Insights into (U)HP metamorphism of the Western Gneiss Region, Norway: A high-spatial resolution and high-precision zircon study

Highlights

• Coupled high-spatial resolution and high-precision techniques characterize zircon (re)crystallization.

• High-precision U–Pb zircon dates record separate (re)crystallization events at ca. 409–407 Ma and ca. 402 Ma.

• U–Pb and trace-element results from zircon reveal that the end of eclogite-facies metamorphism coincides with previous studies.

Abstract

Combining high-spatial resolution and high-precision geochronology and geochemistry of zircon provides constraints on the timing and duration of ultrahigh-pressure (UHP) metamorphism resulting from the collision of Baltica–Avalonia and Laurentia during the Scandian orogeny in the Western Gneiss Region of Norway. Zircons were extracted from a layered eclogite in the Saltaneset region (southern UHP domain) and from an eclogite in the Ulsteinvik region (central UHP domain). Zircons were first analyzed for U–Pb and trace element compositions by laser ablation split-stream (LASS) inductively coupled plasma mass spectrometry (ICP-MS), followed by analysis of those same zircons that yielded Scandian dates by integrated U–Pb isotope dilution–thermal ionization mass spectrometry and Trace Element Analysis (TIMS–TEA). LASS results from a garnet–quartz layer within the Saltaneset eclogite give Scandian dates of ca. 413–397 Ma, with subsequent ID–TIMS analyses ranging from 408.9 ± 0.4 Ma to 401.4 ± 0.2 Ma (2σ). An omphacite-rich layer from the same eclogite yields LASS dates of ca. 414–398 Ma and a single ID–TIMS date of 396.7 ± 1.4 Ma. In comparison, the Ulsteinvik eclogite LASS results give dates spanning ca. 413–397 Ma, whereas ID–TIMS analyses range from 409.6 ± 0.6 Ma to 401.3 ± 0.4 Ma. ID–TIMS zircon data from the eclogites reveals two age populations: 1) ca. 409–407 Ma and 2) ca. 402 Ma. Both in situ and solution trace element data show a distinct pattern for Scandian zircons, with strongly-depleted HREE and weakly-negative Eu anomalies (Eu/Eu*), whereas inherited zircon REE patterns are distinguished by steep HREE slopes and marked negative Eu/Eu*. When coupled with partition coefficients calculated for zircon and garnet, these REE patterns indicate that zircon (re)crystallized during eclogite-facies metamorphism at ca. 409–407 Ma and ca. 402 Ma at two widely separated UHP localities.

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).