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Geochemical evolution of the early Paleozoic collisional magmatism from autochthonous migmatites and granitoids to multiphase granite intrusions (Sharanur and Aya complexes, Baikal Region)
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ScienceDirect Russian Geology and Geophysics 59 (2018) 1616–1625 www.elsevier.com/locate/rgg
Geochemical evolution of the early Paleozoic collisional magmatism from autochthonous migmatites and granitoids to multiphase granite intrusions (Sharanur and Aya complexes, Baikal Region) V.S. Antipin a,*, L.V. Kushch a, N.V. Sheptyakova a, A.G. Vladimirov b,c a
A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia b Novosibirsk State University, ul. Pirogova, 2, Novosibirsk, 630090, Russia c V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia Received 15 February 2018; accepted 26 July 2018
Abstract Overall petrologic and geochemical data indicate that the early Paleozoic magmatism in the Olkhon area of the Baikal Region exhibits diverse types of granitoids, whose time of formation is estimated at a narrow age interval of 500–465 Ma. This magmatism was responsible for the formation of both autochthonous gneiss–migmatite–granitoid suites (Sharanur complex) and multiphase intrusions (Aya complex) emplaced into the upper horizons of the continental crust. In major-element chemistry, K2O/Na2O values, and rare-element composition the migmatite– plagiogranites and calc-alkaline and subalkaline granitoids of the Sharanur complex are similar to the host gneisses and schists, as they were likely derived from melting of the ancient metamorphic substratum of the Olkhon series. In new isotope-geochemical characteristics (ICP MS method) the Sharanur granitoids are close to the first-phase biotite granites of the Aya massif, whose further geochemical evolution was governed mainly by intrachamber magmatic differentiation leading to the production of second-phase leucogranites enriched in HREE and HFSE (in particular, Ta and Nb) and depleted in Sr, Ba, Eu, Li, and LREE. The origin of the autochthonous and intrusive granitoids is related to early Paleozoic collision events within the Olkhon metamorphic terrane, while the formation of syncollisional granitoids is best explained by both melting of the crust protolith (Sharanur complex) and magmatic differentiation (multiphase Aya intrusion). All mineralogical and geochemical characteristics indicate that these granitoids are distinguished from rare-metal pegmatoid granites and Li–F and Rb–Be–Nb pegmatites, whose vein bodies crosscut the granitoids, and are regarded as middle Paleozoic rocks, which mark the transition to within-plate magmatism in the Baikal Region. © 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All right reserved. Keywords: magmatism, granitoids; collision; geodynamics; geochemical types; Paleozoic; intrusion; complexes
Introduction The Central Asian fold belt is characterized by immense volumes of granitic rocks formed as a result of early Paleozoic accretion–collision events, accompanied by the closure of the Paleo-Asian Ocean (Antipin et al., 2014; Distanova, 2013; Dobretsov and Buslov, 2007; Donskaya et al., 2013, 2017; Vladimirov et al., 2011; Yarmolyuk et al., 2000). The collision-related belts occur mainly along the southern margin of the Siberian craton, and are understood as having formed during the accretion of terranes of different geodynamic origins to marginal parts of the Siberian craton. Therefore, the age of major collision events within those terranes is in a range of 500–465 Ma (Gladkochub et al., 2010; Vladimirov et al., 2008). In the junc-
* Corresponding author.
E-mail adress: [email protected] (V.S. Antipin)
tion zone between the Siberian craton and the Barguzin microcontinent the collision-related granitoids are more widespread within the Olkhon terrane (Fedorovsky et al., 1995; Gladkochub et al., 2010; Zonenshain et al., 1990), that covers the Olkhon area (Olkhon Island and Pre-Olkhon’e) and stretches along the western shore of Lake Baikal. As follows from field and geologic studies (Fedorovsky et al., 1995) the Olkhon terrane was produced by the Caledonian collision as a collage of numerous tectonic units composed of rock complexes of different ages originated in various tectonic settings. At present nine complexes which are united into four zones are distinguished within the Olkhon terrane (Donskaya et al., 2017). Previous studies have shown that the early Paleozoic magmatism in the Baikal Region (Khamar-Daban Ridge, Olkhon Island) was responsible for the production of diverse types of granitoids, however, it is close to the mean composition of the continental crust, which confirms the existence of the Olkhon–
1068-7971/$ - see front matter D 201 8, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 8.12.007
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Khamar-Daban block as a single terrane. The granitoids of the Khamar-Daban Ridge were produced in the range of 516– 690 Ma; as regards granitoids of the Sharanur complex on Olkhon Island they were dated as 505–477 Ma (Antipin et al., 2014; Makrygina et al., 2013). The Sharanur and Khamar-Daban granitoids comprise different geochemical types including formations of normal Na-alkalinity (migmatites and plagiogranites), calc-alkaline (K–Na granites and leucogranites) and subalkaline (granosyenites and quartz syenites). Calc-alkaline and subalkaline granitoids are supposed to have been derived from anatectic melting of the ancient gneiss-schistose substratum. In their compositions, these granites are fairly similar to the recent and ancient collision-related granitoids (Himalayas and Central Spain) that points to the inheritance of the protolith composition (Gorlacheva, 2014; Makrygina et al., 2013). In this paper, based on the studies of Sharanur and Aya granitic complexes, we discuss the magmatic and petrologic evolution of the early Paleozoic magmatism represented by both autochthonous migmatite–gneiss suites and multiphase intrusions (Olkhon area, Baikal Region). The major goals for this study are to identify: the age relationships between granitoids and compositions of their crustal sources; role of migmatization processes, melting of compositionally distinct substrata and magmatic differentiation in the formation of the granitoids; and geochemical characteristics which can be used to decipher the geodynamic setting and the nature of the Phanerozoic collision-related igneous rocks. Methods Geological mapping of major granitoid occurrences (Olkhon Island) and collecting of representative whole rock and geochemical samples were the main components of our study. In the process of sampling, a particular attention was given to the Aya intrusion, as precise geochemical data for this pluton were lacking. These new obtained results have been compared with data available for the Sharanur complex, taken as a reference. The silicate analyses were performed using a classical chemical method by analysts G.A. Pogudina, T.V. Ozhogina and XRF by analyst A.L. Finkelstein with inaccuracy 0.5 to 5%. Alkaline elements were analyzed by flame photometry with the precision 5–10% by analysts L.V. Altukhova and I.M. Khmelevs-
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kaya; rare and rare earth elements were defined by L.A. Chuvashova, E.V. Smirnova using the ICP-MS method (σ ± 5–10%), and O.V. Zarubina with inaccuracy 10–20%. All analyses were done at the Analytical Center for Collective Use, Irkutsk Science Center, SB RAS with the use of equipment installed at Center for Collective use, IGC, SB RAS and certified reference samples (Geostandrads…., 1994). Geology, age, structure of collision-related granitoids and relationships with metamorphic sequences Sharanur migmatite–granite complex. On Olkhon Island, the widespread occurrences of voluminous granitoids is a typical feature of the Sharanur pluton, which includes both autochthonous and allochthonous facies. Geologic setting, geochronology, composition of the Sharanur granitoids as well as the relationships between these granitoids and the country metamorphic sequences were previously discussed (Antipin et al., 2014; Donskaya et al., 2013; Kuklei, 1988; Makrygina and Pet rova, 1996; Makrygina et al., 2010; Mikheev et al., 2017; Vla dimirov et al., 2004). Migmatites, granite–gneisses and pegmatoid granites of allochthonous veins were earlier regarded as belonging to the Shranur granitoid complex as well (Pavlovskii and Eskin, 1964). On Olkhon Island our research was concentrated on three large plutons (Sharanur, Tashkinei and Southern Olkhon) and a series of smaller granitoid outcrops in its north and southwest as well as in the Pre-Olkhon area (Fig. 1). According to the recent SHRIMP-II U–Pb data (VSEGEI Institute, St. Petersburg) the weighted average age for the cores of zircon grains from K–Na calc-alkaline Sharanur granitoids is 505 Ma and that for the marginal parts is 477 Ma (Makrygina et al., 2010). The age estimates for the Sharanur granitoids lie within a narrow time interval (465–486 Ma, Table 1). Granosyenites and quartz syenites occurring here yield an age of 495 ± 6 Ma (Gladkochub et al., 2010), which is well consistent with the timing of Sharanur K–Na granitoids formation, thus pointing to the genetic affinity of these calc-alkaline and subalkaline rocks. In Kolokolnya Bay some outcrops of granitoids, varying in composition, have been mapped. They occur within granite-
Fig. 1. Geological map of the Olkhon region (from (Pavlovskii and Eskin, 1964), modified by (Fedorovsky, 2004)). 1, granites, Khaidai complex; 2, gabbro, Ozersky complex; 3, granitoids, Sharanur complex; 4, granites, Aya complex; 5, gneiss domes, Olkhon Island; 6, marbles, schists, quartzites (Olkhon series); 7, schists, marbles, quartzites (Anga sequence); 8, carbonate-silicate rocks, amphibolites, 9, shear zones.
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Table 1. U–Pb isotope ages after zircons from granites and syenites, Olkhon Island and Olkhon area Age, Ma
Geologic object
Reference
Early stage of accretion–collision tectogenesis, granulite facies, granosyenites and enderbites 496 ± 3
Enderbites, Sapshilan pluton, Chernorud zone, Olkhon area
(Khromykh et al., 2005), SHRIMP-II
495 ± 6
Quartz syenite, Southern Olkhon pluton, Olkhon Island
(Gladkochub et al., 2010), SHRIMP-II
485 ± 1.5
Veins of nepheline syenites in gabbro, Ulan-Khargany, Olkhon area
(Khromykh et al., 2005), SHRIMP-II
488.6 ± 8, core 464 ± 11 rim
Subalkaline granosyenite, Tutai pluton, Olkhon area
(Donskaya et al., 2013), SHRIMP-II
Late stage of collision tectogenesis (collapse), synoverthrust nepheline syenites and associated rocks, Tazheran pluton 471 ± 5
Syenites
(Sklyarov et al., 2009;
464 ± 2
Pegmatoid nepheline syenites, Tazheran pluton
(Fedorovsky and Sklyarov, 2010),
451 ± 1
SHRIMP-II
Synoverthrust granites, Sharanur complex and Aya pluton 476 ± 4 500 ± 5
Granites, Sharanur complex, Olkhon Island Zircon cores
(Makrygina et al., 2014), SHRIMP-II
474 ± 1.5
Para-autochthonous granites in metamorphites close to hyperbasite boudinage, Shida. Single grain
(Mekhonoshin et al., 2013), SHRIMP-II
468 ± 6
Granites, Aya pluton, Olkhon area
(Yudin et al., 2005), SHRIMP-II
458 ± 2
Granites, Central dome zone, Olkhon area
(Vladimirov et al., 2011), SHRIMP-II
550 ± 5 455 ± 5
Cross-cutting vein of pegmatoid granites, Shibetskii Cape, Olkhon Island. Protolith Magmatic zircon. Single grain
(Mikheev et al., 2017)
627 ± 6 440 ± 5
Detrital zircon in alkaline syenites. Single grain Alkaline syenite, Budun pluton, Olkhon Island. Single grain
(Makrygina et al., 2013)
390 ± 5
Rare-metal pegmatoid granite, Tashkinei valley, Olkhon Island. Single grain
(Antipin et al., 2014)
gneisses, in places intercalated with marbles and crystalline schists of the hosting sequence (Southern Olkhon massif). Gneisses locally contain plagiogranites (vein, frequently layered concordant bodies) and migmatites with gneiss-like texture and gradual transitions between rocks in marginal parts of numerous small domes. These units are interpreted as belonging to autochthonous facies of granite magmatism. This site is rich in biotite and amphibole–biotite, in places porphyry-like granites producing large allochthonous intrusions along the southwestern coast of Olkhon Island (major granitoid variety). The intrusions are marked by sharp contacts between granites and the host sequence as well as occurrences of small layered bodies of fine-grained leucogranites. Granites occur here along with bodies of amphibole-biotite granosyenites and quartz syenites which extend as a line from the Unshui Cape in the southwest to Tashkinei valley (Antipin et al., 2012, 2014). The largest granite-gneiss dome near Lake Shara-Nur in the central part of Olkhon Island (close to Tashkinei valley) is regarded as a petrotype of Sharanur granitoids. It is hosted by garnet–biotite and amphibole–biotite gneisses and migmatites grading into para-autochthonous granites. As evident from myrmekite rims and plagioclase residues in coarse K-feldspar grains, granite-gneisses show development of microcline after plagioclase. Plagiogranites and migmatites are banded rocks with a higher feldspar content. The plagiomigmatites are of coarse-grained texture and banded structure. These rocks contain, on the average (vol.%): amphibole—15, biotite—5, plagioclase (andesine nos. 30–40)—35, quartz—40. Garnet frequently occurs as well.
