Insights into (U)HP metamorphism of the Western Gneiss Region, Norway: A high-spatial resolution and high-precision zircon study
Introduction
The Western Gneiss Region (WGR) of western Norway is one of the largest and best-exposed ultrahigh-pressure (UHP) terranes on Earth. Because of this, the WGR has been extensively studied to better understand the geodynamics of subduction and subsequent exhumation of 30,000 km2 (5000 km2 of which are UHP rocks) of continental crust (e.g., Krogh et al., 1974, Krogh et al., 2011, Krogh, 1982, Krogh, 1977, Krogh, 1982, Lappin and Smith, 1978, Griffin and Brueckner, 1980, Griffin and Brueckner, 1985, Austrheim, 1987, Tucker et al., 1990, Andersen et al., 1991, Wain, 1997, Cuthbert et al., 2000, Terry et al., 2000a, Terry et al., 2000b, Wain et al., 2000, Root et al., 2005, Hacker, 2007, Kylander-Clark et al., 2009, Hacker et al., 2010). Since the initial discovery of a coesite–eclogite province in the southern WGR (Smith, 1984, Smith, 1988), thermobarometry and identification of microdiamonds, coesite, and polycrystalline quartz grains within eclogite and quartzofeldspathic gneiss have aided in the recognition of three separate UHP domains (Root et al., 2005): the southern (Nordfjord), central (Sørøyane), and northern (Nordøyane) domains (Fig. 1a) (e.g., Dobrzhinetskaya et al., 1995, Wain, 1997, Cuthbert et al., 2000, Terry et al., 2000a, Wain et al., 2000, Carswell and Cuthbert, 2003, Carswell et al., 2003a, Walsh and Hacker, 2004, Young et al., 2007, Butler et al., 2013, Smith and Godard, 2013).
A general increase in peak UHP pressure and temperature (P–T) to the northwest across the WGR points to the coherent nature of the terrane during subduction and exhumation from mantle depths (e.g., Krogh, 1977, Lappin and Smith, 1978, Griffin et al., 1985, Andersen et al., 1991, Cuthbert et al., 2000, Carswell and Cuthbert, 2003, Carswell et al., 2006, Hacker et al., 2010). Characterizing the processes involved in the deep subduction and exhumation of such a large tract of continental crust requires a detailed understanding of the timescales of peak UHP metamorphism (e.g., Terry et al., 2000b, Carswell et al., 2003a, Carswell et al., 2003b, Root et al., 2004, Kylander-Clark et al., 2007, Kylander-Clark et al., 2009, Krogh et al., 2011).
Some (U)HP terranes were likely at mantle depths for tens of millions of years prior to exhumation (Kylander-Clark et al., 2012). This was first recognized in the Dabie Shan of China (Hacker et al., 1998), and then in the WGR of Norway (Kylander-Clark et al., 2008), where geochronological data suggests (U)HP metamorphism from ca. 430–400 Ma (Table 1; Section 2.2). Previous efforts to determine the timing of (U)HP metamorphism in the WGR have relied on techniques that analyze minerals that can be directly linked to the metamorphic evolution of eclogites (e.g., garnet and clinopyroxene) as well as more refractory accessory minerals (e.g., zircon and monazite) (Table 1). However, limitations in some previous geochronological studies of the WGR include: (1) data sets consisting of relatively few analyses (e.g., two-point isochrons); (2) dating of zoned garnet, for which isotopic ages may be averaging dates from multiple, distinct growth zones; (3) dating of multi-grain separates by U–Pb ID–TIMS that may result in inaccurate and/or mixed ages; and (4) ambiguities in how the dated minerals relate to the (U)HP metamorphism. This study builds upon these previous efforts by obtaining U–Pb dates from the same zircon domains via both laser ablation–inductively coupled plasma mass spectrometry (LA–ICP-MS) and from single-grain chemical abrasion–isotope dilution–thermal ionization mass spectrometry (ID–TIMS)—combining a high-spatial resolution technique and a high-precision technique on the same zircon.
