Magmatism and metamorphism of the Mårma Terrane, Kebnekaise region, northern Swedish Caledonides

The Middle Allochthon of the Scandinavian Caledonides represents the Neoproterozoic distal continental passive margin intruded by a dyke swarm with minor Mesoproterozoic and Paleoproterozoic orthogneiss. Locally, it carries early Neoproterozoic plutonic rocks. For this work, we collected geochronological and geochemical data and carried out thermodynamic modelling on a variety of lithologies from the Vássačorru Igneous Complex (VIC) and surrounding rocks of the Mårma terrane of the Seve Nappe Complex (SNC) in the Kebnekaise area, northern Swedish Caledonides.

U-Pb zircon LA-ICP-MS geochronology yielded crystallization ages of c. 864±3 Ma (MSWD=0.92; n=9) and 856±3 Ma (MSWD=2.8; n=10) for the Vistas Granite and a gabbro from the VIC, respectively. A granodioritic intrusion yielded an age of 850±1 Ma (MSWD=1.5; n=38), whereas a granitic dyke and mylonitic orthogneiss yielded ages of 840±7 Ma (MSWD=4.3; n=50) and 835±8 Ma (MSWD=0.71; n=24), respectively. Younger populations of zircon at c. 626–610 Ma were dated in a banded amphibolite and the Aurek gabbro. Rare earth element (REE) geochemistry from felsic lithologies in the VIC indicate lower crustal contamination, while the REE pattern for the VIC gabbro suggests an N-MORB affinity for light REE and enrichment in the heavy REE due to crustal assimilation. The banded amphibolite records pressure-temperature (P–T) conditions in the melt stability field at 10.5–12.0 kbar and 600–680 °C. The Aurek gabbro records high-pressure metamorphism at 11.8–12.6 kbar and 480–565 °C. Phase equilibrium modelling of the peak metamorphic assemblage in the mylonitic orthogneiss yielded 11.2–11.7 kbar and 560–610 °C, while the retrograde assemblage yielded 7.4–8.1 kbar and 615–675 °C. Furthermore, P–T estimates of 6.5–7.5 kbar at 600–625 °C were obtained for the Vistas Granite.

The geochronological data indicate that the Kebnekaise region experienced several magmatic pulses during the Neoproterozoic. These geochronological and geochemical data suggest that the magmatic event responsible for the emplacement of the VIC is related to an attempted break-up of Rodinia between c. 864–835 Ma. The ages obtained from banded amphibolite and the Aurek gabbro represent the emplacement of mafic protoliths during the real break-up at c. 626–610 Ma.

Two metamorphic ages were obtained: one, c. 598 Ma, from the banded amphibolite, is interpreted as the age of the high temperature metamorphism in the melt stability field. The second, c. 443 Ma, from the mylonitic orthogneiss, is interpreted as the age of the amphibolite facies metamorphic condition reached during the collisional stage. The age of the metamorphic peak was not detected. However, the P–T estimates for the mylonitic orthogneiss and the Aurek gabbro are comparable with the results from other lithologies within the Kebnekaise region and in the northern Seve Nappe Complex. For this reason, we hypothesize that the age of the metamorphic peak is at c. 490–480 Ma.

This research is funded by the National Science Centre (Poland) project no. 2019/33/B/ST10/01728 to Majka.