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Precambrian Research 174 (2009) 155–162
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Precambrian Research
journal homepage: www.elsevier.com/locate/precamres
Geochemical and numerical constraints on Neoarchean plate tectonics
Jaana Halla a,∗ , Jeroen van Hunen b , Esa Heilimo c , Pentti Hölttä d
a
Geological Museum, Finnish Museum of Natural History, University of Helsinki, Arkadiankatu 7, P.O. Box 17, 00014 Finland
Department of Earth Sciences, Durham University, United Kingdom
c
Department of Geology, University of Helsinki, Finland
d
Geological Survey of Finland, Espoo, Finland
b
a r t i c l e
i n f o
Article history:
Received 28 October 2008
Received in revised form 17 July 2009
Accepted 20 July 2009
Keywords:
Neoarchean
TTG
Sanukitoid
Geochemistry
Numerical modeling
Geodynamics
a b s t r a c t
This paper discusses early Neoarchean (2.8–2.7 Ga) plate tectonics by integrating knowledge from new
geochemical observations and numerical models. Based on a geochemical dataset of 295 granitoid samples from the Karelian and Kola cratons of the Fennoscandian Shield, we divide Neoarchean juvenile
(extracted from oceanic crust or mantle) granitoids into three groups: (1) low-HREE (heavy rare earth
elements) TTGs (tonalite–trondhjemite–granodiorite) (high SiO2 , low Mg, low-HREE, higher Sr, lower Ybn
and higher Nb/Ta), (2) high-HREE TTGs (slightly lower range of SiO2 , larger range of MgO contents and
higher Cr and Ni contents, high-HREE, lower Sr, higher Ybn and lower Nb/Ta), and (3) high Ba–Sr sanukitoids (medium-HREE, high-Mg and high K–Ba–Sr). The main difference between the low- and high-HREE
groups lies in their pressure-sensitive element contents, which indicates high-pressure melting conditions for the low-HREE group and low-pressure conditions for the high-HREE group. A possible tectonic
scenario for the genesis of the two groups is an incipient hot subduction underneath a thick oceanic
plateau/protocrust. Melting in the lower part of thick basaltic oceanic crust could produce TTGs of the
low-HREE type, whereas low-pressure melting of subducting slab and possible interactions with the mantle wedge at shallow depths would be capable of generating high-HREE TTGs. The third group of Archean
high Ba–Sr sanukitoids was formed after the TTGs. Their low SiO2 and high Mg–K–Ba–Sr contents suggest
an origin by melting in an enriched (metasomatized) mantle source, probably as a result of a slab breakoff
following a continental collision or attempted subduction of a thick oceanic plateau/TTG protocontinent.
Such hypothesis is supported by numerical modeling results that suggest an increased occurrence of slab
breakoff in the Archean, which locally increased temperatures within the mantle wedge. More frequent
breakoff resulted, because subducting plates were weaker (due to rheologically thinner lithosphere and
a thicker basaltic crust), and tensile stresses within the subducting plate were larger (due to a thick crust
that transforms from buoyant basalt to dense eclogite).
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
The study of geodynamics of the early Earth continues to provide
many challenges, and constructing uniform views from different
disciplines on Archean plate tectonic models requires overcoming
several geological, geochemical, and geophysical hurdles. First of all,
time has taken its toll: Archean sites are now relatively sparse, and
data are fragmented. This makes it difficult to recognize the largescale tectonic setting at the time of formation of Archean rocks (de
Wit, 1998; Hamilton, 1998). Furthermore, terminology for volcanic
rocks generated in modern subduction environments (adakites,
sanukitoids) has been adapted to include a variety of Archean granitoids. Finally, the global style of Archean geodynamics is not clear.
Did plate tectonics occur, and if so, was the style similar to mod-
∗ Corresponding author. Tel.: +358 9 191 28760; fax: +358 9 191 22925.
E-mail address: jaana.halla@helsinki.fi (J. Halla).
0301-9268/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.precamres.2009.07.008
ern plate tectonics (Davies, 1992)? Numerical models can provide
insight into the feasibility of certain geodynamic scenarios, but
poor constraints on some of the input parameters (Archean mantle temperature, rheological parameters) necessarily leads to some
uncertainty in the output of those models (e.g., van Hunen and van
den Berg, 2008). The most fruitful procedure to further our knowledge on Archean geodynamics is to combine geochemical data and
geological setting of Archean rocks with geodynamical models.
