COMPORTAMENTO DOS ELEMENTOS TERRAS RARAS PESADAS EM ZIRCÃO, XENOTIMA E TORITA DE GRANITOS E GREISENS DA SUBPROVÍNCIA ESTANÍFERA PARANÃ, GOIÁS

Abstract

Two groups of granitic rocks are present in the Paranã Tin Subprovince, gl and g2. Rocks of gl group are older and display an alkalic affinity. g2 granites are metaluminous to peraluminous. Both groups are formed by A-type granites with anomalous REE concentrations, sometimes IO 3 greater than the observed in chondrites. In the studied granitic massifs, the raain HREE-bearing minerais are zircon, xenotime and thorite. Zircons from gl and g2 granites show an increase in U, Th, Y and REE during the magmatic evolution. Distinct trends of enrichment in yttrium and REE are observed in zircons from both groups, with a lower REE/Y ratio in gl zircons. During greisenization zircon is reequilibrated, with enrichment in REE. The metamictization degree is stronger in zircons from later granite fácies and greisenized rocks. Xenotime is a clearly hydrothermal mineral only found in g1 greisens. Electron-microprobe data indicate the existence of two groups of xenotime with different range of HREE concentration, 8-13% and 16-18%. Thorite is an accessory mineral in granites (group I) and greisens (group II). Group I have lower Y 2 O 3 (1-5%) and REE 2 O 3 (2-6%) contents than group II. Group II thorite is probably a hydrothermally reequilibrated magmatic mineral with an enrichment in yttrium (4-7% Y 2 O 3 ) and REE (7-13% REE 2 O 3 ). Solid solution between zircon and xenotime was absent in analyzed samples, and the entrance of trivalent cations in zircon is allowed by the substitution: (REE,Y) 3+ + (SiO 3 OH) 3- ⇔ Zr 4+ + (SiO 4 ) 4- . Zircon-thorite and zircon-uraninite solid solutions are present in zircons with higher U and Th contents. Xenotime-zircon and xenotime-thorite solid solutions are absent in studied xenotimes, the only identified substitution being Y 3+ <=> REE 3+ . High Zr, U, P, Y and HREE contents in thorite indicate the presence of thorite-zircon, thorite-uraninite and thorite-xenotime solid solutions.