Growth, annealing and recrystallization of zircon and preservation of monazite in high-grade metamorphism: conventional and in-situ U-Pb isotope, cathodoluminescence and microchemical evidence

Zircon and monazite from granulite- to amphibolite-facies rocks of the Vosg es mountains (central Variscan Belt, eastern France) were dated by ion-micr oprobe and conventional U-Pb techniques. Different granulites of igneous (s o-called leptynites) and sedimentary origin (kinzigites) and their leucosom es were dated at 334.9 +/- 3.6, 335.4 +/- 3.6 and 336.7 +/- 3.5 Ma (convent ional age 335.4 +/- 0.6 Ma). Subsequent growth stages of zircon were distin guished by secondary electron (SEM) and cathodoluminescence (CL) imaging: ( 1) subsolidus growth producing round anhedral morphologies and sector zonin g; (2) appearance of an intergranular fluid or melt phase: at incipient deh ydration melting that first resulted in resorption of pre-existing zircons, followed by growth of acicular zircons or overgrowths on round zircons con sisting of planar growth zoning; (3) advanced melting producing euhedral pr ismatic zircons with oscillatory zoning overgrowing the sector zones. Two f urther lithologies, the Kaysersberg granite and the Trois-Epis units, were both formerly considered as migmatites. The intrusion of the Kaysersberg gr anite was dated at 325.8 +/- 4.8 Ma. The Trois-Epis unit was found to be th e product of volume recrystallization of a former granulite, which occurred under amphibolite-facies conditions 327.9 +/- 4.4 Ma ago. The amphibolite- facies overprint of the Trois-Epis zircons led to the complete rejuvenation of most of the zircon domains by annealing and replacement/recrystallizati on processes. Annealing is assumed to occur in strained lattice domains, wh ich are possibly disturbed by high trace element contents and/or large diff erences in decay damage between adjacent growth zones. Investigation of cat hodoluminescence structures reveals that the replacement occurs along curve d chemical reaction fronts that proceed from the surface towards the interi or of the zircon. The monazite U-Pb system still records the age of high-gr ade metamorphism at around 335 Ma. The chemical reagent responsible for the rejuvenation of zircon obviously left the monazite unaffected.