In order to interpret geochronological data obtained from eclogites, it is crucial to link dates to different parts of the P–T path. The trace element composition of zircon can be used as a tool in age interpretations, particularly when coupled with the trace element composition of coexisting garnet. Zircon that (re)crystallizes at high pressure will likely display a flat normalized heavy rare earth element (HREE) pattern (e.g., Lu/Gd ~ < 3), due to the presence of garnet. Moreover, high-pressure zircon may have a flat-to-positive Eu anomaly (e.g., Eu/Eu* > 0.75), indicating (re)crystallization when plagioclase was unstable (Hinton and Upton, 1991, Schaltegger et al., 1999, Hoskin and Ireland, 2000, Rubatto, 2002, Hoskin and Schaltegger, 2003, Rubatto and Hermann, 2003, Rubatto and Hermann, 2007a). In addition, empirically and experimentally determined REE partition coefficients allow assessment of equilibrium between zircon and garnet (e.g., Hinton and Upton, 1991, Van Westrenen et al., 1999, Rubatto, 2002, Whitehouse and Platt, 2003, Kelly and Harley, 2005, Harley and Kelly, 2007, Rubatto and Hermann, 2007b, Taylor et al., 2014), an additional test to determine whether zircon (re)crystallized in the presence of garnet.
This study presents new U–Pb zircon dates from two coesite- and polycrystalline quartz-bearing eclogites to further evaluate the timescales of UHP metamorphism within the southern and central WGR. Trace element analyses of zircon provide insight into the P–T conditions under which zircon (re)crystallization occurred. The results reveal two separate populations of zircon that (re)crystallized under eclogite-facies conditions at ca. 409–407 Ma and ca. 402 Ma within the Ulsteinvik eclogite of the central UHP domain and the Saltaneset eclogite of the southern UHP domain. These results suggest a UHP metamorphic history for the Ulsteinvik eclogite older than the previously recognized 401.6 ± 1.6 Ma age (multi-grain zircon; Carswell et al., 2003a, Tucker et al., 2004), and a UHP history for the Saltaneset eclogite younger than the previously measured 408.3 ± 6.7 Ma age (Sm–Nd isochron; Carswell et al., 2003b).
Section snippets
Western Gneiss Region
The autochthonous basement of the WGR, the Western Gneiss Complex (WGC) (Fig. 1), is a polymetamorphic terrane composed mainly of granodioritic–tonalitic intrusive rocks predominantly formed between ca. 1690–1620 Ma (Brueckner, 1972, Carswell and Harvey, 1985, Tucker et al., 1990, Skår et al., 1994, Skår, 2000, Austrheim et al., 2003, Corfu et al., 2013) during the Gothian orogeny (Gáal and Gorbatschev, 1987). The WGC was intruded by mafic magmas at ca. 1470–1450 Ma and ca. 1260–1250 Ma (Austrheim
Methods
Representative samples of the Ulsteinvik eclogite and the garnet–quartz- and omphacite-rich layers of the Saltaneset eclogite were collected. Zircon was extracted from the whole-rock Ulsteinvik sample, whereas individual garnet–quartz and omphacite-rich layers from the Saltaneset eclogite were separated and then separately crushed to extract zircon from each layer. Polished grain mounts were prepared and imaged by cathodoluminescence (CL) to reveal zoning (Fig. 2). This study utilized two
U–Pb zircon LASS and ID–TIMS geochronology
Zircons extracted from the Ulsteinvik eclogite within the central UHP domain are mostly irregular to sub-rounded, and CL images reveal patchy- and polygonal-sector zoning (Fig. 2). In comparison, zircons from the garnet–quartz and omphacite layers of the Saltaneset eclogite from the southern UHP domain are rounded to sub-rounded and have patchy zoning and homogenous, dark-CL rim overgrowths (Fig. 2). No difference in the CL patterns of the zircons from the two layers was detected. All zircons
Discussion
