Chapter 6Empla ement of mantle peridotiteinto the rust and subsequentevolution

6.1 Introdu tion 1596.1 Introdu tionThe S andinavian Caledonides omprises a sta k of thin-skinned allo hthonous andparauto hthonous nappes thrusted eastwards onto the Balti shield during the Cale-donian ontinent� ontinent ollision between Laurentia and Balti a (Gee and Sturt,1985). In the entral and northern part of the S andinavian Caledonides, the ol-lision has been subdivided into the major orogeni phases: the Finnmarkian at . 500Ma (Stephens and Gee, 1989; Andréasson and Albre ht, 1995), the Trondheimevent at . 480Ma (Eide and Lardeaux, 2002; Roberts et al., 2002b), the Jämtlandianat . 454Ma (Brue kner and Van Roermund, 2004) and the S andian at . 425�400Ma (Gee, 1975; Stephens and Gee, 1989) (Table 1.1, Fig. 1.7). In ontrast, onlythe Jämtlandian (Andersen et al., 1998) and the S andian has been re ognised in thesouthern part of the S andinavian Caledonides, the latter in metagabbro, e logiteand gneiss by U�Pb and Sm�Nd isotope systems (Gri�n and Brue kner, 1980, 1985;Mørk and Mearns, 1986; Tu ker et al., 1987; Carswell et al., 2003b; Walsh et al.,2006). The S andian metamorphism in the SW rea hed UHP metamorphi ondi-tions during the subdu tion of the Balti plate margin into the Dia stability �eld(Dobrzhinetskaya et al., 1995; Van Roermund et al., 2002; Carswell and Van Roer-mund, 2005; Vrijmoed et al., 2006). At present, the te tono-stratigraphi lowermostunit in the nappe pile of the S andinavian Caledonides represents part of the Balti plate margin and is exposed as the Western Gneiss Complex (WGC) in the West-ern Gneiss Region (WGR). E logite en losed within basement ro ks of the WGCre ord an in reasing metamorphi gradient from the SE inland ( . 600 °C) towardsthe NW oastal area (>800 °C) (Krogh, 1977; Gri�n et al., 1985), where three UHPmetamorphi provin es o ur: Nordfjord�Stadlandet, Sørøyane�Hareidlandet andNordøyane�Otrøy (Fig. 1.8) (Smith, 1984; Wain, 1997; Terry et al., 2000b; Carswellet al., 2003b, 2006; Root et al., 2005; Van Roermund et al., 2005). In addition, theentire WGC ontains both LP Chl- and HP Grt-peridotite. In the extreme NW(Otrøy and Nordøyane), Grt-peridotite with majoriti mi rostru tures o urs (VanRoermund and Drury, 1998; Terry et al., 1999) (Fig. 1.8). The ultrama� ro ks are onsidered to be derived and exhumed from the sub- ontinental lithospheri mantle(SCLM) hanging wall during subdu tion and exhumation of the WGR ontinentalbasement (Brue kner, 1998; Brue kner and Medaris, 2000). External e logite fromNordøyane re ords the maximum extent of ontinental exhumation from 3.4�3.9GPaand 820 °C (Terry et al., 2000b), proposed to have started between 407±2Ma (Mnz)(Terry et al., 2000a) and 402±2Ma (Zrn) (Carswell et al., 2003a). The CaledonianUHP metamorphi onditions of the Balti plate margin have re ently been re�nedon external Opx-e logite from Otrøy and Fjørtoft, whi h reveals peak metamorphi onditions of . 4.5�5GPa and 850�900 °C (Van Roermund et al., 2005; Carswellet al., 2006).Although the S andian UHP metamorphism is well onstrained, there is growingeviden e for an early S andian metamorphi evolution: e logitisation ages (Sm�Nd,Rb�Sr) of 425�422Ma (Glodny et al., 2002) and e logite-fa ies rystallization agesof Zrn (U�Pb) of 423±4Ma (Bingen et al., 2004) in the Bergen Ar s (Fig. 1.6), a

160 6. EMPLACEMENT INTO THE CRUSTre ently re-determined rystallization age (Sm�Nd) of the Grytting e logite in theNordfjord�Stadlandet UHP provin e of 422±10Ma (H.K. Brue kner, pers. om.2006; Fig. 1.8) and an impre ise re- rystallization age (Sm�Nd) of Grt-websteritefrom Raudhaugene (Otrøy) of 437±58Ma (Jamtveit et al., 1991,; Fig. 1.10). Theearly S andian isotopi re ord indi ates that the S andian (U)HP metamorphismmay be more omplex than previously thought.This hapter onstraints the timing and depth of the empla ement of the Otrøyperidotite into ontinental basement gneiss of the WGR. It will be demonstratedthat the Nordøyane�Otrøy UHP provin e re ords early S andian metamorphi agesand that the peak UHP metamorphism in the area rea hed higher onditions thanpreviously demonstrated using Al2O3 isopleths in Opx-e logite or (in situ) mi ro-Dia.6.2 Stru tural setting6.2.1 GneissThe peridotite bodies on Otrøy are embedded in e logite and retro-e logite bearing,predominantly amphibolite-fa ies gneiss, whi h omprises strongly deformed to my-loniti dioriti to granodioriti gneiss and less deformed oarse-grained migmatiti or augen gneiss (Fig. 1.10 and 6.1). E logite and retro-e logite bodies o ur morefrequently in proximity to the Ugelvik and Raudhaugene peridotite bodies om-pared to other parts of western Otrøy (Fig. 1.10). The e logite bearing gneiss(northern Otrøy) represents Proterozoi basement of the Balti shield (Carswelland Harvey, 1985), whi h be ame juxtaposed next to allo hthonous e logite-freethrust nappes (southern Otrøy) during the Caledonian orogeny (Fig. 1.10). Thestru tural data presented here were pro essed using the program StereoNet (version3.02) of P.I. Steinsund (1995). Orientations for planar and linear stru tures aregiven in dip dire tion/dip angle and azimuth/plunge, respe tively.

