Adding Rates to P-T Paths: High-Precision CA-TIMS from the Giant Norwegian Ultrahigh-Pressure Terrane

The most common methods for dating zircon from metamorphic rocks are microbeam techniques (SIMS and LA-ICPMS) and single-grain TIMS. Microbeam techniques have the advantage of high spatial precision, an advantage when investigating zoned grains, but lack the analytical precision required to distinguish between different stages of a metamorphic cycle. Single-grain TIMS has the advantage of high analytical precision but requires much larger portions of zircon, which requires the physical separation of metamorphic grains from inherited grains. An alternative technique is chemical-abrasion (CA) TIMS. It is capable of resolving ages with precision as good or better than single-grain TIMS ages, and has the further advantage of stepwise dissolution; each step yields a different portion of the zircon grains, which may result in sufficient spread along a discordia array to yield a high-precision lower intercept (i.e., metamorphic age).

Here we present CA-TIMS data from the Western Gneiss Region ultrahigh-pressure terrane, Norway, which was subducted to and exhumed from depths of ≥150 km. The entire cycle lasted ~40 Myr, from ~420–380 Ma; thus, our ability to precisely pinpoint individual stages within this cycle will yield rates of burial, exhumation, cooling, etc., and increase our understanding of the evolution of ultrahigh-pressure terranes in general.

An undeformed, garnet-bearing aplite dike from within the Blåhø nappe near Volda yields a lower intercept age of 402.3 ± 2.0 Ma from 4 CA-TIMS steps (MSWD=0.85). Decreasing Th-U ratio closer to the lower intercept is consistent with metamorphic growth, and indicates no post-burial, amphibolite-facies deformation. An undeformed pegmatite dike from the Sørøyane UHP domain yielded strongly inherited zircons, which display a lower intercept of 398.9 ± 4.6 Ma (MSWD=0.88) in 4 steps; all of these steps have high Th-U ratios, indicating that amphibolite-facies melting followed quickly on the heels of the peak metamorphism.