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Archaean Kuru-Vaara eclogites in the northernBelomorian Province, Fennoscandian Shield: crustalarchitecture, timing, and tectonic implicationsVictor Balaganskyab, Andrey Shchipanskyc, Alexander I. Slabunovde, Il’ya Gorbunova, SergeyMudruka, Mikhail Sidorova, Pavel Azimovf, Svetlana Egorovad, Alexandera Stepanovad &Anatoly Voloshina

a Geological Institute of the Kola Science Centre, Russian Academy of Sciences, Apatity,Russiab Apatity Branch, Murmansk State Technical University, Apatity, Russiac Geological Institute, Russian Academy of Sciences, Moscow, Russiad Institute of Geology of the Karelian Research Centre, Russian Academy of Sciences,Petrozavodsk, Russiae Mining and Geological Faculty, Petrozavodsk State University, Petrozavodsk, Russiaf Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, SaintPetersburg, RussiaPublished online: 30 Oct 2014.

To cite this article: Victor Balagansky, Andrey Shchipansky, Alexander I. Slabunov, Il’ya Gorbunov, Sergey Mudruk, MikhailSidorov, Pavel Azimov, Svetlana Egorova, Alexandera Stepanova & Anatoly Voloshin (2014): Archaean Kuru-Vaara eclogitesin the northern Belomorian Province, Fennoscandian Shield: crustal architecture, timing, and tectonic implications,International Geology Review, DOI: 10.1080/00206814.2014.958578

To link to this article: http://dx.doi.org/10.1080/00206814.2014.958578

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Archaean Kuru-Vaara eclogites in the northern Belomorian Province, Fennoscandian Shield:crustal architecture, timing, and tectonic implications

Victor Balaganskya,b*, Andrey Shchipanskyc, Alexander I. Slabunovd,e, Il’ya Gorbunova, Sergey Mudruka,Mikhail Sidorova, Pavel Azimovf, Svetlana Egorovad, Alexandera Stepanovad and Anatoly Voloshina

aGeological Institute of the Kola Science Centre, Russian Academy of Sciences, Apatity, Russia; bApatity Branch, Murmansk State TechnicalUniversity, Apatity, Russia; cGeological Institute, Russian Academy of Sciences, Moscow, Russia; dInstitute of Geology of the KarelianResearch Centre, Russian Academy of Sciences, Petrozavodsk, Russia; eMining and Geological Faculty, Petrozavodsk State University,Petrozavodsk, Russia; fInstitute of Precambrian Geology and Geochronology, Russian Academy of Sciences, Saint Petersburg, Russia

(Received 19 March 2014; accepted 22 August 2014)

This paper addresses the relationships between relic amphibolite-eclogites facies (AE) eclogites and their host units,Archaean amphibolites, enveloped by Archaean tonalite–trondhjemite–granodiorite (TTG) gneisses, in the Kuru-Vaarastudy area in the northern Belomorian Province. According to observational constraints, the crystallization of the relicpeak omphacite + Mg-garnet ± kyanite assemblage and the subsequent replacement of omphacite by clinopyroxene–plagioclase symplectite occurred before the earliest deformational, metamorphic, and migmatization events that are recordedin the amphibolites. The amphibolites and their TTG hosts have a shared deformational and metamorphic history that iscomposed of the Archaean and Palaeoproterozoic periods. This history favours the conclusion that the AE metamorphismrecorded in the relic eclogites within the amphibolites occurred during the Mesoarchaean to Neoarchaean periods. Thedeformation and metamorphism of the amphibolite facies of the second period resulted from the Lapland–Kola collisionalorogeny at 1.91–1.93 Ga, which led to eclogite–high-pressure granulite (E–HPG) facies conditions in the lowermostportions of the over-thickened crust in Belomorian Province (the southwestern foreland of the Lapland–Kola collisionalorogen). The Palaeoproterozoic E–HPG overprint was reported from the Palaeoproterozoic Gridino mafic dikes. Althoughthe ages of the oldest low Th/U zircons are close to the time of the Lapland–Kola collision, the low Th/U 1.9–1.8 Ga zirconsreflect a zircon response to regional fluid infiltration in the eclogites during slow exhumation following the Lapland–Kolaorogeny and do not record any metamorphic event. Contrary to the Palaeoproterozoic E–HPG overprint, the arealoccurrence of the 2.7–2.8 Ga AE eclogites with mid-ocean ridge basalt-like chemistry and their paragenetic link with theTTG gneisses suggest a tectonic regime that involves subduction. This research favours concepts suggesting that themodern-style plate tectonics has operated in some places, at least since the late Mesoarchaean.

Keywords: plate tectonics; eclogite; deformational history; Archaean; Palaeoproterozoic; Belomorian Province;Fennoscandian Shield

Introduction

The question of which tectonic processes occurred duringthe early Precambrian is being actively studied. Althoughplate tectonics is the most preferred method of interpreta-tion, the extent of similarities between ancient and modernplate tectonic processes and peculiarities of the transitionfrom the former to the latter is questionable (Stern 2005;Condie and Kröner 2008; Moyen and van Hunen 2012;Korenaga 2013; Brown 2014; Hynes 2014; Sizova et al.2014). Significant insights into early Precambrian tec-tonics can now be provided by using eclogites from theBelomorian Province of the Fennoscandian Shield(Figure 1), which have been mentioned in only a fewcommunications (e.g. Mitrofanov 1996).

In Gridino, Salma, and Kuru-Vaara, the firstcomprehensive case studies of the Belomorian eclogitesindicated that these rocks are found as relics withinArchaean amphibolites, which occurred within an

Archaean tonalite–trondhjemite–granodiorite (TTG)matrix (Figure 1; Volodichev et al. 2004; Mints et al.2010; Konilov et al. 2011; Shchipansky et al. 2012a,2012b). These field observations and the reported zirconages of 2.70–2.72 Ga and 2.87 Ga were interpreted in thestudies to favour an Archaean age of subduction-relatedeclogitic metamorphism. Eclogite relics (‘former eclogites’hereafter) that are represented by clinopyroxene–plagioclasesymplectites contain rare omphacite relics and are wide-spread in the Archaean amphibolites in the northernBelomorian Province (Pozhilenko 2013). Thus, it is impor-tant to understand the roles of the eclogites to furtherunderstand the processes of Archaean tectonics. Apartfrom these numerous former eclogites, a few patches ofeclogitic rocks have been identified in Palaeoproterozoiccoronitic mafic dikes in the Gridino area (Travin andKozlova 2005). In addition, the eclogitized mafic dikestruncate the Archaean eclogite-bearing amphibolites

*Corresponding author. Email: balagan@geoksc.apatity.ru

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(Slabunov et al. 2008). The most recent interpretation of thedikes as Archaean (Dokukina et al. 2014) is under question.

The aforementioned data suggest that the two periodsof eclogitic metamorphism may be established in theBelomorian Province, Archaean and Palaeoproterozoic.However, it is thought that the remnant eclogites in theArchaean amphibolites and the eclogitic patches in thePalaeoproterozoic dikes only resulted from the overprint-ing of Palaeoproterozoic eclogite–high-pressure granulite(E–HPG) (terminology after Brown 2007) facies. ThePalaeoproterozoic age of the eclogitic metamorphism thatwas recorded in the amphibolites is based on U–Pb ages ofca. 1.9 Ga in the zircons, including the zircon rims aroundthe Archaean zircons, which have very low Th/U (<0.01)and MREE–HREE concentrations, and the so-called ʻeclo-giticʼ rare earth element patterns (Skublov et al. 2011). Inaddition, garnets from the same eclogites also yield Lu–Hfages of ca. 1.9 Ga (Herwartz et al. 2012). However, wheretwo or more high-grade metamorphic events that haveoccurred are combined with long-lived P–T evolutionand slow near-isobaric cooling (1–4 °C/Ma), the age ofthe peak metamorphism may be poorly constrained oreven unconstrained by only the zircon data (Harley andKelly 2007). In polymetamorphic complexes, the isotopicmodification of older zircons may reflect later post-peakfluid infiltration events rather than the major high-grademetamorphism recorded in rock mineralogy. Moreover, a

later high-grade overprint with a slow cooling rate maysignificantly reset the Lu–Hf isotope system of the garnetand produce false ages that are much younger than the trueages of the peak metamorphism (Kelly et al. 2011).Indeed, the Belomorian Province is a typical polymeta-morphic complex (Volodichev 1990; Bibikova et al. 2004;Glebovitsky 2005) that experienced high-grade meta-morphic overprint at ca. 1.9 Ga with subsequent long-lived crustal exhumation at a cooling rate of 2–4 °C permillion years (Bibikova et al. 2001).