The most abundant varieties here are biotite granites which are of cataclastic and granoblastic textures. Feldspar biotitic migmatites and granites contain oligoclase; biotite tends to be richer in iron (71.5%). Amphibole–biotite varieties or varieties containing garnet with porphyritic texture form large allochthonous intrusions. The granites contain, on the average (vol.%): plagioclase—30, quartz—25, biotite—5, and microcline—>40. The accessory minerals in the Sharanur pluton are apatite, titanite, magnetite, allanite, and zircon. Zircon is either elongated or oval with complex zoning. Vein series include either small layered or lens-like bodies of fine-grained leucogranites and large veins of pegmatoid granites; the latter usually outcrop outside domes and cut pockets of intercalating marbles, quartzites and diopside schists. Several cross-cutting veins of medium-grained biotite granites up to 3–4 m thick and over 1 km long occur on the north of Olkhon Island (close to Sem’ Sosen Bay). Biotite is oriented across the vein, concordant with the foliation of country gneisses. Granosyenite and quartz syenite exposures extend from the Unshui Cape in the south to Tashkinei valley in the north. These are massive or weakly gneissoid amphibole-bearing rocks with minor amounts of quartz, ferruginous biotite and residues of pyroxene replaced by hastingsite. Feldspar is represented by andesine no. 25 and alkaline feldspar with 5–10% of albite minal. These rocks contain the same paragenesis of accessory minerals as in Sharanur granites, whose petrography and geochemistry are given (Antipin et al., 2014). Aya granite–leucogranite complex. The largest intrusive granitoid body of the Aya complex (Ivanov and Shmakin,
V.S. Antipin et al. / Russian Geology and Geophysics 59 (2018) 1616–1625
Fig. 2. Geologic-petrographic schematic map, Aya pluton, modified by (Ivanov and Schmakin, 1980). 1, alluvial sediments; 2, deluvial-proluvial sediments; 3, calcite graphite-bearing marbles intercalating with dolomite-calcite marbles and amphibole schists; 4, hornblende schists; 5–6, granites, Aya pluton: 5, biotite, 6, leucocratic; 7, pegmatite veins; 8, quartz veins, 9, rupture dislocations.
1980) outcrops between Anga River mouth and Aya Bay in the western Baikal Area (Fig. 2). The pluton, being elongated in the northeast direction and occupying an area of about 2 km2, occurs among marbles of the Anga sequence and includes its large xenolith. As follows from intersecting contacts between different granite types three intrusive phases can be distinguished within the pluton: early phase coarse-grained biotite granites, occupying northeast and south parts, 2nd phase medium- and fine-grained leucogranites, occurring in the west and center and 3rd phase veined granites and pegmatites, occurring amongst granites of earlier phases. Rare veins of rare-metal amazonite-bearing pegmatites, which could be not related to the intrusion, are of special geologic location. In early phase biotite granites alkali feldspar (orthoclase or microcline) prevails over plagioclase (nos. 22–28). Biotite content varies from 2 to 6%; in places the rocks contain muscovite. The accessory minerals here are apatite, magnetite, garnet and in cases monazite. The rocks are of hypidiomorphic textures and massive, in cases, gneissoid structures. The biotite granites contain xenoliths of gneiss-granites and schist with amphibole and biotite. The 2nd phase leucogranites demonstrate wider variations of structural and mineral characteristics. By grain size they vary from medium- to fine-grained which contain small (from 0.5 to 1–2 cm) pegmatoid schlieren, composed mostly of K–Na feldspar, quartz and rare fluorite crystals, 0.2–0.3 mm in size. Leucogranites are composed mainly of feldspars (microcline and acid plagioclase nos. 9–15) in roughly the same proportions. In some places, plagioclase prevails over alkali feldspar. The smoky quartz, occurring as rounded idiomorphic crystals,
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can amount to 30%. The accessory minerals in leucogranites are biotite, garnet, magnetite, zircon, infrequently fine-grained muscovite. Garnet in these granites generally occurs together with muscovite. Distinguishing of albite-like granites as a separate variety within the Aya pluton (Ivanov and Shmakin, 1980) has no grounds. They appear to be the same 2nd phase leucogranites, but possessing a finer-grained texture and demonstrating a prevalence of acid plagioclase over K–Na feldspar. There are generally gradual transitions between the facies; sharp contacts are lacking. Leucogranites and albite-like pegmatites were previously interpreted as petrographically relatively similar. The 3rd phase of vein rocks in the Aya pluton includes dikes of fine-to medium-grained vein granites, infrequently containing biotite and muscovite, as well as pegmatite bodies. The shape of pegmatite bodies is diverse: they tend to form schlieren, lenses and rarely veins. Vein pegmatites are divided into two distinct groups: quartz-feldspar pegmatites, which contain muscovite and garnet and amazonite-bearing variety, which is very scarce. Aya granitoids are spatially confined to a large NE-trending rupture structure which crosses a sublatitudinal fault along the Anga River. New U–Pb SHRIMP-II ages reported for Aya granites are 468 ± 6 (Yudin et al., 2005) and 469 ± 1.5 Ma (Vla dimirov et al., 2008), that is well consistent with the age of Sha ranur granites (Table 1). From the results of 40Ar/39Ar dating, the age spectra of biotite from Aya granite (sample Ol-55) and pegmatite vein (sample Ol-58) that cuts granites, have distinct plateau with ages of 412.8 ± 4.2 and 391.1 ± 3.9 Ma. These ages are likely to reflect the timing of formation of late granite-pegmatites in the Aya intrusion (Yudin et al., 2005). The pegmatoid rare-metal microcline-albite granites bearing beryllium mineralization, discovered on Olkhon Island, Tashkinei valley, yield an age of 390 ± 5 Ma (SHRIMP-II) and can be regarded as middle Paleozoic units and thus are not referred to the Sharanur complex (Antipin et al., 2014; Sheptyakova et al., 2016). By age they are similar to pegmatites from the Aya pluton, which include rare-metal pegmatites containing amazonite. Evolution of early Paleozoic granitoids, their chemistry in relation to composition of host metamorphic sequences, Baikal Region Migmatites and plagiogranites of the Olkhon Island are generally referred to the normal alkalinity series; these are essentially potassium-bearing rocks, and considering sum of (Na2O + K2O) and K2O/Na2O ratio they share the field with hosting gneisses and schists (Table 2, Fig. 3a). Calc-alkaline granitoids of the Sharanur complex contain more K2O than Na2O and have K2O/Na2O values close to 1 (Fig. 3b). It is to be noted that granosyenites and quartz syenites continue the evolutionary trend of compositions of the Sharanur granitoids of calc-alkaline and subalkaline series, thus showing their spatial and possibly genetic relationships. Aya granites of different phases demonstrate insignificant variations in chemical composition in terms of both silica and
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Table 2. Chemical composition of the early Paleozoic granitoids of the Sharanur complex (Olkhon Island) Component
Plagiogranites, migmatites
K–Na granitoids
Granosyenites, quartz syenites
shrn-17
shrn-144
shrn-153
shrn-11
shrn-45
shrn-73
shrn-95
shrn-115
shrn-181
shrn-50
shrn-83
shrn-148
SiO2, wt.%
78.35
78.53
69.35
77.46
75.0
75.77
75.50
75.80
70.30
63.13
65.50
69.18
TiO2
0.02
0.16
0.55
0.06
0.18
0.08
0.08
0.05
0.17
0.13
0.36
0.26
Al2O3
13.43
11.62
15.10
13.10
12.84
13.86
14.0
13.54
16.0
18.40
16.95
16.57
Fe2O3
–
1.61
1,23
0.10
0.70
0.44
0.20
0.45
0.73
2.0
1.65
1,25
FeO
0.41
0.80
2,67
0.42
0.92
0.28
0.73
0.09
1,05
1.30
1.11
0.63
MgO
0.03
0.03
0.11
0.11
0.46
0.05
0.13
0.03
0.31
0.95
1,02
0.86
MnO
0.03
0.59
1.35
0.03
0.04
0.03
0.03
0.05
0.03
0.1
0.06
0.04
CaO
2,54
0.76
2,48
1,86
0.74
0.82
0.40
0.47
1,80
3,41
3,27
2.23
Na2O
4.30
5.27
3.85
2.08
2.46
3.24
3.16
3.16
4.40
6.10
5.10
5.50
K2O
0.85
0.60
2.77
4.35
5.91
5.10
5.30
5.80
3.40
3.72
4.41
2.76
P2O5
0.02
0.02
0.10
0.02
0.09
0.05
0.02
0.02
0.07
0.16
0.02
0.09
LOI
0.37
0.21
0.61
0.61
0.48
0.44
0.46
0.53
0.64
0.28
0.35
0.50
Total
100.3
100.2
100.2
100.1
99.80
100.3
99.96
99.95
99.80
99.81
99.80
99.86
Li, ppm
8.40
11
16
9.0
9.80
6
6
7
28
12
9
14
Rb
12
50
90
95
120
104
76
178
70
60
69
79
Cs
2
2
2
2
2
2
2
2
0.43
2
2
2
Ba
62
943
1010
900
3000
999
3050
351
814
2100
2532
1128
Sr
360
126
413
270
400
227
220
233
711
1800
2056
1081
Be
1.4
0.66
1.35
0.8
0.8
1.2
0.55
1.74
1.74
1.9
1.6
2.1
Sn
0.5
2.34
0.53
0.67
0.95
0.47
0.27
0.9
0.8
1.2
2.0
0.95
W
0.24
0.3
0.2
0.35
0.34
0.7
3.37
0.5
0.2
0.25
3.51
0.2
Pb
1.63
16
22
16
27
35
37
28
29
27
25
24
Zn
1
24
63
6.6
152
20
4.33
51.0
21
38
52
26
Cu
4.63
18
8.3
5.6
20
15.0
3.03
10.7
6,2
10.4
6.4
4.96
Co
0.6
1.47
9.3
1.1
2.98
1.25
1.1
1.55
2.7
6.1
4.6
2.75
Ni
10.0
5.60
13.0
18
28
3.0
9.1
7.7
7.0
19.0
2.02
5.0
Cr
10.8
5.3
32
32
46
2.3
1.21
9.7
5.7
30.0
6.5
7.0
V
0.02
3.67
62
4.88
13.7
1.83
–
6.0
13.7
63
41
15
Y
1.3
19
20
2.15
9.6
6.7
3.5
10.1
4.97
10.4
9.8
10.4
Nb
0.36
5.1
12.0
1.2
4.48
3.7
2.1
3.5
3.75
4.6
8.1
10.6
Ta
0.25
0.072
1.04
0.06
0.19
0.14
0.017
0.3
0.2
0.23
0.16
1.11
Zr
14
193
238
90
357
85
97
135
164
266
264
136
Hf
0.3
6.1
7.0
2.2
7.5
3.47
3.44
4.41
3.75
4.46
6.1
3.74
Th
0.23
1.8
13.2
1.4
30
46
17
7.5
8.8
12.2
11.8
16
U
0.14
0.63
1.73
0.4
1.04
3.63
1.12
1.65
0.76
1.31
1.62
4.13
F
600
170
490
200
600
200
200
200
270
600
400
200
alumina contents and sum of alkali element oxides (Table 4). On classification diagrams (Fig. 3) the major Aya granite varieties produce a compact field within the field of K–Na Sharanur granites. This is quite well correlated with the affinity in age and geological position of the Sharanur and Aya granites. Na2O + K2O values in all varieties of Aya pluton vary from 7.5 to 9.5% that gives grounds to refer them to rocks of normal or slightly higher alkalinity. The rocks of different phases on the Aya pluton including vein-shaped granites are characterized by similar values in the sum of alkali metal oxides. However, 1st phase granites, like
Sharanur granites, contain more K2O than Na2O. At the same time, 2nd phase leucogranites and vein granites are significantly enriched in Na2O. As compared with biotite granites of the early phase, 2nd phase leucogranites show slightly lower CaO, MgO and TiO2 abundancies. It is to be underlined that by the chemical composition the Aya vein and schlieren pegmatites are similar to vein phase granites, thus terminating the formation of the intrusion. However, there is a distinct difference in the composition of amazonite pegmatite vein that contains higher K2O (7.3–8.5%) abundances, resulting from a higher microcline content.
