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Russian Geology and Geophysics 53 (2012) 131­146 www.elsevier.com/locate/rgg

Petrology of Precambrian metaultramafites of the Gridino high-pressure complex (Karelia)
A.A. Morgunova a, A.L. Perchuk b,*
a

Institute of Experimental Mineralogy, Russian Academy of Sciences, ul. Institutskaya 4, Chernogolovka, Moscow Region, 142432, Russia b Lomonosov Moscow State University, Leninskie Gory, Moscow, 119991, Russia Received 16 June 2010; received in revised form 15 December 2010; accepted 1 March 2011

Abstract Along with eclogitized gabbro and gabbronorite bodies, boudinaged metaultramafites such as garnet-pyroxene rocks and orthopyroxenites were revealed in the Archean plagiogneiss strata of the Gridino complex. The garnet-pyroxene rock crope out as a boudin on Vysokii Island. The early stage of the rock evolution is documented by inclusions of diabantite (Fe-Si chlorite), a mineral that occurs in metasomatized peridotites. Diabantite was found in all rock-forming minerals in paragenesis with mineral phases enriched in REE (Ce, Nd, La, etc.), U, and Th. The confinement of ore phases to the inclusion rims and the development of two systems of cracks, radial and concentric, around the inclusions in pyroxenes point to the transformation of the inclusions after their trapping. Thermobarometric studies of the crystal cores revealed that the anhydrous paragenesis garnet + orthopyroxene + clinopyroxene, which replaced the chlorite-bearing rock, formed at ~690 ºC and ~17 kbar. The rims of the rock-forming minerals reflect isothermal decompression to ~12 kbar, which was followed by decompression cooling to ~650 ºC and ~9 kbar with the formation of regressive amphibole-garnet-pyroxene paragenesis. The giant-grained orthopyroxenites compose chains of boudinaged bodies on Izbnaya Luda Island. The orthopyroxene crystals host abundant amphibole, quartz, biotite, and pyrite inclusions pointing to amphibolite metamorphism at the early stage of the rock evolution. There are two types of amphibole: magnesian hornblende and anthophyllite. The hornblende is a primary mineral, whereas the low-temperature anthophyllite forming rims around the quartz inclusions was produced at the regressive stage of metamorphism. There are no indicators of the PT-conditions of the peak metamorphism in the orthopyroxenite. The reaction enstatite + quartz + H2O = anthophyllite allows deciphering water activity of anthophyllite formation, a 0.5. © 2012, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.
Keywords: metaultramafites; eclogite facies metamorphism; garnet-pyroxene rock; Precambrian; Gridino

Introduction High-pressure complexes bear important information about the style of plate-tectonic processes at different stages of the Earth's evolution. The results of numerical petrologo-thermomechanical modeling (Sizova et al., 2009) show that the modern plate tectonics regime was generated in the Neoarchean (2.5 Ga). Nevertheless, at the boundary of ~550 Ma, the number of high-pressure metamorphic complexes specific for subduction zones drastically decreased (Brown, 2006). The reasons are still unclear, but it might have been due to different intensities of regressive transformations (Baldwin et al., 2004; Brueckner and Medaris, 2000; Tsai, 2000). The Gridino eclogite-bearing complex within the Belomorian mobile belt in Karelia is one of the oldest on the Earth

* Corresponding author.
E-mail address: alp@geol.msu.ru (A.L. Perchuk)

(Volodichev et al., 2004). The vivid interest to it is explained not only by the presence of unique Precambrian complexes but also by the debates on the time and character of plate-tectonic processes at the early stages of the Earth's evolution (Baldwin et al., 2004; Brown, 2006), which cannot be elucidated without data on the thermodynamic and fluid regime of metamorphism of corresponding high-pressure rocks. The Gridino complex includes boudinaged eclogite and ultramafite bodies hosted in migmatized gneisses. Until the present time, the ultramafites have been beyond the scope of studies, whereas the eclogites are considered elsewhere (Dokukina et al., 2009; Travin and Kozlova, 2009; Volodichev et al., 2004). Note that there is still no single opinion of the number of high-pressure metamorphic events, their ages, and the geodynamic setting of formation of these eclogites (Dokukina et al., 2009; Slabunov, 2008; Volodichev et al., 2004). Despite the same structural position of the ultramafites and eclogites in the complex, their genesis and metamorphic

1068-7971/$ - see front matter D 201 2 , V . S . S o b o l e v IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. d o i : 10.1016/j.rgg.2011.12+.011


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Fig. 1. A, Tectonic position of the Gridino complex in the eastern Fennoscandian Shield, after Mints et al. (2009), with supplements. Insets show a schematic geologic structure of the region and islands of the Gridino complex (Slabunov, 2008; Travin et al., 2005). B, Schematic geologic structure of Izbnaya Luda Island (Travin et al., 2005). 1, eclogite-bearing complex (Gridino "melange"); 2, supracrustal rocks (amphibolites, metaultrabasites, aluminous and Gt-Bt gneisses); 3, migmatized granitoids of the TTG association; 4, granites; 5, assumed thrusts; 6, dips and strikes; 7, sedimentary cover; 8, Quaternary deposits; 9, basic dikes (a, of unclear localization, b, of lherzolite-gabbronorite complex (~2.45 Ga), c, of coronite gabbro complex (2.12 Ga)); 10, banded gneiss-granites of the Western and Eastern domains; 11, gneisses with brecciform structures of the Central domain; 12, bands of melange bodies (a, amphibolites, b, orthopyroxenites).

evolution might be different. Eclogites in metamorphic complexes are crustal rocks that come out to the Earth's surface as a result of tectonic processes (Dobretsov and Kirdyashkin, 1991; Ernst et al., 1994; Gerya et al., 2002), whereas ultramafites can be of different nature (Medaris, 1999; Reverdatto and Selyatitskii, 2005). For example, ultramafites can be crustal cumulates or metasomatized basalts involved in collision (Liou and Zhang, 1998; Okay, 1994; Reverdatto and Selyatitskii, 2005) or mantle rocks tectonically intruded into oceanic crust before subduction (Yang and Powell, 2008). There also exist ultramafites from a mantle wedge tectonically intruded into a submerging plate under the PT-conditions close to the metamorphism peak (Kadarusman and Parkinson, 2000; Khedr and Arai, 2010; Song et al., 2009).