Thus, to correctly interpret Archaean ages from theformer eclogites within the Archaean Belomorian amphi-bolites, the geological contexts of these eclogites and thereliable isotope ages of their hosts should be considered.These issues are the focus of this paper. The first priorityof this paper is to question whether the omphacite–garnetassemblages postdate the strongly deformed and migma-tized amphibolites or whether the amphibolites haveresulted from the metamorphism, deformation, and mig-matization of the former eclogites.

Geological setting

The Belomorian Province (Figure 1) is a typical earlyPrecambrian polydeformational and polymetamorphicmobile belt that evolved from the late Mesoarchaean tothe late Palaeoproterozoic (Gaál and Gorbatschev 1987;Volodichev 1990; Glebovitsky et al. 1996; Slabunov et al.2006). The prevailing components of the province include2.93 to ca. 2.6 Ga TTG gneisses (Slabunov et al. 2006;Hölttä et al. 2008). The key tectonic units include a ca.2.9 Ga Chupa paragneiss complex, the ca. 2.9 Ga CentralBelomorian greenstone belt and the ca. 2.8 Ga Iringoragreenstone belt (the two belts contain a dismemberedophiolitic unit) (Shchipansky et al. 2004; Slabunov et al.2006), and the Gridino, Salma, and Kuru-Vaara eclogite-bearing complexes. Apart from a granulite facies episodeof 2.72 Ga, a lower geothermal gradient is the character-istic feature of the Archaean and Palaeoproterozoic meta-morphic events that occurred in the amphibolite facies(Glebovitsky et al. 1996). The following major stageswere established: (1) the oceanic stage, ca. 2.9 Ga, (2)the subduction–accretion stage (eclogite facies and amphi-bolite facies metamorphism), 2.9–2.71 Ga, and (3) thecollisional tectonics stage (locally granulite facies meta-morphism), 2.71–2.65 Ga (Bibikova et al. 2004; Slabunovet al. 2006; Hölttä et al. 2008).

The Palaeoproterozoic reworking of the BelomorianProvince was related to the high-grade collisional eventsthat occurred in the core of the Lapland–Kola collisionalorogen, the most important part of which is the LaplandGranulite belt (Figure 1). Because the orogenic core iscomposed of arc-related igneous rocks and meta-greywackes of predominantly Juvenal origin, the orogeniccore represents a thrust-nappe ensemble of the granulite to

Figure 1. Main tectonic units of the Fennoscandian Shield. BP,Belomorian Province; KP, Kola Province (after Balagansky2002; Daly et al. 2006; Slabunov et al. 2006).

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amphibolite facies. The ensemble overlies the BelomorianProvince representing the southwestern foreland of theorogen (Bridgwater et al. 1992; Daly et al. 2006; Patisonet al. 2006; Tuisku et al. 2012). A mid-crustal nappe thatcontains a 10 km-scale sheath fold and overlies the slightlydeformed Archaean rocks was discovered in theKola Province (the northeastern foreland of the orogen;Figure 1), which together with the deep thrusts in theorogenic core suggests the occurrence of Alpine-type tec-tonics (Mudruk et al. 2013). This orogeny occurred 1.97to ca. 1.9 Ga and reached its peak in the orogenic core at1.925 Ga in Kola and 20–30 billion years later in theFinnish Lapland region (Bibikova et al. 1993; Daly et al.2006; Patison et al. 2006; Tuisku et al. 2012). ExtensivePalaeoproterozoic reworking of the Belomorian Provinceoccurred only in a few discrete shear zones (Balagansky2002), which explains the good preservation of theArchaean structures in many locations.

In contrast with the rest of the province, the TTGmatrix of the northern Belomorian Province containsmany more amphibolites, which host rare ultramaficbodies. However, the occurrence of Al-rich paragneissesis very rare (Figure 2). The Palaeoproterozoic deformationwas accommodated by a system of discrete, gently dip-ping, and steep high-grade shear zones that were charac-terized by gently plunging to horizontal stretchinglineations (Balagansky 2002). The Shirokaya SalmaShear Zone (SSSZ) is of great interest because the shear-ing was superimposed on the TTG gneisses and amphibo-lites, with the amphibolites containing relic eclogites(Mints et al. 2010; Konilov et al. 2011). Interestingly,Eskola (1921) interpreted the garnet amphibolites thatoccurred as amphibolized eclogites across an extensivearea around the western region of Kandalaksha (Figure 2).

Analytical methods

Geochemical studies were conducted in the CentralLaboratory of the All-Russian Research GeologicalInstitute, Saint Petersburg. The contents of rock-formingelements were determined on an ARL 9800 X-ray spectralanalyzer with an error not exceeding 2 rel.%. The contentsof trace elements were determined on an ELAN-DRC-61001 ICP MS using 50–100 mg samples and applyingthe technique of their deep acid digestion and fusion withlithium metaborate in a furnace, which ensures the totaldissolution of even poorly soluble minerals. The calibra-tion plots were constructed using the OU-6 InternationalStandard sample. The errors of determination of trace andrare earth elements did not exceed 5 rel.% in accordancewith the IAG.

A U-Pb isotope dating was performed on a SHRIMP IIinstrument in the Centre of Isotope Studies of the All-Russian Research Geological Institute, Saint Petersburg.The manually sampled zircon grains were implanted intoepoxy resin together with TEMORA and 91500 zirconstandard grains. Then the grains were cut-off nearly tohalf of their thickness and polished. To choose datingspots on the surface of the grain sections, we used optical,cathodoluminescent, and backscattered electron images.Isotope measurements were carried out following theaccepted technique (Williams 1998). The intensity of aprimary beam of negative oxygen ions was 4 nA, andthe crater diameter was 25 μm. The errors of singularmeasurements were at the 1σ level, and the errors ofcalculation of the concordant ages were at the 2σ level.The Concordia plots were constructed using theISOPLOT/EX program (Ludwig 1999).

Mineral analyses from Al-rich gneisses were per-formed on a JEOL JSM-6510LA electron microscope

Figure 2. Geological map of the northern Belomorian Province (after Pozhilenko in Mitrofanov et al. 1995; slightly modified).

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equipped with a JED-2200 analyser at the Institute ofPrecambrian Geology and Geochronology RAS (SaintPetersburg). Minerals from gabbro–norites were deter-mined at the Institute of Geology, Karelian ResearchCenter, RAS (Petrozavodsk) using a Tescan Vega II LSHelectron microprobe equipped with an EDS INCA-Energy350 analyser. The analysis’ conditions were conventionalin either case.

Structure of the Kuru-Vaara study area

The study area is composed of TTG gneisses, amphibolites(many contain the former eclogites), ceramic pegmatites,coronitic olivine-bearing gabbro–norite, and minor Al-richgneisses (Figure 3). It can be subdivided into northwesternand southeastern domains. In the southeastern domain,most of the TTG gneisses and the amphibolites have beenstrongly and repeatedly folded, boudinaged, and migma-tized. In this case, metatexite (Figure 4A) and diatexite(Figure 4B) are common, with some amphibolite boudinslooking less migmatized than layers of amphibolites(Figure 4C). In contrast, in the northwestern domain,most of the TTG gneisses are sheared (Figure 4D), displayprominent stretching lineations (Figure 4E), and containmicrocline porphyroblasts. Southern-type and northern-type eclogites occur in the northwestern and southeasterndomains, respectively. However, the gabbro–amphibolitesonly occur in the northwestern domain.

The amphibolites occupy approximately half of thesouthern domain and occur as long deformed bodies(Figure 3A). Two large amphibolite bodies occur in thenorthern margin of this domain. These bodies are sepa-rated by a 20–30 m wide band of sheared and locallymicroclinized granitoids from the rocks in the northerndomain. The band of sheared granitoids truncates a layerof the intensely migmatized and folded TTG gneisses thatseparate the two amphibolite bodies. In addition, the bandis discordant relative to the internal structure of the wes-tern amphibolite body (Figure 3A). The ~15 m-wideamphibolite margins that are adjacent to the band arealso sheared, which suggests that the southern boundaryof the sheared granitoid band is a fault that separates twotectonically juxtaposed sheets that form the southeasternand northwestern domains of the study area. The shearedgranitoids and amphibolites are attributed to the CentralShear Zone (CSZ) (Figure 3A).

Figure 3(A) shows that the structural patterns of theCSZ differ from those of the main bodies of the two tectonicsheets. To confirm this observation, we analysed the orien-tation data. The structural patterns of the main bodies of thetectonic sheets are similar. The poles to the planar fabricsare scattered along great circles, and the π-axis plunges tothe SW 227°/59° in the main body of the northwesterntectonic sheet (Figure 3B) and SW 250°/50° in that of thesoutheastern tectonic sheet (Figure 3C). The average

orientations of the lineations, the hinge lines, and the boudinlong axes in these domains (SW 221°/48° and SW 246°/43°, respectively) are similar to those of the π-axes.