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Table 3. REE in early Paleozoic granitoids of the Sharanur complex (Olkhon Island) Component
Plagiogranites, migmatites
K–Na granitoids
shrn-17
shrn-11
shrn-91
shrn-153
shrn-45
Granosyenites, quartz syenites shrn-73
shrn-95
shrn-115
shrn-181
shrn-50
shrn-83
shrn-148
La
0.98
9.1
30
13
125
14.6
37
30
47
37
55
13.5
Ce
2.32
22
60
18
308
35
58
66
48
116
107
45
Pr
0.5
3.0
6.50
1.64
25.0
3.85
5.1
6.5
8.6
9.2
10.4
3.47
Nd
2.12
14.2
25
5.6
85
14.8
17
22
30
33
37
14.0
Sm
0.44
3.94
4.67
0.72
10.1
2.5
2.3
3.46
3.72
4.54
4.62
3.1
Eu
0.24
0.77
1.17
1.1
1.15
0.82
0.7
0.48
0.74
1.42
1.66
0.7
Gd
0.45
5.25
4.4
0.75
7.6
2.1
2.3
3.12
2.65
3.97
4.12
3.0
Tb
0.06
1.03
0.6
0.08
0.7
0.26
0.24
0.41
0.3
0.44
0.34
0.42
Dy
0.35
7.0
3.80
0.47
2.9
1.5
1.06
2.5
1.21
2.28
2.21
2.56
Ho
0.06
1.6
0.85
0.09
0.47
0.34
0.16
0.48
0.2
0.42
0.4
0.5
Er
0.2
4.87
2.7
0.3
1.4
1.12
0.42
1.61
0.6
1.25
2.21
0.5
Tm
0.03
0.72
0.42
0.05
0.17
0.2
0.04
0.24
0.07
0.18
0.2
0.22
Yb
0.21
5.1
2.82
0.32
1.28
1.51
0.31
1.76
0.5
0.22
1.28
1.44
Lu
0.034
0.8
0.47
0.06
0.23
0.26
0.06
0.27
0.08
0.22
0.21
0.22
REE
4.44
67.1
141.06
39.34
567.4
76.98
122.76
136.95
141.18
208.8
225.5
70.27
La/Yb
4.60
1.78
10.6
40.6
97.6
9.6
119.3
17.04
96.0
29.6
43.0
9.4
As follows from K2O–Na2O diagram (Fig. 3b) granitoids of these two sites demonstrate different petrochemistry. Despite having similar chemical compositions, as illustrated by classification diagrams (Fig. 3a), the major rock varieties from these two complexes produce two different trends: (i) Sharanur rocks show gradual increase in potassium contents and K2O/Na2O ratios from host gneiss through migmatite–plagiogranites to K–Na granites. It points to formation of these rocks in melting of a crustal protolith; (ii) in the Aya pluton, rocks having similar K2O and Na2O contents, or K2O/Na2O < 1 are observed amongst early phase biotite granites marked by K2O/Na2O > 1 which may be due to magmatic melt differentiation.
Therefore, it may be inferred that migmatization and melting of the crustal substratum could have produced the Sharanur granites, while the Aya granites are intrusive formations whose evolution is better explained by intrachamber magmatic differentiation. Comparative geochemistry of autochthonous and allochthonous early Paleozoic granitoids, Western Baikal Region Previous studies in the Olkhon area (Antipin et al., 2014) showed that the Sharanur granites are mostly close in their geo-
Fig. 3. Classification (Na2O + K2O)–SiO2 diagram (а) and K2O–Na2O ratio (b) for granitoids of the Aya and Sharanur complexes, Olkhon Region. Sharanur complex: 1, plagiogranites, plagiomigmatites, Olkhon Island; 2, K–Na granitoids; 3, granosyenites, quartz syenites; Aya pluton, Olkhon area: 4, 1st phase granitoids; 5, 2nd phase granitoids, 6, 3rd phase granitoids; 7, amazonite pegmatites; 8, gneisses and schists, Olkhon Island. Fields on diagram: а, K–Na granitoids, Sharanur complex. Lines with arrows on the diagram: b, evolutionary trends, Sharanur (1) and Aya (2) granitoids.
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Fig. 4. Spider diagrams of rare (a) and rare earth (b) distribution in granitoids of the Aya and Sharanur complexes. Sharanur complex: 1, plagiogranites, plagiomigmatites, Olkhon Island (mean composition); 2, K–Na granitoids (mean composition); 3, granosyenites, quartz syenites (mean composition); Aya pluton: 4, 1st phase granites; 5, 2nd phase granitoids; 6, 3rd phase granitoids; 7, amazonite pegmatites within the Aya pluton.
chemistry to the mean composition of the continental crust and to collisional granitoids of other provinces (the Himalayas, Central Spain). In the geodynamic setting of continental collision S-type granites and migmatites are produced; their coefficient Al2O3/(CaO + Na2O + K2O) > 1.1 (ASI). The granitoids of the Sharanur complex are characterized by a close value (ASI > 1.1), thus their composition is peraluminous. The collisional granites show enrichment in LREE over HREE. As follows from our geochemical data, the crust substratum of hosting gneiss-schistose sequences (Olkhon and Anga) could have served as the source of melts for the early Paleozoic granitoids in the Sharanur complex thus pointing to their formation under collision setting (Antipin et al., 2014; Donskaya et al., 2013; Makrygina and Petrova, 1996; Mikheev et al., 2017; Sheptyakova et al., 2016; Vladimirov et al., 2004). The geological re-
lationships of the Sharanur granitoids showing the transition from gneiss-granites through migmatites and plagiogranites to K–Na granites, forming large gneiss domes and plutons (Sharanur, Tashkinei and Southern Olkhon) indicate wide occurrence of the autochthonous facies of the early Paleozoic granitoid magmatism in the Olkhon area. It has to be underlined that precise analytical data were not sufficient for the allochthonous facies of the granitoid magmatism of the region. In the Olkhon area, it is represented by the multiphase Aya granite pluton, which is close to K–Na Sharanur granites in time of formation and petrochemical features. This paper provides new geochemical data for granitoids of the Baikal Region, given as tables 2–5 and on diagrams (Fig. 4). On spider diagrams (Fig. 4) early phase biotite granites of the Aya pluton show a distribution of rare and rare earth ele-
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Table 4. Chemical composition of granitoids of the Aya complex Component
1st phase granites
2nd phase granites
Amazonite pegmatites
3rd phase veins granites
Aya-582 Aya-581 Aya-585 Aya-587 Aya-588 Aya-576 Aya-578 Aya-579 Aya-580 Aya-610 Aya-589 Aya-596 Aya-611 Aya-612 Aya-595 Aya-598
SiO2, wt.% 74.54
75.94
75.01
75.83
74.48
76.34
76.41
75.93
74.72
76.42
73.50
71.75
74.98
71.47
73.37
74.53
TiO2
0.14
0.02
0.10
0.06
0.12
0.01
0.01
0.01
0.01
<0.02
0.26
0.44
<0.02
0.32
0.03
<0.02
Al2O3
13.38
13.01
13.00
12.86
13.24
13.27
13.52
13.19
13.94
12.98
13.94
13.92
13.84
13.89
14.07
13.40
Fe2O3
0.58
0.66
0.46
0.53
0.54
0.39
0.43
0.26
0.43
0.19
0.70
0.51
0.25
0.19
0.43
0.34
FeO
1.07
0.58
1.13
0.72
1.16
0.50
0.28
0.24
0.45
0.57
1.47
2.46
0.47
2.50
0.77
1.18
MnO
<0.03
0.03
0.02
0.02
0.02
0.01
<0.03
<0.03
<0.03
0.06
<0.03
0.08
0.04
0.05
<0.03
0.12
MgO
0.15
0.04
0.10
0.07
0.13
0.03
0.02
0.02
0.03
0.05
0.34
0.65
0.05
0.38
0.02
0.03
CaO
0.66
0.37
0.73
0.56
0.69
0.03
0.03
0.42
0.15
0.15
0.80
1.63
<0.05
0.71
0.18
0.52
Na2O
4.02
3.90
4.08
4.10
4.04
4.37
4.64
4.85
5.25
4.59
4.58
4.73
4.89
4.57
3.45
5.79
K2O
4.28
4.49
4.67
4.70
4.62
4.54
4.18
4.19
4.05
4.12
3.44
3.13
4.20
4.01
7.30
2.69
P2O5
0.04
0.04
0.03
0.02
0.04
0.02
0.02
0.02
0.02
0.02
0.07
0.13
0.03
0.07
0.02
0.02
LOI
0.38
0.34
0.30
0.48
0.34
0.44
0.44
0.50
0.44
0.55
0.40
0.37
0.48
0.70
0.30
0.26
Total
99.78
99.81
100.17
100.42
99.84
100.40
100.28
100.06
100.01
99.92
100.08
100.12
99.23
99.39
100.65
99.03
Li, ppm
23
34
28
41
29
2
15
8
36
1
56
79
23
140
122
107
Rb
164
200
150
210
160
316
400
350
240
282
169
131
392
270
1122
348
Cs
1
1
1
1
1
1
1
1
1
<1
1
<1
<1
<1
253
51
Ba
215
33
138
41
143
138
16
12.3
10.1
–
739
–
–
–
47
12.4
Sr
65
13
44
18
43
15
4.55
6.3
5.3
–
112
–
–
–
7.4
8.7
Be
–
–
2.75
3.27
–
2.22
4.36
3.7
–
–
3.08
–
–
–
13
14.9
Sn
–
–
1.45
1.72
–
0.59
0.92
1.0
–
–
1.35
–
–
–
44
22
W
–
–
0.22
0.25
–
0.69
3.95
–
–
–
0.29
–
–
–
11.6
7.9
Pb
–
–
30
29
–
27
33
58
–
–
23
–
–
–
106
79
Zn
–
–
26
35
–
90
0.92
30
–
–
36
–
–
–
60
50
Cu
–
–
4.08
12.1
–
5.6
5.9
5.8
–
–
3.03
–
–
–
2.74
5.2
Co
–
–
0.9
0.67
–
0.54
0.36
2.1
–
–
2.08
–
–
–
0.3
0.62
Ni
–
–
77
79
–
80
75
5.7
–
–
67
–
–
–
67
20
Cr
–
–
22
27
–
24
24
9.3
–
–
14
–
–
–
11.5
26
V
–
–
3.22
1.08
–
<ПО
0.93
4
–
–
21
–
–
–
0.051
5.1
Y
11.7
28
14.3
24
16.2
25
21
16.3
26
–
22
–
–
–
99
99
Nb
12.1
10.2
10.5
10.3
11.6
32
100
53
37
–
15
–
–
–
35
135
Ta
0.51
0.36
0.43
0.47
0.54
2.01
7.4
4.9
3.4
–
1.64
–
–
–
28
38
Zr
168
104
166
99
181
138
124
109
105
–
172
–
–
–
54
184
Hf
5.8
4.7
0.22
3.97
5.7
7.5
7.5
6.4
6.2
–
4.64
–
–
–
5.6
15
Th
24
50
29
41
22
12.5
25
16.1
9.3
–
12.6
–
–
––
14.1
50
U
4.9
6.9
6.5
39
4.9
13.8
18
16.2
5
–
9.8
–
–
–
3.47
15
F
400
100
200
200
300
100
200
300
100
<200
400
500
<200
700
1100
700
ments similar to that of K–Na Sharanur granitoids and thus are close to the mean composition of the continental crust. At the same time, Aya granites contain minima of normalized concentrations of Ba, Sr, La and Eu but are enriched in Li. As follows from those differences, the multiphase Aya intrusion underwent magmatic differentiation combined with alkali feldspar and plagioclase fractionation resulting in the minima of the above elements. Second phase Aya leucogranites show a REE distribution close to early phase biotite granites. At the same time, they are depleted in light lanthanoids (La, Се, Pr, Nd, Sm, Eu), as well
as Li, and enriched in HREEs (Table 5, Fig. 4). They also show minima of Ba and Sr normalized concentrations. In the Aya plutons, those geochemical features are good indicators of magmatic differentiation of the multiphase intrusion with a significant fractionation of feldspar. Quite a different REE distribution is shown by amazonite-bearing pegmatites, which crosscut the early phase Aya granites. They are enriched in Cs, Rb, Pb, Be, F, Ta, Sn, Li, Lu, Yb and Y but show minima of Ba, La, Nd, Zr, Sr and Eu concentrations (Fig. 4). Thus, the above geochemical characteristics suggest no genetic relations between Aya granites, pegmatoid rare-metal granites and ama-
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Table 5. REE in early Paleozoic granitoids of the Aya pluton Component
Аya-585
Аya-587
Аya576
Аya-578
Аya-589
Аya-595
Аya-598
La, ppm
25
11.8
2.75
1.17
19
3.69
2.29
Ce
56
30
7.7
3.11
30
7.8
4.91
Pr
6.2
3.94
1.01
0.34
4.07
1.32
0.83
Nd
23
17
4.18
1.62
16
6.7
4.69
Sm
4.72
5.3
1.64
0.80
3.76
3.38
3.12
Eu
0.45
0.19
0.09
0.03
0.56
0.074
0.087
Gd
3.95
5.3
2.41
1.27
4.04
4.91
5.4
Tb
0.52
0.82
0.54
0.34
0.66
0.98
1.22
Dy
2.99
5.1
4.15
3.29
4.17
7.40
10.8
Ho
0.57
0.99
0.96
0.86
0.85
1.75
2.96
Er
1.71
2.93
3.55
3.56
2.69
6.90
14
Tm
0.25
0.43
0.60
0.67
0.41
1.39
3.34
Yb
1.64
2.75
4.77
5.6
2.72
12.9
34
Lu
0.25
0.41
0.70
0.91
0.40
2.20
6.5
REE
127
86.5
35.01
23.62
90
61.41
94.15
La/Yb
15.24
4.29
0.58
0.21
7.00
0.29
0.067
Note. Аya-585, 587, 1st phase granites; Аya-576, 578, 2nd phase granites with fluorite; Aya-589, vein granite in the 1st phase granites; Аya-595, 598, amazonite pegmatites.
