Possibility of relationship between silicate rocks and carbonatites in the Oshurkovski massif (Western Transbaikalia)

 

Ripp G.S., Doroshkevich A.G., Lastochkin E.I., Izbrodin I.A.

Geological Institute SB RAS, Ulan-Ude

 

ripp@gin.bscnet.ru

 

The presence of carbonatites which go with Late Mesozoic interplate rift (Ripp et al., 2000), is one of the South-West Transbaikalia features. Carbonatitic veins displayed in the area of Oshurkovski massif, also concern to them. The relation problem of silica rocks and carbonatites have not been studied because of precursor’s attribution this massif to dioritic, monzo-syenitic series supposed impossibility of generation them the carbonatitic melt. And just after obtainment similar ages of the rocks, the assessment question of them comagmatic features have been risen. The importance of solving this problem more because there are some massifs in the region petrochemically similar to Oshurkovski, which could produce the carbonatitic melt.

The Oshurkovski massif is situated in 20 km from Ulan-Ude, occupies a square some more 12 km² and is strict discordant to host gneiss and gneissoid granites. High concentration of apatite determines its industrial significance. The age of gabbroid formation (SHRIMP II, zircon) is 125.4±2 Ma. It is the only pluton in South-West Transbaikalia for the present, for which Late Mesozoic aging have been made. Other examples of Late Mesozoic basites are just presented by dykes.

Because of middle plagioclase presence some researches attributed them to diorites (Andreev et al., 1972; Kuznetsov, 1980) and higher alkality rocks to gabbro-monzonite-dioritic (Polyakov et al., 1980) and monzodiorit-syenitic (Litvinovsky et al., 2002) series. By other researches the rocks were attributed to gabbro, gabbro-dioritic (Smirnov, 1971), gabbro-ultrabasitic (Kuznetsova ey al., 1995), ultrabasitic alkaline (Yatsenko, 1972) series.

According to petrochemical composition the majority part of the pluton attributes to basites (Petrographical…, 2009), syenites is presented in minor. Composition plots of the first correspond to alkaline gabbro on the TAS diagram. Alkaline-rich rocks are concentrated in alkaline picrites and picrobasalts field, and leucocratic rocks in phonotephrites field. All of them are characterized by high potassium which is explained by presence of biotite, potassium and potassium-sodic feldspars.

Melano-, mezo- and leucocratic varieties are separated among gabbriods, which have sharp contacts between them and gradual transitions. These are low silicate with high titanium and alkality rocks (42-46 wt. % SiO₂, Na₂O+K₂O – 5-8 wt. %). There is nepheline in this rocks and high concentration of silica made for baddeleyit appearance at the starting crystallization stage. The rocks consist of varying amounts of amphibole, biotite, clinopyroxene, apatite, oligoclase (№ 16-18), potassium and potassium-sodic feldspars. Titanite, ilmenite and titanium-rich (up to 17 wt. % TiO₂) magnetite are presented regularly.

Leading hand among major minerals (up to 40-45 wt. %), belongs to hastingsite and biotite. Amphibole belongs to aluminous type, and contains 3.5-4.1 wt. % alkalis, 2-4 wt. % TiO₂. Amounts of titanium reach values (6.6 wt. %) typical to kaersutite.

Amounts of pyroxene (diopside-salite) which contain aegire (10-15%) minal, usually do not exceed 5-7%. Mica (flogopite-annite series) is characterized by high titanium (up to 4-6 wt. % TiO₂) and magnesium (up to 1.8 f.c. Mg). Apatite forms poikilitic quite often resorbed inclusions in mafic minerals and more coarse-grained aggregates in interstitials. Apart from this submonomineral assemblages and vein like bodies, which may be, have liquation formation. Apatite contains up to 1.6 wt. % SrO and 0.8-1.1 wt. % sulphur.

Calcite is presented in gabbroids, forms inclusions in rock-forming minerals and interstitials. It, as in carbonatites, contains higher values of strontium (in average 0.85 wt. % SrO) and up to 0.6 wt. % of magnesium and iron.

