ISSN 18197140, Russian Journal of Pacific Geology, 2015, Vol. 9, No. 6, pp. 399–410. © Pleiades Publishing, Ltd., 2015. Original Russian Text © A.A. Sorokin, A.B. Kotov, N.M. Kudryashov, V.P. Kovach, 2015, published in Tikhookeanskaya Geologiya, 2015, Vol. 34, No. 6, pp. 3–15.
First Evidence of Ediacaran Magmatism in the Geological History of the Mamyn Terrane of the Central Asian Fold Belt A. A. Sorokina, A. B. Kotovb, N. M. Kudryashovc, and V. P. Kovachb a
Institute of Geology and Nature Management, Far East Branch, Russian Academy of Sciences, per. Relochnyi 1, Blagoveshchensk, Amur Region, 675000 Russia email:
[email protected] b Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, nab. Makarova 2, St. Petersburg, 199034 Russia email: abkotov
[email protected] c Institute of Geology, Kola Science Center, Russian Academy of Sciences, ul. Fersmana 15, Apatity, Murmansk Region, 184209 Russia email:
[email protected] Received March 10, 2015
Abstract—U–Pb geochronological studies reveal that the gabbro of the Mikitkin Massif and quartz diorite of the Ust’Garin Massif of the Mamyn Terrane, which were provisionally assigned to the Early Proterozoic Garin Complex, have ages of 583 ± 6 Ma and 607 ± 8 Ma, i.e. Upper Riphean–Lower Vendian. The geochemical features of the magmatic rocks of the studied massifs indicate their suprasubduction origin. The parental melts of the magmatic rocks of the Garin Complex were formed in an active continental margin or ensialic islandarc setting due to melting of the subductionmodified depleted mantle source and the occur rence of contamination and fractional crystallization. These massifs were developed during the Ediacaran Stage of the geological evolution of the Mamyn Terrane and presumably reflect Neoproterozoic convergent geodynamic processes. These processes likely caused the ultimate formation of the Precambrian continental massifs in the eastern Central Asian fold belt, which were subsequently amalgamated into the structure of the epiPaleozoic Amur microcontinent. Keywords: magmatism, geochronology, Ediacarian, Mamyn terrane, Central Asian fold belt DOI: 10.1134/S181971401506007X
Distinguishing of the age boundaries of the magmatic manifestation in the geological evolution of the conti nental massifs of the Central Asian fold belt is necessary for elaborating the overall geodynamic models of the for mation of this major mobile belt of the Earth. Of special significance is reconstruction of the sequence of manifes tation of the late Precambrian magmatic processes, because according to the existing concepts, the orogenic structures of Central Asia began to form in the late Pre cambrian [3, 11, 12, 38. etc.]. The Argun (Argun–Indermeg) and Bureya–Jia musi superterranes (Fig. 1) [1, 12] are among the larg est continental massifs in the eastern Central Asian fold belt. It is believed that the “basements” of these superterranes are magmatic and metamorphic rocks of supposedly Early and late Precambrian age [1, 2, 4, 13, 16, etc.]. However, in spite of the significant volume of recently performed geochronological studies, the presence of Early Precambrian magmatic and meta morphic complexes within these superterranes have not been directly confirmed yet [6–10, 14, 15, 24, 44– 47, 49, etc.]. At the same time, the first direct [24, 42,
46, etc.] and indirect [17, 18, 20, 26, 33, 43, 47–50, etc.] geochronological data suggest wide manifesta tions of Late Precambrian magmatic processes in the geological evolution of the Argun (Argun–Idermeg) and Bureya–Jiamusi superterranes. Unfortunately, these data are insufficient to distinguish the main stage and to reconstruct the geodynamic settings of the Late Precambrian magmatism in the geological evolution of the continental massifs in the eastern Central Asian fold belt. To fill this gap, we have carried out geochro nological, geochemical, and isotope–geochemical studies of gabbro, gabbrodiorite, diorite, and quartz diorite of the geologically oldest Garin magmatic complex of the Mamyn terrane of the Argun (Argun– Idermeg) superterrane, the results of which are dis cussed in this paper. MAIN GEOLOGICAL FEATURES OF THE MAMYN TERRANE In recent geological maps [16, etc.], the Mamyn terrane is distinguished in the eastern part of the Argun
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SOROKIN et al. 108°
60°
132°
144°
129°30′
SEA OF OKHOTSK
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BJ Ink an R.
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Fig. 1. Geological scheme of the central part of the Mamyn terrane. Modified after [4, 16]. (1) Middle Quaternary–Modern sediments; (2) Upper Neogene–Lower Quaternary loose sediments; (3) Early Cretaceous inter mediate and felsic volcanic rocks; (4) Early Cretaceous granitoids; (5) Lower Cretaceous continental terrigenous sediments; (6) Siluarian, Devonian, and Lower Carboniferous terrigenous and terrigenous–carbonate sediments; (7) provisionally Ordovi cian granitoids of the Oktyabr’sky Complex; (8) provisionally Early Ordovician felsic volcanic rocks of the Oktyabr’sky Complex; (9) provisionally Early Precambrian gabbro, gabbrodiorite, diorite, and quartz diorite of the Garinsky Complex; (10) provision ally Proterozoic volcanogenic terrigenous complexes of the South Mongolian–Khingan orogenic belt; (11) faults, (12) localities and numbers of geochronological samples. (U) Ust’–Garinsky, (M) Mikitkin massifs. Inset shows the position of the studied objects in the structure of the eastern Central Asian fold belt: (AR) Argun Superterrane, (BJ) Bureya–Jiamusi superterrane, (MM) Mamyn terrane. Paleozoic–Early Mesozoic fold belt: (SM) South Mongolian–Khin gan, (MO) Mongol–Okhotsk. Asterisk shows the study area.
