Reaction microtextures of monazite: Correlation between chemical and age domains in the Nazaré Paulista migmatite, SE Brazil

Back-scattered imaging, X-ray element mapping and electron microprobe analyzer (EMPA) chemical dating reveal complex compositional and age zoning in monazite crystals from different layers and textural positions in a garnet-bearing migmatite in SE Brazil. Y-rich (variable Y2O3, averaging 2.5 wt.%) relict cores are preserved in mesosome and melanosome monazite, and correspond to 793 ± 6 Ma inherited crystals possibly generated in a previous metamorphic event. These cores are overgrown and widely replaced by two generations of monazite, which are present in all migmatite layers. The first, also Y-rich (average 2.5 wt.% Y2O3), was produced at ~ 635 Ma during prograde metamorphism under subsolidus conditions, while the second has an Y-poor (< 1.5 wt.% Y2O3), low Th/U signature, and precipitated from low Y and HREE anatectic melts produced by reactions in which garnet was inert. Quartz-rich trondhjemitic leucosome represents lower temperature melt (bearing some subsolidus quartz and garnet with included monazite) formed at temperatures below muscovite breakdown; its Y-poor monazite indicates an age of 617 ± 6 Ma. Granitic leucosomes formed close to peak metamorphic conditions (T > 750 °C) above muscovite breakdown have their slightly younger character confirmed by a 609 ± 7 Ma low-Y monazite age. A similar 606 ± 5 Ma age was obtained for low-Y monazite rims and domains in mesosome and melanosome, and reflects the time of monazite saturation in interstitial granitic melt that was trapped in these layers. Our results confirm that inherited monazite crystals can be preserved during partial melting at temperatures above muscovite breakdown. Moreover, careful textural control aided by X-ray chemical mapping may allow monazite generated at different stages in a ~ 25 Myr prograde metamorphic path to be identified and dated using an electron microprobe.