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V.K. POPOV 1 , A.V. GREBENNIKOV 1 , M.D. GLASCOCK 2 , Y.V. KUZMIN 3

Geochemistry, geodynamic regimes, and cesium content of Cenozoic silicic volcanic glasses of Primorye. V.K. POPOV 1 , A.V. GREBENNIKOV 1 , M.D. GLASCOCK 2 , Y.V. KUZMIN 3 1 - Far East Geological Institute Far East Branch Russian Academy of Sciences, Vladivostok, Russia,

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V.K. POPOV 1 , A.V. GREBENNIKOV 1 , M.D. GLASCOCK 2 , Y.V. KUZMIN 3

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  1. Geochemistry, geodynamic regimes, and cesium content of Cenozoic silicic volcanic glasses of Primorye V.K. POPOV 1, A.V. GREBENNIKOV 1, M.D. GLASCOCK 2, Y.V. KUZMIN 3 1 - Far East Geological Institute Far East Branch Russian Academy of Sciences, Vladivostok, Russia, 2 - University of Missouri, Columbia, USA, 3 - Pacific Institute of Geography Far East Branch Russian Academy of Sciences, Vladivostok, Russia.

  2. Introduction • Reconstruction of geodynamic regimes of magmatism is based on research of igneous rocks composition and geochemical typification. Research by G. Pears, G. Cant, S. Taylor, A. White, M.I. Kuzmin, O.N. Volynets and many others have helped to single out basic geochemical types of magmatic rocks and to find connection between their geochemical composition and geodymanic formation conditions. Most information is obtained in research of rare-element rock composition. There have been worked out a significant number of discriminant diagrams for magmatic rocks composition. • G.B. Levashev, Y.A. Martynov, S.V. Rasskazov, S. Okamura, and others have already effected this kind of research when studying Sikhote-Alin Cenozoic basalts. • This report is devoted to the results of geochemical study of Primorye Paleogene-Neogene rhyolite volcanic complexes. Besides, the attempt is made to define acid volcanism geodynamic regimes in Cenozoic period of active continental margin development on basis of silicic volcanic glasses geochemical typification. • There have been studied peculiarities of cesium distribution in volcanic glasses in order to discover cesium-bearing volcanic glasses types. We studied volcanic glasses from Bogopolsk (Paleocene-Eocene), Kraskinsky and Kedrovsky (Eocene-Oligocene), and from Ust-Suifun complexes that cover Late Cretaceous subduction deposits of East Sikhote-Alin volcanic belt. Besides, there have been studied felsic alkali rocks of modern Paektusan volcano illustrating the intraplate magmatism.

  3. The East Sikhote-Alin volcano-plutonic belt Ridge development may be divided into four basic stages: 1 – the main stage of ridge formation:stage of large volumes of felsic ignimbrite eruptions of the Primorsky igneous complex (Turonian - Santonian, 90-85 Ma) 2 – stage of graben-like structures and collapse calderaformation. Bogopolsky volcanic complex (Paleocene – Eocene, 60 – 52 Ma) 3 – stage of basalt-rhyolite volcanism that took place in individual grabensof the rifting type.Kraskinsky (Kedrovsky) complex (Eocene – Oligocene,37 – 21 Ma) 4 – stage of highly explosive felsic volcanism synchronous to the eruption of the Shufansky Plateau basalts. Ust-Suifun suite (Miocene, 11.8-10.8 Ma) 4 3 2 1

  4. Bogopolsky volcanic complex (Paleocene-Eocene) • The initial stages of the Cenozoic felsic volcanism are related with the Bogopolsky complex formation. Rb/Sr isochron ages of extrusive rocks of the complex are 59.7-52.9 Ma. (Grebennikov, 1998; Popov, Grebennikov, 2001). Felsic volcanics of the complex, in contrast to the plateau ignimbrite of Primorsky igneous complex, are more locally developed. They fill graben-like structures (Yakutinskaya, Martelevskaya and others) and collapse caldera in the sublatitudal and northwestern directions.

