(Ultra)High-Pressure Rocks Across the Western Gneiss Region, Norway

Protolith ages and exhumation histories of (ultra)high-pressure rocks across the Western Gneiss Region, Norway Emily O. Walsh† Bradley R. Hacker Phillip B. Gans Department of Geological Sciences, University of California, Santa Barbara, California 93106-9630, USA Marty Grove Department of Earth and Space Sciences, University of California, Los Angeles, 595 Charles Young Drive East, Box 951567, Los Angeles, California 90095-1567, USA George Gehrels Department of Geosciences, University of Arizona, Gould-Simpson Building No. 77, 1040 East 4th Street, Tucson, Arizona 85721, USA ABSTRACT Keywords: ultrahigh pressure, Western Gneiss and Hacker, 2004). Excellent exposure provides Region, secondary ion mass spectrometry, an unparalleled opportunity to investigate not The timing of protolith formation, ultra- 40Ar/ 39Ar, exhumation, laser ablation inductively only the UHP rocks but also the less-studied, high-pressure (UHP) subduction, and subse- coupled plasma–mass spectrometry. high-pressure (HP) rocks that surround the UHP quent exhumation for the ultrahigh-pressure terrane. These lower pressure rocks provide to high-pressure units across the eastern part INTRODUCTION important constraints on both the protolith of the of the Western Gneiss Region, Norway, were HP-UHP terrane and the tectonic processes that assessed using U/Pb zircon, Th/Pb mona- Since the discovery of coesite in regional led to exhumation of the UHP rocks. zite, and 40Ar/39Ar white mica ages. U/Pb metamorphic rocks (Smith, 1984), geologists The purpose of this study is to determine the zircon ages from eclogites demonstrate that have sought to understand how large-scale timing of protolith formation, metamorphism, oceanic and continental allochthons were ultrahigh-pressure (UHP) terranes are formed and subsequent exhumation across the Norwe- emplaced onto the Baltica basement before and exhumed. The study of these terranes has gian HP-UHP terrane—from the foreland into the entire mass was subducted to (ultra)high yielded valuable insights into geological pro- the core of the orogen. We seek to answer the pressure. Eclogites within the allochthons cesses active in the mantle and lower crust, following questions and thus better constrain the across the entire Western Gneiss Region including continental collisions and the recy- mechanisms of UHP rock exhumation: (1) What are Caledonian and show a degree of zircon cling of continental crust. The rates and mecha- are the protolith ages of the high-pressure ter- (re)crystallization that increases with peak nisms of UHP subduction and exhumation rane? (2) Was the high-pressure metamorphism pressure, permitting the interpretation that remain elusive, however, even in well-studied in the eastern part of the Western Gneiss Region the entire region underwent synchronous sub- terranes such as the Western Gneiss Region of coeval with the UHP metamorphism in the west? duction. 40Ar/39Ar white mica ages of 399 Ma southwestern Norway. (3) What is the age of the subsequent high-tem- indicate that the eastern part of the Western The Western Gneiss Region, a ~50,000 km2 perature metamorphism and deformation in the Gneiss Region had been exhumed to shallow terrane of Proterozoic orthogneisses (the autoch- eastern part of the Western Gneiss Region? (4) crustal levels while UHP metamorphism was thonous Western Gneiss Complex, Fig. 1) over- When were the rocks across the entire region ongoing farther west, indicating a westward lain by mixed Proterozoic to Phanerozoic ortho- exhumed into the upper crust? dip to the slab. The 40Ar/39Ar white mica ages gneissic and paragneissic units (Lower, Middle To address these questions, we dated zircon, also show a clear east-to-west gradient across and Upper Allochthons, Fig. 1), contains one of monazite, and muscovite from eclogites and the entire Western Gneiss Region, indicat- the largest known UHP terranes. UHP is evident metapelites within an E-W transect across the ing that the Western Gneiss Region rose dia- from coesite or quartz-pseudomorphs-after- width of the Western Gneiss Region (Fig. 2). chronously to crustal levels from east to west coesite, most common in eclogites that crop out U/Pb ages of zircons were measured by sec- between 399 and 390 Ma. in three distinct zones along the western edge of ondary ion mass spectrometry (SIMS) and by the Western Gneiss Region (Root et al., 2005). laser-ablation multiple-collector inductively These UHP eclogites are surrounded by high- coupled plasma–mass spectrometry (LA-MC- †Present address: Geology Department, Cornell pressure eclogites that stretch at least another ICP-MS, henceforth “ICP”) both of which College, Mount Vernon, Iowa 52314, USA; e-mail: 100 km north, east, and south across the West- provide the spatial resolution necessary for [email protected]. ern Gneiss Region (Cuthbert et al., 2000; Walsh deciphering protolith ages from metamorphic GSA Bulletin; March/April 2007; v. 119; no. 3/4; p. 289–301; doi: 10.1130/B25817.1; 10 fi gures; Data Repository item 