ISSN 1028334X, Doklady Earth Sciences, 2014, Vol. 458, Part 1, pp. 1099–1103. © Pleiades Publishing, Ltd., 2014. Original Russian Text © I.I. Likhanov, V.V. Reverdatto, 2014, published in Doklady Akademii Nauk, 2014, Vol. 458, No. 1, pp. 74–79.
GEOCHEMISTRY
Zoning of Garnets as an Indicator of Metamorphic Evolution in Metapelites of the Yenisei Ridge I. I. Likhanov and Academician V. V. Reverdatto Received March 28, 2014
Abstract—The geochemical patterns of major and trace elements in zonal garnets and the mineral inclusions in them formed by progressive and regressive metamorphism of pelites are established. It is shown that an increase in temperature and pressure led to a decrease in the Y and HREE contents in garnets, and the increase in their contents is related to a decrease in the PTparameters of their formation. A negative corre lation between the CaO and REE contents in garnet indicates their isomorphism. The main reason for the sharp increase in the CaO content in garnets during collision metamorphism is mass transfer between the gar net and the plagioclase. The deviations from this situaiton, which are expressed in simultaneous increase in the grossular component in garnet and the anorthite component in plagioclase, are caused by metamorphic reactions related to the epidote decomposition. The mass transfer of major and trace elements between the reacting phases in metamorphic reactions mostly occurred with preservation of the balance of matter. The mirror shape and the character of the REE patterns of the rockforming minerals relative to the composition of the rock indicate the equilibration of the HREE and Y contents between garnet, the major concentrator of these elements in the rock, and other phases. The balance between the LREEs and HREEs in the rock is achieved by the presence of variable amounts of monazite. DOI: 10.1134/S1028334X14090098
The diagnostic feature of the polymetamorphic processes in metapelites is chemical zoning of garnets expressed in a significant increase (from 1 to 6 wt %) and decrease in the grossular and spessartine compo nents, respectively, synchronously with weak varia tions of other components [1, 2]. Such peculiarities of garnets are typical of the majority of the world thrust areas with polycyclic evolution [3–5]. The elucidation of this problem is of significant theoretical interest, because a redistribution of calcium between minerals of variable composition is the basis for the modern cal ibration of geobarometers [6]. To solve this task, we studied the distribution of major and trace elements in coexisting minerals with real metamorphic reactions in metapelites. Such stud ies are sporadic [7], and no similar works are known in Russia. Metapelites from the riverside outcrops of the Garevka (sample 27) and Yenisei (sample 56) rivers located within the Garevka complex of the Angara region of the Yenisei Range were chosen as objects of study. They include intensely deformed gneisses and crystal schists mostly of Grt + Bt + Ms + Pl + Qz ± St ± Ilm ± Ky ± Chl ± Ep composition (here and hereafter, symbols of minerals are given after [8]). Their poly Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090 Russia email:
[email protected]
metamorphism is clearly traced by the reaction struc tures, chemical zoning of minerals, isotopic dating, and configuration of PTtrends [9]. The specific gar net porphyroblasts with three contrast zones are dis tinctive features of metapelites (Figs. 1a, 1b). The cores are composed of round or ellipsoid garnet with chaotically oriented inclusions of minerals of the matrix. They are rimmed by an intermediate zone of deformed garnet full of black microinclusions of ilmenite and graphite oriented according to the gen eral foliation direction. The outer rim is composed of euhedral garnet. Along with abundant inclusions of the matrix minerals, monazite is present in almost all garnet zones. Epidote and xenotime were found only in the inner zone of garnet from sample 56. The petrological–geochemical studies are based on the contents of major and trace elements in the rocks and minerals analyzed on an Element LAICP MS, a TESCAN MIRA 3 LMU SEM equipped with an INCA Energy 350 EDS, a Jeol JXA8100 micro probe, and a Cameca IMS4f ion microprobe. The inner zones are characterized by an insignificant increase in the content of the grossular (Grs) compo nent at a decrease in the content of the spessartine (Sps) component and in the total Fe# value from the core to the rim (Figs. 1c, 1d). The Crs content strongly increases from Grs3 to Grs12 at the margin between the inner and intermediate zones and is accompanied by a synchronous decrease in the Sps content at weak vari ations in the amount of pyrope and almandine. The
1099
1100
LIKHANOV, REVERDATTO Sample 27 (а)
Sample 56 (b)
B
A
B
A
0.7 mm
1 mm Contents, wt %
(c) 36 35 34 5 4 3 2 1
Grtm
Grtc
(d) Grtr
A 0.5 1.0 1.5 2.0 B Profile length, mm
36 35 FeO 34 MnO 5 MgO 4 CaO 3 2 1
Grtr Grtm Grtc
A
Grtm Grtr
0.4 0.8 1.2 1.6 B Profile length, mm
Fig. 1. Photomicrographs of zonal garnet grains from gneiss of the Garevka (a) and Yenisei (b) areas of the Garevka complex and concentration profiles of major elements (c, d) along the A–B line. Here and hereafter in the text, Grtc, Grtm, and Grtr are the compositions of the inner, medium, and outer parts of the garnet.
