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Lithos 109 (2009) 145–154

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Lithos

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The Sandvik peridotite, Gurskøy, Norway: Three billion years of mantle evolution in the lithosphere

Thomas J. Lapen a,⁎, L. Gordon Medaris Jr.b, Brian L. Beard b, Clark M. Johnson b a Department of Geosciences, University of Houston, Houston TX 77204-5007, USA b Department of Geology and Geophysics, University of Wisconsin–Madison, Madison WI 53704, USA article info abstract

Article history: The Sandvik ultramafic body, Island of Gurskøy, Western Region, Norway, is a mantle fragment that Received 14 February 2008 contains polymetamorphic mineral assemblages and affords a unique view into the response of subcontinental Accepted 17 August 2008 lithospheric mantle to repeated orogenic/magmatic events. The Sandvik peridotite body and nearby outcrops Available online 6 September 2008 record four paragenetic stages: 1) pre-exsolution porphyroclasts of ol+grt+opx (high-Ca )+cpx (low-Ca), which equilibrated at 1100–1200 °C and 6.5–7.0 GPa; 2) kelyphite containing ol+grt+spl+opx (low-Ca)+am Keywords: (high-Al), as well as exsolved pyroxene containing opx+cpx+spl in equilibrium with matrix olivine, at 725 °C Lithospheric mantle Geothermobarometry and 1.5 GPa; 3) granoblastic matrix of ol+spl+opx (low-Ca)+am (high-Al), at 700 °C and 1.0 GPa. A nearby – – Sm–Nd geochronology outcrop contains a fourth assemblage consisting of ol+chl+opx+am. Lu Hf and Re Os model ages of garnet peridotite indicate melt depletion at 3.3 Ga [Beyer, E.E., Brueckner, H.K., Griffin, W.L., O'Reilly, S.Y., Graham, S., Peridotite 2004. mantle fragments in crust, Western Gneiss Region, Norway. Geology 32, 609–612.; Lapen, T.J., Medaris, L.G. Jr., Johnson, C.M., and Beard, B.L., 2005. Archean to Middle Proterozoic evolution of Baltica subcontinental lithosphere: evidence from combined Sm–Nd and Lu–Hf isotope analyses of the Sandvik ultramafic body, Norway. Contributions to Mineralogy and Petrology 150, 131–145.], marking the time of separation from the convecting mantle. Lu–Hf whole rock and mineral isochron ages of constituent garnet peridotite and garnet pyroxenite layers in the Sandvik body reflect cooling and emplacement at ~1.25 Ga and ~1.18 Ga, respectively, whereas Sm–Nd whole rock and mineral ages of the garnet pyroxenite layers and the garnet peridotite are consistent with metasomatic alteration at ~1.15 Ga [Lapen, T.J., Medaris, L.G. Jr., Johnson, C.M., and Beard, B.L., 2005. Archean to Middle Proterozoic evolution of Baltica subcontinental lithosphere: evidence from combined Sm–Nd and Lu–Hf isotope analyses of the Sandvik ultramafic body, Norway. Contributions to Mineralogy and Petrology 150, 131–145.]. The isochron ages likely record lithospheric modification associated with the 1.25–1.00 Ga Sveconorwegian and represent the youngest age of the Stage 1 mineral assemblage equilibration. A 606±39 Ma Sm–Nd isochron age of the Stage 2 kelyphite assemblage is consistent with partial re-equilibration of the porphyroclastic assemblage during continental rifting associated with opening of the Iapetus Ocean between Baltica and Laurentia at ~600 Ma, or extension between Baltica and Siberia that may have been associated with opening of the Ægir Sea. The age of kelyphite, therefore, places the Sandvik peridotite in the uppermost mantle prior to Silurian shortening between the Baltic and Laurentian continents. © 2008 Elsevier B.V. All rights reserved.

1. Introduction orogenic ultramafic bodies, which are commonly exposed in the high and ultra-high pressure of collisional mountain belts, are The initial development of continents, their growth, and modifica- critical for understanding lithosphere evolution because they can tion are often recorded in the thermal, structural, and geochemical preserve mantle structures, pristine mineral assemblages, and/or evolution of subcontinental lithospheric mantle (SCLM). It is lithologic associations that may not be preserved in their associated frequently the case that mantle-derived xenoliths, peridotite massifs, crustal rocks or in mantle xenoliths alone (Medaris, 1999; Brueckner and orogenic mantle fragments show considerable variation in age, et al., 2002, 2004; Beyer et al., 2004, 2006; Lapen et al., 2005; Medaris chemical composition, and mineralogy, reflecting large-scale tecto- et al., 2006; Wittig et al., 2007). Furthermore, orogenic peridotites nothermal events that may also be recorded in superjacent crust record crust–mantle interactions that occur during continental (Medaris, 1999; O'Reilly et al., 2001; Lee, 2006). Many mantle-derived and place critical constraints on the geodynamics of ultra-deep subduction (van Roermund et al., 2002), as well as the ⁎ Corresponding author. Tel.: +1 713 743 6368; fax: +1 713 748 7906. nature of rifting processes and continental break-up (Lemoine et al., E-mail address: [email protected] (T.J. Lapen). 1987; Manatschal, 2004; Montanini et al., 2006). In this paper, we

