Garnet, zircon, and monazite age and REE signatures in (ultra)high-temperature and high-pressure rocks: Examples from the Caledonides and the Pamir

Abstract

Rare earth element (REE) signatures of high-U/Pb and high-Th/Pb accessory minerals are typically used to link their ages to specific petrological processes (‘petrochronology’)—most notably the growth or breakdown of garnet. Although this approach is powerful, gaps in our understandings of REE systematics in high-grade rocks exist, particularly regarding the degree to which these minerals chemically equilibrate at extreme conditions. To investigate this, we performed comparative chronology and REE analysis of garnet, zircon, and monazite in (1) a fluid-rich, ultrahigh-pressure (UHP) migmatite from the Western Gneiss Region, Norway, and (2) dry felsic granulite xenoliths from the Pamir, Tajikistan. Zircon and monazite from the hydrous migmatite provided ages of 450–370 Ma and a range of Gd/Yb values across this time span. A Lu-Hf garnet age of 422 ± 2 Ma coincides with the age at which zircon and monazite exhibit the highest Gd/Yb values, as well as the largest range therein. The degree of dispersion in these values is substantial, especially for monazite. Zircon and monazite in the dry xenoliths provide age clusters between 50 and 11 Ma, recording pulsed growth and recrystallization. The Lu/Hf garnet ages for these samples are 41–38 Ma. The accessory minerals of that age are texturally associated with garnet, yet have the lowest, not the highest, Gd/Yb values. In both cases, there is evidence that co-genetic garnet and accessory minerals achieved REE equilibrium during growth. However, the age and compositional record of chronometric accessory minerals that were co-genetic with garnet are distinctly different between the two case examples. In the hydrous migmatite, supply-limited garnet growth in the presence of fluid phase resulted in strongly zoned garnet and a correspondingly large range in Gd/Yb values among co-genetic zircon and monazite. The range in Gd/Yb values match among these phases and collectively capture the strong fluctuations in the REE composition of matrix micro-domains caused by garnet growth. In this case, it is the range in Gd/Yb values, rather than a specific composition that is diagnostic for garnet growth. In the felsic granulites, garnet and accessory minerals growth occurred in a fluid-limited regime, in which short fluid pulses triggered reactions that had been likely significantly overstepped. The seemingly contradictory pattern of increasing Gd/Yb after garnet growth in these samples is the result of garnet having high Gd/Yb and continued zircon growth forcing further HREE depletion of the matrix with time. The situation that high, rather than low, Gd/Yb values in zircon and monazite indicate equilibrium with garnet may be common in dry granulites and other anhydrous high-temperature rocks. Our findings provide an improved framework for the reliable identification of the accessory minerals that equilibrated with garnet, even when the REE-age record of these minerals is complex, dispersed, and seemingly contradictory. In addition, the combined zircon-, monazite-, and garnet-age and REE-record refines the regional tectonic interpretation of our samples.