U–Pb zircon geochronology of eclogites from the Scandian Orogen, northern Western Gneiss Region, Norway: 14–20 million years between eclogite crystallization and return to amphibolite-facies conditionsThis article is one of a series of papers published in this Special Issue on the theme of <i>Geochronology</i> in honour of Tom Krogh.

Krogh, T. E.; Kamo, Sandra L.; Robinson, Peter; Terry, Michael P.; Kwok, K.

doi:10.1139/e10-076

Cited by 71 publications

(126 citation statements)

References 56 publications

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“…Like most well-exposed and extensively investigated UHP terranes, the WGR is dominated by quartzofeldspathic gneiss; eclogite and (U)HP peridotite comprise only a few volume percent of the Region. The quartzofeldspathic gneiss and eclogite protoliths formed chiefly by magmatism at~1.68-1.64 Ga, 1.29-1.24 Ga, and~990-950 Ma [Corfu and Andersen, 2002;Walsh et al, 2007;Krogh et al, 2011;Corfu et al, 2013]; regional metamorphism [Roffeis and Corfu, 2014] associated with the~1.0-0.9 Ga magmatism reached temperatures of~775-825°C [Spencer et al, 2013]. During the Caledonian orogeny, subductionrelated magmatism related to closure of the Iapetus ocean ended at 430 Ma [Corfu et al, 2006], marginal basin ophiolites and island arcs were emplaced onto the Baltica craton in the Wenlock [Andersen et al, 1990], and the previously hyperextended continental margin [Andersen et al, 2012] began to contract.…”

Section: Geologic Settingmentioning

confidence: 99%

“…During the Caledonian orogeny, subductionrelated magmatism related to closure of the Iapetus ocean ended at 430 Ma [Corfu et al, 2006], marginal basin ophiolites and island arcs were emplaced onto the Baltica craton in the Wenlock [Andersen et al, 1990], and the previously hyperextended continental margin [Andersen et al, 2012] began to contract. The combined thrust stack was subducted, and eclogites [Krogh, 1977;Gee, 1980;Andersen et al, 1991;Hacker and Gans, 2005] and (U)HP peridotites formed between~425 and~405 Ma [Glodny et al, 2008;Kylander-Clark et al, 2009;Krogh et al, 2011;Beckman et al, 2014;Kylander-Clark and Hacker, 2014].…”

Section: Geologic Settingmentioning

confidence: 99%

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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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Section: Geologic Settingmentioning

confidence: 99%

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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“…Ar age patterns, as the titanite U\ \Pb age has been used to constrain the amphibolite-facies recrystallization across the WGR (Krogh et al, 2011;Schärer and Labrousse, 2003;Spencer et al, 2013;Tucker et al, 1987). The conditions of titanite crystallization are controlled primarily by the bulk rock Ca/Al ratio (Frost et al, 2001).…”

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confidence: 99%

Recycling argon through metamorphic reactions: The record in symplectites

McDonald

Regis

Warren

et al. 2018

Lithos

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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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“…This episode continued into the period of orogen-parallel extension superimposed on that detachment (Terry & Robinson, 2003;Robinson et al, 2004Robinson et al, , 2014Osmundsen et al, 2006). The last part of this episode is confidently dated in the northern Western Gneiss Region at ~395 Ma, or late Emsian, in the Early Devonian Krogh et al, 2011).…”

Section: Regional Metamorphic Timingmentioning

confidence: 85%

“…Silurian and earliest Devonian Scandian collision between Laurentia and Baltica (Fig. 13A), with subduction of parts of the Baltican margin, eclogite formation (Krogh et al, 2011), and nappe emplacement. Evidence in the Upper Allochthon for this in the area of Fig.…”

Section: Regional Metamorphic Timingmentioning

confidence: 99%

Metamorphosed cumulate gabbros from the Støren Group of the Upper Allochthon, northern Western Gneiss Region, Norway: petrology and metamorphic record

Hollocher¹,

Robinson²,

Kennedy³

et al. 2015

NJG

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The Støren Group of the Upper Allochthon, in the Scandinavian Caledonides, contains Late Cambrian to Early Ordovician ophiolitic fragments that developed in a tholeiitic oceanic suprasubduction zone setting. These were obducted onto an epicontinental margin (Gula Complex) in the Early Ordovician, and metamorphosed in most areas to greenschist facies. Later, these rocks were thrust onto Baltica as part of the Upper Allochthon during the Silurian-Devonian Scandian Orogeny. Cumulate gabbros in the Støren Group contain the prograde metamorphic assemblage amphibole-plagioclase-diopside-epidote. They have complex textures that include early metamorphic mineral cores and inclusions, later rims, and late vein assemblages. The textures and mineral compositions in nine samples were interpreted iteratively with Perple_X thermodynamic modeling, to reveal three sequential P-T conditions: ~350°C and ~6 kbars for early prograde mineral cores and inclusions, ~530°C and ~6 kbars for peak prograde conditions, and ~425°C and ~3 kbars for an early retrograde vein assemblage. Additionally, minor but ubiquitous, discontinuous Fe 3+ -rich rims on epidote suggest the rocks underwent isothermal decompression immediately after peak prograde conditions, crossing increasingly Fe-rich epidote isopleths predicted by models. We interpret a single-episode, clockwise P-T path representing an Early Devonian phase of Scandian metamorphism. In the study area, prograde metamorphism produced these assemblages only in rocks close to the contact between the Upper and Middle allochthons, which is interpreted as a late-Scandian detachment fault (Agdenes detachment). Prograde Scandian metamorphism of this part of the Upper Allochthon was a form of contact metamorphism, occurring as the detachment fault juxtaposed cool Upper Allochthon rocks against the hotter, exhuming Middle Allochthon.

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Cited by 71 publications

References 56 publications

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

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

Recycling argon through metamorphic reactions: The record in symplectites

Metamorphosed cumulate gabbros from the Støren Group of the Upper Allochthon, northern Western Gneiss Region, Norway: petrology and metamorphic record

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