Download Provenance characteristics of the Brumunddal sandstone in the Oslo

NORWEGIAN JOURNAL OF GEOLOGY
Provenance characteristics of the Brumunddal sandstone in the Oslo Rift
1
Provenance characteristics of the Brumunddal sandstone
in the Oslo Rift derived from U-Pb, Lu-Hf and trace element
analyses of detrital zircons by laser ablation ICMPS
Tom Andersen, Ayesha Saeed, Roy H. Gabrielsen & Snorre Olaussen
Andersen, T., Saeed, A., Gabrielsen, R.H. & Olaussen, S.: Provenance characteristics of the Brumunddal sandstone in the Oslo Rift derived from
U-Pb, Lu-Hf and trace element analyses of detrital zircons by laser ablation ICMPS. Norwegian Journal of Geology, vol. 91, pp 1-19. Trondheim
2011. ISSN 029-196X.
The 800 m thick Brumunddal sandstone is partly an eolian, partly a fluvial sandstone deposited in a fault-bounded basin in the northern part of the
Oslo Rift in Permian time.The sandstone is the youngest rift-related deposit in the northern part of the Oslo Rift. Well rounded detrital zircons are
common accessory mineral grains in the sandstone. U-Pb dating of detrital zircon from a sample of the Brumunddal sandstone by LAM-ICPMS
gives a range of ages from (rare) late Archaean ages to Permian (283±4 Ma). The age and initial εHf pattern of zircons in the sediment match the
main rock- forming events in Fennoscandia from Archaean to Phanerozoic time. This kind of diverse provenance was most likely obtained by
repeated recycling of clastic sediments of Fennoscandian origin, with Silurian, continental sandstones as the most probable direct precursors. Trace
element distributions show a conspicuous absence of patterns with the high level of U and Th enrichment typical of zircon from granitic rocks.
This is consistent with a complex transport and redepositional history: High U-Th, metamict zircons were selectively removed by abrasion during
repeated­transport-deposition-erosion cycles. In addition to recycled material, Caledonian syn-orogenic intrusions and Permian intermediate to
felsic plutonic rocks in the Oslo Rift itself were minor, but still significant sources of detrital zircon.
Tom Andersen & Roy H. Gabrielsen, Department of Geosciences, University of Oslo, PO Box 1047 Blindern, NO-0316 Oslo, Norway (tom.andersen@geo.
uio.no). Ayesha Saeed, GEMOC, Macquarie University, NSW-2109, North Ryde, Australia. Snorre Olaussen, The University Centre in Svalbard, P.O. Box
156, N-9171 Longyearbyen, Norway­
Introduction
The Brumunddal sandstone (informal name) is the
uppermost preserved member of the late Paleozoic, riftrelated, volcanosedimentary succession in the northern
part of the Oslo Rift. It was deposited on a substrate of
rhomb porphyry lava, and is preserved in a ca. 6.5 km by
2.5 km, downfaulted block, the Narud halfgraben (Fig.
1). The Brumunddal sandstone consists of red and yellow, cross-bedded, eolian dune deposits and fluvial channel and floodplain deposits (Rosendal 1929, Larsen et al.
2008). Its maximum depositional age is constrained by
a Rb-Sr mineral isochron age of the underlying rhomb
porphyry lava at 279±9 Ma (Sundvoll et al. 1990). The
absence of overlying, datable rocks leaves the younger
limit for the age of deposition unconstrained. Larsen et
al. (2008) regarded the sandstone as a remnant of graben
fill that formed during the main rifting stage of the Oslo
Rift, which is characterized by widespread plateau lavas
intercalated by continenal deposits.
Through the U-Pb and Lu-Hf isotope systems and trace
element distribution patterns, detrital zircon grains
in a sandstone retain a robust memory of the age,
petro­genetic history and compositional characteristics
of the magmatic or metamorphic rock(s) in which they
formed, i.e. of the protosource (e.g. Fedo et al. 2003,
Kinney & Maas 2003, Belousova et al. 2002, 2006). As
the youngest pre-Quaternary sedimentary deposit in the
northern part of the Oslo Rift, this sandstone may have
sampled a range of Precambrian and Paleozoic protosources in the Fennoscandian Shield, the Caledonian
nappes and within the Oslo Rift itself. However, detrital
zircons in a sedimentary rock may have been recycled
through one or more intermediate reservoirs by erosion
and redeposition of older sedimentary rocks (e.g. Røhr
et al. 2010).
In this paper, we present the results of an analytical study
applying laser ablation inductively coupled mass spectrometry (LAM-ICPMS) and electron microprobe analysis to detrital zircon in the Brumunddal sandstone.
Data include single-zircon U-Pb ages, Lu-Hf isotope
ratios and trace element distribution patterns, which
provide new insights into the sources and recycling patterns of clastic material in Fennoscandia in late Paleozoic
time, and which help to refine the position of the sandstone within the evolutionary history of the Oslo Rift.
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T. Andersen et al.
NORWEGIAN JOURNAL OF GEOLOGY
Figure 1
a)Simplified geological map of the southern part of the Rendal graben segment in the Oslo Graben, Norway (after Høy and
Bjørlykke 1980). Legend: i: Brumunddal sandstone (Mauset Fm), ii-iv: Permian lavas and volcaniclastic sediments (Bjørgeberg Fm.), v: Bruflat Fm. (Upper Silurian), vi: Ekre shale (Silurian). vii: Pentamerus limestone (lower Silurian), viii:
Mjøsa limestone (middle Ordovician), ix: Fluberg Fm (middle Ordovician), x: Cambrian shale and sandstone, xi: Ringsaker quartzite (Neoproterozoic). The broken line is an inferred, unexposed fault.
b)Main geological features of the Oslo Rift, after Larsen et al. (2008). Legend: SG Skagerrak Graben, VG: Vestfold Graben,
AG: Akershus Graben, RG: Rendal Graben. 1: Mesoproterozoic basement, 2: Hedemark Basin (Neoproterozoic). 3: Cambrian to Silurian sedimentary rocks, 4: Permo-Carboniferous volcanic and sedimentary rocks. 5: Permian intrusive rocks.
6: Central volcanoes (cauldrons), 7: Transfer zones, 8: Brittle faults. The red rectangle marks the position of Fig. 1a.
c)The outline of the Oslo Rift system, the preserved parts of the Permian deposits (red and deep purple; after Larsen et al.
2008) and its relation to the tectonostratigraphic units mentioned in the text: Legend: C: Caledonian nappes, H: Hedemark
Basin (Neoproterozoic), D: Dala-Trysil basin (Mesoproterozoic). TIB: Transscandinavian Igneous Belt (Late Paleoproterozoic). O: Orsa sandstone (Silurian), Ö: Öved sandstone (Silurian). On-shore lower Paleozoic sediments in the Oslo Rift are
shown in pale blue.
To evaluate the potential of post-Caledonian continental
sediments outside of the rift area as intermediate repositories for detritus in the Brumunddal sandstone, we have
also analysed U-Pb and Lu-Hf isotopes in detrital zircons
from a sample of the Silurian continental sandstone from
Orsa in central Sweden.
