Apatite and biotite thermochronometers help explain an Arctic Caledonide inverted metamorphic gradient

Biotite generates significant radiogenic Sr and is thus suitable for Rb-Sr dating, which can be achieved directly in thin section via laser ablation and provides a means to directly date growth of this important fabric-forming mineral or its cooling. In contrast, apatite has an extremely low Rb/Sr ratio and hence offers a useful tool to estimate source Sr-87/Sr-86(i). Plagioclase also contains significant Sr and may further assist in understanding the rocks Sr isotopic system. However, the fluid source of growing or recrystallizing apatite, plagioclase, and biotite need not be the same. Compounding the complexity, apatite may gain radiogenic Sr from biotite during recrystallization or dissolution-reprecipitation that accompanies metamorphism. In this contribution, we present a case study from the Kalak Nappe Complex in the Scandinavian Caledonides, Arctic Norway, where a combined geochronology program, critically supported by biotite Rb-Sr dating, helps to resolve the conspicuous inverted metamorphic field gradient found upwards through the nappe stack in this orogen. Apatite U-Pb ages of 417-413 Ma in the upper nappes are interpreted as cooling through the Pb retention zone. Such ages contrast with the suggestion of older, partially reset apatite ages in the lower nappes. Biotite Rb-Sr ages of c. 413 Ma in the upper nappes are similar to U-Pb ages of coexisting apatite but conspicuously different from c. 433 Ma biotite Rb-Sr ages in the lower nappes. Ages of c. 433 Ma are comparable to muscovite 40Ar/39Ar and titanite and monazite U-Pb ages throughout the complex and may directly date mineral growth at peak thermal conditions during the Scandian orogenic event. Apatite Sr-87/Sr-86 ratios also track the influence of metamorphism and imply more radiogenic signatures relative to relic plagioclase in samples which have greater overprinting in the upper nappes of the complex. Phase equilibrium modelling establishes Palaeozoic peak P-T conditions of 610-750 degrees C at <12 kbar in the upper nappes, comparable to peak conditions previously established for the lower nappes. Combined thermal modelling of the post-peak Scandian omgenic history based on published 40Ar/39Ar, paired with new Rb-Sr, and U-Pb data, implies a slower cooling pathway in the upper nappes relative to the lower nappes. We reconcile this thermal history with a geodynamic model of obduction of a hot back-arc, as represented by the Silurian Mageroy Nappe, onto the upper nappes of the Kalak Nappe Complex at c. 430 Ma. These results demonstrate that in-situ laser ablation Rb-Sr dating of biotite provides a powerful tool for resolving orogenic tectonothermal histories in orogens flushed with excess argon.

Automated summary

The study focuses on a geological case study in the Kalak Nappe Complex of the Scandinavian Caledonides in Arctic Norway. By using biotite Rb-Sr dating technique, the growth rate and cooling date of rock fabric were determined, as well as the role of apatite and plagioclase in the Sr isotopic system. It was found that there is a metamorphic gradient in the Kalak Nappe Complex.