THE CHEMICAL-COMPOSITION OF REE-Y-TH-U-RICH ACCESSORY MINERALS IN PERALUMINOUS GRANITES OF THE ERZGEBIRGE-FICHTELGEBIRGE REGION, GERMANY, PART-I - THE MONAZITE-(CE)-BRABANTITE SOLID-SOLUTION SERIES

Peraluminous granites of the Erzgebirge-Fichtelgebirge, Germany, are h osts of various members of the monazite group of minerals that display an unprecedented compositional diversity. The Eibenstock S-type grani te constitutes the third reported occurrence worldwide of brabantite a nd the first occurrence of this mineral in a granite. Many new occurre nces of cheralite-(Ce), as well as a monazite-group mineral intermedia te between monazite-(Ce) and huttonite for which the term huttonitic m onazite is proposed, were discovered. Even ''common'' monazite-(Ce) ma y show extreme ranges of actinide and lanthanide element concentration s. The granites that host brabantite and cheralite-(Ce) are highly dif ferentiated, strongly peraluminous, low-temperature residual melts of S-type affinity, which are rich in fluorine and other volatile constit uents but depleted in thorium and the light rare-earth elements. Such highly evolved, volatile-rich compositions resemble rare-element pegma tites and appear favorable for the precipitation of cheralite-(Ce) and brabantite, but not of monazite with large amounts of huttonitic subs titution. Instead, these minerals occur preferentially in F-poor bioti te and F-rich Li-mica granites of A-type affinity. Irrespective of the level of uranium in silicate melts, which may exceed that of thorium, the substitution of uranium in monazite remains limited. The composit ional data reported here are consistent with complete miscibility in t he monazite-(Ce)-brabantite solid solution series under magmatic condi tions. These granites contain monazites that span almost the entire co mpositional range reported for monazite-group minerals worldwide, and therefore granites appear to be ideal rocks in which to study the crys tal chemistry of this mineral group in general.