Depending on temperature, ultramafites in high- and ultrahigh-pressure metamorphic complexes occur as serpentinites, serpentine-containing schists (Korikovsky et al., 1998; Liou and Zhang, 1998; Tromsdorff et al., 1998), and/or massive igneous rocks (wherlites, garnet lherzolites, pyroxenites, hornblende peridotites, etc.) (Brueckner et al., 1998; Nakamura et al., 2004; Song et al., 2009; Spengler et al., 2006; Zhang et al., 1994; van Roermund et al., 2002). In recent years, massive ultramafites have been more often considered to be the products of prograde metamorphism of serpentinites or their analogs (Enami et al., 2004; Okay, 1994; Song et al., 2009; Yang and Powell, 2008; Zhang et al., 2000). The metamorphic genesis of peridotites is proved by the findings of inclusions of metamorphic minerals--chlorite, sapphirine, talc, phlo-


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gopite, anthophyllite, and serpentinite in garnet or, sometimes, orthopyroxene porphyroblasts, which permits researchers to reconstruct the evolution stages of rocks in detail and determine their source (Kadarusman and Parkinson, 2000; Okay, 1994; Song et al., 2009; Zhang et al., 1997). In this work we present results of the first petrological study of ultramafites from the oldest high-pressure complex in the Earth. Genesis and thermodynamic conditions of the rocks formation and evolution are considered.

Geologic position The Belomorian mobile belt is part of the Belomorian tectonic province bordering the Kola craton in the north and the Karelian craton in the south, which are located within the Fennoscandian Shield (Fig. 1). The Gridino eclogite-including complex is confined to the Keret' nappe localized between the Ket-Lamba and Inari-Kola granite-greenstone belts of the Belomorian province (Mints et al., 2009). The Gridino complex is a 6­7 km wide tectonic plate of NW strike traceable for 60 km in the coastline and on the islands of the White Sea, from Bay Sukhaya in the NW to the Suprotivnye Isles in the SE (Fig. 1, A). The complex is composed mostly of migmatized gneisses with boudinaged bodies of rocks of different sizes and compositions. The boudins are strongly dominated by basic rocks--metabasites (eclogites and amphibolites); the other rocks are metaultramafites, marble, and zoisitic and scapolitic rocks (Slabunov, 2008 and references therein). The complex is cut by Paleoproterozoic gabbroid dikes metamorphosed to the eclogite and upper amphibolite facies and by plagiogranite intrusions and veins (Slabunov, 2008 and reference therein; Volodichev et al., 2004). The ultramafic bodies under study occur on Vysokii and Izbnaya Luda Islands. The latter island is localized 9 km southeast of Gridino Village (Fig. 1). Three domains are recognized there by the kind of deformations and predominant orientation of structural elements (Travin et al., 2005): Central (with exotic brecciform structures), Western, and Eastern. All domains are composed mainly of gray medium-grained migmatized amphibole-biotite gneisses with banding and gneissosity. The gneisses are cut by abundant basite dikes. The set of rocks in the Eastern domain is much more diverse than those in the other two. Its gneisses show a zone with several chains of abundant boudinaged bodies of massive coarsegrained orthopyroxenites (up to 0.2­0.3, sometimes, 0.5 m across). On Vysokii Island, abundant boudinaged bodies of eclogites and amphibolites were discovered among amphibole-biotite gneisses. In the NE, ten meters from the water line, there is also a single ultramafic body formed by a garnet-pyroxene rock. It has the same structural position in the gneisses as the other melanocratic rocks. No geological mapping of the island was made. Based on the results of isotopic dating, four age groups of zircons were recognized (Dokukina et al., 2009; Volodichev

et al., 2004): (1) ~3.00­2.78 Ga (sedimentation), (2) 2.72­ 2.63 Ga (first eclogite metamorphism), (3) 2.47­2.42 Ga (intrusion of dikes of lherzolite-gabbronorite complex and their eclogitization), and (4) 1.9­1.8 Ga (Svecofennian tectonometamorphism). No isotopic dating of the studied ultramafites was carried out. Note that dating of intricately dislocated Precambrian polymetamorphic complexes is a big challenge. Therefore, the above sequence of metamorphism events is supported not by all researchers. The solution of this problem might be favored by findings of omphacite inclusions in zircon and the proof for two stages of eclogitic metamorphism of the most ancient rocks.

Petrography and mineralogy Analytical methods. The chemical composition of rocks (Table 1) was determined by X-ray fluorescent spectrometry (XRF) on a PW2400 Philips Analytical B.V. vacuum successive-operation spectrometer (with dispersion along the wavelength) (Netherlands, www.panalytical.com) at the Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Moscow (analyst A.I. Yakushev). The spectrometer was calibrated using State standard mineral samples. Analysis was carried out by the 439-PC technique (NSAM VIMS (the Analytical Methods Research Board of the All-Russian Research Institute of Mineral Resources)) ensuring the rank III accuracy of results according to Russian Industrial Standard 41-08-205-99. Glass discs for analysis of rock-forming ele-

Table 1. Bulk chemical composition of rocks from the Gridino complex, from data of X-ray fluorescent analysis Component SiO2, wt.% TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Total Cr, ppm Ni Rb Sr Y Zr Nb Ba Orthopyroxenite 54.70 0.17 2.67 15.69 0.29 23.45 2.42 0.32 0.29 0.01 100.01 2537 950 10 2 19 108 3 119 Garnet-pyroxene rock 50.74 0.51 3.05 14.64 0.34 19.21 10.94 0.37 0.08 0.11 99.99 832 1018 5 80 32 74 6 126


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Fig. 2. Garnet-pyroxene rock: a, fragment of rock with a anheudral garnet crystal surrounded by amphibole, b, retrograde fine-grained pyroxene-garnet-amphibole aggregate, c, thin anthophyllite rim over orthopyroxene from the retrograde aggregate. Arrows show lamellae (most likely, of clinopyroxene) in amphibole. Backscattered electron (BSE) image. Hereafter, mineral symbols are after Kretz (1983).

ments were prepared by fusion of a weighted calcined sample with lithium borates at 1300 ºC. The contents of trace elements were determined using tablets prepared by pressing the powdered sample together with a binding agent. Loss on ignition (LOI) was not determined; therefore, the results of analysis of rock-forming elements were reduced to 100%. The chemical composition of the Gridino complex metaultramafites shows that their protolith was rocks of pyroxenitehornblendite family (Bogatikov et al., 1981): The orthopyroxenite has a composition typical of this rock, and the garnet-pyroxene rock is similar in composition to websterite (Table 1).