0024-4937/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2008.08.007 146 T.J. Lapen et al. / Lithos 109 (2009) 145–154 present new age and petrologic data of the Sandvik peridotite body, et al., 1979; Tucker et al., 1990; Austreim et al., 2003). Following this Island of Gurskøy, Western Gneiss Region, Norway which records 3 period of continental growth by magmatic addition and billion years of continental lithosphere growth and modification from accretion is a period of continental rifting, intrusion of dolerite dikes Middle Archean to Paleozoic time. and gabbros, and bimodal volcanism in the period between 1500 and 1250 Ma (Gorbatschev, 1980; Mörk and Mearns, 1986; Mearns, 1986; 1.1. Geological setting Gaal and Gorbatschev, 1987). The period from 1250–900 Ma repre- sents an interval of intense deformation, to - The Western Gneiss Region of Norway (WGR) represents the facies , and magmatism associated with the Sveco- structurally lowest unit of the and is norwegian orogeny (Austreim et al., 2003). Subsequent to, and along composed predominantly of amphibolite- to granulite-facies ortho- the strike of the , a major episode of rifting and paragneiss, pods and lenses of variably retrogressed peridotite, evolved between ~750 and 550 Ma, which generated part of the and (Fig. 1; Krogh and Carswell, 1995; Medaris, 1984; Iapetus Ocean between the margin of Baltica and Laurentia (Torsvik Carswell, 1981). A characteristic feature of the WGR is that measured et al., 1996; Dalziel, 1997; Bingen et al., 1998; Cawood and Pisarevsky, mineral ages of mantle-derived garnet peridotites (Proterozoic) are 2006; Cawood et al., 2007) or part of the Ægir Sea (Torsvik and much older than those of (Silurian) within the same Rehnström, 2001) between Siberia and an inverted Baltica (Hartz and tectonostratigraphic unit (see Brueckner and Medaris, 2000, for a Torsvik, 2002; Cocks and Torsvik, 2005). This rifting event produced discussion). The preservation of Proterozoic and older ages in mantle- extensive mafic dike swarms (Bingen et al., 1998) and resulted in derived garnet peridotites offers a rare opportunity to understand thinning of the continental margin. The thinned continental litho- ancient mantle events. Because of this, the mode and timing of sphere is likely the ‘protolith’ to components of the Seve-Køli eclogite and peridotite emplacement in gneiss, relative to high Complex (Hacker and Gans, 2005) which, like the WGR, contains pressure and ultrahigh pressure metamorphism associated with the Archean mantle fragments (Brueckner et al., 2004). The Caledonian Scandian phase of Caledonian orogeny (420–380 Ma), has been the orogeny, a product of closure of the Iapetus Ocean and continent– subject of considerable study (Lappin and Smith, 1978; Smith, 1980; continent collision between Baltica and Laurentia, resulted in Griffin et al., 1985; Cuthbert and Carswell, 1990; Wain, 1997; Wain eclogite–facies metamorphism across much of the WGR (See Cuthbert et al., 2000). Although recent evidence favors in situ eclogite-facies et al., 2000). This high- and locally ultrahigh-pressure (HP/UHP) metamorphism of metabasic lenses within enclosing quartzofelds- metamorphism is a product of continental subduction during the pathic (for reviews, see: Cuthbert and Carswell, 1990, and Scandian phase of the Caledonian orogeny in the interval of ~420– Krogh and Carswell, 1995), it is uncertain whether peridotite resided 380 Ma (Root et al., 2005; Kylander-Clark et al., 2000) as well as earlier in the crust or mantle prior to the Caledonian orogeny. HP/UHP events in the interval between 500 and 450 Ma (Brueckner The WGR is generally considered part of the and has and van Roermund, 2004). At present, the petrologic and structural experienced several orogenic and magmatic events from about 1750 features in the WGR are the result of Caledonian reworking of earlier to 360 Ma (Gee and Sturt, 1985; Kullerud et al., 1986; Gaal and assembled lithologies. Gorbatschev, 1987; Andersen and Sundvoll, 1995), and this long history is likely imprinted upon the currently enclosed peridotite 1.2. Garnet peridotites bodies. These events include the 1750–1500 , which represents a period of extensive continental growth and magmatism. Peridotite bodies are distributed across the WGR (Fig. 1 MAP) and Gothian-age magmatic rocks represent some of the protoliths to the range in size from a few meters to several kilometers in exposed granitic gneisses in the WGR (Pidgeon and Raaheim, 1972; Lappin extent. There are two main types of peridotite based on major element

Fig. 1. Location map of part of the Western Gneiss Region, modified after Carswell and Cuthbert (2003). T.J. Lapen et al. / Lithos 109 (2009) 145–154 147

Fig. 2. Summary of tectono-magmatic events recorded in the WGR (upper panel) and whole rock model and mineral isochron ages of Mg–Cr peridotites from the WGR (lower panel). Data sources: 1) Re–Os model ages (Beyer et al., 2004), 2) Re–Os isochron age (Brueckner et al., 2002), 3) Sm–Nd model age (Jamtveit et al., 1991), 4) Lu–Hf model age (Lapen et al., 2005), and 5) Sm–Nd (hatched) and Lu–Hf (black) mineral-whole rock ages (Jacobsen and Wasserburg, 1980; Mearns and Lappin, 1982; Mearns, 1986; Rubenstone et al., 1986; Jamtveit et al., 1991; Brueckner et al., 1996; Medaris and Brueckner, 2003; Lapen et al., 2005; Spengler et al., 2006). chemical compositions, a Mg–Cr type derived from the mantle and a small, this outcrop has been the focus of detailed study because it Fe–Ti type derived from mafic (perhaps layered) intrusions emplaced contains several of the numerous mineral assemblages that have been in the crust (Carswell, 1986; Fig. 1). In this investigation, we focus only identified elsewhere in the WGR (Carswell et al., 1983; Medaris, 1984; on the Mg–Cr lithologies because they represent material derived Carswell, 1986; Brueckner et al.,1996; Beyer et al., 2004; Medaris et al., from the subcontinental lithospheric mantle. 2003; Lapen et al., 2005). Although the majority of Mg–Cr peridotites in the WGR are The Sandvik peridotite has a pronounced porphyroclastic texture, extensively recrystallized to chlorite-bearing assemblages by the in which coarse-grained porphyroclasts of olivine, orthopyroxene, interaction of peridotite with hydrous crust during Caledonian clinopyroxene, and garnet occur in a fine-grained, granoblastic orogenesis (Carswell, 1986), fourteen peridotite bodies contain relict groundmass of olivine, orthopyroxene, spinel, and pargasitic mantle assemblages (Fig. 1). Compositionally, the mantle lithologies range from dunite to pyroxenite with a continuum of compositions reflecting various proportions of olivine, clinopyroxene, and ortho- pyroxene. Mineralogically, the peridotites commonly contain several assemblages, which, on the basis of textural relations, typically occur in the following sequence: ol+opx+cpx+grt, ol+opx+cpx+spl, ol+opx+am+spl, and ol+opx+am+chl (Carswell, 1986; Medaris, 1999). Such a sequence reflects recrystallization of the peridotites at different stages, ranging from a high pressure/high temperature garnet-bearing stage to lower pressure/lower tempera- ture spinel-, amphibole-, and chlorite-bearing stages. Note, however, that excursions from a simple PT path of decreasing pressure and temperature have been documented in certain peridotite bodies (Brueckner et al., 2002; van Roermund et al., 2002). The imprint of lithosphere-scale tectonothermal/magmatic events described above is variably preserved in Mg–Cr peridotite bodies across the WGR (Fig. 2). Mineralogical, geochemical, and isotopic data indicate that many of these peridotite bodies record a very long history from the Middle Archean to Paleozoic time (Carswell, 1986; Jamtveit et al., 1991; Beyer et al., 2004, 2006; Lapen et al., 2005; Spengler et al., 2006). Re–Os, Lu–Hf, and Sm–Nd whole rock modal ages for many of the peridotite bodies indicate pre-Gothian (N~1.7 Ga) formation (Jamtveit et al., 1991; Brueckner et al., 2002; Beyer et al., 2004; Lapen et al., 2005). Mineral isochron ages of mantle assemblages indicate Gothian to Sveconorwegian formation, metaso- matism, cooling, and/or recrystallization (Mearns, 1986; Jamtveit et al., 1991; Brueckner et al., 2002; Beyer et al., 2004; Lapen et al., 2005). Pre-Caledonian, post-Sveconorwegian ages (800–600 Ma) are also recorded in garnet-bearing kelyphite assemblages (Brueckner et al., 1996). The Caledonian imprint is locally preserved in high-pressure secondary garnet- and diamond-bearing assemblages (Jamtveit et al., 1991; Brueckner et al., 2002; van Roermund et al., 2002) and widely developed in retrograde hydrous assemblages (e.g., chlorite peridotite).