zonite-bearing pegmatites (Olkhon area and Khamar-Daban Ridge) and are referred as belonging to intraplate magmatic formations (Antipin and Perepelov, 2011). As a result of this study it has been inferred that regarding the composition, K–Na Sharanur granitoids, which inherit the major-and trace composition from the hosting Olkhon gneissschistose sequence, are close to the 1st phase Aya (Olkhon area) granites thus giving an evidence for genetic affinity of autochthonous and intrusive magmatism. It is to be underlined that according to U–Pb zircon dating (Table 1), they were produced within a narrow age interval of 500–465 Ma under collisional setting. Conclusions The geologic and petrologic-geochemical study showed that in the Olkhon area of the Baikal Region the early Paleozoic collision-related granitoid magmatism proceeded within a narrow time span (500–465 Ma). It was responsible for the production of both autochthonous migmatite–granite complexes being close by composition to the Olkhon series host rocks, and multiphase intrusions, emplaced into upper horizons of the continental crust. The Sharanur and Aya granitoids, classified into different geochemical types, involve formations of normal calc-alkaline and subalkaline series. The granitoids of the Olkhon area, grouped in terms of petrographic and petrochemical characteristics, are marked by diverse mineral compositions. Considering the chemical composition and K2O/Na2O ratio, the migmatites–plagiogranites and K–Na calc-alkaline Sha ranur granitoids are close to host crystalline schists and gneisses, that may be a result of formation of these granitoids in melting of an ancient metamorphic substratum of the Olkhon series. On classification diagrams granosyenites and quartz syenites
share the field and common evolutionary trends with K–Na Sharanur granitoids, thus likely pointing to their genetic affinity. However, subalkaline series rocks spatially lie close to the outcrops of crystalline schists and amphibolites, whose melting under collision setting, could have resulted in higher alkalinity of these rocks. The geochemical data from ICP-MS show a complex formation pattern for the multiphase Aya pluton. At early stages, its evolution was possibly due to the composition of the primary magma under melting of gneiss-schistose substratum of the Olkhon series, that is evidenced by petrochemical and petrographic affinity between 1st phase Aya biotite granites and K–Na Sharanur granites (Fig. 3). Later, its geochemical evolution was mainly governed by intrachamber magmatic differentiation. However, the traces of the magmatic differentiation processes were evident during the formation of an early phase represented by biotite granites (Fig. 4). Having similar REE distribution patterns, the 1st phase Aya granitoids as opposed to the Sharanur rocks, contain minima of Sr, Ba and Eu concentrations, which may be due to initial fractionation of feldspar from the magmatic melt. The magmatic differentiation was in particular intense during the formation of 2nd phase leucogranites, depleted in Sr, Ba, Eu, Li, as well as LREE and enriched in HREE and HFSE (Ta, Nb). The age estimates of the autochthonous and intrusive granitoids are well consistent with early Paleozoic collision events within the Olkhon metamorphic terrane. The syncollisional granitoids may be derived from both melting of the crustal protolith (Sharanur complex) and magmatic differentiation (multiphase Aya pluton). By mineralogical-geochemical characteristics, they are not referred to rare-metal pegmatoid granites and Li–F and Rb– Be–Nb pegmatites (Antipin et al., 2014), whose vein bodies
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crosscut the granitoids under study, but represent the middle Paleozoic formations and mark the transition to the withinplate magmatism. The authors highly appreciate assistance of their colleagues V.A. Makrygina, A.M. Fedorov, and E.I. Mikheev provided in the field work and treatment of collected samples. Studies were executed in terms of the Project No. IX.129.1.3 (0350-20160029) and were supported by RFBR (grant No. 15-05-02772а), as well as Program of Ministry of Education (Grant No. 5.1688.2017/Pch). References Antipin, V.S., Perepelov, A.B., 2011. Late Paleozoic rare-metal granitoid magmatism of the Southern Baikal region. Petrology 19 (4), 370–381. Antipin, V.S., Gorlacheva, N.V., Makrygina, V.A., Kushch, 2005. Composition and geochemical typization of granitoids from Ol’khon Island (Sharanurskii Complex). Dokl. Earth Sci. 445 (1), 857–861. Antipin, V.S., Gorlacheva, N.V., Makrygina, V.A., 2014. Geochemistry of Early Paleozoic granitoids of the Baikal region and their geodynamic setting exemplified by the Khamar-Daban Ridge and Olkhon Island. Russian Geology and Geophysics (Geologiya i Geofizika) 55 (2), 228–243 (177–189). Distanova, A.N., 2013. Granitoid magmatism of Early Paleozoic orogens. Russian Geology and Geophysics (Geologiya i Geofizika) 54 (6), 606–612 (787–796). Dobretsov, N.L., Buslov, M.M., 2007. Late Cambrian–Ordovician tectonics and geodynamics of Central Asia. Russian Geology and Geophysics (Geo logiya i Geofizika) 48 (1), 71–82 (93–108). Donskaya, T.V., Gladkochub D.P., Fedorovsky V.S., Mazukabzov A.M., Cho, M., Cheong, W., Kim, J., 2013. Synmetamorphic granitoids (~490 Ma) as accretion indicators in the evolution of the Ol’khon terrane (western Cisbaikalia). Russian Geology and Geophysics (Geologiya i Geofizika) 54 (10), 1205–1218 (1543–1561). Donskaya, T.V., Gladkochub, D.P., Fedorovsky, V.S., Sklyarov, E.V., Cho, M., Sergeev, S.A., Demonterova, E.I., Mazukabzov, A.M., Lepekhina, E.N., Cheong, W., Kim, J., 2017. Pre-collisional (> 0.5 Ga) complexes of the Olkhon terrane (southern Siberia) as an echo of events in the Central Asian Orogenic Belt. Gondwana Res. 42, 243–263. Fedorovsky, V.S., 2004. Geologic Map of Southwestern Part, Olkhon Area. Scale 1:100000 [in Russian]. GIN RAS, Moscow. Fedorovsky, V.S., Sklyarov, E.V., 2010. The Olkhon geodynamic proving ground (Lake Baikal): high resolution satellite data and geological maps of new generation. Geodyn. Tectonophys. 1 (4), 331–418. Fedorovsky, V.S., Vladimirov, A.G., Khain, E.V., Kargopolov, S.A., Gib sher, A.S., Izokh, A.E., 1995. Tectonics, metamorphism and magmatism of collision zones of Central Asian Caledonides. Geotektonika, No. 3, 3–22. Geostandards Newletters, 1994. Spec. Issue 5 (18). Gladkochub, D.P., Donskaya, T.V., Fedorovsky, V.S., Mazukabzov, A.M., La rionov, A.N., Sergeev, S.A., 2010. The Olkhon metamorphic terrane in the Baikal region: An Early Paleozoic collage of Neoproterozoic active margin fragments. Russian Geology and Geophysics (Geologiya i Geofizika) 51 (5), 447–460 (571–588). Gorlacheva, N.V., 2014. Geochemistry and Genetic Fetures of Paleozoic Granitoids, Baikal Region (Khamar-Daban Ridge and Olkhon Island), PhD Thesis, [in Russian]. Irkutsk. Ivanov, A.N., Shmakin, B.M., 1980. Granites and Pegmatites of the Western Cisbaikalia [in Russian]. Nauka, Moscow. Khromykh, S.V., Vladimirov, A.G., Mekhonoshin, V.S., Fedorovsky, V.S., Vol kova, N.N., 2005. Petrology and geochronology of igneous rocks of the Chernorud granulite zone (Western Baikal), in: Proc. All-Russia Conf. on
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Geodynamic Evolution of the Lithosphere of the Central Asian Mobile Belt: from Ocean to Continent [in Russian]. IZK SO RAN, Irkutsk, Vol. 2, pp. 131–134. Kuklei, L.N., 1988. Tectonic Structures of Granitization, Exemplified by the Western Baikal Region [in Russian]. IFZ RAN, Moscow. Makrygina, V.A., Petrova, Z.I., 1996. Geochemistry of migmatites and grani toids of the Olkhon surroundings and Olkhon Island (western Baikal). Geokhimiya, No. 7, 637–649. Makrygina, V.A., Sandimirov, I.V., Sandimirova, G.P., Pakhol’chenko, Yu.A., Kotov, A.B., Kovach, V.P., Travin, A.V., 2010. Nd–Sr systematics of metamagmatic rocks of the Anginskaya and Talanchanskaya Formations, middle part of Lake Baikal. Geochem. Int. 48 (10), 979–987. Makrygina, V.A., Antipin, V.S., Lepekhina, E.N., Tolmacheva, E.V., Gorlacheva, N.V., 2013. Genetic peculiarities and the first data on the U–Pb age of the Solzan granitoid massif, Khamar-Daban (Baikal area). Dokl. Earth Sci. 449 (1), 335–341. Makrygina, V.A., Tolmacheva, E.V., Lepekhina, E.N., 2014. Crystallization history of Paleozoic granitoids in the Ol’khon region, Lake Baikal (SHRIMPII zircon dating). Russian Geology and Geophysics (Geologiya i Geofizika) 55 (1), 33–45 (41–55). Mekhonoshin, A.S., Vladimirov, A.G., Vladimirov, V.G., Volkova, N.I., Kolotilina, T.B., Mikheev, E.I., Travin, A.V., Yudin, D.S., Khlestov, V.V., Khromykh, S.V., 2013. Restitic ultramafic rocks in the Early Caledonian collisional system of western Cisbaikalia. Russian Geology and Geophysics (Geologiya i Geofizika) 54 (10), 1219–1235 (1562–1582). Mikheev, E.I., Vladimirov, A.G., Fedorovsky, V.S., Bayanova, T.B., Mazukab zov, A.M., Travin, A.V., Volkova, N.I., Khromykh, S.V., Khlestov, V.V., Tishin, P.A., 2017. Age of overthrust-type granites in accretionary–collisional system of the early Caledonides (western Baikal region). Dokl. Earth Sci. 472 (2), 152–158. Pavlovskii, E.V., Eskin, A.S., 1964. Specifics of Composition and Structure of Baikal in the Archean [in Russian]. Nauka, Moscow. Sheptyakova, N.V., Antipin, V.S., Kushch, L.V., 2016. Paleozoic collisional and intraplate granitoids of the Baikal area: comparative geochemistry and petrogenesis. Russian Geology and Geophysics (Geologiya i Geofizika) 57 (7), 1027–1039 (1307–1322). Sklyarov, E.V., Fedorovsky, V.S., Kotov, A.B., Lavrenchuk, A.V., Mazukab zov, A.M., Levitsky, V.I., Sal’nikova, E.B., Starikova, A.E., Yakovleva, S.Z., Anisimova, I.V., Fedoseenko, A.M., 2009. Carbonatites in collisional settings and pseudo-carbonatites of the Early Paleozoic Ol’khon collisional system. Russian Geology and Geophysics (Geologiya i Geofizika) 50 (12), 1091–1106 (1405–1423). Vladimirov, A.G., Fedorovsky, V.S., Khromykh, S.V., Dokukina, K.A., 2004. Synmetamorphic stress-granites in deep levels of the Early Caledonian collisional system in the western Baikal region. Dokl. Earth Sci. 397A (6), 771–777. Vladimirov, A.G., Khromykh, S.V., Mekhonoshin, A.S., Volkova, N.I., Tra vin, A.V., Yudin, D.S., Kruk, N.N., 2008. U–Pb dating and Sm–Nd systematics of igneous rocks in the Ol’khon region (Western Baikal Coast). Dokl. Earth Sci. 423 (2), 1372–1375. Vladimirov, A.G., Volkova, N.I., Mekhonoshin, A.S., Travin, A.V., Vladimi rov, V.G., Khromykh, S.V., Yudin, D.S., Kolotilina, T.B., 2011. The geody namic model of formation of Early Caledonides in the Olkhon Region (West Pribaikalie). Dokl. Earth Sci. 436 (2), 203–209. Yarmolyuk, V.V., Kovalenko, V.I., Kuzmin, M.I., 2000. North-Asian activity superplume in the Phanerozoic: magmatism and geodynamics. Geotektoni ka, No. 5, 3–29. Yudin, D.S., Khromykh, S.V., Mekhonoshin, A.S., Vladimirov, A.G., Tra vin, A.V., Kolotilina, T.B., Volkova, M.G., 2005. 40Ar/39Ar age and geoche mical features of syncollisional gabbroids and granites from the Western Baikal Region: evidence from the Birkhin massif and its folded framing. Dokl. Earth Sci. 405 (2), 1261–1265. Zonenshain, L.P., Kuzmin, M.I., Natapov, L.M., 1990. Tectonics of Lithosphere Plates of the USSR Territory [in Russian]. Nedra, Moscow, Vol. 1.
