INTERPRETATION OF MONAZITE AGES OBTAINED VIA IN SITU ANALYSIS

Monazite, (Ce,La,Th)PO₄, is commonly analyzed to obtain geochronologic information from metamorphic rocks. In pelitic lithologies, the mineral appears near the garnet isograd suggesting the combination of pressure-temperature (P-T) conditions and monazite age data is useful to ascertain the evolution of metamorphic terranes. Numerous studies dated monazite in situ (e.g., electron microprobe, ion microprobe), preserving the textural relationships of the grain analyzed and those used to estimate P-T. Despite promising results, a primary issue remains unresolved: how to interpret monazite age data when results from a single rock are inconsistent with a single population. Recently, monazite composition, specifically Y content, has been invoked to address this issue. In this study, monazite grains from several localities in Nepal, China, and Vietnam were compositionally analyzed with an electron microprobe and dated (Th-Pb) with an ion microprobe. Five sources of uncertainty explain complicated age distributions: (1) Pb loss due to prolonged experienced above the closure temperature, (2) dissolution/reprecipitation along the retrograde path, (3) analytical uncertainties, (4) analyses of overlapping age domains, and (5) episodic monazite growth. Chemical variability appears not to reflect timing information and the Y proxy is not useful for these samples. For example, eastern Nepal sample ET26 contains ~500 Ma, ~45 Ma, and ~20 Ma monazite grains, consistent the timing of Pan-African and polymetamorphic Himalayan events. The ~20 Ma grains contain 0.71-1.33 wt% Y₂O₃, ~45 Ma grains have 0.97-3.36 wt%, and a ~500 Ma grain has ~1.2 wt%. Monazite inclusions in garnet from eastern Nepal sample 85H20g contain 0.4-1.4 wt% Y₂O₃, whereas matrix grains have 0-1.88 wt%. Instead of relying on monazite chemical composition, possibly influenced by bulk rock composition, reactions with other accessory minerals, crystal orientation, and the monazite-forming reactant (e.g., allanite), we suggest garnet zoning patterns and peak P-T conditions facilitate age interpretation. Thermobarometric data can assess potential polymetamorphism, retrogression, and in conjunction with diffusion studies, extent of monazite Pb loss.