1.3. Sandvik peridotite

Fig. 3. Polished hand sample (A) and photomicrograph (B) of peridotite sample 4A. The Sandvik ultramafic body is a relatively small outcrop of well Note the porphyroclasts of olivine (ol), orthopyroxene (opx), clinopyroxene (cpx), and foliated garnet peridotite with subordinate layers of garnet pyroxenite garnet (grt) in a fine-grained matrix of olivine, orthopyroxene, spinel, and pargasitic that occur parallel to the foliation in the peridotite. Although it is amphibole (minor). Note the kelyphite (kely) rims around porphyroclastic garnet. 148 T.J. Lapen et al. / Lithos 109 (2009) 145–154 amphibole (Fig. 3). Pyroxene porphyroclasts contain prominent exsolution lamellae of spinel and the complementary pyroxene (Fig. 4). Garnet porphyroclasts are consistently surrounded by kelyphitic rims, which are composed of olivine+garnet+spinel+ orthopyroxene+clinopyroxene±pargasitic amphibole (Fig. 5). Ap- proximately 100 m distant from the Sandvik peridotite is an outcrop of peridotite that has been completely recrystallized to an assemblage of olivine+orthopyroxene+chlorite+tremolitic amphibole. Based on these textural relations, the Sandvik peridotite is thought to record four stages of equilibration (Fig. 6), as follows:

Stage 1, porphyroclasts: ol+grt+opx+cpx (pre-exsolution compositions) Stage 2, kelyphite: ol+grt+Al-spl+opx+cpx±am Stage 2, pyroxene exsolution: ol+Cr-spl+opx+cpx Stage 3, granoblastic: ol+spl+opx+am Stage 4, foliated: ol+chl+opx+am (neighboring peridotite outcrop) The rocks and constituent minerals at Sandvik record a remarkably long history, starting with separation from the convecting mantle at 3.3 Ga (Beyer et al., 2004; Lapen et al., 2005), cooling through the Lu– Fig. 5. A) False color image of Stage 2 kelyphite constructed from Al, Ca, and Mg X-ray maps. Blue, olivine; purple, orthopyroxene; green, amphibole; tan, garnet; pink, spinel. Scale bar is 50 µm. B) Back-scattered electron image of kelyphite, showing fine-grained intergrowth of spinel (spl) with garnet (grt) and with orthopyroxene (opx). Scale bar is 100 µm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Hf closure temperature at about 1.25 Ga, and experiencing cryptic metasomatism at 1.15 Ga (upper age constraint on deep mantle residence; Lapen et al., 2005). These events were followed by development of a kelyphitic mineral assemblage, which reflects exhumation from deep mantle conditions between 1.15 Ga and the ~400 Ma Scandian phase of the Caledonian orogeny. The new petrologic and geochronologic data presented here, in conjunction with previous data (Carswell et al., 1983; Brueckner et al., 1996; Medaris et al., 2003; Beyer et al., 2004; Lapen et al., 2005), indicate that a 3 billion year history is recorded at Sandvik. Because this history is incompletely recorded in the crustal rocks which presently host the