Archean juvenile (extracted from the mantle or oceanic crust)
granitoids has been traditionally classified to TTGs and sanukitoids.
It has been invoked that Archean TTGs are analogues of modern
adakites (e.g., Martin, 1999; Martin et al., 2005). The term adakite,
as originally described by Kay (1978) and Defant and Drummond
(1990), is applied to Cenozoic volcanic rocks thought to be generated by interactions of slab melts with the mantle wedge in a
hot subduction environment, although non-slab melting origins for
adakites have been suggested as well (Macpherson et al., 2006).
Following the original definition, the rocks referred to as adakites
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J. Halla et al. / Precambrian Research 174 (2009) 155–162
should carry geochemical signatures generally related to slab melts
(or other equivalent garnet-bearing basaltic source) [e.g., low-HREE
and high (La/Yb)n ] and mantle interaction (lowered SiO2 , high-Mg,
Mg#, Cr, and Ni). For a thorough review of adakite terminology,
the reader is referred to the papers of Castillo (2006) and Moyen
(in press), and references therein. These papers make obvious that
the term “adakite” is expanded to describe a too large group of
rocks with different petrogenetic processes. Furthermore, Smithies
(2000) pointed out that most early Archean and many Neoarchean
TTGs lack mantle signatures thus showing no evidence for slab
melt–mantle interactions and, therefore, cannot be attributed to
modern-style subduction. Therefore, the term adakite is not recommended to be used for Archean TTGs.
The term Archean sanukitoid suite, first introduced by Shirey
and Hanson (1984), refers to late- to post-tectonic Archean diorites, monzodiorites, granodiorites, and granites with high K and
Mg contents and distinctive geochemical characteristics. These
rocks are considered to represent the addition of enriched mantle wedge melts into the continental crust (e.g., Stern and Hanson,
1991; Lobach-Zhuchenko et al., 2008 and references therein) and
are therefore very important in evaluating crustal growth rates
and understanding the mechanisms of crust formation during the
Archean.
The Archean Karelian and Kola cratons, in the northern part of
the Precambrian Fennoscandian Shield, make one of the largest
exposed nuclei of Archean crust. In terms of crustal growth and
rock series, these cratonic areas represent a complex pattern of different type of granitoids, greenstones, and paragneisses dating back
to 3.5 Ga, with the major peak in crustal growth between 2.9 and
2.6 Ga (Sorjonen-Ward and Luukkonen, 2005, p. 27; Slabunov et
al., 2006). Rapidly increasing geochemical and geophysical research
and data available on these well-exposed cratons makes them an
inviting place to elucidate Archean geodynamics.
This paper divides juvenile Neoarchean granitoids into: (1) lowHREE TTG, (2) high-HREE TTG and (3) high Ba–Sr sanukitoid groups,
and hypothesize on the geodynamic/tectonic settings in which
these granitoids were formed. The study is based on new geochemical evidence from the mainly early Neoarchean (2.9–2.7 Ga)
granitoids of the Karelian and Kola cratons of the Fennoscandian
Shield. To support the hypothesis, we discuss numerical modeling
results.
2. Geochemical data
This study is based on a dataset of 295 analyses (new and published data; averages are given in Table 1) of juvenile granitoids
from ca. 3.1 to 2.7 Ga granite–greenstone terrains in the western
parts of the Archean Karelian and Kola cratons of the Fennoscandian Shield. The samples were collected from Ilomantsi, Iisalmi,
Kianta, Ranua, and Inari terrains in eastern and northern Finland
(Fig. 1).For a detailed description of the terrains, see Sorjonen-Ward
Fig. 1. Geological map of the Karelian and Kola cratons (modified after Sorjonen-Ward and Luukkonen, 2005) showing the locations of the Archean granite–greenstone
terrains in eastern Finland.
J. Halla et al. / Precambrian Research 174 (2009) 155–162
157
Table 1
Average major and trace element data for Archean juvenile granitoids from the
western Karelian and Kola cratons.
