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УДК 552.163
AGE AND METAMORPHIC CONDITIONS OF THE GRANULITES FROM CAPRAL-JEGESSKY SYNCLINORIA, ANABAR SHIELD
Lyudmila Yu. SERGEEVA1, Aleksei V. BEREZIN2, Nikolai I. GUSEV1, Sergei G. SKUBLOV2, Aleksei E. MELNIK2
1 A.P. Karpinsky Russian Geological Research Institute, Saint-Petersburg, Russia
2 Institute of Precambrian Geology and Geochronology RAS, Saint-Petersburg, Russia
The paper presents the results of the isotope, geochemical and thermobarometric study of plagio-crystaUine schist containing in the Upper Anabar series of the Anabar Shield. Granulite complexes of the paleoplatforms are the most important issue in addressing the fundamental problem of the Earth's crust origin and its composition. The early stages of crust formation which correspond to the deeply metamorphosed rocks of the platform basements, available for study within the shields, are of particular interest. The study of the age and metamorphic conditions of granulites by the case of the Upper Ananbar series allows specifying the stages the Anabar Shield's ancient crust formation.
Isotope-geochemical (U-Pb geochronology for zircon and Sm-Nd for garnet-amphibole-WR) and thermobarometric (Theriak-Domino) studies of plagio-crystalline schist allowed to identify two Paleoproterozoic meta-morphism stages within the territory of the Anabar Shield with an age of about 1997 and 1919 million years. The peak conditions of granulite metamorphism are determined as 775±35 °С and 7.5±0.7 kbar, the parameters of the regressive stage are 700 °C and 7 kbar. The sequence of the rocks metamorphic transformations can be assumed: high-thermal metamorphism of the granulite facies (T < 810 °C) and subsequent sub-isobaric (about 7 kbar) cooling to 700 °C with a water activity increase and formation of Grt-Amp paragenesis corresponding to the transition from the granulite to amphibolite facies. Data on the REE and other trace elements distribution in zircon and rock-forming minerals obtained by the ion microprobe analysis contribute significantly to the isotope-geochemical data interpretation.
Key words: granulites, geochemistry, U-Pb age, Anabar Shield
How to cite this article: Sergeeva L.Yu., Berezin A.V., Gusev N.I., Skublov S.G., Melnik A.E. Age and Metamorphic Conditions of the Granulites from Capral-Jegessky Synclinoria, Anabar Shield. Zapiski Gornogo instituta. 2018. Vol. 229, p. 13-21. DOI: 10.25515/PMI.2018.1.13
Introduction. Granulite complexes of the paleoplatforms are the most important issue in ad-dresing the fundamental problem of the Earth's crust origin and composition. The early stages of crust fomation which correspond to the deeply metamorphosed rocks of the platform basements available for study within the shields are of particular interest. The Anabar Shield is characterized by the wide distribution of the granulite facies metamorphic rocks. The study of the age and metamorphic conditions of granulites by the case of the Upper Ananbar series allows specifying the stages the Anabar Shield's ancient crust formation.
The paper presents the results of the isotope, geochemical and thermobarometric study of pla-gio-crystalline schist from the Upper Anabar series of the Anabar Shield.