Fig. 6. Metamorphic evolution and mineral stages recorded at Sandvik. Stages 1 and 2 are discussed in detail in the main text; Stages 3 and 4 represent retrograde assemblages associated with Caledonian recrystallization, retrogression and hydration. Fig. 4. Back-scattered electron images of orthopyroxene (upper panel) and clinopyr- Note that the Stage 4, chlorite-bearing assemblage is not present at the sample locality oxene (lower panel) porphyroclasts. The orthopyroxene contains exsolution lamellae of itself, but is present in nearby outcrops. ‘CV’ refers to the 8-Stage metamorphic clinopyroxene and spinel; scale bar is 200 μm. The clinopyroxene contains exsolved evolution of Mg–Cr peridotites in the WGR (Carswell and van Roermund, 2003), four of orthopyroxene and spinel; scale bar is 100 μm. which are correlated with the four Stages present at Sandvik. T.J. Lapen et al. / Lithos 109 (2009) 145–154 149 peridotite, the Sandvik peridotite represents a unique into factor of 2 increase in ion intensity. Oxygen isotope compositions of the history of Baltic continental lithosphere. 18O/16O=0.002110 and 17O/16O=0.000387 were used to correct the data. Mass analysis was done using a GV Instruments Sector 54 TIMS 2. Analytical methods located in the Radiogenic Isotope Laboratory at the University of Wisconsin–Madison via a three-jump multicollector dynamic analysis Mineral compositions were determined with a Cameca SX-51 and an exponential-law normalization to 146Nd/144Nd=0.7219. electron microprobe at the University of Wisconsin–Madison under During the course of this study, the measured 143Nd/144Nd ratio the following operating conditions: accelerating voltage of 15 kV, a of in-house standards UW AMES-Nd I and UW AMES-Nd II are beam current of 20 nA, and a suite of analyzed natural minerals as 0.512136±18 (2-SD, n =16) and 0.511972±20 (2-SD, n =12), standards; a φ(ρz) data reduction program was used to correct the respectively, and the long-term measured 143Nd/144Nd ratio of USGS data (Armstrong, 1988). The bulk compositions of exsolved pyroxenes rock standard BCR-1 is 0.512647±13 (2-SD, n=7). Samarium was were calculated from the compositions of host and lamellae domains analyzed as Sm+ on Re filaments loaded with silica gel and phosphoric and their modal proportions, as determined by software analysis of acid. All Sm data was internally normalized to 147Sm/152Sm≡0.5608 back-scattered electron images. (derived from Russ, 1973) for isotope dilution calculations. Laboratory Kelyphite from sample 4G-1 of Lapen et al. (2005) was prepared for blanks were b150 pg for Nd and b50 pg for Sm, which are negligible. Sm–Nd analysis by picking ~250 mg of pure material from 250–300 mesh sized crushed whole rock. Special care was taken to avoid 2.1. Mineral chemistry material that contained the stage 1 porphyroclastic assemblage by optical inspection of every grain picked; the stage 1 garnet appears The compositions of minerals in the Sandvik garnet peridotite are gemmy while the kelyphite assemblage is quite dull. The samples generally similar to those in other Mg–Cr garnet peridotites in the were washed in ultra-pure H2O three times to remove fine-grained WGR, consisting of forsterite, low-alumina enstatite, Cr-diopside, and material and some surface contamination. The samples were then pyrope-rich garnet. Olivine grains are homogeneous in composition, placed in Savillex® beakers along with 3 ml 1 M twice-distilled HF and but pyroxene and garnet porphyroclasts tend to be zoned, typically

0.5 ml 7 M distilled HNO3. Samples were capped and heated to ~60 °C characterized by relatively homogeneous cores and an increase in for 60 min followed by removal of the supernatant with a pipette Al2O3 at the rims of pyroxene and an increase in Fe/Mg ratio at the which was saved in another clean Savillex® beaker. Two milliliters of rims of garnet. Representative mean analyses of the cores of Stage 1 2.5 M HCl was added to the sample and it was heated to ~80 °C for (reconstructed) and Stage 2 minerals in the Sandvik garnet peridotite 60 min to dissolve any fluoride salts that may have precipitated during are summarized in Table 2. the HF step. The HCl was removed and transferred to the beaker with The compositions of Stage 1 olivine are highly magnesian, having the HF/HNO3 solution (supernatant from previous step). The HCl step mg-numbers of 92.4 in garnet peridotite and 93.0 in olivine-garnet was repeated another two times and was followed by three rinses pyroxenite. Olivine in Stage 2 kelyphite is more magnesian, with mg- with ultra-pure H2O; all solutions were saved as ‘kelyphite leach 1’ numbers ranging from 93.6 to 94.4. (Table 1). The remaining solids from ‘leach 1’ were leached in 3 ml 2 M Enstatite of the different recrystallization stages has slightly twice-distilled HF and 0.5 ml 7 M HNO3 at ~80 °C for 3 h; the different contents of Al2O3 and CaO, reflecting the different equilibra- supernatant was removed and placed in another beaker as ‘kelyphite tion conditions of the various stages. Stage 2 enstatite, both as the host leach 2’. The HCl and H2O rinses for this leaching step are identical to of exsolved clinopyroxene and as the exsolved phase in clinopyroxene, that of ‘kelyphite leach 1’. The third and final leach was done with 3 ml is relatively low in Al2O3, ~0.50%, and CaO, ~0.20%. In contrast, the twice-distilled 29 M HF and 0.5 ml 7 M HNO3 heated to ~110 °C for bulk composition of Stage 1 enstatite porphyroclasts (re-integrated 30 h. As with the previous leaching steps, the HF/HNO3 supernatant compositions of orthopyroxene host and clinopyroxene and spinel was removed and saved as ‘kelyphite leach 3’; the remaining lamellae; Fig. 4), is slightly more aluminous, 0.57%, and significantly precipitates were subjected to the same HCl and H2O rinses that higher in CaO, 0.67%, than that of the Stage 2 host. Stage 2 enstatite in were applied to the other leaching steps. The remaining solids in the kelyphite has the highest Al2O3 content, 1.80 to 2.85%, and lowest final leaching step were tiny chromite grains that were not dissolved amount of CaO, 0.13 to 0.16%, among the stages. by hot-plate dissolution methods. A 149Sm–150Nd mixed isotope tracer Like enstatite, Cr-diopside of the different stages has different was added to each of the 3 leachates and the samples were taken to contents of Al2O3,Cr2O3, and CaO. Stage 2 diopside, the host to dryness. Bulk separation of REEs from the sample matrix was exsolved orthopyroxene, contains approximately equal amounts of accomplished with cation exchange resin using 2.5 M and 6 M HCl; Al2O3 and Cr2O3, 1.55 to 1.60%, and a relatively high CaO content, α-hydroxyisobuteric acid using cation exchange resin was used to ~23.0%. The bulk composition of Stage 1 diopside porphyroclasts (re- separate Nd and Sm. No precipitates were present in solutions prior to integrated compositions of clinopyroxene host and orthopyroxene column separation of Sm and Nd. and spinel lamellae; Fig. 4) is appreciably higher in Al2O3, 2.18%, and + Neodymium was analyzed as NdO using single Re filaments; Nd Cr2O3, 2.62%, and lower in CaO, 20.12%, compared to the Stage 2 host. was loaded along with silica gel and phosphoric acid. Oxygen was bled Stage 2 diopside in kelyphite has a relatively high CaO content, into the mass spectrometer source near the filament resulting in a ~22.60%, and contains much more Al2O3, 4.50%, and less Cr2O3 , 0.50%, than those in Stages 1 and exsolved Stage 2 Cr-diopside. Stage 1 garnet in peridotite is pyrope rich; the cores of garnet porphyroclasts have a composition of pyrope 71–72, almandine 14–18, Table 1 spessartite 0.5–1.0, and grossularite 10–13. Garnet in Stage 2 kelyphite Measured Sm–Nd isotope data of kelyphite. is also pyrope rich (prp 71, alm 16, sps 1, grs 12), but has much lower Sample name Sm (ng) Nd (ng) 147Sm/144Nd 143Nd/144Nd 2%SE Cr2O3, 0.6%, compared to 2.1 to 3.1% in porphyroclastic garnet. Leachate 1 14.8 21.2 0.3509 0.513659 ±8 Stage 2 spinel occurs as lamellae in enstatite and diopside Leachate 2 47.6 88.0 0.3274 0.513577 ±11 porphyroclasts and intergrown with enstatite and garnet in kelyphite. Leachate 3 11.7 20.2 0.4225 0.513952 ±18 There is a marked compositional contrast for spinel in these two 147 144 143 144 Errors in Sm/ Nd and Nd/ Nd used in age calculations are 0.2% (2σ) and occurrences, where spinel in pyroxene porphyroclasts is less magne- 0.0035% (2σ), respectively, based on long-term external reproducibility of rock and sian and more chrome-rich (mg-number, 60.5 to 61.4; cr-number, 46.3 standard solutions. See text for analytical details. 2%SE errors are from internal, in-run statistics. Concentrations of Sm and Nd are given in total mass of element analyzed to 54.3) than in kelyphite (mg-number, 83.6 to 87.3; cr-number, 6.4 to because the mass of dissolved kelyphite in each leaching step is unknown. 15.8). 150 T.J. Lapen et al. / Lithos 109 (2009) 145–154

