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Russian Geology and Geophysics

2010 year, number 10


I.S. Peretyazhko and E.A. Savina
A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia
Keywords: Porphyritic ongonite, aphyric rocks, melt and fluid inclusions, brine, P-Q solution, immiscible ongonite and calcium fluoride melts, melt convection
Pages: 1110-1125


The paper discusses the formation conditions of the Ary-Bulak ongonite massif (eastern Transbaikalia). Studies of melt and fluid inclusions have shown that, along with crystalline phases and a silicate melt, ongonitic magma contained aqueous-saline fluids of different types, fluoride melts compositionally similar to fluorite, sellaite, cryolite, chiolite, and more complex aluminum fluorides as well as silicate melts with abnormal Cs and As contents. The ongonitic melt crystallized with the participation of P-Q fluids as vapor solutions, presumably NaF-containing and slightly admixed with chlorides. We studied the properties and composition of brine inclusions from Ca- and F-rich rocks on the margin of the massif. Depending on the thermophysical properties of the host rocks and ongonitic melt, the duration of its crystallization has been estimated for a magma chamber with the size and shape of the Ary-Bulak massif. Magma chamber cooling has been modeled, and the density, viscosity, and the Rayleigh criterion have been estimated from the composition of silicate glasses in melt inclusions. These data strongly suggest intense convection in the residual magma chamber lasting for centuries. We have calculated possible fluid overpressure during the crystallization and degassing of the ongonitic melt in a closed magma chamber.
Calcium- and fluorine-rich aphyric and porphyritic rocks on the southwestern margin of the massif might have formed by the following mechanism. Local decompression in the magma chamber quenched an oxygen-containing calcium fluoride melt accumulated at the crystallization front, and these rocks altered during the interaction with fluids. When penetrating the marginal zone, a P-Q magmatic fluid which coexisted with the melt in the residual magma chamber cooled and changed its composition and properties. This caused the fluid to boil and segregate into immiscible phases: a vapor solution and a brine extremely rich in Cl, F, K, Cs, Mn, Fe, and Al. The fluoride and silicate liquids were immiscible; silicate melts had abnormal Cs and As contents; changes in the composition and properties of the magmatic fluids caused them to boil and produce brines. All this is evidence for complex fluid-magma interaction and heterogeneous ongonitic magma during the crystallization of the Ary-Bulak rocks. These processes were favored by the low viscosity and high mobility of the F- and water-rich ongonitic melt, intense melt convection in the residual magma chamber, and rising fluid pressure during its degassing.