Insights into (U)HP metamorphism of the Western Gneiss Region, Norway: A high-spatial resolution and high-precision zircon study

Abstract

Combining high-spatial resolution and high-precision geochronology and geochemistry of zircon provides constraints on the timing and duration of ultrahigh-pressure (UHP) metamorphism resulting from the collision of Baltica–Avalonia and Laurentia during the Scandian orogeny in the Western Gneiss Region of Norway. Zircons were extracted from a layered eclogite in the Saltaneset region (southern UHP domain) and from an eclogite in the Ulsteinvik region (central UHP domain). Zircons were first analyzed for U–Pb and trace element compositions by laser ablation split-stream (LASS) inductively coupled plasma mass spectrometry (ICP-MS), followed by analysis of those same zircons that yielded Scandian dates by integrated U–Pb isotope dilution–thermal ionization mass spectrometry and Trace Element Analysis (TIMS–TEA). LASS results from a garnet–quartz layer within the Saltaneset eclogite give Scandian dates of ca. 413–397 Ma, with subsequent ID–TIMS analyses ranging from 408.9 ± 0.4 Ma to 401.4 ± 0.2 Ma (2σ). An omphacite-rich layer from the same eclogite yields LASS dates of ca. 414–398 Ma and a single ID–TIMS date of 396.7 ± 1.4 Ma. In comparison, the Ulsteinvik eclogite LASS results give dates spanning ca. 413–397 Ma, whereas ID–TIMS analyses range from 409.6 ± 0.6 Ma to 401.3 ± 0.4 Ma. ID–TIMS zircon data from the eclogites reveals two age populations: 1) ca. 409–407 Ma and 2) ca. 402 Ma. Both in situ and solution trace element data show a distinct pattern for Scandian zircons, with strongly-depleted HREE and weakly-negative Eu anomalies (Eu/Eu*), whereas inherited zircon REE patterns are distinguished by steep HREE slopes and marked negative Eu/Eu*. When coupled with partition coefficients calculated for zircon and garnet, these REE patterns indicate that zircon (re)crystallized during eclogite-facies metamorphism at ca. 409–407 Ma and ca. 402 Ma at two widely separated UHP localities.