Group
Low-HREE TTG
High-HREE TTG
Sanukitoids
SiO2
68–76 wt.%
60–74 wt.%
55–70 wt.%
Number of samples (n)
n = 80
n = 45
n = 170
Major elements (wt.%)
SiO2
TiO2
Al2 O3
Fe2 O3
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
70.8
0.31
15.5
2.44
0.03
0.82
2.88
4.81
1.99
0.10
67.3
0.59
15.0
4.81
0.07
1.70
3.76
4.23
1.93
0.17
64.7
0.49
15.8
4.56
0.07
2.48
3.5
4.48
2.74
0.21
Trace elements (ppm)
Ba
Rb
Sr
Pb
Th
U
Hf
Zr
Nb
Ta
Y
Sc
V
Cr
Ni
Co
Cu
Zn
663
60.3
427
8.61
7.44
0.64
3.31
120
3.44
0.19
4.5
3.48
26.5
<30
<20
4.96
113.5
49.1
485
68.40
314
9.15
7.60
1.05
4.32
171
8.26
0.57
18.6
9.37
56.8
43.6
26.6
10.8
30.8
78.2
1182
85.0
729
13.81
6.52
1.21
3.20
135
6.45
0.60
10.9
8.53
69.7
75.0
35.2
12.1
11.7
77.3
38.0
39.3
51.9
Mg#
REE (ppm)
n = 20
n = 20
n = 50
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Ybn
Nb/Ta
(La/Yb)n
(Gd/Er)n
25.6
48.6
4.9
17.0
2.4
0.6
2.0
0.2
1.0
0.2
0.4
<0.1
0.4
<0.1
1.6
18.1
55.2
3.97
26.4
55.5
6.3
24.3
4.6
1.0
4.7
0.7
3.7
0.7
2.1
0.3
2.0
0.3
8.1
14.5
10.5
1.88
37.8
72.8
8.1
30.4
4.8
1.1
3.9
0.5
2.0
0.4
1.0
0.1
0.9
0.1
3.6
10.8
30.1
3.30
Fig. 2. Chondrite-normalized (Taylor and McLennan, 1985) REE patterns of Archean
juvenile granitoids demonstrating their division into high- and low-HREE TTG and
medium-HREE sanukitoid groups. Normalized values for concentrations under the
detection limit are interpolated.
2.1. TTG
There is little difference in the major element compositions of
low- and high-HREE TTGs. The low-HREE TTG group shows slightly
higher SiO2 (68–76 wt.%) and lower MgO contents (<1.2 wt.%)
compared to the larger SiO2 range (60–74 wt.%) and extended
MgO range of the high-HREE group (Fig. 3). The average Mg# is
slightly lower in the low-HREE group than in the high-HREE group
(although the difference is rather meaningless at low MgO and high
SiO2 ). The high-HREE group shows higher contents of mantle compatible elements (V, Cr, Ni, and Co) as well as a slight tendency to
higher Mg# values compared with those of the low-HREE group.
The normalized REE patterns of the low-HREE group are steeply
fractionated [high average (La/Yb)n of 55.2] with significant depletion in HREE [average (Gd/Er)n = 3.97] (Fig. 2), which is thought
to be a consequence of residual garnet retaining the HREE in the
source (Martin, 1987; Moyen and Stevens, 2006). The normalized
REE patterns for the high-HREE group are much less fractionated
Data sources—Ilomantsi terrain: Halla (2005), O’Brien et al. (1993); Iisalmi terrain:
Halla (2005); Kianta terrain: Käpyaho (2006); unpublished data; Ranua terrain:
unpublished data; Inari terrain: unpublished data.
and Luukkonen (2005). New samples were analysed for major and
trace elements by XRF and ICP–MS methods at the Labtium laboratory in Espoo, Finland.
Archean juvenile granitoids in the study area fall into three
groups based on their REE patterns (Fig. 2): (1) low-HREE
TTGs, (2) high-HREE TTGs, and (3) medium-HREE sanukitoids. The first two groups belong to the voluminous, sodic
tonalite–trondhjemite–granodiorite (TTG) series, the major component of the Archean crust, and the third group represents a
slightly later and minor group of potassic Archean sanukitoids.
