RAPID COOLING DURING EXHUMATION OF ULTRAHIGH-PRESSURE TERRANES: U-PB AGES OF TITANITE AND RUTILE FROM THE WESTERN GNEISS REGION, WESTERN NORWAY

The Western Gneiss Region (WGR) of Norway includes one of Earth's giant ultrahigh-pressure (UHP) terranes. Understanding the subduction and exhumation of this >60,000 km² area is relevant to a range of processes, including collisional orogenesis, reworking of the continents, and the global geochemical cycle. Important aspects that remain unanswered include the spatial and temporal variations in the rate of exhumation and the extent of phase transformation-induced reworking during exhumation. To address this issue we have determined the ages of titanites and rutiles of leucosomes, eclogites and orthogneisses within the WGR.

Ages of titanites and rutiles define a discordia array with an upper intercept of either ~990 Ma or ~1.6 Ga, and show increased resetting from east to west with a lower intercept of between 398–383 Ma. Concordant lower intercept ages generally decrease toward the west, coincident with increasing peak temperatures and depths within the slab. This correlation implies progressive east-to-west exhumation. Titanites from leucosomes are similar in age to surrounding gneiss titanites, implying that they were either reset or that decompression melting occurred within a few million years of cooling through titanite closure (~600ºC). Muscovite cooling ages (~400ºC) are within a few million years of these regional titanite cooling ages, indicating that cooling, which occurred at shallow crustal levels, was very rapid (~100ºC/m.y.).