GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 56-10
Presentation Time: 4:15 PM


JIAO, Shujuan, Institute of Geology and Geophysics, Chinese Academy of Sciences, No. 19, Beitucheng Western Road, Beijing, 100029, China, FITZSIMONS, Ian, The Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtin University of Technology, GPO Box 1987, Perth, 6845, Australia, ZI, Jian-wei, TIGeR (The Institute for Geoscience Research), John de Laeter Centre, Department of Applied Geology, Curtin University, Perth, 6102, Australia; State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China, EVANS, Noreen J., TIGeR (The Institute for Geoscience Research), John de Laeter Centre, Department of Applied Geology, Curtin University, Perth, 6102, Australia, MCDONALD, Bradley J., Space Science and Technology Centre, School of Earth and Planetary Science, Curtin University, Perth, 6102, Australia and GUO, Jinghui, Geology, Chinese Academy of Science, Qijiahuozi road, Haidian district, Beijing, China

Understanding the metamorphic formation history of garnet and monazite and correlating their occurrence is vital to geochronological data interpretation and the establishment of a PTt path, however the garnet formation process and relationship to monazite formation during ultrahigh-temperature (UHT) metamorphism are poorly understood. Trace element analyses of garnet and monazite in the Dongpo UHT granulites (Khondalite Belt, North China Craton) informs garnet genesis, and Y and Dy partitioning between garnet and monazite, revealing both garnet growth and resorption during UHT metamorphism. The inclusion-rare garnet formed earliest, before UHT metamorphism, and is characterized by highest CaO contents (0.8–1.7 wt%) and bell-shaped Y zoning (from 330 to 100 ppm). Garnet also formed during the UHT prograde stage, and is characterized by abundant biotite and sillimanite inclusions, rare spinel and sapphirine inclusions, lower CaO (0.5–0.9 wt%) with constant or increased Ca zoning and roughly consistent Y contents (20–100 ppm). The Y-enriched rims indicate garnet resorption and back diffusion during the UHT-decompression stage.

The low-Y monazite inclusions are interpreted to have concurrently formed and achieved chemical equilibrium with their host low-Ca, Y garnet during UHT metamorphism. Yttrium and Dy in equilibrated garnet and monazite pairs are positively correlated. The partition coefficients calculated from 9 monazite–garnet pairs are 21–27 for DYMnz/Grt (average: 25±6 (2σ)) and 48–89 for DDyMnz/Grt (average: 70±30 (2σ)). The quantitative assessment of chemical correlations between monazite and garnet in UHT granulites will be a valuable criterion in relating multiple monazite domains to different metamorphic stages and will lead to a better understanding of the meaning of monazite geochronological data in future granulite studies. More detailed work on empirically-derived monazite-garnet partitioning in natural rocks under a wide range of PT conditions are needed in order to better document the correlation of partition coefficients with other controlling factors, particularly temperature and the presence of apatite.