T56 Metamorphic Petrology Past, Present and Future: Preparing for the Next 100 Years with the Mineralogical Society of America

Sunday, 22 September 2019
1:30 PM - 5:30 PM
Room 126ABC, North Building

GARNET FORMATION HISTORY AND QUANTITATIVE CHEMICAL CORRELATION WITH MONAZITE IN UHT GRANULITE: A CASE STUDY FROM THE KHONDALITE BELT, NORTH CHINA CRATON

Paper number: T56-11
Program: Technical Sessions
Time: 4:15 PM – 4:30 PM

Authors

Abstract

Understanding the metamorphic formation history of garnet and monazite and correlating their occurrence is vital to geochronological data interpretation and the establishment of a P–T–t 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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-336353

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