ISSN 1819-7140, Russian Journal of Pacific Geology, 2016, Vol. 10, No. 4, pp. 239–248. © Pleiades Publishing, Ltd., 2016. Original Russian Text © A.A. Chashchin, A.A. Sorokin, V.A. Lebedev, M.G. Blokhin, 2016, published in Tikhookeanskaya Geologiya, 2016, Vol. 35, No. 4, pp. 3–13.
Age, Main Geochemical Characteristics, and Sources of Late Cenozoic Volcanic Rocks in the Udurchukan Volcanic Area (Amur Region) A. A. Chashchina, d, A. A. Sorokinb, V. A. Lebedevc, and M. G. Blokhina a
Far East Geological Institute, Far East Branch, Russian Academy of Sciences, pr. Stoletiya Vladivostoka 159, Vladivostok, 660022 Russia b Institute of Geology and Nature Management, Far East Branch, Russian Academy of Sciences, Relochnyi per. 1, Blagoveshchensk, 675000 Russia c Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Staromonetny per. 35, Moscow, 119017 Russia dFar East Federal University, Ayaks-10, 12, Engineering School, Russkii Island, Vladivostok, 690922 Russia e-mail:
[email protected] Received January 22, 2015
Abstract—This paper presents new mineralogical, petrographic, geochemical, and isotope–geochronological data on the Cenozoic basaltic trachyandesites from the Udurchukan volcanic area (Amur region), which occupies the watersheds of the Uril, Mutnaya, and Khingan rivers. Based on the available geochrolonological data and new K–Ar dating, the basaltic trachyandesites are middle Miocene in age (18.9–17.1 Ma). Petrogeochemically, they are divided into two groups. These groups differ in the contents of MgO, TiO2, P2O5, as well as Sr, Ba, Nb, Ta, and LREE, which is presumably related to the different degrees of metasomatic reworking of the mantle sources and their melting. In terms of the trace-element distribution and ratios, the basaltic trachyandesites from the Udurchukan area are close to the within-plate rocks and were contributed by enriched lithospheric mantle previously subjected to fluid metasomatism. Keywords: basaltic trachyandesite, mineralogy, geochemistry, Udurchukan volcanic area, Amur region DOI: 10.1134/S1819714016040035
INTRODUCTION The autonomous volcanic areas formed in the continental parts of Central and East Asia in the Late Cenozoic are united, respectively, in the CentralAsian and Far East subprovinces (Fig. 1), which differ in the structural control of the volcanic activity [16]. Recent studies [2, 10, 12, 16, etc.] provided insight into many questions of the composition and geochronology of the Cenozoic volcanism in this region. However, there is as yet no consensus concerning the composition of the magmatic sources, which is related to their insufficient isotope-geochemical study. For the Far East subprovince, modern analytical data are available only for the Cenozoic volcanic fields of Sikhote Alin and South Primorye [6, 8–12]. Petrological information is almost completely lacking for the coeval volcanic rocks located further inland, in the Russian part of the subprovince. In order to fill this gap, we performed mineralogical, geochemical, and isotope–geochronological study of the Cenozoic volcanic rocks of the poorly studied Udurchukan volcanic area (Amur region). The objectives of our study are
basic magmatic rocks, which are the most informative in deciphering the mantle sources and reconstructing the geodynamic settings of magmatic activity. BRIEF GEOLOGY The Udurchukan area includes volcanic cover, which is developed at the watershed of the Uril, Mutnaya, and Khingan rivers and traced as fragments northwestward, up to the Arkhara River basin. Owing to the intense incision by river valleys, the cover acquired a complex shape. In the tectonic respect, it is confined to the zone of the long-lived NE-trending deep-seated Khingan Fault. Initially, the volcanic and volcanosedimentary rocks that compose the Udurchukan area were recognized as an “andesite– basalt succession” (Pavlov, 1993). During later geological surveys, this succession was ascribed by Dobkin (2000) to the Udurchukan Formation. The erupted lavas of the formation lie with angular unconformity both on the pre-Mesozoic metamorphic rocks of the Lesser Khingan terrane (Uril Formation) and
239
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CHASHCHIN et al.