Chemical analyses of minerals were performed at the Institute of Experimental Mineralogy, Chernogolovka, using a Tescan Vega II XMU scanning electron microscope equipped with an energy dispersive X-ray spectrometer (INCAx-sight) with an ATW-2 ultrathin window (10 mm2 in area) favoring recording of characteristic X-ray radiation in the low-energy spectrum region and a quantitative analysis of light elements. The analyses were made in the regime of highly contrasting reflected-electron images (accelerating voltage 20 kEv, absorbed-electron current on a Faraday cup 0.3 nA). A quantitative X-ray analysis was performed on an INCA Energy 450 system (resolution on the MnK1,2 line 133 eV), on horizontal


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Fig. 3. BSE image of chlorite inclusions coexisting with a REE-rich phase: a, in clinopyroxene, b, in orthopyroxene. REE phase occurs on the periphery of inclusions, which are surrounded by two systems of cracks, radial and concentric.

carbon-deposited (15­20 nm film) polished surfaces. The time of spectrum recording was 70 s. As standard samples, we used Na-albite, K-orthoclase, Mg-MgO, Ca-wollastonite, Si-SiO2,

Fig. 4. Different generations of garnet from garnet-pyroxene rock of the Gridino complex on the Mg­Ca­(Mn + Fe) composition diagram. The compositions of garnet from garnet websterites and garnet peridotites are shown for comparison. 1­3, garnet from garnet-pyroxene rock of the Gridino complex: 1, core and 2, rim of large crystals; 3, retrograde mineral; 4, garnet websterite from the eclogite province in northeastern Greenland (Brueckner et al., 1998); 5­7, garnet from crustal peridotites (Fe-Ti type, classification by Carswell et al. (1983) and Medaris (1999)): 5, Eiksunddal complex, Western Norway (Carswell et al., 1983), 6, Kolmannskog massif, Western Norway (Carswell et al., 1983), 7, Ticlinohumite-containing garnet peridotite, Kulet, Kokchetav massif (Zhang et al., 1997); 8­10, garnet from mantle peridotites (Mg-Cr type, classification by Carswell et al. (1983) and Medaris (1999)): 8, Sulawesi Island (Kadarusman et al., 2000), 9, Su-Lu terrane, eastern China (Yang and Jahn, 2000); 11, 12, areas of: 11, mantle rocks (Mg-Cr type), 12, crustal rocks (Fe-Ti type).

Al-Al2O3, Fe-Fe, Mn-Mn, and Ti-Ti. Oxygen corresponded to the stoichiometry. The errors of the analyses were up to 2 rel.% for element concentrations of >10 wt.%, up to 5 rel.% for 5­10 wt.%, and up to 10 rel.% for 1­5 wt.%. The detection limit for the elements in Tables 2­6 is ~0.15­0.2 wt.%. The microprobe analyses of the pyroxenes were recalculated by the method by Cawthorn and Collerson (1974). Formulas of the clinoamphiboles and orthoamphiboles were calculated per 13 and 15 cations, respectively, and 23 oxygens (Leake et al., 1997). The representative analyses of minerals are listed in Tables 2­5. Garnet-pyroxene rock forms an oval body 4 â 5 m in size located in a unit of migmatized gneisses on Vysokii Island. The rock is dark green, coarse- to medium-grained, of heteroblastic and, locally, granoblastic texture, with large porphyritic segregations of pink garnet. It is composed of garnet and clino- and orthopyroxene (Fig. 2). At the retrograde stage, amphibole was developed as rims over garnet and along cracks in clinopyroxene. The latter also hosts a fine-grained paragenesis of amphibole, garnet, and clino- and orthopyroxene (Fig. 2, b). Regressive transformations are most intense on the periphery of the body and are minor in its core. There are two generations of garnet in the rock. One is large (up to 1 cm) anheadral crystals (Fig. 2, a), and the other is small regressive intergrowths of garnet with amphibole and clino- and orthopyroxene (Fig. 2, b). The anheadral garnets contain clinopyroxene and amphibole inclusions on the periphery and isolated chlorite inclusions associated with a REE-, U-, and Th-enriched phase as well as scarce carbonate inclusions in the core (Fig. 3, a). The large garnet crystals show a weak chemical zoning (Table 2): The Fe/(Fe + Mg) value on the periphery is higher by ~10% than that in the core, as is observed in the garnets of the regressive paragenesis (Fig. 4). The garnet has high contents of Fe and Mn, which is typical of garnets from crustal (Fe-Ti-containing) garnet


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Table 2. Representative microprobe analyses and crystallochemical formulas of garnet from garnet-pyroxene rock from the Gridino complex Component SiO2, wt.% TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total O S Ti Al Cr Fe Mn Mg Ca Na K Total X X
Mg** Grt Ca

Core 38.86 0.11 21.15 0.11 21.40 1.82 9.87 5.76 0.08 0.11 99.27 12 2.98 0.01 1.91 0.01 1.37 0.12 1.13 0.47 0.01 0.01 8.00 0.45 0.16

Core 39.07 0.00 21.12 0.45 22.11 1.97 9.02 5.98 0.00 0.16 99.89 12 2.99 0.00 1.91 0.03 1.42 0.13 1.03 0.49 0.00 0.02 8.00 0.42 0.16

Core 38.79 0.16 22.00 0.00 22.54 1.99 9.32 5.14 0.35 0.05 100.34 12 2.96 0.01 1.98 0.00 1.44 0.13 1.06 0.42 0.05 0.00 7.99 0.42 0.14

Core* 39.06 0.05 21.90 0.13 22.23 1.85 9.51 5.99 0.08 0.06 100.94 12 2.96 0.00 1.95 0.01 1.41 0.12 1.07 0.49 0.01 0.01 8.01 0.43 0.16

Rim 38.59 0.21 21.38 0.00 22.63 1.88 8.89 6.35 0.00 0.02 99.95 12 2.96 0.01 1.93 0.00 1.45 0.12 1.02 0.52 0.00 0.00 8.01 0.41 0.17

Rim 38.37 0.00 20.92 0.11 22.72 1.61 9.32 5.69 0.15 0.16 99.05 12 2.97 0.00 1.91 0.01 1.47 0.11 1.07 0.47 0.02 0.02 8.00 0.42 0.16

Rim 38.67 0.07 21.96 0.00 23.05 1.91 9.05 5.78 0.04 0.07 100.59 12 2.95 0.00 1.97 0.00 1.47 0.12 1.03 0.47 0.01 0.01 8.02 0.41 0.16
Grt Ca

Rim* 39.06 0.07 21.67 0.10 22.66 1.82 9.19 6.09 0.10 0.04 100.83 12 2.97 0.00 1.94 0.01 1.44 0.12 1.04 0.50 0.01 0.00 8.00 0.42 0.16

Sympl. 39.19 0.00 21.71 0.41 22.15 2.10 8.95 5.50 0.15 0.00 100.16 12 2.99 0.00 1.95 0.02 1.41 0.14 1.02 0.45 0.02 0.00 7.99 0.42 0.15

Sympl. 38.80 0.00 21.73 0.44 24.05 1.74 8.12 5.12 0.00 0.00 100.00 12 2.99 0.00 1.97 0.03 1.55 0.11 0.93 0.42 0.00 0.00 8.01 0.38 0.14

Sympl. 38.57 0.01 21.09 0.32 23.16 2.28 7.78 5.92 0.00 0.00 99.12 12 3.00 0.00 1.93 0.02 1.50 0.15 0.90 0.49 0.00 0.00 8.00 0.37 0.16

Sympl.* 38.94 0.03 21.49 0.37 22.94 2.22 8.35 5.78 0.14 0.02 100.40 12 2.98 0.00 1.94 0.02 1.47 0.14 0.95 0.47 0.02 0.00 8.00 0.40 0.16

***

Note. Here and in Tables 3­6: * Average value; ** XMg = Mg/(Mg + Fe); *** X

= Ca/(Ca + Fe + Mg); Sympl., Symplectite.