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Geochemical evolution of the early Paleozoic collisional magmatism from autochthonous migmatites and granitoids to multiphase granite intrusions (Sharanur and Aya complexes, Baikal Region) V.S. Antipin a,*, L.V. Kushch a, N.V. Sheptyakova a, A.G. Vladimirov b,c a
A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia b Novosibirsk State University, ul. Pirogova, 2, Novosibirsk, 630090, Russia c V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia Received 15 February 2018; accepted 26 July 2018
Abstract Overall petrologic and geochemical data indicate that the early Paleozoic magmatism in the Olkhon area of the Baikal Region exhibits diverse types of granitoids, whose time of formation is estimated at a narrow age interval of 500–465 Ma. This magmatism was responsible for the formation of both autochthonous gneiss–migmatite–granitoid suites (Sharanur complex) and multiphase intrusions (Aya complex) emplaced into the upper horizons of the continental crust. In major-element chemistry, K2O/Na2O values, and rare-element composition the migmatite– plagiogranites and calc-alkaline and subalkaline granitoids of the Sharanur complex are similar to the host gneisses and schists, as they were likely derived from melting of the ancient metamorphic substratum of the Olkhon series. In new isotope-geochemical characteristics (ICP MS method) the Sharanur granitoids are close to the first-phase biotite granites of the Aya massif, whose further geochemical evolution was governed mainly by intrachamber magmatic differentiation leading to the production of second-phase leucogranites enriched in HREE and HFSE (in particular, Ta and Nb) and depleted in Sr, Ba, Eu, Li, and LREE. The origin of the autochthonous and intrusive granitoids is related to early Paleozoic collision events within the Olkhon metamorphic terrane, while the formation of syncollisional granitoids is best explained by both melting of the crust protolith (Sharanur complex) and magmatic differentiation (multiphase Aya intrusion). All mineralogical and geochemical characteristics indicate that these granitoids are distinguished from rare-metal pegmatoid granites and Li–F and Rb–Be–Nb pegmatites, whose vein bodies crosscut the granitoids, and are regarded as middle Paleozoic rocks, which mark the transition to within-plate magmatism in the Baikal Region. © 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All right reserved. Keywords: magmatism, granitoids; collision; geodynamics; geochemical types; Paleozoic; intrusion; complexes
Introduction The Central Asian fold belt is characterized by immense volumes of granitic rocks formed as a result of early Paleozoic accretion–collision events, accompanied by the closure of the Paleo-Asian Ocean (Antipin et al., 2014; Distanova, 2013; Dobretsov and Buslov, 2007; Donskaya et al., 2013, 2017; Vladimirov et al., 2011; Yarmolyuk et al., 2000). The collision-related belts occur mainly along the southern margin of the Siberian craton, and are understood as having formed during the accretion of terranes of different geodynamic origins to marginal parts of the Siberian craton. Therefore, the age of major collision events within those terranes is in a range of 500–465 Ma (Gladkochub et al., 2010; Vladimirov et al., 2008). In the junc-
* Corresponding author.
E-mail adress: [email protected] (V.S. Antipin)
tion zone between the Siberian craton and the Barguzin microcontinent the collision-related granitoids are more widespread within the Olkhon terrane (Fedorovsky et al., 1995; Gladkochub et al., 2010; Zonenshain et al., 1990), that covers the Olkhon area (Olkhon Island and Pre-Olkhon’e) and stretches along the western shore of Lake Baikal. As follows from field and geologic studies (Fedorovsky et al., 1995) the Olkhon terrane was produced by the Caledonian collision as a collage of numerous tectonic units composed of rock complexes of different ages originated in various tectonic settings. At present nine complexes which are united into four zones are distinguished within the Olkhon terrane (Donskaya et al., 2017). Previous studies have shown that the early Paleozoic magmatism in the Baikal Region (Khamar-Daban Ridge, Olkhon Island) was responsible for the production of diverse types of granitoids, however, it is close to the mean composition of the continental crust, which confirms the existence of the Olkhon–
1068-7971/$ - see front matter D 201 8, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 8.12.007
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Khamar-Daban block as a single terrane. The granitoids of the Khamar-Daban Ridge were produced in the range of 516– 690 Ma; as regards granitoids of the Sharanur complex on Olkhon Island they were dated as 505–477 Ma (Antipin et al., 2014; Makrygina et al., 2013). The Sharanur and Khamar-Daban granitoids comprise different geochemical types including formations of normal Na-alkalinity (migmatites and plagiogranites), calc-alkaline (K–Na granites and leucogranites) and subalkaline (granosyenites and quartz syenites). Calc-alkaline and subalkaline granitoids are supposed to have been derived from anatectic melting of the ancient gneiss-schistose substratum. In their compositions, these granites are fairly similar to the recent and ancient collision-related granitoids (Himalayas and Central Spain) that points to the inheritance of the protolith composition (Gorlacheva, 2014; Makrygina et al., 2013). In this paper, based on the studies of Sharanur and Aya granitic complexes, we discuss the magmatic and petrologic evolution of the early Paleozoic magmatism represented by both autochthonous migmatite–gneiss suites and multiphase intrusions (Olkhon area, Baikal Region). The major goals for this study are to identify: the age relationships between granitoids and compositions of their crustal sources; role of migmatization processes, melting of compositionally distinct substrata and magmatic differentiation in the formation of the granitoids; and geochemical characteristics which can be used to decipher the geodynamic setting and the nature of the Phanerozoic collision-related igneous rocks. Methods Geological mapping of major granitoid occurrences (Olkhon Island) and collecting of representative whole rock and geochemical samples were the main components of our study. In the process of sampling, a particular attention was given to the Aya intrusion, as precise geochemical data for this pluton were lacking. These new obtained results have been compared with data available for the Sharanur complex, taken as a reference. The silicate analyses were performed using a classical chemical method by analysts G.A. Pogudina, T.V. Ozhogina and XRF by analyst A.L. Finkelstein with inaccuracy 0.5 to 5%. Alkaline elements were analyzed by flame photometry with the precision 5–10% by analysts L.V. Altukhova and I.M. Khmelevs-
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kaya; rare and rare earth elements were defined by L.A. Chuvashova, E.V. Smirnova using the ICP-MS method (σ ± 5–10%), and O.V. Zarubina with inaccuracy 10–20%. All analyses were done at the Analytical Center for Collective Use, Irkutsk Science Center, SB RAS with the use of equipment installed at Center for Collective use, IGC, SB RAS and certified reference samples (Geostandrads…., 1994). Geology, age, structure of collision-related granitoids and relationships with metamorphic sequences Sharanur migmatite–granite complex. On Olkhon Island, the widespread occurrences of voluminous granitoids is a typical feature of the Sharanur pluton, which includes both autochthonous and allochthonous facies. Geologic setting, geochronology, composition of the Sharanur granitoids as well as the relationships between these granitoids and the country metamorphic sequences were previously discussed (Antipin et al., 2014; Donskaya et al., 2013; Kuklei, 1988; Makrygina and Pet rova, 1996; Makrygina et al., 2010; Mikheev et al., 2017; Vla dimirov et al., 2004). Migmatites, granite–gneisses and pegmatoid granites of allochthonous veins were earlier regarded as belonging to the Shranur granitoid complex as well (Pavlovskii and Eskin, 1964). On Olkhon Island our research was concentrated on three large plutons (Sharanur, Tashkinei and Southern Olkhon) and a series of smaller granitoid outcrops in its north and southwest as well as in the Pre-Olkhon area (Fig. 1). According to the recent SHRIMP-II U–Pb data (VSEGEI Institute, St. Petersburg) the weighted average age for the cores of zircon grains from K–Na calc-alkaline Sharanur granitoids is 505 Ma and that for the marginal parts is 477 Ma (Makrygina et al., 2010). The age estimates for the Sharanur granitoids lie within a narrow time interval (465–486 Ma, Table 1). Granosyenites and quartz syenites occurring here yield an age of 495 ± 6 Ma (Gladkochub et al., 2010), which is well consistent with the timing of Sharanur K–Na granitoids formation, thus pointing to the genetic affinity of these calc-alkaline and subalkaline rocks. In Kolokolnya Bay some outcrops of granitoids, varying in composition, have been mapped. They occur within granite-
Fig. 1. Geological map of the Olkhon region (from (Pavlovskii and Eskin, 1964), modified by (Fedorovsky, 2004)). 1, granites, Khaidai complex; 2, gabbro, Ozersky complex; 3, granitoids, Sharanur complex; 4, granites, Aya complex; 5, gneiss domes, Olkhon Island; 6, marbles, schists, quartzites (Olkhon series); 7, schists, marbles, quartzites (Anga sequence); 8, carbonate-silicate rocks, amphibolites, 9, shear zones.
