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Precambrian Research 174 (2009) 155–162

Contents lists available at ScienceDirect

Precambrian Research

journa l homepage: www.e lsev ier .com/ locate /precamres

eochemical and numerical constraints on Neoarchean plate tectonics

aana Halla a,∗, Jeroen van Hunen b, Esa Heilimo c, Pentti Hölttä d

Geological Museum, Finnish Museum of Natural History, University of Helsinki, Arkadiankatu 7, P.O. Box 17, 00014 FinlandDepartment of Earth Sciences, Durham University, United KingdomDepartment of Geology, University of Helsinki, FinlandGeological Survey of Finland, Espoo, Finland

r t i c l e i n f o

rticle history:eceived 28 October 2008eceived in revised form 17 July 2009ccepted 20 July 2009

eywords:eoarcheanTGanukitoideochemistryumerical modelingeodynamics

a b s t r a c t

This paper discusses early Neoarchean (2.8–2.7 Ga) plate tectonics by integrating knowledge from newgeochemical observations and numerical models. Based on a geochemical dataset of 295 granitoid sam-ples 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 earthelements) 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 andhigher Cr and Ni contents, high-HREE, lower Sr, higher Ybn and lower Nb/Ta), and (3) high Ba–Sr sanuki-toids (medium-HREE, high-Mg and high K–Ba–Sr). The main difference between the low- and high-HREEgroups lies in their pressure-sensitive element contents, which indicates high-pressure melting condi-tions for the low-HREE group and low-pressure conditions for the high-HREE group. A possible tectonicscenario for the genesis of the two groups is an incipient hot subduction underneath a thick oceanicplateau/protocrust. Melting in the lower part of thick basaltic oceanic crust could produce TTGs of thelow-HREE type, whereas low-pressure melting of subducting slab and possible interactions with the man-tle wedge at shallow depths would be capable of generating high-HREE TTGs. The third group of Archeanhigh 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 breakofffollowing 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 slabbreakoff in the Archean, which locally increased temperatures within the mantle wedge. More frequentbreakoff resulted, because subducting plates were weaker (due to rheologically thinner lithosphere and

nd teyant

a thicker basaltic crust), athat transforms from buo

. Introduction

The study of geodynamics of the early Earth continues to provideany challenges, and constructing uniform views from different

isciplines on Archean plate tectonic models requires overcomingeveral geological, geochemical, and geophysical hurdles. First of all,ime has taken its toll: Archean sites are now relatively sparse, andata are fragmented. This makes it difficult to recognize the large-cale tectonic setting at the time of formation of Archean rocks (de

it, 1998; Hamilton, 1998). Furthermore, terminology for volcanic

ocks generated in modern subduction environments (adakites,anukitoids) has been adapted to include a variety of Archean gran-toids. Finally, the global style of Archean geodynamics is not clear.id 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).

301-9268/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.precamres.2009.07.008

nsile stresses within the subducting plate were larger (due to a thick crustbasalt to dense eclogite).

© 2009 Elsevier B.V. All rights reserved.

ern plate tectonics (Davies, 1992)? Numerical models can provideinsight into the feasibility of certain geodynamic scenarios, butpoor constraints on some of the input parameters (Archean man-tle temperature, rheological parameters) necessarily leads to someuncertainty in the output of those models (e.g., van Hunen and vanden Berg, 2008). The most fruitful procedure to further our knowl-edge on Archean geodynamics is to combine geochemical data andgeological 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 modernadakites (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 gen-erated by interactions of slab melts with the mantle wedge in ahot subduction environment, although non-slab melting origins foradakites have been suggested as well (Macpherson et al., 2006).Following the original definition, the rocks referred to as adakites

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56 J. Halla et al. / Precambria

hould carry geochemical signatures generally related to slab meltsor other equivalent garnet-bearing basaltic source) [e.g., low-HREEnd high (La/Yb)n] and mantle interaction (lowered SiO2, high-Mg,g#, Cr, and Ni). For a thorough review of adakite terminology,

he reader is referred to the papers of Castillo (2006) and Moyenin press), and references therein. These papers make obvious thathe term “adakite” is expanded to describe a too large group ofocks with different petrogenetic processes. Furthermore, Smithies2000) pointed out that most early Archean and many NeoarcheanTGs lack mantle signatures thus showing no evidence for slabelt–mantle interactions and, therefore, cannot be attributed toodern-style subduction. Therefore, the term adakite is not rec-

mmended to be used for Archean TTGs.The term Archean sanukitoid suite, first introduced by Shirey

nd Hanson (1984), refers to late- to post-tectonic Archean dior-tes, monzodiorites, granodiorites, and granites with high K and

g contents and distinctive geochemical characteristics. Theseocks are considered to represent the addition of enriched man-le wedge melts into the continental crust (e.g., Stern and Hanson,991; Lobach-Zhuchenko et al., 2008 and references therein) andre therefore very important in evaluating crustal growth rates

nd understanding the mechanisms of crust formation during therchean.