NORWEGIAN JOURNAL OF GEOLOGY
Provenance characteristics of the Brumunddal sandstone in the Oslo Rift
Geological background
The late Paleozoic Oslo Rift (Fig. 1) encompasses four
separate graben units. The southernmost unit (the
Skagerrak Graben) is entirely situated offshore and features a symmetrical geometry. In contrast, the three onshore units (Vestfold, Akershus and Rendalen grabens)
are half-grabens of contrasting polarities. The graben
units are separated by NNW-SSE- and NNE-SSE-striking transfer zones (Fig. 1b). Volcanic rocks and batholiths dominate the the Vestfold and Akershus grabens,
whereas the Rendalen Graben is devoid of batholiths.
Still, some lava flows exist here, albeit minor in volume
as compared to the southern graben units (Larsen et al.
2008).
The rift evolution can be divided into several distinct
stages (Ramberg and Larsen 1978, Neumann et al. 2004,
Larsen et al., 2008). The proto-rift stage is characterized
by alluvial and minor marine deposits of Pennsylvanian
age, the Asker Group (Olaussen et al., 1994, Larsen et
al. 2008). Volcanism during the main rifting stage produced basaltic lava (B1) which form thick sequences
in the Vestfold Graben, thinning northwards and to
zero around Oslo. These lavas are of late Carboniferous
to earliest Permian age (300 ± 1Ma, Corfu & Dahlgren
2008). Graben subsidence continued and large volumes
of rhomb porphyry lava (latite) and minor basalt erupted
through large fissure volcanoes. Also, thick volcanoclastic and sedimentary units were deposited during this
stage. The composite Larvik pluton in the Vestfold graben intruded in this stage, and has been dated to 298-292
Ma by ID-TIMS U-Pb on zircons (Dahlgren et al., 1996,
1998, Dahlgren 2010). In the subsequent central volcano­
stage, volcanic activity was concentrated in distinct
volcanic­centres with local trends of magmatic evolution,
terminating in caldera collapse. The central­volcanoe­s
are preserved as between 15 and 20 cauldron structures, representing sections through sub­volcanic magma
chambers­, ring-dike systems and downfaulted blocks of
volcanic rocks. U-Pb ages are not yet available for the
rocks that formed during this stage, but Rb-Sr ages suggest a relatively long duration (ca. 280-240 Ma, Sundvoll
et al. 1990). The final stages of rift evolution were characterized by emplacement of intermediate to felsic batholiths,
ranging in composition from larvikite through syenite to
granite. U-Pb ages on zircons from different intrusions
suggest emplacement ages between 272 Ma and 286 Ma
(Pedersen et al. 1995, Haug 2007, Andersen et al. 2009c),
but Rb-Sr isochron ages range down to 241 Ma (Sundvoll
et al. 1990).
The Rendalen Graben is delineated eastwards by the
westerly facing extensional Rendalen fault and is superseded to the north by several smaller sub-graben of varying polarity. One of these, informally termed the Narud
halfgraben, occupies the Brumunddalen area, its Permian sequence being preserved in the rotated hanging wall
fault block of the westerly dipping Gråkvern fault (Fig.
3
1a). The fill of the Narud halfgraben is subdivided into
two formations, namely a lower 250 m thick volcanic
unit, the (Bjørgeberget formation) of Early Permian age
(Sundvoll et al. 1990), and the upper, conformable c.750
m thick undated Brumunddal sandstone unit, which has
been suggested by Eliassen et al. (in prep.) to be renamed
the “Mauset formation” (Fig. 2).
The Bjørgeberg formation rests unconformably on folded
Cambro-Silurian sediments and consists of four rhomb
porphyry lava flows (RPx-u; Rosendal 1928), inter­
calated with up to 30m thick, volcanoclastic conglomerates and sandstones. The conglomerates consist entirely
of rhomb porphyry lava clasts while the sandstones
contain­63-65% quartz, 30-32 % feldspar (mostly plagioclase) and 4-6% lava fragments and hence classify as an
arkosic arenite. The volcanoclastic rocks are interpreted
as alluvial fans derived from evolving basin-bounding
fault escarpments or local highs. Deposition of these fans
took place either in interludes between extrusive activity,
or represent contemporaneous, lateral variations of the
volcanic-sedimentary sequence (Eliassen et al. in prep.).
The Mauset formation (informally the Brumunddal sandstone) is dominated by red and yellow sandstones with
subordinate mudstones and carbonates. The rocks of the
Mauset formation can be subdivided into three units.
The lowermost unit (c. 200 m thick) consists of red sandstone with a mineralogical composition and texture similar to that of the arkosic arenites interlayered with the
lavas in the Bjørgeberg formation. They are interpreted
as wadi deposits characterized by a stable high watertable. This unit is superseded by a 500 m thick sequence
of texturally mature, fine to medium grained subarkosic
arenite with up to 77% quartz, 21% feldspar and less
than 2% lava fragements. The lowermost 250 m of this
unit is interpreted as eolian dune, interdune and sandflat deposits interbedded by fluvial stream and alluvial
plain sandstones and siltstones with evaporite pseudomorphs. The upper 250 m of the middle unit consists
entirely of eolian sands. The Mauset formation is topped
by a 50 m thick sequence of texturally and mineralogically im­
mature sandstones somewhat similar to the
lower unit, but which may have more than 40% feldspar
and less lava fragments. This arkosic arenite is interbedded by siltstones and carbonates that include fresh-water
stromatolites. It was deposited in ephermal lakes, ponds
and sabkhas typical of a mixed playa and alluvial environment and is indicative of a second event marked by
high water-level (Eliassen et al., in prep.).
Taken collectively, the Mauset formation is a typical
eolian/fluvial wadi deposit (Rosendahl, 1929, Olaussen
et al 1994, Eliassen et al. in prep.) much like what has
been described for the Upper Rotliegendes in the Northern and Southern Permian Basins in northern Europe
(e.g Glennie 1990). The limited outcrops do not allow
interpretation of eolian dune types (e.g as barchans,
transverse, seif or dras), nor can reliable determinations
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T. Andersen et al.
NORWEGIAN JOURNAL OF GEOLOGY
Figure 2: Stratigraphy of the Brumunddal sandstone
of paleo-wind direction be made. A similar uncertainty exists for the sediment transport directions of
the ephemeral alluvial stream deposits, which are likely
to have been determined by local dune topography and
structural grain as developed by graben margin faulting (e.g. Gabrielsen et al. 1995). Still, it is considered
unlikely that the 750 m thick eolian/wadi deposit of the
Mauset Formation was constrained to the 6.5 x 2.5 km
half graben in the Brumundal area alone. It is more likely
that the Narud halfgraben is one preserved fragment of
a wider desert area that covered most of the northern
part of the Oslo Rift and its vicinity. Hence the deposits of the Brumundal sandstone might be interpreted as
a northern arm or extension of the Rotliegendes of the
Northern Permian Basin (Olaussen et al. 1994, McCann
1998, Larsen et al. 2008, Eliassen et al. in prep). Consistent with this interpretation is the occurrence of up to 1
m thick beds of red coloured, cross-stratified sandstone
with well-sorted and well-rounded grains interpreted as
eolian or reworked eolian deposits (Olaussen et al 1994).
