Abstract

40Ar/39Ar thermochronology is commonly used to constrain the rates and times of cooling in exhumed metamorphic terranes, with ages usually linked to temperature via Dodson's closure temperature (TC) formulation. Whilst many metamorphic 40Ar/39Ar data are consistent with the timing of crystallisation or cooling within a chronological framework defined by other, higher temperature, chronometers, other 40Ar/39Ar data are more difficult to interpret. We report white mica and biotite single grain fusion and laser ablation 40Ar/39Ar ages from felsic gneisses from the Western Gneiss Region, Norway. The rocks record isothermal decompression from peak eclogite-facies conditions (white mica stable) to amphibolite-facies conditions (biotite stable) at c. 700°C. White mica and biotite yield dispersed single grain fusion dates from 416 to 373Ma and 437 to 360Ma respectively. In-situ laser ablation analyses provide a similar range, with white mica spot ages ranging from 424 to 370Ma and biotite spot ages ranging from 437 to 370Ma. The dates span the duration of the metamorphic cycle suggested by previous studies, and cannot be reconciled with the results of simple models of Ar loss by diffusion during cooling. Samples that show evidence for different physical processes, such as the chemical breakdown of white mica, partial melting, and fluid ingress, generated different age populations to samples that did not experience or record obvious petrological evidence for these processes. Samples that record significant recrystallization and deformation yielded younger white mica (but older biotite) single grain fusion ages than more pristine samples. Amphibolite-facies gneisses that preserve evidence for significant partial melting generated younger biotite ages than samples that recorded evidence for significant hydration. Our data support other reported observations that high-temperature metamorphic mica 40Ar/39Ar dates cannot be assumed to record the timing of cooling through a specific temperature window. Careful assessment of the petrographic context of the dated minerals and consideration of their post-crystallisation history may provide a more robust insight into whether ‘age’ links to ‘stage’ in a temporally meaningful way.

Highlights

  • Understanding the timing of when, and the rate at which, metamorphic terranes are exhumed through the Earth's mantle and crust is important for constraining geodynamic models of tectonic processes. 40Ar/39Ar mica thermochronology is typically employed to constrain exhumation and cooling rates, with age most commonly being linked to temperature via the Dodson closure temperature (TC) formulation (Dodson, 1973)

  • The older biotite age population in more hydrated samples compared to drier samples suggests that fluid released during white mica breakdown acted as a source for ‘excess’ 40Ar in the biotite and that the fluid had limited mobility

  • Felsic gneisses with similar bulk compositions from the Outer Nordfjord area of the ultrahigh-pressure metamorphism (UHP) Western Gneiss Region, Norway, preserve textural and petrological evidence of different stages of their burial and exhumation history. 40Ar/39Ar data show that Ar is incorporated into, hosted by, and lost by white mica and biotite differently in these different rock types despite their shared metamorphic history

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Summary

Introduction

Understanding the timing of when, and the rate at which, metamorphic terranes are exhumed through the Earth's mantle and crust is important for constraining geodynamic models of tectonic processes. 40Ar/39Ar mica thermochronology is typically employed to constrain exhumation and cooling rates, with age most commonly being linked to temperature via the Dodson closure temperature (TC) formulation (Dodson, 1973). This solution to the diffusion equation is only applicable for geological applications under the following boundary conditions: (1) negligible initial lattice-hosted 40Ar during crystallisation We track the incorporation, release, and transport of Ar within and between different minerals during a metamorphic cycle, and especially during the exhumation-related, retrograde metamorphic reactions The study of such processes informs the assessment of the main mechanism(s) for redistributing Ar within minerals, and provides example cases for which 40Ar/39Ar may or may not be reliably linked to temperature via the Dodson TC formulation. Instead the 40Ar concentrations appear to trace other physical processes that affected the rocks during exhumation

Regional geology
Study sites
Petrology and mineral chemistry
Group 1
Group 2
Diffusion modelling
White mica models
Biotite models
New single grain fusion and in-situ data
Findings
Comparison with previously published age data
Conclusions
Full Text
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