The Archean Karelian and Kola cratons, in the northern part ofhe Precambrian Fennoscandian Shield, make one of the largestxposed nuclei of Archean crust. In terms of crustal growth and

ig. 1. Geological map of the Karelian and Kola cratons (modified after Sorjonen-Warderrains in eastern Finland.

arch 174 (2009) 155–162

rock series, these cratonic areas represent a complex pattern of dif-ferent type of granitoids, greenstones, and paragneisses dating backto 3.5 Ga, with the major peak in crustal growth between 2.9 and2.6 Ga (Sorjonen-Ward and Luukkonen, 2005, p. 27; Slabunov etal., 2006). Rapidly increasing geochemical and geophysical researchand data available on these well-exposed cratons makes them aninviting place to elucidate Archean geodynamics.

This paper divides juvenile Neoarchean granitoids into: (1) low-HREE TTG, (2) high-HREE TTG and (3) high Ba–Sr sanukitoid groups,and hypothesize on the geodynamic/tectonic settings in whichthese granitoids were formed. The study is based on new geo-chemical evidence from the mainly early Neoarchean (2.9–2.7 Ga)granitoids of the Karelian and Kola cratons of the FennoscandianShield. To support the hypothesis, we discuss numerical modelingresults.

2. Geochemical data

This study is based on a dataset of 295 analyses (new and pub-

from ca. 3.1 to 2.7 Ga granite–greenstone terrains in the westernparts of the Archean Karelian and Kola cratons of the Fennoscan-dian 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

and Luukkonen, 2005) showing the locations of the Archean granite–greenstone

J. Halla et al. / Precambrian Research 174 (2009) 155–162 157

Table 1Average major and trace element data for Archean juvenile granitoids from thewestern 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 70.8 67.3 64.7TiO2 0.31 0.59 0.49Al2O3 15.5 15.0 15.8Fe2O3 2.44 4.81 4.56MnO 0.03 0.07 0.07MgO 0.82 1.70 2.48CaO 2.88 3.76 3.5Na2O 4.81 4.23 4.48K2O 1.99 1.93 2.74P2O5 0.10 0.17 0.21

Trace elements (ppm)Ba 663 485 1182Rb 60.3 68.40 85.0Sr 427 314 729Pb 8.61 9.15 13.81Th 7.44 7.60 6.52U 0.64 1.05 1.21Hf 3.31 4.32 3.20Zr 120 171 135Nb 3.44 8.26 6.45Ta 0.19 0.57 0.60Y 4.5 18.6 10.9Sc 3.48 9.37 8.53V 26.5 56.8 69.7Cr <30 43.6 75.0Ni <20 26.6 35.2Co 4.96 10.8 12.1Cu 113.5 30.8 11.7Zn 49.1 78.2 77.3

Mg# 38.0 39.3 51.9

REE (ppm) n = 20 n = 20 n = 50

La 25.6 26.4 37.8Ce 48.6 55.5 72.8Pr 4.9 6.3 8.1Nd 17.0 24.3 30.4Sm 2.4 4.6 4.8Eu 0.6 1.0 1.1Gd 2.0 4.7 3.9Tb 0.2 0.7 0.5Dy 1.0 3.7 2.0Ho 0.2 0.7 0.4Er 0.4 2.1 1.0Tm <0.1 0.3 0.1Yb 0.4 2.0 0.9Lu <0.1 0.3 0.1Ybn 1.6 8.1 3.6Nb/Ta 18.1 14.5 10.8(La/Yb)n 55.2 10.5 30.1(

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tion in HREE [average (Gd/Er)n = 3.97] (Fig. 2), which is thoughtto be a consequence of residual garnet retaining the HREE in thesource (Martin, 1987; Moyen and Stevens, 2006). The normalizedREE patterns for the high-HREE group are much less fractionated

Gd/Er)n 3.97 1.88 3.30

ata sources—Ilomantsi terrain: Halla (2005), O’Brien et al. (1993); Iisalmi terrain:alla (2005); Kianta terrain: Käpyaho (2006); unpublished data; Ranua terrain:npublished data; Inari terrain: unpublished data.

nd Luukkonen (2005). New samples were analysed for major andrace elements by XRF and ICP–MS methods at the Labtium labora-ory in Espoo, Finland.