These beds occur within the volcanoclastics interbedded
with the lava pile in the Akershus and Vestfold Graben
further south in the Oslo Rift. In a reconstructed paleogeographic setting, the provenance for the Mauset Formation is suggested to be both of local origin and from
the basin fill of the rift further south.
The Orsa sandstone is an erosional remnant of Silurian
continental sandstone preserved within the Siljan impact
structure in central Sweden (“O” in Fig. 1c). It is the
youngest preserved member of the lower Paleozoic sedimentary sequence in the area, unconformably overlying limestones and shales of Llandowery to Wenlock age
(Thorslund 1960). The deposit consists of a cross-bedded, conglomeratic to coarse sandy lower part, overlain
by a main sequence of loosely consolidated, fine-grained
quartz arenite, which is locally carbonate cemented and
interlayered with clay-rich units (Hjelmqvist 1966). A
NORWEGIAN JOURNAL OF GEOLOGY
Provenance characteristics of the Brumunddal sandstone in the Oslo Rift
sample was included in this study to evaluate the potential of post-Caledonian sediments as intermediate hosts
for detrital material, rather than to establish a detailed
provenance history. The Orsa sandstone was chosen as
an example rather than other Silurian continental sandstones (e.g. the Öved sandstone in Scania (Jeppson &
Laufeld 1986), “Ö” in Fig. 1c; the Ringerrike group in the
Oslo Rift) because it is situated between the site of deposition of the Brumunddal sandstone and potential Paleoproterozoic and older protosources in the Svecofennian
and Archaean domains of Fennoscandia. The sample
studied is a fine-grained, red, friable sandstone without
clay or carbonate (Hjelmqvist 1966).
Zircon petrography
The present study was based on a sample of red, friable
sandstone from the upper part of the Mauset formation (N 60° 54.59’, E 10° 58.89’). The sample is a carbonate-cemented quartz sandstone pigmented by finely
disseminated iron oxide and containing minor to accessory amounts of detrital zircon. Most detrital zircon
Figure 3. Backscattered electron (BSE)
images of detrital zircon from the
Brumunddal sandstone. Each image
is identified by analysis number in
tables of the Supplementary Material, U-Pb age and εHf at the U-Pb
age. Each image is identified by ana­
lysis number, U-Pb age and eHf at the
U-Pb age. Rare Archaean grains (a)
are well-rounded and show little internal structure, except for a weak, BSE
bright overgrowth (lower part). Paleoto Mesoproterozoic grains (b, c, d)
have well developed, more or less complex oscillatory zoning patterns typical
for magmatic zircon, and are commonly well rounded. Permian grains
(e, f) vary in shape from angular to
well-rounded grain shapes, they show
faint oscillatory zoning struc­tures, and
contain abundant, BSE-dark inclusions (feldspar, apatite).
5
grains are well rounded, suggesting significant abration
during transport (Fig. 3). A few grains giving Permian U-Pb ages are less well-abraded (Fig. 3e), but lack
of rounding is not a universal feature of the youngest
group of detrital zircons in the sandstone (compare Fig.
3e with Fig. 3f). Most grains show the low-amplitude,
short-wavelength oscillatory zoning in backscattered
electron (BSE) intensity that is characteristic of magmatic zircons (Fig. 3b, c, d). Complex inner structures
seen in some grains suggest that they are fragments of
twins or intergrown groups of crystals (Fig. 3b). BSEbright overgrowths occur in some grains (e.g. Fig. 3a,c);
these are generally too thin to allow separate analysis
by LAM-ICPMS. Mineral inclusions in zircon (e.g. Fig.
3c) were avoided during U-Pb and Lu-Hf analysis, but
a few were penetrated during trace element ablations.
Inclusions which were large enough to be integrated
separately showed elevated phosphorus and middle or
light REE, and have therefore been tentatively identified
as apatite and monazite, respectively.
6
T. Andersen et al.
Analytical methods
Sample preparation was done at the Department of Geosciences, University of Oslo, and analysis of zircon by
LAM-ICPMS at the GEMOC National Key Centre, Macquarie University and at the Department of Geosciences,
University of Oslo. Zircon grains were separated from
rock samples by standard crushing and mineral separation techniques, followed by hand-picking under a binocular microscope. Selected grains from the two least
magnetic fractions (magnetic and non-magnetic at 1.5
A magnet current in the Franz separator at 15º standard
tilt and slope) were cast in epoxy disks for LAM-ICPMS
analysis. The mounts were ground and polished to expose
the zircons, and thoroughly cleaned before analysis. Zircons were examined in transmitted and reflected light,
and by electron backscatter imaging in a Cameca SX50
electron microprobe. Each imaged zircon was assigned a
number, which also serves to identify analyses in Tables
2, 3 and 4 of Supplementary Material. Standards were
mounted in separate epoxy disks.
LAM-ICPMS U–Pb dating.
U–Pb dating of the Brumunddal sandstone sample
was made with a HP 4500 series 300 inductively coupled plasma quadrupole mass spectrometer (QICPMS),
attached to a Merchantec/New Wave LUV213 Nd-YAG
laser microprobe at GEMOC. Analytical procedures
were as described by Jackson et al. (2004). The analytical protocol allows accurate correction for U/Pb fractionation by sample-standard bracketing. Standard zircon GJ-1 (608 Ma, Jackson et al. 2004) was used for calibration, while standard zircons 91500 (Wiedenbeck et
al. 1995) and Mud Tank (Black and Gulson 1978) were
run frequently as unknowns to provide an independ­
ent control on reproduci­bility and instrument stability.
The GLITTER software (Griffin et al. 2008) was used for
data reduction and calculation of individual U–Th–Pb
ages. The contents­of common lead were estimated from
the 3D discordance pattern in 206Pb/238U – 207Pb/ 235U –
208
Pb/232Th space, and corrected accordingly (Andersen
2002).
Additional U-Pb analyses of detrital zircon from the the
Orsa sandstone were made with a Nu Plasma HR multicollector ICPMS and a NewWave LUV213 laser microprobe at the Department of Geosciences, University of
Oslo, using protocols for data acquisition and standardization described by Rosa et al. (2009).
Lutetium-hafnium isotopes.
Lu-Hf analyses of zircon from the Brumunddal sandstone were made with a NuPlasma multicollector ICPMS
attached to a LUV213 Nd-YAG laser microprobe at
GEMOC, using protocols described by Griffin et al.