Geological framework. The largest outcrops of the Archean granulite formations are located in the central part of the Anabar Shield (Siberian platform) where they occur in the Daldyn and Dzhelindi blocks of sub-meridional extension. The more studied Daldyn block shows, in general, the anticlinorium structure [2]. In its axial part based on the Dalynian series granulites outcrops, the Bekelekh anticlinorium was determined. The Bekelekh anticlinorium passes to the Kotoikansky synclinorium in the west and to the Kapral-Djeges synclinorium (KDS) in the east composed of granulites from the Upper Ananbar series. The KDS is traced for over 200 km through the entire Anabar Shield, in the south of the Jeges River source in the north-western direction to the Kotuikan River headwater and further northward it is hidden under the platform cover. The Upper Ananbar KDS series is mainly composed by hypersthene-, bipyroxene, biotite-hornblende plagiogneisses and gneisses. The interbeds of high-alumina and calc-silicate rocks are presented in combination with quartzites, magnetite-bearing crystalline schist, dioxide-scapolite and graphite-bearing gneisses. The thickest (600-800 m) and laterally extended rocks complexes are confined to the bottoms part of the Upper Ananbar series. In the upper part of the section the paragneisses are less frequent and their thickness decreases. A significant part of the Upper Anabar series is presented by mafic crystalline shales forming lenticular horizons up to 600 m thick. The series is completed by the horizon of biotite-hornblende plagiogneisses with rare paragneses interlayers. The horizon is confined to the
V,Y i \u 2 M 3 I 1 • 1 * 1I 1 4 i i ° A 9 r ° 1 5 V 6 ^174
7
Fig. 1. The geological structure of the right-hand side of the river Bol. Kuonamka in the area of its confluents Kuogastah
and Dzheges (based on A.V.Berezin et. al. [2])
1 - bipyroxene-plagioclase melanocratic gneisses, less frequently crystalline schist; 2 - garnet, biotite-garnet, hypersthene-garnet gneisses and plagio-gneisses; 3 - amphibolites and amphibole crystalline schists; 4 - quartzite and quartzite schists; 5 - garnet-bearing crystalline schists;
6 - alaskite granites; 7 - sampling points for isotope dating The sidebar shows the outline of the Anabar shield and the granulites range within: 1 - Daldyn block; 2 - The Dzhelendin block;
3 - axis of the Dzheges synclinorium; 4 - studied area
core of the Kapral-Dzheges synclinorium and is especially well traced in the interfluve of the Bol. Kuonamka and Jeges rivers. This particular synclinorium core was examined on the right-hand side of the river Bol. Kuonamka from the mouth of its left confluent Kuogastah to approximately 7 km downstream to the mouth of the Dzheges river (Fig.1).
The studied area is distinguished by the extremely monotonous rock set. Dark gray and brownish quartz-bearing melanocratic bipyroxene-plagioclase crystalline schist with a schlieren texture, sometimes turning into mafic lenticular-banded crystalline schist prevail almost everywhere. In the migmatitic species, a coarse banding crushed into simple open folds, with leucosome lenticular areas in the fold hinges. Leucosome is represented by enderbites (hypersthene plagio-gneisses), also involved in folding. The thickness increase is specific to enderbites located in the cores of the antiform folds. Occasionally, there are garnet-bearing crystalline schist with small garnet aggregates up to 1.5 cm across surrounded by quartz-plagioclase leucocratic intergrowth. In addition, elongated garnet grains are at whiles included in polysynthetically twinned plagioclase grains. There is no cli-nopyroxene in the garnet-bearing crystalline schist, and garnet coexists with orthopyroxene only. The imposed amphibolization appears as isolated hornblende areas (brownish in thin sections), varying in size from the first centimeters to lenticular amphibolite segregations, sometimes up to 1 m in diameter, composed of medium-grained hornblende and a plagioclase in various amounts, up to its complete absence. Hornblende replaces both pyroxenes and garnets.
The chemical composition of the main granulites corresponds to the magnesian and meta-alumina mafic rocks of the tholeiitic series, with low and moderate potassium content (K2O -- 0.36-0.87 %, mg# 35-51; A/NK = 2.45-3.20; A/CNK = 0.69-0.92).
