Metamorphic record of monazite in aluminous migmatitic gneisses at Stensjöstrand, Sveconorwegian orogen

Abstract: Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) has been utilised to investigate the age and chemical composition of two texturally different types of monazite in leucosome and mesosome of polymetamorphic aluminous migmatitic gneisses at Stensjöstrand, Halland. Results of radiometric dating are compared with previously published zircon dates. Monazite grains were documented and investigated in three thin sections using polarization microscopy and Scanning Electron Microscopy coupled with Energy Dispersive X-ray (SEM-EDX). Backscatter Electron (BSE) images show that monazite grains in the mesosome have anhedral morphology, embayments, and complex internal zonation. Monazite grains in the leucosome are relatively small, euhedral to subhedral or ovoid, and have relatively distinct internal zoning. Anhedral and ovoid morphologies, embayments, and complex zoning indicate recrystallisation and resorption processes having affected monazite grains during metamorphism. Documented monazite grains were analysed for trace elements Y, Lu, Th, and U, rare earth elements (REE), and radiometrically dated. Results reveal two age groups, c. 1.43 Ga and c. 0.99 Ga, that relate to the Hallandian and Sveconorwegian orogenies, respectively. Both monazite generations occur in leucosome and mesosome, either as single- or double-generation grains. Weighted average calculations of concordant 207Pb/206Pb isotopic dates yield 1434 ± 14 Ma and 988 ± 11 Ma, which can be compared to previously published zircon dates of 1386 ± 7 Ma and 970 ± 5 Ma (Piñán-Llamas et al. 2015). The analytical results may have been affected by data drift owing to downhole fractionation, most distinctly observed in results from Thompson Mine Monazite (TMM) standard as steadily increasing dates throughout the day of analysis. Trends in trace element analyses show relative U enrichment and depletion of Y, Lu, and Th in the 0.99 Ga monazite generation compared to the 1.43 Ga generation, indicating that the bulk of REE content in the rock may have been locked into the Hallandian generation preventing incorporation into the Sveconorwegian generation. In conclusion, obtained U-Pb monazite dates correlate broadly with U-Pb zircon dates, but are c. 50 Ma and c. 18 Ma older, for the Hallandian and Sveconorwegian generations, respectively.