(2000) and Andersen et al. (2002b). Ablation conditions
were: Beam diameter: 55 μm (aperture imaging mode),
NORWEGIAN JOURNAL OF GEOLOGY
pulse frequency: 5 Hz, beam energy density: ca. 2 J/cm2,
static ablation. Each ablation was preceded by a 30 seconds on-mass background measurement. Masses 172 to
180 were measured in Faraday collectors, using the standard collector block of the NU Plasma mass spectrometer. The total Hf signal obtained was in the range 1.53.0 V. Under these conditions, 120-150 seconds of ablation are required to obtain an internal precision of ≤ ±
0.000020 (1SE). Isotopic ratios were calculated using the
Nu Plasma time-resolved analysis software. The raw data
were corrected for mass discrimination using an exponential law, the mass discrimination factor for Hf was
determined assuming 179Hf/177Hf= 0.7325, the observed
2 SE uncertainty of the Hf mass discrimination factor
was better than 0.5%, and was propagated through to
the error of the final result. The isobaric interferences
on 176Hf by 176Lu and 176Yb were corrected from measurements of interference-free 172Yb and 175Lu. The observed
mass discrimination for Hf was used as a proxy for those
of Lu and Yb. 176Lu/175Lu= 0.02669 was used for correction of Lu interference on mass 176 (DeBievre & Taylor
1993). The 176Yb/172Yb ratio used in the correction procedure (0.5870) was determined from multiple runs of
Yb-doped JM475 Hf standard solutions. Lu-Hf analyses of zircon from the Orsa sandstone were made with
the Nu Plasma HR instrument in Oslo, using procedures
described by Andersen et al. (2009a) and Heinonen et al.
(2010).
A value for the decay constant of 176Lu of 1.867 x 10-11
a-1 has been used in all calculations (Söderlund et al.
2004; Scherer et al. 2001, 2007). For the calculation of
εHf values we used present-day chondritic 176Hf/177Hf =
0.282785 and 176Lu/177Hf = 0.0336 (Bouvier et al. 2008).
We have adopted the depleted mantle model of Griffin
et al. (2000), modified to the λ176Lu and chondritic composition used, which produces a present-day value of
176
Hf/177Hf (0.28325, εHf = +16.4) similar to that of average
MORB over 4.56 Ga, from chondritic initial hafnium at
176
Lu/177HfDM = 0.0388.
Trace elements.
Trace elements in zircon were analysed using an Agilent­
7500 quadrupole ICPMS and a New Wave Research
LUV213 laser microprobe at GEMOC. Ablation
conditions­were: Beam diameter: 40 μm (aperture imaging mode), pulse frequency: 5 Hz, beam energy density:
ca. 5 J/cm2, giving a laser energy at the sample of 0.07 mJ.
A fast scanning protocol was employed, analysing masses
from 29 (Si) to 238 (U), with 10 ms dwell times on all
masses analysed except 206 (15 ms), 207 (30 ms) and 238
(15 ms). Ablations lasted for 120 seconds, and each was
preceded by a 60 seconds background measurement with
the laser off. NIST 610 silicate glass was used as external standard, with 29Si as the internal standard, assuming a stoichiometric composition for zircon with 32.78
weight % SiO2. Data reduction was made off-line using
the software package GLITTER 4.4 (Griffin et al. 2008).
NORWEGIAN JOURNAL OF GEOLOGY
0
a
4
8
12
16
20
24
0.20
2800
0.20
0.16
2800
0.16
0.12
Pb/ 206Pb
2000
0.12
2000
0.08
1200
207
Figure 4. U-Pb data for detrital zircon
from the Brumunddal sandstone.
a:Tera Wasserburg diagrams showing
data for zircon crystals in the magnetic and non-magnetic fractions from
the Franz magnetic separator at 1.5 A
separately. Note that Permian zircons
occur only in the magnetic fraction.
b:
Conventional concordia diagram
showing the Permian fraction. The
three grains represented by white
fill define a concordia age at 285±4
Ma, the fraction as such a weighted
average 206Pb/238U age at 283±4 Ma,
which is the preferred estimate of
the crystallization age of the source
rock(s).
7
Provenance characteristics of the Brumunddal sandstone in the Oslo Rift
0.08
0.04
Magnetic fraction
400
1200
0.04
400
Non-magnetic fraction
0
4
8
12
238
U/
b
0.052
16
20
24
206
Pb
Concordia Age = 285 ±4 Ma
320
(2 , decay-const. errs ignored)
MSWD of concordance and equivalence = 1.6
238
U
300
0.047
206
Pb/
280
0.042
0.037
0.26
260
Weighted average 206Pb/ 238U age:
283±4 (95% conf.), 9 of 10 grains
Wtd by data-pt errs only, 1 of 10 rej.
MSWD = 1.7
240
0.28
0.30
0.32
207
Pb/
Accuracy and external precision of the individual elements estimated from repeated analyses of reference zircon GJ-1 are given in Table 1 of Electronic Supplement.
Results
Uranium-lead ages of detrital zircons in the Brumunddal
sandstone.
Of a total of 105 grains dated by U-Pb (Table 2 of Electronic Supplement), 94 were concordant or less than 10
% normally discordant (central discordance). The zircons yield a large range of U-Pb ages, from late Archaean
0.34
0.36
0.38
235
U
(2.6 Ga) to Permian (Fig. 4a), with a pronounced frequency maximum at ca. 1 Ga, and subordinate maxima
at ca. 1400 Ma, 1600 Ma, 1800 Ma and in the Phanerozoic, with marked hiatuses in the Neoproterozoic and
early Paleoproterozoic (Fig. 5a). Although the magnetic
and non-magnetic zircon fractions show largely similar
age distribution patterns, a distinct group of late Paleozoic zircons (270-310 Ma) has been observed only in
the magnetic fraction (Fig. 4a). Of eleven zircons in this
group, eight are 10 % or more discordant. The three concordant zircons give a concordia age of 285±4 Ma, and 9
zircons define a weighted average 206Pb/238U age of 283±4
Ma (Fig. 4b, 5b), indistinguishable from the concordia
age. Another group of similar size (9 grains) give ages
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T. Andersen et al.
NORWEGIAN JOURNAL OF GEOLOGY
a
Orsa sandstone, Sweden (Silurian)
Brumunddal sandstone
20
Relative probability
Number
15
10
5
0
200
600
1000
1400
1800
2200
2600
Figure 5. Accumulated probability distribution diagrams showing:
a):The whole population of dated zircons (207Pb/206Pb ages for t> 600
Ma, 206Pb/238U ages for t< 600 Ma),
analyses with more than ± 10 %
discordance excluded. The colourfilled background histogram shows
the age distribution of detrital zircons in the Silurian Orsa sandstone,
Sweden (Table 5a of Supplementary
material).
b):
Permian zircons (n=11, including
discordant grains). Broken outline
indicates zircons that are not used to
calculate ages.
3000
Detrital zircon age, Ma
5
b
Weighted average 206Pb/238U
age: 283±4 Ma
Relative probability
Number
4
3
2
1
0
250
260
270
Zircon
280
206
Pb/
290
300
310
238
U age, Ma
between 526 and 398 Ma, with a peak at 440 Ma, which
is compatible with igneous or metamorphic source rocks
in Caledonian nappes (e.g. Gee et al. 2008). Two analyses
give ages of 346±4 Ma and 361±4 Ma. These grains do
not differ from the Caledonian fraction in terms of trace
element distributions and Lu-Hf characteristics (below),
and it is therefore suggested that they are Caledonian
grains which have suffered sufficient lead-loss in Permian or recent time to modify their ages, while remaining
concordant within error.