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Table 1. U–Pb isotope ages after zircons from granites and syenites, Olkhon Island and Olkhon area Age, Ma
Geologic object
Reference
Early stage of accretion–collision tectogenesis, granulite facies, granosyenites and enderbites 496 ± 3
Enderbites, Sapshilan pluton, Chernorud zone, Olkhon area
(Khromykh et al., 2005), SHRIMP-II
495 ± 6
Quartz syenite, Southern Olkhon pluton, Olkhon Island
(Gladkochub et al., 2010), SHRIMP-II
485 ± 1.5
Veins of nepheline syenites in gabbro, Ulan-Khargany, Olkhon area
(Khromykh et al., 2005), SHRIMP-II
488.6 ± 8, core 464 ± 11 rim
Subalkaline granosyenite, Tutai pluton, Olkhon area
(Donskaya et al., 2013), SHRIMP-II
Late stage of collision tectogenesis (collapse), synoverthrust nepheline syenites and associated rocks, Tazheran pluton 471 ± 5
Syenites
(Sklyarov et al., 2009;
464 ± 2
Pegmatoid nepheline syenites, Tazheran pluton
(Fedorovsky and Sklyarov, 2010),
451 ± 1
SHRIMP-II
Synoverthrust granites, Sharanur complex and Aya pluton 476 ± 4 500 ± 5
Granites, Sharanur complex, Olkhon Island Zircon cores
(Makrygina et al., 2014), SHRIMP-II
474 ± 1.5
Para-autochthonous granites in metamorphites close to hyperbasite boudinage, Shida. Single grain
(Mekhonoshin et al., 2013), SHRIMP-II
468 ± 6
Granites, Aya pluton, Olkhon area
(Yudin et al., 2005), SHRIMP-II
458 ± 2
Granites, Central dome zone, Olkhon area
(Vladimirov et al., 2011), SHRIMP-II
550 ± 5 455 ± 5
Cross-cutting vein of pegmatoid granites, Shibetskii Cape, Olkhon Island. Protolith Magmatic zircon. Single grain
(Mikheev et al., 2017)
627 ± 6 440 ± 5
Detrital zircon in alkaline syenites. Single grain Alkaline syenite, Budun pluton, Olkhon Island. Single grain
(Makrygina et al., 2013)
390 ± 5
Rare-metal pegmatoid granite, Tashkinei valley, Olkhon Island. Single grain
(Antipin et al., 2014)
gneisses, in places intercalated with marbles and crystalline schists of the hosting sequence (Southern Olkhon massif). Gneisses locally contain plagiogranites (vein, frequently layered concordant bodies) and migmatites with gneiss-like texture and gradual transitions between rocks in marginal parts of numerous small domes. These units are interpreted as belonging to autochthonous facies of granite magmatism. This site is rich in biotite and amphibole–biotite, in places porphyry-like granites producing large allochthonous intrusions along the southwestern coast of Olkhon Island (major granitoid variety). The intrusions are marked by sharp contacts between granites and the host sequence as well as occurrences of small layered bodies of fine-grained leucogranites. Granites occur here along with bodies of amphibole-biotite granosyenites and quartz syenites which extend as a line from the Unshui Cape in the southwest to Tashkinei valley (Antipin et al., 2012, 2014). The largest granite-gneiss dome near Lake Shara-Nur in the central part of Olkhon Island (close to Tashkinei valley) is regarded as a petrotype of Sharanur granitoids. It is hosted by garnet–biotite and amphibole–biotite gneisses and migmatites grading into para-autochthonous granites. As evident from myrmekite rims and plagioclase residues in coarse K-feldspar grains, granite-gneisses show development of microcline after plagioclase. Plagiogranites and migmatites are banded rocks with a higher feldspar content. The plagiomigmatites are of coarse-grained texture and banded structure. These rocks contain, on the average (vol.%): amphibole—15, biotite—5, plagioclase (andesine nos. 30–40)—35, quartz—40. Garnet frequently occurs as well.
The most abundant varieties here are biotite granites which are of cataclastic and granoblastic textures. Feldspar biotitic migmatites and granites contain oligoclase; biotite tends to be richer in iron (71.5%). Amphibole–biotite varieties or varieties containing garnet with porphyritic texture form large allochthonous intrusions. The granites contain, on the average (vol.%): plagioclase—30, quartz—25, biotite—5, and microcline—>40. The accessory minerals in the Sharanur pluton are apatite, titanite, magnetite, allanite, and zircon. Zircon is either elongated or oval with complex zoning. Vein series include either small layered or lens-like bodies of fine-grained leucogranites and large veins of pegmatoid granites; the latter usually outcrop outside domes and cut pockets of intercalating marbles, quartzites and diopside schists. Several cross-cutting veins of medium-grained biotite granites up to 3–4 m thick and over 1 km long occur on the north of Olkhon Island (close to Sem’ Sosen Bay). Biotite is oriented across the vein, concordant with the foliation of country gneisses. Granosyenite and quartz syenite exposures extend from the Unshui Cape in the south to Tashkinei valley in the north. These are massive or weakly gneissoid amphibole-bearing rocks with minor amounts of quartz, ferruginous biotite and residues of pyroxene replaced by hastingsite. Feldspar is represented by andesine no. 25 and alkaline feldspar with 5–10% of albite minal. These rocks contain the same paragenesis of accessory minerals as in Sharanur granites, whose petrography and geochemistry are given (Antipin et al., 2014). Aya granite–leucogranite complex. The largest intrusive granitoid body of the Aya complex (Ivanov and Shmakin,
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Fig. 2. Geologic-petrographic schematic map, Aya pluton, modified by (Ivanov and Schmakin, 1980). 1, alluvial sediments; 2, deluvial-proluvial sediments; 3, calcite graphite-bearing marbles intercalating with dolomite-calcite marbles and amphibole schists; 4, hornblende schists; 5–6, granites, Aya pluton: 5, biotite, 6, leucocratic; 7, pegmatite veins; 8, quartz veins, 9, rupture dislocations.
1980) outcrops between Anga River mouth and Aya Bay in the western Baikal Area (Fig. 2). The pluton, being elongated in the northeast direction and occupying an area of about 2 km2, occurs among marbles of the Anga sequence and includes its large xenolith. As follows from intersecting contacts between different granite types three intrusive phases can be distinguished within the pluton: early phase coarse-grained biotite granites, occupying northeast and south parts, 2nd phase medium- and fine-grained leucogranites, occurring in the west and center and 3rd phase veined granites and pegmatites, occurring amongst granites of earlier phases. Rare veins of rare-metal amazonite-bearing pegmatites, which could be not related to the intrusion, are of special geologic location. In early phase biotite granites alkali feldspar (orthoclase or microcline) prevails over plagioclase (nos. 22–28). Biotite content varies from 2 to 6%; in places the rocks contain muscovite. The accessory minerals here are apatite, magnetite, garnet and in cases monazite. The rocks are of hypidiomorphic textures and massive, in cases, gneissoid structures. The biotite granites contain xenoliths of gneiss-granites and schist with amphibole and biotite. The 2nd phase leucogranites demonstrate wider variations of structural and mineral characteristics. By grain size they vary from medium- to fine-grained which contain small (from 0.5 to 1–2 cm) pegmatoid schlieren, composed mostly of K–Na feldspar, quartz and rare fluorite crystals, 0.2–0.3 mm in size. Leucogranites are composed mainly of feldspars (microcline and acid plagioclase nos. 9–15) in roughly the same proportions. In some places, plagioclase prevails over alkali feldspar. The smoky quartz, occurring as rounded idiomorphic crystals,
