Electron microprobe technique for U-Th-Pb and REE chemistry of monazite, and its implications for pre-, peak- and post- metamorphic events of the Lutzow-Holm Complex and the Napier Complex, East Antarctica

Abstract

Monazites in high-grade metapelites from the Lutzow-Holm Complex and Napier Complex have been examined in terms of U, Th, Pb and rare earth element (REE) chemistry using an electron microprobe. The studied samples include four granulite-facies garnet-biotite-bearing metapelites from Skallen within the Lutzow-Holm Complex, and a re-hydrated garnet-sillimanite gneiss from the Mt. Riiser-Larsen area within the UHT zone of the Napier Complex. Two out of four garnet-bearing metapelitic samples from Skallen gave simple 560-500Ma monazite U-Th-Pb ages, whereas the other two samples yielded two age populations, i.e., 560-500Ma and 650-580Ma. The younger age group is consistent with the 550-520Ma metamorphic ages reported by SHRIMP. The older>580Ma monazites are relatively enriched in Nd, Sm, Gd, Dy (MREE) and depleted in Si (Ca and Th) compared with the younger (560-500Ma) ones. These older monazites possibly formed through M-HREE-enriched conditions such as garnet-free conditions, suggesting that the growth of these monazites pre-dated the peak metamorphism. Garnet-sillimanite gneiss from the Mt. Riiser-Larsen area shows various post-UHT re-hydration textures such as biotite-sillimanite aggregates, and fine-grained biotite flakes around or intracrystalline fractures within garnet porphyroblasts. Monazites enclosed within garnet cores have 2480-2440Ma U-Th-Pb ages consistent with the reported zircon and monazite SHRIMP dates. On the other hand, those associated with re-hydrated zones gave fluctuating 2200-700Ma ages. These younger ages are thought to reflect the incomplete chemical disturbance during the post-UHT crustal processes