The structure of the CSZ (Figure 3D) is different and ischaracterized by the preferred orientation of the statisti-cally vertical planar fabrics which form an elongated max-imum. The centre of the maximum plunges to the NW325°/0° and reflects the general strike of the CSZ to theNE 55–60° (Figure 3A). However, the orientation of thelinear fabrics is similar to the orientation of the two tec-tonic sheets, with a slightly steeper plunge (SW 225°/62°)that appears to be connected with the vertical position ofthe shear planar fabrics.

The northwestern margin of the southeastern tectonicsheet is considered as a separate domain. The stereoplotconstructed for the domain (Figure 3E) demonstrates thesimilarity of the structure of this margin with the structureof the main body, and the discordance between the strikeof the rocks in this margin and in the sheared adjacentgranitoids (compare Figure 3(E) with Figures 3(C) and3(D), respectively). This similarity is demonstrated bythe average orientation of the linear fabrics in domains C(SW 246°/43°) and E (SW 246°/48°).

Background rock units of the Kuru-Vaara study area

TTG granitoids

The TTG gneisses form the matrix in which all the othertypes of rocks occur (Figure 3A). These gneisses are per-aluminous (A/CNK index = 1.1–1.27), have Al2O3 con-tents of 13.7 to 17.4 wt.% (Mg# = 0.40–0.65 and LaN/YbN > 10), and are classified as calc-alkalic magnesiangranitoids (Shchipansky et al. 2012b). The igneous precur-sors of the TTG gneisses were generated from the depletedmantle and are younger than ca. 3.0 Ga (Timmerman andDaly 1995; Mints et al. 2010; Morozova et al. 2012). Amagmatic zircon from a trondhjemitic gneiss yielded an ageof 2805 ± 11 Ma (U–Pb SIMS; Shchipansky et al. 2012b),which coincides with the ages of similar TTG granitoids inthe northern Belomorian Province (2.78 Ga, Mitrofanovet al. 1995; 2.81 Ga, Morozova et al. 2012). Metatexiteand diatexite are common in these rocks.

Amphibolites

Two types of amphibolites occur in the study area: (i)amphibolites, which are intensely foliated, boudinaged,migmatized, and contain relics of eclogites; and (ii) prac-tically non-migmatized and moderately foliated gabbro–amphibolites, which contain a few bodies of non-migmatized sanukitoid-type granitoids (Shchipanskyet al. 2012a). Metatexite and diatexite can be distinguishedin the amphibolites. The size of the amphibolite boudinsvaries from tens of centimetres to tens of metres. The

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Figure 3. (A) Geological map of the Kuru-Vaara study area; ceramic pegmatites not shown (compiled by V.V. Balagansky, I.A.Gorbunov, S.V. Mudruk, M.Yu. Sidorov and T.V. Kartushinskaya). (B–E) Stereoplots illustrating the orientations of linear and planarfabrics (equal area, lower hemisphere); the software ‘OpenStereo 0.1.2 devel’ by C.H. Grohmann and G.A.C. Campanha was used.

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distances between the solitary rounded boudins are usuallylarge (Figure 4C), suggesting that they experienced con-siderable displacement and are intensely rotated.

Al-rich gneisses

A few outcrops of Al-rich gneisses occur in the northern-most domain of the Kuru-Vaara quarry among the TTGrocks. A representative Al-rich gneiss sample is composedof garnet (pyrope, grossular, and spessartine molar parts upto 30%, up to 10%, and 2–3%, respectively), biotite(Ti content up to 0.16 apfu), kyanite, plagioclase (An35–45), and quartz. The mineral assemblage plagioclase +quartz + biotite + kyanite + garnet is overprinted by laterrims of cordierite (XMg = 0.77–0.80) and basic plagioclase(An55–90) around the garnet porphyroblasts and kyanitegrains. The rims were formed at 585–610°С and 5.0–5.5 kbar (multiequilibrium thermobarometry technique

TWEEQU; Berman 1991; Berman and Aranovich andBerman 1996; Aranovich 1996).

Gabbro–norite dike

A coronitic gabbro–norite dike truncates the stronglysheared TTG gneisses and the amphibolites in the CSZ(Figure 3A). The western part of the dike exhibits a smallstrain that only occurred in the narrow marginal zones(usually a few tens of centimetres thick). In contrast, theeastern part of the dike is thin (approximately 1 m), whichpotentially reflects its primary morphology or resultedfrom a much higher strain.

The thickest part of the dike preserves chilled margins insome places. Its internal domains consist of amphibolitizedmedium-grained gabbro–norites with locally preservedigneous minerals, such as orthopyroxene (XMg = 0.86),which contains inclusions of olivine (Fo83) with Cr-spinel

Figure 4. (A) Folded and intensely migmatized (metatexite) TTG gneiss (photo width approximately 1 m). (B) Intensely migmatized(diatexite) amphibolite (compass width 8.5 cm). (C) Boudinaged and migmatized (metatexite and diatexite) amphibolites in the TTGmatrix (photo width approximately 50 m). (D) Intensely sheared TTG gneisses (hammer 55 cm). (E) Intensely lineated TTG gneisses(Guest Editor Vinod Singh; height 168 cm).

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(Cr# = 66), clinopyroxene, central portions of which are richin tiny Fe-oxide inclusions, and brownish plagioclase. Theinternal domains contain up to 7% olivine. Biotite is a sub-ordinate mineral. Accessories include ilmenite, sulphides,zircon, and baddeleyite. Interstitial quartz-plagioclase gran-ophyre occurs in the central portion of the dike.

The igneous orthopyroxene is enveloped by theamphibole rim. The reaction coronas around the olivineexhibit a multizonal structure. The oldest, inner zone iscomposed of prismatic secondary orthopyroxene grains(XMg = 0.80), which have been completely replaced bythe olivine in some areas. The younger, intermediate zoneconsists of symplectites of garnet (Alm27Prp47Grs15) andamphibole (edenite). The youngest, outer zone is com-posed of edenite grains with spinel inclusions. The crystal-lization of similar multizonal coronas was experimentallydetermined to occur at P = 5 kbar and T = 670–700°C(Larikova and Zaraisky 2009).

Ceramic pegmatites

These coarse-grained to giant-grained rocks are composedof quartz, microcline, and plagioclase and have been

explored by the ceramic industry. The age of the mainpegmatite phase is 1.84 Ga (Skublov et al. 2011) andpostdates both high-grade and low-grade deformationalevents in Kuru-Vaara.

Deformational history of the Kuru-Vaara study area

The field observations and the results of the geometricalanalysis suggest that the TTG gneisses and eclogite-bearing amphibolites have a common deformational his-tory that consists of two major deformational periods. Acrucial component of distinguishing between the defor-mations of the first and second periods is the gabbro–norite dike that postdates the older deformations andpredates the younger deformations.

Deformations of the first period

These deformations occurred under amphibolite faciesconditions and resulted in folding and the formation offoliation and stromatic migmatites in all of the rocks andthe strong boudinage of the amphibolites. This periodconsists of at least three episodes (Table 1). The first

Table 1. Main events in the geological history of the Kuru-Vaara study area.

Events Age

Ceramic pegmatites 1.84 Gaa

● Deformational period 2. Local foliation and folding, microcline-bearing leucosomes and granites;latest event of amphibolite facies metamorphism at T = 585 ± 5°С and P = 5.3 ± 0.1 kbar

ca. 1.9 Gab

Dike of the coronitic olivine-bearing gabbro–norites >2.15 Gad (2.42 Ga?c)

● Deformational period 1. Episode 3. Shearing (mylonites, lineation, sheath folds); amphibolite faciesmetamorphism; juxtaposition of tectonic sheets containing the southern and northern types of eclogites;the development of the Central Shear Zone; change of relics of eclogites into amphibolites

Episode 2. Deformation of intensely boudinaged amphibolites into open and tight isoclinal folds;amphibolite facies metamorphism, migmatization; change of relics of eclogites into amphibolites

ca. 2.7–2.55 Gac,e

Episode 1. Foliation; complex folding; migmatization; boudinage (change of the overwhelming majority ofeclogites into amphibolites); net of trondhjemitic veins survived only within relics of eclogite; obliteration ofclinopyroxene–plagioclase symplectites

Formation of clinopyroxene–plagioclase symplectites, complete metamorphic assemblage isclinopyroxene + orthopyroxene + hornblende + plagioclase + quartz (transitional area betweenamphibolite facies and granulite facies conditions)

Net of trondhjemitic veins within eclogites f

Formation of northern-type eclogites (eclogite facies metamorphism). 2.72 Gae

Magmatic protoliths of eclogites and trondhjemitic lithology of TTG gneisses; southern-type eclogites(earliest episode of eclogite facies metamorphism)

(>2.77)–2.82 Gac–e

Notes: aSkublov et al. (2011).bShchipansky et al. (2012b), and re-interpreted ages from Skublov et al. (2011), and Li et al. (2013).cLi et al. (2013).dThis study.eShchipansky et al. (2012b).fShchipansky et al. (2012a).

