Initial tectonothermal evolution within the Scandinavian Caledonide Accretionary Prism: constraints from ⁴⁰Ar/³⁹Ar mineral ages within the Seve Nappe Complex, Sarek Mountains, Sweden

Abstract

In the Caledonide orogen of northern Sweden, the Seve Nappe Complex is dominated by rift facies sedimentary and mafic rocks derived from the Late Proterozoic Baltoscandian miogeocline and offshore-continent-Iapetus transition. Metamorphic breaks and structural inversions characterize the nappe complex. Within the Sarek Mountains, the Sarektjikki Nappe is composed of c. 600-Ma-old dolerites with subordinate screens of sedimentary rocks. These lithological elements preserve parageneses which record contact metamorphism at shallow crustal levels. The Sarektjlkki Nappe is situated between eclogite-bearing nappes (Mikka and Tsakkok nappes) which underwent high-P metamorphism at c. 500Ma during westward subduction of the Baltoscandian margin. 40Ar/39Ar mineral ages of c. 520-500 Ma are recorded by hornblende within variably foliated amphibolite derived from mafic dyke protoliths within the Sarektjikki Nappe. Plateau ages of 500 Ma are displayed by muscovite within the basal thrust of the nappe and are consistent with metamorphic evidence which indicates that the nappe escaped crustal depression as a result of detachment at an early stage of subduction. Cooling ages recorded by hornblende from variably retrogressed eclogites in the entire region are in the range of c. 510-490 Ma and suggest that imbrication of the subducting miogeocline was followed by differential exhumation of the various imbricate sheets. Hornblende cooling ages of 470-460 Ma are recorded from massive dyke protoliths within the Sarektjlkkl Nappe. These are similar to ages reported from the Seve Nappe Complex in the central Scandinavian Caledonides. Probably these date imbrication and uplift related to Early Ordovician arrival of outboard terranes (e.g. island-arc sequences represented by structurally lower horizons of the Koli Nappes).Metamorphic contrasts and the distinct grouping of mineral cooling ages suggest that the various Seve structural units are themselves internally imbricated, and were individually tectonically uplifted through argon closure temperatures during assembly of the Seve Nappe Complex. The cooling ages of 520-500 Ma recorded within Seve terranes and along terrane boundaries of the Sarek Mountains provide evidence of significant accretionary activity in the northern Scandinavian Caledonides in the Late Cambrian-Early Ordovician.

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

“…The results in this study follow a common pattern in the Scandinavian Caledonides: mica ages are well defined around 425-430 Ma, whereas amphibole yields older and less well-defined ages (see, for example, Dallmeyer, 1988;Dallmeyer, Andréasson & Svenningsen, 1991). However, the younger ages presented here derive from undeformed rocks which are known to be at least 180 Ma older than the obtained Ar dates.…”

“…The interior of the nappe shows no signs of having experienced the high-pressure metamorphism that the over-and underlying units went through, and it is not evident that they were juxtaposed at the time when eclogite facies conditions were prevailing in the Mikka/Tsäkkok and Vaimok nappes, which occurred at 503 ± 18 and 503 ± 14 Ma respectively (Mørk, Kullerud & Stabel, 1988). The oldest metamorphic ages recorded in the Sarektjåkkå Nappe are the 464 ± 4 to 469 ± 5 Ma hornblende Ar ages from its marginal parts (Dallmeyer, Andréasson & Svenningsen, 1991). What then, caused the degassing of the structurally intact metasedimentary rocks at Favorithällen?…”

“…4). All yield 36 Ar/ 40 (Dallmeyer, 1990;Dallmeyer, Andréasson & Svenningsen, 1991;Dallmeyer, Gee & Beckholmen, 1985;Dallmeyer & Gee, 1986;Dallmeyer & Gee, 1988;Dallmeyer & Stephens, 1991;Page, 1992). (b) shows the same data filtered using the following criteria: plateau age (Fleck, Sutter & Elliot, 1977), isochron age not deviating by more than ± 20 Ma from the plateau age, and an MSWD for the inverse isochron ≤ 2.5; all samples not meeting these criteria have been excluded.…”

“…The last main episode of thrusting in this part of the Caledonides occurred around 430-420 Ma, as indicated by the lack of any radiometric ages or any other evidence for events younger than c. 430-420 Ma in this eastern part of the orogen (Dallmeyer, 1990;Dallmeyer, Andréasson & Svenningsen, 1991;Dallmeyer, Gee & Beckholmen, 1985;Dallmeyer & Gee, 1986Dallmeyer & Stephens, 1991;Gee, 1978;Page, 1992;Roberts & Gee, 1985). Thus the ages obtained from Favorithällen must somehow be related to this event.…”

“…For sample RF93-HB, 36 Ar is below the detection limit for most heating steps, making isochron plotting undoable. and hornblende separates from the basal Sarektjåkkå Nappe at one locality yielded ages differing by 28 Ma, with the muscovite giving the older age, even though hornblende's closure temperature is ~200°C higher than muscovite's (Dallmeyer, Andréasson & Svenningsen, 1991 (1991)). It is therefore argued that the technically poorer hornblende age in fact provides a better indication of the cooling history of the rocks than the muscovite age, which should be interpreted with utmost caution until confirmed or rejected by more work.…”