Composition, structure and age of the Naryn massif of alkaline syenites: towards the diversity of Sangilen rare metal rocks
U.A. Moroz1, A.V. Nikiforov1, V.V. Yarmolyuk1,2, A.M. Kozlovsky1, A.A. Ivanova3, E.V. Salnikova3, V.M. Savatenkov3, U.V. Plotkina3, А.К. Khertek2, N.A. Polyakov1
1 Institute of Geology and Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, Russia
2 Tuvinian Institute for Exploration of Natural Resources of Siberian Branch of the Russian Academy of Sciences, Kyzyl, Russia
3 Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, St. Petersburg, Russia
Keywords: nepheline syenites, Central Asian Orogenic Belt, magma differentiation, isotopic composition, geochronology, Sangilen, rare-metal magmatism.
Abstract
Massifs of alkaline granitoids and nepheline syenites are quite widespread in the territory of Eastern Tuva. Rare metal deposits and ore occurrences, which were formed mainly at the end of the Carboniferous period and in the Permian, are often associated with them. The Naryn massif includes nepheline syenites of the main phase, contact zone and a complex of 3 types of foidite microsyenite dikes. The age of the massif determined by zircon from the rocks of the main phase (SIMS – 315±3 million years) and dikes of microsyenites-1 (CA ID TIMS – 318±1 million years) showed an earlier time of its formation in the middle of the Carboniferous. Mineralogical, geochemical and Nd-isotope characteristics of the massif rocks suggest that the diversity of its rocks is associated with the same type of differentiation of a single initial magma, but occurring at different hypsometric levels. Crystallization of biotite-pyroxene-amphibole nepheline syenites of the main phase and feldspar liebnerite syenites of the marginal zone occurred at the upper level. From the lower chamber with the differentiating melt, dykes of apo-amphibole nepheline microsyenites-1, pyroxene-biotite microsyenites-2 and biotite nepheline microsyenites-3 were successively intruded. The evolution of melts, both at the upper and lower levels, occurred due to the precipitation of mafic minerals (amphibole), apatite and feldspar. With an increase in the alkalinity of the melts, accumulation of rare elements (Zr, Nb, Ta, REE) occurred, which are included in their own minerals at the late stages of rock crystallization. These data indicate that the foidite series rocks, widespread in Eastern Tuva, may be potentially ore-bearing, and the middle of the Carboniferous period should be considered as a phase of alkaline magmatism of the East Sayan rare-metal metallogenic zone. The Nd isotopic composition of the Naryn massif rocks (Nd(T) +6.3 – +7.1) does not suggest significant participation of the continental crust in the source of melts, therefore, sublithospheric alkaline-basaltoid magmas probably served as parental rocks for all the Naryn massif rocks.
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