outer zone of the garnet grows with a gradual decrease in the Grs and Sps contents with simultaneous increase in the Alm component toward the rim (Figs. 1c, 1d). The increase in the anorthite content from XAn = 0.14 to XAn = 0.25 coupled with the increase in the Grs content in the garnets is a typical peculiarity of plagioclase inclusions of the Yenisei area. The reverse correlation is observed in the concentration profiles of plagioclase from the Garevka area, where the increase in XAn in plagioclase is accompanied by a drop in the Grs content in garnet. Epidote in the rocks of the Yenisei area is a typical pistacite with the Fe/(Fe + Al) content of 0.22. Three metamorphic stages distinct in age, thermo dynamic regimes, and values of metamorphic gradi ents were distinguished and dated in situ in the U–Th bearing minerals on the basis of study of the chemical zoning of garnets from these rocks [9]. At the meta morphic gradient dT/dH = 20–30°C/km typical of orogenesis, lowpressure zonal metamorphic com plexes were formed at the first stage, which is con trolled by zoning of the inner garnet zone, at the end of the Mesozoic and the beginning of the Neoprotero zoic (1050–850 Ma) during the Grenville Orogeny. At the second stage, these rocks underwent Late Riphean (801–793 Ma) collision metamorphism of moderate
pressures with low gradient dT/dH ≤ 10°C/km. These and other peculiarities (strong increase in the Grs con tent with a weak variation in the Fe# value and Prp and Alm components of garnet during transitions to the intermediate Grt generation) are typical of collision metamorphism, which is caused by tectonic thicken ing of the crust as a result of thrusts with further exhu mation to the surface [2, 10]. The last stage was accompanied by synexhumation retrograde dyna mometamorphism (785–776 Ma) with dT/dH ≤ 12°C/km, which reflects the tectonic settings of the fast uplift in the shear zones. The analysis of the behavior of the REEs in meta morphic garnets shows that their patterns depend on the type of zoning. The garnets are characterized by significant depletion in LREEs and enrichment in HREEs, which exceeds the chondrite Yb/La ratio by 10 000 times. The garnets of the progressive metamor phic stage are characterized by a strongly differenti ated REE pattern with a regular increase from LREEs to HREEs, somewhat enrichment in MREEs, La, and Ce, and depletion in HREEs from the core to the intermediate zone (Fig. 2a). A noticeable decrease in the REE sum in the intermediate zone relative to the center by 2.5–3 times due to the decrease in the HREE content is a distinctive feature of this process. DOKLADY EARTH SCIENCES
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ZONING OF GARNETS AS AN INDICATOR OF METAMORPHIC EVOLUTION Mineral/Chondrite 1000
1101
(a) 1Grtc 2Grtc 1Grtm 2Grtm 1Grtr 2Grtr Ep 1Plc 2Plc 1Plm 2Plm 1Plr 2Plr Rock
100
10
1
0.1
0.01
0.001
La
Ce
Pr
Nd
Sm
Eu
Gd
Dy
Er
Yb
Lu
6
10
(b)
105 104
Grt Pl Mnz Ep Ilm Qz Ms Bt Ky St Chl Rock
103 102 10 1 10−1 10−2 10−3
La
Ce
Pr
Nd
Sm
Eu
Gd
Dy
Er
Yb
Lu
Fig. 2. Chondritenormalized [8] REE patterns in zonal garnet, plagioclase, epidote (a), and other minerals (b) participating in metamorphic reactions in comparison with the trace element composition of the rock. 1Grt and 1Pl, sample 56; 2Grt and 2Pl, sample 27. Light and dark gray fields are the garnet and plagioclase spectrums, respectively.
