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GEODYNAMICS & TECTONOPHYSICS
PUBLISHED BY THE INSTITUTE OF THE EARTH'S CRUST SIBERIAN BRANCH OF RUSSIAN ACADEMY OF SCIENCES
2017 VOLUME 8 ISSUE 3 PAGES 455-459 ISSN 2078-502X
https://doi.org/10.5800/GT-2017-8-3-0261
Proceedings of the Second Russia-China International Meeting on the Central Asian Orogenic Belt (September 6-12, 2017, Irkutsk, Russia)
Geochronology and Sr-Nd isotope geochemistry of Late Paleozoic collisional granitoids of Undinsky complex (Eastern Transbaikal region)
S. I. Dril1, Yu. V. Noskova1, K.-L. Wang2, 3, V. A. Belyaev1, 2, S. Yu. Skuzovatov1, D. A. Grigoriev1, D. A. Belkov1
1 A.P. Vinogradov Institute of Geochemistry, Siberian Branch of RAS, Irkutsk, Russia
2 Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
3 Department of Geosciences, National Taiwan University, Taipei, Taiwan
For citation: Dril S.I., Noskova Yu.V., Wang K.-L., Belyaev V.A., Skuzovatov S.Yu., Grigoriev D.A., Belkov D.A., 2017. Geochronology and Sr-Nd isotope geochemistry of Late Paleozoic collisional granitoids of Undinsky complex (Eastern Transbaikal region). Geodynamics & Tectonophysics 8 (3), 455-459. doi:10.5800/GT-2017-8-3-0261.
There are several geodynamic models of the Central Asian Orogenic Belt (CAOB) development [§engor et al., 1993, Zorin, 1999; Parfenov et al., 1999, 2003; Willem et al., 2012; and others]. The Mongol-Okhotsk Orogenic Belt (MOB) represents important part of CAOB. All geodymanic models of Late Riphean to Paleozoic structures of CAOB emphasize significance of subduction processes along Northern Asian craton margin at that time. Collage of CAOB terrains formed as a result of accretion of island arc, accretionary wedge, turbidite, and continental margin terrains to the Siberian paleoconti-nent. These terrains became a substrate for the further granitoid magmatism. Formation of large volumes of the granite magmas within orogenic belts is often related to widespread terrain displacement, when the settings of active continental margin and collision turn
into transform margin of lithospheric plates. Such geo-dynamic change probably occurred within MOB during Permian to Early Triassic [Parfenov et al., 2013]. At that time, extended Gobi-Khingan belt of palingenic granitoids, attributed to the Undin complex in Eastern Transbaikalia, formed along northern margin of the Argun superterrain [Kozlov et al., 2003; Parfenov et al., 2003]. Numerous massifs of these granitoids of grano-diorite-granite series and genetically related small leu-cogranite intrusions are located both within the Onon accretionary wedge terrain of the MOB and the Argun superterrain (Fig. 1). New Rb-Sr and U-Pb geochrono-logical data specify the age of the Undin complex. Based on Sr-Nd isotope systematics, involvement of different crustal protoliths into petrogenesis of collisional granitoids is evaluated.
Fig. 1. Geological scheme of the Undin complex location in the Transbaikalian sector of the Mongol-Okhotsk Belt (MOB).
1 (I) - Western Stanovoi metamorphic terrain of MOB; 2 (II) - Kamensk island arc terrain (MOB); 3 (III) - Onon accretionary wedge terrain (MOB); 4 (IV) - Argun terrain of the MOB passive margin; 5 - massifs of the Undin complex; 6 - limit of the Vend-Cambrian carbonate cover of the Argun terrain; 7 - faults bordering Mongol-Okhotsk suture zone; 8 - state border. Numbers in circles denote granitoid massifs: 1 - Ust-Telegnui, 2 - Verkhneundinsk, 3 - Margutseksk, 4 - Krasnokovylin.
Geochronology and Sr-Nd isotope geochemistry.
The four studied massifs of the Undin complex are located in different parts of MOB [Fig. 1). The Ust-Teleg-nui massif [# 1 on Fig. 1) is located within the Onon accretionary wedge terrain. The Verkhneundin massif [# 2 on Fig. 1) is located in marginal part of the Argun superterrain adjacent to the Onon terrain. The Margutseksk and Krasnokovylin massifs of leucocratic granites [# 3 and 4 on Fig. 1) are located within internal part of the Argun superterrain nearby its Vendian-Cambrian carbonate cover. Isotope Rb-Sr [TIMS) dating of the granitoids yielded following results: 1) The Ust-Telegnui massif: 257±3 Ma, I[0)Sr=0.70598±6, MSWD= =3.6; 2) The Krasnokovylin massif: 244±35 Ma, I(0)Sr= =0.7047±0.0047, MSWD=14; 3) The Margutseksk massif: 248±3 Ma, I[0)Sr=0.70535±0.00022, MSWD=0.64. Zircon from the Verkhneundin massif was dated by
U-Pb method. Zircon is represented by idiomorphic elongated grains of hyacinth habit 150-600 |im in size.