superterrane (Argun–Idermeg) (Kerulen–Argun– Mamyn continental massif after [2]) of the Central Asian fold belt [12, etc.] (Fig. 1). The oldest supracrustal rocks of this terrane include biotite– hornblende, biotite and hornblende gneisses, amphib olites, marbles, and quartzites of the Elna sequence, as well as metabasalts, metaandesites, chlorite– amphibole, sericite, quartz–sericite schists, metasandstones, and marbled limestones of the Gar Group, which as suggested, are Late Archean and Paleoproterozoic in age, respectively. The highest stratigraphic position with respect to the metamorphic rocks of the Elna Sequence and Gar Group is occu
pied by the Vendian (?) and Cambrian terrigenous and terrigenous–carbonate rocks and volcanic intermedi ate and felsic rocks of the Oktyabr’skaya sequence. Up to now, the age of this sequence has been considered as Late Proterozoic [4, etc.] or Early Ordovician [16]. However, the results of recently performed U–Pb geo chronological studies [23] indicate that the Okty abr’skaya sequence includes volcanic rocks of different age (Vendian–Cambrian and Late Cambrian). The sedimentary and volcanic sequences of the Mamyn terrane are crowned by the Permian–Triassic interme diate and felsic volcanic rocks of elevated alkalinity of
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the Manegr Sequence and faunally characterized Sil urian, Devonian, and Carboniferous sediments [16]. A significant part of the Mamyn terrane is occupied by exposures of compositionally diverse magmatic rocks of the Depsk, Garin, Oktyabr’skii, Tyrma– Bureya, and Kharin complexes. It is believed that the ultrabasic rocks of the Dep Complex, as well as the gabbro, gabbrodiorite, diorite, and quartz diorite intrusions of the Gar Complex, are traditionally ascribed to the Paleoproterozoic stage in the geologi cal evolution of the Mamyn terrane [16]. The grani toid massifs of the Oktyabr’skii Complex intrude the magmatic rocks of the Gar Complex and the volcanic rocks of the Oktyabr’skaya Sequence, which was formed during the Vendian–Cambrian stage of the geological evolution of the Mamyn terrane [23]; the massifs with erosion are overlain by the Silurian sedi mentary rocks of the Mamyn Formation. U–Pb geo chronological studies have shown that the granitoids of this complex are of Late Cambrian age [22]. According to existing concepts [1, 2, 16, etc.], the gabbrodiorite–granodiorite–granite intrusions of the Tyrma–Bureya Complex are Middle–Late Carbonif erous in age, while the syenites, subalkaline leucog ranites, and granites of the Kharin Complex are Per mian–Triassic in age. Unfortunately, there are no reli able geochronological data on the magmatic rocks of these complexes, the massifs of which are localized within the Mamyn terrane. The petrotype granitoid massifs of the Tyrma–Bureya and Kharin complexes of the Turan terrane are of late Triassic–Early Jurassic age [19, 21]. Summing up the geological characteristics of the Mamyn terrane, it should be noted that the above described sedimentary and volcanic sequences and magmatic complexes are overlain by Mesozoic and Cenozoic terrigenous sequences and Late Mesozoic volcanic rocks [1, 2, 16]. As seen, the presently accumulated geological and geochronological data indicate that the oldest mag matic rocks of the Mamyn terrane are the ultrabasic rocks of the Dep complex and the gabbro, gabbrodior ite, and quartz diorite of the Gar complexes. Natu rally, the most promising objects for dating magmatic manifestations at the early stages of the evolution of the Mamyn terrane are the magmatic rocks of the Garin Complex. The most typical massifs of this com plex are the Ust’Garin and Mikitka massifs. The former is situated in the nearmouth part of Mikitkin Creek (left tributary of the Orlovka River) (Fig. 1). Both the massifs represent large xenoliths among the Early Paleozoic granitoids of the Oktyabr’skii Com plex, while significant parts of them are overlain by Cenozoic loose deposits. The constituent gabbro, gab brodiorite, diorite, and quartz diorite show variable traces of cataclasis, mylonitization, and the appear ance of gneissose. RUSSIAN JOURNAL OF PACIFIC GEOLOGY