Figure 6.1: Field image showing a tight fold in gneiss in proximity to the Raudhaugene peridotite.Coin diameter 21mm.

6.2 Stru tural setting 161

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O(a) Proximal gneiss: poles to fo-liation (n+=131) and lineation(n◦=40, nN=12). (b) Simpli�ed geologi al mapof part of western Otrøy (sub-set of Fig. 1.10 with legend atpage 29).

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( ) Distal gneiss: poles tofoliation (n+=59) and lin-eation (n◦=18) (reprodu edFig. 1.11).?

6?

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gneiss

Figure 6.2: Stru tural data from western Otrøy basement gneiss. (a) Proximal gneiss within . 500m to Ugelvik and Raudhaugene peridotite bodies (area overed by the arrow on the left of(b); lower hemisphere, equal area proje tion, + poles to foliation, ◦ mineral lineation, N fold axis).Compiled dataset from the work of A. Haker and the author. ( ) Data from distal gneiss (area overed by the arrows on the right of (b)).In the e logite bearing gneiss, the shape preferred orientation of Fsp, Qtz, Bt,Amp and ±Grt de�nes the main foliation (S), whi h is tight to iso linally foldedwith axial planes parallel to S in the fold limbs (Fig. 6.1). Mineral lineation (L) isformed by elongated poly rystalline Qtz-Fsp-aggregates and the grain shape of Fsp.The basement gneiss surrounding the Ugelvik and Raudhaugene peridotite bod-ies within . 500m (proximal gneiss, Fig. 6.2(a)) shows a superposition of two stru -tural re ords in ontrast to the more distal basement gneiss (Fig. 6.2( )). Thefoliation in the proximal gneiss (S5 ‖ S6) trends WSW�ENE ±45 ° and has dom-inantly steep dip (Fig. 6.2(a)). The mineral lineation forms a girdle distribution(Fig. 6.2(a)). The majority of the mineral lineation data has subhorizontal plungeand WSW�ENE azimuth (L6), the minority has moderate to steep plunge (L5).Fold axes (FA) follow a girdle distribution, whi h overlaps that of L5,6 (Fig. 6.2(a)).Plunges vary between shallow (FA6) and steep (FA5). Fold axes in dire t vi inityto the peridotite bodies are typi ally steep (FA5), but the foliation involved onsistof amphibolite-fa ies minerals.

162 6. EMPLACEMENT INTO THE CRUSTThe foliation in the distal e logite bearing gneiss north and south of the Ugelvikand Raudhaugene peridotite bodies, S6, is less variable in trend, WSW�ENE ±25 °,and has dominantly steep dip (Fig. 6.2( )). L6 plunges subhorizontal only and hasWSW�ENE azimuth (Fig. 6.2( )).6.2.2 PeridotiteTwo km-sized Grt-peridotite bodies o ur at the lo alities Ugelvik and Raudhaugene(Carswell (1968), Fig. 6.3 , En losures 1�3). The Ugelvik body is exposed with anelongated shape, 400×1000m in size and extends at the western margin into thesea. The Raudhaugene body is res ent in shape, has a bent length of 1800m andde reases in width from 400m in the west to 150m in the east. Part of this bodyextends into a lake (Innafor Straumen). Another small Grt-peridotite body out ropsat the lo ality Midsundvatnet, is lenti ular in shape, 25×50m in size and extendsat its western margin into a lake (Midsundvatnet, M in Fig. 6.2(b) , En losure 4).The Ugelvik and Raudhaugene bodies were subdivided for stru tural analyses intoa western and eastern part, and the Raudhaugene peridotite additionally into a entral part (Fig. 6.3).Previous �eld studies at Raudhaugene and Ugelvik have shown that both peri-dotite bodies form a omposite of dis rete, lithologi al distin t layers (Carswell,1968; Van Roermund et al., 2000b; Drury et al., 2001). The ompositional layering(S2) is internally folded, de�ning fold axes (FA3) and an axial plane foliation (S3)(Fig. 6.4). Re-folded planar and linear stru tures re ord a se ond folding stage.This is explained below.Compositional layeringThe ompositional layering in the peridotite, S2, is de�ned by variable modes ofOl, Grt, Opx, Cpx, Spl, their repla ement produ ts Srp, Chl, green Amp, kelyphiteand stret hed lusters of Grt and Spl within some layers. The peridotite layers varyin thi kness between a few mm to several m. Grt- linopyroxenite, Grt-websteriteand Grt-bearing orthopyroxenite form layers parallel to S2, whi h vary in thi knessbetween a few mm and . 20 m (Fig. 6.4(a)). In addition, Grt-bearing orthopy-roxenite (with ±Cpx) o urs in form of rare layer-parallel boudins several m insize and isolated lenses, whi h rea h m-size (Carswell, 1973). Garnetite o urs asisolated lenses a few m to . 25 m in size.S2 in the Raudhaugene peridotite body has dominantly steep dip and varies instrike systemati ally from the western to the eastern subdomains (Fig. 6.5). Polesto S2 luster in the subdomain W2 at W�NW (with a density maximum at 266/17),in W1 at SE (126/04), in C at SSE (158/16), in E1 at SSE (161/14) and in E2 ateastern and western dire tions (254/09, Fig. 6.5). The density maxima for poles ofplanar stru tures were determined by ontouring (not shown).S2 in the Ugelvik peridotite ontrasts with moderate NE and N dip (212/61 and176/57) in the western and eastern subdomain, respe tively (Fig. 6.5).

6.2 Stru tural setting 163

m

WE

W1W2 C E1 E2

Figure 6.3: Stru tural and out rop map of the Raudhaugene and Ugelvik peridotite bodies.Stru tural data in the gneiss is a ompilation of the work of A. Haker and the author. Dashedlines mark subdivisions of the peridotite bodies with W(1,2) � west, C � entre and E(1,2) � east.