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deformational episode resulted in intense boudinage, fold-ing, and migmatization. One important feature of thisperiod is that the lineations, hinge lines, and boudinswere rotated into parallelism. These linear fabrics plungeto the SW at an angle of approximately 45° (see above).During the second deformational episode, the intenselyboudinaged amphibolites were deformed into open andtight isoclinal folds. The orientation of the hinge lines ofthe later folds is similar to those of the older folds.

The third deformational episode resulted in the forma-tion of the CSZ and an extensive shearing in the north-western tectonic sheet and in the adjacent narrow marginalzone of the southeastern tectonic sheet. The shearingoccurred under amphibolite facies conditions and resultedin the high strain in the TTG gneisses and amphibolitesand the prominent lineations in many places (Figure4D–E). A characteristic feature of this shearing is thesubvertical component in the shear direction, whichresulted in subvertical foliations of several sheath foldswith subvertical hinge lines and a steeper attitude of theolder linear fabrics in the CSZ.

The deformation and metamorphism that occurred dur-ing this period resulted in the nearly complete obliterationof the eclogite facies assemblages and the later clinopyr-oxene–plagioclase symplectites.

Deformations of the second period

The second deformational period includes the deforma-tions that affected the gabbro–norite dike. These deforma-tions produced the local foliations along the contacts of thedike, which were combined with the dike’s amphiboliza-tion. These foliated margins have linear fabrics that aresubparallel to the lineations of the first period. Thestrongly sheared TTG gneisses are bent into the sparseopen folds with the axial plane leucosomes of the micro-cline granite. These folds, and a few the latest shear zones,were potentially formed during the second period.

Eclogites

The predominant morphologies of the eclogite remnantsinclude lenses and boudins with sizes of a few metres(Figure 5A) to approximately 10–15 m (Figure 5B).These remnants have been found within amphibolite bou-dins and lenses, whose thicknesses vary from a few metresto one hundred metres.

Southern-type and northern-type eclogites were estab-lished in the study area (Shchipansky et al. 2012a, 2012b).The present study confirms this subdivision. The southern-type eclogites are coarse grained as observed in samples(Figure 6A) and thin sections (Figure 7A), whereas thenorthern-type eclogites are relatively fine grained (grainsize < 1 mm), which is also observed in samples(Figure 6B) and thin sections (Figure 7B). The southern-

type eclogites are widespread throughout the southeasterndomain of the study area, whereas the remnants of thenorthern-type eclogites are rare and only occur in the north-western domain of the study area. The southern-type eclo-gites usually exhibit a strong retrogression in which theomphacite was generally replaced by clinopyroxene–plagi-oclase symplectites (Figure 7C). In contrast, the northern-type eclogites display a lower degree of retrogression, andthe well-preserved omphacite contains quartz exsolutionlamellae (Figure 7D). Quartz lamellae have been observedin the Na–clinopyroxene from both types of eclogites(Konilov et al. 2011; Shchipansky et al. 2012a).

In the southern-type eclogites, the metamorphic peakis characterized by the garnet + clinopyroxene + quartz ±rutile ± kyanite assemblage. The garnet forms large euhe-dral and semi-euhedral porphyroblasts that are enriched inthe pyrope minal (up to 58 mol.%) and has distinct pro-grade zonation in the less retrogressed eclogites. The clin-opyroxene is commonly omphacite, with the Jadeitecontent of 26–31 mol.%.

In the northern-type eclogites, the peak mineral assem-blage is kyanite free and is interpreted as garnet + clin-opyroxene + hornblende ± orthopyroxene ± rutile. In thenorthern-type eclogites that have the Mg-number identicalto that in the southern-type eclogites, the garnet con-tains ≤ 42 mol.% pyrope, has higher grossular and loweralmandine contents toward the grain edges, and displaysno distinct zonation in the pyrope content.

The clinopyroxene is commonly omphacite, with ajadeite content of 20–25 mol.%. Orthopyroxene comprisesscarce fine inclusions in garnet and small rounded grainswithin the clinopyroxene–plagioclase symplectites. Thehornblende at textural equilibrium with clinopyroxene isedenite, with a composition that is similar to that of a lateramphibole from the coronas around garnet.

The minimum P–T values for the metamorphism of theeclogite facies are 750–775°C and 14.0–14.3 kbar(Konilov et al. 2011). The peak metamorphism of theeclogite facies is suggested to have occurred atP ≥ 20 kbar and T = 750–780°C for the southern-typeeclogites and P ≥ 20 kbar and T = 680–720°C for thenorthern eclogite (Shchipansky et al. 2012a). A recentdiscovery of majoritic garnet indicates that the P–T peakpotentially occurred in the ultrahigh-pressure field, whichwas confirmed by the P–T pseudosection calculations(ThermoCalc software) for a kyanite-free eclogite(P ≥ 27 kbar and T < 900°C; Liu and Zhang 2014).Thus, these eclogites can correspond to at least the AEmetamorphism according to Brown (2007).

The decompression of the eclogites was accompaniedby increasing temperatures when the P–T path passedthrough the granulite facies field (P ≈ 10–14 kbar;Shchipansky et al. 2012a; Liu and Zhang 2014). Thesubsequent cooling and decompression passed into theamphibolite facies field at P < 10 kbar and T = 690–

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640°C, during which the strong amphibolization and mig-matization of the eclogites occurred (Shchipansky et al.2012a).

Kyanite- and corundum-bearing pegmatoids

The southern-type retrogressed eclogites host specific kya-nite- and corundum-bearing pegmatoid veins that are up to2.5 m long and 30 cm thick. One vein is located at theboundary between an amphibolized eclogite and a migma-tized, sheared TTG gneiss and truncates a shear foliationin the amphibolized eclogite. The veins are massive andexhibit no deformation. The kyanite crystals vary in sizefrom a few tens of microns to a few centimetres. Thedescription below concerns only the veins with kyaniteand corundum grains at the tens of microns scale.

The major mineral in these veins is bull quartz(55–70%), which forms anhedral grains of up to 15 mmacross and forms the matrix for specific mineral aggre-gates. These aggregates are morphologically similar tolarge mica flakes (Figure 8A) and consist of symplectites

of biotite (5–7%), orthoclase (7–10%), and corundum(1–2%) (Figure 8B). The symplectitic flakes of biotiteare often subparallel to the longer axis of the hostingmineral aggregates. Corundum forms needles that areoriented parallel to the biotite flakes (Figure 8B). Theseneedles are separated from the quartz matrix by rims, andno contact has been observed between the corundum andquartz. At the margins of the veins, the rims consist ofplagioclase (An25–40, 5–7%) and kyanite (3–4%).However, in the central domains, the rims are composedof orthoclase. In addition, garnet (3–5%), hornblende(3–4%), clinopyroxene (3–4%), epidote (replacing plagio-clase) (1–2%), and rare grains of kyanite occur in themarginal domains. The accessories are represented byrutile (containing ilmenite relics), tourmaline, andchalcopyrite.

The same rocks locally contain dumortierite and phen-gite (Dokukina 2013). Phengite occurs as a relic inside thesymplectitic aggregates, indicating that the symplectiticaggregates are pseudomorphs of large phengite flakes.Both of these minerals are characteristic of HP and UHP

Figure 5. (A) Intensely migmatized amphibolites exhibiting a pinch-and-swell structure; the central part of the swell is composed ofmassive or weakly banded retrogressed (symplectitic) eclogites; contacts between these rocks are gradational. (B) Intensely deformedamphibolite layer containing a large relic of massive retrogressed eclogite (photo width approximately 70 m).

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rocks (Carswell and Zhang 2000; Ferrando et al. 2009; ElKorh et al. 2011; Janák et al. 2012) including abyssalpegmatites (Cempirek and Novák 2007).

Relationships between the eclogites and amphibolites

The most important feature of the former eclogites is thatthe contacts between the retrogressed eclogites in thecentre of the remnants and their host, the slightly foliatedamphibolites with relics of garnet and clinopyroxene–plagioclase symplectites (coarse and fine), are grada-tional. The slightly foliated amphibolites, in turn, displaygradational contacts with the enveloping foliated andmigmatized amphibolites. We will discuss one key out-crop that illustrates these relationships.