  5. Dacite-rhyolite volcanism of this period is characterized by a wide occurrence of volcanic glasses (perlites) composing the extrusive (tuffs and ignimbrites) and intrusive (dikes) bodies. The most important feature of the Bogopolsky complex magmatism is the absence of a visible spatial-temporal and genetic relation with basaltic volcanism

  6. Bogopolsky volcanic complex (Paleocene-Eocene) • The chemical composition of the silicic volcanic glasses shows that they belong to the calc-alkaline rock series with differentiated distribution of K and Na that is reflected in the variations of trace element composition. As a whole, the perlites are enriched in alkaline and alkaline-earth elements – Cs, Rb, Ba, Sr, and Zr - and characterized in absence of a clear Ta-Nb gap. Relative to the average composition of the continental upper crust, they have a higher content of LREE and HREE. Among the Bogopolsky perlites there have been distinguished the volcanic glasses of the Yakutinskaya volcanic structure that show anomalously low Ba, Sr, and Eu, forming clear minimums on the spider-diagrams, and high concentrations of Th and Ta and HREE. Values of the 87 Sr/86Sr initial isotope ratios in hyaloignimbrites of the volcanic structure are 0.7066-0.7081.

  7. On the discrimination geochemical diagrams, the figurative points of the Bogopolsky extrusive composition are placed between the fields of the island-arc and intraplate granitoids. The geodynamic regime of this period’s volcanism may be considered as transitional from the subduction one (initial volcanism stages – Maastrichtian) to that of the transform continental margins (closing volcanism stages – Paleocene-Early Eocene).

  8. Volcanic glasses of Kraskinsky complex (Eocene-Oligocene) The volcanic glasses of the Kraskinsky (Kedrovsky) complex, found in volcanic depressions of the rifting type, are spatially and paragenetically related with basaltic andesite volcanism. Geochronological K-Ar age datings of the silicic volcanic glasses are 36.7 and 21 Ma. (Popov et al., 2001). Rb-Sr isochrones on whole rocks are 33.5±1.1 Ma at (87Sr/86Sr)0 = 0.70467±0.00003 (Rasskazov et al., 2003).

  9. Volcanic glasses of Kraskinsky complex (Eocene-Oligocene) • In chemical composition, the Kraskinsky volcanic glasses are similar to the Bogopolsky complex perlites and differ from them in lower concentrations of Zr, Hf, Yb, HREE, as well as Nd, Sm, and Eu. An insignificant Th-Ta maximum appears in them. Relative to the average composition of the continental upper crust, the volcanic glass have higher Rb, Ta, and Th and lower Sr, Ba, and REE.

  10. Volcanic glasses of Kraskinsky complex (Eocene-Oligocene) • On the discrimination diagram by S.D. Velikoslavinsky (2003), compositions of the silicic volcanic rock fall in the field of the intraplate granitoids. On the discrimination geochemical diagrams by Pearce and Harris, the points of the Kraskinsky volcanic glasses lie on the boundary field of the island-arc and intraplate granites. This allows them to be distinguished as a geochemical type similar to the Late Quaternary rhyolites of the Koso volcanic province, California. The geodynamic setting of this period volcanism represents a distinctive variety of the transform continental margins of California type (Khanchuk et al., 1987).

  11. Volcanic glasses of Ust-Suifun suite (Miocene) The Late Cenozoic (Miocene-Pliocene) magmatic stage is characterized by insignificant manifestation of felsic volcanism synchronous to the eruption of the Shufansky Plateau basalts. The felsic volcanism of this period was highly explosive. This resulted in the pyroclastic deposits widely developed in the Ust-Suifun suite in southwestern Primorye. The absolute age (fission track dating method) of the volcanic glasses from the sequences of the volcanogene-sedimentary deposits is 11.8 and 10.8 Ma, correspondingly (Pavlyutkin et al., 1985).