164 6. EMPLACEMENT INTO THE CRUST

(a) (b)S3

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( ) (d)1mm 150µmGrt3 Spl3

Grt3+Spl3Grt2Ol3Cpx3+Grt3+Spl3

Figure 6.4: Du tile deformation in peridotite and pyroxenite from the Raudhaugene C domain.(a) Field image showing a tight fold hinge in ompositionally layered peridotite (S2) with inter- alated pyroxenite (pen for s ale is 15 m). Note weak axial plane foliation S3. (b) Opti al lightmi rograph of a polished hand spe imen of Grt(red)- linopyroxenite (sample DS0295), whi h isderived lose to a fold hinge. The sample has a porphyro lasti , e logite-fa ies re rystallized tex-ture, whi h forms a foliation (S3) angular to the ompositional layering (S2). ( ) Opti al lightmi rograph of Spl-Grt-peridotite (PPL, sample NRK10a from H.L.M. van Roermund). Porphy-ro lasti Grt2 has tails of re rystallized Grt3 and Spl3 in paragenesis with Ol3, whi h de�ne thefoliation S3 to be formed in the Spl-Grt-peridotite stability �eld. (d) Subset of ( ).FoliationThe preferred orientation of re rystallized e logite-fa ies assemblages, in luding Ol,Opx, Cpx, Grt and Spl, de�ne a foliation S3 in both peridotite bodies (Fig. 6.4). S3 isweakly developed or absent in duniti peridotite. The orientation of S3 is dominantlysubparallel to that of S2. Therefore it is often di� ult to distinguish between them atout rop s ale (Fig. 6.6( )). The northwestern part of the subdomain RaudhaugeneC is an ex eption in that many tight folds exhibit an angular relationship betweenS2 and S3. This angle is re ognised in some folds of Raudhaugene C to in reasetowards the hinges to 90 °, whi h de�nes S3 as the axial plane foliation.Fig. 6.5 shows the distribution of S3 obtained in the entral subdomain of Raud-haugene. Poles to S3 are subhorizontal with SSE azimuth (arithmeti mean of154/00), whi h is subparallel to that of poles to S2 in the same subdomain (densitymaximum at 158/16).

6.2 Stru tural setting 165o

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Figure 6.5: Stru tural data of the Raudhaugene and Ugelvik peridotite bodies plotted intostereonets of the various subdomains (lower hemisphere, equal area proje tion): • � poles to the ompositional layering (S2), x � poles to foliation (S3), ◦ � mineral lineation (L3).

166 6. EMPLACEMENT INTO THE CRUSTFolds and lineationsS2 is tight to iso linally folded with `Z'-shaped folds (looking down the fold axisFA3, Fig. 6.6). Fold wavelengths vary between several m and several m. Fold axialplanes have a small angle to S2 or are subparallel. No systemati di�eren es inorientation have been observed between tight and iso linal folds. Folds are frequentin the Raudhaugene peridotite body and rare in the Ugelvik peridotite body. Onlytwo FA3 were measured in the Ugelvik peridotite body.Mineral lineations (L3) are de�ned by elongated Pyx rystals in pyroxenite.Pyroxenite layers o ur in all subdivisions, but are less frequent in RaudhaugeneW1,2 and intensely retrogressed in Raudhaugene E1,2. Measurements of L3 andFA3 are plotted into stereographi proje tions shown in Fig. 6.5 and 6.7 for theRaudhaugene and Ugelvik peridotite bodies.Raudhaugene Raudhaugene FA3 have steep dip (Fig. 6.7(a)). Azimuths varysystemati ally with an arithmeti mean towards NE (058/67) in the eastern parts(E1+E2), towards NW (313/75) in the entral part (C) and towards WSW (245/69)in the western parts (W2+W2). The angles between the means are 31 ° and 20 °,

(a) (b)

( ) (d)Figure 6.6: Field images showing folded S2. (a)�(b) `Z'-shaped (looking down the FA3) tightfolds in peridotite, Raudhaugene C. ( ) Iso linally folded pyroxenite layer in peridotite with S2subparallel to S3, Ugelvik E. (d) Iso linally folded and retrogressed pyroxenite layer in peridotite,Raudhaugene E2. Coin diameter 21mm.

6.2 Stru tural setting 167

(a) Raudhaugene FA3 (n=27)1.3 %

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( ) Ugelvik W and E S2 + L3 (d) Raudhaugene C and Ugelvik E S2 + L3

E1,2CW1,2 rpFA ∼220 ° W2 W1CE1

E2

πSAπSB91 °

80 °

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34 °CEC E

κRU60 °65 °

Figure 6.7: Systemati orientation relationships of planar and linear stru tures between di�erentsubdomains (lower hemisphere, equal area proje tion). (a) Raudhaugene FA3 (H � W2, H � W1,△ � C, N � E1, N � E2) and orresponding arithmeti mean FA3 (⋆) des ribe a small ir le, onsistent with a km-s ale rotation of . 220 ° between the subdomains W2 and E2 around a steeprotation pole axis (rpFA). (b) Contoured poles to S2 for all Raudhaugene subdomains (shadedareas) and poles to S2 density maxima for ea h single subdomain (©) de�ne two π- ir les withsteep sub-parallel π-pole axes (πSA,B). The variation in orientation of S2 (whi h is (sub-)parallelto S3) from W2 to E1 orresponds to a km-s ale ylindri al folding by 80 ° around πSA , that of E1to E2 by 91 ° around πSB . ( ) Ugelvik W and E density maxima of poles to S2 (©) and arithmeti means of L3 (•©) share a small ir le rotation pole axis (rpSL), whi h indi ates an angle of 24�34 °between both Ugelvik subdomains formed by rotation around a shallow SSW plunging axis. (d)Raudhaugene C and Ugelvik E density maxima of poles to S2 (©) and arithmeti means of L3 (•©)share a small ir le rotation pole axis (κRU). The Ugelvik and Raudhaugene subdomains forman angle of 60�65 °, whi h is onsistent with a km-s ale non-symmetri al folding of the peridotitebodies around a moderately ESE plunging axis.