In this exposure, a lens occurs in the highly foliatedand migmatized amphibolites (Figure 5A). The inner por-tion of this lens is massive, but it locally displays a weakbanding that is formed by bands that are enriched in quartzor hornblende (Figure 6C). Rare quartz veinlets are paral-lel to the banding. The lens consists of garnet, coarseclinopyroxene–plagioclase symplectites (Figure 7E), and

skeletal grains of brownish hornblende filled with plagio-clase that are similar to symplectites (Figure 7F). Nometamorphic rims have been observed at the contactsbetween the garnet and brownish hornblende or betweenthe clinopyroxene and hornblende. However, the garnet isseparated from the clinopyroxene by plagioclase rims.Thus, this rock is identified as an eclogite that is retro-gressed under high-grade conditions.

The amount and size of the brownish hornblendegrains increase towards the margins of the lens, whilethe proportion of skeletal grains decreases. The non-ske-letal hornblende grains display a very weak foliation, butthe rock has preserved its massive appearance (Figure 6D).The replacement of clinopyroxene by brownish hornble-nde is common. Despite this recrystallization, the rock isunambiguously interpreted as a retrogressed eclogite.

Closer to the margin, brownish hornblende is replacedby green hornblende. The amount and the size of the greenhornblende grains increase markedly; the skeletal formsare nearly absent and the prismatic grains display a pre-ferred orientation (Figure 7G) that is observed in expo-sures as a weak foliation and is parallel to the margins of

Figure 6. (A–B) Well-preserved eclogites of (A) southern type and (B) northern type. (C–F) Eclogites retrogressed to a different extentshowing gradational contacts between them. See the text for other explanation.

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the lens (Figure 5A). The proportion of clinopyroxene–plagioclase symplectites decreases (clinopyroxene is lar-gely replaced by green hornblende), and the symplectitesoccur as rare relics. Large grains of the green hornblendesurround the garnet grains and are separated from the

garnet by plagioclase, whereas small grains occur aroundand within the garnet crystals and have partly replacedthem (Figure 7H). The proportion of garnet decreases.This variety of retrogressed eclogite represents a weaklyfoliated garnet amphibolite, which contains fine

Figure 8. Photomicrographs of corundum-bearing pegmatoid veins: (A) internal structure of flake-like symplectitic aggregates and (B)corundum-orthoclase-biotite symplectites. All minerals were determined by the X-ray refraction technique in the Geological Institute ofthe Kola Science Centre RAS (Apatity).

Figure 7. Photomicrographs of eclogites showing a different extent of retrogression (A–H, Kuru-Vaara; I, Shirokaya Salma ShearZone). Hereafter, mineral abbreviations are given after Kretz (1983). See the text for other explanation.

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clinopyroxene grains. However, the gradational contactswith the retrogressed eclogites suggest that the clinopyr-oxene-bearing garnet amphibolite is also a retrogressedeclogite.

Further from the margin of the massive lens that arecomposed of retrogressed eclogite, weakly foliated clino-pyroxene-bearing garnet amphibolites are intruded bynumerous veins of quartz diorite (SiO2 = 63.68 wt.%).These rocks can be classified as stromatic migmatitesand are shown in the left lower part of Figure 5(A). Themigmatized amphibolites have gradational contacts withthe non-migmatized varieties in which relics of the clin-opyroxene–plagioclase symplectites occur. The migma-tized amphibolites are truncated by veins of youngerleucogranite (SiO2 = 74.93%), which resulted in a newgeneration of stromatic migmatites (Figure 5A). Thin

leucogranite veins and their amphibolite hosts are stronglysheared, whereas the thick ones locally exhibit a massiveappearance. The amphibolite (mesosome) consists ofgreen hornblende (50–55 wt.%), biotite, plagioclase,minor microcline, and sparse relics of clinopyroxene.The only evidence that this rock developed from the retro-gressed eclogites is the gradational contact.

The geochemistry of the eclogites displaying a differentextent of their change into amphibolites

Geochemical data on the eclogites and their reworkedvarieties are given in Table 2. Samples of both types ofthe well-preserved eclogites correspond to primitive (Mg#[Mg2+/(Mg2+ + Fe2+)] = 0.6–0.7) olivine-normativelow-K and high-Mg tholeiites (the average MgO content

Table 2. Major, trace, and rare earth element data for the Kuru-Vaara well-preserved eclogites and their amphibolized varieties.

Oxide andelement

Southern eclogite Northern eclogite Northern eclogites amphibolized

Average(n = 9) Std

Average(n = 4) Std

SlightlyKV-09/12

ModeratelyKV-08/12

CompletelyKV-07/12

Major elements (wt.%; XRF)SiO2 47.74 1.57 47.90 1.78 49.43 49.53 52.22TiO2 0.58 0.13 0.65 0.39 1.07 1.06 0.78Al2O3 14.96 2.19 12.82 2.28 16.59 16.71 15.60Fe2O3 (total) 11.44 1.24 12.25 0.50 13.36 13.33 8.50MnO 0.21 0.08 0.22 0.03 0.16 0.16 0.12MgO 11.09 1.80 11.27 3.06 8.16 7.73 7.77CaO 11.45 0.73 12.81 2.70 9.66 9.68 8.65Na2O 1.86 0.48 1.52 0.78 2.18 2.40 2.84K2O 0.24 0.23 0.29 0.19 0.27 0.27 2.45P2O5 0.03 0.01 0.06 0.03 0.06 0.04 0.49

Trace and rare earth elements (ppm; ICP–MS)Th 0.16 0.08 0.30 16 0.31 1.2 3.5Nb 2.8 2.2 2.3 0.9 3.1 3.7 6.9Sr 83 37 63 3 68.7 59 499Zr 29 7 37 20 45.6 62.6 93Hf 1.02 0.34 1.14 0.52 1.5 1.9 2.4Y 18 9 15 6 23.8 26.2 20.6La 1.36 0.54 1.63 1.45 2.2 5.3 68.2Ce 4.21 2.11 4.64 3.93 7.6 12.5 156.5Pr 0.74 0.41 0.77 0.58 1.2 1.8 19.7Nd 3.77 1.91 3.96 2.70 6.1 8.3 80.0Sm 1.44 0.79 1.44 0.79 2.3 2.6 12.2Eu 0.49 0.18 0.50 0.25 0.88 0.83 2.7Gd 2.27 1.53 1.97 0.79 3.7 4.1 7.2Tb 0.47 0.27 0.38 0.15 0.67 0.74 1.0Dy 3.11 1.90 2.49 0.78 4.3 4.7 4.5Ho 0.69 0.36 0.57 0.21 0.95 1.0 0.81Er 2.03 1.03 1.66 0.54 2.8 3.0 2.2Tm 0.29 0.14 0.26 0.08 0.41 0.30 0.16Yb 1.89 0.91 1.68 0.54 2.7 2.8 1.9Lu 0.29 0.12 0.27 0.28 0.41 0.45 0.29

Notes: Std, standard deviation.XRF, X-ray fluorescence method.ppm, parts per million.ICP–MS, inductively coupled plasma mass-spectrometry method.

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is 11 wt.%). It suggests that the primary mantle-derivedmelts did not experience substantial fractional crystalliza-tion. Regarding their composition, the eclogites are similarto the melts derived from the primitive mantle rather thanthe melts produced from the partial melting of the moderndepleted upper mantle (Figure 9A). Both types of eclogitesshowed pronounced and positive Nb anomalies(Figure 9A), indicative of melts that were not contaminatedby crustal materials or subduction inputs (Jochum et al.1991). In this case, the precursor for the Kuru-Vaara eclo-gite was likely the subducted Archaean oceanic crust.Petrological modelling of the eclogites (Shchipansky2012) revealed that the eclogitic protoliths are composition-ally consistent with the experimental and parameterized

primary melts from the late Archaean oceanic crust, whichare likely derived by partial melting of the ambient perido-titic mantle source hotter than the modern ambient mantleby 150–250 K (Herzberg et al. 2010).

The geochemical affinity of the mid-ocean ridge basalt(MORB) is only exhibited in the well-preserved eclogites.Our study demonstrates that the prolonged retrogression ofthe eclogites, which is accompanied by amphibolization,changed the MORB signature to a trace and rare earthelement pattern that was similar to the island arc signature.Massive, slightly amphibolized, symplectitic eclogite KV-09/12 that forms a lens within the amphibolites (Figure5A; the eclogite is shown in Figure 6(C)) and consists ofgarnet and coarse clinopyroxene–plagioclase symplectites(Figure 7E) has a MORB signature (Figure 9B).Enveloping and nearly massive amphibolite KV-08/12,with relics of the symplectites and garnet (the amphiboliteis shown in Figure 6(A), and its texture in Figure 7(G–H)),exhibits a markedly different geochemical signature(Figure 9B). Furthermore, the garnet-free foliated andmigmatized amphibolite KV-07/12 surrounded by amphi-bolite KV-08/12 exhibits an island arc-like signature(Figure 9B). These findings clearly demonstrate the retro-gression of the eclogites proceeded in an open system.Thus, the Kuru-Vaara eclogites have been re-equilibratedby fluid-induced element transfer under retrograde P–Tconditions at the crustal level. It indicates that dating byusing a mineral isochron to constrain the age of eclogiticmetamorphism is ineffective.

