PETROLOGICAL FEATURES OF THE MAKSYUTOV AND THE ATBASHI ECLOGITE-GLAUCOPHANE-SCHIST COMPLEXES WITHIN THE URAL-

MONGOLIA FOLD BELT

FEDKIN, Valentin V., NOVIKOV, Gennady V., and FEDKIN, Alexey V. Institute of Experimental Mineralogy, Russian Academy of Sciences

Chernogolovka, 142432, Russia, fedkin@iem.ac.ru

300 400 500 600 700 8000

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Grt-Chl

Grt-Gl

Grt-Cpx

And

Sill

Ky

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kba

r

Temperature, oC

118-1

Abstract:

According to Miyashiro's concept of paired metamorphic belts, the Maksyutov and the Atbashi eclogite-glaucophane-schist complexes (Russia) belong to the united Ural-Mongolia Fold Belt, but are related to its different branches – outside and inside, respectively. Hence, our main task was to study the metamorphic evolution of these regions in comparison on the basis of petrological, experimental, and theoretical data on mineral equilibria.

Preliminary geochemical data obtained for eclogite samples from the Maksyutov complex indicate that the eclogite protolith most likely corresponds to the MORB basalts rather then island arc basalt. The prominent negative Eu anomaly in the Atbashi rocks suggests that plagioclase was a subliquidus phase during the basalt formation, which contradicts the hypothesis of the magmatic origin of the eclogitic garnet. So, magmatic origin is suggested for the Maksyutov eclogite-glaucophane-schist complex. On the other hand, the REE spectra of rock samples do not provide an answer to the question if the complexes were formed under identical geological and tectonic conditions. In this study, we attempted to use the REE distribution in rock-forming minerals to get answer for this problem.

We used the analytical data on the major rock-forming mineral compositions and geothermobarometry database to restore the P-T history of the eclogite-glaucophane-schist complexes. The “clockwise” P-T path with a low geothermal gradient (about 10 deg. per km), typical for collision zones, was determined for the first metamorphic stage of the Atbashi complex. At least, allochemical metamorphic conditions were determined for the Atbashi eclogite-glaucophane-schist complex.

The extensive studies of both Maksyutov and Atbashi clinopyroxenes by X-ray powder diffraction and Moessbauer spectroscopy revealed several structural modifications in the Jd-Aeg-Di solid solution, which have different elastic properties. More detailed information was obtained on the local structure of these modifications. The studied omphacites have the perfect structure, characterized by local cation ordering in M1 and M2 sites (apparently, sp. gr. P2/n), which could be saved under changing metamorphic conditions. These data may be resulted in corrections in clinopyroxene thermodynamic properties used in thermobarometry.

Fig.1. Tectonic map of Eurasia (by A.L.Yanshin, 1966) and locations of the Maksyutov (Mks) and the Atbashi (At) eclogite-glaucophane-schist complexes.

Fig.2. Prograde and retrograde P-T paths for the Maksyutov (a) (Leech & Ernst, 1998) and the Atbashi (b) (Fedkin, 2004) complexes. The solid curve is based on possible presence of coesite and graphite pseudomorphs after diamond; dashed and dotted curves are based on the thermobarometric calculations and petrographic studies of eclogites and related rocks.

(a)

(b)

Eclogite-glaucophane-schist complexes mark the major suture-collision zones in the Ural–Mongolia Fold Belts and can be reliable indicators of the principal collision stages, which predetermines acute interest in these rocks. The Maksyutov metamorphic complex in the Southern Urals (Mks) formed during early collision of the East European Platform and a variety of microcontinents in the Late Paleozoic. Isotopic dates for various rocks and minerals from this complex obtained by various methods constrain the age interval to 390–365 Ma. The mineral assemblages, compositions of minerals, and mineralogical thermometric data indicate that the eclogite-glaucophane-schist rocks were metamorphosed at P=15–23 kbar and T=550–700°C (Beane et al., 1995; Lennykh et al., 1995; Dobretsov et al., 1996; Hetzel et al., 1998; Schulte and Blümel, 1999; Volkova et al., 2004). Considering the occurrence of quartz pseudomorphs after coesite (Chesnokov and Popov, 1965; Dobretsov and Dobretsov, 1988) and graphite cuboids after diamond (Leech and Ernst, 1998), the pressure during the early metamorphic stage could have reached 27-32 kbar.

