Rare-metal alaskites of the Mankhambo massif (Northern Urals): U-Pb (SIMS) data and isotope-geochemical characteristics

The study focuses on rare-metal alaskites discovered within the Mankhambo granitoid massif (Northern Urals). Petrographic-mineralogical and petro-geochemical analyses were conducted. The crystallization age was determined using U-Pb dating (SIMS) on single zircons; the Hf isotopic composition was analysed by LA ICP MS. The rocks are highly differentiated alaskites (Zr/Hf<25), formed in an intraplate geodynamic setting. They exhibit elevated concentrations of Ga, Nb, Ta, Th, HREE along with low Zr/Hf, Nb/Ta, Y/Ho ratios. Singenetic rare-metal mineralization (fergusonite, aeschynite , Nb-Ta-bearing titanite, brannerite) was identified. Three zircon types were identified: xenogenic (527–512 Ma and 1479 Ma); ‘reticulate’ and ‘porous’ (337.6±7.3 Ma) crystals. The latter show low Th/U ratios, La anomalies in REE patterns indicating the Earth’s fluid activity. The composition points of zircons are located outside the magmatic/hydrothermal fields, which is related either to the disturbance of the U-Pb system or to the specificity of rare-metal melts. The crystallization temperature of rocks ranges from 790 to 677 °C. Positive εHf(t) values (from +0.38 to +6.8) indicate a juvenile mantle source. The TDM2 model age (1.11–0.82 Ga) means the involvement of ancient crustal material in melt generation. The Mankhambo alaskites represent a late-stage pulse of granitic melt derived from a deep juvenile source, which is genetically distinct from the main magmatic phase of the massif. Their crystallization from highly fractionated melts in upper-crustal chambers resulted in unique mineralogical and geochemical features. By the estimated age, the alaskites belong to the Ponino alkaline hypabyssal complex C3-Ppn.