Long-lived, cold burial of Baltica to 200 km depth

https://doi.org/10.1016/j.epsl.2009.02.001Get rights and content

Abstract

The collision of two continents causes subduction of one of the continental margins temporarily below the other into the diamond stability field (> 3.8 GPa or > 120 km depth), coeval ultra-high pressure (UHP) metamorphism of the continental crust followed by exhumation of UHP metamorphic terrains. Recent thermo-mechanical models propose that this subduction–exhumation cycle is short-lived (< 15 Ma), contradicting the range in metamorphic ages observed in several high pressure/UHP metamorphic terrains. Here we use microstructures, mineral chemistry, Sm–Nd geochronology and Nd–Sr isotope systematics to show that the micro-diamond bearing Western Gneiss Region in the Scandinavian Caledonides of western Norway was subjected to UHP conditions for c. 30 Ma during a long-lived cycle of subduction and exhumation related to the Scandian phase of the Caledonian orogeny. Orogenic peridotite bodies on Otrøy and Flemsøy islands are interpreted as mantle wedge fragments tectonically emplaced into Baltic continental crust during the prograde continental subduction of Baltica underneath Laurentia after c. 438 Ma. Subduction related deformation and associated strain-induced recrystallization of the mantle fragments partially to completely destroyed pyroxene exsolution microstructures in cm-scale garnet within layers of garnet–pyroxenite that have a Palaeoarchaean origin (3.33 ± 0.19 Ga, 2σ, 5 point whole rock). Millimeter scale recrystallized orthopyroxene in garnet–websterite has low Al cores (≥ 0.10 wt.% Al2O3) showing recrystallization conditions at UHP, 6.3 ± 0.2 GPa, and at sub-geotherm temperatures of Archaean areas, 870 ± 50 °C. Three mineral isochrons of 2 to 5 points from recrystallized mineral assemblages of garnet–pyroxenite indicate overlapping, early Scandian ages (429.5 ± 3.1 Ma, 2σ, weighted mean) showing that the Archaean mantle fragments record the prograde subduction of Baltica to 200 km depth underneath Laurentia in c. 8 Ma (25 mm a 1 vertical subduction rate). Contrasting 87Sr/86Sr in recrystallized clinopyroxene from Otrøy and Flemsøy (0.7016–0.7023 and 0.7131, respectively) indicates strain-induced recrystallization occurred partly at dry (fluid absent) conditions. Subsequent metamorphic conditions during peridotite retrogression record exhumation through c. 120 km depth (3.8 GPa), overlapping maximum metamorphic conditions recorded in regional country-rock eclogites dated at c. 400 Ma. A slow average vertical exhumation rate of 3.6 mm a 1 is implied for the diamond phase stability. Most external eclogites crystallized or re-equilibrated, probably triggered by deformation and fluids, during crustal exhumation c. 30 Ma after the peridotites crystallized early Scandian garnets. Crustal micro-diamond — formed by long-lived but cold UHP metamorphism, and found almost exclusively in Phanerozoic orogens — suggests a change in the nature of collisional tectonics within a cooling earth.

Introduction

Ancient exhumed continent–continent collision zones preserve evidence for the temporarily burial, down to mantle depth > 120 km, of one continental margin below the other. Evidence includes metamorphic pressure (P) and temperature (T) gradients normal to the collision front (Krogh, 1977, Griffin et al., 1985), metamorphic index minerals like coesite (Coe) and micro-diamond (Dia; other mineral abbreviations after Kretz, 1983) within the subducted continental crust (Sobolev and Shatsky, 1990, Dobrzhinetskaya et al., 1995) and orogenic (mantle and crustal) peridotites embedded in the subducted continental crust (Brueckner and Medaris, 2000, Liou et al., 2007). In addition, parts of the cycle of burial (subduction), reversal and exhumation of positively buoyant continental lithosphere have successfully been modelled in thermo-mechanical/thermo-tectonic numerical studies (Ranalli et al., 2000, Warren et al., 2008). The first part of the cycle, the subduction of continental lithosphere, transforms near-surface low pressure (LP) rocks to high pressure (HP) and ultra-high pressure (UHP) metamorphic crustal rocks. These include eclogite (Eskola, 1915), Coe bearing gneiss (Chopin, 1984, Smith, 1984) and micro-Dia bearing gneiss (Sobolev and Shatsky, 1990, Dobrzhinetskaya et al., 1995), which are believed to preserve the maximum depth of plate burial. The third stage, the exhumation, is generally assumed to overprint to varying degrees the HP/UHP mineral assemblages. The duration of both the first and third part of the cycle is constrained indirectly by palaeomagnetic and numerical models and directly by combined isotope and mineral chemical studies that all suggest fast rates for both continental plate subduction and crustal exhumation. Reported rates for vertical plate movement range from 7 to more than 80 mm a 1 in a single orogen (Torsvik et al., 1996, Terry et al., 2000a, Root et al., 2004, Camacho et al., 2005, Kylander-Clark et al., 2008) that all would lead to estimates for the total burial time of less than 15 Ma.

