NIOBIAN RUTILE AND ITS ASSOCIATIONS AT JOLOTCA, DITRAU ALKALINE INTRUSIVE MASSIF, EAST CARPATHIANS, ROMANIA

The Nb-rutile at Jolotca, situated in Ditrau alkaline intrusive complex occurs as intergrowths with ilmenite, Mn-ilmenite, Fe-pyrophanite and has ferrocolumbite, manganocolumbite, aeshynite-(Ce), aeshynite-(Nd), fergusonite-(Y), euxenite-(Y) and polycrase-(Y) exsolutions. The textural relations in this association show the replacement of niobian rutile by ilmenite and Mn ilmenite. Niobian rutile is the oldest mineral. Ilmenite and Mn-ilmenite occur as lamellar exsolutions in niobian rutile and as veins, and separately, in grains as solid solution with Fe-pyrophanite. The range of Nb2O5 content in Nb rutile varies generally from 2 to 15% wt. Sometimes, the values of Nb2O5 (up to 37.5% wt) are higher than any previously recorded for rutile from alkaline suites, pegmatites and carbonatites, having a chemical composition similar to that of old name „ilmenorutile”. Because of such a big difference in chemical composition, and the different kind of appearances of the two rutiles, they can be separated into two Nb rutile generations. The first niobian rutile (niobian rutile I) formed on old rutile, has low Nb2O5 (10-15wt%), and oscillatory composition. Its composition is characteristically close to stoichiometric TiO2. The Nb-rutile II, less widespread, has no zonations, has a smooth surface and it is Nb-richer than early Nb rutile I. Some low Cr2O3 content of Jolotca Nb-rutile and low Ta concentrations indicate a relatively high-pressure regime of its genesis (Tollo & Haggerty). The occurrence of niobian rutiles could be linked with a carbonatite (?)/alkaline phase of the alkaline intrusive Ditrău complex. The presence of Cr2O3 in its composition suggests a mantle origin. It is difficult to establish the possible precursors of such ilmenite-rutile intergrowths and their Nb, REE(Y) mineral suite exsolutions. The mineralizing process was multi-staged which was supported by textural relationships, where there are replacements of earlier rutile, by Nb rutile I, then the appearance of richer Nb rutile II. The last stage could be the appearences of Mn-columbite/Fe-columbite, yttrocolumbite-(Y), aeshynite-(Ce), aeshynite-(Nd), polycrase-(Y) and euxenite-(Y) as exsolutions in niobian rutile/ilmenite intergrowth, from a common solid solution. The original precursor product from the upper mantle was probably a mixture linked by a high pressure crystalographically sheard rutile structure (Tollo & Haggerty, 1987). When the temperature and pressure fall, these compounds from the original mixture become unstable and their exsolutions occur.