COMPOSITION, STRUCTURE, AND AGE OF THE NARYN ALKALINE SYENITE MASSIF: INSIGHTS INTO THE DIVERSITY OF RARE-METAL ROCKS IN SANGILEN
U.A. Moroz1, A.V. Nikiforov1, V.V. Yarmolyuk1,2, A.M. Kozlovsky1, A.A. Ivanova3, E.B. Salnikova3, V.M. Savatenkov3, Yu.V. Plotkina3, A.K. Khertek2, N.A. Polyakov1
1Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, Moscow, Russia
2Tuvinian Institute for Exploration of Natural Resources, Siberian Branch of the Russian Academy of Sciences, Kyzyl, ul. Internatsional’naya 117a, Russia
3Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, St. Petersburg, Russia
Keywords: Nepheline syenite, magma differentiation, isotopic composition, geochronology, rare-metal magmatism, Central Asian orogenic belt, Sangilen
Abstract
Massifs of alkaline granitoids and nepheline syenite are widespread in Eastern Tuva and are frequently associated with rare-metal deposits and occurrences, which formed mainly during the Late Carboniferous and Permian. The Naryn massif comprises nepheline syenite of the main phase, rocks of the contact zone, and a complex of three types of foidite microsyenite dikes. Zircon U-Pb geochronology yields ages of (SIMS - 315 ± 3 Ma) for the main phase and (CA-ID-TIMS - 318 ± 1 Ma) for the microsyenite-1 dikes, indicating that the massif was emplaced during the Middle Carboniferous. The mineralogical, geochemical, and Nd isotopic characteristics of the massif rocks suggest that their diversity results from the differentiation of a single parental magma, a process that occurred at different hypsometric levels. Crystallization of biotite-pyroxene-amphibole nepheline syenite of the main phase and feldspar liebnerite syenite of the marginal zone occurred at the upper level. Dikes of apo-amphibole nepheline microsyenite-1, pyroxene-biotite microsyenite-2, and biotite nepheline microsyenite-3 were emplaced sequentially from a deeper chamber containing the differentiating melt. The evolution of the melts at both levels was driven by the fractionation of mafic minerals (amphibole), apatite, and feldspar. As melt alkalinity increased, rare elements such as Zr, Nb, Ta, and REE accumulated, eventually forming their own mineral phases during the late stages of crystallization. These findings indicate that the widespread foidite-series rocks of Eastern Tuva may be potentially ore-bearing, and the Middle Carboniferous should be considered as a distinct phase of alkaline magmatism within the East Sayan rare-metal metallogenic zone. The Nd isotopic composition of the Naryn massif rocks (εNd( T ) = 6.3-7.1) precludes significant involvement of continental crust in their source. Therefore, sublithospheric alkaline basaltoid magmas are interpreted to be the parental magmas for all rocks of the Naryn massif.
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