, 3 U %  K-feldspar, 10% biotite, 1- 
2% hornblende and I-2% garnet with accessories of apatitc, 
zircon, monazite and opaques; the secondary minerals are chlor- 
ite, sericite and epidote. 
This gneiss forms persistent layers, u p  to 5 m in each thick- 



A n  Early Proterozoic U - P b  zircon age from an Eskolabreen Formation 149 

Sarrbreen Fm 
Vassfaret Fm. 
Bangenhuk Fm. 
Rittervatnet Fm. 
Harkerbreen Fm. 

Fig. 2 .  Morphotypes of the zircon grains. scanning electro-micrographs. Upper left, morphotype 1 ,  x480, upper right, morphotype 
1 x IhX; lower left and lower right morphotypc 2, both x 240. 

D s 
3 
(D 
iii 

3 

v) 

Table 1. Lithostratigraphic schemes of Middle and Early Proterozoic successions in Ny Fricsland, aftcr Harland ct a l .  (19%) and 
Krasil'sfikov (1Y73) 

Smutsbreen Fm. 
Eskolabreen Fm. 

Harland et al. 1966 

Planetfjellet Group 

2 
I' 

I 
0, 

S ~ r b r e e n  Fm. 
Vassfaret Fm. 0 2  
Bangenhuk Fm. i3e 
Rittervatnet Fm. 6 s  
Polhem Fm. 3 (D 

Krasilscikov 1973 

Mossel series 



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A n  Early Proterozoic U - P b  zircon age f r o m  an Eskolabreen Formation 151 

ness, altcrnating with pink granitic gncisses and thin amphibo- 
lites of hornblende-biotite gncisscs and is located about l00m 
below thc uppcr boundary of the Eskolabreen Formation in the 
core of the Atomfjclla antiform. Many isoclinal fold hinges 
occur within layers and the gneissosities arc all of tcctonic- 
metamorphic in origin. 

Zircons 
Two morphotypes of zircon grains arc distinguished in the rock 
(Fig. 2): 

( I )  Grains with facets of ( 1 1 1 ) .  (311). (110). (100) and wide 
bulging margins between them. having apparent oval or 
roundcd outlines with the width/lcngth ratios 1.5-2.5. They 
have smooth surfaces and arc commonly transparent. yellow to 
brownish in colour. Some grains have thin overgrowths at thc 
pinacles. 

(2) Prismatic grains with ( 1  1 I )  and (100) facets. These have 
relativcly rough surfaces and the width/length ratios are 2-3.5. 
The colours are similar to morphotypc I .  but darker and not as 
clear duc to thc rougher surfaccs. 

Zircons of both morphotypcs have some inclusions of brown 
nccdlcs. probably thorite. and glass-liquid bubbles. The surfaces 
of grains do not show any abrasion pattern as usually seen on 
the surfaces of detrital grains. Thermal-ion mass-spectrometry 
studies on both types of zircon grains show a large difference 
in 2"7Pb/zHPb age (ca. 2.36 and 1.87 Ga), indicating at least two 
generations of the grains. The zircons of morphotype 2 have 
not been analysed in this preliminary study because they are 
considered to have a complex history of crystallization and 
would therefore give results difficult to interpretate. 

Six size fractions of morphotypc 1 zircon grains have been 
chosen for the prcsent U/Pb analyscs. Only transparent grains 
without ovcrgrowth at the pinacles have bccn selected. Fraction 
4 was air-abraded to remove the inner part and fraction 6 was 
additionally milled. 

Isotope anulyses and results 
Extraction of U and Pb was performed by Krogh's (1973) 
method. Aftcr being washed with diluted nitric acid, the zircon 
grains were dissolved in 2 ml H F  in teflon capsules and heated 
in aluminium bombs in an oven at 185°C for some days. Aftcr 
dissolution and aliquotation. thc ID part was spiked with a 
mixed 235U-208Pb tracer. U and Pb wcrc separated by ion- 
exchange using 6.2 M HCI and water. Total blank during the 
experiments were 0.05 ng for U and 0.1-0.5 ng for Pb. 

Thc sample was loaded with nitric acid on a double Re 
filament for U and with silica gel and phosphoric acid on a 
single Re filament for Pb. Both wcrc analyscd with a MI- 
1201-T single collector mass spectrometer. The results of the 
measurements were corrcctcd for blanks of 0.5 ng for Pb and 
0.05 ng of U .  and common lead using Stacey & Kramer's model 
(1975) for an age of 2.450 Ma. 

The results of U and P b  isotope analyses are given in Table 
2 and the obtained isochron is illustrated in Fig. 3. The four 
fractions define a discordia with an upper intercept of 
2.415 + -34 Ma and a lower intercept of 624 + -68 Ma, with 
MSWD = 6.9. Two fractions, 4 and 6, are off from the discordia, 
probably duc to later thermal effects and/or contamination 
during the air-abrasion for fraction 4. 

