Records of Garnet Growth in Zircon and Monazite from High-Grade Rocks

Age data from accessory minerals in conjunction with rare earth element (REE) signatures are commonly used to estimate the timing of garnet growth or breakdown in metamorphosed rocks. This approach may or may not be reliable, depending on the efficiency of (re-)equilibration in rocks. To test the systematics of 'petrochronology', we performed a trace-element and multi-chronology analysis, involving Lu-Hf garnet dating and in-situ LA-ICPMS U-(Th-)Pb zircon and monazite dating. The analyses were done on two high-grade rock samples: fluid-rich, ultrahigh-pressure (UHP) migmatites from the Western Gneiss Region, Norway, and dry felsic granulite xenoliths from the Pamir, Tajikistan.

Zircon and monazite from the hydrous migmatites yield a range of Gd/Yb ratios and ages with a large degree of dispersion. Highest Gd/Yb values occur at 420-410 and c. 420 Ma for monazite and zircon, respectively. Garnet in this rock yielded a Lu-Hf bulk age of c. 422 Ma. In contrast, accessory minerals in the dry Pamir xenoliths grew in distinct pulses between 50 and 11 Ma. Consistent with the age of garnet growth inferred from texturally controlled U-(Th)-Pb dating (42-37 Ma [1]), the samples yielded Lu-Hf garnet ages of 41-38 Ma. Highest Gd/Yb values for zircon and monazite do not occur at this age, but instead at 30 Ma or younger.

This study shows that fluid saturation strongly controls the ability of accessory minerals to re-equilibrate and record mineral reactions involving garnet. Even when fluids were present and these minerals reacted extensively, Gd/Yb values still differ greatly, including grains that grew synchronously with, and in close proximity to, garnet. The upper envelope of Gd/Yb-age data, not each individual data point, reliably tracks garnet growth in such case. In dry granulites, accessory minerals appear to re-equilibrate during short pulses, without apparent chemical interaction with garnet. High-Gd/Yb zircon and monazite may then provide a minimum age for the matrix REE depletion caused by garnet growth. Reliable age estimates for garnet growth from accessory phases can be obtained in each cases. The extremes from large dataset of Gd/Yb-age appear to best reflect the equilibration between garnet and accessory minerals, even in well-equilibrated and fluid-saturated samples.

[1] Kooijman et al. (2017) EPSL 465, 59-69.