Each group shows diagnostic geochemical signatures (Table 1 and
Figs. 3 and 4).
Fig. 3. (Gd/Er)n vs. MgO plot illustrating different groups of Archean juvenile granitoids. The hypothetic source end-members are garnet-bearing [high (Gd/Er)n ] or
garnet-free [low (Gd/Er)n ] basaltic crust (low MgO) or mantle (high MgO).
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J. Halla et al. / Precambrian Research 174 (2009) 155–162
and Sr concentrations of sanukitoids are observed to be independent of the SiO2 contents; therefore this signature is generally
interpreted to come from the mantle. Because high-HREE TTGs
also show mantle characteristics but lack the extremely high Ba–Sr
signature (Ba + Sr > 1200 ppm) typical of sanukitoids (Fig. 4), it is
concluded that the mantle source of sanukitoids was enriched by
an allochthonous high Ba–Sr fluid/melt flux.
3. Discussion
3.1. TTG subdivisions
Fig. 4. Na2 O/K2 O vs. Ba + Sr plot for discriminating the high Ba–Sr sanukitoid group
from the TTG groups. The hypothetic source end-members are enriched mantle (high
Ba + Sr, low Na2 O/K2 O) and primitive basaltic source (low Ba + Sr, high Na2 O/K2 O).
Data for the Mesozoic Rogart pluton are from Fowler et al. (2001).
[average (La/Yb)n = 10.5] and show relatively flat HREE profiles
[average (Gd/Er)n = 1.88] indicating the absence of garnet in the
residue (Table 1 and Fig. 2). According to Moyen (in press), there
are also other processes that can produce the high La/Yb signature:
melting of a high La/Yb source, fractional crystallization, or interactions of felsic melts with the mantle. However, deep garnet-present
melting provides the best explanation for the low-HREE content
[high (La/Yb)n ].
The low-HREE group shows increasing (Gd/Er)n values for low
MgO contents (Fig. 3), whereas the high-HREE group shows an
opposite trend; increasing MgO contents for low (Gd/Er)n values
(Fig. 3). The high-HREE contents and low (Gd/Er)n values point to
a garnet-free residual. High MgO, V, Cr, Ni, and Co contents are
generally attributed to a contribution from a mantle source.
The low-HREE group shows Eu anomalies from slightly negative (Eu* 0.8) to positive (Eu* 2.3). Eu anomalies are attributed
to Eu’s tendency to be incorporated into plagioclase preferentially
over other minerals. Negative Eu anomalies indicate plagioclase in
the source or feldspar fractionation, whereas positive Eu anomalies indicate accumulation of plagioclase. The Eu-positive subgroup
is illustrated in Fig. 2 by a separate field. Positive Eu anomalies
are observed to correlate with high Sr concentrations supporting
plagioclase accumulation.
The other important differences observed in trace elements are
the lower Sr contents, higher Yb and lower Nb/Ta of the high-HREE
group compared with those of the low-HREE group.
2.2. Sanukitoids
The third group of medium-HREE sanukitoids has uniform REE
patterns that are more fractionated [average (La/Yb)n = 30.1] and
show lower HREE ends [average (Gd/Er)n = 3.30] than those of the
high-HREE group (Fig. 2). The sanukitoid group shows the largest
variation in SiO2 contents (55–70 wt.%) and the highest V, Cr, Ni,
and Co contents (Table 1). They show a high range of MgO contents and variable (Gd/Er)n values (Fig. 3) pointing to a mantle
component in the source. A distinctive feature of the sanukitoid
group is their low Na2 O/K2 O ratio and high enrichment in fluidmobile elements Ba and Sr (Fig. 4). Both low- and high-HREE TTG
groups show an opposite trend: increasing Na2 O/K2 O ratios with
respect to low concentrations of Ba + Sr (<1200 ppm). The high Ba
3.1.1. Previous divisions
The distribution of TTGs into two geochemical groups was first
noticed by Barker (1979) who divided TTGs into low- and highAl2 O3 groups (separated at 15% Al2 O3 and 70% silica) and suggested
that the groups were produced by fractionation crystallization or
different degree of partial melting.