168 6. EMPLACEMENT INTO THE CRUSTrespe tively. All FA3 means lay on a small ir le, whi h de�nes a steeply plungingrotation pole axis (rpFA) at 143/82 and rotation angles of 83 ° and 53 °, in total136 ° for a minimum rotation of FA3 between the eastern and western limbs of theRaudhaugene peridotite body (Fig. 6.7(a)). The variation in orientation of all singleFA3 in Raudhaugene indi ate a systemati rotation of . 220 ° (Fig. 6.7(a)).The variation in orientation of S2 between the Raudhaugene subdomains pre-serves a similar rotation relationship. Poles to S2 from all subdomains form a half-girdle distribution, similar to a girdle distribution with a pau ity of data with NW�SE strike (Fig. 6.7(b)). The poles to S2 density maxima de�ne two great ir les (π- ir les) with orresponding rotation pole axes (πSA and πSB) at 033/66 and 018/72for W2�W1�C�E1 and E1�E2, respe tively, onsistent with a km-s ale ylindri alfolding of the Raudhaugene peridotite body of 80+91 °, respe tively (Fig. 6.7(b)).The pau ity of stru tural data in Fig. 6.7(b) in between the subdomains E1 andE2 orrelates with minor peridotite exposure in this area (Fig. 6.3). The south-easternmost limb of E2 has S2 with NE�SW strike (Fig. 6.3). If in luded, then atotal folding of the Raudhaugene body of 80+91+ . 50= . 220 ° around a steeplyplunging axis an be obtained, similar to values indi ated by the FA3 (Fig. 6.7(a)).This large s ale folding phase, whi h re-folded FA3 and S2,3, postdates thee logite-fa ies M3 re rystallization of the peridotite. The minor orientation dif-feren e between πSA,B and rpFA may be related to low statisti s on FA3 in theeastern and western subdomains. These onstru ted, steeply plunging rotation axesare similar in orientation to steeply plunging FA5 and L5 in the proximal gneiss(Fig. 6.2(a)) and are distin t to the subhorizontal plunging lineation L6 in the dis-tal gneiss (Fig. 6.2( )).L3 in the Raudhaugene body has moderate to steep dip and western azimuth(Fig. 6.5). The dataset shows sub-parallelism of L3 and FA3 in the entral subdo-main, where the arithmeti means of L3 (278/64) and of FA3 (313/75) form a smallangle of 16 ° (Fig. 6.7(a) and (d)).Ugelvik and Raudhaugene L3 in the Ugelvik peridotite body ontrasts with �atto moderate plunge and NNW azimuth (with an arithmeti mean of 346/22) in thewestern subdomain and NNE azimuth (011/37) in the eastern subdomain (Fig. 6.5and 6.7( )). The means of L3 and the density maxima of poles to S2 share a small ir le rotation pole axis (rpSL) at 202/12 showing that the orientation di�eren eswithin the Ugelvik body are small, 24�34 ° (Fig. 6.7( )), but learly distin t to theorientations in the Raudhaugene body (Fig. 6.7(a)�(b)). This signi� ant di�eren ein the orientation of L3 and S2 between both peridotite bodies an be geometri allyre onstru ted for Ugelvik E and Raudhaugene C by a small- ir le rotation of bothstru tural elements by 60�65 ° around a moderately plunging rotation pole axis(κRU) at 115/38 (Fig. 6.7(d)). If related to du tile deformation, su h a rotation is onsistent with a oni al (non- ylindri al) km-s ale folding of the peridotite bodieswith κRU the entral axis of the in lined one (Gamond, 1972). The dire tion ofthe tip of the oni al fold is indi ated by the onvex dire tion of the π- ir le in

6.3 Petrography of samples 169

(a) (b) ( )κRU ≈ ‘FA4A'

L3± ‖ FA3

FA3 πS ∼ FA6S3± ‖ S2

Figure 6.8: S hemati summary diagram that portrays the three major folding phases re orded inthe Raudhaugene (R) and Ugelvik (U) peridotite bodies. (a) Intensive and penetrative deformation aused iso linal folding of S2 with FA3 oriented sub-parallel to L3. (b) Km-s ale non- ylindri alfolding of peridotite formed an angle of 60�65 ° between the M3 stru tures exposed at Raudhaugeneand Ugelvik. ( ) Late ylindri al folding around a steep FA6 formed stru tural subdomains.the lower hemisphere stereographi proje tion, WNW (Vialon et al., 1976). Thisfolding postdates the formation of L3 and FA3, but predates the late re-folding ofFA3 around a steeply plunging axis (πSA,B, rpFA). In onsequen e, the moderatelyplunging κRU orresponds to `FA4A'. This is s hemati ally summarized in Fig. 6.8 .6.3 Petrography of samples6.3.1 PeridotitePeridotite has a porphyro lasti texture with porphyro asts (M2) of Ol2, Grt2,±Opx2, ±Cpx2 and ±Spl2. The matrix minerals are de�ned by re rystallized Ol3,Grt3, Opx3, Cpx3 and Spl3 (Fig. 6.4( )�(d)). If reworked during amphibolite- andgreens hist-fa ies onditions, the e logite-fa ies assemblages were repla ed in ludingthe phases Amp, Chl, Mag and Srp. Coroniti rea tion textures around Grt2 areundeformed and not parallel to S3 (Fig. 6.9(a)), whi h is in agreement with thee logite-fa ies origin of S3 (Fig. 6.4).The dominating Al-phase in Grt-peridotite is porphyro lasti Grt2. This Grt2is surrounded by omplex rea tion textures ≤300µm thi k (Fig. 6.9). Peridotite is learly serpentinised by approximately 20�50%, whi h makes the study of re rys-tallized phases, su h as Ol and Opx, di� ult. Three peridotite samples des ribedin Chapter 5 are hosen for further analyses as the rim of porphyro lasti Grt2 pre-serves a metamorphi re ord. These samples are Spl-bearing low-Cr Grt-peridotiteDS0263 and Spl-bearing high-Cr Grt-peridotite DS0208 and DS0260. Sample lo a-tions are shown in Fig. 5.1 and sample ompositions are given in Tab. 5.1.The three previously studied Spl-Grt-peridotite samples ontain Grt2 porphyro- lasts, whi h have omplex mineral repla ement textures formed by the rea tions:High-Cr Grt2 + Ol3 → Spl4A + Grt4A + Cpx4A (6.1)