110°
120°
5 6 7
1 2 3 4
130°
140° 6
5
7
50°
4 3
CA
8
9
FE
2
11 10
20.6 ± 0.8
1
18.7 ± 0.4
40° 12
1
R. ur Am
13
U 17.1 ± 0.5
2
49°
3 4 5 6 0
17.1 ± 0.5
25 km 129°
130°
7
131°
Fig. 1. Geological position of the Cenozoic volcanic rocks of the Udurchukan area. Simplified after [3]. (1) Cambrian terrigenous, terrigenous–carbonate, and conditionally Precambrian metamorphic complexes; (2) Paleozoic and Early Mesozoic granitoids; (3) Cretaceous volcanogenic and volcanosedimentary complexes; (4) Cenozoic loose sediments; (5) Neogene basalts and basaltic andesites; (6) faults; (7) location of samples with K–Ar datings, Ma. U denotes the Udurchukan area. The inset shows a scheme of the Cenozoic volcanic fields of Central and East Asia. Compiled after [16]. (1) Lava fields; (2) Far East subprovince; (3) Central Asian subprovince; (4) boundary of the Amur Plate after [7]; (5) Faults; (6) boundary between the Central Asian (CA) and Far East (FE) subprovinces; (7) studied area. Circled numbers are volcanic provinces and areas: (1) Dariganga; (2) South Khangai; (3) Southern Baikal; (4) Vitim; (5) Udokan; (6) Tokin Stanovik; (7) Sovgavan; (8) Udurchukan, (9) Udalianchi; (10) Jingbo–Mudanjiang; (11) Shkotovo–Shufan; (12) Changbaishan; (13) Hannuoba.
on the Mesozoic magmatic rocks of the Khingan– Olonoi volcanic zone (Obmanii Complex). According to [4], the formation is made up of basaltic andesite, basaltic trachyandesite, basalt, andesite, tuffite, tuffaceous siltstone, tuffstone, and pebble. The volcanosedimentary rocks are weakly or not at all lithified. Drilling works showed that the formation succession consists of four lava flows 4–27 m thick, which are separated by interbeds of volcanosedimentary rocks. Lenses of pebbles (up to 10 m) and volcanic glasses are observed at the base of the cover. The maximum thickness of the Udurchukan Formation reaches 180 m. The available age determinations for the volcanic rocks of the Udurchukan area, in general, correspond to the Miocene determinations previously obtained using spore-pollen assemblages in the intervening tuffaceous–sedimentary beds (Pavlov, 1993). In particular, K–Ar datings on the basaltic andesite vary within 18.6–22.6 Ma and correspond to the first half
of the Miocene (Vas’kin, 1998). In the Udurchukan volcanic field, the basalts rest on clays bearing Early– Middle Miocene palynocomplexes. We carried out additional K–Ar dating of the basaltic trachyandesite from the western flank of the Udurchukan area. The obtained values (17.1–18.9 Ma) confirmed the Middle Miocene age of these magmatic rocks, which is much older than the age of the trachybasalts (9 Ma) located in the Bureya–Arkhara–Uril river interfluve [5]. According to the obtained data, the andesites developed on the northwestern flank of the volcanic field were formed 20.6 Ma (Table 1). With allowance for the new age and chemical data on the basalts of the Udurchukan volcanic area, the studied rocks were subdivided into two groups: (1) basaltic trachyandesite-I with an age of 18.9 ± 0.4 Ma and (2) trachybasaltic andesite-II with an age of 17.1 ± 0.5 Ma. Unfortunately, the poor exposure of the territory and a few prospecting pits did not allow us to under-
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Table 1. Results of K–Ar dating of the rocks of the Udurchukan volcanic area Ordinal Sample no. 1 2 3 4
С-998-1 С-1000 С-1001 Арх-2
Coordinates latitude
longitude
Potassium, wt %
49°12′16.0″ 49°14′39.6″ 49°14′39.6″ –
130°42′10.4″ 130°38′03.5″ 130°38′03.5″ –
2.65 1.27 2.29 0.67
Rock Basaltic trachyandesite Basaltic trachyandesite Basaltic trachyandesite Andesite
40
Arrad, ng/g 3.446 1.670 2.721 0.972
Age, Ma 18.7 ± 0.4 18.9 ± 0.6 17.1 ± 0.5 20.6 ± 0.8
(1–2) basaltic trachyandesites of group I; (3) basaltic trachyandesite of group II; (4) andesite sample collected on the northwestern flank of the Udurchukan area. For sample location, see Fig. 1.
stand the relationships between the exposed flows of the basaltic trachyandesites of both groups.
international standards (NBS-28). The measurement accuracy for δ18O was no less than ±2‰.