U-Pb geochronology

We dated sample E-Sa11/19 from the olivine-bearinggabbro–norite dike which truncates the TTG gneissesand amphibolites that were jointly deformed during thefirst deformational period. The yield of zircon from thissample was very small; however, two types of zirconwere recognized. The first type includes small (up to100 µm) grains. Several of these grains have baddeleyitecores (Figure 10A–C), and one of them was observed ina thin section. It was used to exclude probable contam-ination during the separation. The oldest U–Pb SIMS agefrom the baddeleyite cores was 2147 ± 28 Ma (Table 3;Figure 11). The second zircon type includes large (up to250 µm) grains that consist of oscillatory cores andunzoned rims (Figure 10D). These rims, together withthe zircons rims around the baddeleyite cores, yielded abadly constrained isochron age of 1881 ± 60 Ma(Figure 11). The 207Pb/206Pb ages of the older coresvaried from 2.65 to 2.74 Ga (Table 3).

The Shirokaya Salma Shear Zone

The SSSZ (Figure 12) is located a few hundred metres tothe south of the Shirokaya Salma eclogite and

Figure 9. (A) Primitive mantle-normalized (Hofmann 1988)trace element averaged abundance patterns of southern-type(n = 9) and northern-type eclogites (n = 4) from the Kuru-Vaarastudy area; modern N-MORB abundances are shown for compar-ison. (B) Primitive mantle-normalized trace element abundancepatterns of southern-type eclogite shown in Figure 5(A) suggestthe generation of apparent arc signatures for the oceanic MORBprotolith during protracted retrogression from the eclogite faciesto amphibolite facies. Inset is enlarged abundance patterns fortrace elements used for mineral chronometry, which indicate theirsignificant redistribution and thus the behaviour of isotopes inmineral phases within a rock. Primitive mantle and N-MORB aregiven after Hofmann (1988).

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approximately 4 km to the north of the Uzkaya Salmaeclogite (Mints et al. 2010; Konilov et al. 2011). Thiszone exposes TTG gneisses and subordinate amphibolitesthat are identical to those in the Kuru-Vaara study area.These rocks are migmatized and sheared in amphibolitefacies. However, the structural patterns formed by theserocks in the SSSZ significantly differed from those inKuru-Vaara. Thick layers of banded amphibolites thathave experienced much higher shear strain during thelatest deformation have a consistent W–E strike and dis-played gentle pinch-and-swell structures (Figure 12).Neither relic eclogites nor clinopyroxene symplectiteshave been found in these banded amphibolites. Thinamphibolite layers have been boudinaged, and the boudinshave been displaced by long distances from each other.This distribution is similar to the distribution of the amphi-bolite boudins in Kuru-Vaara. The small rounded boudins(a few decimeters to a few metres across) are very elon-gated and display a statistically horizontal orientation thatis identical to that of the very prominent aggregate andmineral lineations, the x-axes of the isoclinal sheath folds

Figure 10. Back-scattered electron and cathodoluminescenceimages of zircons with baddeleyite and zircon cores fromcoronitic olivine-bearing gabbro–norite E-Sa-11/19; SHRIMPspot location labelled with approximate spot concordia age(A–B) and 207Pb/206Pb age (C–D).

Table

3.U-Th-PbSHRIM

PIIdata

onzircon

andbadd

eleyite

(b)from

Kuru-Vaara

gabb

ro–n

orite

E-Sa-11/19.

Spo

tU,pp

mTh,

ppm

232Th/

238U

206Pb*

,pp

ma

206Pb c

%a,b

207Pb*

/206Pb*

±%c

207Pb*

/235U

±%c

206Pb*

/238U

±%c

Err.corr.d

Dsc,%

e207Pb/

206Pbage,

Maf

2.1

152

220.15

68.9

0.13

0.19

080.59

13.84

1.6

0.52

621.5

0.93

41

2749

±9.7

4.1

230

960.43

97.2

0.08

0.17

914

0.5

12.13

1.5

0.49

111.5

0.94

73

2645

±8.3

4.2

745

0.07

20.4

0.15

0.1162

1.7

5.09

2.4

0.31

81.6

0.68

47

1898

±31

5.1

297

237

0.82

133

0.05

0.18

674

0.42

13.41

1.7

0.52

091.6

0.96

90

2714

±6.9

6.1

213

108

0.52

70.8

0.65

0.1163

1.2

6.15

20.38

41.5

0.77

3−9

1899

±22

7.1

157

117

0.77

52.5

0.34

0.12

071.1

6.45

1.9

0.38

781.6

0.82

7−7

1966

±19

8.1

345

930.28

111

0.54

0.1118

0.97

5.71

1.8

0.37

041.5

0.83

9−10

1828

±18

9.1

8918

32.14

27.1

0.72

0.1102

1.8

5.37

2.4

0.35

351.6

0.67

0−8

1802

±33

10.1

95114

1.24

31.9

0.78

0.116

1.8

6.19

2.4

0.38

691.6

0.67

1−10

1895

±32

11.1

122

130

1.10

38.3

0.80

0.1123

1.6

5.59

2.2

0.36

121.6

0.69

7−8

1837

±29

12.1

408

138

0.35

131

0.13

0.1145

40.66

5.90

91.6

0.37

421.5

0.91

2−9

1873

±12

13.1b

153

340.23

52.3

0.32

0.13

350.92

7.3

1.7

0.39

691.5

0.84

90

2144

±16

15.1

164

800.50

46.8

0.45

0.1117

1.3

5.09

2.3

0.33

031.8

0.81

7−1

1827

±24

Notes:a Pb c

andPb*

indicate

thecommon

andradiog

enic

portions,respectiv

ely.

bCom

mon

Pbwas

correctedusingmeasured

204Pb.

c Errorsare1σ

.dErr.corr.,errors’correlation.

e Dsc,discordance.

f Errorsare2σ

.Error

instandard

calib

ratio

nwas

0.54%

(not

included

inaboveerrors

butisrequired

whencomparing

data

from

differentmounts).

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(apical angles of 5–20°), and the hinge lines of asymme-trical tight to isoclinal folds (stereoplot in Figure 12).

However, the long axes of the large rounded boudins(up to 20–30 m across) are inclined in some places to

subvertical positions (stereoplot in Figure 12). The centralpart of one of these boudins exhibits its own inner struc-ture, a weak banding that is oriented normal to the contactof this boudin with the enveloping, strongly sheared TTGgneisses (arrow in Figure 12). The banding is composed ofrough chains of elongated grains and aggregates of horn-blende occurring in the garnet–clinopyroxene–plagioclasematrix and with clinopyroxene and plagioclase exhibitingsymplectitic intergrowths (Figure 7I). Thus, the weaklybanded amphibolite is a retrogressed eclogite that is simi-lar to the weakly banded retrogressed eclogite that makesup a lens in the Kuru-Vaara study area (Figure 5A). Thebanding postdates the considered eclogite facies meta-morphism, but predates the development of the higheststrain in the SSSZ.

Finally, two deformational periods can be establishedin the Shirokaya Salma locality. The second deformationalperiod resulted in the nearly complete obliteration of theolder structural forms or their rotation into parallelismwith the horizontal WE-trending shear direction. Theonly older structures that are composed of strongly retro-gressed eclogites are the boudins and were steeply inclinedbefore shearing. The orientation of these boudins is similarto that in the Kuru-Vaara study area and suggests that the

Figure 11. Concordia diagram for zircons and baddeleyite fromthe olivine-bearing gabbro–norite dike. Data-point error ellipsesare 2σ; age error is 2σ; decay constant errors are included;however, the Concordia is represented by a curve of the constantthickness.

Figure 12. Geological map of the Shirokaya Salma study area (after Kozlova in Balagansky and Kozlova 1992). Stereoplot is an equal-area projection on the lower hemisphere.

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boudinage occurred before shearing and that the steeplyplunging boudins belong to the structural forms of the firstdeformational period.