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ANALYTICAL TECHNIQUES The contents of major elements and Zr were deter mined by XRF at the Institute of Geology and Nature management of the Far East Branch of the Russian Academy of Sciences (Blagoveshchensk) on an Xray Pioneer 4S spectrometer. Trace elements (Li, Ga, Rb, Sr, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Nb, Ta, Th, U, Pb, Sc, V, Cr, Co, Ni, Cu, and Zn) were analyzed by ICPMS on an Elan 6100 DRC mass spectrometer at the Institute of Tec tonics and Geophysics of the Far East Branch of the Russian Academy of Sciences (Khabarovsk). Powdered samples for XRF analysis were homoge nized by their fusion with lithium metaborate and tet raborate in a muffle furnace at T = 1050–1100°C. The values of the intensities of the analytical bands during analysis were corrected for background and the effects of absorption and secondary fluorescence. The recov ery of samples for ICPMS determination of trace ele ments was carried out by acid digestion. The sensitivity of the mass spectrometer over the entire mass scale was calibrated by standard solutions, including all ana lyzed elements. The relative error in the determination of the contents of major and minor elements was 3– 10%. Geochronological studies (U–Pb zircon method) were performed at the Geological Institute of the Kola Science center of the Russian Academy of Sciences (Apatity). Accessory zircon was extracted using mag netic separation and heavy liquids. Microimages of zircon crystals were performed in a secondary electron mode at the Institute of Geology and Nature Manage ment of the Far East Branch of the Russian Academy of Sciences (Blagoveshchensk) on a JSM6390 LV JEOL electron microscope and in a cathodolumines cent mode at the Geological Institute of the Kola Sci ence Center of the Russian Academy of Sciences (Apatity) on a LEO1450 electron microscope equipped with a PANA CL. The zircons selected for U–Pb geochronological studies were subjected to multistage removal of surface pollution in alcohol, acetone, and 1 M HNO3. The chemical decomposition of the zircon and extraction of U and Pb was performed using the modified Krogh technique [29]. In some cases, differential dissolution of zircon was applied to decrease the degree of discor dance. For this purpose, a zircon aliquot of 0.7 mg weight was loaded in the HF and heated for one hour at T = 205°C. After cooling, the solution was removed, while the remaining zircon again was placed in the HF and heated at T = 205°C for three days. After complete dissolution of zircon, the following procedures were carried out using the Krogh technique [29]. Isotope studies were performed using a 235U–202Pb mixed isotope tracer. Isotope analyses were performed on a multichannel Finnigan MAT262 (RPQ) mass spectrometer. All isotope ratios were corrected for mass fractionation (0.12 ± 0.04%) calculated for rep No. 6
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5 Syenite Monzonite
8 6
1 2
Quartz monzonite
Monzo diorite Monzo gabbro
Gabbro
2 45
50
Gabbro Diorite diorite
55
2 Calcalkaline series
0
Granodiorite
60 65 SO2, %
3
1
Granite
4
Tholeiitic series
4 FeO*/MgO
K2O + Na2O, %
10
45
70
75
80
Fig. 2. (K2O + Na2O)–SiO2 classification diagram [30] for magmatic rocks of the Ust’Garin and Mikitkin massifs (1) gabbro from the Mikitkin Massif; (2) gabbrodiorite, diorite, and quartz diorite from the Ust’Garin Massif.
licate analyses of SRM981 and SRM982. The U and Pb contents and U/Pb ratios were determined with error of 0.5–0.7%. Blanks were less than 100 pg for Pb and 50 pg for U. Experimental data were processed using the PbDAT and ISOPLOT softwares [31, 32]. Ages were calculated using conventional decay con stants of U [40]. Correction for common lead was introduced according to model values [39]. All errors in the table are given at 2σ level. Sm–Nd and Rb–Sr isotope geochemical studies were performed at the Institute of Precambrian Geol ogy and Geochronology (St. Petersburg). Sm and Nd were extracted using the technique described in [5], which is close to that described in [37]. Rb and Sr were extracted using conventional techniques with ion exchange resins. The Sm, Nd, Rb, and Sr isotope compositions were measured on multichannel Finni gan MAT261 and TRITON TI mass spectrometers in static mode. The measured 143Nd/144Nd ratios were normalized to 146Nd/144Nd = 0.7219 and adjusted to 143Nd/144Nd = 0.511860 in the La Jolla Nd standard. The weighted average 87Sr/86Sr in the JNdi1 La Jolla standard during measurements was 0.512108 ± 7 (n = 10). Sr isotope ratios were normalized to 87Sr/86Sr = 8.37521. The weighted average 87Sr/86Sr in the SRM 987 Sr standard during measurements was 0.710270 ± 8 (n = 12). The determination accuracies were ±0.5% for Sm, Nd, Rb, and Sr concentrations; ±0.5% for 147Sm/144Nd; ±0.005% for 143Nd/144Nd; ±0.5% for 87Rb/86Sr; and ±0.05% for 87Sr/86Sr (2σ). Blanks were no more than 0.2 ng for Sm, 0.5 ng for Nd, 0.05 ng for Rb, and 0.7 ng for Sr. The values of εNd(t) and model ages TNd(DM) were calculated using presentday CHUR values after [27] (43Nd/144Nd = 0.512638, 147Sm/144Nd = 0.1967) and DM after [25] (143Nd/144nd = 0.513151, 147Sm/144Nd = 0.2136).
50
55 SiO2, %
60
65
Fig. 3. FeO*/MgO–SiO2 diagram [35] for magmatic rocks of the Ust’Garin and Mikitkin massifs. Symbols are shown in Fig. 2.