METHODS
MINERALOGY AND GEOCHEMISTRY OF THE ROCKS
The compositions of the rock-forming and accessory minerals in the groundmass glass were determined using a JXA-8100 electrone microprobe at the Far East Geological Institute of the Far East Branch of the Russian Academy of Sciences (Vladivostok, analysts N.I. Ekimova and G.B. Molchanova). Major oxides were analyzed by X-ray fluorescence at the Institute of Geology and Nature Management (Blagoveshchensk, analysts A.A. Zenevich and E.V. Ushakova), while REE and trace element contents were analyzed by ICP-MS at the Kosygin Institute of Tectonics and Geophysics of the Far East Branch of the Russian Academy of Sciences (Khabarovsk, analysts A.V. Shtareva and L.S. Bokovenko). Isotope (K–Ar) whole-rock dating of the volcanic rocks was carried out at the Laboratory of Isotope Geochemistry and Geochronology of the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences. The content of radiogenic 40Ar in the samples was analyzed on a MI-1201 IG mass spectrometer (SELMI, Ukraine) using isotope dilution with the 38Ar monoisotope as a spike. The potassium concentrations were analyzed by flame spetrophotometry. Ages were calculated using international decay constants and potassium isotope abundance values. Age values are given with an error of ±2σ. The oxygen isotope composition was determined at the Laboratory of Stable Isotopes of the Far East Branch of the Russian Academy of Sciences (Vladivostok, analysts N.P. Konovalova and E.S. Ermolenko). The analyzed samples (2–3 mg in weight) were heated using an infrared СО2 laser in BrF5 atmosphere. Next, the released oxygen was purified using cryogenic separation and a chemical method using KBr. The content of oxygen isotopes was analyzed on a Finnigan MAT-252 mass spectrometer using a double inlet system. The method was calibrated using laboratory and RUSSIAN JOURNAL OF PACIFIC GEOLOGY
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Petrographic–Mineralogical Characteristics The basaltic trachyandesites of the Udurchukan area are dark gray or black dense rocks with massive or weakly porous structure and aphyric, rarely subaphyric texture. In terms of mineral composition, they are subdivided into plagioclase–olivine–clinopyroxene and plagioclase–clinopyroxene–orthopyroxene varieties; the latter is more typical of group-II basaltic trachyandesites. Plagioclase (Pl) is the major rock-forming mineral in all petrographic varieties. It forms small columnar crystals and groundmass microlites corresponding in composition to labradorite (An51.7–56.92) or andesine (An40.76–48.8). The content of the orthoclase molecule in the mineral varies from 1.69 to 4.46 mol %. The labradorite and andesine frequently reveal sharply expressed normal zoning (An54.34–43.9 in the core and An40.03–18.22 in the rim). In addition to andesine, the groundmass of the plagioclase–two-pyroxene trachybasaltic andesite also contains rare small grains of anorthoclase (An22.14–7.07Or12.13–23.81) and oligoclase (An18.40–17.33Or10.52–11.79), the rims of which are made up of sodic sanidine (An4.75–3.03Or36.38–43.11). Clinopyroxene (Cpx) is present as single small tabular or acicular grains and microlites in the groundmass. In composition, it corresponds to augite (Wo40.12–36.14En43.97–47.41Fs15.91–16.45). Individual minerals show normal zoning (En48.78–44.49Fs13.86–15.93 in the core and En43.80–38.23Fs14.27–21.63 in the rim). It is noteworthy that in addition to augite, the plagioclase–olivine–clinopyroxene basaltic trachyandesite also contains salite (Wo46.72–45.18En29.40–36.79Fs20.76–23.88), while the plagioclase–two-pyroxene varieties contain pigeonite (Pig) (Wo6.52–11.0En43.20–58.42Fs50.28–38.28), which is observed as microlites in the groundmass or forms narrow zones around the augite. No. 4
