CHRONOLOGY OF CALEDONIAN HIGH-PRESSURE GRANULITE-FACIES METAMORPHISM,UPLIFT, AND DEFORMATION WITHIN NORTHERN PARTS OF THE WESTERN GNEISS REGION, NORWAY

Citation
Rd. Dallmeyer et al., CHRONOLOGY OF CALEDONIAN HIGH-PRESSURE GRANULITE-FACIES METAMORPHISM,UPLIFT, AND DEFORMATION WITHIN NORTHERN PARTS OF THE WESTERN GNEISS REGION, NORWAY, Geological Society of America bulletin, 104(4), 1992, pp. 444-455
Citations number
89
Language
INGLESE
art.tipo
Article
ISSN journal
0016-7606
Volume
104
Issue
4
Year of publication
1992
Pages
444 - 455
Database
ISI
SICI code
0016-7606(1992)104:4<444:COCHGM>2.0.ZU;2-C
Abstract
Sm-Nd, U-Pb, and Ar-40-Ar-39 age constraints are presented for the P-T and deformation history of rocks exposed in central Vestranden, Norwa y, where supracrustal sequences were infolded with Precambrian granito id orthogneisses during associated Caledonian metamorphism up to at le ast upper-amphibolite-facies conditions. The peak of metamorphism reac hed high-pressure granulite facies in many sequences throughout west-c entral Vestranden. Interlayered orthogneisses and supracrustal rocks s tructurally overlie the Roan Window in western Vestranden. Geometric r elations require tens of kilometers of thrust displacement. Rocks both within and above the window record maximum P-T conditions of approxim ately 860-degrees-C and 12-14.5 kbar, suggesting that large-scale thru sting 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 ba sic, high-pressure granulite within the window is interpreted to close ly date attainment of peak metamorphic conditions. These sequences exp erienced subsequent, nearly isothermal uplift into medium-to low-press ure granulite- and amphibolite-facies conditions. The time of subseque nt cooling is reflected by Ar-40/Ar-39 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 Ar- 40/Ar-39 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 u ndeformed granodioritic pegmatitic dike provides a lower age limit for the deformation recorded within orthogneiss and supracrustal complexe s. The P-T evolution suggests that the rocks experienced initial rapid uplift with little cooling. This was followed by rapid cooling with m inimal uplift. Small differences in the closure ages recorded by hornb lende and muscovite are consistent with late rapid cooling. Tectonic u nroofing likely was the main factor responsible for the rapid exhumati on of Vestranden.