Archean juvenile granitoids in the study area fall into threeroups based on their REE patterns (Fig. 2): (1) low-HREETGs, (2) high-HREE TTGs, and (3) medium-HREE sanuki-oids. The first two groups belong to the voluminous, sodic

onalite–trondhjemite–granodiorite (TTG) series, the major com-onent of the Archean crust, and the third group represents alightly later and minor group of potassic Archean sanukitoids.ach group shows diagnostic geochemical signatures (Table 1 andigs. 3 and 4).

Fig. 2. Chondrite-normalized (Taylor and McLennan, 1985) REE patterns of Archeanjuvenile granitoids demonstrating their division into high- and low-HREE TTG andmedium-HREE sanukitoid groups. Normalized values for concentrations under thedetection limit are interpolated.

2.1. TTG

There is little difference in the major element compositions oflow- and high-HREE TTGs. The low-HREE TTG group shows slightlyhigher SiO2 (68–76 wt.%) and lower MgO contents (<1.2 wt.%)compared to the larger SiO2 range (60–74 wt.%) and extendedMgO range of the high-HREE group (Fig. 3). The average Mg# isslightly lower in the low-HREE group than in the high-HREE group(although the difference is rather meaningless at low MgO and highSiO2). The high-HREE group shows higher contents of mantle com-patible elements (V, Cr, Ni, and Co) as well as a slight tendency tohigher Mg# values compared with those of the low-HREE group.

The normalized REE patterns of the low-HREE group are steeplyfractionated [high average (La/Yb)n of 55.2] with significant deple-

Fig. 3. (Gd/Er)n vs. MgO plot illustrating different groups of Archean juvenile gran-itoids. The hypothetic source end-members are garnet-bearing [high (Gd/Er)n] orgarnet-free [low (Gd/Er)n] basaltic crust (low MgO) or mantle (high MgO).

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ig. 4. Na2O/K2O vs. Ba + Sr plot for discriminating the high Ba–Sr sanukitoid grouprom the TTG groups. The hypothetic source end-members are enriched mantle (higha + Sr, low Na2O/K2O) and primitive basaltic source (low Ba + Sr, high Na2O/K2O).ata for the Mesozoic Rogart pluton are from Fowler et al. (2001).

average (La/Yb)n = 10.5] and show relatively flat HREE profilesaverage (Gd/Er)n = 1.88] indicating the absence of garnet in theesidue (Table 1 and Fig. 2). According to Moyen (in press), therere also other processes that can produce the high La/Yb signature:elting of a high La/Yb source, fractional crystallization, or interac-

ions of felsic melts with the mantle. However, deep garnet-presentelting 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

gO contents (Fig. 3), whereas the high-HREE group shows anpposite trend; increasing MgO contents for low (Gd/Er)n valuesFig. 3). The high-HREE contents and low (Gd/Er)n values point to

garnet-free residual. High MgO, V, Cr, Ni, and Co contents areenerally attributed to a contribution from a mantle source.

The low-HREE group shows Eu anomalies from slightly neg-tive (Eu* 0.8) to positive (Eu* 2.3). Eu anomalies are attributedo Eu’s tendency to be incorporated into plagioclase preferentiallyver other minerals. Negative Eu anomalies indicate plagioclase inhe source or feldspar fractionation, whereas positive Eu anoma-ies indicate accumulation of plagioclase. The Eu-positive subgroups illustrated in Fig. 2 by a separate field. Positive Eu anomaliesre observed to correlate with high Sr concentrations supportinglagioclase accumulation.

The other important differences observed in trace elements arehe lower Sr contents, higher Yb and lower Nb/Ta of the high-HREEroup compared with those of the low-HREE group.