Table 2 Table 3 Representative mineral compositions of stages 1 and 2 assemblages. Comparison of T–P estimates.

Mineral ol ol opx opx cpx cpx grt grt grt Lithology Sample Stage Geothermometer Iterative solutions to various Stage 1 2 1 2 1 2 1 1 2 geothermobarometers Sample 4A 4G 4A 4G 4A 4G 4A 4G 4G NG85 BK90 BBG07 wt.% oxides T, °C P, GPa T, °C P, GPa T, °C P, GPa

SiO2 41.38 40.99 58.25 57.55 52.97 53.63 42.24 42.36 41.88 peridotite 4A 1 T98 1239 6.4 1240 7.0 1223 6.0 a TiO2 ––0.01 0.04 0.01 0.65 0.01 0.23 0.02 peridotite 4A 1 OW79 996 5.0 1003 5.2 976 4.5

Al2O3 ––0.57 1.86 2.18 4.50 21.66 22.33 24.00 peridotite 4G-F 2 H84 713 1.7 715 1.7 677 1.0

Cr2O3 ––0.30 0.19 2.62 0.50 3.12 2.21 0.62 peridotite 4C-A1 2 H84 720 1.7 726 1.8 692 1.1 FeO 7.44 5.54 4.87 4.52 2.02 1.27 7.17 7.75 8.30 peridotite 4C-A2 2 H84 761 1.8 764 1.8 731 1.3 MnO 0.09 0.06 0.11 0.07 0.05 0.07 0.42 0.34 0.38 peridotite 4C-C2 2 H84 709 1.3 721 1.5 683 0.8 MgO 50.40 52.82 35.75 36.23 19.14 15.96 20.28 21.01 20.02 ol 4I-D1 2 H84 713 1.6 713 1.6 676 0.9 NiO 0.39 0.38 ––––––– pyroxenite CaO ––0.67 0.16 20.12 22.63 5.28 4.16 4.79 ol 4I-D2 2 H84 725 1.2 725 1.2 675 0.3

Na2O ––0.01 0.01 0.79 1.22 ––– pyroxenite Sum 99.70 99.81 100.81 100.64 99.90 100.42 100.18 100.41 100.01 BK90: Brey and Köhler, 1990; BBG07: Brey, Vulatov and Girnis, 2007; H84: Harley, 1984; NG85: Nickel and Green, 1985; OW79: O'Neill and Wood, 1979; T98: Taylor, 1998.P-T Cations data for sample 4C is based on unpublished data of L.G. Medaris. Si 1.006 0.989 1.985 1.954 1.920 1.926 3.004 2.995 2.970 a Spurious P–T results for Stage 1. Ti ––0.000 0.001 0.000 0.018 0.000 0.012 0.001 Al ––0.023 0.074 0.093 0.190 1.816 1.861 2.006 Cr ––0.008 0.005 0.075 0.014 0.176 0.124 0.035 (Brey and Köhler, 1990; Taylor, 1998), combined with three different Al- Fe 0.151 0.112 0.139 0.128 0.061 0.038 0.427 0.458 0.492 in-orthopyroxene geobarometers (Nickel and Green, 1985; Brey and Mn 0.002 0.001 0.003 0.002 0.002 0.002 0.025 0.021 0.023 Köhler, 1990; Brey et al., 2008). The re-integrated compositions of Mg 1.827 1.901 1.817 1.834 1.034 0.854 2.150 2.214 2.117 porphyroclasts yield values of 1220–1240 °C and 6.0–7.0 GPa (Taylor, Ni 0.008 0.007 ––––––– 1998; Table 3), which place the Stage 1 assemblage in the majorite field Ca ––0.024 0.006 0.781 0.870 0.402 0.315 0.364 Na ––0.001 0.001 0.056 0.085 ––– (Fig. 7), although garnet porphyroclasts in the Sandvik peridotite show Sum 2.994 3.011 4.000 4.005 4.023 3.997 8.000 8.000 8.009 no evidence of a majoritic composition. Results from application of the Mg # 92.4 94.4 92.9 93.5 94.4 95.7 83.4 82.8 81.1 Brey and Köhler geothermometer (not tabulated) are closely similar to Mineral spl spl spl spl those from the Taylor method. Stage 2 2 2 2 Ordinarily, P–T estimates for garnet peridotites are obtained from Sample 4A 4A 4G 4G application of the olivine-garnet Fe–Mg exchange geothermometer Location in opx in cpx inner kelyphite outer kelyphite (O'Neill and Wood, 1979), combined with an Al-in-orthopyroxene wt.% oxides geobarometer, as recommended by Brenker and Brey (1997), the