PETROGRAPHY, GEOCHEMISTRY, AND ISOTOPEGEOCHEMICAL FEATURES OF THE MAGMATIC ROCKS OF THE MIKITKIN AND UST’GARIN MASSIFS The Mikitkin Massif is made up of fineto mediumgrained gabbro, which have massive or gneis sose structures and hypidiomorphic texture. The main rockforming minerals of these rocks are plagioclase (55–60%), clinopyroxene (30–35%), and hornblende (up to 10%). The main accessory minerals are apatite, titanite, and zircon. Plagioclase (labradorite) is replaced by epidote, sericite, chlorite, and carbonate, while clinopyroxene is practically completely replaced by green hornblende and chlorite. The Ust’Garin Massif consists of fineto mediumgrained gabbrodiorite, diorite, and quartz diorite, with massive or gneissose structure and hypid iomorphic texture. They are made up of andesine (55– 60%), hornblende (25–30%), biotite (5–10%), and quartz (up to 2%). The accessory minerals are apatite, ilmenite, and zircon. Plagioclase (andesine) is usually replaced by an aggregate of epidote, sericite, and chlo rite, while common hornblende is replaced by biotite. In terms of silica–alkali ratios, the gabbros of the Mikitkin Massif (SiO2 = 48.1–50.7%, K2O = 1.0– 1.6%, K2O + Na2O = 3.8–4.5%) are ascribed to the lowK calcalkaline series (Table 1, Fig. 2). At the same time, they have high FeO*/MgO = 1.38–2.65, which is typical of the magmatic rocks of the tholeiitic series (Fig. 3). The rocks in the MgO–(FeO* + TiO2)–Al2O3 also define a tholeiitic trend (Fig. 4). The REE distribution in the gabbro of the Mikitkin Massif shows weak differentiation (Fig. 5) ([La/Yb]n = 4.0– 5.7) with a weakly expressed Eu anomaly Eu/Eu* = 0.83–0.92. The gabbrodiorite, diorite, and quartz diorite of the Ust’Garin Massif in terms of silica–alkali relations (SiO2 = 55.9–59.9%, K2O = 1.2–2.0%, K2O + Na2O = 4.9–6.1%) are also ascribed to the rocks of
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Sample no., component
C1199
C11991
C11992
C11993
C11994
C11995
C11996
C11997
C1202
C12021
C12022
C12023
C12024
C12025
Table 1. Chemical composition of representative samples of magmatic rocks of the Mikitkin and Ust’Garin massifs
1
2
3
4
5
6
7
8
9
10
11
12
13
14
SiO2 TiO2 Al2O3
49.62 0.81 16.52
50.69 0.56 17.95
48.98 1.19 17.21
49.79 0.87 15.94
49.33 1.10 17.48
48.57 0.69 18.28
49.9 1.18 16.81
48.14 1.03 17.05
58.58 0.86 16.75
59.28 0.84 16.96
57.21 0.88 16.56
59.83 0.98 16.61
58.45 0.99 16.78
55.90 0.84 15.83
Fe 2 O 3* 10.84 9.16 12.58 11.7 12.62 10.35 12.77 MnO 0.21 0.19 0.23 0.22 0.25 0.2 0.27 MgO 5.94 5.95 4.35 5.92 4.44 6.40 4.33 CaO 9.87 8.38 8.72 8.89 6.62 7.96 7.82 2.18 3.25 3.43 2.66 3.17 2.46 3.64 Na2O 1.59 1.07 1.08 1.24 1.32 1.09 1.05 K2O P2O5 0.16 0.11 0.29 0.16 0.23 0.13 0.26 L.O.I. 1.97 2.44 1.54 2.32 2.06 2.82 1.57 Total 99.71 99.75 99.6 99.71 98.62 98.95 99.6 Li 6.2 9.9 4.4 5.7 5.4 9.1 4.6 Ga 15 14 18 16 18 16 18 Rb 37 31 22 26 62 50 24 Sr 436 525 477 450 494 362 489 Ba 402 317 287 314 523 472 331 La 11.17 9.63 16.55 11.11 15.67 10.41 19.33 Ce 24.52 19.66 40.17 25.79 35.73 23.81 43.15 Pr 3.29 2.53 5.29 3.37 4.50 2.93 5.65 Nd 14.35 10.43 24.84 15.58 20.77 13.01 26.30 Sm 3.45 2.39 5.92 3.77 4.80 3.00 6.14 Eu 1.11 0.71 1.91 1.14 1.51 0.88 1.94 Gd 3.81 2.62 6.64 4.27 5.48 3.40 6.98 Tb 0.53 0.37 0.90 0.59 0.74 0.46 0.94 Dy 3.05 2.14 5.33 3.53 4.46 2.77 5.57 Ho 0.63 0.43 1.05 0.70 0.89 0.55 1.11 Er 1.74 1.25 3.05 2.06 2.62 1.62 3.21 Tm 0.24 0.18 0.42 0.28 0.36 0.23 0.43 Yb 1.57 1.15 2.71 1.86 2.37 1.48 2.83 Lu 0.23 0.17 0.41 0.28 0.36 0.22 0.42 Y 15 11 25 17 22 14 27 Zr 60 74 48 49 51 53 48 Nb 1.6 1.8 2.2 1.7 1.9 1.9 2.3 Ta 0.13 0.13 0.09 0.15 0.15 0.15 0.17 Th 1.75 1.99 1.37 0.68 0.55 1.14 0.59 U 0.33 0.46 0.25 0.19 0.14 0.29 0.15 Pb 7.7 4.7 4.4 4.8 2.9 4.6 4.3 Sc 48 31 40 42 35 38 37 V 339 229 366 387 382 272 412 Cr 46 44 30 36 23 48 14 Co 29 28 25 38 28 34 27 Ni 20 27 12 26 10 29 12 Cu 26 25 58 63 71 52 40 Zn 100 78 67 88 82 81 97
12.42 0.23 5.28 9.21 2.56 1.33 0.24 2.17 99.66 5.4 17 33 453 352 12.86 30.68 4.03 18.60 4.51 1.37 5.14 0.71 4.23 0.83 2.40 0.33 2.19 0.33 20 48 1.5 0.14 0.49 0.15 4.3 40 401 28 30 15 112 90
8.91 0.15 2.47 4.86 3.76 1.67 0.20 1.63 99.84 6.7 18 48 437 423 21.30 47.25 5.58 23.51 5.04 1.40 5.69 0.77 4.57 0.93 2.78 0.40 2.64 0.40 24 144 11.0 0.71 8.48 2.34 8.5 20 145 46 15 7 22 69
8.73 0.14 2.37 4.88 3.77 1.68 0.20 1.05 99.9 7.9 18 62 529 542 25.40 55.54 6.32 25.59 5.23 1.38 6.01 0.78 4.71 0.94 2.82 0.39 2.63 0.39 24 158 10.7 0.71 8.84 1.77 7.2 19 141 50 16 7 36 72