The Atbashi eclogite-glaucophane-schist complex (At) is located in the Southern Tien-Shan and connected with the Main Kansk-Atbashi Depth Fault between the Northern and Southern Tien-Shan. It is involved in the Hercynian folded terrain and forms Precambrian basement of the region. Detailed petrographic studies of the eclogite-glaucophane and other associated rocks included in the Cheloktore suite, indicate that these rocks were formed at high pressure and intermediate temperature conditions (Fig. 2b) as a result of the joint action of metamorphic and metasomatic processes. The available petrochemical data and the observed sequence of the mineral reactions that took place during the transformations of the high grade metamorphic rocks of the complex (eclogites and eclogite-like rocks with garnet-clinopyroxene mineral assemblages) to glaucophane-bearing and muscovite-quartzitic schists, testify that the chemical composition of the rocks was changing during the Atbashi complex formation. These changes appeared as the decrease of Ca, Mg, and Fe contents in the rocks, smooth increase of Si, Al, and K contents, and increase of sodium activity at the different stages of metamorphism. These processes testified against the background of the total prograde metamorphic evolution of the complex.

Isotopic data for various rocks and minerals from the Atbashi complex correspond to a wider age interval for different rock formation than for Maksyutov’s rocks: 292-427 Ma. However, there are also older data – 567-568 Ma, and even 1100 Ma (Dobretsov, 1974).

Intensive microprobe studies of the main rock-forming mineral composition, mineral zoning, inclusions, and contact zones (more than 450 analyses) indicate different prograde-retrograde P-T trends for both primary (garnet-clinopyroxene) and secondary (garnet-glaucophane) mineral assemblages, as well as for greenschist assemblages (Fig.2b). The prograde zoning of garnet and the retrograde transformation of rocks (glaucophane growth, muscovitization, carbonatization etc.) are interpreted in terms of abundant acid leaching metasomatism and metamorphic processes.

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Sample 1-157b

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- Grt-Cpx, c, m- Grt-Cpx, r, ct- Cpx-Am, c, m- Cpx-Am, r, ct- Grt-Am, c, m- Grt-Am, r, ct- Grt-Chl, c, m- Grt-Chl, r, ct

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Sample At-054

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- Grt-Cpx, c, m- Grt-Cpx, r, ct- Cpx-Am, c, m- Cpx-Am, r, ct- Grt-Am, c, m- Grt-Am, r, ct- Grt-Chl, c, m- Grt-Chl, r, ct

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Sample At-052

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Sample B-24k

Ky- Grt-Cpx, c, m- Grt-Cpx, r, ct- Cpx-Am, c, m- Cpx-Am, r, ct- Grt-Am, c, m- Grt-Am, r, ct- Grt-Chl, c, m- Grt-Chl, r, ct

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Fig. 2b. Representative samples (blue) and final PT paths for the various Atbashi rocks.

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whitlockite

Sphene

Grt

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AT-032

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rutile

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II. Protolith crystallization sketch for the representative samples of the Atbashi and Maksyutov eclogites, calculated by the MELTS program (Ghiorso & Sack, 1995).

(Equilibrium calculation in closed dry system)

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Potassium feldspar can be formed during crystallization of the Maksyutov protolith at high pressure (near 20 kb, Fig. 7c ) , while Pl is usually formed in the Atbashi protolith (Fig. 7b). Minor K-Na feldspar, referred as Fsp1, could occur in the Atbashi protolith (Fig. 7d).