2007063. For permission to copy, contact [email protected] 289 © 2007 Geological Society of America Walsh et al. Tectonostratigraphy large thrust sheets exposed in the foreland. Pre- 5°E viously (Walsh and Hacker, 2004), we showed Devonian that the eclogites in this swath recrystallized at Averøya Trondheim Nordøyane peak temperatures and minimum pressures of ~700 °C and ~1.8 GPa, with two rocks exhib- iting evidence of UHP. The eclogites and their UHP domain Upper Allochthon quartzofeldspathic and pelitic host gneisses then Ophiolitic rocks underwent a late amphibolite-facies metamor- 62°N Stadtlandet phism at 650–750 °C and ~1.1 GPa (Walsh and Raudkleivane Middle Allochthon Hacker, 2004); we call this a supra-Barrovian Western edge metamorphism because the pressures are higher of Baltica or Hornelen microcontinent than classic Barrovian metamorphism, culmi- Basin study area nating in kyanite-stable rather than sillimanite- Lower Allochthon stable assemblages (Fig. 3). This was followed Nordfjord-Sogn Jotun Nappe Detachment Sedimentary cover by or transitioned into a second, low-pressure, of Autochthon and Parautochthon Buchan-style amphibolite-facies metamor- Solund phism at 650–750 °C and ~0.6 GPa (Fig. 9 of Basin Parautochthon & Walsh and Hacker, 2004). This same sequence Autochthon of supra-Barrovian amphibolite-facies overprint Western Gneiss and Buchan-style amphibolite- to granulite- Bergen Complex/ 100 km 10°E Baltic Shield basement facies overprint is also seen in the UHP domains 60°N eclogite (Terry et al., 2000b; Root et al., 2005), indicat- Figure 1. Geologic map of the Western Gneiss Region. Study area lies within the black box. ing that it is characteristic of the (U)HP Western UHP—ultrahigh pressure. Gneiss Region as a whole. The amphibolite- facies fabric, including a strong foliation, isocli- nal folds with E-W axes, and a moderately to ages. High-temperature metamorphic ages were Nordfjord-Sogn Detachment Zone (Hacker et al., shallowly E- or W-plunging stretching lineation, obtained by ICP analysis of Th/Pb decay in 2003; Johnston et al., in press). is defi ned by quartz, biotite, amphibole, plagio- monazite from metapelites, and 40Ar/39Ar analy- In the foreland east of the Western Gneiss clase, or muscovite; throughout the bulk of the sis of muscovite collected from metapelites and Region (Fig. 1) the Western Gneiss Complex is study area, this amphibolite-facies fabric is only quartzites across the region provided the timing overlain by demonstrably allochthonous units. weakly overprinted by greenschist-facies defor- of exhumation into the upper crust. Zircon ages There the allochthonous units include ca. 480 Ma mation or metamorphism in the study area. demonstrate that the high-pressure metamor- ophiolites, ca. 440 Ma ophiolites (Upper Alloch- The age of the eclogite-facies event in the UHP phism in the east was broadly coeval with the thon), and the telescoped former margin of the domains is constrained by two three-point Sm/ UHP metamorphism in the west. Exhumation Baltica craton (Middle and Lower Allochthons; Nd isochrons of 408 ± 8 Ma (Mearns, 1986) and through the upper crust began in the east while e.g., Hacker and Gans, 2005). These units have 400 ± 16 Ma (Mørk and Mearns, 1986), U-Pb UHP metamorphism was occurring in the west, been traced westward across the entire Western zircon ages of 401.6 ± 1.6 Ma (Carswell et al., and continued from east to west over ~9 m.y. Gneiss Region and correlated with composition- 2003) and 405–400 Ma (Root et al., 2004), and a ally similar mixed orthogneiss and paragneiss monazite Th-Pb age of 415 ± 6.8 Ma (Terry et al., GEOLOGIC SETTING units in the UHP core of the orogen (Krill, 1985; 2000a). There were, however, at least three other Rickard, 1985; Robinson, 1995; Terry et al., (U)HP events in the Scandinavian Caledonides— Closure of the Iapetus Ocean initiated the 2000a); throughout this paper we shall refer to ca. 423 Ma, ca. 452–450 Ma, and ca. 503 Ma Scandian Orogeny at ca. 435 Ma, resulting in the these rocks as allochthons, following the above (see summary in Brueckner and van Roermund, emplacement of a series of allochthons onto the authors, although alternative interpretations 2004)—making it crucial to determine the ages Baltic Shield basement (the Autochthon or West- cannot be refuted. The UHP eclogites crop out of the eclogites that span the eastern part of the ern Gneiss Complex, Fig. 1) by ca. 415 Ma (Rob- within three culminations along the western Western Gneiss Region before constructing tec- erts, 2003). These allochthons include (from top edge of the Western Gneiss Region; similar UHP tonic models for their exhumation. to bottom) the eastern margin of Laurentia (the and HP eclogites occur within both the West- There are also insuffi cient constraints on the Uppermost Allochthon, not present in the study ern Gneiss Complex basement and the inferred ages of the two amphibolite-facies events that area), Iapetus ophiolites (the Upper Allochthon), allochthonous rocks (Root et al., 2005).

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