Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by BARKOV, A. Y. Articles by MEN'SHIKOV, Y. P. Search for Related Content GeoRef GeoRef Citation

Sr-Na-REE titanates of the crichtonite group from a fenitized megaxenolith, Khibina alkaline complex, Kola Peninsula, Russia

¹ Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada
² Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada
³ Geological Institute, Kola Science Centre, Russian Academy of Sciences, 14 Fersman Street, 184200 Apatity, Russia

^* E-mail: abarkov{at}eps.mcgill.ca, mfleet{at}uwo.ca, bobm{at}eps.mcgill.ca

^** Present address: Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada.

An extensive, nearly continuous and hitherto unreported solid-solution series is observed in Sr-Na-REE titanates in a fenitized megaxenolith, Khibina alkaline complex, Kola Peninsula, Russia. These titanate minerals, related to crichtonite, landauite and davidite-(Ce,La), may represent potentially new members of the crichtonite group. They display compositional zoning, and are associated with diverse oxide minerals, albite and alkali feldspar. Our compositions (EMP) suggest that the "large cations" (Sr, Na, K, and the rare-earth elements, REE), along with minor Ca, occupy fully the A (M₀) site in their structure; in contrast, Ca enters dominantly the B (M₁) site, probably via a coupled substitution of the type [(Ca²⁺+Ti⁴⁺) = (Zr⁴⁺+Fe²⁺)]. The extensive Na⁺-for-Sr²⁺ substitution observed at the A site is coupled with a Ti⁴⁺-for-Fe²⁺ substitution at the C (M₃-M₄-M₅) site. The tetrahedral T (M₂) site is dominated by Fe. Among the REE, only the light REE are present in substantial amounts (up to 7.37 wt.% REE₂O₃); they are positively intercorrelated, indicating an ordered distribution at the A site. The incorporation of the REE³⁺, which probably replace Sr²⁺ and K⁺, is controlled by a corresponding decrease in Zr⁴⁺ (and by relative increase in divalent Mn) at the B site in order to maintain charge balance. The observed presence of up to 1.87 wt.% Cr₂O₃ in the Sr-Na-REE titanate minerals at Khibina indicates that rocks of mafic affinity were the protolith for the mineralized megaxenolith. The high Na contents of these minerals are clearly related to the geochemical environment (i.e., Na-metasomatism). The contrasting association of Cr and Nb (up to 1.06 wt.% Nb₂O₅) in these minerals undoubtedly involves derivation from two different sources. We suggest that the Ti-(Fe)-Nb-REE oxide mineralization formed in the megaxenolith as a result of interaction of a pre-existing mafic rock(s), probably Proterozoic rocks of the Imandra-Varzuga Supergroup, with metasomatizing oxidizing fluids of alkaline affinity.

Key-words: crichtonite, landauite, davidite-(Ce), titanates, oxide minerals, solid solutions, fenites, xenoliths, Ti-(Fe)-Nb-REE mineralization, alkaline rocks, Khibina complex, Kola Peninsula, Russia.