Arguing that the composition of TTGs has changed through time
(their SiO2 contents have decreased and Mg contents increased),
Martin and Moyen (2002) proposed a model that attributes the
secular change in the TTG composition to the transformation from
flat to deep subduction due to the cooling of the Earth (Abbott
et al., 1994). This would have enabled the formation of a mantle
wedge and thus allowed interactions between slab melts and peridotite. However, this model was challenged by Smithies (2000),
who pointed out that most early Archean and many Neoarchean
TTGs lack mantle signatures thus showing no evidence for slab
melt–mantle interactions and, therefore, cannot be attributed to
modern-style subduction. Furthermore, numerical modeling suggests that flat subduction was not physically feasible in the hotter
Archean mantle (van Hunen et al., 2004). Based on these arguments
and taking account that most TTGs of this study lack the high-Mg
signature, secular division by Mg contents is not supported here.
Moyen and Stevens (2006) gave experimental constraints on
TTG petrogenesis. They suggested, on the basis of pressure-sensitive
element concentrations, that the TTG data support a continuum of
conditions for melting from low- to high-pressures. Low-pressure
TTGs (ca. 10 kbar) are Sr poor (<400 ppm), relatively undepleted in
Yb (Ybn = 5–10) with slightly negative Eu anomaly, and low Nb/Ta
(ca. 1.0 GPa). High-pressure TTGs (2.0–2.5 GPa) show the opposite
characteristics. The average data in Table 1 show that high- and
low-HREE groups are consistent with the low- and high-pressure
TTG groups, respectively. Furthermore, experimental studies have
shown that there are no obvious differences in the major element
compositions of a melt as a function of pressure (Rapp and Watson,
1995).
Moyen (in press) observed the dual nature of Archean “adakites”
expressed by their contrasting Sr/Y and La/Yb ratios. He related
the origin of the high Sr/Y series (equivalent to the low-HREE
TTGs) to deep melting of basaltic source (>2.0 GPa) and that of the
low Sr/Y series (equivalent to the high-HREE TTGs) to the shallow
melting of basaltic source (1.0 GPa). As the pressure increases, plagioclase becomes unstable releasing Sr whereas garnet becomes
stable retaining Y.
The compositional differences between the low- and high-HREE
groups presented here are similar to those between the high- and
low-Al groups as well as the high- and low-pressure groups of
Moyen and Stevens (2006). The differences in the major element
compositions are not as large as would be expected if the two groups
were formed as a consequence of fractionation crystallization or
different degrees of melting. Because the differences between the
groups lie mainly in the pressure-sensitive elements, we maintain
that the two TTG groups were produced as a function of melting at
different pressures and, consequently, at different depths. Therefore
J. Halla et al. / Precambrian Research 174 (2009) 155–162
159
Fig. 5. (a) (HREE)n vs. SiO2 and (b) Al2 O3 vs. SiO2 diagrams for low- and high-HREE groups.
we suggest naming these groups by their HREE content, which has
been widely regarded as the best indicator of pressure conditions
of melting.
3.1.2. Justifications for the division by HREE
Depletion in HREE is acknowledged as one of the most prominent feature in TTGs. It is generally understood that the HREE
contents of magmas are controlled by garnet stability in the source,
which requires high-pressure conditions. The degree of melting
exerts relatively little influence, because garnet is generated in the
melting process (Moyen and Stevens, 2006). The more melt is produced, the greater proportion of garnet remains in the residue.
The (HREE)n vs. SiO2 plot of the Karelian TTGs in Fig. 5a implies
that the HREE content is not dependent on the SiO2 content and
thus is not related by magmatic evolution or fractionation processes.
The subdivision of TTG by HREE content presented in this paper
argues for previous classifications. In Fig. 5b, the division into
low- and high-HREE TTGs is compared with that into low and
high Al2 O3 TTGs. The plot indicates that there are two genetically
different groups of TTGs. Low-HREE TTGs follow the high-Al2 O3
trend, whereas high-HREE TTG show lower Al2 O3 contents at
given silica content. This is consistent with initial observations of
Hammarstrom and Zen (1986) that in granitoids, the total amount of
Al that can be accommodated in hornblende increases as a function
of pressure.
