SYENITES AND MONZONITES IN THE SIYA-USHPINSKY INTERFLUVE OF GORNY ALTAI: ABSOLUTE AGE, GEOCHEMISTRY AND PETROLOGY

Abstract

The relevance of the research is caused by the need to clarify the absolute age and to study the features of petrology and geochemistry of moderately alkaline rocks – syenites and monzonites of the Siya-Ushpinsky interfluve in the northeastern Altai mountains (Russia) – in order to determine the formation patterns of the associated gold mineralization. The problem of the origin of alkaline igneous rocks is still interesting for scientists in many countries due to the fact that these are the rocks used to confirm the mantle-crustal interaction and to determine the nature of the evolution of the continental crust. Today, there is still no single position on their origin. The aims of the research are to clarify the absolute age of alkaline syenites of the Ulmen ore field and to study the petrological and geochemical features of syenitoids and monzonites of the Siya-Ushpinsky interfluve using a complex of petrological diagrams which help to reveal the physicochemical features of petrogenesis. The main objects of the study were syenitoids and monzonites of the Ulmen pyroxenite-syenite-monzonite-gabbro complex of the Middle Cambrian, located in the Siya-Ushpinsky interfluve. This territory is located within the anomalous tectonic block formed at the junction of the Gorno-Altai, Gorno-Shor and Salair structures. Among the minerals, the objects of study are zircons from alkaline syenites of the western section of the Ulmen ore field. Methods. The absolute age of the rocks was determined by the U-Pb secondary ion mass spectrometry method using a SHRIMP-II ion microprobe on zircon in the Laboratory of the Russian Geological Research Institute (Saint-Petersburg). Geochemical data were interpreted using a set of petrological diagrams. The tetrad effect of fractionation of REE (TE1.3) was calculated by the method of V. Irber. Results. Syenites in the Siya-Ushpinsky interfluve have the absolute age of 501 ±4 Ma, which corresponds to the end of the Middle Cambrian. Data on the composition of syenites and monzonites of the Bolshoi Lichim, Ushpa, Ulmeni, Upper Karakan, Salazan areas within the Siya-Ushpinsky interfluve were obtained. The authors established the sequence of formation of rocks with the separation of three phases: phase 1 – monzonite, phase 2 – alkaline syenite, phase 3 – syenite. Monzonites and syenites belong to the rocks of the intermediate composition of the moderately alkaline series. In addition, the composition of the monzonites and syenites studied by us corresponds to the shoshonite series of rocks. High ratios of Nb/Ta (from 19,0 to 31,4) and Zr/Hf (from 30,4 to 60,6) in syenitoids and monzonites of the Siya-Ushpinsky interfluve indicate the involvement of lower crust material in melting. Relatively low concentrations of Ni (from 2,3 to 4,1 ppm), Cr (from 1,0 to 3,2 ppm) and MgO (from 0,27 to 1,39 %) in syenites and monzonites indicate that they could not be the product of direct melting of mantle magma and the result of fractionation of mafic minerals from the evolved melt. Consequently, in the generation of syenites and monzonites of the Siya-Ushpinsky interfluve, the main role was played by melts formed as a result of partial melting of the lower thickened crust. In the experimental diagrams that allow the restoration of the source of melting for igneous rocks, the compositions of syenitoids and monzonites of the Siya-Ushpinsky interfluve fall or close to the melting fields of amphibolites and graywackes. In the studied rocks, the tetrad effect of fractionation of M-type REEs was revealed, which indicates their saturation with fluids, among which such volatile components as CO2 and H2O, played an important role. The latter were of key importance for gold transfer. The author's diagram Au–TE1,3 shows the trend of increase in gold concentrations with the growth of the tetrad effect of fractionation of M-type REEs. Spatially, the area of ​​distribution of syenites and monzonites is associated with numerous manifestations of gold, gold-bearing placers, as well as the Ulmen gold-copper-skarn deposit. In the latter, the richest mineralization tends to to the syenite dikes.

Keywords

• alkaline magmatism
• syenites
• monzonites
• petrology
• zircon
• tetrad effect of fractionation of rare earth elements
• gold
• gorny altai