Cpx of the Atbashi protolith is predominantly Ca-rich, whereas a wider range in Cpx composition from Ca-rich to Ca-poor (clino-enstatite) is more typical for the Maksyutov complex.

Aqueous minerals (Bt, Mu) appear during crystallization of water bearing (1.5 wt. %) protolith system at Ptotal=10 kb (Sample At-030) (Fig. 7f).

Protolith of the least altered eclogite sample (At-030) is a rock poor in silica: Ol is the First mineral on liquidus, Sp and Lc are present sometime. However, Qtz appears in water system at Ptotal=10 kb (Fig. 7e, 7f).

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Fig. 7. Results of the MELTS program calculations for the Atbashi and the Maksyutov least altered eclogites.

The MELTS program calculation was carried out on the assumption that the rock compositions were not changed during metamorphic processes. The following characteristics were found for the Maksyutov and the Atbashi complexes:

The protolith of the Atbashi eclogites contains Cpx as the liquidus phase, whereas Ol (small) or Opx would be the liquidus phases for the Maksyutov protolith.

(a) (b) (c)

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Mks-Cpx

Atb-Grt

Mks-Grt rim

Mks-Grt core

Atb-Grt

Atb-Cpx

Mks-Cpx

Mks-Grt

Atb-Cpx

I. Protolith composition for the Maksyutov and Atbashi rocks.Estimates based on 26 analyses for major, REE and trace elements

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Ba

Rb

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Mks-complex Atb - complex(a) (b) (c)

Fig. 5. Representative samples of the least altered Mks and Atb eclogites (a); and all studied patterns (b)-(c) in the REE diagrams, normalized to the C1 chondrite.

The prominent negative Eu anomaly in the Atbashi rocks suggests that plagioclase was a subliquidus phase during the basalt formation, which contradicts the hypothesis of the magmatic origin of the eclogitic garnet and magmatic nature on the complex.

On the other hand, the REE spectra of the Maksyutov rocks do not change with alteration, whereas the Atbashi rocks become more LREE-rich with retrograde processing.

(a) Mks-complex (b) Atb-complex

Fig. 4. The C1 Chondrite-normalized spider diagrams for trace elements.

The spider spectra for trace elements are approximately the same for both the Maksyutov and the Atbashi complexes and are not principally changed in differently altered rocks: the least altered eclogites (red), Grt-Cpx-Gl moderately altered rocks (blue), and retrograde most altered Grt-Chl-Mc schists, quartzites etc (green).

Fig. 6. REE spectra for Grt and Cpx normalized to the C1 chondrite.

Both Grt and Cpx REE spectra demonstrate the same tendency: the Mks-minerals fortified by the REE in comparison with the Atb-minerals. At the same time, the REE are accumulated in the rims of the Grt grains more than in the cores.

Garnet always shows the tendency of concentrating HREE rather than LREE, whereas clinopyroxene is symmetrically depleted in both HREE and LREE.

CAB

IAT

MORB

OIT

OIA

MnO*10 P2O5*10

TiO2

CAB

IAT

MORB

OIT

OIA

MnO*10 P2O5*10

TiO2

(b) Atb (a) Mks

48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 650.0

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Calc-Alkaline

SiO2

FeO

t/MgO

Mks

Atb

(c)

Tholeiitic

Calc-Alkaline

Na2O+K2O MgO

FeOt

(d)

Fig. 3. Major elements discrimination diagrams for the Maksyutov and the Atbashi differently altered rocks: solid symbols –

the least altered eclogites, half-empty symbols – moderately altered rocks, and open symbols – retrograde most altered schists.

Proposed protolith composition is slightly different for the Maksyutov and the Atbashi complexes. The Maksyutov rock composition points mainly fall around the MORB area, but the Atbashi rock points mostly correspond to the Island Arc Tholeiites (IAT) or the Calc Alkaline Basalt (CAB) fields.

