No abstract

Based on a comparative study of geochemistry of metavolcanics and metasediments of two large terranes, Baikal-Muya and Khamar-Daban-Ol’khon, as well as of the Baikal-Patom passive margin and Olokit accretionary wedge, we have recognized volcanosedimentary series accumulated in the settings of island arcs of different maturities and fragments of volcanosedimentary complexes of back-arc and fore-arc basins. Metabasalts of the Medvezhy and Tyya Formations in the basement of passive-margin sequence and the Olokit Group are similar in geochemistry to plateau basalts and mark the beginning of rifting on the platform periphery. The abundance of metavolcanics and turbidites in the Olokit Group permits this structure to be considered an accretionary wedge of the Baikal-Muya island arc. According to the metavolcanics composition, the Baikal-Muya terrane formed in the environment of oceanic ensimatic island arcs and back-arc and inter-arc basins with the minimum amounts of sediments and contains ophiolite slices. The geochemistry of metavolcanics and metasediments of the Ol’khon, Talanchan, and Slyudyanka complexes evidences their formation in the environment of ensialic back-arc sediment-rich basin (Slyudyanka, Ol’khon, and Svyatoi Nos series), mature island arc (Anga-Talanchan paleoarc, Anga and Talanchan Groups), and fore-arc basin (Khangarul’ Group). According to chemistry and evolution history, all these complexes must be assigned to the Khamar-Daban-Ol’khon terrane.

No abstract

Oxyvanite has been identified as an accessory mineral in Cr-V bearing quartz-diopside meta morphic rocks of the Slyudyanka Complex in the southern Baikal region, Russia. The new mineral was named after constituents of its ideal formula (oxygen and vanadium). Quartz, Cr-V bearing tremolite and micas, calcite, clinopyroxenes of the diopside-kosmochlor-natalyite series, Cr bearing goldmanite, eskolaite-karelianite dravite-vanadiumdravite, V bearing titanite, ilmenite, and rutile, berdesinskiite, schreyerite, plagioclase, scapolite, barite, zircon, and unnamed U-Ti-V-Cr phases are associated minerals. Oxyvanite occurs as anhedral grains up to 0.1-0.15 mm in size, without visible cleavage and parting. The new mineral is brittle, with conchoidal fracture. Observed by the naked eye, the mineral is black, with black streak and resinous luster. The microhardness (VHN) is 1064-1266 kg/mm 2 (load 30 g), and the mean value is 1180 kg/mm 2 . The Mohs hardness is about 7.0-7.5. The calculated density is 4.66 (2) g/cm 3 . The color of oxyvanite is pale cream in reflected light, without internal reflections. The measured reflectance in air is as follows (λ, nm-R, %): 440.7. Oxyvanite is monoclinic, space group C2/c; the unit cell dimensions are a = 10.03(2), b = 5.050(1), c = 7.000(1) Å, β = 111.14(1)°, V = 330.76(5) Å 3 , Z = 4. The strongest reflections in the X ray powder pattern [d, Å, (I in 5 number scale)(hkl)] are 3.. The chemical composition (electron microprobe, average of six point analyses, wt %): 14.04 TiO 2 , 73.13 V 2 O 3 (53.97 V 2 O 3calc , 21.25 VO 2calc ), 10.76 Cr 2 O 3 , 0.04 Fe 2 O 3 , 0.01 Al 2 O 3 , 0.02 MgO, total is 100.03. The empirical formula is ( Cr 0.30 ) 2.0 ( Ti 0.41 ) 1.0 O 5 . Oxyvanite is the end member of the oxyvanite-berdesinskiite series with homovalent isomorphic substitution of V 4+ for Ti. The type material has been deposited at the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.

Chromium and vanadium-bearing metamorphic derivatives of siliceous-carbonate sediments in the Sludyanka granulite-facies complex, southern Baikal, contain high-Cr clinopyroxenes that belong to the diopside–kosmochlor–natalyite join (CaMgSi2O6–NaCrSi2O6–NaVSi2O6). The ternary join includes isomorphic binary joins of which the diopside–kosmochlor join with 94% kosmochlor (Kos) end-member has been studied in detail and found to be virtually complete. As indicated by signature of diopside–eskolaite reactions in the minerals, kosmochlor and high-Cr members of the join formed at the expense of metamorphic eskolaite as a result of Na input after progressive metamorphism. There is no miscibility gap along the diopside–kosmochlor join; the incompleteness of the kosmochlor formation reaction and the coexistence of its compositionally different members are rather due to kinetic factors associated with low mobility of Cr. Thus, the mechanism of Cr incorporation in clinopyroxenes does not depend on PT-conditions of metamorphism.