.2. Sanukitoids

The third group of medium-HREE sanukitoids has uniform REEatterns that are more fractionated [average (La/Yb)n = 30.1] andhow lower HREE ends [average (Gd/Er)n = 3.30] than those of theigh-HREE group (Fig. 2). The sanukitoid group shows the largestariation in SiO2 contents (55–70 wt.%) and the highest V, Cr, Ni,nd Co contents (Table 1). They show a high range of MgO con-ents and variable (Gd/Er)n values (Fig. 3) pointing to a mantleomponent in the source. A distinctive feature of the sanukitoid

roup is their low Na2O/K2O ratio and high enrichment in fluid-obile elements Ba and Sr (Fig. 4). Both low- and high-HREE TTG

roups show an opposite trend: increasing Na2O/K2O ratios withespect to low concentrations of Ba + Sr (<1200 ppm). The high Ba

arch 174 (2009) 155–162

and Sr concentrations of sanukitoids are observed to be indepen-dent of the SiO2 contents; therefore this signature is generallyinterpreted to come from the mantle. Because high-HREE TTGsalso show mantle characteristics but lack the extremely high Ba–Srsignature (Ba + Sr > 1200 ppm) typical of sanukitoids (Fig. 4), it isconcluded that the mantle source of sanukitoids was enriched byan allochthonous high Ba–Sr fluid/melt flux.

3. Discussion

3.1. TTG subdivisions

3.1.1. Previous divisionsThe distribution of TTGs into two geochemical groups was first

noticed by Barker (1979) who divided TTGs into low- and high-Al2O3 groups (separated at 15% Al2O3 and 70% silica) and suggestedthat the groups were produced by fractionation crystallization ordifferent 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 thesecular change in the TTG composition to the transformation fromflat to deep subduction due to the cooling of the Earth (Abbottet al., 1994). This would have enabled the formation of a mantlewedge and thus allowed interactions between slab melts and peri-dotite. However, this model was challenged by Smithies (2000),who pointed out that most early Archean and many NeoarcheanTTGs lack mantle signatures thus showing no evidence for slabmelt–mantle interactions and, therefore, cannot be attributed tomodern-style subduction. Furthermore, numerical modeling sug-gests that flat subduction was not physically feasible in the hotterArchean mantle (van Hunen et al., 2004). Based on these argumentsand taking account that most TTGs of this study lack the high-Mgsignature, secular division by Mg contents is not supported here.

Moyen and Stevens (2006) gave experimental constraints onTTG petrogenesis. They suggested, on the basis of pressure-sensitiveelement concentrations, that the TTG data support a continuum ofconditions for melting from low- to high-pressures. Low-pressureTTGs (ca. 10 kbar) are Sr poor (<400 ppm), relatively undepleted inYb (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 oppositecharacteristics. The average data in Table 1 show that high- andlow-HREE groups are consistent with the low- and high-pressureTTG groups, respectively. Furthermore, experimental studies haveshown that there are no obvious differences in the major elementcompositions 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 relatedthe origin of the high Sr/Y series (equivalent to the low-HREETTGs) to deep melting of basaltic source (>2.0 GPa) and that of thelow Sr/Y series (equivalent to the high-HREE TTGs) to the shallowmelting of basaltic source (1.0 GPa). As the pressure increases, pla-gioclase becomes unstable releasing Sr whereas garnet becomesstable retaining Y.

The compositional differences between the low- and high-HREEgroups presented here are similar to those between the high- andlow-Al groups as well as the high- and low-pressure groups ofMoyen and Stevens (2006). The differences in the major elementcompositions are not as large as would be expected if the two groups

different degrees of melting. Because the differences between thegroups lie mainly in the pressure-sensitive elements, we maintainthat the two TTG groups were produced as a function of melting atdifferent pressures and, consequently, at different depths. Therefore

J. Halla et al. / Precambrian Research 174 (2009) 155–162 159

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e suggest naming these groups by their HREE content, which haseen widely regarded as the best indicator of pressure conditionsf melting.

.1.2. Justifications for the division by HREEDepletion in HREE is acknowledged as one of the most promi-

ent feature in TTGs. It is generally understood that the HREEontents of magmas are controlled by garnet stability in the source,hich requires high-pressure conditions. The degree of melting

xerts relatively little influence, because garnet is generated in theelting process (Moyen and Stevens, 2006). The more melt is pro-

uced, the greater proportion of garnet remains in the residue.he �(HREE)n vs. SiO2 plot of the Karelian TTGs in Fig. 5a implieshat the HREE content is not dependent on the SiO2 content andhus is not related by magmatic evolution or fractionation pro-esses.

