REPLACEMENT OF PRIMARY MONAZITE BY APATITE-ALLANITE-EPIDOTE CORONAS IN AN AMPHIBOLITE FACIES GRANITE GNEISS FROM THE EASTERN ALPS

Accessory monazite crystals in granites are commonly unstable during a mphibolite facies regional metamorphism and typically become mantled b y newly formed apatite-allanite-epidote coronas. This distinct textura l feature of altered monazite and its growth mechanism were studied in detail using backscattered electron imaging in a sample of metagranit e from the Tauern Window in the eastern Alps. It appears that the oute r rims of the former monazites were replaced directly by an apatite ri ng with tiny thorite intergrowths in connection with Ca supply through metamorphic fluid. Around the apatite zone, a proximal allanite ring and a distal epidote ring developed. This concentric corona structure, with the monazite core regularly preserved in the center, shows that the reaction kinetics were diffusion controlled and relatively slow. Q uantitative electron microprobe analyses suggest that the elements rel eased from monazite breakdown (P, REE, Y, Th, U), were diluted and red istributed in the newly formed apatite, allanite, and epidote overgrow th rings and were unable to leave the corona. This supports the common hypothesis that these trace elements are highly immobile during metam orphism. Furthermore, microprobe data suggest that the preserved monaz ite cores lost little, possibly none of their radiogenic lead during m etamorphism. Thus, metastable monazite grains from orthogneisses appea r to be very useful for constraining U-Th-Pb protolith ages. On the ba sis of these findings and a review of literature data, it seems that m onazite stability in amphibolite facies metamorphic rocks depends stro ngly on lithologic composition. While breaking down in granitoids, mon azite may grow during prograde metamorphism in other rocks such as met apelites.