Decoupling of monazite petrochronology from P–T evolution during garnet hydrothermal dissolution–reprecipitation

Monazite U–Th–Pb ages are commonly coupled with Y and heavy rare earth element (HREE) contents to link reactions, particularly those involving the formation and breakdown of garnet, with pressure–temperature (P–T) paths to constrain and uncover orogenic processes. This link assumes that changes in garnet modal abundances reflect changes in P–T and no other processes. Here we show that protracted and cryptic fluid-driven dissolution-reprecipitation of garnet and monazite disturbed the P–T–time(t) relationship between these two minerals.

We collected samples of metapelitic schists from the Yardoi gneiss dome, southern Tibet. The central portion of the Yardoi dome comprises orthogneiss and metapelites intruded by Eocene (43–35 Ma) to Miocene (17 Ma) granitoids, providing a critical window investigate fluid–rock interaction in mid-crustal metamorphic rocks. Our data constrain prograde to peak metamorphism from 5.7 kbar and 550°C to 7.5–8.5 kbar and 650–750 °C, followed by retrograde metamorphism at 5.5–6.5 kbar and 650–700°C. Low Y+HREE monazite domains dated to 41–46 Ma indicate peak metamorphism in the presence of garnet at this time, whereas high Y+HREE contents between 23 and 15 Ma indicate the timing of garnet breakdown during isothermal decompression. These data indicate 20–30 Myr at 650–700°C, consistent with near complete resetting of garnet major element zoning in most samples.

One sample near the core of the dome displays atoll garnets along with biotite, muscovite, plagioclase, quartz, and rutile. Annuli of plagioclase, quartz, and biotite grains separate the garnet core from the rim. Monazite from this sample show a quasi-continuous age spread from 50 Ma to 19 Ma, with an increase in Y + HREE between 45 and 38 Ma, followed by a decrease after ~38 Ma. These data suggest a period of garnet breakdown followed by (re-)growth between 45 and 20 Ma. Phase diagram models show very limited variation in garnet modal abundance at >7 kbar and 600–700 °C, indicating that changes in P–T are unlikely to influence garnet modes. Additionally, there are no other phases like staurolite, which may have reacted with garnet.

We propose instead cryptic dissolution-reprecipitation of garnet. Maps of grossular content (X_Grs) shows a pebbly texture with interconnected moats of low X_Grs garnet surrounding islands of high X_Grs garnet, whereas other endmember fractions show flat garnet cores and mantles with slight increases in spessartine at the rim. We suggest that garnet recrystallization was driven by the release of magmatic-hydrothermal fluids from nearby 43–35 Ma granite intrusions. We suggest that fluid-assisted recrystallization can generate age-composition trends that mimic monazite zoning patterns of P–T path controlled garnet breakdown or growth. Misinterpretations of such data would propagating significant errors into tectono-metamorphic reconstructions, emphasizing the necessity of microstructurally constrained petrochronology when interpreting monazite U–Th–Pb ages.