The preservation of (U)HP eclogites and mantle peridotites across 30,000 km2 of the WGR provides irrefutable evidence for the deep subduction and exhumation of a large body of continental crust during the late stages of the Caledonian orogeny. Previous geochronological investigations have concluded that the rocks remained at eclogite-facies depths from ca. 425–400 Ma. In order for the subducted material to have remained at eclogite-facies conditions for > 20 Myr prior to exhumation, studies have
Conclusions
This study combines high-spatial resolution (LASS) and high-precision (ID–TIMS–TEA) techniques on the same zircon from two UHP eclogites within the Western Gneiss Region of Norway. The results capture metamorphism from 409.6 ± 0.6 Ma to 401.3 ± 0.4 Ma and 409.0 ± 0.4 Ma to 401.4 ± 0.2 Ma, with the data suggesting two (U)HP zircon (re)crystallization events at ca. 409–407 Ma and ca. 402 Ma. Trace-element analyses from both populations of zircon show flat, depleted HREE signatures and weak negative Eu
Acknowledgments
This work was supported by NSF Grants EAR-1019709, EAR-1062187 (Gordon), and EAR-1219942 (Hacker). High-precision mass spectrometry at MIT is possible because of an Instrumentation and Facilities grant (EAR-0931839 to S.A.B.) and the collective sharing of knowledge by the EARTHTIME community. Thanks to Jahan Ramezani for providing assistance in the MIT Isotope Laboratory. Helpful reviews from F. Corfu and H. Brueckner greatly improved the manuscript.
References (110)
Eclogitization of lower crustal granulites by fluid migration through shear zones
Earth Planet. Sci. Lett.
(1987)- et al.
The Proterozoic Hustad igneous complex: a low strain enclave with a key to the history of the Western Gneiss Region of Norway
Precambrian Res.
(2003) - et al.
A SHRIMP U–Pb and LASS trace element study of the petrogenesis of garnet–cordierite–orthoamphibole gneisses from the Central Zone of the Limpopo Belt, South Africa
Lithos
(2006) - et al.
Coesite inclusions and the U–Pb age of zircons from the Hareidland Eclogite in the Western Gneiss Region of Norway
Lithos
(2003) - et al.
Eclogites and eclogites in the Western Gneiss Region, Norwegian Caledonides
Lithos
(2000) - et al.
An outline of the Precambrian evolution of the Baltic shield
Precambrian Res.
(1987) - et al.
The age and origin of some Norwegian eclogites: a U–Pb zircon and REE study
Chem. Geol.
(1985) - et al.
Diffusion v. recrystallization processes in Rb–Sr geochronology: isotopic relicts in eclogite facies rocks, Western Gneiss Region, Norway
Geochim. Cosmochim. Acta
(2008) - et al.
U–Pb dates and trace-element geochemistry of zircon from migmatite, Western Gneiss Region, Norway: significance for history of partial melting in continental subduction
Lithos
(2013) - et al.
REE, Rb–Sr and Sm–Nd studies of Norwegian eclogites
Chem. Geol.
(1985)
U-Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling-Dabie Orogen, China, Earth Planet
Sci. Lett.
High-temperature deformation during continental-margin subduction & exhumation: the ultrahigh-pressure Western Gneiss Region of Norway
Tectonophysics
The impact of zircon–garnet REE distribution data on the interpretation of zircon U–Pb ages in complex high-grade terrains: an example from the Rauer Islands, East Antarctica
Chem. Geol.
The chemistry of zircon: variations within and between large crystals from syenite and alkali basalt xenoliths
Geochim. Cosmochim. Acta
Metamorphic evolution of Norwegian country-rock eclogites, as deduced from mineral inclusions and compositional zoning in garnets
Lithos
Improved accuracy of U–Pb zircon dating by creation of more concordant systems using air abrasion technique
Geochim. Cosmochim. Acta
Coupled Lu–Hf and Sm–Nd geochronology constrains prograde and exhumation histories of high- and ultrahigh-pressure eclogites from western Norway
Chem. Geol.
Slow exhumation of UHP terranes: titanite and rutile ages of the Western Gneiss Region, Norway
Earth Planet. Sci. Lett.