peridotites and uncommon for mantle (Mg-Cr-containing) garnet peridotites (Fig. 4). Orthopyroxene composing the rock matrix has elongate crystals (up to 6 mm) (Fig. 2, b). It occurs in paragenesis with regressive garnet and amphibole (Fig. 2, c) and contains inclusions of chlorite, apatite, ilmenite, magnetite, pyrite, and zircon. The matrix orthopyroxene is chemically homogeneous: Al2O3 = 0.76 wt.%, XMg = 0.74 (Table 3). But at the crystal edge, at the boundary with the amphobilitization front (and at the contact with garnet), the Mg-number decreases and the Al2O3 content increases: Al2O3 = 1.31 wt.%, XMg = 0.72. The orthopyroxene from the regressive paragenesis is similar in composition to the edges of the matrix crystals: Al2O3 = 1.40 wt.%, XMg = 0.70 (Fig. 5, Table 3). Clinopyroxenes occurs as crystals in the matrix, as inclusions in garnet, and as xenomorphous grains in garnet-amphibolite intergrowths. It is represented by augite with a low content of the jadeite end-member (XMg = 0.84­0.86, XJd ~ 0.01­0.05). The matrix clinopyroxene contains inclusions of chlorite, apatite, ilmenite, magnetite, and zircon. It is more magnesian than the clinopyroxene from the fine-grained aggregate (Table 4, Fig. 6). Thus, the clinopyroxene of

different generations, like the orthopyroxene and garnet, is of different compositions. According to the classification by Leake et al. (1997), amphibole in the matrix, in intergrowths with garnet, and in inclusions in garnet falls into the field of magnesian hornblende (Fig. 7) with XMg = 0.83­0.98, Ti < 0.04; CaB = 1.78­1.91, (Na + K)A = 0.05­0.52, and Si = 6.48­7.67 (Fig. 7). The clinopyroxene cracks are often filled with tremolite. Sometimes, there are finest reactionary rims of late anthophyllite around the orthopyroxene in symplectites (Table 5). Chlorite occurs only as inclusions in the main rock-forming minerals--garnet and clino- and orthopyroxenes. The inclusions are oval isometric and, sometimes, angular. There are always radiate cracks around the inclusions in garnet and radial and concentric cracks in pyroxenes (Fig. 3). The inclusions also contain phases enriched in REE (mainly Ce, Nd, La, etc.), U, and Th. The microprobe analysis showed that these are ferrodollaseite (epidote group mineral) (SiO2 = 21.96; TiO2 = 0.46; Al2O3 = 8.81; FeO = 15.06; MnO = 0.94; MgO = 4.94; CaO = 3.53; Na2O = 0.18; K2O = 0.15; La2O3 = 4.95; Ce2O3 = 12.98; Nd2O3 = 6.30; Pr2O3 = 1.68; Sm2O3 = 1.02 wt.%) and ferrotornebomite (SiO2 = 14.12; TiO2 = 0.89; Al2O3 = 3.71; FeO = 4.45; MgO = 1.41; CaO = 2.71; Na2O =


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A.A. Morgunova and A.L. Perchuk / Russian Geology and Geophysics 53 (2012) 131­146 Table 3. Representative microprobe analyses and crystallochemical formulas of orthopyroxene from garnet-pyroxene rock from the Gridino complex Component SiO2, wt.% TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total O S Ti Al Cr Fe Mn Mg Ca Na K Total X
Mg**

137

Core 54.49 0.04 0.82 0.00 16.99 0.72 25.73 0.25 0.02 0.07 99.14 6 1.99 0.00 0.04 0.00 0.52 0.02 1.40 0.01 0.00 0.00 3.99 0.73

Core 54.30 0.15 0.72 0.08 17.06 0.43 26.62 0.19 0.06 0.15 99.76 6 1.98 0.00 0.03 0.00 0.52 0.01 1.44 0.01 0.00 0.01 4.01 0.74

Core 54.87 0.00 0.73 0.04 16.29 0.28 26.81 0.33 0.05 0.02 99.49 6 1.99 0.00 0.03 0.00 0.49 0.01 1.45 0.01 0.00 0.00 4.00 0.75

Core* 54.63 0.05 0.76 0.07 17.13 0.47 27.11 0.28 0.09 0.02 100.65 6 1.97 0.00 0.03 0.00 0.52 0.01 1.46 0.01 0.01 0.00 4.01 0.74

Rim 53.28 0.00 1.27 0.15 18.05 0.54 25.73 0.37 0.13 0.02 99.52 6 1.96 0.00 0.05 0.00 0.55 0.02 1.41 0.01 0.01 0.00 4.02 0.72

Rim 53.59 0.00 1.56 0.00 18.67 0.59 25.00 0.28 0.00 0.05 99.74 6 1.96 0.00 0.07 0.00 0.57 0.02 1.37 0.01 0.00 0.00 4.00 0.70

Rim 54.67 0.09 1.42 0.00 17.32 0.46 26.69 0.19 0.00 0.00 100.86 6 1.97 0.00 0.06 0.00 0.52 0.01 1.43 0.01 0.00 0.00 4.00 0.73

Rim* 54.20 0.06 1.31 0.07 18.27 0.54 26.03 0.31 0.11 0.03 100.94 6 1.96 0.00 0.06 0.00 0.55 0.02 1.40 0.01 0.01 0.00 4.01 0.72

Sympl. 53.66 0.00 1.49 0.04 19.60 0.23 25.45 0.10 0.08 0.00 100.65 6 1.95 0.00 0.06 0.00 0.60 0.01 1.38 0.00 0.01 0.00 4.02 0.70

Sympl. 53.24 0.00 1.21 0.00 19.30 0.85 24.47 0.38 0.10 0.00 99.55 6 1.97 0.00 0.05 0.00 0.60 0.03 1.35 0.02 0.01 0.00 4.01 0.69

Sympl. 53.48 0.00 1.60 0.00 19.84 0.69 24.72 0.55 0.16 0.00 101.04 6 1.95 0.00 0.07 0.00 0.60 0.02 1.34 0.02 0.01 0.00 4.02 0.69

Sympl.* 54.00 0.04 1.40 0.04 19.62 0.60 25.34 0.33 0.09 0.02 101.55 6 1.96 0.00 0.06 0.00 0.59 0.02 1.37 0.01 0.01 0.00 4.02 0.70

0.20; K2O = 0.29; La2O3 = 8.78; Ce2O3 = 26.16; ThO2 = 0.92; UO2 = 0.22; Nd2O3 = 10.71; SrO = 0.81; Pr2O3 = 2.61; Sm2O3 = 0.58 wt.%). It is remarkable that ferrodollaseite and ferrotornebomite are localized predominantly on the periphery

Fig. 5. Al2O3­XMg diagram for orthopyroxene: 1, core and 2, rim of large crystals; 3, retrograde mineral.

