The Eskolabreen granitoids revisited -: an ion microprobe study of complex zircons from late Palaeoproterozoic granitoids within the Ny Friesland Caledonides, Svalbard

Zircons from one orthogneiss, previously dated by conventional U-Pb to 1766 +/- 10 Ma, and one sample of migmatite neosome, previously dated to 1734 +/-5 Ma, all from the Eskolabreen Complex of the Ny Friesland Caledonides, Svalbard, have been analysed by the NORDSIM ion microprobe in order to determine the nature of Caledonian overprinting in these zircons, leading to lower intercept ages close to 400 Ma, and the age of migmatization in the neosome sample. Most ion microprobe analyses fall along a discordia with an upper intercept at 1738 +/- 15 Ma, interpreted to reflect magmatic crystallization, and a lower intercept at 385 +/- 51 Ma, interpreted to reflect Caledonian overprinting. There is a concentration of near-concordant analyses with an average Pb-207/Pb-206 age of 1733 +/-5 Ma. Discordant ages appear to be caused by Pb loss from the outer parts of zircon crystals, and sometimes from metamict cores, during Caledonian metamorphism. No distinct new zircon rims yielding concordant Caledonian ages were recorded, neither in the orthogneiss, nor in the neosome sample. This observation would speak against a Caledonian migmatization age for the nesosome sample, and favour a late Palaeoproterozoic migmatization event, closely related to the magmatism forming the protoliths of these rocks.