Syenites are minor presented in the massif. According to (Litvinovsky et al., 1998) they were formed by fractional crystallization. By other researches the syenites are related to metasomatic (Kuznetsov, 1980) and assimilatic (Smirnov, 1971) processes.

Carbonatites formed low-width (up to 0.6-1.0 m) different oriented veins, traced on strike to 100 m. These are fine-, medium-grained calcitic rocks with striation, conformed to vein orientation. Their contacts with host rocks are sharp and often tectonized. The bodies of carbonatites also consist of splinters in fine-grained detritus floury matrix. Selvages are usually bordered by phlogopite and enriched by magnetite, sometime by alkali feldspar, titanite. The rocks contain apatite, high-strontium (up to 10-14 wt. % SrO) barite, accessory monazite, zircon, allanite. Their age (SHRIMP II, zircon) is 126±0.85 Ma. Isotopic compositions of oxygen (average ‰ δ¹⁸ SMOW) in magnetite (0.7), phlogopite (4.9), apatite (4.6), titanite (3.6), amphibole (5.2), and sulphur in sulphates (6.4 ‰ δ³⁴ CTD) are similar to other compositions of magmatic carbonatites, and values of δ¹³C-δ¹⁸O in calcites plots in field, which typical to mantle carbonatites. Mineral formation temperatures, estimated by isotopic-oxygen thermometers, are 932-946°C for a couple magnetite-phlogopite, and 625°C for a couple magnetite-calcite.

More late silicification and calcite recrystallization are founded in some veins. Magnetite is martitizated here, new formed allanite was appeared and SrO values were decreased in some calcites to less than 0.1 wt. % (1.5-2 et. % originally). Isotopic shift to weighting of oxygen (up to 16.4 ‰ δ¹⁸O SMOW) and reduction of weight of carbon (up to -13.1 ‰ δ¹³C PDB) is founded in hydrothermal recrystallized carbonates.

Postmagmatic biotitization, amphibolization are displayed in the massif. Greenstone alteration, zeolitization, carbonatization, silicification are fragmentary founded. In brecciation zones assemblages of calcite with chlorite and quartz present in gabbroids. This calcite enriched by light isotope (-2.8/-7.3 δ¹⁸O SMOW), indicative of vadose water contribution to their formation. To some researchers (Smirnov, 1971; Kuznetsov, 1980) mind postmagmatic processes were attended by redistribution of apatite.

Similar ages, spatial association, isotopic strontium ratio in non-rubidium minerals (apatite, pyroxene, calcite, barite) indicate integrated comagmatic of silica rocks and carbonatites. Ratio values ⁸⁷Sr/⁸⁶Sr denote high potassium of parent center, and features of calcite association indicate syngenesis their formation with silica rock-forming minerals. Isotopic composition of oxygen in minerals of all studied rocks more accentuates their genetic relationship. Carbonatites inherited from gabbroids high values of strontium, barium, phosphorus and sulphur. Relationship of these rocks also expressed in similarity of normalize curves configuration, value of La/Yb.

Defectiveness of carbonatite formation was made for formation in gabbroids on the early stage of great values calcium bearing minerals (apatite, pyroxene, hastingsite and then plagioclase). While high fluid concentration of the melt, which due to presence of magmatic apatite and water-bearing minerals (amphibole and phlogopite), determines lower temperatures of its crystallization and carbonatite formation ability in the issue of crystallization differentiation, escaped liquation on silica and carbon liquids. Evidence of this can be a character of calcite association in gabbros, similar to assemblages formed during crystallization of the melt. The grains are included in hastingsite, plagioclase, biotite, and form clusters in interstitials without any reactionary interaction.

The presence of negative Eu anomaly in carbonatites (Eu/Eu*=0.6-0.8), and about 1 in gabbroids, also evidences probability of carbonatite formation during crystallization differentiation. One knows these values do not typical for liquitic liquids.

 

This study was financially supported by RFBR grant 08-05-98028, Integration project SB RAS 14.2, NSch-3848.2010.5 and МK-2873.2010.5.

 

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