Coupled Lu–Hf and Sm–Nd geochronology constrains prograde and exhumation histories of high- and ultrahigh-pressure eclogites from western Norway

Abstract

Eclogites from the Western Gneiss Region (WGR), Norway, that formed over a wide range in temperature (∼ 600 °C to > 850 °C) and pressure (high- and ultrahigh-pressure conditions; UHP) have a diverse range of Lu–Hf and Sm–Nd ages that correlate with their P–T histories. Three eclogites (∼ 676–850 °C and ∼ 2.0–2.85 GPa) yield Lu–Hf ages of 419.5 ± 4.3 Ma, 416.3 ± 3.7 and 412.0 ± 4.7 Ma, and two Sm–Nd ages of 402.7 ± 4.6 and 398.3 ± 5.5 Ma. The former are interpreted to date early garnet growth and the latter are interpreted to date near-peak metamorphism. These results support previous proposals that, for eclogites which recrystallized below Lu–Hf and Sm–Nd closure temperatures, the differences in ages for the two isotopic systems constrain the period of prograde garnet growth. In the case of the WGR, garnet growth is estimated to have occurred over a period of at least 20 m.y., beginning at ∼ 425–420 Ma. In contrast, two UHP eclogites (> 800 °C and > 3 GPa) yield significantly younger and overlapping Lu–Hf and Sm–Nd ages of 390–370 Ma; these ages are significantly younger than ages that are generally accepted for peak eclogite-facies conditions in the WGR, although they overlap 40Ar/ 39Ar ages that have been previously determined on white mica from the same samples or localities, indicating that the Lu–Hf and Sm–Nd ages may reflect rapid exhumation and cooling during the latest stages of HP and UHP metamorphism in the WGR.