Crystals are transparent to semi-transparent and light-
yellow-colored. CL-images of selected zircons presented in Fig. 2 demonstrate oscillatory and sectorial zoning. U-Pb isotope studies were conducted by LA-ICP-MS with Photon Machines Geolas ArF 193 nm laser ablation system connected to Agilent-7900 mass-spectrometer installed at the Institute of Earth Sciences (IES) in Taipei, Taiwan. 36 zircon grains were analyzed. For most concordant analyses (discordance <5 %) 206Pb/238U age is 249±4 Ma (MSWD=0.75) (Fig. 3). Similar age estimate of 254±2 Ma (MSWD=1.2) is calculated for all concordant (within uncertainty) results (n=21). Hf isotope analyzes of these zircons obtained by MS-LA-ICP-MS using Photon Machines Analyte G2 193 nm excimer laser connected to Nu Plasma multi-collector
mass-spectrometer at IES yielded sHf(T)= -2.5 to +3.6 and Tdm=1050-1410 Ma.
Therefore, time period of granitoid formation covers Late Permian to Early Triassic. Notable, age of regional metamorphism in the Onon accretionary wedge terrain of MOB was regarded as Permian [Rutshtein, 1997]. Probably, granitoid massifs of the Undin complex intruding the Onon terrain formed during retrogressive stage of the regional metamorphism. The origin of the Krasnokovylin and Margutseksk massifs located within internal part of the Argun terrain is less evident. This part of the Argun superterrain is supposed to be stable
in Paleozoic and was not affected by the processes which took place in Mongol-Okhotsk suture during Late Paleozoic. The magma generation processes in this stable part probably were influenced by transpression-transtension regime characteristic of transform margin of lithospheric plates [Khanchuk, 2006]. In this case the sources of melts formed within metamorphosed accre-tionary wedge (Onon terrain) and stable block (Argun terrain) should be different.
Granitoids of the massifs located within the Onon acretionary wedge terrain and periphery of the Argun superterrain have moderately negative sNd(257MA)= -4.3
| Fig. 3. Concordia diagram for zircons of UN-1-4 granite from the Verkhneundin massif. Concordant age is 249±4 Ma.
Fig. 4. Sr-Nd isotope diagram for the Undin complex granitoids from the Eastern Transbaikalia. 1 - Ust-Telegnui and Verkhneundinsk granitoid massifs; 2 -Margutseksk and Krasnokovylin granitoid massifs.
(Ust-Telegnui massif) h sNd(254MA)— -5.0 to -1.9 (Verkh-neundin massif). These characteristics correspond to those of metaterrigenous rocks of the MOB accretio-nary wedge with sNd(385-475)— -6.3 to +2.8 which can be considered as a protolith for the granite magmas generation. This conclusion is supported by the model ages of granitoids TNd(DM-2)—1210-1460 Ma corresponding to those of metasediments of Kulinda and Onon formations with TNd(DM-2)—1161-1575 Ma. Granitoids of the massifs located within internal part of the Argun terrain have more contrast isotope characteristics. Granites of the Margutseksk and Krasnokovylin massifs have sNd(250MA)— -8.9, TNd(DM-2)—1775 Ma, and sNd(245MA) — +1.3, TNd(DM-2)—926 Ma, respectively.
Sr-Nd isotope systematics of the Undin complex granitoids is shown on Fig. 4. The granitoids emplaced into the accretionary wedge form rather extended trend of increasing radiogenic Sr isotope composition at nearly constant moderately negative sNd, corresponding to the trend of metasedimenary rocks of the MOB accretionary wedge, and to the granitoids of SW Japan, originated from subduction-accretionary com-
plexes [Jahn, 2010]. This indicates participation of metasedimentary rocks in the granitoid genesis. Compositions of the leucogranites within the Argun terrain have lower Sr isotope ratios at moderately negative or close-to-zero sNd. This can argue for a mafic lower crustal source of these granites.
Conclusion. Collison in the Mongol-Okhotsk Orogen led to formation of the Gobi-Khingan belt of palingenic granitoids along northern margin of the Argun superterrain. In the Eastern Transbaikalia these granitoids, known as the Undin complex, formed in Late Permian to Early Triassic (from new Rb-Sr and U-Pb geochrono-logical data), constraining time of collision in the Mongol-Okhotsk Orogenic Belt. Similar Sm-Nd and Sr-Nd isotope characteristics of the granitoids and Mongol-Okhotsk accretionary wedge metasediments indicate participation of the latter in the sources of granite magmas.
Acknowledgements. The study was supported by the Russian Foundation for Basic Research, Project 1505-05079.
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