of inclusions (Fig. 3). Chlorite corresponds to diabantite with XMg = 0.43­0.56 and a low Al2O3 content. The Al2O3- and Fe-richest diabantite was found in garnet (Table 5, Fig. 8). Orthopyroxenite occurs as abundant chains of boudins 5 to 40 cm across in amphibole-biotite gneisses on Izbnaya Luda Island. The rock is massive giant-crystalline. Tabular crystals of orthopyroxene sometimes reach 10 cm and more in length. The mineral abounds in inclusions of biotite, amphibole, quartz, and ore minerals -- magnetite, pyrite, and ilmenite; therefore, the orthopyroxenite looks like a usual granoblastic rock in thin section (Fig. 9). In composition this orthopyroxene (XMg = 0.74­0.77; XCa 0.01; Al2O3 = 1.07­1.61 wt.%) is similar to the orthopyroxene from the garnet-pyroxene rock (Table 6). Amphibole is present as two varieties -- magnesian hornblende (XMg = 0.82­1.0, Ti = 0.02­0.07 f.u., CaB = 1.59­1.77 f.u., (Na + K)A = 0.06­0.25 f.u., Si = 7.09­7.40 f.u.) and anthophyllite (XMg = 0.73­0.76) (Fig. 7). Anthophyllite is often developed at the boundary between the quartz inclusions and the host orthopyroxene, which evidences the regressive nature of the latter (Fig. 10, a). Biotite occurs in intergrowths with hornblende and quartz. The magnesian hornblende and biotite are enriched in Cr (Cr2O3 = 0.45­1.17 and 0.91­1.20 wt.%, respectively). The orthopyroxene abounds in fluid primary and secondary inclusions (Fig. 10, b), whose chains are broken at the boundary with solid-phase


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Table 4. Representative microprobe analyses and crystallochemical formulas of clinopyroxene from garnet-pyroxene rock from the Gridino complex Component SiO2, wt.% TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total O Si Ti Al Cr Fe Fe
3+ 2+

Core 54.65 0.00 0.65 0.10 4.90 0.00 15.80 23.35 0.38 0.03 99.86 6 1.98 0.00 0.03 0.00 0.03 0.13 0.01 0.87 0.92 0.04 0.00 4.01 0.85 0.03

Core 53.95 0.12 0.91 0.15 5.14 0.00 15.97 23.06 0.62 0.01 99.93 6 1.98 0.00 0.04 0.00 0.03 0.13 0.00 0.87 0.91 0.04 0.00 4.00 0.87 0.01

Core 54.37 0.02 0.84 0.00 4.68 0.38 15.92 23.66 0.33 0.03 100.22 6 1.99 0.00 0.04 0.00 0.00 0.14 0.01 0.87 0.93 0.02 0.00 4.00 0.86 0.02

Core* 53.99 0.04 0.87 0.08 5.12 0.11 15.89 23.60 0.42 0.04 100.29 6 1.98 0.00 0.04 0.00 0.02 0.14 0.00 0.87 0.93 0.03 0.00 4.00 0.86 0.01

Rim 54.30 0.00 0.69 0.04 5.42 0.19 15.73 23.06 0.23 0.00 99.66 6 2.00 0.00 0.03 0.00 0.00 0.17 0.01 0.86 0.91 0.02 0.00 3.99 0.84 0.02

Rim 53.32 0.00 1.10 0.09 5.27 0.07 15.64 23.79 0.45 0.05 99.79 6 1.97 0.00 0.05 0.00 0.02 0.14 0.00 0.86 0.94 0.03 0.00 4.02 0.86 0.01

Rim 52.92 0.00 1.14 0.00 5.66 0.21 15.17 24.39 0.36 0.06 99.91 6 1.96 0.00 0.05 0.00 0.02 0.16 0.01 0.84 0.97 0.03 0.00 4.02 0.84 0.01

Rim* 54.16 0.00 1.04 0.06 5.27 0.17 15.81 23.63 0.41 0.04 100.59 6 1.98 0.00 0.04 0.00 0.01 0.15 0.01 0.86 0.93 0.03 0.00 4.01 0.85 0.02

Sympl. 53.84 0.06 1.04 0.00 5.87 0.32 15.72 23.56 0.31 0.06 100.78 6 1.97 0.00 0.04 0.00 0.01 0.17 0.01 0.86 0.92 0.02 0.00 4.01 0.83 0.01

Sympl. 53.41 0.14 1.03 0.21 6.02 0.25 15.53 23.75 0.30 0.00 100.64 6 1.96 0.00 0.04 0.01 0.02 0.16 0.01 0.85 0.93 0.02 0.00 4.01 0.84 0.00

Sympl. 52.67 0.06 0.82 0.00 5.89 0.30 15.40 23.00 0.25 0.00 98.39 6 1.98 0.00 0.04 0.00 0.01 0.18 0.01 0.86 0.92 0.02 0.00 4.01 0.83 0.01

Sympl.* 53.42 0.11 0.96 0.08 5.72 0.24 15.55 23.54 0.34 0.04 100.02 6 1.97 0.00 0.04 0.00 0.02 0.16 0.01 0.85 0.93 0.02 0.00 4.01 0.84 0.00

Mn Mg Ca Na K Total X X
Mg** Jd

inclusions. This might be related to the recrystallization of solid inclusions after trapping.

Thermobarometry The thermodynamic parameters of metamorphism of the Gridino metaultramafites were fitted using the TWQ program (Berman, 1991) with an updated thermodynamic database located on the official site of the Geological Service of Canada (http://gsc.nrcan.gc.ca/index_e.php). Ferric iron (Fe2O3) in the minerals was taken into account in the calculations. For thermobarometric studies, we used the compositions of the crystal core and rim (classical petrological approach (e.g., Perchuk et al. (1983)). Figure 11 shows parameters fitted with the TWQ program for the garnet-pyroxene rock and orthopyroxenite. The computations were made for the average mineral compositions given in Tables 2­4. The average compositions of the core of garnet (15 runs), orthopyroxene (18 runs), and clinopyroxene (12 runs) from the garnet-pyroxene rock yield an intersection of four equilibrium lines: Alm + 3Di = Py + 3Hed, Alm + 3En = Py + 3Fs, Alm = 3Fs + Opx, Py = 3En + Opx, respectively, at 690 ºC

and 17 kbar (Fig. 11, Table 7). These parameters are interpreted as the peak of metamorphism. They coincide with the estimates of 700 ºC and 16 kbar for the plagioclase-free kyanite eclogites from Bezymyannyi Island of the same complex (Morgunova et al., 2008). Note that the temperatures estimated earlier for the eclogites from Stolbikha Island are somewhat higher, 740­865 ºC, but the pressures are close, 14­17 kbar (Volodichev et al., 2004). The rim compositions of coexisting garnet (20 runs), clinopyroxene (10 runs), and orthopyroxene (15 runs) point to isothermal decompression with a pressure decrease to 12 kbar (Fig. 11, Table 7). A subsequent decrease in P-T conditions to 650 ºC and 9 kbar is reconstructed from the compositions of these minerals (Grt--12 runs, Opx--10 runs, Cpx--8 runs) in the regrade fine-grained aggregate. Thus, the retrograde metamorphic P-T path was reconstructed from the compositions of coexisting minerals (Fig. 11). The results obtained using the TWQ program were compared with the temperature and pressure estimated with mineralogical thermometers and barometers. The thermobarometric studies were performed using the garnet-pyroxene (Powell, 1985), garnet-orthopyroxene (Harley, 1984), and two-pyroxene (Brey and Kohler, 1990) geothermometer and