Size and exhumation rate of ultrahigh-pressure terranes linked to orogenic stage
Earth Planet. Sci. Lett.
Laser-ablation split-stream ICP petrochronology
Chem. Geol.
Zircon U–Pb chemical abrasion (“CA–TIMS”) method: combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages
Chem. Geol.
Sm–Nd ages for Norwegian garnet peridotite
Lithos
Sm–Nd isotopic systematics of a gabbro–eclogite transition
Lithos
Timing of the Permian–Triassic biotic crisis: implications from new zircon U–Pb age data (and their limitations)
Earth Planet. Sci. Lett.
Fabric development during exhumation from ultrahigh-pressure in an eclogite-bearing shear zone, Western Gneiss Region, Norway
J. Struct. Geol.
The Scandinavian Caledonides: event chronology, palaeogeographic settings and likely modern analogues
Tectonophysics
Zircon geochronology and ca. 400 Ma exhumation of Norwegian ultrahigh-pressure rocks: an ion microprobe and chemical abrasion study
Earth Planet. Sci. Lett.
Zircon trace element geochemistry: distribution coefficients and the link between U–Pb ages and metamorphism
Chem. Geol.
Zircon formation during fluid circulation in eclogites (Monviso, Western Alps): implications for Zr and Hf budget in subduction zones
Geochim. Cosmochim. Acta
Experimental zircon/melt and zircon/garnet trace element partitioning and implications for the geochronology of crustal rocks
Chem. Geol.
A new method integrating high-precision U–Pb geochronology with zircon trace-element analysis (U–Pb TIMS–TEA)
Geochim. Cosmochim. Acta
Campaign style titanite U–Pb dating by laser-ablation ICP: implications for crustal flow, phase transformations and titanite closure
Chem. Geol.
Long-lived, cold burial of Baltica towards 200 km depth
Earth Planet. Sci. Lett.
Subduction and eduction of continental crust: major mechanisms during continent–continent collision and orogenic extensional collapse, a model based on the south Norwegian Caledonides
Terra Nova
The tectonic significance of pre-Scandian 40Ar/39Ar phengite cooling ages in the Caledonides of western Norway
J. Geol. Soc. Lond.
Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea
Nature
Metamorphic zircon formation at the transition from gabbro to eclogite in Trollheimen–Surnadalen, Norwegian Caledonides
Trace element signature and U–Pb geochronology of eclogite-facies zircon, Bergen Arcs, Caledonides of W Norway
Contrib. Mineral. Petrol.
The quartz–coesite transformation: a precise determination and the effects of other components
J. Geophys. Res.
Quartz–coesite transition revisited: reversed experimental determination at 500–1200 °C and retrieved thermochemical properties
Am. Mineral.
Engineering cyber infrastructure for U–Pb geochronology: Tripoli and U–Pb redux
Geochem. Geophys. Geosyst.
Interpretation of Rb–Sr Ages from the Precambrian and Paleozoic rocks of southern Norway
Am. J. Sci.
Precambrian ages from the Geiranger–Tafjord–Grotli area of the Basal Gneiss Region, west Norway
Nor. Geol. Tidsskr.
Dunk tectonics: a multiple subduction/eduction model for the evolution of the Scandinavian Caledonides
Tectonics
Discovery of coesite–eclogite from the Nordøyane UHP domain, Western Gneiss Region, Norway: field relations, metamorphic history, and tectonic significance
J. Metamorph. Geol.
Ultrahigh pressure metamorphism in the Western Gneiss Region of Norway
The intrusive history and tectonometamorphic evolution of the Basal Gneiss Complex in the Moldefjord area, west Norway
The timing of stabilisation and exhumation rate for ultrahigh-pressure rocks in the Western Gneiss Region of Norway
J. Metamorph. Geol.
Scandian ultrahigh-pressure metamorphism of Proterozoic basement rocks on Fjørtoft and Otrøy, Western Gneiss Region, Norway
Int. Geol. Rev.
Atlas of zircon textures
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