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Table 2. Representative microprobe analyses of the rock-forming minerals from the basaltic trachyandesite of the Udurchukan volcanic area S-998
S-1000
S-999
first group
second group
PlIc
PlIr
OlIc
OlIr
OlcII
OlcII
CpxcI
CpxrI
CpxcI
CpxcI
OpxcI
PigcII
PlcI
PlrI
CpxcI
СpxrI
PigcII
SiO2
54.49
58.42
39.00
37.79
37.26
33.82
52.87
51.64
47.64
50.90
52.44
51.30
56.20
60.87
52.01
52.33
50.55
TiO2
0.0
0.0
0.0
0.0
0.0
0.0
0.79
1.33
3.32
0.89
0.69
0.61
0.0
0.0
1.07
1.04
0.00
Al2O3
28.05
25.73
0.0
0.0
0.0
0.0
1.70
1.84
3.40
1.26
0.80
0.61
27.13
23.37
1.44
1.36
0.00
Cr2O3
0.0
0.0
0.0
0.0
0.0
0.0
0.62
0.52
0.0
0.0
0.0
0.0
0.0
0.0
0.43
0.0
0.00 31.04
FeO*
0.62
0.79
18.30
23.80
28.70
44.40
8.39
9.08
12.47
14.07
19.38
21.76
0.89
0.53
10.11
11.33
MnO
0.0
0.0
0.33
0.33
0.53
0.69
0.00
0.0
0.0
0.31
0.31
0.41
0.0
0.0
0.0
0.0
0.70
MgO
0.0
0.0
42.91
38.30
33.75
20.66
16.74
15.27
11.14
14.24
23.94
19.54
0.0
0.0
16.10
15.72
14.84
CaO
10.94
8.47
0.0
0.0
0.0
0.40
19.17
20.57
21.65
18.31
2.21
4.56
9.72
4.86
19.17
0.0
1.88
Na2O
5.31
6.48
0.0
0.0
0.0
0.0
0.00
0.0
0.64
0.45
0.0
0.0
5.79
7.94
0.0
0.56
0.0
0.46
0.81
0.0
0.0
0.0
0.0
0.44
0.47
0.0
0.0
0.0
0.66
1.28
0.0
0.0
K2O Σ Wo
99.87 100.71 100.51 100.17 100.56 100.01 100.73 100.73 100.25 100.43 –
–
–
–
–
–
39.15
42.07
46.17
37.26
0.0 99.77 4.37
98.79 100.39
98.86 100.33 100.87
0.0 99.02
9.33
–
–
38.74
37.63
4.04
En
–
–
–
–
–
–
47.53
43.42
33.03
40.32
65.76
55.77
–
–
45.30
44.40
44.14
Fs
–
–
–
–
–
–
13.33
14.51
20.76
22.42
29.87
34.9
–
–
15.96
17.97
51.82
An, %
51.86
40.03
–
–
–
–
–
–
–
–
–
–
46.32
23.42
–
–
Or, %
2.6
4.56
–
–
–
–
–
–
–
–
–
–
3.75
7.34
–
–
Fo, %
–
–
80.70
74.20
67.7
45.3
–
–
–
–
–
–
–
–
–
–
Indices near mineral abbreviations: (I) phenocrysts, (II) groundmass microlites; (c) core, (r) rim. * All iron as FeO.
Orthopyroxene (Opx) was found only in the plagioclase–two-pyroxene basaltic trachyandesite. It forms single small rounded and angular grains, which in composition correspond to hypersthene or ferrohypersthene (Table 2). The mineral is characterized by normal zoning (En65.76Fs29.87 in the core and En28.38Fs68.16 in the rim). Oivine (Ol) occurs mainly in the plagioclase–olivine–clinopyroxene varieties. It is represented by small, more rarely large (up to 1 mm across) shortprismatic, rounded, or equant crystals corresponding in composition to chrysolite or hyalosiderite (Fo80.7–71.6). Most of the olivine phenocrysts are characterized by normal zoning (Fo80.7–75.5 in the core and Fo75.1–73.0 in the rim). In the groundmass, it occurs as microlites or skeletal crystals usually having hyalosiderite (Fo69.3–64.9), more rarely hortonolite (Fo40.1–45.3) composition. Frequently, the olivine phenocrysts contain inclusions of Cr-magnetite. Ore minerals were mainly found in the groundmass, where they are represented by small equant and tabular crystals of Ti-magnetite, more rarely ilmenite, and sometimes Cr-magnetite, which is observed only in the plagioclase–olivine–clinopyroxene basaltic trachyandesite.
