Chemical composition,volatile components,and trace elements in the magmatic melt of the Kurama mining district,Middle Tien Shan: Evidence from the investigation of inclusions in quartz |
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| Authors: | V B Naumov V A Kovalenker V L Rusinov |
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| Institution: | 1.Vernadsky Institute of Geochemistry and Analytical Chemistry,Russian Academy of Sciences,Moscow,Russia;2.Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM),Russian Academy of Sciences,Moscow,Russia |
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| Abstract: | Melt and fluid inclusions were investigated in six quartz phenocryst samples from the igneous rocks of the extrusive (ignimbrites
and rhyolites) and subvolcanic (granite porphyries) facies of the Lashkerek Depression in the Kurama mining district, Middle
Tien Shan. The method of inclusion homogenization was used, and glasses from more than 40 inclusions were analyzed on electron
and ion microprobes. The chemical characteristics of these inclusions are typical of silicic magmatic melts. The average composition
is the following (wt %): 72.4 SiO2, 0.06 TiO2, 13.3 Al2O3, 0.95 FeO, 0.03 MnO, 0.01 MgO, 0.46 CaO, 3.33 Na2O, 5.16K2O, 0.32 F, and 0.21 Cl. Potassium strongly prevails over sodium in all of the inclusions (K2O/Na2O averages 1.60). The average total of components in melt inclusions from five samples is 95.3 wt %, which indicates a possible
average water content in the melt of no less than 3–4 wt %. Water contents of 2.0 wt % and 6.6 wt % were determined in melt
inclusions from two samples using an ion microprobe. The analyses of ore elements in the melt inclusions revealed high contents
of Sn (up to 970 ppm), Th (19–62 ppm, 47 ppm on average), and U (9–26 ppm, 18 ppm on average), but very low Eu contents (0.01
ppm). Melt inclusions of two different compositions were detected in quartz from a granite porphyry sample: silicate and chloride,
the latter being more abundant. In addition to Na and K chlorides, the salt inclusions usually contain one or several anisotropic
crystals and an opaque phase. The homogenization temperatures of the salt inclusions are rather high, from 680 to 820°C. In
addition to silicate inclusions with homogenization temperatures of 820–850°C, a primary fluid inclusion of aqueous solution
with a concentration of 3.7 wt % NaCl eq. and a very high density of 0.93 g/cm3 was found in quartz from the ignimbrite. High fluid pressure values of 6.5–8.3 kbar were calculated for the temperature of
quartz formation. These estimates are comparable with values obtained by us previously for other regions of the world: 2.6–4.3
kbar for Italy, 3.7 kbar for Mongolia, 3.3–8.7 kbar for central Slovakia, and 3.3–9.6 kbar for eastern Slovakia. Unusual melt
inclusions were investigated in quartz from another ignimbrite sample. In addition to a gas phase and transparent glass, they
contain spherical Feoxide globules (81.2 wt % FeO) with high content of SiO2 (9.9 wt %). The globules were dissolved in the silicate melt within a narrow temperature range of 1050–1100°C, and the complete
homogenization of the inclusions was observed at temperatures of 1140°C or higher. The combined analysis of the results of
the investigation of these inclusions allowed us to conclude that immiscible liquids were formed in the high-temperature silicic
magma with the separation of iron oxide-dominated droplets. |
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