TiO2 0.00 0.00 0.05 0.01 rationale being that Fe–Mg diffusion is relatively slow in garnet, Al Al2O3 29.16 24.29 53.65 61.90 diffusion is slow in orthopyroxene, and the composition of olivine, Cr2O3 37.53 43.09 14.97 6.32 which is the predominant phase in peridotite, will change little by Fe– V2O3 0.00 0.35 0.03 0.01 FeO 15.90 16.33 8.19 6.49 Mg exchange with garnet during cooling. This approach, using the MnO 0.00 0.17 0.01 0.00 compositions of Stage 1 olivine, garnet cores, and re-integrated MgO 13.35 12.90 21.17 22.82 orthopyroxene, yields values of 975–1005 °C and 4.5–5.2 GPa ZnO 0.00 0.19 0.03 0.01 (Table 3), which lie within the upper part of the P–T array for WGR NiO 0.00 0.02 0.16 0.31 Sum 95.94 97.35 98.27 97.87

Cations Ti 0.000 0.000 0.001 0.000 Al 1.064 0.897 1.672 1.863 Cr 0.918 1.067 0.313 0.128 V 0.000 0.009 0.001 0.000 Fe 0.411 0.428 0.181 0.139 Mn 0.000 0.005 0.000 0.000 Mg 0.616 0.603 0.835 0.869 Zn 0.000 0.004 0.001 0.000 Ni 0.000 0.001 0.003 0.006 Sum 3.009 3.013 3.006 3.005 Mg # 61.4 60.5 83.6 87.3 Cr# 46.3 54.3 15.8 6.4

