T82 Unravelling Timescales of Magmatism, Metamorphism, and Crustal Evolution I

Monday, 16 October 2023
8:00 AM - 12:00 PM
327

LEVERAGING SHEAR ZONE-HOSTED TITANITE, RUTILE, AND QUARTZ TO EXTRACT A DETAILED EXHUMATION HISTORY OF THE NORDØYANE UHP DOMAIN, WESTERN GNEISS REGION, NORWAY

Paper number: T82-10
Program: Technical Sessions
Time: 10:20 AM – 10:35 AM

Authors

Abstract

The Western Gneiss Region (WGR) of Norway is one of Earth’s largest and most extensively studied ultrahigh-pressure metamorphic (UHP) terranes. Previous work in the WGR delineates peak pressures attained during Caledonian subduction of Baltica beneath the Laurentian continental margin, the multistage exhumation of Baltica basement, and a detailed record of Caledonian titanite growth and recrystallization of inherited (pre-UHP) titanite grains. However, the relationship between deformation, P-T conditions, and complex U-Pb titanite dates and compositions is not well understood.

To investigate the connection between titanite petrochronology and the P-T-d record of deformation, we sampled a lithologically diverse, meters-scale shear zone on Otrøya, within the deepest exposed UHP domain in the WGR (reported pressures of 3.2–3.6 GPa and temperatures of ~850 °C). Evidence for syn-exhumation deformation is found in variably retrogressed and variably deformed eclogites and gneisses. Application of tetravalent cation thermobarometry, outcrop-scale mapping, and U/Pb-geo/thermochronology indicate an early fabric in retrogressed eclogite that formed at ~825°C and ~2.0 GPa at ~402 Ma. At ~392 Ma, early titanite growth was associated with partial melting of eclogite at ~740 °C and ~1.3 GPa. Younger, ~382–375 Ma dates from zircon, monazite, and titanite correspond to recrystallization accompanied by constrictional strain at ~ 740°C and ~1.0 GPa. More localized deformation overprints earlier constrictional fabrics, and the rocks impacted by this late deformation contain quartz that yields Ti-in-quartz temperatures <600°C. The P-T-t-d results add context to the disparate Caledonian U-Pb titanite dates reported for rocks in the WGR and demonstrate the importance of trace elements for identifying titanite (re)crystallization events in apparently nondispersed U-Pb datasets. Furthermore, the detailed P-T-t-d path produced from Otrøya samples support a multi-stage exhumation history for the WGR.

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-395646

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