170 6. EMPLACEMENT INTO THE CRUST

Grt2Spl2Kelyphite4B

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N Grt2Spl4A+Grt4A+Cpx4A 200 µm(e) Kelyphite4 repla ement front in Grt2

Cpx4A

Grt4ASpl4A

(f) Detail of Kelyphite4A in Grt2Figure 6.9: BSE-mi rographs showing omplex rea tion textures on porphyro lasti Grt2. (a)High-Cr Grt2 (≥4wt% Cr2O3) is asso iated with oarse Spl2, has an outer orona of Kelyphite4A(Spl4A+Opx4A+Cpx4A±Grt4A) and an inner orona of Kelyphite4B (Cpx4B+Opx4B+Spl4B). ABshows the position of the orresponding ompositional pro�le in Fig. 5.7 . (b) Low-Cr Grt2 (≤4wt%Cr2O3 with the maximum at the rims) with a marginal Cpx2 in lusion is surrounded by an outer orona of Kelyphite4A (Spl4A+Cpx4A+Amp4A+Grt4A) and an inner orona of Kelyphite4B. (d)High-Cr Grt2 is lo ally extensively repla ed by Kelyphite4A (Spl4A+Grt4A+Cpx4A±sulphide) andis partly surrounded by Kelyphite4B. (e) Lobate interfa es (bla k arrows) between porphyro lasti Grt2 and Kelyphite4A (large subset in (d)). (f) Small subset in (d).

6.3 Petrography of samples 171High-Cr Grt2 + Ol3 ± H2O→ Spl4A + Grt4A + Cpx4A ± Amp4A ±Opx4A (6.2)Grt2 + Ol3 → Spl4B + Opx4B + Cpx4B (6.3)with high-Cr Grt2 representing Grt ompositions with a minimum of 4wt% Cr2O3.These repla ement textures surround and penetrate Grt2 and an be subdividedinto a relatively oarse grained Grt-bearing Kelyphite4A and a �ne grained Grt-freeKelyphite4B (Fig. 6.9).Kelyphite4A is dominantly ≤200µm in grain size, granular and forms an outer orona of Spl4A, Grt4A, Cpx4A, ±Amp4A, ±Opx4A and ±sul�de (Fig. 6.9(a)�( ), re-a tion 6.2). In addition, this Kelyphite4A repla ed porphyro lasti high-Cr Grt2internally and non- on entri ally with single grains dominantly ≤50µm in size(Fig. 6.9( )�(f)). The internal Kelyphite4A onsists dominantly of Spl4A, Cpx4A,Grt4A and ±sul�de (rea tion 6.1), has a granular mi rostru ture and forms an ir-regular lobate onta t relationship with the porphyro lasti host Grt2 (Fig. 6.9(e)).Kelyphite4B has grain width of ≤10µm, shows radially oriented mineral inter-growth and forms an inner orona of Spl4B, Opx4B and Cpx4B (Fig. 6.9(a), rea tion6.3).The re rystallized matrix assemblage of the studied harzburgite and dunite on-sists of sub-mm sized grains of Ol3,4A and minor Cpx3,4A, Opx3,4A, Amp4A andSpl3,4A.6.3.2 PyroxenitePyroxenite o urs throughout both peridotite bodies but most frequently in theRaudhaugene entre and eastern subdomains. Folded and foliated pyroxenite showsthat the basi layers shared the same deformation history as the host peridotite,although Grt2 re rystallized to Grt3 mu h more in pyroxenite than in peridotite. In ontrast to peridotite, pyroxenite is less altered ex ept that exposed in the easternsubdomains of the Raudhaugene body.Otrøy pyroxenite an be subdivided into three types based on the paragene-sis and the olour of Grt2,3: Grt(red)- linopyroxenite, Grt(purple)-websterite andGrt(purple)-orthopyroxenite. The olour of Grt2,3 is darker (green or bla k) forpyroxenite o urring in brown weathering (metasomatised) perdotite (Chapter 5).Grt-websterite is the dominating pyroxenite type in the peridotite bodies at Raud-haugene and Ugelvik.Nine pyroxenite samples were studied in detail: Grt(red)- linopyroxeniteDS0295, DS0380, DS0384 and 99NR6 (sample from H.L.M. van Roermund),Grt(purple)-websterite DS0246, DS0288, DS0346 and DS03AO and Grt-orthopyroxenite DS0429. Sample DS0246 omes from the Midsundvatnet peridotitebody, all others from the Raudhaugene and Ugelvik peridotite bodies. 99NR6 isderived from the entral subdomain of Raudhaugene, all other sample lo ations aregiven in Fig. 6.10. Sample DS0295 and DS0380 are derived from the same layer;DS0429 and U95 (Carswell, 1973) are derived from the same lens.

