Peak and post-peak development of UHT metamorphism at Mather Peninsula, Rauer Islands: Zircon and monazite U-Th-Pb and REE chemistry constraints

Zircon and monazite in ultrahigh temperature (UHT) metamorphic rocks from the Rauer Islands of Prydz Bay in East Antarctica were investigated in terms of U-Th-Pb and rare earth elements (REE) chemistry along with textural context. All five analyzed samples, three from the Mather Paragneiss UHT unit and two from the host orthogneiss unit yield 522-517 Ma concordant zircon ages, with older protolith/inherited zircon ages of 3268 and 2800-2400 Ma along with highly discordant Mesoproterozoic to Neoproterozoic ages. Our data confirm the Archaean protolith age for the host orthogneiss surrounding the UHT Mather Paragneiss. The Archaean and Mesoproterzoic components of the Rauer Islands were not amalgamated in the Rauer Tectonic Event at 1030-990 Ma, and deposition of the Mather Paragneiss was considered at some time after the Rauer Tectonic Event. In contrast to the well-defined 520 Ma ages obtained from the zircons in the UHT rocks, monazite grains measured by electron microprobe show a distinct internal zonation, from 580-560 Ma dark-backscattered electron image (BSE) cores enriched in middle rare earth elements (MREE) and heavy rare earth elements (HREE) to 550-520 Ma mid-BSE mantles and 510-500 Ma bright-BSE rims. From the chemical and textural evidence we infer that the MREE-HREE-rich 580-560 Ma monazite cores may have formed through the decomposition of garnet during decompression just after the UHT event, whereas the MREE-HREE-depleted 550-500 Ma monazite grains/rims formed or recrystallized in reactions associated with subsequent extensive hydration during the upper-amphibolite to granulite-facies main Prydz Tectonic Event, which also caused marked recrystallization of zircon. The above data strongly support the interpretation that the UHT metamorphism occurred prior to 590-580 Ma.