Geochemical Characteristics In terms of alkali contents (Table 3), all of the studied volcanic rocks are ascribed to the mildly alkaline series, plotting in the basaltic trachyandesite field in the diagram SiO2–(Na2O + K2O) (Fig. 2). They are ascribed to the K–Na (K2O/Na2O = 1.17–2.34) highalumina (al = 1.06–1.20) rocks. As mentioned above, the considered rocks may be subdivided into two groups: basaltic trachyandesite-I and basaltic trachyandesite-II. The basaltic trachyandesite-I is characterized by moderate contents of MgO (4.34–5.25 wt %) and high magnesium number Mg# = Mg/(Mg + Fe2+) = 56– 60 (in at %), as well as elevated TiO2 (1.60–1.70 wt %) and P2O5 (0.30–0.46 wt %). In addition, this rock is characterized by moderate to elevated K2O contents, which vary between 1.63 and 3.13 wt %; high contents of large-ion lithophile elements (Sr, Rb, Ba, Cs); and some depletion in high-field-strength elements (Th, U). The rare-earth element (REE) distribution in the basaltic trachyandesite-I (Fig. 3a) is characterized by a high degree of LREE fractionation (La/Yb = 17.46– 22.58). The trace-element pattern of the rocks has a weakly expressed Eu minimum (Eu/Eu* = 0.84– 0.93), which indicates a limited role of plagioclase in fractionation. Note that an increase of K2O in the basaltic trachyandesite of this group is accompanied
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Table 3. Major- (wt %) and trace-element (ppm) composition of Late Cenozoic volcanic rocks of the Udurchukan area S-998
S-998/1
S-1000
S-1000/1
S-999
first group
S-1001
S-1001/1
S-999/1
second group
SiO2
54.32
54.30
53.76
54.58
54.45
54.32
55.44
54.56
TiO2
1.70
1.68
1.71
1.60
1.94
1.87
1.88
1.97
Al2O3
15.78
15.60
15.93
15.21
15.89
15.67
16.03
15.76
Fe2O3 MnO MgO CaO Na2O
9.08 0.12 5.25 5.98 3.67
9.09 0.12 5.20 5.97 3.64
9.81 0.12 4.83 6.51 3.81
10.02 0.11 4.34 5.86 4.00
9.12 0.10 4.02 5.62 4.01
8.91 0.10 3.86 5.50 4.10
8.80 0.11 4.19 5.51 4.09
9.16 0.10 3.99 5.73 3.99
K2O
3.13
3.05
1.63
2.22
2.75
2.83
2.97
2.76
P2O5
0.46 0.52 100.00 92.09 119.21 23.13 69.19 46.21 513.54 12.82 113.72 19.12 0.67 514.87 20.32 42.78 5.29 23.11 4.90 1.43 5.34 0.60 3.01 0.47 1.25 0.12 0.90 0.10 2.96 1.10 3.81 2.39 0.50
0.45 0.90 99.98 95.86 116.27 23.15 68.85 48.52 534.32 13.32 120.15 19.72 0.74 532.91 20.79 43.46 5.35 23.52 4.96 1.46 5.39 0.61 3.11 0.49 1.30 0.13 0.93 0.10 3.01 1.06 3.89 2.46 0.51
0.33 1.56 99.99 90.36 96.80 24.22 68.88 24.53 420.35 12.68 92.43 13.84 0.17 345.44 14.23 29.38 3.68 17.12 4.06 1.31 4.58 0.54 2.84 0.46 1.19 0.11 0.84 0.09 2.21 0.68 2.57 1.22 0.21
0.30 1.69 99.94 86.16 79.59 23.11 60.78 34.26 367.72 14.17 119.65 16.00 0.36 361.24 17.24 35.05 4.27 18.92 4.37 1.29 4.94 0.59 3.10 0.51 1.34 0.14 0.99 0.11 2.99 0.80 3.51 1.70 0.31
0.64 1.47 99.99 87.53 66.85 22.02 58.99 43.31 704.12 13.35 128.61 37.56 0.12 719.87 30.23 59.29 7.44 31.58 6.17 1.82 6.72 0.72 3.40 0.49 1.21 0.10 0.70 0.06 2.98 2.09 3.61 3.09 0.57
0.59 2.17 99.92 87.31 92.14 21.61 69.12 49.70 581.48 12.42 135.02 32.92 0.22 707.75 27.53 54.22 6.75 28.03 5.58 1.63 6.06 0.65 3.02 0.44 1.13 0.09 0.70 0.07 3.15 1.65 3.74 2.98 0.57
0.60 0.39 99.99 89.97 103.33 21.74 69.45 53.45 624.85 13.00 144.67 34.52 0.57 727.88 28.97 57.05 7.00 29.63 5.88 1.71 6.36 0.68 3.13 0.47 1.18 0.10 0.74 0.07 3.31 1.73 4.27 3.14 0.60
0.64 1.28 99.94 85.48 65.44 22.19 59.48 41.46 699.82 13.28 129.73 37.47 0.14 724.82 30.18 59.18 7.41 31.22 6.09 1.79 6.60 0.71 3.31 0.48 1.18 0.09 0.68 0.06 2.93 2.03 3.74 3.07 0.56
L.O.I. Σ V Cr Co Ni Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U
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I II
Tephriphonolite
Tephrite
5
Basanite
0 35
Trachyte Rhyolite
3
2 Basalt
1
Trachyandesite
(a)
100
OIB
10
1
Dacite
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Rock/primitive mantle
Phonotephrite
Foidite
Andesite
10
Basaltic andesite
Na2O + K2O, wt %
Phonolite
Rock/primitive mantle
244
(b)
100
45
55 SiO2, wt %
65
75
Fig. 2. Classification diagram (Na2O + K2O)–SiO2 [18] for the Late Cenozoic volcanic rocks of the Udurchukan area. Numbers denote the fields: (1) picrobasalt; (2) trachybasalt; (3) basaltic trachyandesite. (I–II) volcanic rocks: (I) basaltic trachyandesite of group I, (II) basaltic trachyandesite of group II. Contents of major oxides are calculated to a water-free basis.
