Sequence and timing of mineral replacement reactions during albitisation in the high-grade Bamble lithotectonic domain, S-Norway

New data on mineral replacement combined with TIMS U-Pb ages give increased knowledge on the characterization and evolution of Ti-phases and associated minerals during albitisation. The study examines scapolite metagabbros and albitites in the Kragerø area of the Bamble lithotectonic domain, S-Norway. Zircon data yield a U-Pb age of 1272 ± 16 Ma for the protolith of the albitites and also record a distinct metamorphic overprint at around 1100 Ma. Variations in albitite lithology, mineral assemblages and replacement reactions indicate that albitisation occurred under changing physical conditions over a wide range of temperatures <550 °C, where P-T pseudosection modeling of clinopyroxene-bearing albitite indicates equilibration at 410-420 °C at mid-crustal levels. Ilmenite present in the protolith reacted to rutile or to rutile and titanite during scapolitisation. Titanium was then incorporated into titanite during albitisation. Additional titanite was also produced by chloritisation of phlogopite and amphibole. The titanite U-Pb data yield two groups of ages at 1104-1100 Ma and 1094-1093 Ma. Subsequent albitisation affecting the clinopyroxene-bearing albitite replaced titanite by rutile in aggregates with calcite and quartz. The U-Pb data for rutile in two albitite samples are disturbed and not very precise but indicate a range between 1110 and 1070, with a higher probability at around 1080 Ma. Monazite also occurs in different generations, the oldest at 1101 ± 5 Ma and a younger one at 1078 Ma, reflecting different reactions. Titanite and rutile connected through replacement reactions, together with monazite U-Pb-geochronology, shed light on the complex age evolution, the time scale of the albitisation and the relationship between the metamorphism and metasomatism. The results show that these metasomatic processes spanned a protracted period of time of over 20 million years at the tail end of a 50 million year long multistage and complex metamorphic development of this segment of the Sveconorwegian orogen.