Na-Enriched Titanite in Apoquartzite Fenites From Azov Area (Ukraine)

Abstract

The Na-enriched titanite (up to 3.53 % Na2O) was found in the apoquartzite fenites of the Tunikova gully (Eastern part of the Azov megablock, Ukrainian Shield). It is observed in association with alkaline amphiboles, aegirine, low-Al and high-F phlogopites, rutile, bastnaesite-(Ce), monazite-(Ce), albite, and Ba-enriched potassium feldspar. This mineral association, as well as the agpaitic specificity of apoquartzite fenites (peralkaline index >2), is the result of the influence of undiscovered intrusion of alkaline igneous rocks (silicate or carbonatite) that were the source of alkaline fluids. In general, the Na-enriched titanite contains (wt. %): Y2O3 (up to 3.48), Ce2O3 (up to 0.98), Nd2O3 (up to 1.13), increased F (up to 1.94), but moderate Nb2O5 (0.2-2.8) and negligible content of FeO and MnO, in the almost complete absence of Al2O3. It is assumed that the Na-enriched titanite substitutes the primary rutile of quartzites and reflect high concentration of Na2O, CaO, REE, Y, F introduced by alkaline fluids. The Na → Ca isomorphism was accompanied by parallel substitution Y, REE → Ca, and partially O → F. Other schemes of substitution intrinsic for titanites of alkaline rocks, such as Na+Nb → Ca+Ti or Nb+Fe(Al) → 2Ti, are practically not manifested that obviously due to the low Nb content in the primary rutile and introduced alkaline fluids, as well as the depletion of quartzite in alumina and iron.