Crystal chemistry of monazite and xenotime from Saxothuringian-Moldanubian metapelites, NE Bavaria, Germany
Franz, Gerhard; Andrehs, Gerhard; Rhede, D.
European Journal of Mineralogy Volume 8 Number 5 (1996), p. 1097 - 1118
published: Oct 30, 1996
manuscript accepted: Apr 18, 1996
manuscript received: Jul 18, 1995
ArtNo. ESP147050805016, Price: 29.00 €
Abstract Monazite and xenotime from a regional metamorphic area were analyzed by electron microprobe technique. The rocks are metapelites-psammites with a major and trace element composition characteristic for volcanic arc sediments. Textural observations on grain size and shape indicate a continuous growth of both monazite and xenotime from greenschist to granulite fades temperatures. The whole rock REE distribution pattern for garnet-poor assemblages is dominated by these accessory phases. Monazite incorporates the LREE La-Sm as major elements and the HREE in minor or trace amounts. An increase of YPO4 solid solution with increasing 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 preferentially incorporated into xenotime, though GdPO4 crystallizes in the monazite structure. Th and U strongly fractionate into monazite and xenotime, respectively. KD values (calculated from REE concentration in monazite/REE concentration in xenotime) allow a distinction between "monazite-REE" and "xenotime-REE". The occurrence of metamorphic grown monazite (and xenotime) in greenschist fades rocks, their zoning, which is preserved up to high tenperatures, and the presence of texturally and chemically different types opens the possibility to date prograde metamorphic histories by in situ techniques.