Analytical methods. The garnet-orthopyroxene plagio-crystalline schist from the Upper Anabar series was under detailed isotope geochemical study. The composition of rock-forming minerals was determined at the Institute of Precambrian geology and geochronology RAS (by O.L.Galankina) using a JEOL JSM-6510LA scanning electron microscope with an energy dispersive detector JED-2200. The zircon U-Pb dating was carried out in the A.P.Karpinsky Russian Geological Research Institute on the SHRIMP-II ion microprobe by standard methods. The content of REE and
other trace elements in zircon and rock-forming minerals was determined on the ion microprobe analyzer Cameca IMS-4f in the Yaroslavl Branch of the Institute of Physics and Technology (by S.G.Simakin, E.V.Potapov) using the released methods [4, 5, 9]. The zircon chemical composition was analyzed at the particular points of previous U-Pb dating. In the REE spectra scheduling, the composition of zircon and rock-forming minerals was normalized to the chondrite CI composition [12]. The list of mineral abbreviations is given in Ref. [18].
Results.
Minerals composition. The garnet-orthopyroxene plagio-crystalline schist (sample 174) is composed of Pl (53 %), Amp (32 %), Opx (10 %), Grt (4 %). Subhedral plagioclase grains reach 0.5 mm in diameter and correspond to andesine with anorthite content up to 43 %. All K-feldspar grains contain about 5 % albite components and adjoin smaller (up to 0.2 mm) plagioclase grains.
The composition of orthopyroxene euhedral grains which are up to 0.5 mm in size is a mixture of ferrosilicon and enstatite in equal proportions with an insignificant value of the wollastonite component. The Al content in orthopy-roxene does not exceed 0.1 apfu. Clino-pyroxene with a magnesia component exceeding 0.6 is almost always associated with orthopyroxene and contains no more than 6 % of the jadeite component.
The xenomorphic garnet aggregates probably fill out a weakened zone (cracks) with a thickness of up to 1 mm. There is also single garnet grains formed inside the plagioclase matrix together with amphibole. The pyrope-grossular-almandine composition of garnet is specified by Alm component of about 60 % with an increased content of Sps component - 4 %, which indicates retrograde metamorphic conditions. The Grs component (17-19 %) prevails over Py (14-17 %).
Garnet/Chondrite 1000
100
10 .
0,1
0,01
• 22 ♦ 23 □ 28 A 29
—1-1-1-1—
La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu
Pyroxene/Chondrite 10
0.1
0.01
La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu
Amphibole/Chondrite 1000 :
100
10
♦ 24 □ 25 ▲ 30 O 31
La Ce Pr Nd
Sm Eu Gd
Dy
Er
Yb Lu
Fig.2. REE distribution spectra in garnets (a), pyroxenes (b) and amphiboles (c) from garnet-orthopyroxene plagio-crystalline schist (sample 174) The points numbers correspond to Table 1
1
b
1
c
The garnet crystals are generally homogeneous in the CaO, FeO, MnO, and MgO distribution. There is only a slight decrease in the MnO content from core to rim. Zoning of this type tends to be characteristic for garnets formed at high metamorphic grade settings [1] and also implies retrograde formation.
Amphibole replacing pyroxenes with magnesia content of about 0.5 refers to the calcium group and can be represented as a mixture of 70 % pargasite, 20 % ferrotschermakite and 10% glauco-phane components. Amphiboles of this composition are characteristic of the amphibolite facies metamorphism, which does not contradict its paragenesis with a garnet, rich in an almandine component.
REE in minerals. The rare earth elements study of garnets from plagio-crystalline schist revealed typical enrichment in HREE with a minimum at Eu (Eu/Eu = 0.63) (Fig.2, a). There is no zoning for LREE and HREE within the garnet crystals, but only enrichment in HREE in going from core to rim.
The distribution of REE in orthopyroxenes is slightly differentiated (Fig.2, b) with the total REE content not exceeding 1 ppm. The magnitude of the Eu anomaly varies up to it the disappearance (Eu/Eu* = 0.51-1.01).
The total content of REE in amphibole is 334-371 ppm with the predominance of LREE (291-327 ppm), significant differentiation ((La/Yb) N = 5.19-7.59), and moderate negative Eu-anomaly (Eu/Eu = 0.57-0.71) (Fig.2, c). Amphibole is also characterized by increased contents of Ti (17997-20704 ppm), V (495-700 ppm), and Zr (127-172 ppm) (Table 1).
