Synchysite-(Y)–synchysite-(Ce) solid solutions from Markersbach, Erzgebirge, Germany: REE and Th mobility during high-T alteration of highly fractionated aluminous A-type granites

Summary

The Ca,REE-fluorocarbonate synchysite, which forms anhedral grains hosted in albite, was identified in the Markersbach pluton (Erzgebirge, Germany) composed of F-rich, highly fractionated and autometasomatically altered granites. These granites were emplaced in a post-collisional setting and are of aluminous A-type affinity. The grains were identified by electron microprobe analysis as intermediate members of the probably complete solid-solution series between synchysite-(Y) and synchysite-(Ce). The rareearth elements likely were liberated upon complete destruction of magmatic monazite and xenotime during interaction with a F-CO₂-Ca-bearing, late-magmatic fluid at relatively high temperatures. The geochemical patterns of different granite samples from Markersbach indicate that the REE and Th were not trapped immediately at the site of their release but mobilized over distances of at least decimeters. During transport of the elements in the fluid, presumably as fluoride (HREE) and chloride (LREE) complexes, no significant fractionation took place among the REE.