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Fig. 6. AFM diagram for clinopyroxene of different structural positions in the rock. Symbols follow Fig. 5.

garnet-orthopyroxene geobarometer (Brey and Kohler, 1990) widely applied in petrological investigations. Pressures and temperatures were calculated for the same mineral compositions as in the TWQ computations. The obtained P and T

values agree (within the thermometer accuracy of 50 ºC) with the results of the TWQ computations (Table 7). Thus, the presence of two stages of metamorphism under the conditions of the eclogite and amphibolite facies is confirmed by two independent methods. Note that the garnet-chlorite thermometers (Ghent et al., 1987; Perchuk, 1991) for diabantite inclusions in garnet yield abnormally high temperatures (>1100 ºC and >3000 ºC, respectively). This might be due to the low alumina content of chlorite (Al2O3 = 8.9­12.8 wt.%), which was not taken into account on the thermometer calibration, or to the poor re-equilibration between the chlorite and the garnet produced at different stages of the rock evolution. Reliable sensors of temperature and pressure are lacking in the orthopyroxenite. We succeeded in reconstructing only a fragment of the retrograde rock evolution. For this, we made a TWQ computation of the position of the equilibrium En + Qtz + H2O = Ath on the P­T diagram for the magnesian pyroxene and amphibole end-members at different water activities (Fig. 11). These end-members were chosen because the Mg-number values of anthophyllite and enstatite in the rock are nearly the same (0.73 and 0.74, respectively). Assuming that the orthopyroxenite underwent the same evolution as the garnet-pyroxene rock, we concluded that the activity of water participating in the formation of anthophyllite

Table 5. Representative microprobe analyses and crystallochemical formulas of amphibole and chlorite from garnet-pyroxene rock from the Gridino complex Component SiO2, wt.% TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total O Si Ti Al Cr Fe Fe
3+ 2+

Hbl 48.80 0.16 9.78 0.19 8.09 0.00 17.13 11.79 1.41 0.02 97.38 23 6.83 0.01 1.61 0.02 0.72 0.23 0.00 3.58 1.77 0.38 0.00 15.16 0.94

Tr 53.07 0.14 3.48 0.14 7.10 0.00 19.62 12.01 1.08 0.12 96.76 23 7.46 0.01 0.58 0.01 0.52 0.32 0.00 4.11 1.81 0.29 0.02 15.12 0.93

Ant 56.63 0.13 2.12 0.20 14.33 0.28 24.26 0.18 0.34 0.21 98.68 23 7.82 0.01 0.35 0.02 0.00 1.67 0.03 5.00 0.03 0.09 0.04 15.04 0.75

Chl inclusion in Cpx Chl inclusion in Opx Chl inclusion in Grt 36.78 0.08 9.62 0.35 24.11 0.96 15.30 0.50 0.20 0.29 88.19 14 3.79 0.01 1.17 0.03 ­ 2.08 0.08 2.35 0.06 0.04 0.04 9.64 0.53 36.70 0.00 8.91 0.64 25.39 1.06 12.83 0.37 0.54 0.28 86.72 14 3.88 0.00 1.11 0.05 ­ 2.25 0.10 2.02 0.04 0.11 0.04 9.61 0.47 35.09 0.08 11.51 0.15 26.57 1.31 12.08 0.46 0.29 0.18 87.73 14 3.69 0.01 1.43 0.01 ­ 2.34 0.12 1.89 0.05 0.06 0.02 9.62 0.45

Mn Mg Ca Na K Total XMg**

Note. Here and in Table 6: Dash, not calculated.


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Fig. 7. Classification diagrams (Leake et al., 1997) for amphibole: a, clinoamphibole, b, orthoamphibole. Dark circles, garnet-pyroxene rock, light circles, orthopyroxenite.

wa s this into fluid

<0.5 (Fig. 11). A hydrous fluid which is necessary for reaction, most likely, penetrated along the microcracks the host mineral, as evidenced from abundant chains of inclusions in the orthopyroxene (Fig. 10, b).

Fig. 8. Classification diagram (Hey, 1954) for chlorite from inclusions in garnet-pyroxene rock. The chlorite composition falls in the field of diabantite. Fe*, total Fe.

Discussion Interpretation of mineral inclusions in metaultramafites of gneiss complexes. Inclusions of hydrous phases in rockforming minerals of igneous metaultramafites were described in (ultra)high-pressure complexes of different regions all over the world (Enami et al., 2004; Kadarusman and Parkinson, 2000; Khedr and Arai, 2010; Malaspina et al., 2006; Song et al., 2009; Yang and Jahn, 2000; Yang and Powell, 2008; Zhang et al., 1994). The inclusions are usually interpreted as relics of earlier mineral assemblages related to the metamorphic evolution of the mantle rocks or crustal cumulates. An example of a crustal cumulate is an orthopyroxenite lens from the Maowu area, Dabie ultrahigh-pressure complex, eastern China (Liou and Zhang, 1998; Okay, 1994). The garnets hosted by these rocks bear inclusions of clinochlore, gedrite, hornblende, talc, phlogopite, and high-temperature granulite assemblage--sapphirine, enstatite, and corundum. Mantle ultramafites with mineral inclusions occur more often than those from the crust. There are different hypotheses of formation of these inclusions. According to one of them,

the inclusions are hydrated rocks of a mantle wedge, which tectonically intruded into a subducting plate. An example of such rocks is the garnet peridotites from Sulawesi Island, Indonesia, with inclusions of pargasitic amphibole and biotite in paragenesis with olivine and clino- and orthopyroxene (Kadarusman and Parkinson, 2000). A similar scenario is proposed for the garnet peridotites from the Su-Lu ultrahighpressure terrane, eastern China, which bear inclusions of clinochlore, hornblende, Na-gedrite, Na-phlogopite, talc, spinel, and pyrite (Yang and Jahn, 2000), and for the ultrahigh-pressure peridotites from the Sanbagawa complex, Japan, with inclusions of edenitic amphibole, clinochlore, and magnetite (Enami et al., 2004). In the latter case, alternative tectonometamorphic evolution is not ruled out, which suggests that peridotite underwent low-pressure metamorphism within the oceanic plate before its subduction (Enami et al., 2004). A similar scenario was proposed for the garnet peridotites from the North Qaidam Mts., western China, with inclusions of amphibole, serpentine, clinohumite, and carbonate (Yang and Powell, 2008). Later, however, Song et al. (2009) assumed that all the inclusions can be classified as pseudoinclusions because they formed at the regressive stage. But this hypothesis was not supported by any facts. The garnet peridotites were