Table 1
The content of major (wt%), trace and rare-earth elements (ppm) in garnets, orthopyroxenes and amphiboles
from plagio-crystalline schist (sample 174)
Compound/ element Garnet Orthopyroxene Amphibole
22 core 23 rim 28 rim 29 rim 26 27 32 33 24 25 30 31
SiO2 37.6 38.0 37.7 38.3 51.0 50.8 50.0 50.7 42.1 41.8 42.6 42.0
TiO2 - - - - - - - - 2.59 2.40 2.99 2.81
AI2O3 21.9 21.5 21.4 20.8 0.76 1.32 1.28 1.14 13.0 13.2 12.4 12.4
FeO 27.7 27.9 28.5 28.1 30.1 29.9 31.0 30.8 17.9 18.2 17.8 18.9
MnO 1.65 2.03 2.18 1.87 0.71 0.73 0.71 0.69 0.32 0.18 - 0.27
MgO 4.15 3.81 3.99 4.25 16.9 16.8 16.6 16.0 9.29 9.14 9.23 9.29
CaO 6.92 6.80 6.25 6.65 0.47 0.48 0.46 0.60 11.5 11.9 11.6 11.1
Na2O - - - - - - - - 1.79 1.67 1.88 1.63
K2O - - - - - - - - 1.59 1.55 1.58 1.49
La 0.02 0.05 0.05 0.02 0.02 0.01 0.08 0.02 33.4 36.7 37.9 35.7
Ce 0.29 0.19 0.22 0.19 0.08 0.05 0.06 0.04 109 125 126 122
Pr 0.15 0.11 0.10 0.11 0.01 0.01 0.01 0.01 19.3 21.4 21.8 21.2
Nd 2.78 1.73 2.02 2.29 0.04 0.05 0.09 0.05 102 113 113 107
Sm 3.73 3.33 3.36 3.47 0.04 0.05 0.05 0.04 23.3 24.2 24.1 23.7
Eu 1.42 1.13 1.19 1.24 0.01 0.01 0.01 0.01 4.13 4.20 4.32 4.64
Gd 10.8 9.62 10.3 10.9 0.05 0.04 0.03 0.05 21.1 20.1 18.2 16.7
Dy 21.1 24.1 29.3 26.6 0.09 0.09 0.08 0.08 12.6 13.4 15.1 14.8
Er 18.5 28.1 39.6 28.9 0.08 0.08 0.14 0.10 4.99 6.28 5.94 5.51
Yb 25.6 45.0 66.8 43.9 0.22 0.25 0.31 0.33 2.99 4.80 4.01 4.29
Lu 3.57 6.00 8.99 5.91 0.03 0.05 0.04 0.04 0.87 1.03 0.97 0.87
Ti 508 248 292 354 487 619 515 596 17997 18248 20704 20302
V 120 121 113 123 57.9 83.3 68.9 77.3 658 700 518 495
Cr 49.5 44.5 68.4 75.9 151 156 146 150 147 161 152 133
Sr 0.38 0.54 0.31 0.28 0.50 0.26 0.57 0.25 141 185 189 211
Y 177 227 288 238 0.59 0.60 0.79 0.82 50.9 61.8 61.4 61.0
Zr 21.8 13.0 15.4 16.6 2.44 4.13 2.53 2.88 127 172 156 171
End of Table 1
Compound/ element Garnet Orthopyroxene Amphibole
22 core 23 rim 28 rim 29 rim 26 27 32 33 24 25 30 31
Nb 0.05 0.04 0.04 0.02 0.03 0.04 0.04 0.04 23.3 25.7 28.9 28.1
Hf 7.05 8.20 9.19 8.00 0.08 0.08 0.12 0.10 7.58 8.98 8.51 8.28
Eu/Eu* 0.68 0.61 0.62 0.61 0.86 0.51 1.01 0.66 0.57 0.58 0.63 0.71
(La/Yb)N 0.0006 0.0007 0.0005 0.0004 0.06 0.03 0.17 0.04 7.59 5.19 6.42 5.65
(Sm/Nd)N 4.14 5.96 5.12 4.67 2.53 3.11 1.85 2.50 0.71 0.66 0.66 0.68
(Yb/La)N 1814 1405 1831 2805 15.6 32.4 5.75 25.1 0.13 0.19 0.16 0.18
XREE 88.0 119 162 124 0.67 0.68 0.92 0.77 334 370 371 357
XHREE 79.6 113 155 116 0.47 0.51 0.60 0.60 42.6 45.6 44.1 42.1
XLREE 8.40 6.54 6.95 7.33 0.20 0.17 0.31 0.18 291 324 327 314
Dash - the element content is b.d.l.
