GEOCHEMISTRY OF HIGH-PHOSPHORUS ZIRCON FROM THE UPPER RIPHEAN METASANDSTONES OF THE SOUTHERN TIMAN

Abstract

A detailed mineralogical and geochemical study (using the EPMA and SIMS methods) of zircon from metasandstones of the Dzhezhim Formation in the South Timan made it possible to establish an anomalously high content of phosphorus in it (up to 10.27 wt % P2O5 according to EPMA data), which correlates with an increased content of other non-formula admixture elements, the main of which are Y, REE, Ca, Fe, Al, Ti, Sr, Ba, Th, U. Of particular note is the significant amount of volatile components in zircon (up to 0.49 wt % water and up to 0.26 wt.% fluorine, determined by the method SIMS). The total content of non-formula admixture elements can exceed 20 wt %, which is a characteristic feature of the composition of zircon exposed to fluid or formed as a result of hydrothermal-metasomatic processes. The main mechanism for the incorporation of admixture elements into the composition of zircon was xenotime-type heterovalent isomorphism, when the presence of quinquevalent phosphorus is compensated by the participation of trivalent Y and REE. Of subordinate importance was the isomorphism scheme, which ensures the entry of hydrogen (water). The areas of zircon enriched in phosphorus and other adulterants are confined to the edge of the grains, or to systems of scissures and fluid-permeable areas. Granitoids served as a possible source of zircon, and the process of transformation of its composition (recrystallization and enrichment of local areas, less often whole grains) probably occurred already in the sandstone sequence, during metamorphism and/or hydrothermal process.