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3+ 2+

141

Opx 54.28 0 1.53 0.04 16.71 0.34 26.67 0.13 0.21 0.01 99.91 6 1.97 0 0.07 0 ­ 0.51 0.01 1.44 0.01 0.01 0 4.01 0.74

Ant inclusion 55.74 0 0.75 0 14.61 0.37 23.45 0.73 0.39 0.16 96.2 23 7.91 0.09 0.13 0 0.13 1.61 0.04 4.96 0.11 0.11 0.03 15.03 0.76

Hbl inclusion 51.44 0.71 6.78 0.96 6.65 0.29 19.04 11.07 1.53 0.31 98.78 23 7.07 0.07 1.1 0.05 0.67 0.09 0.03 3.9 1.63 0.41 0.06 15.1 0.92

Bt inclusion 39.67 2.3 14.82 1.16 7.49 0.12 19.93 0.17 0.66 9.59 95.91 11 2.84 0.12 1.25 0.07 ­ 0.45 0.01 2.13 0.01 0.09 0.08 7.86 0.83

Mn Mg Ca Na K Total X
Mg**

regarded by the authors as mantle cumulates carried by a viscous asthenospheric flow to depths of more than 160 km in the mantle hanging wall (Kincaid and Sacks, 1997). This brief review shows that most of researchers accept the existence of prograde metamorphism of ultramafites. The most debatable questions are related to the kind of early metamorphic stage and the nature of protolith. It is admitted that at the early stage of evolution, ultramafites might be localized in different tectonic positions: in continental crust, in oceanic crust, or in mantle wedge. Depending on the tectonic position, the progressive metamorphism of metaultramafites is controlled by subduction, collision, or a viscous mantle flow caused by the motion of downgoing plates. Let us consider the evolution of the Gridino metaultramafites in this context. The nature of mineral inclusions in the garnet-pyroxene rock. The main minerals of the garnet-pyroxene rock bear isolated diabantite inclusions in paragenesis with ore minerals (ferrodollaseite and ferrotornebomite). Depending on the host mineral, the inclusions are surrounded by one (in garnet) or two (in clino- and orthopyroxenes) systems of cracks. To our knowledge, these inclusions have no analogs neither in chemical composition nor in the type of viscoelastic interaction with the host mineral; therefore, it is worth discussing

their nature. Note that diabantite in ultramafites was described only as a metasomatism product (Spiridonov and Pletnev, 2002). Thus, we interpret the isolated diabantite inclusions associated with REE-, U-, and Th-rich phases as the evidence for the early stage of intensive either metasomatic or metamorphic transformations. One more specific feature of the studied inclusions (Fig. 3) is the presence of REE phases on the periphery of vacuoles. Such a spatial orientation of ore phase could hardly have been inherited from the initial rock. We regard it as the result of transformation of the inclusions after their trapping. Savko et al. (2010 and references herein) consider one of the mechanisms of this replacement by the example of the Paleoproterozoic carbonaceous shales of the Tim-Yastrebovka structure in the Voronezh crystalline massif. The authors describe the continuous reactions of REE-rich matrix chlorite breakdown that produce REE phases such as bastnesite and allanite during progressive metamorphism. Above, we noted one more specific feature of the inclusions, namely, radial and concentric cracks (Fig. 3). As known from mechanics (Gillet et al., 1984; van der Molen and van Roermund, 1986; Yang and Jahn, 2000), radial cracks originate from rupture of the host mineral due to the ovepressure within the inclusion. Concentric cracks are, on the contrary, caused by the sagging of the host-mineral walls as a result of the underpressure in the inclusion. Earlier numerous petrological studies showed that such effects arise mainly during phase transformations with great volume changes. A typical example of this process is radiate cracks arising on decompression around coesite inclusions that are replaced by quartz in garnet (clinopyroxene) (Chesnokov and Popov, 1965; Chopin, 1984). Two systems of cracks around the inclusions in mineral are a unique phenomenon. Probably it is due not only to the difference in the PVT-properties of chlorite and the host garnet but also to the partial melting of inclusions. The thermodynamic conditions of diabantite stability are still unknown. But we cannot rule out the mineral decomposition during progres-

Fig. 9. Abundant inclusions of biotite, amphibole, and quartz in a single crystal of orthopyroxene from orthopyroxenite. Photomicrograph of a thin-section segment.


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Fig. 10. Inclusions in a single crystal of orthopyroxene from orthopyroxenite. a, Reaction rims of anthophyllite around quartz inclusions (BSE image), b, primary and secondary fluid inclusions breaking at the contact with mineral inclusions (transmitted-light photo).

sive metamorphism, which might have led to the dehydration melting of inclusions under the PT-conditions of the metamorphism peak (Table 7). This process is known both from experimental works and from studies of natural materials (Perchuk et al., 2005, 2008a,b, 2009). The hypothesis of melting of inclusions agrees with their morphology and localization of the REE phase along the periphery of vacuoles. Note that the great volume effects caused by the melting/crystallization of inclusions explain the nature of radial and concentric cracks around the inclusions. Radial cracks are the result of the pressure decrease in the inclusion (a high pressure arises when the specific volume of inclusion increases during its melting), and concentric cracks are initiated by a reverse process during the melt crystallization. Note that none of the considered versions (metamorphic and magmatic) yields an unambiguous interpretation of all observed facts. Nevertheless, it is obvious that the inclusions considerably transformed during the metamorphic evolution of the rock in the closed system. Therefore, we must again emphasize that the studied inclusions are isolated and the included minerals are lacking beyond the host minerals. Inclusions in orthopyroxenite. Conservation of unaltered fragments of earlier formed parageneses in mineral inclusions is a widespread phenomenon in metamorphic complexes of different ages and depths of occurrence (Perchuk et al., 1985; Thompson et al., 1977). Thus, the mineral inclusions in the orthopyroxenite reflect the peculiarities of the early stage of its evolution. For example, the set of minerals in the inclusions (biotite, amphibole, quartz, and pyrite) evidences that the orthopyroxenite was developed after quartz-biotite-amphibole rock typical of the middle stages of metamorphism. This indicates that in the orthopyroxenite, as in the garnet-pyroxene rock, the water-bearing mineral assemblage was changed by a paragenesis of anhydrous minerals at the progade stage of

Fig. 11. Generalized P-T evolutionary path of metaultramafites of the Gridino complex, computed using the TWQ program (Berman, 1991). The computation was made from the compositions of the cores (stage I) and rims (stage II) of large crystals of garnet and clino- and orthopyroxene and from the compositions of minerals from the retrograde paragenesis (stage III). Numerals mark the number of mineral reactions shown on the diagram. For explanations, see the text.