As noted before, the major differences between the high and
low-HREE groups are mainly restricted to the pressure indicator
elements such as HREE, which reflects differences in the melting
conditions and thus in the site of melting rather than fractionation
processes or different degrees of melting.
Division of TTGs by HREE content is suggested here because: (1)
the contrasting HREE content is a prominent feature in TTGs and
the data presented show no overlap between low- and high-HREE
groups, (2) lots of experimental results on the behavior of HREE
are available in the literature, and (3) the HREE content seems to
be largely a pressure-dependent feature. The “adakitic” division by
Sr/Y and La/Yb (the “adakitic signature”) cannot be favored before
clearing up the confusion of the term. Furthermore, a high Sr content might be a result of metasomatism, at least in some cases.
The old division by Al content is based only on one major element that is affected also by fractionation processes, and the Mg
content (and Mg#) is too variable to be used as a basis of classification.
3.2. Geodynamic setting of low- and high-HREE TTGs
The data presented in this study show that two coeval and
contrasting groups of Neoarchean juvenile TTGs with low- and
high-HREE characteristics can be found in the western parts of
the Karelian and Kola cratons. The simultaneous occurrence of the
two types of TTGs in the Karelian craton has been demonstrated
by Samsonov et al. (2005). The similar temporal occurrence but
contrasting pressure indicator element contents suggests that the
magmas were generated under different melting pressure conditions from separate sources (but with similar bulk geochemistry)
during the same large-scale tectonic event. A detailed analysis of
spatial and temporal relations of different geochemical groups is
needed and remains to be carried out.
The typical features of the low-HREE group (high SiO2 , lowHREE, high (Gd/Er)n , and low MgO) are attributed to fluid-absent
partial melting of amphibolites in the garnet stability field (Barker
and Arth, 1976; Defant and Drummond, 1990; Martin, 1987, 1994).
The high-HREE TTGs show elevated MgO and lack the high-pressure
signatures related to slab melting in garnet stability depths. The
low-HREE granitoid group is consistent with high-pressure melting
of deep oceanic crust or plateaus, whereas high-HREE granitoids
favor a shallower, low-pressure source possibly with a mantle
involvement.
A possible tectonic setting where these two types of sources
may exist at the same time is an incipient subduction zone below
the margin of an oceanic plateau, where the mantle wedge is
unusually hot. White et al. (1999) proposed such an origin for a
tonalitic batholith (∼85 Ma) in the island of Aruba in the southern
Caribbean. They present a model involving derivation of tonalite
by contributions from the mantle wedge, the subducting slab and
the overlying plateau crust. This environment provides an attractive
hypothesis also for the Karelian and Kola TTG formations: low-HREE
magmas were produced by partial melting at high-pressures in
the deep lower part of a thick oceanic crust, whereas high-HREE
TTG magmas were generated by interactions between subducting oceanic slab and mantle wedge at low-pressures and shallow
depths.
A third possible mechanism for TTG generation is delamination
of the lower eclogitic part of an oceanic plateau, which has been proposed for Middle Archean cratons by Zegers and van Keken (2001).
This model was originally proposed as an alternative for plate tectonics, but it might be possible that early Archean oceanic plateaus
were first converted to TTG protocontinents by this mechanism and
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then accreted to continents by convergent tectonics that initiated
in the late Archean.
3.3. Archean high Ba–Sr sanukitoids
Some authors have suggested that Archean sanukitoids, which
are identified by their enriched mantle signature (variable SiO2
and high Mg, Mg#, Ni, Cr, K, Ba, and Sr), are possible analogues to
adakites, at least to those with lower silica contents (e.g., Martin et
al., 2005). However, there are several arguments against this view.
