Abstract:
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 Kbibina 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. © 2006 E. Schweizerbart'sche Verlagsbuchhandlung.
