MONAZITE-XENOTIME MISCIBILITY GAP THERMOMETRY .1. AN EMPIRICAL CALIBRATION

Rare earth element (REE) and yttrium concentrations of coexisting mona zite and xenotime were determined from a suite of seven metapelites fr om the Variscan fold belt in NE Bavaria, Germany. The metapelites incl ude a continuous prograde, mainly low-P (3-5 kbar) metamorphic profile from greenschist (c. 400 degrees C) to lower granulite facies conditi ons (c. 700 degrees C). The LREE (La-Sm) are incorporated preferential ly in monoclinic monazite (REO(9) polyhedron), whereas the HREE plus Y are concentrated in tetragonal xenotime (REO(8) polyhedron). The majo r element concentrations of both phases in all rocks are very similar and do not depend on metamorphic grade. Monazite consists mainly of La , Ce and Nd (La-0.20-0.23, Ce-0.41-0.45, Nd-0.15-0.18)PO4, all other e lements are below 6 mol%. Likewise, xenotime consists mainly of YPO4 w ith some Dy and Gd solid solutions (Y-0.76-0.80, Dy-0.05-0.07, Gd-0.04 -0.06) In contrast, the minor HREE concentrations in monazite increase strongly with increasing metamorphic grade: Y, Dy and Gd increase by a factor of 3-5 from greenschist to granulite facies rocks. Monazite c rystals often show zonation with cores low in HREE and rims high in HR EE that is interpreted as growth zonation attained during prograde met amorphism. Similarly, Sm and Nd in xenotimes increase by a factor of 3 -4 with increasing metamorphic grade. Prograde zonation in single crys tals of xenotime was not observed. The X(HREE+Y) in monazite and X(LRE E) in Xenotime of the seven rocks define two limbs along the strongly asymmetric miscibility gap from c. 400 degrees C to 700 degrees C. The empirical calibration of the monazite miscibility gap limb coexisting with xenotime is appropriate for geothermometry. Due to its contents of U and Th, monazite has often been used for U-Pb age determination. The combination of our empirical thermometer on prograde zoned monazit e along with possible age determination of zoned single crystals may p rovide information about prograde branches of temperature-time paths.