Exhumation of the High‐Pressure Tsäkkok Lens, Swedish Caledonides: Insights From the Structural and White Mica 40Ar/39Ar Geochronological Record

Abstract

Integrated structural, geochemical, and geochronological investigations were conducted on metasedimentary rocks in the eclogite-bearing Tsäkkok Lens of the Seve Nappe Complex (Scandinavian Caledonides) to resolve its exhumation history. Three deformation events are defined. D1 is likely related to the prograde to peak-metamorphic stages, represented by a locally preserved S1. D2 resulted in vertical shortening and is defined by a pervasive S2 and cm-/m-scale F2 closed folds. D2 terminated with Scandian thrusting, which emplaced the overlying Köli Nappe Complex. D3 records NE-SW shortening and constitutes m-/km-scale F3 open folds that deformed the Tsäkkok Lens and Köli Nappe Complex together. In situ white mica ⁴⁰Ar/³⁹Ar geochronology was conducted on select metasedimentary samples possessing S1 or S2 to resolve the timing of exhumation. Postdecompression cooling of the Tsäkkok Lens is best recorded by samples containing S1 or S2 that yield homogeneous white mica chemistry and ⁴⁰Ar/³⁹Ar dates. The timing of cooling is resolved to 477.2 ± 4.1 Ma (S1) and 475.3 ± 3.5 Ma (S2). Vertical shortening of the lens during exhumation may have proceeded until 458.1 ± 9.0 Ma. Later-stage deformation during Scandian thrusting penetrated the Tsäkkok Lens at 429.9 ± 9.0 Ma, or younger. This resulted in noncoaxial deformation of the metasedimentary rocks, producing heterogeneous white mica chemistry and partially reset the older ⁴⁰Ar/³⁹Ar cooling record. Temperatures for deformation are resolved to the upper greenschist‐lower amphibolite facies. Altogether, the Tsäkkok Lens records rapid exhumation from eclogite‐facies conditions to midcrustal depths or shallower, followed by emplacement of the overlying Köli Nappe Complex.