9.68 0.15 2.72 3.9 3.6 1.69 0.20 2.24 98.83 8.8 19 72 318 898 27.03 58.38 6.54 26.04 5.12 1.27 5.81 0.76 4.54 0.90 2.74 0.39 2.62 0.39 23 136 11.9 0.77 9.54 2.44 4.4 18 149 66 20 9 14 91
8.37 0.16 2.38 4.05 4.61 1.5 0.22 1.12 99.83 7.7 16 48 389 415 21.50 46.63 5.36 22.47 4.68 1.23 5.21 0.70 4.14 0.83 2.51 0.36 2.44 0.36 21 180 12.8 0.77 8.69 1.67 5.4 18 133 51 21 9 38 62
9.19 0.16 2.72 4.86 3.4 1.8 0.21 1.07 99.54 7.8 18 65 412 485 19.82 43.37 5.05 21.02 4.32 1.20 4.89 0.65 3.91 0.81 2.42 0.35 2.38 0.36 21 150 12.3 0.76 9.16 1.69 10.4 17 146 52 20 8 27 80
8.44 0.19 2.68 6.03 3.66 1.25 0.19 3.30 98.31 7.3 17 28 190 745 19.05 41.87 4.91 20.94 4.33 1.27 4.88 0.66 3.99 0.80 2.39 0.35 2.37 0.35 21 124 10.9 0.71 8.13 1.73 8.4 16 146 39 18 7 18 76
Contents of major elements are given in wt %; trace elements, in ppm. (1–8) gabbro from the Mikitkin Massif; (9–14) gabbrodiorite, diorite, and quartz diorite from the Ust’Garin Massif. RUSSIAN JOURNAL OF PACIFIC GEOLOGY
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SOROKIN et al. Rock/primitive mantle 1000
FeO* + TiO2
(a)
100 10 TH ite des An
1 1000 (b)
e cit Da
e cit Da
Rhyolite
100
CA
alt Bas ite des An
lite yo Rh
10
Al2O3
MgO 1
Fig. 4. Classification diagram MgO–(FeO* + TiO2)– Al2O3 [28] for magmatic rocks of the Ust’Garin and Mikitkin massifs.
lowK calcalkaline series (Table 1, Fig. 2) and have high FeO*/MgO = 2.83–3.24, corresponding to the tholeiitic magmatic rocks (Fig. 3); their compositions reveal a tholeiitic trend in the MgO–(FeO* + TiO2)– Al2O3 diagram (Fig. 4). As compared to the gabbro of the Mikitkin Massif, they are characterized by higher contents and more differentiated distribution of REE [La/Yb]n = 5.4–7.0 (Table 1, Fig. 5) and a clearly expressed Eu anomaly (Eu/Eu* = 0.71–0.84). Rock/chondrite 1000
(a)
100 10 1 (b) 100 10 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Fig. 5. REE distribution in gabbro from the Mikitkin Mas sif (a) and gabbrodiorite, diorite, and quartz diorite from the Ust’Garin Massif (b). Chondrite after [42].
Rb Th Nb K Ce Pr Nd Sm Ti Tb Yb Ba U Ta La Pb Sr Zr Eu Gd Y Lu Fig. 6. Traceelement distribution in gabbro of the Mikit kin Massif (a), and gabbrodiorite, diorite, and quartz dior ite of the Ust’Garin Massif (b). Primitive mantle after [41].
The magmatic rocks of the considered massifs are enriched in Rb (up to 72 ppm), Ba (up to 898 ppm), Sr (up to 529 ppm), LREE (La up to 27 ppm, Ce up to 58 ppm), and Pb (up to 10 ppm), with a clear deficit of Nb, Ta, and Ti (Table 1, Fig. 6). It should also be noted that the diorite and quartz diorite of the Ust’Garin Massif, as compared to the gabbro of the Mikitkin Massif, are enriched in Th (up to 9.5 ppm) and U (up to 2.4 ppm). The results of isotopegeochemical (Sm–Nd, Rb– Sr) studies of the magmatic rocks of the Mikitkin and Ust’Garin massifs are listed in Table 2. The gabbro of the Mikitkin Massif is characterized by positive εNd(t) from +1.4 to 1.6 and 87Sr/86Sr(0) = 0.70480–0.70593. The biotite–amphibole diorite of the Ust’Garin Massif has negative εNd(t) from –1.2 to –1.3, tNd(DM) = 1.5 Ga, and values 87Sr/86Sr(0) = 0.70548–0.70593. RESULTS OF U–PB GEOCHRONOLOGICAL STUDIES U–Pb geochronological studies were carried out for the gabbro of the Mikitkin (sample C1199) and biotite–amphibole quartz diorite (sample C12002) of the Ust’Garin Massif. The sampling localities are shown in Fig. 1, while the obtained results are pre sented in Table 3 and Figs. 7–9. The Mikitkin Massif. The accessory zircon extracted from the gabbro of the Mikitkin Massif
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(87Sr/86Sr)0
(±2σmeas)
87Sr/86Sr*
87Rb/86Sr
Sr, ppm
Rb, ppm
tNd(DM),Ga
εNd(t)
Nd, ppm
εNd(0)
Age, Sm, Ma ppm
143Nd/144Nd* (±2σmeas)
Sample
147Sm/144Nd
Table 2. Results of Sm–Nd and Rb–Sr isotopegeochemical studies of magmatic rocks from the Mikitkin and Ust’Garin massifs
Gabbros from the Mikitkin Massif C1199
583
C11991 583
3.65 15.33 0.1439 0.512519 ± 3
–2.3
1.6
–
38
370 0.2984 0.707285 ± 5 0.70480
2.56 11.40 0.1358 0.512477 ± 2 –3.1
1.4
–
32
472 0.1975 0.707569 ± 4 0.70593
Quartz diorites from the Ust’Garin Massif 607
5.07 23.7
0.1296 0.512302 ± 2 –6.6
–1.3
1537
50
383 0.3750 0.709182 ± 4 0.70593
C12021 607
5.28 25.7
0.1242 0.512287 ± 3 –6.8
–1.2
1468
64
456 0.4030 0.708977 ± 5 0.70548
C1202
* Errors (2σ) correspond to the last significant digits.