The decrease in the content of HREEs, MREEs, and REE sum along with the increase in LREE content from the intermediate zone to the rim in garnets with regressive zoning of the Yenisei area is clearly corre lated with an increase in the content of all REEs in the newly formed plagioclases (Fig. 2a). In contrast, the garnets of the Garevka area are characterized by an increase in the content of most REEs (except for Y, DOKLADY EARTH SCIENCES
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Dy, Gd) at a coupled decrease in the REE content in the newly formed plagioclases. In both cases, however, the increase in temperature and pressure led to a decrease in the HREE and Y content in garnets and a decrease in these parameters had the opposite effect, which is consistent with peculiarities of the REE pat tern in garnets of the Lapland–White Sea belt of the Baltic Shield [11]. Epidote, the major REE contribu
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REE content (ppm) in minerals of the Garevka complex Minerals Element Grtc La Ce Pr Nd Sm Eu Gd Dy Y Er Yb Lu
2Grt
1Grt Grtm
0.01 0.25 0.04 0.51 0.00 0.08 0.08 0.55 0.51 1.17 0.53 0.91 21.0 7.96 134.8 34.9 1212 321.0 151.0 41.8 182.8 61.5 28.1 7.52
1Pl
2Pl
Grtr
Grtc
Grtm
Grtr
Plc
Plm
Plr
Plc
Plm
Plr
0.00 0.02 0.00 0.07 0.19 0.17 3.17 22.1 274.2 33.4 47.4 5.92
0.00 0.01 0.00 0.09 0.16 0.05 1.93 27.4 336.4 61.1 105.0 11.7
0.01 0.02 0.00 0.05 0.31 0.11 6.01 18.4 198.6 17.3 13.4 1.98
0.07 0.15 0.01 0.14 0.40 0.41 2.97 12.9 124.4 21.0 26.8 3.42
0.05 0.08 0.00 0.04 0.04 0.08 0.01 0.01 0.24 0.00 0.00 0.00
1.87 2.99 0.32 0.84 0.13 1.23 0.11 0.07 0.54 0.02 0.00 0.00
3.05 4.81 0.38 1.41 0.24 3.46 0.28 0.14 0.59 0.02 0.01 0.00
0.09 0.19 0.02 0.11 0.06 0.29 0.02 0.02 0.27 0.00 0.00 0.00
3.98 5.68 0.49 1.60 0.23 2.91 0.17 0.16 0.71 0.03 0.03 0.00
0.83 1.35 0.16 0.51 0.10 0.72 0.05 0.04 0.49 0.01 0.00 0.00
Minerals Element La Ce Pr Nd Sm Eu Gd Dy Y Er Yb Lu
Ep
Chl
Qz
Ms
Bt
St
Ky
Mnz
Ilm
2.529 11.52 2.199 13.01 4.488 4.282 4.888 4.257 31.22 3.434 3.194 0.520
0.028 0.121 0.000 0.028 0.037 0.005 0.012 0.015 0.055 0.003 0.003 0.000
7.249 12.23 0.000 5.494 1.259 1.574 0.246 0.359 1.070 0.336 0.160 0.000
0.555 0.108 0.000 0.535 0.018 1.143 0.025 0.245 0.044 0.291 0.035 0.000
0.317 0.433 0.000 0.179 0.083 0.118 0.043 0.120 0.044 0.034 0.020 0.000
0.004 0.025 0.000 0.007 0.005 0.001 0.002 0.046 0.003 0.004 0.091 0.000
0.032 0.049 0.000 0.008 0.011 0.001 0.012 0.016 0.079 0.009 0.078 0.000
57561 119512 12818 47142 7758 0.000 6505 2177 6300 437 0.000 0.000
0.194 0.086 0.000 0.047 0.913 0.254 0.169 0.079 0.000 0.074 0.585 0.000
1Grt and 1Pl, sample 56; 2Grt and 2Pl, sample 27. c, m, r, composition of the inner, medium, and outer parts of the zonal minerals. 0.000, not analyzed.