The subdivision of TTG by HREE content presented in this paperrgues for previous classifications. In Fig. 5b, the division intoow- and high-HREE TTGs is compared with that into low andigh Al2O3 TTGs. The plot indicates that there are two geneticallyifferent groups of TTGs. Low-HREE TTGs follow the high-Al2O3rend, whereas high-HREE TTG show lower Al2O3 contents ativen silica content. This is consistent with initial observations ofammarstrom and Zen (1986) that in granitoids, the total amount ofl that can be accommodated in hornblende increases as a functionf pressure.

As noted before, the major differences between the high andow-HREE groups are mainly restricted to the pressure indicatorlements such as HREE, which reflects differences in the meltingonditions and thus in the site of melting rather than fractionationrocesses or different degrees of melting.

Division of TTGs by HREE content is suggested here because: (1)he contrasting HREE content is a prominent feature in TTGs andhe data presented show no overlap between low- and high-HREEroups, (2) lots of experimental results on the behavior of HREEre available in the literature, and (3) the HREE content seems toe largely a pressure-dependent feature. The “adakitic” division byr/Y and La/Yb (the “adakitic signature”) cannot be favored beforelearing up the confusion of the term. Furthermore, a high Sr con-

ent might be a result of metasomatism, at least in some cases.he old division by Al content is based only on one major ele-ent that is affected also by fractionation processes, and the Mg

ontent (and Mg#) is too variable to be used as a basis of classifica-ion.

2 diagrams for low- and high-HREE groups.

3.2. Geodynamic setting of low- and high-HREE TTGs

The data presented in this study show that two coeval andcontrasting groups of Neoarchean juvenile TTGs with low- andhigh-HREE characteristics can be found in the western parts ofthe Karelian and Kola cratons. The simultaneous occurrence of thetwo types of TTGs in the Karelian craton has been demonstratedby Samsonov et al. (2005). The similar temporal occurrence butcontrasting pressure indicator element contents suggests that themagmas were generated under different melting pressure condi-tions from separate sources (but with similar bulk geochemistry)during the same large-scale tectonic event. A detailed analysis ofspatial and temporal relations of different geochemical groups isneeded and remains to be carried out.

The typical features of the low-HREE group (high SiO2, low-HREE, high (Gd/Er)n, and low MgO) are attributed to fluid-absentpartial melting of amphibolites in the garnet stability field (Barkerand Arth, 1976; Defant and Drummond, 1990; Martin, 1987, 1994).The high-HREE TTGs show elevated MgO and lack the high-pressuresignatures related to slab melting in garnet stability depths. Thelow-HREE granitoid group is consistent with high-pressure meltingof deep oceanic crust or plateaus, whereas high-HREE granitoidsfavor a shallower, low-pressure source possibly with a mantleinvolvement.

A possible tectonic setting where these two types of sourcesmay exist at the same time is an incipient subduction zone belowthe margin of an oceanic plateau, where the mantle wedge isunusually hot. White et al. (1999) proposed such an origin for atonalitic batholith (∼85 Ma) in the island of Aruba in the southernCaribbean. They present a model involving derivation of tonaliteby contributions from the mantle wedge, the subducting slab andthe overlying plateau crust. This environment provides an attractivehypothesis also for the Karelian and Kola TTG formations: low-HREEmagmas were produced by partial melting at high-pressures inthe deep lower part of a thick oceanic crust, whereas high-HREETTG magmas were generated by interactions between subduct-ing oceanic slab and mantle wedge at low-pressures and shallowdepths.

A third possible mechanism for TTG generation is delamination

of the lower eclogitic part of an oceanic plateau, which has been pro-posed for Middle Archean cratons by Zegers and van Keken (2001).This model was originally proposed as an alternative for plate tec-tonics, but it might be possible that early Archean oceanic plateauswere first converted to TTG protocontinents by this mechanism and

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hen accreted to continents by convergent tectonics that initiatedn the late Archean.