III. OMPHACITES: STRUCTURE FEATURES, LOCAL FIELDS AND THE LOCAL ORDERING EFFECTS

To discover the physical motives of phase transformations in natural Cpx-bearing systems, the elastic properties of pyroxene structural modifications and distribution of local fields in the key positions M1 were studied by the X-ray powder diffraction and the NGR methods.

Fig.8. Topological -niches of C2/c-I, C2/c-II, C2/c-III, C2/c-IV and P pyroxene structures and three parts of the general trend – V for natural pyroxene solid solution Jd-Aeg-Di, corresponding to three structural modifications: Jd, P-Omphacite (Omph. Р2/n – great rhomb) and Di.

The niches {(a-, b-, c-, ) – V} of topological stability of distinct monoclinic structural modifications of Ge-Si pyroxene solid solution (Li,Na,Ca,Mg,Fe,Al)2(Si,Ge)2O6, having different elastic properties (Fig.8), were studied in [1-4].

Three main constituents could be distinguished in any pyroxene structure – the metal-oxygen polyhedra M1, M2 and the tetrahedra SiO4. Due to common corners and edges these polyhedra form the “multistage hinge” (like the two-angle transmission system in the car). The most conservative subsystem is the zigzag chain of regular octahedra M1. The roles of M2 polyhedra and Si-tetrahedra – to get the topological stability of the pyroxene structures, when possible, and just the “multi-hinge” chain M1-M2-T determines the elastic properties of structural modification of the chain structure.

Fig.9. X-trends “Q.s. – I.s.” for the main M1 and, M1a, M1b doublets: Omphacites and Clinopyroxenes. 300K: HP-CFs (C2/c-I and P21/c), HP-Hd, Jad, C2/c, Na- and Fe-substituted Ge-Hd (C2/c-II). 90-300K: Cpx P21/c, Hd (C2/c-II), Ge-Hd (C2/c-II), Fe- substituted Ge-Hd (C2/c-II; Ca0.8Fe

0.2 ).

The QS-IS trends of Fe2+(M1) in the Ge-Si solid solutions were studied recently in detail [1-4]. In NaxCa1-xFeGe2O6 solid solution Fe2+ ions should occupy only the M1 position. This allows, using the 57Fe gamma-resonance spectroscopy, studying the distribution of the electron structure of Fe2+(M1) at gradual substitution of Ca ions by Na ions in the M2 position. Results of such research at T=300K are presented in the Fig. 9. To improve the resolution of spectra, the original N-procedure [5] was used. It permitted to resolve some of overlapped components at the cost of signal-to-noise ratio and allowed to make a number of general conclusions [1].

On replacement of Ca ions by Na ions in the М2 position, while the fraction of sodium is < 0.5, the main part of ions Fe2+ in the М1 position keeps their electronic structure, characteristic for hedenbergite and for its Ge-analog. The similar behavior of main doublet М1 was found [1] for the Ca-rich members of the FexCa1-xFeGe2O6 solid solution by detailed studies at low temperatures (88 - 300К). Except for a

doublet of the Fe2 + ions, dominating in the М1 position and keeping their electronic structure, in a spectrum, at substitution Ca Na in M2 positions, appears one more doublet (М1а) having greater quadrupole splitting.

Three doublets of Fe2+ with distinct HFS in the M1 polyhedra were found in omphacite spectra [6]. It proves the existence of M1 octahedra with three distinct distortions, induced by “guest-atoms” in their nearest M2 site.

The “host” cation of M1 polyhedra predetermines the aggregate size and form in C2/c pyroxenes, having complex composition. If two “host” cations exist in the M1 sites, local ordering of cations with contrast properties could induce the hybrid structural P-modifications due to their cooperative M1-M2 ordering with different degree of order, depending on the “P-T story” of such pyroxenes, as omphacites.