tor among other coexisting minerals of metapelites, is characterized by a flat REE pattern, which exceeds that of chondrite by 10–30 times, and by a positive Eu anomaly. It is remarkable that the flat spectrum of epi dote almost corresponds to the REE pattern of the rock and is somewhat lower and slightly higher in the LREE and HREE areas, respectively. In contrast to garnet, monazite is characterized by the opposite (steep negative) REE patterns caused by strong enrichment in LREEs relative to HREEs. Other min erals are depleted in almost all REEs relative to their contents in the rock (Fig. 2b). The type of chemical zoning is a result of the matter redistribution caused by the change in the metamor phic PTparameters. The chemical reactions respon
sible for these mineral transformations are calculated using the matrix algebra in the MATHEMATICA 5.0 program following [12]. The mineral transformations in the course of the regional metamorphism at Р = 4.5–5 kbar and Т = 560–570°С occurred at the expense of the reaction Grt + Chl + Ms → St + Bt + Pl + Qz ± Ep + H2O, which is typical of the zoning of the And–Sil facial series on the petrogenetic lattice for ferrous–alumi nous metapelites [13]. The mineral transformations of two rocks from the studied areas are distinct only in the presence of epidote in the newly formed assemblages (sample 56) at almost equal proportions between the stoichiometric coefficients of reacting and newly formed minerals. DOKLADY EARTH SCIENCES
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ZONING OF GARNETS AS AN INDICATOR OF METAMORPHIC EVOLUTION
Taking into account the peculiarities of variable zoning of garnet and plagioclase in the course of pro gressive collision metamorphism of the rocks of the Garevka area for the KFMASHNaCaTiMn system, the following mineral reaction may be written for Р = 9.3 kbar and Т = 660°С: 0.16Grtc + 0.69Plc + 0.98Qz + 0.11Ms + 0.22St = 0.42Grtm + 0.67Plm + 0.05Bt + 0.07Ilm + 0.221Ky + 0.35H2O. The course of the reac tion is in agreement with synchronous increase in the Grs component in garnet at a decrease in the An con tent in plagioclase at the boundary between the inner and intermediate zones. For the rocks of the Yenisei area with a coupled increase in the Grs content in gar net and the An content in plagioclase, the increase in the CaO content is not a result of redistribution of Ca and Al only between the garnet and plagioclase. Taking into account the observations on the stability of epi dote in the regional metamorphic rocks of the Garevka area and its absence in the products of colli sion metamorphism, it is possible to calculate the fol lowing mineral reaction of progressive metamorphism at Р = 7.9 kbar and Т = 625°С: 0.134Grtc + 0.040Plc + 0.144Qz + 0.011Ms + 0.017Ep + 0.102St = 0.248Grtm + 0.064Plm + 0.010Bt + 0.02Ilm + 0.089Ky + 0.136H2O. This mass balance equation reflects the proportion of the mineral phases at the final stage, when epidote completely disappears in the rock. According to experimental data [14], in metapelites, in contrast to the wide area of stability of epidote in metamafic rocks, the field of its stability significantly decreases under reducing conditions at the expense of an increase in the stability of garnet and plagioclase. At low oxygen fugacity close to the QFM buffer, the boundary of the P–Tstability of epidote in metapelites is less than T = 600°C at P = 5 kbar. The low oxygen fugacity (lower than the QFM buffer) upon formation of the rocks of the Garevka complex is evident from the joint presence of ilmenite and graph ite and also from the high Fe# values in the Fe–Mg minerals [15]. The zoning