Eclogites and eclogites in the Western Gneiss Region, Norwegian Caledonides

The 40 Ar/ 39 Ar ages of metamorphic micas that crystallized at high temperatures are commonly interpreted as cooling ages, with grains considered to have lost 40 Ar via thermally-driven diffusion into the grain boundary network. Recently reported laser-ablation data suggest that the spatial distribution of Ar in metamorphic micas does not always conform to the patterns predicted by diffusion theory and that despite high metamorphic temperatures, argon was not removed efficiently from the local system during metamorphic evolution. In the Western Gneiss Region (WGR), Norway, felsic gneisses preserve microtextural evidence for the breakdown of phengite to biotite and plagioclase symplectites during near isothermal decompression from c. 20-25 to c. 8-12 kbar at~700°C. These samples provide an ideal natural laboratory to assess whether the complete replacement of one K-bearing mineral by another at high temperatures completely 'resets' the Ar clock, or whether there is some inheritance of 40 Ar in the neocrystallized phase. The timing of the high-temperature portion of the WGR metamorphic cycle has been well constrained in previous studies. However, the timing of cooling following the overprint is still much debated. In-situ laser ablation spot dating in phengite, biotite-plagioclase symplectites and coarser, texturally later biotite yielded 40 Ar/ 39Ar ages that span much of the metamorphic cycle. Together these data show that despite residence at temperatures of~700°C, Ar is not completely removed by diffusive loss or during metamorphic recrystallization. Instead, Ar released during phengite breakdown appears to be partially reincorporated into the newly crystallizing biotite and plagioclase (or is trapped in fluid inclusions in those phases) within a close system. Our data show that the microtextural and petrographic evolution of the sample being dated provides a critical framework in which local 40 Ar recycling can be tracked, thus potentially allowing 40 Ar/ 39 Ar dates to be linked more accurately to metamorphic history.

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“…The calculated HP conditions overlap with the data presented for the same area (e.g. Cuthbert et al, 2000;Wain, 1997).…”

Section: Metamorphic Evolutionsupporting

confidence: 49%

Recycling argon through metamorphic reactions: The record in symplectites

McDonald

Regis

Warren

et al. 2018

Lithos

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“…Andersen et al, 1991;Terry et al, 2000b). This is indicated by a continuous increase of Caledonian eclogite facies P,T conditions towards the NW (e.g., Krogh, 1977;Cuthbert et al, 2000), by the regional SE-NW trend of amphibolite facies muscovite Ar-Ar ages (reviewed in Hacker, 2007), and large-scale consistency of structural grain (Krabbendam and Dewey, 1998;Labrousse et al, 2004). A remarkable feature within the WGR is the spatially highly variable metamorphic grade of the prevalent lithologies.…”

Section: The Western Gneiss Regionmentioning

confidence: 98%

Diffusion versus recrystallization processes in Rb–Sr geochronology: Isotopic relics in eclogite facies rocks, Western Gneiss Region, Norway

Glodny

Kühn

Austrheim

2008

Geochimica et Cosmochimica Acta

106

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“…2) in the core of the orogen [see geothermometry reported below and summaries in Cuthbert et al, 2000;Ravna and Terry, 2004;Hacker, 2006;Spencer et al, 2013]. Ultrahigh-pressure (UHP) rocks with coesite, diamond, or equivalent metamorphic pressures [e.g., Smith, 1984;Vrijmoed et al, 2008] are restricted to three main domains in the hinterland (pink in Fig.…”

Section: Geologic Settingmentioning

confidence: 99%

Monazite response to ultrahigh-pressure subduction from U–Pb dating by laser ablation split stream

et al. 2015

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To assess the response of monazite during subduction of continental crust to mantle depths, U-Pb isotopic ratios and elemental abundances were measured simultaneously by laser-ablation split-stream inductively-coupled plasma mass spectrometry (LASS) in rocks from the ultrahigh-pressure Western Gneiss Region of the Scandinavian Caledonides. Nearly seventy different samples of quartzofeldspathic basement and overlying metasedimentary rocks were studied. Pre-subduction monazite (chiefly 1.6 Ga and 1.0 Ga) is preserved locally in the structurally lowest, basement rocks because earlier, Precambrian tectonism produced coarse-grained, high-grade rocks that were resistant to further recrystallization in spite of syn-subduction temperatures and pressures of 650-800°C and 2-3.5 GPa. A few of the monazite in the metasedimentary rocks atop the basement preserve syn-subduction U-Pb dates, but the majority continued to recrystallize during post-subduction exhumation and record a general westward decrease in age related to westward-progressing exhumation. The absence of Precambrian monazite in the metasedimentary rock atop the basement suggests that sedimentation postdated the 1.0-0.9 Ga high-grade metamorphism and was late Proterozoic to early Paleozoic.

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Eclogites and eclogites in the Western Gneiss Region, Norwegian Caledonides

Cited by 194 publications

References 71 publications

Recycling argon through metamorphic reactions: The record in symplectites

Recycling argon through metamorphic reactions: The record in symplectites

Diffusion versus recrystallization processes in Rb–Sr geochronology: Isotopic relics in eclogite facies rocks, Western Gneiss Region, Norway

Monazite response to ultrahigh-pressure subduction from U–Pb dating by laser ablation split stream

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