.3. Archean high Ba–Sr sanukitoids

Some authors have suggested that Archean sanukitoids, whichre identified by their enriched mantle signature (variable SiO2nd high Mg, Mg#, Ni, Cr, K, Ba, and Sr), are possible analogues todakites, at least to those with lower silica contents (e.g., Martin etl., 2005). However, there are several arguments against this view.irstly, sanukitoids show high K2O contents and distinctively highnrichment in both Ba (>1000 pm) and Sr (av. 729 ppm) (Fig. 4 andable 1). Secondly, sanukitoids are 10–100 Ma younger than theajor phase of the TTG magmatism within the same tectonic region

f the Karelian craton (Bibikova et al., 2005; Käpyaho et al., 2006),hich attests their late- to post-tectonic setting, whereas adakites

re regarded as pre- to syntectonic. In eastern Finland, the majorhase of TTG magmatism occurred between 2.83 and 2.74 Ga andas followed by a brief period of sanukitoid magmatism between

.73 and 2.70 Ga (Käpyaho et al., 2006). Therefore, sanukitoids can-ot be regarded as analogues of subduction-related adakites. Aetter recent analogy for sanukitoids should be looked for amonghe post-collisional granitoids rather than among subduction-elated volcanics. In fact, there is a group of mainly Mesozoic andaleozoic post-collisional high–Ba–Sr granitoids (Tarney and Jones,994; Fowler et al., 2008) that can be considered to be analoguesf sanukitoids. To understand the characteristics of the high Ba–Srranitoids, we need to discuss the source of Ba and Sr.

High Ba and Sr concentrations of sanukitoids are independent ofheir SiO2 contents and are thus thought to derive from the mantle.commonly suggested source for Ba and Sr is a slab-melt metasom-

tized mantle wedge (Stern and Hanson, 1991; Lobach-Zhuchenko

t al., 2008). A recent study of Mogarovskii et al. (2007) highlightshe possibility that Ba and Sr derived from deep astenosphericources, can accumulate in the upper mantle as a consequencef mantle metasomatism and melting in the metasomatized man-le. It is possible that the upper mantle source of sanukitoids was

ig. 6. Numerical model results of a slab breakoff event during subduction in a hotter Eat present. Panels (a) and (b) show the viscosity structure (in colour) and location of basabreakoff event. Panels (c) and (d) show the corresponding thermal structures in a closeedge temperatures. After van Hunen and van den Berg (2008).

arch 174 (2009) 155–162

enriched in these fluid-mobile elements at the time of their for-mation. The mantle source may have been metasomatized andmelted as a consequence of a post-collisional mantle upwellingtriggered by a slab breakoff or delamination of the lower part of thecrust. A slab breakoff has been suggested as the trigger of sanuki-toid magmatism by e.g., Calvert et al. (2004), Whalen et al. (2004),and Lobach-Zhuchenko et al. (2008). Sanukitoids show also somecharacteristics that probably derived from previous subduction-related metasomatism, such as crustal isotopic signatures (Halla,2005; Lobach-Zhuchenko et al., 2008). The geochemical signaturesof sanukitoids can be best explained by two separate events: (1)subduction-related metasomatism in the mantle source and (2) slabbreakoff-related metasomatism (Ba–Sr flux) and partial melting atdifferent depths (variable pressures) in the twice metasomatizedmantle.

Slab breakoff is a process that follows an attempted subduc-tion of buoyant continental lithosphere during continental collision(e.g., Wortel and Spakman, 2000). Oceanic lithosphere detachesfrom continental lithosphere, and hot asthenosphere upwells intothe tearing slab causing melting and metasomatism in the overrid-ing mantle lithosphere (Davies and von Blanckenburg, 1995). Slabbreakoff-induced melting in the enriched mantle is expected togenerate linear belts of high-K calc-alkaline plutons with enrichedmantle signature (high K, Ba, Sr, Mg, Cr, Ni) within a narrow timeinterval (Atherton and Ghani, 2002). These characteristics havebeen observed in Archean sanukitoids as well as in high Ba–Srgranitoids of British Caledonides (Fowler et al., 2008) and otherMesozoic–Paleozoic provinces worldwide. Another mechanismthat can produce astenospheric mantle upwelling is delaminationof a mafic lower continental crust into underlying convecting man-tle, as suggested for Mesozoic high Ba–Sr granitoids found in theNorth 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 afterthe main tectonic events towards the end of the Caledonian orogeny(Atherton and Ghani, 2002). Similar time gaps have been observedbetween Archean TTGs and sanukitoids in the Karelian craton

rth, for a mantle potential temperature of 1550 ◦C approximately 200 K hotter thanltic (black) and eclogitic (white) crust of a subducting slab, before and shortly after-up of the subduction zone mantle wedge, which illustrates that increased mantle