(sample C1199) forms a transparent brown bipyrami dal–prismatic habit (Fig. 7, I–IV). The crystals are shaped by the faces of prisms {110} and pipyramids {111}. Crystal size varies from 50 to 150 μm, Kel = 2.0– 3.0. Their inner structure is characterized by well
I
expressed “thin” magmatic zoning (Fig. 7, V–VIII), and the presence of small amounts of solidphase inclusions. In the cathodoluminescence mode (Fig. 7, V–VIII), the central parts of some zircon crystals have dark color, which is presumably related to U enrichment.
II
III
50µm
50µm
V
50µm
50µm
50µm
VII
VI
50µm IX
50µm
50µm X
50µm
VIII
50µm
XI
50µm XIII
IV
50µm XIV
XII
50µm
XV
50µm
50µm
XVI
50µm
Fig. 7. BSE and cathodoluminescent microimages of accesory zircon crystals. (I–VIII) from gabbro of the Mikitkin Massif (sample C1199); (IX–XVI) from quartz diorite of the Ust’Garin Massif (sample C1202). RUSSIAN JOURNAL OF PACIFIC GEOLOGY
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SOROKIN et al. 0.102
0.096 583 ± 6 Ma MSWD = 0.21
0.092
C1199/3
560
C1199/4
0.090 0.088
C1199/2 C1199/1
540
0.086
C1199/5
0.71
0.73
0.094 0.090 0.086
–47 ± 1460 Ma 0.084 0.69
0.098 206Pb/238U
206Pb/238U
0.094
580
607 ± 8 Ma MSWD = 0.092 590
610 C1202/IID C1202/ID
570 C1202/4 C1202/1 C1202/7 550 C1202/5 C1202/2 C1202/8 C1202/3 C1202/6
–5 ± 130 Ma 0.75
0.77
0.79
207Pb/235U
Fig. 8. Concordia diagram for accessory zircon from gab bro of the Mikitkin Massif (sample C1199). Numbers in the diagram correspond to ordinal numbers of fractions in Table 3.
U–Pb geochronological studies were performed for five zircon aliquots taken from differentsized frac tions (Table 3). In the Concordia diagram (Fig. 8), the position of their data points are approximated by Dis cordia, with the upper intercept at 583 ± 6 Ma (MSWD = 0.21) and lower intercept at –47 ± 160 Ma; the positioning presumably reflects the presentday lead loss. The data points of the isotope composition of the zircon taken from the size fraction from –100 + 75 μm (C1192, Table 3) are plotted almost on Con cordia. The morphology of the accessory zircon from the gabbro of the Mikitkin Massif indicates its crystalliza tion from the melt. Thus, the age estimate of 583 ± 6 Ma may be considered as the formation age of the massif. Ust’Garin Massif. The accessory zircon from the biotite–amphibole quartz diorite of the Ust’Garin Massif (sample C1202) is ascribed to two morpholog ical types. Type I (30%) is represented by transparent prismatic crystals (Fig. 7, IX, X) of pinkish color, while type II is transparent colorless equant zircon (Fig. 7, XI–XII). The crystals of the type I zircon are shaped by prisms {100}, {110} and dipyramids {111} and {311}. The shaping of the type II zircon is determined by a combination of dipyramids {311}, {111}, and prisms {100} and {110}. The size of the zircon crystals of types I and II varies from 50 to 200 μm, Kel = 3.0–4.0. Their internal structure is characterized by wellexpressed magmatic zoning (Fig. 7, XIII–XVI). U–Pb geochronological studies were performed for four aliquots of type I zircon and four aliquots of type II zircon taken from different size fractions (Table 3). In addition, we analyzed residues of zircons of types I and II after preliminary acid treatment
0.082 0.69 0.71 0.73 0.75 0.77 0.79 0.81 0.83 0.85 207Pb/235U Fig. 9. Concordia diagram for accessory zircon from biotite–amphibole quartz diorite of the Ust’Garin Massif (sample C1202). Numbers in the diagram correspond to the ordinal num bers of the fractions in Table 3.