of garnet and plagioclase changes at the boundary between the intermediate and inner zones: the Grs contents decrease at a simultaneous increase in the An content that is related to the regressive low temperature metamorphism. For the KFMASHCa system, which models the mineral transformations between the minerals–participants of reactions in the course of exhumation of the highly metamorphosed blocks to the surface at Р = 4.6–4.8 kbar and Т = 500– 560°С, such mineral transformations for sample 56 may be written as follows: 0.584Grtm + 0.221Plm + 0.328Qz + 0.148Bt + 0.034H2O = 0.494Grtr + 0.467Plr + 0.131Ms. The Garevka area is character ized by similar proportions between the stoichiometric coefficients of reacting and newly formed minerals. The following conclusions may be drawn on the basis of our studies. The garnets of metapelites from the Yenisei Range demonstrate different types of pro gressive and regressive zoning. An increase in temper DOKLADY EARTH SCIENCES
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ature and pressure led to a decrease in the HREE and Y contents in garnets, and the decrease in PTparam eters is related to the increase in the content of these elements. The negative correlation between the CaO and REE contents in garnet shows that they occupy the same crystal chemical position [9] and isomorphi cally replace each other in metamorphism. The main reason for the strong increase in the CaO content in garnet in collision metamorphism is its gain from pla gioclase, which became more sodic in composition. The deviations from this rule expressed in a simulta neous increase in the grossular component of garnets and the anorthite component of plagioclases are caused by the reaction with epidote (its decomposi tion). The mirror character of the REE patterns of the rockforming minerals relative to the composition of the rock indicates the equilibration of the HREE and Y concentrations between the garnet, which is a major concentrator of these elements in the rock, and all other phases. The balance of the LREEs and MREEs in the rock is achieved due to the presence and varia tion in the amount of monazite (Fig. 2b). A stricter analysis of mass transfer is possible only by using data on the volume proportion of phases. REFERENCES 1. I. I. Likhanov and V. V. Reverdatto, Russian Geol. Geophys. 55 (3), 299–322 (2014). 2. I. I. Likhanov and V. V. Reverdatto, Intern. Geol. Rev. 53 (7), 802–845 (2011). 3. I. I. Likhanov, O. P. Polyanskii, V. V. Reverdatto, et al., Geol. Geofiz. 42 (8), 1205–1220 (2001). 4. I. I. Likhanov, P. S. Kozlov, N. V. Popov, et al., Dokl. Earth Sci. 411 (1), 1313–1317 (2006). 5. I. I. Likhanov, V. V. Reverdatto, P. S. Kozlov, et al., Petrology 16 (2), 136–160 (2008). 6. E. D. Ghent and M. Z. Stout, Contrib. Mineral. Petrol. 76, 92–97 (1981). 7. F. Nehring, S. F. Foley, and P. Holtta, Contrib. Mineral. Petrol. 159, 493–519 (2010). 8. I. I. Likhanov, Geol. Geofiz. 44 (4), 305–306 (2003). 9. I. I. Likhanov, V. V. Reverdatto, P. S. Kozlov, et al., Petrology 21 (6), 561–578 (2013). 10. I. I. Likhanov, V. V. Reverdatto, P. S. Kozlov, et al., Rus sian Geol. Geophys. 50 (12) 1034–1051 (2009). 11. C. G. Skublov, REE Geochemistry of the RockForming Metamorphic Minerals (Nauka, St. Petersburg, 2005) [in Russian]. 12. I. I. Likhanov, V. V. Reverdatto, and I. Memmi, Eur. J. Mineral. 6, 133–144 (1994). 13. I. I. Likhanov, V. V. Reverdatto, and A. Yu. Selyatitskii, Petrology 13 (1), 73–83 (2005). 14. S. Poli and M. W. Schmidt, Rev. Mineral. Geochem. 56, 171–195 (2004). 15. I. I. Likhanov and V. V. Reverdatto, Geochem. Intern. 52 (1), 1–21 (2014).
Translated by I. Melekestseva