2.2. Pressure and temperature conditions

Numerous geothermometers and calibrations of the Al-in-orthopyr- oxene geobarometer are available for application to garnet peridotites, but the determination of meaningful P–T values for such rocks is challenging due to polymetamorphism, compositional zoning in Fig. 7. Pressure and temperature diagram depicting the four paragenetic Stages minerals, and the inherent problem of utilizing a combination of recorded at Sandvik and nearby outcrops. The P–T array for WGR Mg–Cr peridotites exchange and net transfer reactions, which may have frozen in at (modified from Medaris, 1999) represents a metamorphic field gradient which is different times under different P–T conditions. Because the Stage 1 pre- similar to an inferred cratonic geotherm. Stages 1 and 2 have been directly dated, whereas stages 3 and 4 are related to Caledonian orogeny at 400 Ma. Mineral equilibria exsolution pyroxene compositions are most likely to record the highest shown for reference are diamond-graphite (Bundy, 1980), coesite-quartz (Bohlen and – temperatures in the Sandvik peridotite, P T estimates for Stage 1 have Boettcher, 1982), garnet-spinel peridotite (O'Hara et al., 1971), and majorite-in-garnet been obtained by application of two, two-pyroxene geothermometers isopleths from Gasparik (2003). T.J. Lapen et al. / Lithos 109 (2009) 145–154 151 garnet peridotites (Fig. 7). However, these results are significantly result in leachates with variable 147Sm/144Nd ratios reflecting various lower than those obtained above, most likely due to the core mixtures of dissolved minerals. Fractionation of Sm and Nd during compositions of garnet having been modified by Fe–Mg exchange leaching is very unlikely given their similar chemical behavior in HF and with olivine during cooling from the Stage 1 high temperatures of HCl as shown by extensive partial dissolution tests. These tests included 1220–1240 °C. Thus, the P–T results based on garnet and olivine partial dissolution of garnet-bearing whole-rock samples and garnet compositions, in this instance, are spurious, and do not represent the separates, which indicate that Sm and Nd are not fractionated and initial Stage 1 P–T conditions. that the primary (either protolith or metamorphic) relationship Although olivine is present in the Stage 2 kelyphite assemblage, it between 147Sm/144Nd ratios and 143Nd/144Nd ratios are not disturbed is not volumetrically predominant, and application of the olivine- during leaching (Mahlen et al., 2008). This approach resulted in garnet Fe–Mg exchange thermometer yields improbably low tem- leachates with measured 147Sm/144Nd ratios that are correlated to peratures, most likely reflecting loss of Fe from olivine to garnet 143Nd/144Nd ratios, which define a statistically meaningful 3-point Sm– during post-Stage 2 cooling. Consequently, P–T conditions for the Nd isochron age of 606±39 Ma (2σ;MSWD=0.88;Fig. 8). Stage 2 kelyphite assemblage have been derived from the orthopyr- oxene-garnet Fe–Mg exchange geothermometer (Harley, 1984), 3. Discussion combined with the three Al-in-orthopyroxene geobarometers (Table 3). Note that pressures for the Stage 2 kelyphite assemblage Carswell (1986) and Carswell and van Roermund (2003) outlined estimated from the Brey et al. (2008) geobarometer are considerably the various mineral assemblages preserved in several fresh peridotite lower than those from the other two geobarometers, which are in bodies across the WGR that reflect their extremely long history. Based close agreement with each other. Because the Brey et al. (2008) on petrologic analyses by several workers (Carswell, 1986; Medaris calibration is based largely on experiments at 6.0 to 10.0 GPa, its and Carswell, 1990; Jamtveit et al., 1991; Medaris et al., 2003; Carswell application to the relatively low pressure kelyphite assemblage may and van Roermund, 2003), there are 8 mineral assemblages that are not be appropriate. Accordingly, we have used the Brey and Köhler recognized in peridotites in the WGR, including 2 that are kelyphitic. (1990) and Nickel and Green (1985) geobarometers, which were At Sandvik and a nearby outcrop, however, only 4 paragenetic stages calibrated at 1.0 to 6.0 GPa, to obtain values for kelyphite of 710–760 °C are present. Fig. 6 summarizes the assemblages at Sandvik, discussed and 1.2–1.8 GPa (Table 3), which lie close to the boundary between below, and compares those to the Stages recorded across the WGR. garnet and spinel peridotite (Fig. 7), and are consistent with the The Stage 1 mineral assemblage at Sandvik (Fig. 6) is similar to that presence of both garnet and spinel in the kelyphite assemblage. of Mg–Cr peridotites elsewhere in the WGR. Local differences include the presence of majoritic garnet at Otrøy (van Roermund et al., 2001; 2.3. Kelyphite age Spengler et al., 2006), which is lacking at Sandvik, even though estimated pressures are within the ‘majorite field’ (Fig. 7). Overall Samarium-neodymium isotope analyses were carried out on similarities in WGR peridotites are that the Stage 1 assemblages kelyphite from sample 4G-1 of Lapen et al. (2005). Because the minerals record a high-pressure paragenesis (Carswell and van Roermund, that constitute the kelyphite assemblages are very small, 5–40 μm 2003). Based on Lu–Hf, Sm–Nd, and Re–Os isotope systematics of the (Fig. 8), physical separation of the minerals is impossible. We therefore whole rocks and high mg-numbers of Stage 1 olivine, the Stage 1 performed a chemical leaching method (described earlier) that assemblages have preserved the geochemical imprint of extensive relied upon the relative refractory nature of the phases in the kelyphite (~20–30%) melt depletion during the Middle Archean (e.g., Beyer et assemblage. For example, garnet and spinel are more refractory al., 2004; Lapen et al., 2005; Spengler et al., 2006). These Middle phases in HF than amphibole and pyroxene, and these minerals have Archean ages are over a billion years older than any known crustal widely different Sm/Nd concentration ratios; thus step-leaching will rocks which host these bodies, and the nearest crust of Middle Archean age in Baltica occurs over 1000 km to the east of the WGR (Slabunov et al., 2006; Peltonen and Brügmann, 2006). The Stage 1 porphyroclastic assemblage at Sandvik, as well as elsewhere in the WGR, represents deep lithospheric mantle condi- tions consistent with residence in the subcontinental lithospheric mantle. Sm–Nd and Lu–Hf mineral isochron ages of the Stage 1 assemblages from peridotites across the WGR typically yield Proter- ozoic ages (Fig. 2 and references therein) which is in sharp contrast to the Archean whole rock model ages determined from the same samples (Jamtveit et al., 1991; Lapen et al., 2005; Spengler et al., 2006). The spread in ages have been interpreted as due to variable re- equilibration during cooling from a widespread ~1.7 Ga equilibration age (Jamtveit et al., 1991; Brueckner et al., 2002; Medaris and Brueckner, 2003; Lapen et al., 2005), metasomatic re-equilibration of Archean and/or Proterozoic protoliths (Lapen et al., 2005), or formation ages associated with high-pressure trapped melt compo- nents. At Sandvik, however, the Stage 1 assemblage variably records cooling through the Lu–Hf closure temperature and metasomatic alteration between 1.25 and 1.15 Ga (Lapen et al., 2005), which corresponds to the age of widespread deformation, amphibolite- to granulite-facies metamorphism, and magmatism in the crust asso- ciated with the Sveconorwegian orogeny. This suggests that ancient Fig. 8. Sm–Nd isochron diagram of Stage 1 and Stage 2 assemblages of the sample 4G-1. melt depletion ages are relicts that have been variably overprinted The Stage 1 assemblage of garnet (gt) and clinopyroxene (cpx), along with the whole during the Sveconorwegian (Lapen et al., 2005). The imprint of this rock (wr) define a 3-point isochron age of 1147 Ma (data and discussion in Lapen et al., orogenic event is very strong in the WGR and suggests that the 2005), whereas the step-leaching of the Stage 2 kelyphite (kel) yields an age of 606 Ma. All ages are reported with errors at the 2-standard deviation level. Data points (black Sandvik peridotite was part of the Baltica subcontinental lithosphere dots) are much larger than the errors at the scale of the diagram. that experienced this lithosphere-scale orogenic-magmatic event. 152 T.J. Lapen et al. / Lithos 109 (2009) 145–154