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Bibikova et al., 2005). These granitoids show a high K2O–Ba–Srrend similar to that of sanukitoids (for comparison, a plot for a

esozoic high Ba–Sr granitoid, the Rogart pluton, is shown in Fig. 4)nd high Mg# (50–59). Therefore we suggest that slab breakoff pro-ides a viable explanation for the genesis of the Archean high Ba–Sranukitoid series. Lobach-Zhuchenko et al. (2008) also proposedlab breakoff and subsequent mantle upwelling as a possible trig-er for partial melting in the enriched subcontinental lithosphericantle beneath the Karelian craton.

.4. Numerical models

Our conclusions based on the geochemical data on sanuki-oids are supported by numerical models suggesting frequent slabreakoffs in the Archean. Fig. 6 illustrates that hotter mantle con-itions during the Archean could have lead to the occurrence ofpontaneous slab breakoff (van Hunen and van den Berg, 2008).his is because the hotter mantle conditions had two dynami-ally important effects. Firstly, the more buoyant decompressionelting at the mid-ocean ridge lead to a thicker crustal layer,

ossibly as thick as 20–25 km. Secondly, the large temperatureependence of mantle material made a hotter mantle significantlyeaker. Since crustal material is significantly weaker than man-

le material under the same conditions, oceanic lithosphere withthickened crust is relatively weak. And, although more melting

eads to a thicker depleted (and therefore probably dehydratednd stiffer) mantle lithosphere, this effect is probably dominatedy the purely thermal decrease in mantle strength. In addition,rustal material transforms from buoyant basalt to dense eclogite atdepth of approximately 30–40 km, which leads to tensile stressesithin the subducting plate, which are proportional to the crustal

hickness. All these conditions promote slab breakoff in a hotterantle. Fig. 6 furthermore illustrates that mantle wedge temper-

tures increase after breakoff, providing favorable conditions toreate the Archean high Ba–Sr sanukitoid series. In the Archean,uch slab breakoffs may have followed continental collisions or anttempted subduction of thick oceanic plateaus possibly convertedo TTG protocontinents.

. Conclusions

Juvenile early Neoarchean granitoids in the western Karelia andola cratons of the Fennoscandian Shield can be divided into threeroups: (1) low-HREE TTGs (high SiO2, low Mg, low-HREE, higherr, lower Ybn and higher Nb/Ta), (2) high-HREE TTGs (slightly lowerange of SiO2, larger range of MgO contents and higher Cr and Niontents, high-HREE, lower Sr, higher Ybn and lower Nb/Ta), and (3)igh Ba–Sr sanukitoids (medium-HREE, high Mg and high K–Ba–Sr).

The low-HREE TTG group is consistent with high-pressure par-ial melting (>2.0 GPa) of a garnet-bearing basaltic source, whereashe high-HREE group suggests low-pressure melting (1.0 GPa) of aarnet-free basaltic crust and interaction with the mantle. A possi-le tectonic scenario for the genesis of the two groups is an incipientot subduction zone underneath a thick oceanic plateau/protocrust.eep melting in the lower part of thick basaltic oceanic crust

stacked crust or plateau) could produce low-HREE TTGs, whereaselting of subducting slab and possible interactions with theantle wedge in shallow depths would be capable of generating

igh-HREE TTGs.Archean late- to post-tectonic high Ba–Sr sanukitoids point to

elting in the enriched subcontinental lithospheric mantle wedge

elow the TTG crust, probably triggered by slab breakoff andsthenosphere mantle upwelling.

Early stage hot subduction at the margin of a thick oceaniclateau provides a tectonic setting where melting in deep oceanicrust and shallow oceanic crust-mantle interactions may occur

arch 174 (2009) 155–162 161

simultaneously. An attempted subduction of a thick oceanicplateau/protocrust may cause slab breakoff and mantle upwellingresulting in the generation of sanukitoid magmas.

Numerical model results support a slab breakoff origin for theformation of post-tectonic sanukitoids as a consequence of frequentArchean slab breakoff events.

Acknowledgements

We thank H. Rollinson and R.H. Smithies for their critical andvaluable reviews which greatly improved the manuscript. Thisstudy was supported by the Geological Survey of Finland, FinnishMuseum of Natural History, and the University of Helsinki.

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