Discussion

Observational and isotope constraints

The presented data allow us to conclude the development ofthe eclogites, whose remnants occur in amphibolites pre-ceded by all of the deformational and metamorphic events,including migmatization, which was recorded in the amphi-bolites and their TTG hosts in the Kuru-Vaara andShirokaya Salma study areas. A comparison of the defor-mational and metamorphic histories of these areas revealstheir similarities. The oldest metamorphic events occurredunder the eclogite facies conditions, and the followingdeformation and metamorphism resulted in the transforma-tion of the eclogites into amphibolites. Next, the amphibo-lites and their TTG host experienced two periods ofdeformational and metamorphic reworking. However, theKuru-Vaara and Shirokaya Salma study areas have distinctstructural patterns. The former study area shows littlereworking during the second period, whereas the latter ischaracterized by almost complete obliteration of the olderstructures during the shearing of the second period. The firstand second periods are characterized by different kinematicplans. These periods are separated by the emplacement ofthe coronitic olivine-bearing gabbro–norite.

The geological history of the northern BelomorianProvince can generally be subdivided into two deforma-tional periods that correspond to the Archaean andPalaeoproterozoic periods. These deformational periodsare separated by the emplacement of the numerous coro-nitic mafic rocks that are attributed to the lherzolite–gab-bro–norite complex (Stepanova and Stepanov 2010) datedat 2.39–2.44 Ga (Bibikova et al. 2004; Krivolutskayaet al. 2010; Slabunov et al. 2011). The long axes of theboudins, hinge lines, and lineations formed during theArchaean period are characterized by their moderately tosteeply inclined attitudes in the domains that experiencedlittle or no Palaeoproterozoic deformation (Balagansky2002). This period is constrained by ages of ca. 2.85–2.6 Ga (Mitrofanov et al. 1995; Bibikova et al. 2004;Glebovitsky 2005; Slabunov et al. 2006). In contrast, thePalaeoproterozoic Lapland–Kola collisional orogenyresulted in a set of discrete shear zones that were charac-terized by the statistically horizontal positions of the linearfabrics in the Archaean rocks in the Belomorian Provinceand the Palaeoproterozoic rocks in the orogenic core(Balagansky 2002; Daly et al. 2006). The moderately tosteeply inclined Archaean linear fabricsrotates towardsparallelism with the horizontal shear direction in thePalaeoproterozoic shear zones in the amphibolite facies(Balagansky 2002). The formation of the horizontal

stretching lineations was related to the major collisionalevents of the Lapland–Kola Orogeny that were con-strained by ages of 1906–1925 Ma in the Russian portionof the Lapland–Kola Orogen (Bibikova et al. 1993; Dalyet al. 2006; Balagansky and Mudruk 2013).

A comparison of the common geological histories ofthe Kuru-Vaara and Shirokaya Salma study areas and theBelomorian Province shows that these histories are simi-lar. This similarity is noted by the dike of the coroniticolivine-bearing gabbro–norite, truncating the structures ofthe first deformational period. According to its rock miner-alogy and geochemistry, this dike is practically identical tothe aforementioned ca. 2.4 Ga lherzolite–gabbro–noritecomplex and plays a crucial role in constraining the timeof the eclogitic metamorphism that is recorded in the reliceclogites in the amphibolites.

The oldest concordant U–Pb baddeleyite age from thegabbro–norites is 2147 ± 28 Ma. Baddeleyite is an idealmineral for determining the crystallization age of maficand ultramafic intrusions (Heaman and LeCheminant1993). However, the analysed area of our sample waswider than the baddeleyite core and included the youngerzircon host (Figure 10A–B), and the baddeleyite age of2147 ± 28 Ma should be interpreted as the minimum ageof the magmatic crystallization of the gabbro–norites.Thus, the age of the eclogitic metamorphism must beolder. If the gabbro–norites are related to the lherzolite–gabbro–norite complex, then the eclogitic metamorphismshould have occurred earlier at ca. 2.4 Ga.

The Archaean zircon cores have morphologies andinternal structures that are typical for granitoids (Corfuet al. 2003) and are interpreted as xenocrysts that areentrained from the TTG host. The age of 1881 ± 60 Mafrom the zircon rims is poorly constrained and hardlycorresponds to any thermal event.

Thus, both observational and isotope age data haveconfirmed the Archaean age of the eclogite facies meta-morphism that was recorded in the remnants of the eclogitesin the amphibolites of the northern Belomorian Province(late Mesoarchaean to early Neoarchaean; Mints et al.2010; Konilov et al. 2011; Shchipansky et al. 2012a).

Interpretation of ca. 1.9 Ga zircon ages

Trace element distributions between zircon and neighbour-ing silicate minerals are important for understanding theorigin of zircon (Harley and Kelly 2007). Thus, we ana-lysed the distributions of the REE contents of the zirconsand garnet in the former eclogites from the Kuru-Vaarastudy area. Based on the MREE–HREE data from thegarnets and the ‘eclogitic’ low Th/U zircons (Skublovet al. 2011), we calculated the apparent trace elementdistribution coefficients (DREE Zrn/Grt) and comparedthem with the experimentally derived distribution valuesthat were inferred to be in the equilibrium (Rubatto and

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Hermann 2007). The DREE (Zrn/Grt) values calculated forthe ca. 1.9 Ga zircons that were coupled with core garnetsslightly decreased from the MREEs to the HREEs(Figure 13A). This decrease is highly inconsistent with aremarkable increase in the MREE–HREE DREE values inthe zircon–garnet assemblage that was equilibrated at theappropriate T ≈ 750–800°C, which represent the tempera-ture values inferred for the peak metamorphism of theeclogite facies in the Kuru-Vaara eclogites. Hence, theca. 1.9 Ga old zircons cannot be correctly ascribed tothe consumption of garnet. Therefore, these zircons cannotbe assigned to the metamorphism of the eclogite facies.

In contrast, the distributions of MREEs and REEsbetween the zircons and garnet rims significantly vary,with some exhibiting a flat pattern with very low DREE

(Zrn/Grt) values (Figure 13B). This strong discordance inthe distribution value patterns suggests that the 1.9 Gazircons should have crystallized after the uplift of theeclogites to a higher crustal level, where the eclogiticgarnet was apparently modified during the growth of thereaction rims. A similar flat distribution was reported byHarley and Kelly (2007), who argued that the low Th/Uzircon was ‘blind’ to the high-grade metamorphism andconcluded that the zircon was influenced by a later post-peak fluid infiltration event.

Li et al. (2013) reported an age interval of 1920–1860 Ma for the Salma zircons having an extreme low U(a few tens of parts per million) and Th concentrations(<1 ppm), and the very low ∑REE values (~10 ppm) wereformed at temperatures of approximately 620°C. Liu andZhang (2014) published U–Pb zircon ages of 1884 ± 9,1815 ± 5, 1809 ± 12, and 1767 ± 10 Ma from the Kuru-Vaara study area. Moreover, in the Belomorian Province,the Palaeoproterozoic U–Pb ages of titanites are a few tensof millions years older than those of rutiles, and the agestend to become younger southwestwards and display aregional lateral distribution (Bibikova et al. 2001). It indi-cates that slow cooling occurred after Palaeoproterozoichigh-grade metamorphism, which is the characteristic ofmany deeply eroded Precambrian orogens (Willigers et al.2002). Although the ages of the oldest low Th/U zirconsoccurred close to the time of the Lapland–Kola collision,the low Th/U 1.9–1.8 Ga zircons reflected a zirconresponse to the regional fluid infiltration in the eclogitesthat occurred during the slow exhumation after theLapland–Kola orogeny rather than recording any meta-morphic event. This fluid infiltration was also indicatedby the Palaeoproterozoic pegmatites, which are wide-spread across the entire Belomorian Province, wereformed at ca. 1880‒1840 Ga (Bibikova et al. 2004), andare not linked with any granites. This process could bepartially responsible for the significant disturbance of thetrace and rare earth element patterns of the eclogites dur-ing their retrogression (see above).

Tectonic implications

Many geologists suggest that modern-style tectonic pro-cesses were operational on early Earth. These discus-sions mainly considered when and how the pre-platetectonics were changed by tectonic mechanisms basedcompletely or partly on the modern-style tectonics. Thetransition to plate tectonics that operated at a globalscale, involved subduction, and was near to theArchaean–Proterozoic transition (ca. 2.5–2.7 Ga), ispotentially the most acceptable (e.g. Condie and Kröner2008). Nevertheless, Hynes (2014) noted that serioustheoretical reasons for precluding the operation of platetectonics in Archaean times are lacking. Furthermore,Korenaga (2013) noted that geological records and the-oretical considerations permit the initiation of plate tec-tonics, even in the Hadean. In contrast, Stern (2005) callsattention to the lack of complete ophiolites, blueschists,and ultra-high pressure eclogites in orogenic belts thatare older than ca. 1.0 Ga and attributes the onset of theplate tectonics to this age. A common statement is thatthe modern-style tectonics began in some crustal blocksand progressively propagated into others from the

Figure 13. Diagrams showing (A) zircon/garnet (core) and (B)zircon/garnet (rim) mid- to heavy REE partition coefficientsestimated for low Th/U ca. 1.9 Ga zircons in a retrogressedeclogite (data after Rubatto and Hermann 2003, 2007; Harleyand Kelly 2007; Skublov et al. 2011).

