Monazite geochronology of UHP and HP metamorphism, deformation, and exhumation, Nordøyane, Western Gneiss Region, Norway

Abstract

U-Th-Pb monazite geochronology is combined with previous structural analysis and quantitative estimates of metamorphic conditions to date the thermotectonic evolution of UHP and HP plates (820 °C, 39 kbar; 780 °C, 18 kbar) metamorphosed during the Late Silurian-Early Devonian collision between Baltica and Laurentia. The Upper Plate contains a microdiamond-bearing, kyanitegarnet- graphite gneiss and associated kyanite eclogites, independently indicating probable diamond-forming conditions. In situ dating of monazite in the microdiamond sample, using the SHRIMP II at the Geological Survey of Canada, yielded ages of 415 ± 6.8 Ma for those included in garnet and 398 ± 6 Ma for those in the matrix. These ages compare to 408.0 ± 5.6 and 397.5 ± 4.4 Ma determined using the electron microprobe at the University of Massachusetts. Both methods also identified complexly zoned detrital cores up to 150 micrometers in diameter with ages of 1100-950 Ma and scattered grains with ages of 900-500 Ma, but no ages of 1680-1650 Ma equivalent to the local Baltica basement were found. Agreement between the two techniques allowed evaluation of monazite age domains (198 analyses) from the microdiamond rock and a kyanite-garnet-sillimanite mylonite produced from it, using a combination of high-resolution element imaging and trace-element analysis of U, Th, Pb, and Y. This comparison yielded three mean ages of 407.0 ± 2.1 Ma, 394.8 ± 2.3 Ma, and 374.6 ± 2.7 Ma.