UDC 550.42.14+551.21+552.333+(51)

https://doi.org/10.26516/2541-9641.2025.4.141

EDN: PUHFZV

Geochemical Assessment of Differentiation in Lava Layers and Spatial Change in Volcanic Sources for pre-Holocene Tephrite-Trachybasalt Series in the High Great Sayan Range and Oka Basin

S.V. Rasskazov^1,2, D.I. Parfenov², Z.B. Galsanov³, B.D. Sharastepanov³, A.P. Papaev³, S.V. Snopkov^2,4, T.A. Yasnygina¹, I.S. Chuvashova¹

¹Institute of the Earth's Crust SB RAS, Irkutsk, Russia

²Irkutsk State University, Irkutsk, Russia

³Orlik Secondary School, Orlik, Russia

⁴Irkutsk National Research Technical University, Irkutsk, Russia

Abstract. We present the results of comparative geochemical studies of pre-Holocene trachybasalts and tephrites in the northern part of the late Cenozoic Oka volcanic field — the Ust-Zhombolok lava cover in the Oka Basin and lava flows and cinder cones of the upper reaches in the Zhombolok River and the Khi-Gol valley at the axial part of the Great Sayan Range. From surface morphology of lava layers in both areas, it is assumed that residual melts were concentrated in pipe-like channels, followed by their removal and roof collapse. Outcrops of the Ust-Zhombolok lava flow of the Oka Basin reveal a trend toward decreasing CaO/Al₂O₃ with a slight relative decrease in magnesium number (Mg#) of transitional facies rocks between the quench and residual melt facies of the "pipe." The trend reflects a control of component redistribution by co-crystallization of titanaugite and plagioclase at the early stage of peripheral solidification of lava layers. In the residual melts of the pipe-like channels within lava layers of the Ust-Zhombolok cover, SiO₂ increases and Sr decreases. In the rocks of the upper parts of the uneroded lava flows of the axial part of the Bolshoi Sayan Range accessible for sampling, trends in olivine fractionation with the participation of Ca-plagioclase (with elevated CaO/Al₂O₃) and without the participation of Ca-plagioclase (with low CaO/Al₂O₃) are distinguished. The latter trend corresponds to the olivine + titanaugite cotectic. The trend of the "pipe" facies (titanium-augite + plagioclase cotectic with variable CaO/Al₂O₃ and a narrow range of Mg#) in the rocks of the axial part of the Bolshoi Sayan Range is not identified. Changes in the geochemical signatures of rocks are revealed, indicating shallow melting of slightly amphibolized, garnet-free pyroxenite material beneath the Oka Basin and deeper, amphibole-enriched, garnet-bearing, peridotite one beneath the axial part of the Great Sayan Range. The importance of the last (pre-Holocene) influx of magmatic melts from the depths of the crust-mantle transition (CMT) and underlying mantle in the Baikal Rift System (BRS) is emphasized. Attention is drawn to the differences between sources of a final volcanism in the northern Oka volcanic field and Chulutyn zone of Central Mongolia: involvement in melting of the depleted lithosphere in the former, and displaying of OIB-like material in the latter.

Keywords: basalts, late Pleistocene, Holocene, Eastern Sayans

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Rasskazov Sergei Vasilevich,

doctor of geological and mineralogical sciences, professor,

664025, Irkutsk, st. Lenina, 3,

Irkutsk State University, Faculty of Geology,

Head of Dynamic Geology Char,

664033, Irkutsk, st. Lermontova, 128,

Institute of the Earth's Crust SB RAS,

Head of the Laboratory for Isotopic and Geochronological Studies,

tel.: (3952) 51–16–59,

email: rassk@crust.irk.ru

 

Parfenov Daniil Igorevich,

664025, Irkutsk, st. Lenina, 3,

Irkutsk State University, Faculty of Geology,

second year bachelor's student,

email: parff0315@mail.ru

 

Galsanov Zolto Belikovich,

Orlik settlement, Sayot district, Buryat Republic,

Orlik Secondary School,

school student,

 

Sharastepanov Bair Dasheevich

Candidate of Geography,

Orlik settlement, Sayot district, Buryat Republic,

Orlik Secondary School,

Director,

email: orlik_school@mail.ru

 

Papaev Alexey Purboevich,

Candidate of Geography,

Orlik settlement, Okinsky district, Buryat Republic,

Orlik Secondary School,

Deputy Director for Educational Work,

email: papaev13@rambler.ru

 

Snopkov Sergey Viktorovich,

Candidate of Geological and Mineralogical Sciences,

664025, Irkutsk, Lenin st., 3,

Irkutsk State University, Faculty of Geology

assistant professor,

664074, Irkutsk, Kurchatov st., 3,

Siberian School of Geosciences, Irkutsk National Research Technical University,

Leading Researcher,

email: snopkov_serg@mail.ru

 

Yasnygina Tatyana Alexandrovna,

candidate of geological and mineralogical sciences,

664033, Irkutsk, st. Lermontova, 128,

Institute of the Earth's Crust SB RAS,

Senior Researcher,

tel.: (3952) 51–16–59,

email: ty@crust.irk.ru

 

Chuvashova Irina Sergeevna,

candidate of geological and mineralogical sciences,

664033, Irkutsk, st. Lermontova, 128,

Institute of the Earth's Crust SB RAS,

Senior Researcher,

tel.: (3952) 51–16–59,

email: chuvashova@crust.irk.ru

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