Lutetium-Hafnium isotopes of detrital zircon in the
Brumund­dal­ sandstone
In Fig. 6 (data from Table 3 of Electronic Supplement),
initial 176Hf/177Hf for 82 concordant or near-concordant zircons are plotted at the preferred U-Pb age (i.e.
207
Pb/206Pb ages for zircons older than 600 Ma, 206Pb/238U
ages for younger zircons).
The late Paleozoic zircons form a coherent group with
distinct initial Hf character. When zircons that are more
than 10% discordant in U-Pb are included in the calculation, nine zircons give a weighted average 176Hf/177Hf=
NORWEGIAN JOURNAL OF GEOLOGY
0.2835
Oslo Rift
magmas
DM
0.2830
Provenance characteristics of the Brumunddal sandstone in the Oslo Rift
Sveconorwegian granites
+10
Initial 176Hf/177Hf
Granite and rhyolite
Telemark block
-10
0.2820
Silurian sandstone from Orsa, Sweden
Age range of Caledonian
magmatism
CHUR 0
0.2825
"Gothian" arc magmatism
Transscandinavian
Igneous Belt (TIB)
Magmatic zircon
TIB crust
0.2815
Inherited zircon in
Sveconorwegian
granites
0.2810
0.2805
0
9
500
Inherited zircon in TIB,
Intrusions in the
Archaean Domain
1000
1500
Late Arcahean
granitoids, Finland
2000
2500
3000
Detrital zircon age, Ma
Figure 6. Initial 176Hf/177Hf plotted agains U-Pb or Pb-Pb age for detrital zircons in the Brumunddal sandstone (white fill:
Non-magnetic fraction, grey fill: Magnetic fraction), compared to fields of initial Hf isotope composition for magmatic
rocks in Fennoscandia: Late Archaean granitoids: Patchett et al. (1981), Lauri et al. (2011). Intrusions in the Archaean
domain, Finland: Patchett et al. (1981). Transscandinavian Igneous Belt, and inherited zircons therein: Andersen et al.
(2009a). Mesoproterozoic “Gothian” arc magmatism, Telemark magmatism and Sveconorwegian granitoids: Andersen et
al. (2002, 2004, 2007, 2009b), Pedersen et al. (2009). Oslo Rift magmas: Haug (2007), Andersen et al. (2009c). The fields
with diagonal ruling are the observed fields of detrital zircons in a sample of Neoproterozoic sandstone from the Hedmark
Basin published by Bingen et al. (2005a). The ages of syn-orogenic Caledonian magmatism are constrained by U-Pb data
on subduction-related igneous rocks in the Caledonian nappes (Gee et al. 2008 and references therein). The Hf isotopic
composition of these rocks remains unconstrained by data. The broad, open arrow represents the evolution of TIB and
related Paleoproterozoic granitioids with time, based on observed 176Hf/177Hf in zircon, and an assumed average 176Lu/177Hf
= 0.010. Broken arrows are trends of apparent evolution of 176Hf/177Hf in zircons which have lost all or part of their radiogenic lead during secondary, ancient events. Thin, broken lines are contours of 176Hf/177Hf at 2 epsilon units interval from
-10 to +10. The colour-filled background points are zircons from the Silurian Orsa sandstone, Sweden (data from Table 5b
of Supplementary Material).
0.28272±0.00004 (95% confidence) or εHf=4.2±1.5, with
no rejections. The spread of initial 176Hf/177Hf in this
group is comparable to the analytical error, suggesting that these zircons originated from one or more protosource rocks within the rift with homogeneous Lu-Hf
characteristics.
Zircons older than these show considerable variation in
initial 176Hf/177Hf, ranging from well below CHUR (i.e.
εHf<0) to values near the depleted mantle growth curve at
the crystallization age (Fig. 6), without simple correlation
with age. The “Caledonian” age group shows internal
variation from εHf= -11 to +1.4, indicating that this age
fraction must have come from sources with different
crustal histories. An even larger internal variation is seen
in the large group of Sveconorwegian zircons, which
span 20 epsilon units (-11 to +9). Zircons in the 14001500 Ma age group have positive εHf, except for a small,
coherent group with εHf= -3 to -8. These may be early
Mesoproterozoic or older zircons that have been affected
by lead loss around 1500 Ma. Some of the Paleoproterozoic zircons (1600-2000 Ma) have low-positive initial εHf,
but zircons with negative εHf are also present in this age
group. Unfortunately, only one single late Archaean grain
10
T. Andersen et al.
could be analysed for Hf isotopes, and yielded nearchondritic 176Hf/177Hf at its 207Pb/206Pb age.
Trace element compositions of detrital zircon in the Brumunddal sandstone
The trace element distribution patterns observed in
the analysed samples are typical for magmatic zircon
(Hoskin and Schaltegger 2003, Belousova et al. 2002),
with positive anomalies for Hf, U, Th and Ce (Fig. 7, data
from Table 4 of Electronic Supplement). Hf concentrations range from ca. 0.6 to ca. 2.4 percent, and total REE
from 33 to 2000 ppm. Source-dependent trace element
combinations spread from the less U and LREE enriched
part of the field of variation of zircon from granitoids
towards that of zircon from mafic rocks (Fig. 8, fields
from Belousova et al. 2002). This is reflected by the
CART classification algorithm of Belousova et al. (2002),
which gives a dominance of “dolerite” and different types
of “granitoids” as suggested source rocks (Table 1). A few
zircon analyses indicate source rock types such as “kimberlite” and “carbonatite”. Although both rock types
are known to exist in Fennoscandia (e.g. O’Brien et al.
2005, Rukhlov & Bell 2010), they are rare, and unlikely
to have yielded significant amounts of detrital material
to the Brumunddal sandstone. Zircon grains indicating
uncommon source rock types show anomalous REE patterns with low HREE and /or HREE / LREE ratios, combined with Eu/Eu* > 0.5 (NM15A-02, 04, 17, 21, 29, 32,
45, 66, Fig. 9) and probably formed by crystalli­zation
from hydrothermal solutions or during metamorphism
rather than from magma; the relatively flat HREE pattern in NM15A-32 suggests coexistence with garnet (e.g.
Hoskin & Schaltegger 2003).
Chondrite normalized REE distribution patterns are
strongly enriched in the heavy REE, as is common in zircon; all grains analysed have a marked positive Ce anomaly and a variable, negative Eu anomaly (Fig. 9). A majority of the analyses have (Yb/Sm)CN= 30 to 200, and deep,
negative Eu anomalies (Eu/Eu* < 0.4). With the exception of a few grains with apparent LREE enrichment due
to the presence of solid inclusions (see below), none of
the zircons analysed from the Brumunddal sandstone
show the characteristic flattening of the LREE pattern
observed in zircons from “granitoids” by Belousova et al.
