Chemical microprobe Th-U-Pb age dating of monazite from rare-metal pegmatites of the Burpala massif (North Baikal)

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Аннотация

The chemical composition and zoning of monazite from a dike of rare-metal pegmatites of the Britholite zone of the Burpala massif (Northern Baikal region) was studied by electron probe microanalysis (EPMA). It has been established that monazites are represented by Ce and La varieties. Grains of monazite-(Ce) are characterized by an increased concentration of Nd2O3 and ThO2 compared to monazite-(La). The weighted average age for monazites, determined by the U-Th-Pb CHIME method, was 251±31 Ma. The age calculated by the isochrone method corresponds to 273±69 Ma. The obtained age values of monazite indicate that the pegmatites of the Britholite zone were intruded after the pegmatites of the northwestern area and suggest the existence of several stages in the formation of rare-metal pegmatites in the massif.

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Авторлар туралы

A. Spivak

D.S. KorzhinskyInstitute of Experimental Mineralogy, Russian Academy of Sciences

Хат алмасуға жауапты Автор.
Email: spivak@iem.ac.ru
Ресей, Chernogolovka

I. Sotnikova

A. P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences

Email: spivak@iem.ac.ru
Ресей, Irkutsk

A. Viryus

D.S. KorzhinskyInstitute of Experimental Mineralogy, Russian Academy of Sciences

Email: spivak@iem.ac.ru
Ресей, Chernogolovka

M. Kuzmin

A. P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences

Email: spivak@iem.ac.ru

Academician of the RAS

Ресей, Irkutsk

E. Zakharchenko

D.S. KorzhinskyInstitute of Experimental Mineralogy, Russian Academy of Sciences

Email: spivak@iem.ac.ru
Ресей, Chernogolovka

T. Kolotilina

A. P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences

Email: spivak@iem.ac.ru
Ресей, Irkutsk

N. Alymova

A. P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences

Email: spivak@iem.ac.ru
Ресей, Irkutsk

Әдебиет тізімі

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2. Fig. 1. Schematic geological map of the Burpalinsky massif. 1 – Quaternary deposits; 2 – sandstones, siltstones of the Kholodninsk formation; 3 – hornstones; 4 – shonkinites; 5 – nepheline syenites; 6 – striped trachytoid syenites; 7 – quartz syenites; 8 – Mariupol; 9 – foyaites; 10 – rare metal pegmatites; 11 – apatite-fluorite vein; 12 – carbonatites; 13 – alkaline granites; 14 – sampling site for geochronological Th-U‒Pb studies on monazite (1–Drill 310/9); 15 - sampling sites for U-Pb geochronological studies on zircon (2 – Drill 305/10 sample, 3 – Drill 313/10 sample [2,3]).

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3. Fig. 2. BSE images of monazite grains of sample 310-9 (in reflected electrons) with the designation of the analysis points (digits are the sequence number of the analysis).

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4. Fig. 3. Image of BSE (in reflected electrons). Inclusions of monazite and other minerals in apatite grains from rare metal pegmatites of the central part of the Burpalinsky massif. (a) – the main grain of apatite with inclusions of monazite; Ce-bastnesite, amphibole, albite with inclusion of zircon; (b) – grain of apatite with inclusions of monazite, amphibole, K-feldspar, albite; (c) – monazite in association with Ce-bastnesite, fluorite and quartz in apatite; (d) – monazite in apatite in association with carbonesite, fluorite, zircon and quartz. Designations: Ab – albite, Amp – amphibole, Ap – apatite, Bsn-Ce – bastnesite-(Ce), Flr – fluorite, Kfs– K- feldspar, Mnz – monazite, Qz – quartz, Zrn – zircon.

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5. Fig. 4. The spread of point ages relative to the weighted average value of the age of the studied monocytes; item n. a. – the sequence number of the analysis corresponding to Table 4 (SQUAW = 2.2; the error was calculated according to the 2σ criterion).

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6. Fig. 5. The ThO2*‒PbO isochron, constructed by the CHIME method, based on the results of the analysis of monocytes by the EZRS method (SQUAW = 1; the error is calculated by 2σ-criterion).

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