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

Dating mylonitic overprinting of ancient rocks

“…Detailed analytical procedures are provided in Supplementary Methods and all Rb-Sr data are in Supplementary Data 2. Reference biotite CK001 yielded an age of 420 ± 6 Ma ( 87 Sr/ 86 Sr i = 0.681 ± 0.066; MSWD = 0.8, n = 53) identical, within uncertainty, to the published value of 413 ± 3 Ma and 87 Sr/ 86 Sr i of 0.709 ± 0.001 65,66 (Supplementary Figure 1). Titanite U-Pb isotope (Supplementary Data 3) and trace element concentration data (Supplementary Data 4) were also obtained via LA-ICPMS analysis.…”

“…EDS spectra consist of one thousand counts, are standardized to a pure Mn reference, and are statistically compared to a mineral spectra database for identification purposes. Operational conditions 65 consisted of a beam energy of 25 keV, a beam intensity of ~19 keV, a probe current of ~5.3 nA, a spot size of ~80 nm, and a nominal working distance of 15 mm. High-resolution maps of thin sections are provided in Supplementary Data 1.…”

Mechanisms and durations of metamorphic garnet crystallization in the lower nappes of the Caledonian Kalak Nappe Complex, Arctic Norway

“…The peak T of 630°C obtained here for the metamorphism of 28HF18 on Sværholt Peninsula is close to the peak conditions reported for the nappes structurally above the Kolvik Nappe of Porsanger Peninsula, to the west of the study area (e.g., Rice, 1984a). There, the presence of staurolite, kyanite and sillimanite has been interpreted to indicate the inversion of the metamorphic gradient (e.g., Kirkland et al, 2021; Rice, 1985; Rice et al, 1989; Roberts, 1985), and a higher metamorphic grade than that experienced by the rocks from Sværholt. Our garnet growth modelling indicates that 28HF18 has also been metamorphosed up to upper amphibolite facies conditions but that it lacks these index minerals as a result of the evolution of the effective bulk‐rock composition during chemical fractionation associated with garnet growth along the metamorphic P‐T path.…”

“…In addition, our results suggest that repeated, Barrovian-type garnet growth within 7.5 Myr, as inferred by Gaidies et al (2022), is a geologically reasonable scenario for the Scandian phase of the Caledonian Orogeny in Finnmark. Kirkland et al (2021) proposed thermal trajectories for the upper and lower nappes of the KNC based on thermochronological and geochronological constraints using mainly micas and accessory phases (e.g., Kirkland et al, 2005Kirkland et al, , 2009Kirkland et al, , 2021Kirkland, Daly, Chew, & Page, 2008;Kirkland, Daly, Eide, & Whitehouse, 2006, 2008. According to the authors, at circa 430 Ma, the lower nappes of the KNC experienced a brief residence time of circa 5 Myr at peak T of $630 C, whereas the upper nappes remained longer at these conditions, with slower initial cooling rates from peak T. Those constraints on cooling rates are mainly based on Rb-Sr biotite ages, where most of the cooling ages in the upper nappes are circa 413 Ma, contrasting with circa 433 Ma for the lower nappes (Dallmeyer, 1988;Kirkland, Daly, Eide, & Whitehouse, 2007).…”

“…These metamorphic zones are usually delimited by low‐angle shear zones, indicating that the apparent metamorphic inversion is rather a result of nappe emplacement and overthrusting (Rice, 1984a, 1984b, 1987). Kirkland et al (2021), based on thermochronological studies, suggest that the inverted field gradient resulted from different residence times at higher T . According to that study, the lower nappes cooled faster than the upper nappes because the latter were affected by obduction of back‐arc material, remaining hotter for a longer duration.…”