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Eoarchaean to the Neoarchaean (Condie and Kröner2008) and potentially passed though some intermediatestage (Cagnard et al. 2011).

A principal feature of the modern plate tectonics isdeep subduction, which is responsible for the formationof ultra-high-pressure metamorphic rocks. Based on thegeological records and numeric modelling, Sizova et al.(2014) concluded that even Palaeoproterozoic orogenieswere associated with shallow slab breakoff, which madethe development of ultra-high-pressure metamorphicrocks impossible. However, van Hunen and van denBerg (2008) and Moyen and van Hunen (2012) per-formed numerical modelling, analysed Archaeandomains, and concluded that short-term subductioncould have immersed the Archaean oceanic crust backinto the mantle, at least as deep as 200 km (i.e. at depthswhere ultra-high-pressure metamorphism can occur).These different results of the well-constrained numericmodelling clearly demonstrate the imperfections in ourknowledge regarding the fundamental principles of platetectonics in the early Precambrian. It is likely that furtheradvance in this field of geology will occur by directlystudying the features of the geological records ratherthan by theoretical modelling (Hynes 2014). The eclo-gite-bearing Belomorian Province provides a uniquepiece of this record and significant input to our under-standing of early Precambrian tectonics.

First, apart from the Archaean eclogitic metamorph-ism, a local Palaeoproterozoic E–HPG overprint has beenreported from the Palaeoproterozoic Gridino dikes. In theKuru-Vaara locality, this overprint could be represented bythe kyanite- and corundum-bearing pegmatoids. The tec-tonic setting of the Palaeoproterozoic E–HPG overprintcould be explained as satisfactory. The core of theLapland–Kola collisional orogen consists of the Laplandgranulite nappes ensemble that was thrust over theBelomorian Province (Figure 1; Bridgwater et al. 1992;Daly et al. 2006; Patison et al. 2006; Tuisku et al. 2012).The basal portion of this ensemble was metamorphosed inthe HP and HT granulite facies at depths of approximately40–45 km (Mints et al. 2007) and contains relics ofeclogites (Tuisku and Makkonen 1999). Consideringtheir crustal thickness, the Belomorian rocks that underliethe ensemble of the Lapland granulite nappes should haveoccurred at depths of >50 km (i.e. in the field of E–HPGmetamorphism). We consider the Palaeoproterozoic E–HPG overprint as an analogue of the eclogitization thatoccurs in the lowermost portions of the young collisionalorogens with an over-thickened crust (Johnson and Harley2012).

The eclogites that were precursors of the Archaeanamphibolites are widespread in the northern BelomorianProvince, which is likely formed in a principally distincttectonic setting. This suggestion is based on two facts.First, the former eclogites have the areal occurrence in

the northern Belomorian Province. Second, the eclogite-bearing amphibolites and their TTG host have at least aparagenetic link. To explain these facts, the subduction-related model has been suggested for determining theformation of the eclogites and TTG gneisses, with thelatter being interpreted as the products of melting of theeclogites (Mints et al. 2010; Konilov et al. 2011;Shchipansky et al. 2012a, 2012b). The same tectonicmodel was proposed for the Gridino eclogites(Volodichev et al. 2004). Figure 14 illustrates the tectonicsetting of the Belomorian eclogites and their position inthe geological history of the Belomorian Province.

Archaean eclogites were unknown before the discoveryof the Belomorian eclogites, and the existence of these rocksand the possibility of deep subduction in the Archaean washypothetical. The case study of the Archaean Belomorianeclogites demonstrates that the eclogites were formed insome places during the Mesoarchaean to Neoarchaean peri-ods. Their formation could have occurred by means of asubduction-involving mechanism, for example, similar tothat by Moyen and van Hunen (2012). This research showsthat the geological observations are just as important as themodern sophisticated techniques. Seeking for further obser-vational constraints, including mineral indicators of UHPmetamorphism, both the Belomorian Province and otherArchaean domains will continue to be vital.

Conclusions

(1) The relic amphibolite-eclogite facies eclogites arewidespread within amphibolites of the Kuru-Vaarastudy area. According to the observational con-straints, the crystallization of the relic peakomphacite + Mg-garnet ± kyanite assemblage andthe subsequent replacement of omphacite by clino-pyroxene–plagioclase symplectite occurred beforethe earliest deformational, metamorphic, andmigma-tization events that are recorded in the amphibolites.

(2) The amphibolites and their TTG hosts have ashared deformational and metamorphic historythat is composed of the Archaean andPalaeoproterozoic periods. This history favoursthe conclusion that the AE metamorphismrecorded in the relic eclogites within the amphibo-lites occurred during the Mesoarchaean toNeoarchaean periods (Mints et al. 2010; Konilovet al. 2011; Shchipansky et al. 2012a, 2012b).

(3) The deformation and metamorphism of the amphi-bolite facies of the second period resulted from theLapland–Kola collisional orogeny at 1.91–1.93 Ga,which led to E–HPG facies conditions in the lower-most portions of the over-thickened crust in theBelomorian Province (the southwestern foreland ofthe Lapland–Kola collisional orogen). The

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Figure 14. Cartoons illustrating the tectonic settings and position of the Belomorian eclogites in the early Precambrian geologicalhistory of the Belomorian Province (vertical and horizontal scales not proportional) (data and tectonic concepts after Berthelsen andMarker 1986; Volodichev 1990; Bridgwater et al. 1992; Melezhik and Sturt 1994; Mitrofanov et al. 1995; Glebovitsky et al. 1996;Tuisku and Makkonen 1999; Myskova et al. 2000; Bibikova et al. 2004; Shchipansky et al. 2004, 2012a, 2012b; Stern 2004; Volodichevet al. 2004; Glebovitsky 2005; Travin and Kozlova 2005; Daly et al. 2006; Patison et al. 2006; Slabunov et al. 2006, 2008, 2011; Mintset al. 2007, 2010; Krivolutskaya et al. 2010; Stepanova and Stepanov 2010; Konilov et al. 2011; Skublov et al. 2011; Herzberg andRudnick 2012; Moyen and van Hunen 2012; Li et al. 2013; Shirey et al. 2013; Liu and Zhang 2014, and this study).

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Palaeoproterozoic E–HPG facies overprint wasreported from the Palaeoproterozoic Gridino maficdikes (Travin and Kozlova 2005; Slabunov et al.2008). This overprint seems to be recorded in thekyanite- and corundum-bearing pegmatoids in theKuru-Vaara study area. Although the ages of theoldest low Th/U zircons are close to the time of theLapland–Kola collision, the low Th/U 1.9–1.8 Gazircons reflect a zircon response to regional fluidinfiltration in the eclogites during slow exhumationfollowing the Lapland–Kola orogeny and do notrecord any metamorphic event.

(4) Contrary to the Palaeoproterozoic E–HPG faciesoverprint, the areal occurrence of the 2.7–2.8 Gaamphibolite–granulite facies eclogites withMORB-like chemistry and their paragenetic linkwith the TTG gneisses suggests that a tectonicregime involves subduction. This research favoursconcepts suggesting that the modern-style platetectonics has operated in some places, at leastsince the late Mesoarchaean.

AcknowledgementsWe appreciate I.I. Yakovenko, head of the former Kovdor-SlyudaJoint-Stock Company, S.N. Khalliulova, head of the Kuru-Vaaraenterprise, and the Kuru-Vaara staff (especially L.O. Guseinova,N.P. Suleimanova and T.V. Yas’kina) for permission to conductfieldwork in the quarry and for help with the logistics of this study.We thank S.L. Presnyakov for his assistance in isotope dating andE.A. Selivanova for her help in identifying minerals using the X-ray refraction technique. Furthermore, we thank T.N. Bazhanov, G.A. Bezhev, L.A. Cherednik, T.V. Kartushinskaya, and M.Yu. Raudfor their help with the fieldwork and treatment of the materials. M.Brown is greatly appreciated for a discussion of the Belomorianeclogites during the meeting and the geological excursion‘Precambrian High-Grade Mobile Beltsʼ (June 2014,Petrozavodsk–Gridino–Chupa–Kuru-Vaara–Salma, Russia). M.Väisänen and one anonymous reviewer are thanked for their con-structive criticisms and notes that have resulted in significantimprovements of the manuscript. We are thankful to V. Singhand R.J. Stern for their editorial support.

FundingThis research was supported by RFBR under grants [12-05-00597],[12-05-01080], [13-05-91162], and [14-05-31137-mol-a], and bythe Department of Earth Sciences at the Russian Academy ofSciences (program ONZ-6).

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