THE IMPORTANCE OF UHT METAMORPHISM FOR CRUSTAL DIFFERENTIATION

Stabilization of Earth’s most enduring tracts of continental crust over billion-year timescales requires that the crustal inventory of heat-producing elements (HPE), U, Th and K, are concentrated close to Earth’s surface. A long-held view is that melting of the deep crust drives this chemical differentiation via the dissolution of key accessory minerals, such as monazite and zircon, and subsequent upward melt transport [e.g. 1]. However, a growing body of empirical evidence challenges this paradigm and implies that monazite, in particular, remains a restitic phase during melting at granulite-facies conditions [e.g. 2]. This contribution characterizes the effect of ultrahigh-temperature metamorphism on the redistribution of HPEs. Peraluminous UHT metasediments of the Ivrea Zone, NW Italy, are depleted in U, Th, LREE and enriched in HFSE and HREE relative to non-UHT compositions [3; this study]. Heat production decreases abruptly from 2-3 µW/m³ in the non-UHT granulites (peak-T ~900 °C) to <0.2 µW/m³ in UHT samples. Conservative behaviour of Ti during melting, due to the stabilization of rutile at the expense of high-Ti biotite, allows for mass-balance constraints to be imposed on the volumetric strain associated with melt-loss across the terrane. Despite losing ~40 vol.% melt, the highest-T granulites preserve heat productivities similar to sub-solidus amphibolite-facies metapelites, whereas the UHT metasediments underwent only 5 to 10 % more melt-loss and yet experienced almost complete removal of HPE. Additional processes to the temperature-dependent dissolution of monazite in granitic melt are required to account for this observation. Restitic UHT metapelites from the Ivrea Zone exhibit bulk-rock Ti/Th > 1000, primarily controlled by the stability of rutile and dissolution of monazite. Peraluminous metasedimentary xenoliths from the Basin and Range, the Rio Grande Rift and val Malenco exhibit similar bulk-rock systematics, implying that the attainment of temperatures >900 °C is key to the effective differentiation and stabilization of continental crust.

[1] Rudnick and Gao 2003, Treatise on Geochemistry, 3, 1-65; [2] Alessio et al 2018, Geology, 46(4), 335-338; [3] Ewing et al 2014, EPSL, 389, 106-118