(2002), and the patterns in general resemble those of zircon from “dolerite” and “basaltic” sources (Fig. 9).
Anomalous LREE-enrichment seen in a few analyses (e.g. NM15A-57 and -67) are from ablations that
have picked up minor amounts of LREE-rich inclusions
(monazite?) in the zircon. Other anomalous analyses
show relative enrichment of middle REE and a shallow
Eu anomaly (e.g. NM15A-37, Fig. 9). Again, this is due
to accidental ablation of small inclusions in the zircon, in
this case most probably of apatite.
The zircons of late Paleozoic age form a consistent
NORWEGIAN JOURNAL OF GEOLOGY
group with relatively low HREE enrichment (Yb/Sm)CN
= 20-30). Of the 11 grains in this group, four have distinctly shallower Eu anomalies than the rest (Eu/Eu*>0.4,
compared to Eu/Eu*≤ 0.3, Fig. 9). The patterns of the
latter resemble those of Oslo Rift larvikite analysed by
Belousova et al. (2002) in terms of pattern curvature
and Eu anomaly, but at an overall lower REE enrichment level. The differences in concentration level may,
however, be an effect of the different methods of internal standardization used in the two studies (assumed
stoichiometric SiO2 value in this study vs. Hf analysed by
electron microprobe).
U-Pb and Lu-Hf characteristics of Silurian sandstone
The age pattern of detrital zircon in the Silurian Orsa
sandstone mimics that of the Brumunddal sandstone
with age maxima in the late Paleoproterozoic, at ca. 1500
Ma and in the late Mesoproterozoic (Fig. 5a, data from
Table 5a of Electronic Supplement). There is also a small
contribution of Phanerozoic zircon, presumably from a
Caledonian source. The variation of initial Hf isotope
character also overlaps that of the Brumunddal sandstone (Fig. 6, data from Table 5b of Electronic Supplement l). The clearest difference between the two distribution patterns is due to a single, highly U-Pb discordant
zircon with εHf=-21 at 1376 Ma (Orsa-38, Table 4); this
is most probably a late Archaean or Paleoproterozoic zircon that has lost a significant amount of lead in a thermal
event in Mesoproterozoic time.
Discussion
U-Pb, Lu-Hf and potential protosources of detrital zircon
in the Brumundal sandstone.
The detrital zircon ages from the Brumunddal sandstone
range from late Archaean to Phanerozoic, with initial Hf
istope signatures from near depleted mantle to crustal
ratios. The distribution of age and initial 176Hf/177Hf or εHf
in Fig. 6 is, however, far from random, and can be related
to known crustal evolution events in Fenno­scandia.
Precambrian protosources. The near-CHUR initial
Hf/177Hf of the single late Archaean detrital zircon analysed for Lu-Hf falls within the age and 176Hf/177Hf range
of late Archaean granitoids from eastern Fennoscandia
(Patchett et al. 1981, Lauri et al. 2011). Archaean rocks
are exposed only in E Finland, NE Sweden N Norway
and NW Russia, at considerable distance from their sites
of deposition.
176
The Precambrian rocks of the Fennoscandian shield
immediately bordering the Oslo Rift (Fig. 1c) belong
to the late Paleoproterozoic Transscandinavian Igneous
Belt (TIB, Högdahl et al. 2004) and the early Mesoproterozoic (“Gothian”) magmatic arc fragments (Andersen
et al. 2004). The TIB was formed by multiple episodes
NORWEGIAN JOURNAL OF GEOLOGY
11
1.0 6
ppm in zircon / ppm in chondrite
Figure 7. Spider diagrams showing the overall variation of zircon
compositions in the
Brumunddal sandstone,
normalized to chondritic
values of McDonough
and Sun (1995). The
shaded background field
is the interquartile range
of zircon compositions in
granitoids (Belousova et
al. 2002).
Provenance characteristics of the Brumunddal sandstone in the Oslo Rift
1.0 4
1.0 2
1.0
1.0-2
Pb La Ce Pr Nd Sm Eu Gd Th Dy Y Ho Er U Yb Lu Hf Nb Ta Ti
25000
106
105
104
Granitoids
15000
Mafic rocks
Y ppm
Hf ppm
20000
10000
10
1
10 2
10 3
Y ppm
10
10 4
1
10 -2
10 5
106
105
105
104
104
103
102
10
10
0.1
1
Nb/Ta
10
1
10 2
10 3 10 4
10 5
103
102
1
10 -1
U ppm
106
Y ppm
Y ppm
103
102
5000
0
P
10
100
1
1
10
Yb/Sm
100
103
Figure 8. Source-indicat­ing element and element­-ratio plots showing that a significant proportion of the detrital zircons analysed falls between the main fields of zircon derived from granitoid and mafic (i.e. basalt, dolerite) sources (reference fields
from Belousova et al. 2002).
12
T. Andersen et al.
104
NORWEGIAN JOURNAL OF GEOLOGY
La Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd
< 320 Ma
10
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
320-900 Ma
3
103
NM15A-37
102
NM15A-21
NM15A-32
10
1
102
10
1
10-1
ppm zircon / ppm chondrite
104
Larvikite,
interquartile range,
Belousova et al. (2002)
NM15A-04
104
104
900-1250 Ma
900-1250 Ma
10
10-1
3
NM15A-35: inclusion
M15A-01
102
103
NM15A-29
NM15A-67
102
NM15A-57
10
10
1
1
10-1
10-1
Basalt
Dolerite
Granitoids
104
1400-1500 Ma
10
104
> 1600 Ma
3
102
103
102
NM15A-26
10
10
1
1
10-1
10-2
NM15A-62
La Ce Pr Nd
10-1
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
10-2
Figure 9. Chondrite normalized REE patterns for zircon from the Brumunddal sandstone, divided into six groups (<320 Ma;
320-900 Ma; 900-1250 Ma divided in groups with deep and shallow Eu anomalies, respectively; 1400-1500 Ma; >1600 Ma,
see text for further explanation). Reference fields for all except the <320 Ma groups are interquartile ranges of variation of
zircon from granitoids, dolerites and basalts; for the <310 Ma group, the interquartile range of zircon from the Oslo Graben
larvikite is shown instead (all reference data from Belousova et al. 2002). Analyses have been normalized to the chondrite
values of Boynton (1984).