172 6. EMPLACEMENT INTO THE CRUST

0 2 4 6 m

DS0246

Lithology (uncertain)

Peridotite body margin

Undifferentiated outcrop

Midsundvatnet peridotite(centre)

Ugelvik peridotite(west)

Raudhaugene peridotite(centre)

LegendGeological overview map of

western Otr yø

Figure 6.10: Geologi al maps of parts of the Ugelvik (U), Raudhaugene (R) and Midsundvatnet(M) peridotite bodies with lo ations of 8 pyroxenite samples. The geologi al overview map ofwestern Otrøy (upper right) is a simpli�ed subset of Fig. 1.10 (with legend at page 29) showingthe peridotite map positions (white re tangles).Grt(red)- linopyroxenite layers vary in width within a few mm to several mand are essentially bi-minerali in omposition with Grt2,3 light red and Cpx2,3 lightgreen in olour. A essory phases (<1%) are Ol4, Ilm2,3,4, Rt4 and FeNi-sul�des.The major mineral phases Grt2,3 and Cpx2,3 in the studied samples o ur in a modalratio of approximately unity and form a porphyro lasti texture (Fig. 6.4(b) and6.11(a)). Approximately 50% of the Grt phase is porphyro lasti Grt2, porphy-ro lasti Cpx2 is subordinate. Grt2 porphyro lasts are ir ular to oval in shape,>1.5mm in size and ontain in the ores oriented exsolution lamellae of Cpx2 as-sembled together with Ilm2 (Fig. 6.11(a) and ( )). Sample DS0384 has ex eptionallyhigh amounts of Ilm in both Grt2 ores (Ilm2) and the re rystallized matrix (Ilm3) ompared to other samples (Fig. 6.11(b)�( )).

6.3 Petrography of samples 173

(a) DS0295 (b) DS0384 ( ) DS03841mmGrt3Grt2 Cpx3 700µmGrt3 Cpx3

Ilm3

20µmGrt2Cpx2

Ilm2Prg-

Figure 6.11: Opti al light and BSE mi rographs of Grt(red)- linopyroxenite. (a) Porphyro lasti Grt2 with Pyx2 mi rostru ture is surrounded by re rystallized Grt3+Cpx3 (almost XPL). (b)Dynami ally re rystallized matrix phases (BSE image). ( ) Detail of a porphyro lasti Grt2 orewith multiphase in lusions asso iated with (late?) Prg (BSE image).The re rystallized M3-grains are equigranular to shortprismati in rystal shapeand form aligned grain aggregates parallel S3 with single grain sizes of dominantlyless than 0.5 × 1mm (Fig. 6.4(b) and 6.11(b)). Boundaries of neighbouring grainshave angles of 120 °. Re rystallized Grt3 la ks exsolution mi rostru tures. Re rys-tallized Cpx3 has undulatory extin tion in thin se tions under XPL.A essory Ol4B forms part of the re rystallized matrix assemblage, has 120 ° grainboundary angles to neighbouring grains and ontains a symple ti intergrowth ofIlm4B and ±Rt4B (Fig. 6.12). The symple ti intergrown phases are similar tobreak-down produ ts of Ti-Chu:Fe17TiSi8O34(OH)2 → 8Fe2SiO4 + FeTiO3 + H2O (6.4)where Mg an substitute for Fe (Risold et al., 2001). The formula for Ti-Chu an bewritten in a more general form by dividing the stru ture into an Ol-like and Br -likepart: (M2SiO4)4 ×M1−nTin(OH,F)2−2nO2n (6.5)where M is dominantly Mg with minor amounts of Fe2+, Mn, Ni, Ca and 0 < n ≤ 0.5(Weiss, 1997). A Ti-undersaturated Chu#1 (n < 0.5) will �rst de ompose to a Ti-saturated Chu#2 (n = 0.5) and Ol and Br before the break down of the Ti-saturatedChu starts after the rea tion:2[(M2SiO4)4 ×M0.5Ti0.5(OH,F)O]→ 8M2SiO4 + MTiO3 + H2O (6.6)where again M is dominantly Mg substituted by minor divalent ations (Weiss,1997). Rt4 o urs in small amounts in the symple tite (Fig. 6.12( )), but annot beexplained by rea tion 6.6 . This suggests that the pre ursor phase ould have beenslightly oversaturated in Ti (n & 0.5), whi h an be expressed in the hypotheti alequation:

2[(M2SiO4)4 ×M0.45Ti0.55(OH,F)0.9O1.1]→8M2SiO4 + 0.9MTiO3 + 0.2TiO2 + 0.9H2O (6.7)

174 6. EMPLACEMENT INTO THE CRUST

(a) (b)Grt2

Grt3Cpx3symple ti Ilm4B

-

R

Ol4B

Cpx3

Grt3Ilm4B

� 100 µm

( )Rt4B

Rt4B

Ilm4B�

-

-Figure 6.12: BSE mi rographs of symple ti intergrowths of Ol4B+Ilm4B±Rt4B after Ti-Chu3 inpyroxenite 99NR6. (a) Porphyro lasti Grt2 is surrounded by a re rystallized matrix assemblage,whi h in ludes symple tite after Ti-Chu3. (b) Matrix Ol4B with symple ti stru ture has straightgrain boundaries to re rystallized Grt3+Cpx3. Break-down of Ti-Chu postdates the e logite-fa iesre rystallization. ( ) Detail of symple ti Ilm4B (grey) ±Rt4B (bright) in Ol4B (dark).Grt(purple)-websterite layers vary in thi kness from a few mm to . 40 m.Their essentially tri-minerali mineral omposition onsists of either purple (Raud-haugene, Ugelvik) or red (Midsundvatnet) oloured Grt2,3, light green Cpx2,3 andlight brown Opx2,3. A essory phases (<1%) are Ol3, Ilm3 and FeNi-sulphides. LateAmp formed along grain boundaries in some samples. The three major omponentso ur in similar proportions. All samples show a porphyro lasti texture. Porphy-ro lasti Grt2 is ir ular to oval in shape, is >1.5mm in size and ontains in the ores oriented exsolution lamellae of Cpx2 and Opx2 or reli ts of them (Fig. 6.13(a),( ) and (e)). Pyx2 porphyro lasts have not been observed.The re rystallized matrix onsists of Grt3, Cpx3 and Opx3 with either equigran-ular to short prismati (Fig. 6.13(b)) or both short and long prismati grains shapes(Fig. 6.13(d)�(e)). The equigranular to short prismati grains are dominantly lessthan 0.5×1mm in size. Long prismati grains ontrast with larger, dominantly lessthan 1×2mm grain size. Long prismati Pyx3 di�ers to short prismati Pyx3 in thatthe former exhibits lear and the latter weak undulose extin tion in thin se tions

6.3 Petrography of samples 175

(a) DS0288 (b) DS0288 ( ) DS0288Grt2Pyx2

?