(Table 3). It is seen in Fig. 9 that the data points of all of the studied zircon aliquots, including the acid treat ment residue, fall in the Concordia, with the upper intercept age of 607 ± 8 Ma (MSWD = 0.092) and lower intercept of –5 ± 130 Ma. The morphological features of the accessory zircon from the biotite–amphibole quartz diorite from the Ust’Garin Massif indicate their magmatic origin. Hence, there are grounds to suggest that the obtained age of 607 ± 8 Ma corresponds to the crystallization age of the parental melts of the magmatic rocks of this massif. DISCUSSION The results of U–Pb geochronological studies show that the emplacement of the gabbro from the Mikitkin (583 ± 6 Ma) and quartz diorite from the Ust’Garin (607 ± 8 Ma) massifs, which on the mod ern geological maps are ascribed to the Paleoprotero zoic Garin magmatic complex [16], is related to the Ediacaran stage of the geological evolution of the Mamyn terrane of the Argun (Argun–Idermeg) super terrane from the Central Asian fold belt. As mentioned above, information on manifestations of the Neoprot erozoic magmatism in the geological evolution of the continental massifs of the eastern Central Asian fold belt remains limited. The available data include several age estimates for the Early and Middle Neoprotero zoic magmatic complexes of the Argun and Hankai terranes [24, 42, 46, etc.]. The available data on the age of the Precambrian detrital zircons from the Pale ozoic deposits of the considered region [17, 18, 20, 34, 43, 47, 49] show that they are dominated by the Mid dle Mesoproterozoic zircons, whereas the late
RUSSIAN JOURNAL OF PACIFIC GEOLOGY
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–100 + 75, 0.8, pr.
C1202/2
947
2561
795
2767
4285
–200 + 150, 0.3, pr. 25.8 248.0
–75 + 50, 0.4, pr. 12.0 116.9
C1202/7
C1202/8
334
490
970
207Pb/235U
0.2288 ± 2 0.09047 ± 27 0.7420 ± 37 0.67
0.2642 ± 2 0.09224 ± 18 0.7572 ± 53 0.43
0.2373 ± 2 0.09464 ± 28 0.7759 ± 39 0.62
0.2430 ± 1 0.09362 ± 46 0.7668 ± 38 0.90
0.07463 ± 11 0.2805 ± 3 0.08641 ± 35 0.7089 ± 57 0.46
0.06504 ± 7
0.07760 ± 8
0.06459 ± 6
0.06270 ± 5
Rho
0.1946 ± 2 0.08876 ± 17 0.7358 ± 44 0.49
0.2001 ± 2 0.09087 ± 18 0.7532 ± 45 0.44
0.1849 ± 2 0.08645 ± 26 0.7167 ± 57 0.43
0.1508 ± 2 0.09049 ± 27 0.7499 ± 30 0.72
0.07331 ± 7
0.2096 ± 2 0.09833 ± 25 0.8154 ± 82 0.46
0.10627 ± 11 0.2201 ± 2 0.09691 ± 48 0.8020 ± 99 0.49
0.10283 ± 10 0.2813 ± 2 0.08788 ± 44 0.7276 ± 98 0.53
0.08931 ± 17 0.2465 ± 5 0.09084 ± 36 0.7530 ± 90 0.48
0.07486 ± 7
0.06564 ± 9
0.06552 ± 13 0.1526 ± 3 0.09101 ± 27 0.7548 ± 43 0.73
0.06749 ± 13 0.1617 ± 3 0.08769 ± 18 0.7271 ± 58 0.45
0.06669 ± 7
0.07127 ± 7
605 ± 2
596 ± 3
543 ± 3
560 ± 2
534 ± 2
558 ± 2
562 ± 2
542 ± 1
554 ± 1
561 ± 1
534 ± 2
558 ± 2
569 ± 1
583 ± 2
577 ± 3
606 ± 6
598 ± 9
555 ± 8
570 ± 7
549 ± 4
568 ± 2
571 ± 3
555 ± 4
560 ± 3
570 ± 3
544 ± 4
564 ± 3
572 ± 4
583 ± 3
578 ± 3
609 ± 9
604 ± 12
605 ± 12
608 ± 8
608 ± 6
607 ± 2
609 ± 2
608 ± 4
608 ± 5
608 ± 6
585 ± 22
585 ± 9
587 ± 18
584 ± 10
582 ± 6
206Pb/238U 207Pb/235U 207Pb/206Pb
Age, Ma
* Iisotope ratios corrected for blank and total lead; Rho—correlation coefficient of 207Pb/235U–206Pb/238U ratios; pr.—prismatic crystals of zircon; (eq) equant crystals of zircon. Errors (2σ) correspond to last significant digits. 1202/ID and C1202/IID are fractions analyzed after preliminary acid treatment.
1080
–200 + 150, 1.0, eq. 18.2 193.8
C1202/6
2572
C1202/IID –100 + 75, 0.7, eq. 19.3 179.1
–75 + 50, 2.0, eq. 33.8 355.2
C1202/5
2656
310
–75 + 50, 2.6, eq. 36.6 379.7
C1202/4
1934
2158
1280
8.5 76.9
–100 + 75, 1.7, eq. 21.8 234.8
C1202/3
32.1 335.1
–150 + 100, 0.8, pr. 30.0 301.2
C1202/ID –100 + 75, 0.7, pr.
206Pb/238U
Gabbro from the Mikitkin Massif (sample C1199)
206Pb/204Pb* 207Pb/206Pb* 208Pb/206Pb*
Isotope ratios
Biotite–amphibole quartz diorite from the Ust’Garin Massif (sample C1202)
81.3 802
81.4 807
66.4 616
82.1 774
156.2 1482
Pb
Content, ppm
C1202/1
–50, 0.3
C1199/5
–75 + 50, 0.4
C1199/3
+100, 0.8
–100 + 75, 0.6
C1199/2
C1199/4
–150 + 100, 0.7
C1199/1
Sample Fraction size (μm) no./fraction and weight (mg) no.