Firstly, sanukitoids show high K2 O contents and distinctively high
enrichment in both Ba (>1000 pm) and Sr (av. 729 ppm) (Fig. 4 and
Table 1). Secondly, sanukitoids are 10–100 Ma younger than the
major phase of the TTG magmatism within the same tectonic region
of the Karelian craton (Bibikova et al., 2005; Käpyaho et al., 2006),
which attests their late- to post-tectonic setting, whereas adakites
are regarded as pre- to syntectonic. In eastern Finland, the major
phase of TTG magmatism occurred between 2.83 and 2.74 Ga and
was followed by a brief period of sanukitoid magmatism between
2.73 and 2.70 Ga (Käpyaho et al., 2006). Therefore, sanukitoids cannot be regarded as analogues of subduction-related adakites. A
better recent analogy for sanukitoids should be looked for among
the post-collisional granitoids rather than among subductionrelated volcanics. In fact, there is a group of mainly Mesozoic and
Paleozoic post-collisional high–Ba–Sr granitoids (Tarney and Jones,
1994; Fowler et al., 2008) that can be considered to be analogues
of sanukitoids. To understand the characteristics of the high Ba–Sr
granitoids, we need to discuss the source of Ba and Sr.
High Ba and Sr concentrations of sanukitoids are independent of
their SiO2 contents and are thus thought to derive from the mantle.
A commonly suggested source for Ba and Sr is a slab-melt metasomatized mantle wedge (Stern and Hanson, 1991; Lobach-Zhuchenko
et al., 2008). A recent study of Mogarovskii et al. (2007) highlights
the possibility that Ba and Sr derived from deep astenospheric
sources, can accumulate in the upper mantle as a consequence
of mantle metasomatism and melting in the metasomatized mantle. It is possible that the upper mantle source of sanukitoids was
enriched in these fluid-mobile elements at the time of their formation. The mantle source may have been metasomatized and
melted as a consequence of a post-collisional mantle upwelling
triggered by a slab breakoff or delamination of the lower part of the
crust. A slab breakoff has been suggested as the trigger of sanukitoid magmatism by e.g., Calvert et al. (2004), Whalen et al. (2004),
and Lobach-Zhuchenko et al. (2008). Sanukitoids show also some
characteristics that probably derived from previous subductionrelated metasomatism, such as crustal isotopic signatures (Halla,
2005; Lobach-Zhuchenko et al., 2008). The geochemical signatures
of sanukitoids can be best explained by two separate events: (1)
subduction-related metasomatism in the mantle source and (2) slab
breakoff-related metasomatism (Ba–Sr flux) and partial melting at
different depths (variable pressures) in the twice metasomatized
mantle.
Slab breakoff is a process that follows an attempted subduction of buoyant continental lithosphere during continental collision
(e.g., Wortel and Spakman, 2000). Oceanic lithosphere detaches
from continental lithosphere, and hot asthenosphere upwells into
the tearing slab causing melting and metasomatism in the overriding mantle lithosphere (Davies and von Blanckenburg, 1995). Slab
breakoff-induced melting in the enriched mantle is expected to
generate linear belts of high-K calc-alkaline plutons with enriched
mantle signature (high K, Ba, Sr, Mg, Cr, Ni) within a narrow time
interval (Atherton and Ghani, 2002). These characteristics have
been observed in Archean sanukitoids as well as in high Ba–Sr
granitoids of British Caledonides (Fowler et al., 2008) and other
Mesozoic–Paleozoic provinces worldwide. Another mechanism
that can produce astenospheric mantle upwelling is delamination
of a mafic lower continental crust into underlying convecting mantle, as suggested for Mesozoic high Ba–Sr granitoids found in the
North China craton (Gao et al., 2004; Qian et al., 2003; cf. Zegers
and van Keken, 2001).
Caledonian high Ba–Sr granites were emplaced 40–50 Ma after
the main tectonic events towards the end of the Caledonian orogeny
(Atherton and Ghani, 2002). Similar time gaps have been observed
between Archean TTGs and sanukitoids in the Karelian craton
Fig. 6. Numerical model results of a slab breakoff event during subduction in a hotter Earth, for a mantle potential temperature of 1550 ◦ C approximately 200 K hotter than
at present. Panels (a) and (b) show the viscosity structure (in colour) and location of basaltic (black) and eclogitic (white) crust of a subducting slab, before and shortly after
a breakoff event. Panels (c) and (d) show the corresponding thermal structures in a close-up of the subduction zone mantle wedge, which illustrates that increased mantle
wedge temperatures. After van Hunen and van den Berg (2008).