by an increase of Cs, Rb, Ba, Ta, Th, U, and LREE, whereas the contents of the other trace elements remain at almost the same level. The basaltic trachyandesite-II is chemically close to the basaltic trachyandesite of the first group, but differs in somewhat lowered MgO (Mg# = 53–55) and elevated TiO2 and P2O5 (Table 3). The rocks of these two groups are similar in the contents of compatible elements (Ni, Co, V), as well as Zr, Hf, and Cs. At the same time, the basaltic trachyandesite-II shows higher Sr, Ba, Nb, Ta, as well as LREE, and slightly lowered HREE (La/Yb = 39.11–44.37). The analyzed samples of the basaltic trachyandesites-II are characterized by a weakly expressed Eu minimum (Eu/Eu* = 0.85– 0.87). The specifics of the incompatible element distribution in the basaltic trachyandesites of the groups are well seen in the primitive mantle-normalized patterns. All of the normalized incompatible element distribution patterns for the basaltic trachyandesite-I have clearly expressed positive Rb, Ba, Sr, and K anomalies and negative Th and U anomalies and weakly expressed negative Hf, Ta, and Nb anomalies (Fig. 3b), which is regarded as a typomorphic feature of the suprasubduction rocks. The basaltic trachyandesite-II, in addition to the above-mentioned features, reveals a weakly expressed negative Ti anomaly and the absence of negative Nb and Ta anomalies. At the same time, the data points of the basaltic trachyandesite-I in the discriminant diagrams Th–Hf/3–Ta and Th–Hf/3–Nb/16 fall in the field of E-MORB basalts and within-plate tholeiites, whereas the basaltic trachyandesite-II lies in the field of
OIB
10
1 Cs Ba U Nb La Sr Hf Sm Ti Dy Yb Rb Th K Ta Ce Nd Zr Eu Gd Y Lu Fig. 3. Concentrations of rare-earth (a) and incompatible (b) elements normalized to primitive mantle [24] in the Late Cenozoic volcanic rocks of the Udurchukan area. Symbols are shown in Fig. 2. Composition of ocean island basalts (OIB) are shown after [24].
within-plate alkali basalts (Fig. 4). Since all of the analyzed basaltic trachyandesites have high Th/Yb (1.5–4.4) and Ta/Yb (0.80–2.99) ratios, their data points in the Th/Yb–Ta/Yb diagram (Fig. 5) are confined to the field of within-plate basalts. It should be noted that the basaltic trachyandesite-I falls near the average composition of ocean island basalts (OIB), whereas the basaltic trachyandesite-II is slightly shifted toward an enriched mantle source. The similarity of the basaltic trachyandesites to ocean island basalts also follows from the other trace element ratios. In particular, both these groups are characterized by high Nb/U (38–67) and Ce/Pb (10–16) ratios and low Zr/Nb (3.42–7.47) ratios, which are close to those of OIB-type basalts (47, 25, and 5.8, respectively [22]). In general, the basaltic trachyandesites of the Udurchukan area differ from the average OIB in lower Th, Ta, Nb, Hf, Zr, and REE and higher Rb, Ba, and K. However, it should be noted that the basaltic trachyandesite-II in terms of LREE (La, Ce, Pr, Nd) and Cs, Th, Ta, Nb, and Sr contents approaches oceanisland basalts. Thus, the basaltic trachyandesites of the Udurchukan area bear geochemical characteristics of both island arc and within-plate lavas.
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Fig. 4. Th–Hf–Ta diagram [27] for the Late Cenozoic volcanic rocks of the Udurchukan area. Symbols are shown in Fig. 2. Fields in the diagram: (A) N-type MORB; (B) E-type MORB and within-plate tholeiites; (С) within-plate alkali basalts; (D) island-arc basalts.