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Table 7. P-T estimates made using the TWQ software, geothermometers, and geobarometers for garnet and clino- and orthopyroxenes produced at different stages of metamorphism Stage Mineral Grt XMg X I (core) II (rim)
Ca

Software Opx Cpx
Mg

Geothermometers Grt-Opx, H84** Grt-Cpx, P85**

Geobarometers Cpx-Opx, BK90** Grt-Opx, H84** P, kbar 17* 12* 9* T, °C 690* 685* 650* Grt-Opx, BK90** P, kbar 23 13 6

TWQ, B91**

XMg Al2O3 X

XJd T, °C P, kbar T, °C 17 12 9 674 673 647

P, kbar T, °C 17* 12* 9* 669 608 573

P, kbar T, °C 17* 12* 9* 778 660 647

P, kbar T, °C 20 15 14 690* 685* 650*

0.43 0.16 0.74 0.76 0.42 0.16 0.72 1.31

0.86 0.01 690 0.85 0.02 685 0.84 0.00 650

III 0.39 0.16 0.70 1.40 (retrograde paragenesis )

* Given. ** B91, (Berman, 1991); H84, (Harley, 1984); P85, (Powell, 1985); BK90, (Brey and Kohler, 1990).

metamorphism. But in contrast to the garnet-pyroxene rock, the inclusions of primary minerals in the orthopyroxenite show no signs of decomposition during the progressive metamorphism. Remind that the P and T parameters of the metamorphism peak are highly uncertain. Probably, they corresponded to the PT-conditions of the garnet-pyroxene rock. Note one more specific feature of the inclusions in the orthopyroxenite. At a first glance, they can be taken as isolated. But a careful examination revealed that their isolated state was disturbed at the regressive stage of metamorphism, when a fluid penetrated into the vacuoles through the walls of the host minerals (Fig. 10, b). This resulted in reactionary rims of low-temperature anthophyllite. Thus, both types of rocks show transformation of inclusions after their trapping, but in the orthopyroxenite this process is open and in the garnet-pyroxene rock it is close. The tectonometamorphic evolution of metaultramafites of the Gridino complex and the protolith nature. The issues of genetic classification based on geochemical features have been developed in most detail for garnet peridotites and pyroxenites of Phanerozoic fold belts (Brueckner and Medaris, 2000; Carswell et al., 1983; Medaris, 1999; Reverdatto et al., 2008; Zhang et al., 1997). According to this classification, pyroxenites/peridotites (ultramafites) with high contents of Mg and Cr preserve geochemical features of mantle rocks (Mg-Cr type) intruded into the lithosphere as mantle melts or tectonic fragments. Pyroxenites/peridotites of crustal genesis (Fe-Ti type) reflect the composition of subsurface sills and dikes that intruded into the upper Earth's crust before the subduction but were metasomatized and then metamorphosed during the subduction, when they subsided deep into the mantle (Reverdatto et al., 2008). The geochemical difference between these groups of rocks are depicted on the diagrams (Fig. 12) constructed from the literature data (Reverdatto et al., 2008). It is remarkable that the compositions of the Gridino complex metaultramafites either fall into the areas of crustal rocks or located between the areas of mantle and crustal ultramafites. Thus, the geochemical data agree with the conclusion that the rocks experienced considerable transformations during several stages of metamorphism. The formation of anhydrous parageneses in the ultramafites of the Gridino complex took place at the prograde stage of

metamorphism and was accompanied by intense dehydration. The large-scale removal of components from the garnet-pyroxene rock during the metamorphism is evidenced from the presence of chlorite and REE-minerals. Assuming that these minerals composed much of the rock earlier and have been preserved only as inclusions in the rock-forming minerals, we conclude about a valuable fluid reworking of the rock at the prograde stage, which favored the removal of these components from the rock. Obviously, the bulk chemical composition of the garnet-pyroxene rock (which, in addition, underwent regressive metamorphism under amphibolite facies conditions) does not correspond in full measure to the protolith composition. According to Slabunov (2008), the metaultramafites of the Central Belomorian greenstone belt are similar in geochemistry to the metaultramafites of the ophiolite complexes in West Greenland (Jacobson et al., 1984; Jagoutz et al., 1979). There are no geochemical data on REE in the ultramafites of the Gridino complex; therefore, their belonging to ophiolites is still questionable. Moreover, the high-pressure parageneses of the Gridino complex are often associated with continental collision (Dokukina et al., 2009), which agrees with the localization of the eclogites and metaultramafites in a gneiss substrate. Note that the garnet-pyroxene rock is of the same mineral composition as the garnet websterite from the eclogite province in Northeast Greenland (Brueckner et al., 1998). The isotope characteristics (Nd = ­5­16; 87Sr/86Sr = 0.708­0.715) and field relationships of this garnet websterite indicate that its protolith was an integral part of continental crust before the high-pressure metamorphism (Brueckner et al., 1998). Thus, there is much indirect evidence for the collisional nature of the Gridino ultramafites. Nevertheless, as in the case with the Gridino eclogites, this question is still open. Hence, we can only state that the considered prograde transformations of the ultramafites might have proceeded in a downgoing block of both continental and oceanic crust. It is obvious that the established PT-conditions could not have been realized in the abyssal rocks of the mantle wedge, since they are characterized by a higher-temperature regime (Gerya et al., 2002).


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Fig. 12. Binary diagrams (Reverdatto et al., 2008) of a geochemical difference in the contents of rock-forming oxides (wt.%) and trace elements (ppm) between mantle and crustal peridotites/pyroxenites. The contents of rock-forming oxides are not normalized. 1, 2, rocks of the Gridino complex: 1, orthopyroxenite, 2, garnet-pyroxene rock; 3­6, data from Reverdatto et al. (2008): 3, 5, pyroxenites, 4, 6, peridotites; 7, 8, areas of: 7, mantle rocks (Mg-Cr type), 8, crustal rocks (Fe-Ti type).

Conclusions The petrological studies of the metaultramafites of the Gridino complex showed that the primary intrusive igneous rocks of an ophiolite or gneiss complex intensely transformed before the magmatism peak. For example, orthopyroxenite formed after quartz-biotite-amphibole rock at the prograde stage of metamorphism. Garnet-pyroxene rock formed after chlorite-bearing paragenesis under the PT-conditions of eclogitic metamorphism. The metamorphism reactions that proceeded at the prograde stage led to intense dehydration, whereas the regressive stage was characterized by the formation of parageneses including water-bearing minerals. Thus, the studied rocks show an intricate history of transformations with a repeated change of parageneses. We thank O.I. Volodichev and A.I. Slabunov for organization of the field works and fruitful discussions, A.A. Viryus for help with the microprobe analyses, E.M. Spiridonov for determination of REE-minerals in the inclusions, and S.P. Korikovskii for critical comments on the early version of the manuscript and valuable advice. The paper was improved by critical comments by V.V. Reverdatto and E.V. Sklyarov. This work was supported by grants 09-05-01217 and 09-05-00991 from the Russian Foundation for Basic Research.

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Editorial responsibility: V.S. Shatsky