T
0.520
0.518
0.516
0.514
0.512
0.510
Amp
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
y
Grt(H)
/
Sm-Nd isotopic data. Sm-Nd isotopic data for whole rock and bulk garnet, clinopyroxene, and amphibole probes were analyzed using TIMS method, with an error ± 0.5 % on a multicolumn mass spectrometer TRITON in the IPGG RAS. The mineral grains were separated using heavy liquids, with subsequent handpicking under a binocular microscope (about 100 mg of garnet, clinopyroxene, and amphibole).
The inclusions with low Sm/Nd ratio (apatite, monazite, and others) frequently occur in garnet, affect the accuracy of the Sm-Nd dating [15]. Therefore, selective digestion of possible inclusions with low Sm/Nd ratio included the sulphuric acid leaching technique. This technique used here for the garnets, preliminary crushed in an agate mortar, consists of only one step: leaching for 24-25 hours at 180 °C in 96 % H2SO4 [6]. This processing method significantly increases the range of 147Sm/144Nd ratio variations, hence, the accuracy of dating [3]. The isochron construction, as well as the age calculation, was carried out in the Isoplot toolkit [11].
A garnet-orthopyroxene plagio-crystalline schist (sample 174) yields a three-point isochron (WR, Amp, Grt) corresponding to an age of 1919 ± 13 Ma (MSWD = 0.75, Fig.3).
The addition of orthopyroxene isotopic data to the age calculations do not affect the Sm-Nd age (1914 ± 49 Ma) but increases the MSWD value to 13.
Zircon geochemistry and U-Pb age. Zircon from garnet-orthopyroxene plagio-crystalline schist (sample 174) is represented by grains with smoothed ribs with an elongation ratio 1.3-2.7. Grains are gray and dark gray in CL with prevalent sector zoning and also shaded thin-banded growth zoning (Fig.4).
A concordant age of 1997 ± 10 Ma was obtained by U-Pb zircon dating (10 measurements) (Fig.4). Zircon of the Pa-leoproterozoic age is characterized by a low and average U (106-394 ppm) and low Th (mean value is 66 ppm) contents. The Th/U ratio mean value is 0.31 (Table 2).
The mean value of REE content in zircon is 520 ppm and shows HREE enrichment with positive Ce-anomaly and negli-
/
/
WR
s = +0,7
Age = 1919 ± 13 Ma '3Nd/144Nd = 0.510186 ± 0.000018 MSWD = 0.75
/
0
0.2
0.4 147Sm/144Nd
0.6
0.8
Fig.3. Sm-Nd isochron for garnet-orthopyroxene plagio-crystalline schist (sample 174) WR - whole rock; Amp - amphibole; Grt (H) - garnet after the sulfuric acid leaching
0.39
0.37
Ph
0.35
0.33 5.5
Concordant age = 1997 ± 10 Ma
MSWD = 0.58 Concordance probability = 0.45
5.7
5.9
6.1 6.3
207Pb/235U
6.5
6.7 6.9
Fig.4. CL images (a) and concordia diagram (b) of zircon from garnet-orthopyroxene plagio-crystalline schist.