E'a#] 0.44 

Fig. 3 .  U-Pb concordia diagram. Number of sample refers to 
Table 2. 

Discussion 
Close structural observations of the Stubendorffbreen 
Supergroup in this area show that all layered and banded 
structures in the rocks are of tectonic origin: no stratigraphic 
(scnsu stricto) definition is therefore possible and the suc- 
cessions must be defined as lithostratigraphic of tectono- 
stratigraphic units (Manby 1990; Gee et al. in press). 

The protoliths of the rocks of the Harkcrbrcen and Fin- 
landveggcn Groups have been considered to be areno-argil- 
laceous and arkosic sediments and hi-modal igneous rocks of 
mostly eruptive origin. The volcanigenic origin of somc of these 
rocks has also been discussed by Manhy (IWO) and Sirotkin 
(unpubl. report). The latter author suggested a difference in a 
sedimentary environment for these groups, an extensional deep 
sea basin for the Finlandveggen Group and a transgressional 
shallow sea - subaerial basin for thc Harkerbrcen Group. 

All previous authors agreed that these two groups bclong to 
a large volcani-sedimentary succcssion without any orogenic 
break within it. though all observed contacts bctwccn thc two 
groups are cut by thrust o r  steep fault in southern Ny Friesland. 

The obtained U-Pb upper interccpt age can be considered as 
a metamorphic zircon age, judging from the transparent colour. 
smooth surfaces lacking any trace of abrasion and the absence 
of any clearly zoned structure. This age is applicable to both 
the Eskolabreen and the Smutsbreen Formations. since these 
are conformable both in structure and metamorphism. but this 
age is not applicable to the Harkerbreen Group since this group 
is separated by faults from the Smutsbreen Formation in this 
area. This U-Pb upper intercept age suggests an older tec- 
tonothermal event than the emplacement of the Flitan granite 
at ca. 1.7 G a  (Gee et al. 1992) in Ny Friesland. Similar ages of 
ca. 2.4-2SGa have also been obtained recently from U-Pb 
upper intercepts of zircon from a quartz porphyryrhyolite clasts 
in a conglomerate-pyroclastic unit of the northwestern 
Hornsund (Balashov et al. 1992). 



152 Yu. A. Balashou et al. 

Thc Caledonian and Precambrian rocks of Ny Fricsland have 
becn compared with those of East Greenland (Harland 1985; 
Gee et al. 1992). There the 4 1 0 k m  thick Krummcdal sup- 
racrustal sequence (Higgins 1988) has been metamorphosed 
during the Grenvillian period (e.g, Peucat et al. 1985), while 
much older U-Pb zircon uppcr intcrccpt agcs. ca. 1.9-3.OGa 
(Hanscn & Friderichsen 1987; Hansen el al. 1987) and ca. 
2.5 Ma (Peucat et al. 1985) have also bccn rcportcd. Howevcr. 
thcsc rocks are involved in complex gneiss-migmatite structures 
and the discrimination between Middle and Early Proterozoic 
rocks has not yet been completed. 

Thc palinspastic position of Svalbard before Cenozoic ocean 
spreading is not too far from northern Ellcsmcrc Island. and a 
structural unit of Calcdonian affinity, Pcarya, has bccn rccog- 
nizcd in that area (Trettin 1987). Recent U-Pb zircon studies 
on the Caledonian and older rocks in Pcarya yielded older agcs 
than thc Grcnvillian, ca. 2.1 and ca. 2.2Ga (Trettin et al. 
1992). Both are considered to be from inherited zircons, but no 
geological constraints arc known yct. 

No correlative geological event has cvcr bccn observed in 
the present area which corresponds with thc lower intercept 
age, ca. 625 Ma. But a fcw similar Rb-Sr and K-Ar ages arc 
known in the northern parts of Nordaustlandct (Hamilton & 
Sandford 1964; Ohta 1992). A similar agc for cclogitic meta- 
gabbroic rock has been obtained from Biskayerhalveya (Peucat 
ct al. 1989). This period corresponds to the time of the Baikalian 
event in the southeastern Barcnts Sea rcgion: however, all these 
agcs in Svalbard are from igneous rocks and no distinguishing 
structural event of this period has yet hccn rccognizcd. 

Acknowledgements. - T h e  authors arc gratcful for critical com- 
ments to the manuscript given by D. G. Gee, University of 
Lund. Sweden, and A. Johansson. Swedish Museum of Natural 
History, Stockholm. Wc would also likc to thank A .  A. 
Krasil'SEikov, Polar Marine Geological Expedition. Lomono- 
sov. Russia, for organizing of the projcct. 

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