FROM FAILED RIFTING TO CONTINENTAL COLLISION: THE WILSON CYCLE RECORDED IN THE MÅRMA TERRANE, NORTHERN SCANDINAVIAN CALEDONIDES

The Seve Nappe Complex (SNC) of the Scandinavian Caledonides, composed of the ancestral outermost margin of Baltica, is an excellent natural laboratory to investigate Wilson Cycle tectonics and re-activation of pre-existing crustal structures. The Mårma terrane is a key locality for studying the northern SNC tectonic record in the Wilson Cycle context. In part, it consists of metasedimentary rocks hosting a bimodal igneous suite (Vássačorru Igneous Complex; VIC) as well as solitary mafic bodies (e.g., the Aurek gabbro).

U-Pb zircon geochronology of felsic and mafic intrusive rocks from the VIC provided crystallization ages from c. 864–840 Ma. A mylonitic orthogneiss provided a crystallization age of c. 836 Ma, and is interpreted as equivalent to the VIC rocks. In contrast, samples from both pristine and metamorphosed Aurek gabbro and a migmatitic variety of the Vierručohkka amphibolite yielded zircon with crystallization ages from c. 615–600 Ma with the younger age interpreted as partial melting due to heating by Aurek gabbro emplacement.

Geochemistry of felsic VIC lithologies indicate S-type granites sourced from lower crustal rocks. The VIC gabbro has an N-MORB affinities with variable degrees of crustal assimilation.

Phase equilibrium modelling of the mylonitic orthogneiss suggests peak metamorphic conditions at c. 11.2–11.7 kbar and 560–610 °C, followed by retrograde metamorphic condition of c. 7.4–8.1 kbar and 615–675 °C. This metamorphic evolution is broadly resolved in time by U-Pb monazite geochronology to c. 443 Ma.

These new data reveal three tectonic events recorded by the Mårma terrane: 1) emplacement of the VIC, related to failed rifting of Rodinia between c. 864–835 Ma, 2) Successful rifting and opening of the Iapetus Ocean starting at c. 626–610 Ma, and 3) Localized metamorphism and deformation of the VIC rocks and the Aurek gabbro during the Caledonian orogeny in a subduction regime. This sequence of events demonstrates how continental breakup and ocean formation can nucleate on a pre-existing crustal structure (i.e., failed rift) that are later incorporated into the collisional zone of two continents.

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