Ar-Ar dating of Caledonian and Grenvillian rocks from northeasternmost Svalbard - evidence of two stages of Caledonian tectonothermal activity in the high Arctic?
Ar-Ar analyses have been carried out on Caledonian and Grenvillian rocks from the northernmost Barents Shelf (approx. 80"N) on Nordaustlandet
and Kvitøya, northeast Svalbard. Five muscovites and one biotite from Caledonian granitoids, three muscovites and one biotite from Grenvillian
granitoids, and two samples of hornblende from mafic rocks of uncertain age, were analysed using stepwise laser heating of single grains. The
Grenvillian granitoids, as well as the mafic rocks, yield ages between 410 and 425 Ma, interpreted to date cooling after Caledonian regional metamorphism; they show no traces of a Grenvillian argon component. Thus, Caledonian metamorphism in central and eastern Nordaustlandet must have reached above the closure temperature for argon in these minerals. In the Caledonian granitoids, two age groups may be discerned: one with Ar-Ar ages in muscovite between 415 and 430 Ma, and another with muscovite and biotite ages around 400 Ma. The first group is related to granitic and aplitic rocks having Late Ordovician to Early Silurian U-Pb monazite ages (ca. 440-460 Ma); the second group includes the large Rijpfjorden granite with an Early Devonian U-Pb monazite age of 412 Ma. Thus, prolonged tectonothermal activity during the Caledonian orogeny may be discerned in northeasternmost Svalbard, possibly divisible into two stages: one stage with regional metamorphism and syn-tectonic magmatism occurring in the Late Ordovician to Early Silurian, and a second stage with late- or post-tectonic magmatism in the Early Devonian. For each stage, the Ar-Ar age follows the U-Pb age with a delay of 10-20 Ma, corresponding to rapid cooling of 20-45 °C per million years from near-magmatic temperatures to the closure temperature of argon in these minerals.