Ellipses dimensions are in the 2a interval
gible negative Eu-anomaly (mean value Eu/Eu* = 0.43) (Fig.5, a). The LuN/LaN ratio varies over a wide range of values from 871 to 9672 (except analytical points 5,1 and 8,1). Zircon Hf content mean value is 7351 ppm, while Y, P and Ca concentrations are variable (Table 2).
Zircon yields almost constant Ti contents of the mean value 11 ppm. The average value of calculated Ti-in-zircon temperature is 755 °C [16], which corresponds to the parameters of the granu-lite facies metamorphism.
Table 2
The content of trace and rare-earth elements (ppm) in zircon from garnet-orthopyroxene from plagio-crystalline schist
(sample 174)
b
a
Compound/ Analytical point number (Fig.4, a)
element 1,1 2,1 3,1 4,1 5,1 7,1 8,1 9,1 10,1
La 0.03 0.08 0.80 0.15 1.01 0.08 0.04 0.13 1.11
Ce 2.33 10.5 9.45 4.26 8.10 6.78 5.16 7.52 13.8
Pr 0.01 0.21 0.51 0.10 0.75 0.35 0.06 0.39 0.32
Nd 0.07 2.71 4.18 0.79 4.21 4.44 0.70 5.02 2.55
Sm 0.15 2.88 3.90 1.27 1.29 5.88 1.13 5.95 3.06
Eu 0.04 0.91 1.11 0.34 0.58 1.53 0.38 1.51 0.86
Gd 1.28 13.7 17.4 6.05 4.19 23.8 5.32 26.1 14.7
Dy 6.37 54.2 68.4 24.5 15.3 79.2 23.6 90.8 78.0
Er 23.2 147 178 64.8 46.3 179 79.2 206 264
Yb 77.9 398 406 167 129 372 250 430 614
Lu 17.1 76.6 72.7 31.8 26.8 67.6 54.0 77.8 116
Li 5.23 2.40 2.63 0.79 4.45 0.39 3.46 0.70 3.58
P 22.6 105 191 61.2 75.7 166 72.3 89.0 512
Ca 0.16 0.39 13.1 0.94 24.0 0.52 1.27 1.03 311
Ti 13.3 11.7 8.44 14.0 7.65 9.69 11.6 14.5 10.3
Sr 0.08 0.27 0.67 0.19 0.82 0.41 0.33 0.33 1.81
Y 121 954 1026 371 260 1056 433 1161 1383
Nb 145 122 84.7 73.7 95.2 66.3 47.6 46.1 41.1
Ba 1.35 0.97 1.69 1.02 2.30 1.55 0.54 0.82 0.60
Hf 7450 8697 6873 6820 8925 6362 7142 7119 6768
Th 19.2 194 118 29.6 42.3 97.2 64.7 120 163
U 376 743 426 136 328 266 378 329 668
Th/U 0.05 0.26 0.28 0.22 0.13 0.37 0.17 0.37 0.24
Eu/Eu* 0.30 0.44 0.41 0.38 0.76 0.39 0.47 0.37 0.39
Ce/Ce* 28.1 19.8 3.57 8.20 2.26 10.0 25.7 8.17 5.60
End of Table 2
Compound/ element Analytical point number (Fig.4, а)
1,1 2,1 3,1 4,1 5,1 7,1 8,1 9,1 10,1
ZREE 129 707 762 301 237 740 419 852 1109
ZLREE 2.44 13.5 14.9 5.30 14.1 11.7 5.96 13.1 17.8
ZHREE 126 690 742 294 221 721 412 831 1087
LuN/LaN 5301 9672 871 2002 256 8429 12576 5905 1003
LuN/GdN 109 45.3 33.7 42.5 51.8 22.9 82.1 24.1 63.7
SmN/LaN 7.84 60.5 7.77 13.3 2.04 122 43.8 75.0 4.41
7(Ti), °C 769 757 728 774 720 740 756 777 746
The zircon points on the La-(Sm/La)N diagram [10, 14] fall into the fields of unaltered mag-matic and porous zircons (Fig.5, b). The distinctive features of porous zircon are the increased content of La, Fe, Ca, Al and an abnormally low (Sm/La)N ratio [9]. At points 3.1 and 5.1. Ca content is noticeably higher than in unaltered zircons (Table 2), and at point 10.1 this value reaches 311 ppm with also low (Sm/La)N ratio = 2-8.
