ISSN 1028334X, Doklady Earth Sciences, 2014, Vol. 458, Part 1, pp. 1067–1072. © Pleiades Publishing, Ltd., 2014. Original Russian Text © T.V. Donskaya, D.P. Gladkochub, A.M. Mazukabzov, E.V. Sklyarov, E.N. Lepekhina, Tao Wang, Lingsen Zeng, Lei Guo, 2014, published in Doklady Akademii Nauk, 2014, Vol. 458, No. 3, pp. 306–312.
GEOLOGY
Continuity of Late Paleozoic–Early Mesozoic Magmatism in the Western Transbaikal Region T. V. Donskayaa, D. P. Gladkochuba, A. M. Mazukabzova, Corresponding Member of the RAS E. V. Sklyarova, b, E. N. Lepekhinac, Tao Wangd, Lingsen Zengd, and Lei Guod Received April 28, 2014
DOI: 10.1134/S1028334X14090268
Late Paleozoic–Early Mesozoic magmatic com plexes are widespread in the northern segments of the Central Asian fold belt (CAFB), which is located north of the Mongol–Okhotsk suture zone [1]. Two discrete stages of magmatic activity are defined for the Western Transbaikal part of this segment: Late Paleo zoic and Early Mesozoic. They are separated by a calm period lasting over 40 Ma long [2–5]. The first stage (Late Carboniferous–Early Permian, approximately 310–270 Ma ago) was marked by the formation of the Angara–Vitim batholith and Western Transbaikal vol canoplutonic belt, and the second stage (Late Triassic, approximately 230–210 Ma ago) saw development of the Mongol–Transbaikal volcanoplutonic belt [2–5]. The discrete patterns (or discontinuity) of mag matic activity allowed the authors of [2–5] to assume that magmatism in the Western Transbaikal part of the Central Asian fold belt was controlled by postcolli sional or intraplate extension (rifting) [2–5]. At the same time, the assumption of magmatism discontinu ity in the western Transbaikal region is inconsistent with the magmatism pattern in adjacent areas of the northern segment of CAFB, where magmatic events are widely manifested during the period of 270– 230 Ma ago. For example, the Middle–Late Permian and Early–Middle Triassic igneous rocks are known from western and northern Mongolia (large Khangai batholith and Erdenetuin Obo magmatic center [6, 7]) and in the eastern Transbaikal region [8]. In addition, one reliable Middle–Late Permian date (261 ± 5 Ma) a
Institute of the Earth’s Crust, Siberian Branch, Russian Academy of Sciences, Irkutsk, Russia b Far East Federal University, Vladivostok, Russia c Karpinskii AllRussia Institute of Geology (VSEGEI), St. Petersburg, Russia d Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China email:
[email protected],
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was also obtained for gneissose granites of the western Transbaikal region [9], although it was never men tioned in the abovementioned models of magmatism in this region [2–5]. The ambiguity in interpretations of magmatic activity in the Western Transbaikal part of the northern segment of the CAFB determined the necessity for additional investigation of this problem. Granitoids of unclear age attributed to the Zagan Complex were selected to serve as an object for this study (Fig. 1). For a long time, these rocks were con sidered as being Paleoproterozoic in age. After defin ing of metamorphic core complexes (MCCs) in the Transbaikal region, it appeared, however, that most granitoids of the Zagan Complex are confined to the lower parts of these structures [10]. The authors of [10] concede that the Precambrian age assumed for these rocks based on their more intense metamorphic and tectonic reworking as compared with the surrounding rocks may be substantially younger. The age of gneis sose granites and granodiorites from the lower part of the Zagan MCC was determined by the Rb–Sr whole rock method as 289 ± 23 Ma, while slightly gneissose granites and granosyenites that intrude the last com plex were dated back to 151 and 160 Ma, respectively [10]. Thus, there were grounds for the assumption that granitoids from the lower part of the Zagan MCC may be Late Paleozoic–Mesozoic or, partly, Middle–Late Permian in age, which predetermined their selection as an object for this study. Granitoids of the Zagan complex from the lower part of the Zagan MCC include massive and gneissose varieties. By their composition, they correspond to monzonites, quartz monzonites, granosyenites, gran ites, and leucogranites. The leucogranite and monzo nite samples (09130 and 09124, respectively) were taken for estimating the age of these rocks by the U– Pb zircon method (Fig. 1). Leucogranite represents a rock composed (%) of K feldspar (40), quartz (30), plagioclase (28), biotite (1), ore mineral (1), and accessory zircon. Its chemical composition is as fol
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Fig. 1. Schematic geological structure of the Zagan metamorphic core complex ([10], modified). (1) Quaternary sediments; (2) Cenozoic basalts; (3–7) rocks constituting the upper part of the Zagan metamorphic core complex: (3) Early Cretaceous sedimentary rocks, (4) Early Cretaceous volcanics and sediments, (5) Late Triassic granitoids, (6) Late Tri assic volcanics and sediments, (7) Early Permian granitoids; (8–10) rocks constituting the lower part of the Zagan metamorphic core complex: (8) Jurassic granitoids, (9) Late Triassic volcanics and sediments, (10) Middle/Late Permian–Early Triassic gran itoids of the Zagan complex; (11) milonitized rocks constituting the lower part of the Zagan metamorphic core complex; (12) detachment zone (a), faults (b); (13) bedding (a), foliation, gneissose patterns (b); lineation (c); (14) sampling site for the geochronological investigation. The inset shows the location of the Zagan metamorphic core complex.
lows (wt %): SiO2, 75.46; TiO2, 0.06; Al2O3, 13.80; Fe2O3, 0.24; FeO, 0.95; MnO, 0.01; MgO,