�

Pyx3 Grt3 Pyx3

Grt2Pyx2

j

(d) DS0246 (e) DS0346 (f) DS0346Grt3 Grt3Cpx3

Opx3 Grt3Cpx3 Pyx4A

±Grt4A

Cpx3

Grt3Grt4A

Figure 6.13: Opti al light mi rographs of Grt(purple)-websterite, whi h shows three e logite-fa ies assemblages (almost XPL, ex ept ( ) and (f) PPL). (a) Porphyro lasti Grt2 with reli ts ofPyx2 mi rostru ture in the ore is surrounded by re rystallized granular to short prismati Pyx3.(b) Re rystallized matrix of granular to short prismati Grt3 and Pyx3, whi h form elongatedgrain aggregates. ( ) Detail of porphyro lasti Grt2 ore showing oriented Pyx2 mi rostru ture.(d) Re rystallized matrix of long and short prismati Grt3 and Pyx3. (e) Long prismati unduloseCpx3 (right) be ame partly repla ed by equigranular Cpx3 (left), before all M3 phases re rystallizedpartly to �ne grained M4A a few tens of µm in size (lower entre and upper right; ompositemi rograph) (f) Subset of a Grt3 rim, whi h is surrounded by �ne grained Grt4A.under XPL (Fig. 6.13(e)). Re rystallized Grt3 asso iated with both long and shortprismati Pyx3 la ks exsolution mi rostru tures. Boundaries of neighbouring grainsform angles of 120 °. Several re rystallized M3 phases form elongated monominerali grain aggregates (Fig. 6.13(b)) and re rystallized monominerali layers. The shapepreferred orientation of both elongated M3 rystals (Fig. 6.13(d)) and elongatedM3 grain aggregates (Fig. 6.13(b)) de�ne S3. In addition, sample DS0246 ontainssymple tite of Ol4B+Ilm4B±Rt4B after Ti-Chu3 as in sample 99NR6.Sample DS0346 ontains a third e logite-fa ies mineral assemblage (M4A) inaddition to porphyro lasti M2 and M3 Grt and Pyx (Fig. 6.13(e)�(f)). M4A onsistsof �ne grained Grt4A, Cpx4A and Opx4A, whi h have grain sizes of <100µm andwhi h show repla ive onta t relationships to the phases of M2 and M3. This latere rystallization aused previously re rystallized Pyx3 to be ome porphyro lasti (Fig. 6.13(e)). Long prismati Pyx3 porphyro lasts show a lear undulose extin tionin thin se tions under XPL and subgrain formation, omposite equigranular Pyx3porphyro lasts have weak undulose extin tion in XPL (Fig. 6.13(e)).

176 6. EMPLACEMENT INTO THE CRUST

(a) DS0429Opx2Grt2 Cpx2 z

�i

5mm(b) DS0429 600 µmGrt2 Cpx2

Amp4 Opx2Opx2�

( ) DS0429 600 µmOpx2

Grt2Pyx2

�

j

**

Figure 6.14: Opti al light and BSE mi rographs, whi h show Pyx mi rostru tures in Grt(purple)-orthopyroxenite. (a) Cm-sized Opx2 with in lusions of Cpx2 has a orona of mm-sized Grt2 and± Cpx2 (PPL). (b) Conta t between Opx2 and oroniti Grt2 showing that Opx2 ontains Cpx2lamellae and Grt2 ontains Opx2 in lusions. Late Amp4 de orates the onta t between Opx2 andthe orona (BSE image). ( ) Detail of a oroniti Grt2 ore showing that en losed Pyx2 o urs asgrains and as lamellae (almost XPL).Grt(purple)-orthopyroxenite layers vary in width between a few mm to several m. The mineral omposition is essentially tri-minerali . Opx2 ( . 80%) dominatesthe paragenesis with Grt2 and Cpx2 ( . 10% ea h). Amp4 o urs as an a essory,later phase at some grain margins. Orthopyroxenite layers are re rystallized and ontain Opx2 porphyro lasts, several mm to a few m in size. Orthopyroxeniteboudins in dm-s ale are less deformed. The studied sample DS0429 is part of am-s ale lens (Carswell, 1973, sample U95), whi h re ords minor deformation, butdoes not indi ate a penetrative re rystallization (Fig. 6.14(a)).Opx2 in DS0429 is m in size, has undulose extin tion in thin se tions underXPL, ontains oriented lamellae of Cpx2 and sub-mm sized granular in lusions ofCpx2 and Grt2 (Fig. 6.14(a)�(b)). Mm-sized grains of Grt2 and minor Cpx2 �llsas a orona assemblage the inter rystalline positions in between the m-s ale Opx2 rystals. A few Grt2 grains at triple jun tions o ur in m size and preserve in the ore reli ts of oriented Pyx lamellae (Fig. 6.14( )). In addition, the Grt2 grainsen lose sub-mm s ale Cpx2 and Opx2 grains (Fig. 6.14( )). This mi rostru tureis similar to that in a Grt-websterite lens at Bardane (Fjørtoft) (Brue kner et al.,2002; Van Roermund et al., 2002; Carswell and Van Roermund, 2005).