PT-parameters of metamorphism. Sample 174 is chosen for the metamorphism parameters study since it contains garnet phase, which substantially increases the number of the reactions suitable for the temperature calculation. The TWQ method gives T= 695±15 °C and P = 6±1 kbar (based on three independent reactions) for the late paragenesis of Amp-Grt-Pl-Qz. Obtained PT parameters correspond to the boundary conditions of the amphibolite and granulite facies [13] and. as can be seen, are characterized by the incomplete equilibrium of coexisting minerals (Fig. 6).
The equilibrium is incomplete because amphibole defining pressure-dependent reaction (e.g. 3Ab + 2Grs + Prp + 3Ts = 3Prg + 6bQtz + 6An and 3Ab + 2Grs + Prp + 4fTs = 3fPrg + T + 6bQtz + + 6An) and garnet, determining a temperature (e.g. 3Prg + 4Alm = 4Prp + 3fPrg) formed with a certain time interval. An additional point is that significant uncertainty makes the inaccuracy of the thermodynamic parameters for the amphibole group end members. The water activity in the system was not less than 0.8, which is consistent with the observed orthopyroxene and amphibole phase's ratios.
To verify the obtained P-T values. pseudo-section and isopleths calculations were performed using Theriak-Domino software package with the JUN92d.bs database, which is an analog of the thermodynamic database used in TWQ [17]. Since a local inhomogeneity in the minerals distribution and composition is not rare in polymetamorphic rock complexes, the most appropriate method
a
10000 -1000 -
<u
I § 100 -
o
13 10 -
0.1
0.01
o
Unaltered magmatic zircon
1 Xi * m
La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu
100
100
£ ^ 100
10
1.0
U, ppm 2000 3000
« 1,1 (2011 ± 15) ■ 2,1 (1996 ± 14) O 3,1 (1980 ± 14) X 4,1 (1965 ± 28) « 5,1 (2000 ± 16) •k 7,1 (1991 ± 21) 8,1 (2015 ± 18) A 9,1 (1986 ± 16) O 10,1 (2002 ± 11)
Hydrothermal zircon
0.1 0.001
0.01
0.1
1
10
100 La, ppm
Fig.5. The distribution of REE (a) in zircon from a garnet-orthopyroxene plagio-crystalline schist (sample 174) and the position of zircon figurative points on discrimination diagrams (b)
b
1
0.10
0
11 "
S
Ph
600
700
800
900
Temperature, °C
Fig.6. The stability field of mineral paragenesis in sample 174 (gray fill), calculated in the Theriak-Domino software package [8] for the MnNCMFATSH system at aH2O = 0.8. The regions of the minimum PT-parameters (~720 °C h 7 kbar) for the Grt + Opx + Pl + Amp association and the maximum PT-parameters (~775 °C h 7,5 kbar) for the Opx + Pl association are shown by ellipses (black fill and bold
dotted line, correspondingly). The boundaries of the amphibolite and granulite facies are shown by Oh and Liou [13]. The sidebar shows the convergence of the mineral reaction lines for the Grt + Opx + Pl + + Amp association, calculated in the TWQ program [7]
for estimating the rock's bulk composition with equilibrium mineral paragenesis is to calculate it by the actual minerals compositions ratios. The isopleths of garnet (Alm, Py, Grs) and plagioclase (An) end members, magnesia and Al content in orthopyroxene (apfu) were calculated and constructed by the actual method. Further, with a comparison of the calculated compositions and measured ones, was constructed a rather compact region (the black ellipse in Fig.6), showing the area, where minerals compositions are the closest. The obtained PT-parameters (720±10 °C. 7.0±0.2 kbar) are almost equal to the TWQ ones.
