Caledonian metamorphism of Proterozoic Seve rocks on Mt. Åreskutan, southern Swedish Caledonides
Titel:
Caledonian metamorphism of Proterozoic Seve rocks on Mt. Åreskutan, southern Swedish Caledonides
Auteur:
Claesson, Stefan
Verschenen in:
GFF
Paginering:
Jaargang 103 (1982) nr. 3 pagina's 291-304
Jaar:
1982-01-15
Inhoud:
Åreskutan is a type locality for the complex of Seve nappes, which covers vast areas in the Swedish Caledonides. Here, an upper tectonic unit, the Åreskutan Nappe, composed of migmatitic gneisses and metabasites metamorphosed in granulite facies, is separated by a thrust from a lower Seve unit. Various rocks in the Åreskutan Nappe have been investigated using Rb-Sr and U-Pb techniques. Conventional Rb-Sr whole-rock analyses fail to yield isochrons but show that the rocks are clearly Precambrian. Small-scale whole-rock samples from the neosome part of a 2 kg migmatized metabasite specimen define an isochron with the age 414 ± 27 Ma, while paleosome samples from the same specimen scatter about this isochron. The age is considered to date the migmatization. A late, cross-cutting pegmatite gives an age of 395 ± 40 Ma, and minerals separated from a gneissic granite give 390 ± 45 Ma. Zircon from gneissic granite and from metabasite give a discordia line with lower and upper intercept ages of 386 ± 12 and 1570 ± 25 Ma, respectively. All zircons plot closer to the lower than to the upper intercept. The old Pb component present in the zircon is situated in cores, the overgrowth of which took place during the high-grade Caledonian metamorphism. This conclusion is supported by the result of a stepwise dissolving experiment carried out on a core-bearing zircon sample. The lower intercept age, which dates this overgrowth, is somewhat low due to later, minor Pb-loss. Monazite from garnet-sillimanite gneiss has almost concordant U-Pb ages conforming to a lower intercept age of 402 Ma.
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