Evidence for the duration of the intermediate stage two, i.e. the integrated mantle residence time between early subduction (1) and late exhumation (3) is sparse. The reversal of crustal rock movement is generally believed to occur rapidly as the buoyancy contrast between crustal and mantle lithologies increases with depth. Eclogites preserve large age ranges that suggest a larger duration of eclogite-facies conditions of up to 25 Ma (Griffin and Brueckner, 1985, Mattinson et al., 2006, Kylander-Clark et al., 2007). In addition, micro-Dia bearing UHP metamorphic terrains commonly enclose orogenic Grt-peridotite bodies that record systematically higher peak P and T conditions (Hirajima and Nakamura, 2003) than do eclogites and thus suggest deeper subduction. It follows that chronological data from crustal rocks may also systematically underestimate the time of peak metamorphism, and therefore the residence time at mantle conditions. Here we show that orogenic peridotite enclosed in Baltica basement gneiss at Otrøy and Flemsøy islands in western Norway (Fig. 1) records the prograde subduction and early retrograde exhumation history of the Baltica plate margin, indicating that this margin resided in the Dia stability field for c. 25 Ma, in the Coe stability for c. 30 Ma, much longer than generally accepted for any exhumed UHP metamorphic terrain.

Section snippets

Geological setting and sample description

The Caledonides in Scandinavia formed as a result of the Palaeozoic continent–continent collision between Laurentia and Baltica and consists of a pile of tectonic nappes translated eastwards onto the Baltica continental margin (Roberts and Gee, 1985). The Western Gneiss Region (WGR) of Norway hosts a tectonic window through this nappe pile and exposes at the lowest tectonostratigraphic position a large segment of the former craton of Baltica that is dominated by Palaeo- to Mesoproterozoic,

Methods

Major element compositions of minerals were obtained by electron microprobe analysis (EMPA) using a JEOL JXA8600 superprobe at Utrecht University. The superprobe operated at standard conditions (15 kV, 20 nA, 30 s counting time in WDS mode, external calibration against international standards).

Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) at Utrecht University was used to measure mineral rare earth element (REE) contents in recrystallized M3 Grt and Cpx from two

PT estimates

Major element profiles of mineral grains > 0.5 mm reveal that recrystallized (M3) and porphyroclastic (M2) mineral grain cores are homogeneous in composition; compositional variations (zoning) occur at grain rims. Core compositions of small grains (< 0.5 mm across) depend on the grain size. Cores of recrystallized M3 Opx > 0.5 mm preserve plateaus of low Al2O3 content, 0.10–0.24 wt.%, that suggest equilibration (Fig. 3, supplementary Table S1). These Al2O3 contents are lower than in any of the

REE

The REE content of M3 Grt and Cpx matrix grains in three samples from Otrøy was measured to characterize trace element partitioning in the recrystallized mineral assemblages (Table S2). M3 Grt and Cpx in clinopyroxenite (DS0380, − 84) and websterite (DS03AO) show a normal fractionation of REE with light REE (LREE) predominantly hosted in Cpx and heavy REE in Grt (Fig. 4a). The REE content of each of the two minerals is higher in clinopyroxenite than in websterite reflecting distinct whole rock

Isotopes

We analysed Sm–Nd and Sr isotopes on fractions of M2 and M3 minerals to determine the timing of recrystallization (Table S3). Three of the Otrøy samples (DS0346, DS0380, DS0384) contained relics of M1 Grt (M2 Grt + enclosed M2 Pyx lamellae) in sufficient quantity for analysis. Plotting these Grt with the associated whole rock on Sm–Nd isochron diagrams generates lines (‘two point isochrons’). If the slopes of these lines are taken to have age significance, they indicate variable but clearly

Pressure–temperature–time evolution

All the microstructural, mineral chemical and isotope data record major differences between the M2 and M3 mineral assemblages in the pyroxenites on Otrøy and Flemsøy (summarized in Fig. 7). The M2 mineral assemblages, characterized by a granular, partially coronitic texture in Grt-websterite, but also in garnetite and websteritic garnetite in earlier studies (Carswell, 1973, Van Roermund and Drury, 1998, Spengler et al., 2006), contain exsolution lamellae of Pyx in Grt (Fig. 2b, c) and record a

Conclusion and implications

Orogenic peridotite bodies on Otrøy and Flemsøy are mantle fragments that preserve evidence for a prolonged, relatively cold, residence in the Dia and Coe UHP stability fields, for c. 25 and c. 30 Ma respectively, together with the enclosing Baltica basement. Such long mantle residence times for continental rocks have not been recognized before from other UHP metamorphic areas nor has it been proposed in geodynamic models of collisional tectonics. Destruction of mineral exsolution

Acknowledgements

This project was initiated during the NWO-PIONEER project of M.R. Drury and subsequently funded by the Utrecht University Institute of Geodynamic Research (GOI). Additional support provided NSF grant EAR-0000937 and grants from the Research Foundation of the City University of New York. The manuscript is an extended version of part of the senior author's Ph.D. study (http://igitur-archive.library.uu.nl/dissertations/2006-1114-200554/index.htm) and benefited from financial support from the Japan

References (69)

  • RanalliG. et al.