The natural and synthetic Jd, omphacites and Di (Hd) give the general trend, which consists of three nearly linear parts. The short inner part of the trend corresponds to P- and two others – to C-structures. Together with the data on Ge-analogs of the Ca-Na pyroxene structures the trend of omphacites forms the general Si-Ge niche of P-structures, having the double chain T.

In natural omphacites P2/n [6] the chains M1 and M2 are also splinted (each in two). Apparently, two such “complex hinges”: M1a-M2a-Ta and M1b-M2b-Tb exist in this specific structure. The studies of local fields in M1 positions support this suggestion [1, 3, 4, 6].

Comparative analyses of local fields in natural Omphacite, Jadeite and Di-Aeg solid solutions

Two distinct satellites of Fe2+ (M1) doublet: M1a and M1b?

Influence of M2 neighbor on the electron state of ion Fe2+ in distinct М1 polyhedra?

1 .10 1 .1 5 1 .2 0 1 .25 1 .3 0I .s ., m m /s

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., m

m/s

C 2 /c -IH P -C F s

O m -11 0

Ja d (C 2 /c) -

O m (P 2 /n ) - O m (C 2 /c ) -

P 2 1 /cH P -C F s

j1 3 9N a .o 2C a .o 8

j1 3 9

J a d

M 1 a

M 1

M 1 b

3 8 0 4 0 0 4 2 0 4 4 0 4 6 0 4 8 0 5 0 0V (A 3)

1 0 3

1 0 5

1 0 7

1 0 9

G e-H d

H P -C F s

G e-D i

H d

G e-A eg .J a d .C 2 /c -III

A eg .

D i

(N a -L i)-A eg . (N a -L i) G e-A eg .

G e-C F g

P 2 /n

Conclusion and remarks.The Atbashi and the Maksyutov eclogite-glaucophane-schist complexes related to different branches of the Ural-Mongolia Fold Belts are characterized by the following features:

Both the Maksyutov outer and the Atbashi inner complexes were generally formed during a single global Earth event - the Hercynian Orogeny. The age estimates for the Atbashi complex cover wider time interval than the Maksyutov complex. However, it might result from more intense retrograde metamorphic processes and smaller depth of the complex formation.

Considering the occurrence of quartz pseudomorphs after coesite and graphite cuboids after diamond, the Maksyutov complex is characterized by much deeper formation during the early metamorphic stage. Perhaps, the outer position of the complex within the Ural-Mongolia Fold Belts is a reason for that.

According to different discrimination and trace element spider diagrams, proposed compositions of the protolith for both complexes are not clearly associated with any certain type of basalts. The least altered eclogite samples of the Maksyutov terrain were formed perhaps from the Mid Ocean Ridge Basalts, whereas the Atbashi eclogitic and related rocks were probably formed from the Island Arc Tholeiites or the Ocean Floor Basalts.

Hypothesis of magmatic origin of the Maksyutov eclogites seems to be more preferable than the model of the descent of the Earth crust terrain. This idea is supported by physicochemical modeling (MELTS program) of its formation, mineralogical evidence, and proposed protolith composition.

Two satellites of main Fe2+(M1) doublet with distinct quadrupole splitting, found in omphacite NGR spectra, prove the existence of M1 octahedra with three distinct distortions, induced by Ca, Mg, Fe2+ or Na cations in the nearest M2 site. The local M1-M2 ordering phenomenon induces the P2/n =>C2/c transition, which depends on T and P and could be used for analyses of the structural features of cation ordering in natural omphacites, their “P-T story”, as well as their thermodynamic properties. 

Acknowledgements

Authors are very grateful to Ekaterina Bazilevskaya and Mark Fedkin (Penn State University) for technical support and help with translation.

We also thank Edward S. Grew (University of Maine) for constructive review of the abstract.

This study was supported by the Russian Foundation for Basic Research, project # 05-05-64561 and 05-07-90318.

Our participation in the GSA Meeting was kindly supported by the International Division of the Geological Society of America.

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