DISCUSSION Role of Crustal Contamination Taking into account the fact that the basaltic trachyandesites of the Udurchukan area were formed in relatively thick continental crust (25–35 km [4]), crustal contamination played a definite role in the magma genesis of the described rocks. However, the obtained isotope–geochemical data indicate an insignificant crustal contribution. In particular, the oxygen isotope composition of the basaltic trachyandesites of both these groups varies within δ18О = 7.4–7.8‰, which is close to the range of δ18О observed in unaltered mantle rocks (6–8‰ [23]). In addition, the basaltic trachyandesites are characterized by elevated Ce/Pb (10–16) and Th/Nb (0.08–0.12) and low La/Nb (0.8–1.1) ratios as compared to continental crust (3.8, 0.44, and 2.2, respectively [26]), which also suggests an insignificant influence of the crustal protolith on the melt composition. Mineralogically, this is expressed in the absence of reversely zoned rockforming minerals (Pl, Px, Ol) and disequilibrium mineral assemblages. Fractional Crystallization The presence of plagioclase, pyroxene, and olivine phenocrysts with well-expressed normal zoning in the basaltic trachyandesites of the Udurchukan area indicates that the studied volcanic rocks genesis was controlled by fractionation. This is also confirmed by some geochemical features, for instance, moderate Mg# (53–60) and relatively low contents of Ni (61– 70 ppm), Cr (65–120 ppm), Co (22–24 ppm) in the RUSSIAN JOURNAL OF PACIFIC GEOLOGY
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Fig. 5. Th/Yb–Ta/Yb diagram [20] for the Late Cenozoic volcanic rocks of the Udurchukan area. Symbols are shown in Fig. 2. Fields in the diagram show the compositions of the island-arc (IA) and active- continentalmargin (ACM) basalts. Data points of reference settings: E-MORB and N-MORB are the compositions of “enriched” and “normal” basalts of the mid-ocean ridges, respectively; (OIB) composition of ocean island basalts are shown after [24]. (DMS) depleted mantle; (EMS) enriched mantle. Vectors of rock alteration with contribution of: (S) subduction component, (C) contamination by continental crust, (W) within-plate (mantle) enrichment by lithophile elements, (F) fractional crystallization.
basaltic trachyandesites, which indicates the fractionation of mafic minerals (Px, Ol). However, fractionation crystallization cannot explain the steady difference between the basaltic trachyandesites of the first and second groups in terms of Ва, Nb, Ta, and REE. Based on the above data, we may suggest that the observed geochemical differences between the basaltic trachyandesites of these groups are presumably related to either different composition of their mantle protoliths or conditions of magma generation. Composition of Magmatic Sources The composition of the magmatic sources and the degree of their enrichment or depletion are usually estimated using concentrations of so-called “conservative” incompatible elements (Nb, Ta, Zr, Hf, Y, Yb), whose content is controlled by the composition of the melting protolith [22]. In addition, the mantle sources are frequently reconstructed using the Nb/Sm, Nb/Yb, and Zr/Yb ratios [9]. It is believed that these element pairs do not usually fractionate from each other during melting and crystallization differentiation, and correspondingly, their ratios are constant in the rocks formed by melting of the same source. Figure 6 demonstrates the N-MORB-normalized incompatible element patterns of the studied basaltic trachyandesites of the Udurchukan area. The line No. 4
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8% Grt
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Rb Th Nb K Ce Pr Nd Hf Eu Gd Dy Ho Tm Ba U Ta La Pb Sr Zr Sm Ti Tb Y Er Yb
Fig. 6. N-MORB-normalized [24] incompatible element distribution patterns in the Late Cenozoic volcanic rocks of the Udurchukan area. The dashed line connects “conservative” elements, which are immobile with respect to the fluid phase and have low mineral–melt bulk partition coefficient.
connecting the values of the normalized contents of “conservative” elements in this diagram makes it possible to estimate the degree of depletion of the mantle source [21, 22]. In our case, the basaltic trachyandesites of both groups were derived by melting of a more enriched protolith as compared to the source of depleted N-MORB type oceanic basalts, which follows from the steep negative slope of the line connecting these elements, as well as from the high (>1) normalized concentrations of high-field strength elements and MREE (Sm, Eu, Gd). The basaltic trachyandesite-II is characterized by higher normalized HFSE (Nb, Ta, Zr) as compared to the basaltic trachyandesite-I, which indicates the contribution of a more enriched mantle source in their genesis. This assumption is also supported by the high Nb/Sm (5.87–6.16), Nb/Yb (46.65–55.10), and Zr/Yb (183.73–195.5) ratios in the basaltic trachyandesiteII, which significantly exceed the similar values in the basaltic trachyandesite-I (3.41–3.97, 16.16–21.24, and 11.01–129.19, respectively). The same conclusion can be drawn from the Ce/Y–Zr/Nb diagram (Fig. 7), which shows that the basaltic trachyandesite-I was formed by moderate moderate degree melting (>2%) of mantle peridotite with garnet content up to 4%, whereas the basaltic trachyandesite-II was formed by a slightly lower melting
Fig. 7. Compositions of the basalatic trachyandesites of the Udurchukan area in the Ce/Y–Zr/Nb diagram, showing variations of melt composition depending on the content of modal garnet and degree of mantle melting, after [25]. The solid lines correspond to the compositions of melts from the mantle source enriched in incompatible elements (two times chondrite). The numbers near the lines show the garnet content. Ticks indicate the degree of melting, in percent. Symbols are shown in Fig. 2.
