CRYSTAL-CHEMISTRY OF MONAZITE AND XENOTIME FROM SAXOTHURINGIAN-MOLDANUBIAN METAPELITES, NE BAVARIA, GERMANY

Monazite and xenotime from a regional metamorphic area were analyzed b y electron microprobe technique. The rocks are metapelites-psammites w ith a major and trace element composition characteristic for volcanic are sediments, Textural observations on grain size and shape indicate a continuous growth of both monazite and xenotime from greenschist to granulite facies temperatures. The whole rock REE distribution pattern for garnet-poor assemblages is dominated by these accessory phases. M onazite incorporates the LREE La-Sm as major elements and the HREE in minor or trace amounts. An increase of YPO4 solid solution with increa sing metamorphic temperatures was found. The brabanite (CaTh(PO4)(2)) component is another important constituent. Xenotime is essentially a solid solution between YPO4 and HREE-PO4, with minor or trace amounts of LREE, and an important USiO4 (coffinite) component. Gadolinium is p referentially incorporated into xenotime, though GdPO4 crystallizes in the monazite structure. Th and U strongly fractionate into monazite a nd xenotime, respectively. K-D values (calculated from REE concentrati on in monazite/REE concentration in xenotime) allow a distinction betw een ''monazite-REE'' and ''xenotime-REE''. The occurrence of metamorph ic grown monazite (and xenotime) in greenschist facies rocks, their zo ning, which is preserved up to high temperatures, and the presence of texturally and chemically different types opens the possibility to dat e prograde metamorphic histories by in situ techniques.