    Time dependence of negative buoyancy and the subduction of continental lithosphere

    J. Geodyn.

    (2000)
  • RobertsD.

    The Scandinavian Caledonides: event chronology, palaeogeographic settings and likely modern analogues

    Tectonophysics

    (2003)
  • RootD.B. et al.

    Zircon geochronology and ca. 400 Ma exhumation of Norwegian ultrahigh-pressure rocks: an ion microprobe and chemical abrasion study

    Earth Planet. Sci. Lett.

    (2004)
  • TorsvikT.H. et al.

    Continental break-up and collision in the Neoproterozoic and Palaeozoic — a tale of Baltica and Laurentia

    Earth-Sci. Rev.

    (1996)
  • WarrenC.J. et al.

    Modelling tectonic styles and ultra-high pressure (UHP) rock exhumation during the transition from oceanic subduction to continental collision

    Earth Planet. Sci. Lett.

    (2008)
  • WennbergO.P.

    Superimposed fabrics due to reversal of shear sense: an example from the Bergen Arc Shear Zone, western Norway

    J. Struct. Geol.

    (1996)
  • BeyerE.E. et al.

    Archean mantle fragments in Proterozoic crust, Western Gneiss Region, Norway

    Geology

    (2004)
  • BreyG.P. et al.

    Geobarometry for peridotites: experiments in simple and natural systems from 6 to 10 GPa

    J. Petrol.

    (2008)
  • BreyG.P. et al.

    Geothermobarometry in four-phase lherzolites II. New thermobarometers, and practical assessment of existing thermobarometers

    J. Petrol.

    (1990)
  • BrownM.

    Duality of thermal regimes is the distinctive characteristic of plate tectonics since the Neoarchean

    Geology

    (2006)
  • BruecknerH.K.

    Sinking intrusion model for the emplacement of garnet-bearing peridotites into continent collision orogens

    Geology

    (1998)
  • BruecknerH.K. et al.

    A general model for the intrusion and evolution of ‘mantle’ garnet peridotites in high-pressure and ultra-high-pressure terranes

    J. Metamorph. Geol.

    (2000)
  • BruecknerH.K. et al.

    Dunk tectonics: a multiple subduction/eduction model for the evolution of the Scandinavian Caledonides

    Tectonics

    (2004)
  • CamachoA. et al.

    Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids

    Nature

    (2005)
  • CarswellD.A. et al.

    The timing of stabilisation and the exhumation rate for ultra-high pressure rocks in the Western Gneiss Region of Norway

    J. Metamorph. Geol.

    (2003)
  • CarswellD.A. et al.

    Norwegian orthopyroxene eclogites: calculated equilibration conditions and petrogenetic implications

  • CarswellD.A. et al.

    On multi-phase mineral inclusions associated with microdiamond formation in mantle-derived peridotite lens at Bardane on Fjørtoft, west Norway

    Eur. J. Mineral.

    (2005)
  • CarswellD.A. et al.

    Scandian ultrahigh-pressure metamorphism of Proterozoic Basement rocks on Fjørtoft and Otrøy, Western Gneiss Region, Norway

    Int. Geol. Rev.

    (2006)
  • CartignyP. et al.

    Early Proterozoic ultrahigh pressure metamorphism: evidence from microdiamonds

    Science

    (2004)
  • ChopinC.

    Coesite in pure pyrope in high grade blueschist of the western Alps

    Contrib. Mineral. Petrol.

    (1984)
  • DallmeyerR.D. et al.

    Chronology of Caledonian high-pressure granulite-facies metamorphism, uplift, and deformation within northern parts of the Western Gneiss Region, Norway

    Geol. Soc. Amer. Bull.

    (1992)
  • De AstisG. et al.

    Transition from calc-alkaline to potassium-rich magmatism in subduction environments: geochemical and Sr, Nd, Pb isotopic constraints from the island of Vulcano (Aeolian arc)

    Contrib. Mineral. Petrol.

    (2000)
  • DobrzhinetskayaL.F. et al.

    Microdiamonds in high-grade metamorphic rocks from the Western Gneiss Region, Norway

    Geology

    (1995)
  • DunningG.D. et al.

    U/Pb ages of ophiolites and arc-related plutons of the Norwegian Caledonides: implications for the development of Iapetus

    Contrib. Mineral. Petrol.

    (1988)
  • Cited by (0)

    View full text