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.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology
- Isotope dilution-thermal ionization mass spectrometry
- Laser ablation split-stream
- Ultrahigh-pressure metamorphism