Chronology of Caledonian high-pressure granulite-facies metamorphism, uplift, and deformation within northern parts of the Western Gneiss Region, Norway
Sm-Nd, U-Pb, and 40 Ar- 39 Ar age constraints are presented for the P-T and deformation history of rocks exposed in central Vestranden, Norway, where supracrustal sequences were infolded with Precambrian granitoid orthogneisses during associated Caledonian metamorphism up to at least upper-amphibolite-facies conditions. The peak of metamorphism reached high-pressure granulite facies in many sequences through-out west-central Vestranden. Interlayered orthogneisses and supracrustal rocks structurally overlie the Roan Window in western Vestranden. Geometric relations require tens of kilometers of thrust displacement. Rocks both within and above the window record maximum P-T conditions of ∼860 °C and 12-14.5 kbar, suggesting that large-scale thrusting occurred prior to and(or) during attainment of peak metamorphic conditions. A Sm-Nd isochron (opx-cpx-wr-gt) of 432 ± 6 Ma from a basic, high-pressure granulite within the window is interpreted to closely date attainment of peak metamorphic conditions. These sequences experienced subsequent, nearly isothermal uplift into medium-to low-pressure granulite- and amphibolite-facies conditions. The time of subsequent cooling is reflected by 40 Ar/ 39 Ar hornblende ages. Six hornblende concentrates from supracrustal rocks yield well-defined plateau ages ranging from 396 ± 2 to 406 ± 3 Ma. Seven hornblende ages from the orthogneiss complex range between 386 ± 6 and 413 ± 3 Ma. Six 40 Ar/ 39 Ar muscovite ages from orthogneisses and supracrustal rocks date later cooling through appropriate closure temperatures between 390 ± 2 and 395 ± 2 Ma. A U-Pb age of 398 ± 3 Ma for zircons from an undeformed granodioritic pegmatitic dike provides a lower age limit for the deformation recorded within orthogneiss and supracrustal complexes. The P-T evolution suggests that the rocks experienced initial rapid uplift with little cooling. This was followed by rapid cooling with minimal uplift. Small differences in the closure ages recorded by hornblende and muscovite are consistent with late rapid cooling. Tectonic unroofing likely was the main factor responsible for the rapid exhumation of Vestranden.