The evaluation of the early high-temperature metamorphism PT-parameters is a much more complex task since only the most basic plagioclase and orthopy-roxene with maximum alumina content can be assigned to the minerals, formed in this conditions. Using the principle of effective bulk composition [8] corresponding to the retrograde metamor-phism, garnet and amphibole were removed from the calculations. Then iso-
pleths for the anorthite component in plagioclase, and the tschermakite component and magnesia in orthopyroxene were calculated. Using the above-described method, a region corresponding to the maximum PT-parameters (dotted ellipse in Fig.6) with a temperature of 775±35 °C and a pressure of 7.5±0.7 kbar was constructed. This approximate estimate indicates that the protolith at an early stage experienced metamorphism under granulite-facies conditions. The additionally calculated area of coexistent real mineral paragenesis (gray fill in Fig.6) indicates the possibility of observed paragenesis formation in the extended in temperature, but limited in pressure interval.
Thus, the sequence of the rock metamorphic transformations can be traced: high-thermal granulite facies metamorphism (T<810 °C and subsequent sub-isobaric (about 7 kbar) cooling to 700 °C with the water activity increase and Grt-Amph paragenesis formation. corresponding to the granulite to amphibolite facies transitional phase [13].
Conclusions. The detailed study of plagio-crystalline schist from the Upper Anabar series provides new information on the age, geochemical features, and PT-conditions of the rocks formation. The plagio-crystalline schist protolith is the basic rock not containing primary magmatic zircon. The age of 1997±10 Ma, obtained by the U-Pb zircon dating corresponds to the time of granulite facies metamorphism. Sm-Nd age of 1919±13 Ma obtained from the whole rock, bulk garnet, and amphibole probes indicates the regressive amphibolite facies metamorphism. The granulite facies meta-morphism peak conditions are determined as 775±35 °C and 7.5±0.7 kbar. The regressive stage of metamorphism, which led to the Grt-Amph paragenesis formation, occurred at the temperature of about 700 °C and pressure of 7 kbar.
Acknowledgements. The authors are grateful to O.L.Galankina, E.S.Bogomolov (IPGG RAS), S.G.Simakin, E. V.Potapov (IPT RAS) and colleagues of the VSEGEI Institute for analytical studies. The study was carried out with the financial support of the Russian Foundation for Basic Research (grants 17-35-50002, 16-35-60092, 18-35-00229).
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Authors: Lyudmila Yu. Sergeeva, Leading Engineer, [email protected] (A.P. Karpinsky Russian Geological Research Institute, Saint-Petersburg, Russia), Aleksei V. Berezin, Candidate of Geological and Mineralogical Sciences, Research, [email protected] (Institute of Precambrian Geology and Geochronology RAS, Saint-Petersburg. Russia), Nikolai I. Gusev, Head of Eastern Siberia ores Department, [email protected] (A.P. Karpinsky Russian Geological Research Institute, Saint-Petersburg, Russia), Sergei G. Skublov, Doctor of Geological and Mineralogical Sciences, Professor, Chief Researcher, [email protected] (Institute of Precambrian Geology and Geochronology RAS, Saint-Petersburg. Russia), Aleksei E. Melnik, Candidate of Geological and Mineralogical Sciences, Junior Researcher, [email protected] (Institute of Precambrian Geology and Geochronology RAS, Saint-Petersburg. Russia).
The article was accepted for publication on 22 January, 2018.
