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Zirconium minerals frome carbonatites of the Tiksheozero massive (North Karelia, Russia)

Frantz N.A.*, Skublov S.G.**, Rodionov N.V.***, Lokhov K.I.*

*Saint -Petersburg State University, St.Petersburg, Russia, nfrantz@mail.ru; ** Institute of Precambrian Geology and Geochronology, St.Petersburg, Russia; ***Centre of Isotopic Research, Russian Geological Research Institute (VSEGEI), St. Petersburg, Russia.


               The two principal Zr minerals, zircon and baddeleite, occurs in carbonatites of the Tiksheozero massive. There are four types of zircons based on morphological, petrographical and geochemical feature. 1. "Magmatic" zircon (I type) forms resorbed grains, in optical cathodoluminescence images (OM-CL) has a mosaic structure. It is most enriched in REE, Y and depleted in Nb, compared to other types. 2. "Magmatic" zircon (II  type) is  bright in  OM-CL images. This type has lower Nb content and Th/U ratio. REE has a fractional distribution,  positive Ce anomaly  and  correspond to magmatic zircons from carbonatites of the world. 3. Zoning zircon is the most common type. It contains apatite, calcite and baddeleite inclusions.  There are zoning structure in OM-CL images, black metamictic core and bright white rim. Some content of  Ca, P, Nb and U is contained in this zircon.  Y, Th, U and REE content is decreased from core to rim, Hf content is increased. 4. "Metasomatic" zircon occur as the overgrowth of baddeleite, black in OM-CL images. Chemistry of this zircon is contrasting compare to other type. P, Ca, Ti, Sr, Y, Nb, U and REE contents show high value of an 2-3 order. REE has a low fractional distribution and  correspond to metasomatic zircons. Baddeleite from tiksheozero carbonatites occur as a unique elongate prismatic transparent brown crystal or opaque black crystal, forms intergrowth grains with zircon or micro inclusions in zoning zircon. A notable feature of baddeleite is trace elements, Hf, U and REE, some Ca content and high  Nb value. REE has a fractional distribution with positive Ce anomaly.

         Concordant baddeleyite U-Pb data correspond to an age of 199711 Ma. (UThPb SIMS  SHRIMP-II, 20 analysis of 11 baddeleyite grains were used.) Isotopic data on the subconcordant zoning zircons  core give exhibit 199213 Ma crystallization age . Isotopic data on zoning zircons rims and metasomatic zircon (zircons overgrows on baddeleyite) show high discordant U/Pb ratios. Clearly geochemical characteristics of zircons, indicate their magmatic-metasomatic origin,  leading to a disturbance of U-Pb isotopic system.