Olivine megacrysts in alkali-basaltoid dikes of the Western Sangilen, Southeastern Tuva

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Abstract

Large (up to 4 cm) crystals of euhedral olivine and its intergrowths with clinopyroxene were discovered in the alkali-basaltoid dikes of the Western Sangilen, South-Eastern Tuva. The composition of olivine (Mg# 81–83, NiO 0.2–0.3 wt. %) differs sharply from the composition of olivines from mantle and gabbroid parageneses. It is shown that olivine is a liquidus phase that crystallized from an alkali basaltic melt at pressures of 10–13 kbar in intermediate magma chambers located at depths of 34–43 km, which corresponds to the level of the crust-lithospheric mantle boundary for the Western Sangilen.

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About the authors

A. E. Izokh

Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences

Author for correspondence.
Email: izokh@igm.nsc.ru
Russian Federation, Novosibirsk

V. V. Egorova

Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences

Email: izokh@igm.nsc.ru
Russian Federation, Novosibirsk

R. A. Shelepaev

Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences

Email: izokh@igm.nsc.ru
Russian Federation, Novosibirsk

Ya. Yu. Shelepov

Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences

Email: izokh@igm.nsc.ru
Russian Federation, Novosibirsk

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Supplementary files

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2. Fig. 1. Scheme of arrangement of camptonite dikes of the Agardag complex of Western Sangilen (southeastern Tuva) according to [15]. 1 – Precambrian metamorphosed terrigenous-carbonate rocks of the Moren complex; 2 – Cambrian volcanogenic-sedimentary rocks; 3 – hyperbasites; 4 – gabbroids of the Pravotarlashkinsky and Bashkymugursky massifs; 5 – monzodiorites of the Bashkymugursky massif; 6 – Ordovician granitoids; 7 – dikes of camptonites of the Agardag complex, a dike with olivine megacrysts is shown as a large star; 8 – faults.

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3. Fig. 2. Photograph of olivine (Ol) megacryst in camptonite (dike 5H). The crystal size is 3.5 cm in length, it has a clear crystallographic facet.

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4. Fig. 3. Composition of olivines from the Agardag complex dikes: 1 – megacrysts; 2 – cores of olivine phenocrysts in camptonites; 3 – marginal parts of olivine phenocrysts in camptonites; 4 – intergrowths of clinopyroxene and olivine megacrysts; 5 – olivine xenocrysts; 6 – olivine from lherzolite xenoliths carried out by camptonite dikes; 7-8 – olivines from gabbroids: 7 – Pravotarlashkinsky massif, 8 – Bashkymugursky massif. Mg# = Mg*100/ (Mg+Fe), at.%.

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5. Fig. 4. (a) Results of thermobarometric calculations according to [18, 19]: 1 – for clinopyroxene megacrysts, 2 – for clinopyroxene and olivine intergrowths; 3 – for gabbroid xenoliths from dikes of the Agardag complex according to [15], 4 – experimental data according to [20]. (b) Formation pressures and composition of clinopyroxenes of the megacryst association of alkali basaltoids from different regions: 1-2 – from camptonites of the Agardag complex: 1 – megacrysts, 2 – intergrowths with olivine; 3 – Mongolia; 4 – Vietnam; 5 – Kyrgyzstan.

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