Table 3. Results of U–Pb geochronological studies of accessory zircons from magmatic rocks of the Mikitkin and Ust’Garin massifs
FIRST EVIDENCE OF EDIACARAN MAGMATISM IN THE GEOLOGICAL HISTORY 407
408
SOROKIN et al. 10
Shoshonite Calcalkaline rocks
EMS
Th/Yb
1
0.1
0.01
Tholeites
DMS
0.1
1
10
Ta/Yb Fig. 10. Th/Yb–Ta/Yb tectonic diagram [36] for mag matic rocks of the Mikitkin and Ust’Garin massifs. Symbols are shown in Fig. 2. (EMS) enriched mantle source, (DMS) depleted mantle source. Arrows show the enrichment trend above the subduction zone.
Neoproterozoic zircons are much less common. This is presumably related to the fact that either the Ediaca ran magmatic complexes are of limited abundance or the magmatic rocks of these complexes, due to their composition, could not provide a significant amount of detrital zircons. The gabbro of the Mikitkin Massif and gabbrodior ite, diorite, and quartz diorite of the Ust’Garin Com plex are characterized by similar geochemical features. They are classified as lowK rocks and characterized by a high FeO*/MgO ratio, LILE enrichment, and depletion in some highfieldstrength elements (HFSE), primarily, Nb, Ta, and Ti (Fig. 6). In the dis criminant tectonic diagrams, for instance, Th/Yb– Ta/Yb (Fig. 10), their data points define trends of enrichment typical of suprasudbuction rocks. All these facts indicate that the formation of the magmatic rocks of the Garin Complex occurred in a supra subduction setting. Geochemical and isotope data make it possible to suggest that the formation of the parental melts of the gabbro of the Mikitkin Massif (εNd(t) from +1.4 to +1.6 and 87Sr/86Sr = 0.70480– 0.70592) was related to the melting of the depleted mantle source modified by slabderived fluids and melts, whereas the formation of the gabbrodiorite– diorite–quartz diorite of the Ust’Garin Massif with εNd(t) from –1.2 to –1.3, 87Sr/86Sr(0) = 0.70548– 0.70593, was related to the contamination by conti nental crust and fractional crystallization. It is highly possible that the formation of the magmatic rocks of the Garin Complex was related to the geodynamic set ting of the active continental margin or ensialic crust.
CONCLUSIONS (1) The magmatic rocks of the Mikitkin and Ust’ Garin complexes are Late Proterozoic in age, rather than Paleoproterozoic, as suggested previously [16]. (2) The geochemical features of the magmatic rocks of the MIkitkin and Ust’Garin massifs indicate their suprasubduction nature. The parental melts of the magmatic rocks of the Garin Complex were gener ated in the active continental margin or ensialic islandarc setting due to melting of the subduction modified mantle source, as well as with participation of contamination and fractional crystallization. (3) The emplacement of the Mikitkin and Ust’ Garin massifs occurred during the Ediacaran stage of the geological evolution of the Mamyn terrane and presumably reflects Neoproterozoic convergent geo dynamic processes. These processes presumably led to the final formation of the Precambrian continental massifs in the eastern part of the Central Asian fold belt, which later were inserted into the structure of the epiPaleozoic Amur microcontinent. ACKNOWLEDGMENTS We are grateful to collaborators of the analytical laboratories of the Institute of Geology and Nature Management of the Far East Branch of the Russian Academy of Sciences (Candidates in Physics and Mathematics V.I. Rozhdesntin, A.I. Palazhchenko, E.S. Sapozhnik, and E.V. Ushakova) and the Geolog ical Institute of the Kola Science Center of the Rus sian Academy of Sciences (Candidate in Geology and Mineralogy L.M. Lyalin) for performance of analyti cal studies. We are grateful to reviewers D.P. Gladko chub and I.K. Kazakov for careful analysis of the manuscript and constructive comments. The studies were supported by the Russian Foun dation for Basic Research (project no. 130500116). REFERENCES 1. Geodynamics, Magmatism, and Metallogeny of East Rus sia, Ed. by A.I. Khanchuk (Dal’nauka, Vladivostok, 2006) [in Russian]. 2. Geological Map of the Amur Region and Adjacent Territo ries. 1 : 2500000: Explanatory Notes (VSEGEI, St. Petersburg, 1999) [in Russian]. 3. A. N. Didenko, A. A. Mossakovskii, D. M. Pecherskii, S. V. Ruzhentsev, S. G. Samygin, and T. N. Kheraskova, “Geodynamics of Paleozoic oceans of Central Asia,” Geol. Geofiz., Nos. 7⎯8, 59–75 (1994). 4. V. F. Zubkov and M. T. Turbin, Geological Map of the Baikal–Amur Mainline. 1 : 500000. N52G, Ed. by M.G. Zolotov (VSEGEI, Leningrad, 1984) [in Rus sian]. 5. A. B. Kotov, V. P. Kovach, E. B. Sal’nikova, V. A. Gle bovitskii, S. Z. Yakovleva, N. G. Berezhnaya, and T. A. Myskova, “Stages in the formation of conti nental crust of the central Aldan granulite–gneiss ter
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Recommended for publishing by A.N. Didenko Translated by M. Bogina
RUSSIAN JOURNAL OF PACIFIC GEOLOGY
Vol. 9
No. 6
2015