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can amount to 30%. The accessory minerals in leucogranites are biotite, garnet, magnetite, zircon, infrequently fine-grained muscovite. Garnet in these granites generally occurs together with muscovite. Distinguishing of albite-like granites as a separate variety within the Aya pluton (Ivanov and Shmakin, 1980) has no grounds. They appear to be the same 2nd phase leucogranites, but possessing a finer-grained texture and demonstrating a prevalence of acid plagioclase over K–Na feldspar. There are generally gradual transitions between the facies; sharp contacts are lacking. Leucogranites and albite-like pegmatites were previously interpreted as petrographically relatively similar. The 3rd phase of vein rocks in the Aya pluton includes dikes of fine-to medium-grained vein granites, infrequently containing biotite and muscovite, as well as pegmatite bodies. The shape of pegmatite bodies is diverse: they tend to form schlieren, lenses and rarely veins. Vein pegmatites are divided into two distinct groups: quartz-feldspar pegmatites, which contain muscovite and garnet and amazonite-bearing variety, which is very scarce. Aya granitoids are spatially confined to a large NE-trending rupture structure which crosses a sublatitudinal fault along the Anga River. New U–Pb SHRIMP-II ages reported for Aya granites are 468 ± 6 (Yudin et al., 2005) and 469 ± 1.5 Ma (Vla dimirov et al., 2008), that is well consistent with the age of Sha ranur granites (Table 1). From the results of 40Ar/39Ar dating, the age spectra of biotite from Aya granite (sample Ol-55) and pegmatite vein (sample Ol-58) that cuts granites, have distinct plateau with ages of 412.8 ± 4.2 and 391.1 ± 3.9 Ma. These ages are likely to reflect the timing of formation of late granite-pegmatites in the Aya intrusion (Yudin et al., 2005). The pegmatoid rare-metal microcline-albite granites bearing beryllium mineralization, discovered on Olkhon Island, Tashkinei valley, yield an age of 390 ± 5 Ma (SHRIMP-II) and can be regarded as middle Paleozoic units and thus are not referred to the Sharanur complex (Antipin et al., 2014; Sheptyakova et al., 2016). By age they are similar to pegmatites from the Aya pluton, which include rare-metal pegmatites containing amazonite. Evolution of early Paleozoic granitoids, their chemistry in relation to composition of host metamorphic sequences, Baikal Region Migmatites and plagiogranites of the Olkhon Island are generally referred to the normal alkalinity series; these are essentially potassium-bearing rocks, and considering sum of (Na2O + K2O) and K2O/Na2O ratio they share the field with hosting gneisses and schists (Table 2, Fig. 3a). Calc-alkaline granitoids of the Sharanur complex contain more K2O than Na2O and have K2O/Na2O values close to 1 (Fig. 3b). It is to be noted that granosyenites and quartz syenites continue the evolutionary trend of compositions of the Sharanur granitoids of calc-alkaline and subalkaline series, thus showing their spatial and possibly genetic relationships. Aya granites of different phases demonstrate insignificant variations in chemical composition in terms of both silica and
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Table 2. Chemical composition of the early Paleozoic granitoids of the Sharanur complex (Olkhon Island) Component
Plagiogranites, migmatites
K–Na granitoids
Granosyenites, quartz syenites
shrn-17
shrn-144
shrn-153
shrn-11
shrn-45
shrn-73
shrn-95
shrn-115
shrn-181
shrn-50
shrn-83
shrn-148
SiO2, wt.%
78.35
78.53
69.35
77.46
75.0
75.77
75.50
75.80
70.30
63.13
65.50
69.18
TiO2
0.02
0.16
0.55
0.06
0.18
0.08
0.08
0.05
0.17
0.13
0.36
0.26
Al2O3
13.43
11.62
15.10
13.10
12.84
13.86
14.0
13.54
16.0
18.40
16.95
16.57
Fe2O3
–
1.61
1,23
0.10
0.70
0.44
0.20
0.45
0.73
2.0
1.65
1,25
FeO
0.41
0.80
2,67
0.42
0.92
0.28
0.73
0.09
1,05
1.30
1.11
0.63
MgO
0.03
0.03
0.11
0.11
0.46
0.05
0.13
0.03
0.31
0.95
1,02
0.86
MnO
0.03
0.59
1.35
0.03
0.04
0.03
0.03
0.05
0.03
0.1
0.06
0.04
CaO
2,54
0.76
2,48
1,86
0.74
0.82
0.40
0.47
1,80
3,41
3,27
2.23
Na2O
4.30
5.27
3.85
2.08
2.46
3.24
3.16
3.16
4.40
6.10
5.10
5.50
K2O
0.85
0.60
2.77
4.35
5.91
5.10
5.30
5.80
3.40
3.72
4.41
2.76
P2O5
0.02
0.02
0.10
0.02
0.09
0.05
0.02
0.02
0.07
0.16
0.02
0.09
LOI
0.37
0.21
0.61
0.61
0.48
0.44
0.46
0.53
0.64
0.28
0.35
0.50
Total
100.3
100.2
100.2
100.1
99.80
100.3
99.96
99.95
99.80
99.81
99.80
99.86
Li, ppm
8.40
11
16
9.0
9.80
6
6
7
28
12
9
14
Rb
12
50
90
95
120
104
76
178
70
60
69
79
Cs
2
2
2
2
2
2
2
2
0.43
2
2
2
Ba
62
943
1010
900
3000
999
3050
351
814
2100
2532
1128
Sr
360
126
413
270
400
227
220
233
711
1800
2056
1081
Be
1.4
0.66
1.35
0.8
0.8
1.2
0.55
1.74
1.74
1.9
1.6
2.1
Sn
0.5
2.34
0.53
0.67
0.95
0.47
0.27
0.9
0.8
1.2
2.0
0.95
W
0.24
0.3
0.2
0.35
0.34
0.7
3.37
0.5
0.2
0.25
3.51
0.2
Pb
1.63
16
22
16
27
35
37
28
29
27
25
24
Zn
1
24
63
6.6
152
20
4.33
51.0
21
38
52
26
Cu
4.63
18
8.3
5.6
20
15.0
3.03
10.7
6,2
10.4
6.4
4.96
Co
0.6
1.47
9.3
1.1
2.98
1.25
1.1
1.55
2.7
6.1
4.6
2.75
Ni
10.0
5.60
13.0
18
28
3.0
9.1
7.7
7.0
19.0
2.02
5.0
Cr
10.8
5.3
32
32
46
2.3
1.21
9.7
5.7
30.0
6.5
7.0
V
0.02
3.67
62
4.88
13.7
1.83
–
6.0
13.7
63
41
15
Y
1.3
19
20
2.15
9.6
6.7
3.5
10.1
4.97
10.4
9.8
10.4
Nb
0.36
5.1
12.0
1.2
4.48
3.7
2.1
3.5
3.75
4.6
8.1
10.6
Ta
0.25
0.072
1.04
0.06
0.19
0.14
0.017
0.3
0.2
0.23
0.16
1.11
Zr
14
193
238
90
357
85
97
135
164
266
264
136
Hf
0.3
6.1
7.0
2.2
7.5
3.47
3.44
4.41
3.75
4.46
6.1
3.74
Th
0.23
1.8
13.2
1.4
30
46
17
7.5
8.8
12.2
11.8
16
U
0.14
0.63
1.73
0.4
1.04
3.63
1.12
1.65
0.76
1.31
1.62
4.13
F
600
170
490
200
600
200
200
200
270
600
400
200
alumina contents and sum of alkali element oxides (Table 4). On classification diagrams (Fig. 3) the major Aya granite varieties produce a compact field within the field of K–Na Sharanur granites. This is quite well correlated with the affinity in age and geological position of the Sharanur and Aya granites. Na2O + K2O values in all varieties of Aya pluton vary from 7.5 to 9.5% that gives grounds to refer them to rocks of normal or slightly higher alkalinity. The rocks of different phases on the Aya pluton including vein-shaped granites are characterized by similar values in the sum of alkali metal oxides. However, 1st phase granites, like
Sharanur granites, contain more K2O than Na2O. At the same time, 2nd phase leucogranites and vein granites are significantly enriched in Na2O. As compared with biotite granites of the early phase, 2nd phase leucogranites show slightly lower CaO, MgO and TiO2 abundancies. It is to be underlined that by the chemical composition the Aya vein and schlieren pegmatites are similar to vein phase granites, thus terminating the formation of the intrusion. However, there is a distinct difference in the composition of amazonite pegmatite vein that contains higher K2O (7.3–8.5%) abundances, resulting from a higher microcline content.
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Table 3. REE in early Paleozoic granitoids of the Sharanur complex (Olkhon Island) Component
Plagiogranites, migmatites
K–Na granitoids
shrn-17
shrn-11
shrn-91
shrn-153
shrn-45
Granosyenites, quartz syenites shrn-73
shrn-95
shrn-115
shrn-181
shrn-50
shrn-83
shrn-148
La
0.98
9.1
30
13
125
14.6
37
30
47
37
55
13.5
Ce
2.32
22
60
18
308
35
58
66
48
116
107
45
Pr
0.5
3.0
6.50
1.64
25.0
3.85
5.1
6.5
8.6
9.2
10.4
3.47
Nd
2.12
14.2
25
5.6
85
14.8
17
22
30
33
37
14.0
Sm
0.44
3.94
4.67
0.72
10.1
2.5
2.3
3.46
3.72
4.54
4.62
3.1
Eu
0.24
0.77
1.17
1.1
1.15
0.82
0.7
0.48
0.74
1.42
1.66
0.7
Gd
0.45
5.25
4.4
0.75
7.6
2.1
2.3
3.12
2.65
3.97
4.12
3.0
Tb
0.06
1.03
0.6
0.08
0.7
0.26
0.24
0.41
0.3
0.44
0.34
0.42
Dy
0.35
7.0
3.80
0.47
2.9
1.5
1.06
2.5
1.21
2.28
2.21
2.56
Ho
0.06
1.6
0.85
0.09
0.47
0.34
0.16
0.48
0.2
0.42
0.4
0.5
Er
0.2
4.87
2.7
0.3
1.4
1.12
0.42
1.61
0.6
1.25
2.21
0.5
Tm
0.03
0.72
0.42
0.05
0.17
0.2
0.04
0.24
0.07
0.18
0.2
0.22
Yb
0.21
5.1
2.82
0.32
1.28
1.51
0.31
1.76
0.5
0.22
1.28
1.44
Lu
0.034
0.8
0.47
0.06
0.23
0.26
0.06
0.27
0.08
0.22
0.21
0.22
REE
4.44
67.1
141.06
39.34
567.4
76.98
122.76
136.95
141.18
208.8
225.5
70.27
La/Yb
4.60
1.78
10.6
40.6
97.6
9.6
119.3
17.04
96.0
29.6
43.0
9.4
As follows from K2O–Na2O diagram (Fig. 3b) granitoids of these two sites demonstrate different petrochemistry. Despite having similar chemical compositions, as illustrated by classification diagrams (Fig. 3a), the major rock varieties from these two complexes produce two different trends: (i) Sharanur rocks show gradual increase in potassium contents and K2O/Na2O ratios from host gneiss through migmatite–plagiogranites to K–Na granites. It points to formation of these rocks in melting of a crustal protolith; (ii) in the Aya pluton, rocks having similar K2O and Na2O contents, or K2O/Na2O < 1 are observed amongst early phase biotite granites marked by K2O/Na2O > 1 which may be due to magmatic melt differentiation.
Therefore, it may be inferred that migmatization and melting of the crustal substratum could have produced the Sharanur granites, while the Aya granites are intrusive formations whose evolution is better explained by intrachamber magmatic differentiation. Comparative geochemistry of autochthonous and allochthonous early Paleozoic granitoids, Western Baikal Region Previous studies in the Olkhon area (Antipin et al., 2014) showed that the Sharanur granites are mostly close in their geo-
Fig. 3. Classification (Na2O + K2O)–SiO2 diagram (а) and K2O–Na2O ratio (b) for granitoids of the Aya and Sharanur complexes, Olkhon Region. Sharanur complex: 1, plagiogranites, plagiomigmatites, Olkhon Island; 2, K–Na granitoids; 3, granosyenites, quartz syenites; Aya pluton, Olkhon area: 4, 1st phase granitoids; 5, 2nd phase granitoids, 6, 3rd phase granitoids; 7, amazonite pegmatites; 8, gneisses and schists, Olkhon Island. Fields on diagram: а, K–Na granitoids, Sharanur complex. Lines with arrows on the diagram: b, evolutionary trends, Sharanur (1) and Aya (2) granitoids.
