Thermal history of thrust sheets in an orogenic wedge: ⁴⁰Ar/³⁹Ar data from the polymetamorphic Seve Nappe Complex, northern Swedish Caledonides

Abstract

The Seve Nappe Complex in the Scandinavian Caledonides contains the fragmented late Precambrian continent-ocean transition between Baltica and the Iapetus Ocean. This passive margin was fragmented and thrust eastwards over the Baltic Shield during Caledonian orogenesis. The individual thrust sheets in the Seve Nappe Complex went through different P-T-t evolutions, resulting in dramatic metamorphic contrasts: eclogite-bearing nappes are juxtaposed with nappes showing no evidence of Caledonian deformation or metamorphism in their interiors. Strain localization to the marginal parts of the thrust sheets left records of both pre-orogenic (rift) and early orogenic (subduction and subsequent uplift) processes in the thrust sheets of the Seve Nappe Complex. Even though it has been transported several hundred kilometres, only the margins of the eastern part of the Sarektjåkkå Nappe are affected by penetrative Caledonian deformation. This part of the Sarektjåkkå Nappe is dominated by pristine tholeiitic dykes and cross-bedded sandstones. The dykes are 608 ± 1 Ma old and make up 70-80 % of the nappe. Widely spaced thin shear zones of the Ruopsok fault system represent the only Caledonian penetrative deformation in the interior of the nappe. Previously published Ar-Ar dates indicate cooling below the closure temperature of hornblende at c. 470 Ma, but numerous ages have been recorded. Ar dating of biotite and muscovite from a cross-laminated metapsammite in the Sarektjåkkå Nappe gave well-defined ages of 428.5 ± 3.6 and 432.4 ± 3.8 Ma, respectively. Muscovite from a shear zone in the Ruopsok Fault System gave 428.2 ± 4.0 Ma, whereas hornblende from the same locality did not yield interpretable data. The results indicate that these rocks were completely degassed at some unknown time, presumably at the emplacement of the dyke swarm. No subsequent excess argon contamination can be detected. A likely candidate for the degassing event is the emplacement of the dykes at 608 Ma. The interior of the nappe, and thus the entire nappe complex, cooled below ~350°C at around 430 Ma. Cooling from more than 500°C at c. 470 Ma to 350°C at c. 430 Ma suggests an average cooling rate of ≤ 4°C/Ma. A prolonged period of slow cooling (≈exhumation?) following the initial, rapid uplift of the eclogitebearing nappes and Early Ordovician construction of the Seve Nappe Complex is suggested. *

Continental collisions and the creation of ultrahigh-pressure terranes: Petrology and thermochronology of nappes in the central Scandinavian Caledonides

“…Rather old hornblende ages of 500 Ma to 448 Ma come from the Seve Nappes in the Norrbotten (n ≈ 15; north of Fig. 1), Tømmerås, and Åre areas (n = 4), with a cluster in the 469-463 Ma time range (Dallmeyer et al, 1985;Dallmeyer and Gee, 1986;Dallmeyer, 1990;Dallmeyer and Stephens, 1991;Page, 1992;Svenningsen, 2000). This range of older hornblende ages suggests that a major amphibolite-facies metamorphism in the Seve Nappes ended by ca.…”

“…Muscovite 40 Ar/ 39 Ar ages from the Seve Nappes and Middle Allochthon in Norrbotten are ~35 m.y. younger than the hornblende ages, at 444-425 Ma (most are 434-425 Ma) (Dallmeyer and Gee, 1986;Dallmeyer and Stephens, 1991;Page, 1992;Svenningsen, 2000), indicating slow cooling rates of ~3 °C/m.y.…”

Root zone of a continental rift: the Neoproterozoic Kebnekaise Intrusive Complex, northern Swedish Caledonides

“…482 Ma (Root and Corfu, 2012) and ca. 430 Ma (Svenningsen, 2000), respectively. Although evidence of a metamorphic overprint of the KIC above greenschist grade is absent in the Tarfala Valley, radiogenic lead loss due to Caledonian orogenesis, which would results in a shift of SIMS age analyses down the concordia, cannot be discounted.…”

Preservation of a magnetization carried by magnetite-inclusions during amphibolite grade metamorphic conditions? An example from the Sarek Dykeswarm, Northern Swedish Caledonides

“…Comparison of data from different dykes in the complex here offers an approach when discussing ages of the different components. Muscovite yielded an 39 Ar– 40 Ar age of 430 Myr (Svenningsen 2000), and thus both intrusions (U–Pb) and metamorphic events (Ar–Ar) are well dated. This article discusses the above problem on behalf of palaeomagnetic data from that outcrop.…”

“… (a) Geological map of The Favorithällen Outcrop (modified after Svenningsen 2000). Sampling points were randomly spread within the shaded areas.…”