Melt and fluid inclusion study

Quartz phenocrysts of the dyke rocks contain melt (MI) and fluid inclusions (FI). The phenocrysts from Chechek and Akhmirovka belts contains similar FI and MI assemblages. At room temperature MI consist of aggregate of silicate daughter minerals. The studied inclusions often contain fluid segregation. The absence of glass in the MI indicate that their entrapment and subsequent cooling occur under hypabyssal conditions and was slow enough to provide complete crystallization of the entrapped melt. MI sizes vary significantly and reach 30-50 mkm. Large and medium size MIs are surrounded by decrepitation haloes of tiny fissures and fluid inclusions. Primary FIs are composed of low-salinity (1,5-8 wt% NaCleq) aqueous solution and gas bubble.

On heating under atmospheric pressure the majority of MIs were decrepitated due to a high internal fluid pressure. Homogenization experiments were conducted in autoclave under external water pressure 2 kbar by a quenching method. Stepwise heating within 660-625⁰С with quench at each step was applied in order to register behavior of inclusions on heating. Melt inclusions homogenize into silicate melt at temperatures ≤ 625⁰C. Low crystallization temperature is apparently caused by highly evolved melt compositions with high concentrations of fluorine and water, which decrease solidus temperature.

Chemical composition of homogeneous and re-melted MI was studied by EDS and EMP analyses. The quenched MI glasses contain moderate silica (68 – 70 wt%) and elevated F (up to 1.5 wt%) and phosphorus (P₂O₅ up to 0,4 wt%). Assuming that deficiency of analytical totals of EMP analyses is due to water content about 4-7 wt%. The majority of glasses are peraluminious (A/CNK =1,1-1,4) with sodium predominating over potassium. In general compositions of melt inclusions correspond to the bulk rocks. It is important to note that compositional differences in the bulk rock compositions is reflected by MI glasses (Fig. C,D). For instance MIs from phenocrysts of rare-metal enriched rocks of Chechek belt have the highest P and F content.

Fig. A - Chondrite normalized REE spectrum of rocks of Chechek and Akhmirovsky dyke belts, на которых видно выделение трах типов пород. B –Histogram of Li+Rb+Cs sum distribution in rocks. C, D – phosphorus and fluorine content in rocks (C) and melt inclusions (D).

1 – rocks of Akhmirovsky dyke belt, 2 – low rare-metal rocks of Chechek dyke belt, 3 – high rare-metal rocks of Chechek dyke belt. In the fig. D – composition of melt inclusions in the corresponding rocks.

Discussion

The data obtained in this study indicate that Chechek and Akhmirovka dyke belts in general have similar geochemistry and are built up of the ongonitic dykes. However on the basis of REE geochemistry and rare metal contents the dyke rocks are divided into three different groups. This indicates that in spite of similar geochemical character their magmatic sources have separate differentiation history. The difference between Chechek and Akhmirovka rock compositions may be related to differentiation of spatially divided magmatic sources. However within Chechek belt compositional variations can be explained by intrusions of magma from the sources located at different geological levels.

The study of melt inclusions revealed that quartz phenocrysts crystallized at relatively low temperatures (~625°C) and high fluid pressure. The crystallizing magma was saturated in water and fluid phase coexisted with the silicate melt. In the course of magma ascent some large MI decrepitated. This resulted in appearance of fissure haloes containing inclusions of magmatic aqueous fluid phase. The similarity of MI and bulk rock compositions, however, indicates that there was no significant melt crystallization after the MI entrapment before the dyke emplacements. Thus the formation of the dyke belts is related to relatively shallow magma sources.

Variations in compositions of rare-metal rich dyke belts indicate complex history of the final stages of rare metal magmatism in the North-Western part of the Greater Altay. The data obtained indicate that the formation of dykes and their source evolution take place under different conditions, fluid regime and probably within relatively long time period.

This work was supported by RF President grant for young scientists МК-1753.2012.5.

References:

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2. Maslov V.I., Kozlov M.S., Dovgal’ V.N., Distanova A.N. Complex of ongonites and lithium-fluorine granites in south-west Altay // Petrology. 1994. Vol. 2, № 3, p.331-336.

3. Pushko E.P., Shamanaeva V.V., Stapanov A.V., Shipovalov Yu.V. About finding of Li-F granite-porfires (analogs of ongonites) in Kalba-Narym ore belt // Geology, geochemistry and mineralogy of rare element deposits. 1978. Vol.5, p. 3-19.

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