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Fig. 4. Spider diagrams of rare (a) and rare earth (b) distribution in granitoids of the Aya and Sharanur complexes. Sharanur complex: 1, plagiogranites, plagiomigmatites, Olkhon Island (mean composition); 2, K–Na granitoids (mean composition); 3, granosyenites, quartz syenites (mean composition); Aya pluton: 4, 1st phase granites; 5, 2nd phase granitoids; 6, 3rd phase granitoids; 7, amazonite pegmatites within the Aya pluton.
chemistry to the mean composition of the continental crust and to collisional granitoids of other provinces (the Himalayas, Central Spain). In the geodynamic setting of continental collision S-type granites and migmatites are produced; their coefficient Al2O3/(CaO + Na2O + K2O) > 1.1 (ASI). The granitoids of the Sharanur complex are characterized by a close value (ASI > 1.1), thus their composition is peraluminous. The collisional granites show enrichment in LREE over HREE. As follows from our geochemical data, the crust substratum of hosting gneiss-schistose sequences (Olkhon and Anga) could have served as the source of melts for the early Paleozoic granitoids in the Sharanur complex thus pointing to their formation under collision setting (Antipin et al., 2014; Donskaya et al., 2013; Makrygina and Petrova, 1996; Mikheev et al., 2017; Sheptyakova et al., 2016; Vladimirov et al., 2004). The geological re-
lationships of the Sharanur granitoids showing the transition from gneiss-granites through migmatites and plagiogranites to K–Na granites, forming large gneiss domes and plutons (Sharanur, Tashkinei and Southern Olkhon) indicate wide occurrence of the autochthonous facies of the early Paleozoic granitoid magmatism in the Olkhon area. It has to be underlined that precise analytical data were not sufficient for the allochthonous facies of the granitoid magmatism of the region. In the Olkhon area, it is represented by the multiphase Aya granite pluton, which is close to K–Na Sharanur granites in time of formation and petrochemical features. This paper provides new geochemical data for granitoids of the Baikal Region, given as tables 2–5 and on diagrams (Fig. 4). On spider diagrams (Fig. 4) early phase biotite granites of the Aya pluton show a distribution of rare and rare earth ele-
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Table 4. Chemical composition of granitoids of the Aya complex Component
1st phase granites
2nd phase granites
Amazonite pegmatites
3rd phase veins granites
Aya-582 Aya-581 Aya-585 Aya-587 Aya-588 Aya-576 Aya-578 Aya-579 Aya-580 Aya-610 Aya-589 Aya-596 Aya-611 Aya-612 Aya-595 Aya-598
SiO2, wt.% 74.54
75.94
75.01
75.83
74.48
76.34
76.41
75.93
74.72
76.42
73.50
71.75
74.98
71.47
73.37
74.53
TiO2
0.14
0.02
0.10
0.06
0.12
0.01
0.01
0.01
0.01
<0.02
0.26
0.44
<0.02
0.32
0.03
<0.02
Al2O3
13.38
13.01
13.00
12.86
13.24
13.27
13.52
13.19
13.94
12.98
13.94
13.92
13.84
13.89
14.07
13.40
Fe2O3
0.58
0.66
0.46
0.53
0.54
0.39
0.43
0.26
0.43
0.19
0.70
0.51
0.25
0.19
0.43
0.34
FeO
1.07
0.58
1.13
0.72
1.16
0.50
0.28
0.24
0.45
0.57
1.47
2.46
0.47
2.50
0.77
1.18
MnO
<0.03
0.03
0.02
0.02
0.02
0.01
<0.03
<0.03
<0.03
0.06
<0.03
0.08
0.04
0.05
<0.03
0.12
MgO
0.15
0.04
0.10
0.07
0.13
0.03
0.02
0.02
0.03
0.05
0.34
0.65
0.05
0.38
0.02
0.03
CaO
0.66
0.37
0.73
0.56
0.69
0.03
0.03
0.42
0.15
0.15
0.80
1.63
<0.05
0.71
0.18
0.52
Na2O
4.02
3.90
4.08
4.10
4.04
4.37
4.64
4.85
5.25
4.59
4.58
4.73
4.89
4.57
3.45
5.79
K2O
4.28
4.49
4.67
4.70
4.62
4.54
4.18
4.19
4.05
4.12
3.44
3.13
4.20
4.01
7.30
2.69
P2O5
0.04
0.04
0.03
0.02
0.04
0.02
0.02
0.02
0.02
0.02
0.07
0.13
0.03
0.07
0.02
0.02
LOI
0.38
0.34
0.30
0.48
0.34
0.44
0.44
0.50
0.44
0.55
0.40
0.37
0.48
0.70
0.30
0.26
Total
99.78
99.81
100.17
100.42
99.84
100.40
100.28
100.06
100.01
99.92
100.08
100.12
99.23
99.39
100.65
99.03
Li, ppm
23
34
28
41
29
2
15
8
36
1
56
79
23
140
122
107
Rb
164
200
150
210
160
316
400
350
240
282
169
131
392
270
1122
348
Cs
1
1
1
1
1
1
1
1
1
<1
1
<1
<1
<1
253
51
Ba
215
33
138
41
143
138
16
12.3
10.1
–
739
–
–
–
47
12.4
Sr
65
13
44
18
43
15
4.55
6.3
5.3
–
112
–
–
–
7.4
8.7
Be
–
–
2.75
3.27
–
2.22
4.36
3.7
–
–
3.08
–
–
–
13
14.9
Sn
–
–
1.45
1.72
–
0.59
0.92
1.0
–
–
1.35
–
–
–
44
22
W
–
–
0.22
0.25
–
0.69
3.95
–
–
–
0.29
–
–
–
11.6
7.9
Pb
–
–
30
29
–
27
33
58
–
–
23
–
–
–
106
79
Zn
–
–
26
35
–
90
0.92
30
–
–
36
–
–
–
60
50
Cu
–
–
4.08
12.1
–
5.6
5.9
5.8
–
–
3.03
–
–
–
2.74
5.2
Co
–
–
0.9
0.67
–
0.54
0.36
2.1
–
–
2.08
–
–
–
0.3
0.62
Ni
–
–
77
79
–
80
75
5.7
–
–
67
–
–
–
67
20
Cr
–
–
22
27
–
24
24
9.3
–
–
14
–
–
–
11.5
26
V
–
–
3.22
1.08
–
<ПО
0.93
4
–
–
21
–
–
–
0.051
5.1
Y
11.7
28
14.3
24
16.2
25
21
16.3
26
–
22
–
–
–
99
99
Nb
12.1
10.2
10.5
10.3
11.6
32
100
53
37
–
15
–
–
–
35
135
Ta
0.51
0.36
0.43
0.47
0.54
2.01
7.4
4.9
3.4
–
1.64
–
–
–
28
38
Zr
168
104
166
99
181
138
124
109
105
–
172
–
–
–
54
184
Hf
5.8
4.7
0.22
3.97
5.7
7.5
7.5
6.4
6.2
–
4.64
–
–
–
5.6
15
Th
24
50
29
41
22
12.5
25
16.1
9.3
–
12.6
–
–
––
14.1
50
U
4.9
6.9
6.5
39
4.9
13.8
18
16.2
5
–
9.8
–
–
–
3.47
15
F
400
100
200
200
300
100
200
300
100
<200
400
500
<200
700
1100
700
ments similar to that of K–Na Sharanur granitoids and thus are close to the mean composition of the continental crust. At the same time, Aya granites contain minima of normalized concentrations of Ba, Sr, La and Eu but are enriched in Li. As follows from those differences, the multiphase Aya intrusion underwent magmatic differentiation combined with alkali feldspar and plagioclase fractionation resulting in the minima of the above elements. Second phase Aya leucogranites show a REE distribution close to early phase biotite granites. At the same time, they are depleted in light lanthanoids (La, Се, Pr, Nd, Sm, Eu), as well
as Li, and enriched in HREEs (Table 5, Fig. 4). They also show minima of Ba and Sr normalized concentrations. In the Aya plutons, those geochemical features are good indicators of magmatic differentiation of the multiphase intrusion with a significant fractionation of feldspar. Quite a different REE distribution is shown by amazonite-bearing pegmatites, which crosscut the early phase Aya granites. They are enriched in Cs, Rb, Pb, Be, F, Ta, Sn, Li, Lu, Yb and Y but show minima of Ba, La, Nd, Zr, Sr and Eu concentrations (Fig. 4). Thus, the above geochemical characteristics suggest no genetic relations between Aya granites, pegmatoid rare-metal granites and ama-
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Table 5. REE in early Paleozoic granitoids of the Aya pluton Component
Аya-585
Аya-587
Аya576
Аya-578
Аya-589
Аya-595
Аya-598
La, ppm
25
11.8
2.75
1.17
19
3.69
2.29
Ce
56
30
7.7
3.11
30
7.8
4.91
Pr
6.2
3.94
1.01
0.34
4.07
1.32
0.83
Nd
23
17
4.18
1.62
16
6.7
4.69
Sm
4.72
5.3
1.64
0.80
3.76
3.38
3.12
Eu
0.45
0.19
0.09
0.03
0.56
0.074
0.087
Gd
3.95
5.3
2.41
1.27
4.04
4.91
5.4
Tb
0.52
0.82
0.54
0.34
0.66
0.98
1.22
Dy
2.99
5.1
4.15
3.29
4.17
7.40
10.8
Ho
0.57
0.99
0.96
0.86
0.85
1.75
2.96
Er
1.71
2.93
3.55
3.56
2.69
6.90
14
Tm
0.25
0.43
0.60
0.67
0.41
1.39
3.34
Yb
1.64
2.75
4.77
5.6
2.72
12.9
34
Lu
0.25
0.41
0.70
0.91
0.40
2.20
6.5
REE
127
86.5
35.01
23.62
90
61.41
94.15
La/Yb
15.24
4.29
0.58
0.21
7.00
0.29
0.067
Note. Аya-585, 587, 1st phase granites; Аya-576, 578, 2nd phase granites with fluorite; Aya-589, vein granite in the 1st phase granites; Аya-595, 598, amazonite pegmatites.
zonite-bearing pegmatites (Olkhon area and Khamar-Daban Ridge) and are referred as belonging to intraplate magmatic formations (Antipin and Perepelov, 2011). As a result of this study it has been inferred that regarding the composition, K–Na Sharanur granitoids, which inherit the major-and trace composition from the hosting Olkhon gneissschistose sequence, are close to the 1st phase Aya (Olkhon area) granites thus giving an evidence for genetic affinity of autochthonous and intrusive magmatism. It is to be underlined that according to U–Pb zircon dating (Table 1), they were produced within a narrow age interval of 500–465 Ma under collisional setting. Conclusions The geologic and petrologic-geochemical study showed that in the Olkhon area of the Baikal Region the early Paleozoic collision-related granitoid magmatism proceeded within a narrow time span (500–465 Ma). It was responsible for the production of both autochthonous migmatite–granite complexes being close by composition to the Olkhon series host rocks, and multiphase intrusions, emplaced into upper horizons of the continental crust. The Sharanur and Aya granitoids, classified into different geochemical types, involve formations of normal calc-alkaline and subalkaline series. The granitoids of the Olkhon area, grouped in terms of petrographic and petrochemical characteristics, are marked by diverse mineral compositions. Considering the chemical composition and K2O/Na2O ratio, the migmatites–plagiogranites and K–Na calc-alkaline Sha ranur granitoids are close to host crystalline schists and gneisses, that may be a result of formation of these granitoids in melting of an ancient metamorphic substratum of the Olkhon series. On classification diagrams granosyenites and quartz syenites
share the field and common evolutionary trends with K–Na Sharanur granitoids, thus likely pointing to their genetic affinity. However, subalkaline series rocks spatially lie close to the outcrops of crystalline schists and amphibolites, whose melting under collision setting, could have resulted in higher alkalinity of these rocks. The geochemical data from ICP-MS show a complex formation pattern for the multiphase Aya pluton. At early stages, its evolution was possibly due to the composition of the primary magma under melting of gneiss-schistose substratum of the Olkhon series, that is evidenced by petrochemical and petrographic affinity between 1st phase Aya biotite granites and K–Na Sharanur granites (Fig. 3). Later, its geochemical evolution was mainly governed by intrachamber magmatic differentiation. However, the traces of the magmatic differentiation processes were evident during the formation of an early phase represented by biotite granites (Fig. 4). Having similar REE distribution patterns, the 1st phase Aya granitoids as opposed to the Sharanur rocks, contain minima of Sr, Ba and Eu concentrations, which may be due to initial fractionation of feldspar from the magmatic melt. The magmatic differentiation was in particular intense during the formation of 2nd phase leucogranites, depleted in Sr, Ba, Eu, Li, as well as LREE and enriched in HREE and HFSE (Ta, Nb). The age estimates of the autochthonous and intrusive granitoids are well consistent with early Paleozoic collision events within the Olkhon metamorphic terrane. The syncollisional granitoids may be derived from both melting of the crustal protolith (Sharanur complex) and magmatic differentiation (multiphase Aya pluton). By mineralogical-geochemical characteristics, they are not referred to rare-metal pegmatoid granites and Li–F and Rb– Be–Nb pegmatites (Antipin et al., 2014), whose vein bodies
V.S. Antipin et al. / Russian Geology and Geophysics 59 (2018) 1616–1625
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