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