NORWEGIAN JOURNAL OF GEOLOGY
Provenance characteristics of the Brumunddal sandstone in the Oslo Rift
of magmatic intrusion in the period 1.88 to 1.68 Ga; the
TIB intrusions close to the Oslo Rift belong to the youngest part of the belt, known as TIB3, with intrusive ages
around 1.68-1.67 Ga (Larson and Berglund 1992, Heim
et al. 1996, Andersen et al. 2009a). Despite the large size
and long time of emplacement of the TIB province, the
initial Hf signature of magmatic zircon from TIB granitoids shows little variation, which can be represented
by initial εHf= +2±3 at 1.88 Ga and 176Lu/177Hf≈ 0.015
(Andersen et al. 2009a). Given the short distance to
potential source rocks within the TIB, the low abundance
of TIB-like detrital zircons in the Brumunddal sandstone
(Fig. 5a, 6) is somewhat surprising. On the other hand,
a small group of Paleoproterozoic zircons with negative εHf overlaps the field of inherited zircon of possible
Archaean origin in a TIB intrusion at the eastern margin
of the belt in Sweden (Andersson et al. 2006, Andersen
et al. 2009a). Furthermore, Paleoproterozoic zircons with
such initial Hf characteristics are found in Paleoproterozoic granites within the Archaean domain of Finland
(Patchett et al. 1981, Andersen & Lauri 2010). Paleoproterozoic crust with Lu-Hf properties similar to the rocks
of the Transscandinavian Igneous Belt or their source in
the deep crust has contributed source material to most of
the Mesoproterozoic crustal magmatic events that can be
recognized in southwestern Fennoscandia, even far west
of the present TIB outcrop area (Andersen et al. 2001,
2002a, 2009b, Andersen 2005, Pedersen et al. 2009).
Detrital zircons of early Mesoproterozoic age with initial
Hf/177Hf approaching the DM curve resemble zircons
from both Mesoproterozoic continental margin arc complexes (Andersen et al. 2002b, 2004) and those from felsic rocks from the Telemark block, which are still poorly
constrained by Lu-Hf data. A small, coherent group
of 1.45-1.50 Ga zircons with εHf= -3 to -8 can either be
derived from a coeval, crustal source, or they can be TIBderived zircons that have lost lead in a Mesoproterozoic
tectonothermal event.
176
During the Sveconorwegian period, southwestern Fennoscandia was affected by several episodes of mafic and
felsic magmatism in different tectonic settings, ranging from ca. 1300 Ma to 920 Ma (Bingen and van Breemen 1998, Andersen et al. 2001, 2002a,b, 2007, 2009b,
Vander Auwera et al. 2003, Pedersen et al. 2009). Magmatic events involving crustal underplating with mafic
material with a depleted mantle Hf isotope signature
(εHf ≈ +12) at 1.3 and 1.22 Ga have been documented in
the Telemark block; after each of these events, the initial
Hf isotope composition of zircon from magmatic rocks
evolved towards less radiogenic compositions due to
increasing effects of crustal source components (Andersen et al. 2007, Pedersen et al. 2009). Initial εHf of the
large group of Sveconorwegian zircons overlaps well
with the combined range of magmatic and inherited zircon from Sveconorwegian granitoids (Andersen et al.
2002a, 2007, Pedersen et al. 2009).
13
Caledonian protosources. Synorogenic intrusions in the
nappes of the Scandinavian Caledonides that could be
sources of early Paleozoic detrital zircon have been dated
to 430-500 Ma (Gee et al. 2008 and references therein).
The Caledonian mountain chain was mainly levelled
by mid-Carboniferous time (Gabrielsen et al. 2005,
2010), but late Permian – Triassic uplift is likely to have
enhanced relief to the west and northwest of the Oslo
Graben system (Rohrman et al. 1995, Gabrielsen et al.
2010). By the early Triassic, sediments of Caledonian origin were transported southwards to become deposited
in the Central European Basin (Paul et al. 2009). Unfortunately, no Hf isotope data are available from Caledonian intrusions. The considerable range of εHf (+ 1.4
to -11) observed suggests, however, that these zircons
either crystallized from magmas with variable amounts
of a TIB-like crustal component, or that they are zircons
from Sveconorwegian protoliths that have lost lead in
Caledonian­ metamorphism.
Within-rift sources. Permian grains make up ca. 10 % of
the analysed detrital zircon population (Fig. 5). Rocks
from the Larvik pluton have been dated to 292-298 Ma
by U-Pb on zircon and baddeleyite (Dahlgren et al. 1998,
Dahlgren 2010). This event was, however, followed by
the emplacement of syenitic to granitic plutons around
280 Ma (Pedersen et al. 1995, Haug 2007). Magmatic
zircon from larvikite, nordmarkite and biotite granite in the ca. 280 Ma Sande and Drammen plutons have
εHf in the range - 4 to +6, whereas zircons from pegmatites in the Larvik pluton are slightly more radiogenic
at εHf = +8 ± 2 (Andersen et al. 2009b); no evidence has
so far been found of hafnium derived from a late Paleozoic depleted mantle reservoir (εHf ≈ + 15) in any of these
rocks. The 176Hf/177Hf at 280 Ma of the late Paleozoic
zircons falls within the age and Hf isotope range of zircon in the younger group of intermediate to felsic intrusions in the rift. Two distinct types of REE patterns have
been observed, with “deep” and “shallow” Eu anomalies, respectively (Fig. 9). The first group has REE pattern
geometry similar to that of zircon from larvikite reported
by Belousova et al. (2002), and therefore most likely originates from intermediate intrusive rocks. The other type
must originate from other syenitic or (alkali) granitic
plutonic rocks, which cannot be identified.
Recycling of sedimentary rocks
Most of the main documented crust-forming events in
Fennoscandia are represented in the detrital zircon population of the Brumunddal sandstone, including accessory late Archaean and Paleoproterozoic zircons that
must have originated in protosources east of the Transscandinavian Igneous Belt. It is highly unlikely that zircon from such a wide range of sources could have been
brought into the rift in a single event of one-way transport from protosource to depositional basin. Rather, zircon must have been recycled through deposition in intermediate, sedimentary reservoirs followed by erosion and
14
T. Andersen et al.
NORWEGIAN JOURNAL OF GEOLOGY
new transport. This has eventually caused a relatively
well mixed sample of the entire Fennoscandian surface to
be brought into the Oslo Rift, including far-transported
material from eastern Fennoscandia.
sediments with a long and complex transport history.
The close similarity in age and initial 176Hf/177Hf distribution of detrital zircon in the Brumunddal and Orsa sandstones indicates that Silurian sandstone is a very likely
immediate precursor. Silurian, continental deposits may
have had a wide distribution in western and central Fennoscandia in post-Caledonian time; outside of the Oslo
Rift they are now preserved only as small erosional remnants. Like the Orsa sandstone, the late Silurian Ringerrike Group within the Oslo Rift itself (Davies et al. 2005
and references therein) contains material derived from
both Fennoscandian and Caledonian sources (Turner &
Whitaker 1976, Dahlgren & Corfu 2001).
The Neoproterozoic was a quiet period in Fennoscandia, with little or no production of zircon-fertile igneous
rocks. Nevertheless, Neoproterozoic detrital zircon ages
have been reported from both the Neoproterozoic Hedmark basin (Bingen et al. 2005a) and from the Carboniferous Asker Group within the Oslo Rift (Dahlgren &
Corfu 2001). In the Brumunddal sandstone, such zircons
are conspicuously absent. A non-Fennoscandian source
for Neoproterozoic zircon has been suggested (Dahlgren
& Corfu 2001), but the range of 176Hf/177Hf in Neoproterozoic zircons in the Hedmark Basin with the observed
U/Pb ages is fully compatible with Neoproterozoic loss of
radiogenic lead in zircon from Sveconorwegian granito­
ids (Fig. 6). This could be caused by surface weathering prior to deposition, or by interaction with diagenetic
fluids­within the Hedmark Basin.