J. Halla et al. / Precambrian Research 174 (2009) 155–162
161
(Bibikova et al., 2005). These granitoids show a high K2 O–Ba–Sr
trend similar to that of sanukitoids (for comparison, a plot for a
Mesozoic high Ba–Sr granitoid, the Rogart pluton, is shown in Fig. 4)
and high Mg# (50–59). Therefore we suggest that slab breakoff provides a viable explanation for the genesis of the Archean high Ba–Sr
sanukitoid series. Lobach-Zhuchenko et al. (2008) also proposed
slab breakoff and subsequent mantle upwelling as a possible trigger for partial melting in the enriched subcontinental lithospheric
mantle beneath the Karelian craton.
simultaneously. An attempted subduction of a thick oceanic
plateau/protocrust may cause slab breakoff and mantle upwelling
resulting in the generation of sanukitoid magmas.
Numerical model results support a slab breakoff origin for the
formation of post-tectonic sanukitoids as a consequence of frequent
Archean slab breakoff events.
3.4. Numerical models
We thank H. Rollinson and R.H. Smithies for their critical and
valuable reviews which greatly improved the manuscript. This
study was supported by the Geological Survey of Finland, Finnish
Museum of Natural History, and the University of Helsinki.
Our conclusions based on the geochemical data on sanukitoids are supported by numerical models suggesting frequent slab
breakoffs in the Archean. Fig. 6 illustrates that hotter mantle conditions during the Archean could have lead to the occurrence of
spontaneous slab breakoff (van Hunen and van den Berg, 2008).
This is because the hotter mantle conditions had two dynamically important effects. Firstly, the more buoyant decompression
melting at the mid-ocean ridge lead to a thicker crustal layer,
possibly as thick as 20–25 km. Secondly, the large temperature
dependence of mantle material made a hotter mantle significantly
weaker. Since crustal material is significantly weaker than mantle material under the same conditions, oceanic lithosphere with
a thickened crust is relatively weak. And, although more melting
leads to a thicker depleted (and therefore probably dehydrated
and stiffer) mantle lithosphere, this effect is probably dominated
by the purely thermal decrease in mantle strength. In addition,
crustal material transforms from buoyant basalt to dense eclogite at
a depth of approximately 30–40 km, which leads to tensile stresses
within the subducting plate, which are proportional to the crustal
thickness. All these conditions promote slab breakoff in a hotter
mantle. Fig. 6 furthermore illustrates that mantle wedge temperatures increase after breakoff, providing favorable conditions to
create the Archean high Ba–Sr sanukitoid series. In the Archean,
such slab breakoffs may have followed continental collisions or an
attempted subduction of thick oceanic plateaus possibly converted
to TTG protocontinents.
4. Conclusions
Juvenile early Neoarchean granitoids in the western Karelia and
Kola cratons of the Fennoscandian Shield can be divided into three
groups: (1) low-HREE TTGs (high SiO2 , low Mg, low-HREE, higher
Sr, lower Ybn and higher Nb/Ta), (2) high-HREE TTGs (slightly lower
range of SiO2 , larger range of MgO contents and higher Cr and Ni
contents, high-HREE, lower Sr, higher Ybn and lower Nb/Ta), and (3)
high Ba–Sr sanukitoids (medium-HREE, high Mg and high K–Ba–Sr).
The low-HREE TTG group is consistent with high-pressure partial melting (>2.0 GPa) of a garnet-bearing basaltic source, whereas
the high-HREE group suggests low-pressure melting (1.0 GPa) of a
garnet-free basaltic crust and interaction with the mantle. A possible tectonic scenario for the genesis of the two groups is an incipient
hot subduction zone underneath a thick oceanic plateau/protocrust.
Deep melting in the lower part of thick basaltic oceanic crust
(stacked crust or plateau) could produce low-HREE TTGs, whereas
melting of subducting slab and possible interactions with the
mantle wedge in shallow depths would be capable of generating
high-HREE TTGs.
Archean late- to post-tectonic high Ba–Sr sanukitoids point to
melting in the enriched subcontinental lithospheric mantle wedge
below the TTG crust, probably triggered by slab breakoff and
asthenosphere mantle upwelling.
Early stage hot subduction at the margin of a thick oceanic
plateau provides a tectonic setting where melting in deep oceanic
crust and shallow oceanic crust-mantle interactions may occur
Acknowledgements
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