Relations between granitoid magmatism and migmatization:: U-Pb geochronological evidence from the Western Gneiss Complex, Norway

Gneiss and migmatite samples from the southern part of the Western Gneiss Complex of Norway define a single U-Pb zircon discordia line with lower and upper intercepts at 1633 +/- 8 Ma and 969 +/- 8 Ma. The upper intercept is interpreted as the magmatic age of the protoliths, showing that the basement rocks were generated during the Gothian orogeny (1750-1500 Ma). The migmatites yield titanite ages that are similar to the lower intercept zircon age, and we interpret this to be the age of migmatization during the Sveconorwegian orogeny. U-rich titanite cores yield an age of 973 +/- 3 Ma whereas less U-rich rims gave a slightly younger age of 961 +/- 4 Ma. A granitoid batholith in the area gave a U-Pb zircon age that is coeval with the migmatite age (966 +/- 2 Ma). The metaluminous granitic magma of the pluton formed by melting at relatively high temperatures in the lower crust. In contrast, the migmatites formed during upper amphibolite-facies conditions at relative high crustal levels. The close temporal and spatial relations between the migmatites and the granite batholith suggest that migmatite formation in the lower parts of the upper crust was related to the heat and mass flux linked to the rise of granitic magma from the lower crust.