degree ~1% at a higher garnet content (6%) in the mantle, i.e., from a more enriched magmatic source. As mentioned previously, the characteristic features of the described rocks are the elevated contents of LILE (Rb, Ba, Sr, K) and low U, Th, and in some cases, weak depletion in Ta, Nb, and Ti. Such geochemical signatures are typical of the island arc rocks, whose formation is related to the contribution of continental lithospheric mantle metasomatically reworked by previous subduction events. It should also be recalled that the basaltic trachyandesite-II has higher contents of large-ion lithophile elements (Ba and Sr) as compared to the basaltic trachyandesite-I, which may indicate their formation from mantle subjected to strong metasomatic reworking. Taking into account the localization of the studied rocks of the Udurchukan area within the Khingan– Olonoi volcanic zone, it is more reasonable to suggest that the metasomatic reworking of the mantle was caused by tectonomagmatic processes related to the formation of this volcanic zone. However, recent geochemical and isotope-geochronological studies of the Early and middle Cretaceous magmatic associations of the Khingan–Olonoi volcanic zone has shown that they were formed on a Californian-type transform continental margin [13, 14]. Hence, the metasomatic reworking of the mantle of the considered region cannot be directly related to the evolution of the Khingan–Olonoi volcanic zone. At the same time, the Late Permian gabbroids (Karagai Massif) found in the northern part of the Lesser Khingan terrane are in their geochemistry ascribed to typical suprasubduction rocks [1]. This fact indicates that the lithospheric
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mantle of the Lesser Khingan terrane was modified by pre-Cretaceous subduction processes. However, this problem requires independent and detailed study, which is beyond the scope of this paper. CONCLUSIONS Geochemical and geochronological studies of the Middle Miocene basic and intermediate magmatic rocks of the Udurchukan volcanic area have allowed us to draw the following conclusions. (1) The basaltic trachyandesites of the Udurchukan area are subdivided into two groups differing in the contents of major (MgO, TiO2, and P2O5) and trace (Sr, Ba, Nb, Ta, REE) elements. It is suggested that the observed differences between the compositions of the basaltic trachyandesites of both these groups are caused by different degrees of melting of the mantle sources, which also differed in composition and degree of metasomatic reworking. (2) The trace element distribution and ratios indicate that the middle Miocene basaltic trachyandesites of the Udurchukan area are close to the within-plate rocks and were contributed by continental lithospheric mantle metasomatically reworked during previous, presumably, pre-Cenozoic tectonomagmatic events in this region. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (project no. 13-05-12090) and the Far East Branch of the Russian Academy of Sciences (project no. 15-I-2-042). REFERENCES 1. I. V. Buchko, A. A. Sorokin, and N. M. Kudryashov, “Late Paleozoic gabbroids of the Lesser Khingan terrane of the eastern Central-Asian Fold Belt: age, geochemistry, and tectonic setting,” Russ. J. Pac. Geol. 7 (3), 189–198 (2013). 2. Geodynamics, Magmatism, and Metallogeny of East Russia, Ed. by A. I. Khanchuk (Dal’nauka, Vladivostok, 2006) [in Russian]. 3. State Geological Map of the Russian Federation. 1 : 1000000. Sheet M-52 (53). Blagoveshchensk, Ed. by E.M. Zablotskii (VSEGEI, St. Petersburg, 1995) [in Russian]. 4. State Geological Map of the Russian Federation. 1 : 1000000 (Third Generation). Dal’nevostochnaya Series. Sheet M-52. Blagoveshchensk: Explanatory Notes, Ed. by A. S. Vol’skii (VSEGEI, St. Petersburg, 2012) [in Russian]. 5. I. M. Derbeko and Yu. V. Koshkov, “Manifestations of the Cenozoic volcanism in the Amur region,” in Mesozoic and Cenozoic Magmatic and Metamorphic Rocks of Far East (Khabarovskgeologiya, Khabarovsk, 2001), pp. 3–6 [in Russian]. RUSSIAN JOURNAL OF PACIFIC GEOLOGY
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Recommended for publishing by Yu.A. Martynov
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