The Silurian continental sediments make up the youngest of several cover sequences in Fennoscandia, ranging
in age from Paleoproterozoic (e.g. Claesson et al. 1993,
Lundqvist & Persson 1999, Lahtinen et al. 2002, Bergman
et al. 2008), through Mesoproterozoic (e.g. Bingen et al.
2001, 2003, 2005b, Kuhonen & Rämö 2005, Andersen et
al. 2004, Lamminen & Köykkä 2010) and Neoproterozoic (e.g. Nystuen et al. 2008, Bingen et al. 2005a, Lamminen et al. 2008, 2009) to Phanerozoic (e.g. Thorslund
1960, Bjørlykke 1974a, Turner & Whitaker 1978, Davies
et al. 2005). Each of the successive cover sequences must
have contained clastic material recycled from older
deposits, as well as detritus derived directely from crystalline protosource rocks.
Basins such as the late Vendian – Cambrian Peri-Thornquist, and the Pre-Baltic sub-basins (Sliaupa et al. 1997,
Poprawa et al. 1999), the southwestern part of the Baltic Basin (Lazauskiene et al. 2002), as well as the Scandinavian (Bjørlykke 1974b, Worsley et al. 1983) and German-Polish Caledonian foreland basins (Lazuskiene et
al. 2002) may have acted as intermediate repositories for
far-transported detritus, including Paleoproterozoic and
Archaean detrital zircons of east Fennoscandian origin.
The Caledonian forebulge may have promoted uplift and
erosion of these basins, thus releasing zircons of an easterly origin.
The most zircon-fertile rocks within each of the protosource-terranes are granitic. Nevertheless, the trace element patterns are more in line with mafic to intermediate
source rocks than with granite. Zircons with a relatively
less steep LREE pattern are also likely to be more heavily enriched in uranium and thorium (Fig. 7; Belousova
et al. 2002), and would therefore more easily become
metamict due to internal radiation damage than less U
and Th enriched (and hence also more strongly LREE
depleted) ones. Metamict zircon is brittle and much
more likely to be destroyed by abrasion during transport
than grains with an intact crystal structure. A scarcity of
typical “granitoid” zircon is therefore to be expected in
The significance of Neoproterozoic detrital zircon ages in
southwestern Fennoscandia.
Time of deposition and tectonostratigraphic status of the
Brumunddal sandstone
The weighted average U/Pb age of 283±4 Ma (Fig. 3b)
confirms the previous estimate of the maximum age limit
for sedimentation based on Rb-Sr ages of the underlying
rhomb porphyry lavas (Sundvoll et al. 1990, Larsen et
al. 2008). By the time of deposition of the Brumunddal
sandstone, rocks belonging to the ca. 280 Ma “batholith
stage” of rift evolution must have been exposed to erosion at the surface, which suggests that deposition took
place during the final stages of rift evolution (stage 6 in
the chronology of Larsen et al. 2008), rather than during
or immediately after rhomb porphyry volcanism (i.e. in
stage 3), which preceded emplacement of the younger,
batholitic bodies. Plausible sources of Permian zircon are
only exposed to the south of the site of deposition, in the
intrusive complexes of the Akershus and Vestfold graben
segments. These zircon grains must therefore have been
transported from south to north along the rift axis. The
observation that some of these zircons are quite severely
rounded due to abrasion during transport indicates significant transport for some, but not for all of the young
zircons observed.
The present data suggest that sediment transport took
place from south to north along the graben axis, perhaps
suggesting a topographic gradient in this direction. It is
of particular interest to note that one of the REE-patterns
shows the presence of a deep (plutonic) source, suggesting that plutons became exposed and eroded in the
southern part of the graben simultaneously with ongoing
volcanism in the Brumunddal area. This situation is not
unique and is paralleled in the present East African rift
system (Barberi et al. 1970, 1972).
NORWEGIAN JOURNAL OF GEOLOGY
Provenance characteristics of the Brumunddal sandstone in the Oslo Rift
Why are Permian zircons only found in the magnetic
fraction?
Permian zircons were only found in the magnetic
fraction­at 1.5 A. With ca. 10% of the total number
of analyzed­zircons, this fraction is relatively large.
However­, if each of the two fractions from the magnetic
separator are considered individually, their sizes are not
sufficient to guarantee detection of even relatively large
age fractions in each (Andersen 2005). However, it is still
fully possible that the absence of syn-rift zircons from the
non-­magnetic fraction reflects a real physical property of
these zircons. If so, relatively standard use of magnetic
separation has efficiently removed them from the least
magnetic­fraction. The tendency of magnetic­mineral­
separation to introduce undesirable bias in detrital zircon populations was pointed out by Sircombe and Stern
(2002), and the present results could imply that such
effects can be due to even a moderate use of the magnetic­
separator.
Conclusions
The Brumunddal sandstone was deposited during
the final stages of evolution of the Oslo Rift, at a time
when plutonic rocks formed during the ca. 280 “batholith” stage were exposed to erosion. The sandstone contains detrital zircon ranging in age from late Archaean to
Permian. Combined U-Pb and Lu-Hf data suggest that
detrital zircons from most of the region-wide petrogenetic events in Fennoscandia are represented in this single sediment, including zircons formed in events not represented by rocks within reasonable distance from the
site of deposition. The likely explanation for this broad
distribution of detrital zircon is recycling of material
from Phanerozoic sedimentary cover on Fennoscandian
basement, which may in turn contain material recycled
from still older sedimentary rocks. Repeated transport
and deposition has effectively mixed detrital zircon from
a wide range of protosources, and depleted the detritus in
metamict, high U zircons with typical “granitoid” trace
element distribution patterns, leaving a population with
a large proportion of zircons having trace element distributions more characteristic of “mafic” source rocks.
Acknowledgments. - The research reproted her could be carried out
thanks to economic support from the University of Oslo and Macquarie University. Thanks are due to Trine-Lise Knudsen for assistance in
the field, Gunborg Bye-Fjeld for technical assistance with mineral sepa­
ration, Justin Payne for help with the LAM-ICPMS, to Elena Belousova­
and Norman Pearson for many helpful discussions, and to Bernard
Bingen­and an anonymous reviewer for helpful comments. T.A. wants
to thank GEMOC and its directors S.Y. O’Reilly and W.L. Griffin­
for repeated­invitations to spend fruitful research time in Australia­.
This is publication no. 17 from the Isotope Geology Laboratory at the
Department­of Geosciences, University of Oslo and contribution 696
from the Australian­Research Council National Key Centre for the
Geochemical­Evolution and Metallogeny of Continents (http://www.
gemoc.mq.edu.au).
15
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