The Stage 2 kelyphite assemblage (Fig. 6) is of particular importance with opening of the Iapetus Ocean or Ægir Sea. This process could be because it records uppermost mantle/lower crustal P–Tconditions, analogous to, though not as extreme as, models for the rifted margin of probably associated with the emplacement of these peridotite bodies the Ligurian Tethys (Lemoine et al., 1987; Manatschal, 2004), where into the crust or uppermost mantle (Brueckner et al.,1996; Carswell and deep lithospheric mantle is exhumed to such a degree that it becomes van Roermund, 2003). Although two generations of kelyphite assem- structurally associated with felsic crust. blages can be recognized in certain peridotites of the WGR (Carswell and During Neoproterozoic rift-related exhumation and cooling, Meso- van Roermund, 2003), a 5-phase assemblage containing garnet (Griffin proterozoic mineral porphyroclasts were locally preserved, whereas and Heier, 1973; Carswell, 1986) represents the earliest assemblage that kelyphite recrystallization records the ~600 Ma age of rifting. will provide critical constraints on their maximum residence time in the Subsequent Caledonian subduction and associated eclogite-facies lithospheric mantle (not including Caledonian subduction). The new metamorphism, as well as tectonic modification during burial and age and P–T data of the Stage 2, 5-phase kelyphitic assemblage places exhumation, likely resulted in final emplacement of mantle fragments additional constraints on the history of the Sandvik peridotite. The Sm–Nd into crustal rocks, during which hydrous ultramafic mineral assem- isochron age of 606±39 Ma is similar to a two-point Sm–Nd isochron age blages were stabilized (Fig. 6). This scenario explains how peridotites in of 746±26 Ma for kelyphite also from Sandvik (Brueckner et al., 1996). the WGR retain much older ages than associated eclogites, which is an Metamorphic conditions during garnet-bearing kelyphite formation are unusual feature of WGR peridotites compared to orogenic peridotites in 710–760 °C and 1.2–1.8 GPa (Table 3; Fig. 7). These conditions represent other collisional orogens (Brueckner and Medaris, 2000). the stage at which the Sandvik peridotite was at depths of 40 to 60 km, depending on the thickness and composition of crustal and mantle 4. Conclusions lithologies, placing these rocks in the lowermost crust or uppermost mantle lithosphere in the Late Proterozoic. Lu–Hf, Sm–Nd, and Re–Os isotope and petrologic analyses of the An important concern regarding the Sandvik kelyphite age data is Sandvik peridotite indicate an exceptionally long history from whether the 606±39 Ma Sm–Nd age represents crystallization, Archean to Paleozoic time (Beyer et al., 2004; Lapen et al., 2005). cooling, or partial equilibration during Caledonian metamorphism. In This history involves the following stages: 1) Initial melt depletion at order to evaluate this, we must consider the age data in the context of 3.3 Ga, a finding that is consistent with Archean melt depletion ages the following constraints. 1) It is clear that the kelyphite-forming determined elsewhere in the WGR (Brueckner et al., 2002; Beyer et al., reaction occurred after elemental and isotopic equilibration of the 2004). Trace element modeling of the same garnet peridotite indicates 1220–1240 °C and 6.0–7.0 GPa Stage 1 assemblages at 1.15 Ga, which that the original melt depletion event involved up to 25% partial represents the maximum possible age of the Stage 2 assemblage melting (Lapen et al., 2005), which is consistent with Mg# in olivine (Lapen et al., 2005). 2) The metamorphic conditions recorded in the as high as 93. A similar high degree of Archean partial melting is also Stage 2 assemblage are 710–760 °C and 1.2–1.8 GPa, ~250 °C and inferred for another WGR peridotite body at Otrøy north of Sandvik ~5 GPa lower than conditions recorded in Stage 1, reflecting (Spengler et al., 2006). 2) Cooling and metasomatic alteration exhumation of at least 150 km. And 3), there are significant between 1.25 Ga and ~1.15 Ga is associated with the 1.25–1.00 Ga differences in mineral compositions between the Stage 1 and Stage Sveconorwegian orogeny (Lapen et al., 2005). This event records the 2 assemblages (Table 2). We interpret the Stage 2 assemblage at maximum age for the Sandvik Stage 1 assemblage (1220–1240 °C and Sandvik to be the result of continental rifting, because the kelyphite- 6.0–7.0 GPa). 3) Further cooling and decompression is recorded by the forming event must have involved significant exhumation (~150 km) Stage 2 kelyphitic assemblage (710–760 °C and 1.2–1.8 GPa). The of the mantle lithosphere. Such rifting may have been associated with kelyphite-forming event is ~600 Ma, based on a leaching method that opening of the Iapetus Ocean in the Late Proterozoic or perhaps produced a 3 point isochron with a Sm–Nd age of 606±39 Ma (2σ). related to the opening of the Ægir Sea, although distinguishing This age is incompatible with kelyphite formation during the ~400 Ma between these two scenarios is beyond the scope of this paper. Scandian phase of the Caledonian orogeny, but it is consistent with It is possible that the 606 Ma age represents a cooling age from an formation during continental rifting associated with opening of the earlier event, particularly since the grain size in the kelyphite is small Iapetus Ocean at ~600 Ma (Torsvik et al., 1996; Bingen et al., 1998; (~5–40 μm diameter). However, the only lithosphere-scale event likely Cawood and Pisarevsky, 2006; Cawood et al., 2007), or perhaps to exhume large sections of deep lithospheric mantle between 1.15 and associated with rifting along the margin of the Ægir Sea (e.g. Cocks and 0.6 Ga in the WGR is continental rifting associated with the breakup of Torsvik, 2005). And, 4) Continent–continent collision between the Baltica and Laurentia, or perhaps Baltica and Siberia. If the kelyphite Baltic and Laurentian margins at about 420–400 Ma resulted in a formed during HP metamorphism in the Caledonian (~400 Ma), the strong penetrative fabric associated with the development of mineral Sm–Nd age of 606 Ma would indicate partial re-equilibration of the Stages 3 and 4 at Sandvik and vicinity, as well as in most peridotite Stage 2 from the Stage 1 assemblage. Partial re-equilibration is not bodies in the WGR. likely the case, because the kelyphite phases are chemically distinct The four mineral stages and isotope compositions of mantle- from those of Stage 1 (Table 2), the kelyphite texture indicates that the derived rocks at Sandvik record the evolution of Baltic lithospheric reactant Stage 1 garnet was completely recrystallized (Fig. 5), and mantle from Archean to mid-Paleozoic time. This long record is either there is no evidence for the presence of relict Stage 1 phases within the incompletely preserved or obscured in the surrounding crustal rocks, kelyphite that would result in mixed ages. making these peridotite bodies unique windows into the timing of The kelyphite assemblages in other peridotites of the WGR have been continental growth and its modification. attributed to Caledonian orogeny that is perhaps associated with crust- mantle interactions during early exhumation of subducted continental Acknowledgements crust and peridotite (Brueckner, 1998; Brueckner and Medaris, 2000). The Sm–Nd age at Sandvik, however, raises the possibility that the We thank Dr. John Fournelle for his expert assistance in the UW- formation of kelyphite in some peridotites across the WGR may be Madison electron microprobe laboratory. We would also like to thank related to Neoproterozoic exhumation and continental rifting and not to the organizers of the 2007 International Eclogite Field Symposium and the ~400 Ma Scandian phase of the Caledonian orogeny. Because rifting Workshop of the Task Force IV, International Lithosphere Program, is an alternative explanation which provides a viable mechanism for Portree, Scotland, where a session entitled “Ultra-deep Continental decompression and cooling of mantle rocks (Robinson et al., 2003), Crust Subduction” was dedicated to the memory of the late Tony peridotites of the WGR may therefore have been significantly exhumed Carswell. This contribution is also dedicated to Tony, our friend and from depths greater than 150 km during continental rifting associated colleague of many years, who pioneered the investigation of WGR T.J. Lapen et al. / Lithos 109 (2009) 145–154 153 peridotites. We also thank two anonymous reviewers whose com- Dalziel, I.W.D., 1997. 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