  12. Volcanic glasses of Ust-Suifun suite (Miocene) • The volcanic glasses are represented by perlites with obsidian cores (marekanites). In chemical composition they are similar to potassic rhyolites. Remnant marecanites in perlite from pebbles of Ust-Suifunskaya Site (Source “Razdolnaya River”).

  13. Volcanic glasses of Ust-Suifun suite (Miocene) • In chemical composition, the Ust-Suifun volcanic glasses are similar to the Bogopolsky complex perlites and differ from them in lower concentrations of Nd, Sm, and Eu. An insignificant Th-Ta-Nb maximum appears in them. Relative to the average composition of the continental upper crust, the volcanic glass have higher Rb, Ta, and Th and lower Sr, Ba, and REE. • On the discrimination petrochemical diagram by Pearce and Harris, the figurative points of the Miocene rhyolite composition lie on the boundary field of the island-arc and intraplate granites.

  14. Paektusan Volcanic glasses of Paektusan volcano (Holocene) Modern volcano of Paektusan is located in the central part of Changbaishang Plateau, on the border between China and North Korea. Obsidian-perlites of the Paektusan (Baitoushan) modern volcano originated in a closing formation stage of a thick volcanic edifice composed of alkaline basalts, trachytes, pantellerites, and comendites. Obsidian abruption (black stratum) in crater’ wall of Paektusan Volcano.

  15. Volcanic glasses of Paektusan volcano (Holocene) In chemical composition obsidians belong to alkaline rocks (pantellerites and comendites), and in some domes they are rhyolites of a normal alkalinity. Relative to the volcanic glasses of the Bogopolsky and Kraskinsky complexes, they have higher concentrations of Rb, Zr, Y, Th, Ta, Nb, and LREE and very low contents of Sr, Ba, and Eu. With respect to the average composition of the continental upper crust, the Paektusan volcanic glasses have high concentration of REE (with the exception of Eu forming a clear minimum) and K, Rb, Th, Ta, Nb, Ce, Zr, and Hf.

  16. On the discrimination diagrams by Pearce and Harris, the points of the volcanic glass composition of Paektusan volcano are placed in the field of the intraplate granites showing close similarity to alkaline rhyolites (comendites) of the alkaline-basalt-trachyte-comendite series of west Kamchatka (Cherpuk, Ichinsky, and Belogolovsky volcanoes), which were conformable (Volynets et al., 1987) in the geodynamic setting of the intraplate type.

  17. Bogopolsky volcanic complex: The cesium-bearing perlites make up the peripheral bodies of the extrusive domes, developed at the felsic volcanism terminating phases. High cesium concentrations (61-481 ppm) are characteristic of perlites of the Yakutinskaya, Bogopolskaya, and Brusilovskaya volcanic structures. • Kraskinsky complex:The cesium content in perlites is 3-5 ppm and rarely exceeds 20 ppm. • Ust-Suifun suite:The cesium content in volcanic glasses not exceed 5 ppm. • Paektusan volcano:The cesium content of volcanic glasses is about 4-6 ppm.

  18. Conclusion • The data obtained testify that the rocks of the studied complexes belong to three geochemical types of magmas. Comparison of them with the "typical" modern volcanic series that characterized different geodynamic settings, indicates a regular change of geodynamic regimes of magma generation on the territory of Primorye in Cenozoic time. • The Bogopolsky complex volcanic glasses (Paleocene-Eocene) demonstrate geochemical composition similar to that of the rocks formed in the subduction geodynamic setting and the rocks of the transform continental margin. In the volcanic glasses of the Kraskinsky complex (Eocene-Oligocene), the geochemical features of volcanites of the transform continental margins (California type) are clearly defined. The volcanic glasses of the Ust-Suifun complex and Pektusan volcano unambiguously belong to the intraplate volcanic series. • Cesium-bearing perlites are related with the last phases of felsic volcanism of the Bogopolsky complex (54-52 Ma) and are considered the most promising from the point of view of cesium mineralization.

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