From ID-TIMS U-Pb dating of single monazite grain to APT-nanogeochronology: application to the UHT granulites of Andriamena (North-Central Madagascar)

Monazite and zircon grains from the ultra-high temperature (UHT) granulite of the Andriamena unit in the centre north of Madagascar, previously investigated using micro-drilled single grain isotopic dissolution geochronology and in-situ electron microprobe chemical dating by Paquette et al. (2004), are reinvestigated using high spatial resolution techniques: atom probe tomography (APT), transmission electron microscopy (TEM), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS). The dated grains are located in quartz, garnet belonging to the peak UHT metamorphic assemblage and in retrograde coronitic texture composed of biotite, cordierite and orthoamphibole. Monazite grains show complex discordant trends with several radiogenic Pb (Pb*) loss dispersions, that were previously interpreted as recording at least four geologic episodes occurring from the Archean (~2.7 Ga) to the Proterozoic-Cambrian (~0.5 Ga). Multiscale investigations with SIMS, TEM and APT have been carried out on monazite and LA-ICP-MS on zircon. Zircon grains hosted in garnet and those in quartz record concordant ages at 2758 ±28 Ma and 2609 ±51 Ma, respectively. Monazite grains show age dispersion and a Pb*-loss trend correlated with the grain petrographic position. Grains located in garnet and quartz present less Pb*-loss than those located in the coronitic texture. The discordia indicates a crystallisation age at 2555 ±71 Ma and monazite grains hosted in quartz and garnet record a disturbance event at 1053 ±246 Ma. TEM investigations show that monazite located in quartz and garnet contain numerous Pb-bearing nanophases compared to monazite located in coronitic texture. There is an inverse correlation between the number of Pb-bearing nanophases and the percentage of Pb*-loss in monazite grains. 232Th/208Pb ratio obtained with APT in monazite matrix (i.e. excluding Pb-bearing nanophases) located in quartz and garnet indicated a mean age of 1092 ±127 Ma. This date is interpreted as a hitherto undetected geochronological record in Andriamena unit.