6.4 Mineral hemistry 1776.4 Mineral hemistryMineral ompositions were analysed in situ for major elements on a �ve spe trometerJEOL JXA8600 EMP at standard onditions (15 kV, 20 nA, beam ∅ 1µm) and ounting times of 30�50 s at Universiteit Utre ht (Appendix A.6).6.4.1 PeridotitePorphyro lasti GrtPorphyro lasti Grt2 preserves a omplex ompositional zoning with shallow to mod-erate hemi al gradients in Grt2 ores ( ore-zoning, des ribed in Chapter 5) andsteep hemi al gradients at the outermost . 200µm Grt2 rims (rim-zoning). Thesteep rim-zoning is similar in trend in purple, green and bi- oloured Grt (Fig. 4.6(a),5.6 and 5.7). Detailed ompositional pro�les of Grt2 rims are presented in Fig. 6.15for the bi- oloured Grt bearing sample DS0263 and in Fig. 6.16 for the green Grtbearing sample DS0208. Kelyphite4B repla ed the Grt2 rims in sample DS0260(Fig. 6.9(a)). Representative major element ompositions for outer rims of Grt2 aregiven in Table 6.1 .Sample DS0263 DS0263 DS0208 DS0208 DS0208 DS0208 DS0208 DS0208Mineral Grt1-or Grt2-or Grt1-or Grt2-or Grt3-or Grt2- Spl2- Cpx2- l o w - C r G r t h i g h - C r G r t K e l y p h i t e 4Awt% n=3 n=3 n=3SiO2 40.49 40.24 40.75 41.07 40.83 40.32 0.11 54.04TiO2 0.02 0.03 0.03 0.02 0.06 0.07 0.06 0.07Al2O3 21.00 21.10 21.01 21.57 21.47 20.32 33.32 1.32FeO 13.40 12.49 12.44 12.04 11.94 11.57 17.42 1.96MnO 0.88 0.75 0.84 0.81 0.78 0.79 0.36 0.05MgO 14.62 15.02 14.82 15.01 14.73 12.47 13.11 16.95CaO 6.65 7.23 7.63 7.70 7.90 11.30 0.23 23.83Na2O 0.00 0.01 0.00 0.01 0.00 0.04 n.a. 0.46Cr2O3 1.94 1.73 1.88 1.26 1.46 2.77 34.03 0.54NiO 0.00 0.03 0.00 0.01 0.00 0.01 0.07 0.03Total 98.99 98.63 99.41 99.50 99.16 99.65 98.72 99.26mol%Alm 27.4 25.3 25.1 24.3 24.3 23.6 � �Prp 53.3 54.3 53.4 54.1 53.5 45.3 � �Sps 1.8 1.5 1.7 1.7 1.6 1.6 � �Gau 17.4 18.8 19.8 19.9 20.6 29.5 � �atomi ratiosMg# 0.66 0.68 0.68 0.69 0.69 0.66 0.57 0.94Cr# 0.06 0.05 0.06 0.04 0.04 0.08 0.40 0.21Table 6.1: Major element mineral ompositions in Spl-Grt-peridotite representative for porphyro- lasti Grt2 rims and average ores of Kelyphite4A phases Grt4A, Spl4A and Cpx4A, whi h repla edinternal parts of Grt2 shown in Fig. 6.16(b) (or � outer rim, � average ore, n.a. � not analysed).

178 6. EMPLACEMENT INTO THE CRUSTOuter rims of Grt in DS0208and DS0263 have higher on entra-tions of MgO (14.6�15.0wt%), FeO(11.9�13.4wt%), MnO (0.75�0.88wt%)and higher atomi Mg# (dominantly0.68�0.69) than the inner Grt rims(11.6�12.2wt%, 10.1�11.4wt%, 0.58�0.76wt% and 0.66�0.67, respe tively).Cr2O3 and CaO de rease from innerrims (3.6�4.1 and 11.3�13.5wt%) toouter rims (1.3�1.9 and 6.6�7.9wt%). A orresponding zoning with steep hem-i al gradients has not been dete ted inother mineral phases.Kelyphite4AAverage ore ompositions of therepla ive assemblage Spl4A, Grt4A andCpx4A in one Grt2 rystal of sampleDS0208 are given in Tab. 6.1 . Themajor element average omposition ofGrt4A (in wt%: MgO 12.5, FeO 11.6,MnO 0.79, CaO 11.3 and Cr2O3 2.8)is in between those for inner and outerrims of porphyro lasti Grt2. AverageCpx4A is ri h in CaO (23.8wt%) and lowin Cr2O3 (0.54wt%), Al2O3 (1.3wt%),FeO (2.0wt%) and Na2O (0.46wt%).Spl4A has average Cr2O3 of 33.3wt%and average Cr# of 0.40 .The hemistry of Grt2 preservesasymmetri al ompositional pro�les. Forexample, on entrations of Cr2O3 andCaO are higher in Grt2 at the onta tto Kelyphite4A (point D in Fig. 6.16(b)and point B in Fig. 6.16(a) and 5.7)than in the outer Grt2 rims (point C inFig. 6.16(b) and point A in Fig. 6.16(a)and 5.7). Con entrations of Al2O3 andMgO are vi e versa. This asymmetryshows that the M4A assemblage post-dates the steep ompositional zoning inthe outer rims of Grt2, whi h in turnpostdates the moderate ore zoning ofGrt2 (Chapter 5).Figure 6.15: Major element ompositionalpro�les of a bi- oloured Grt2 rim (CD) show-ing a narrow outer rim ( . 30 µm) low in Cr2O3and elevated in Mg# (AB in the PPL photomi- rograph orresponds to a ompositional pro�lein Fig. 5.6).

6.4 Mineral hemistry 179

(a) DS0208-Grt1 (b) DS0208-Grt2Figure 6.16: Major element ompositional pro�les a ross green Grt2 rims (CD). (a) Wide outerrim ( . 150 µm) shows in reased Mg# and de reased Cr2O3 (PPL photomi rograph indi ates pro�leposition; AB orresponds to a ompositional pro�le given in Fig. 5.7). (b) Asymmetri ore�rimpro�le showing a narrow outer rim ( . 30 µm) with low Cr2O3, low CaO and elevated Mg# atthe onta t to the matrix in ontrast to high Cr2O3, high CaO and low Mg# at the onta t toKelyphite4A (Spl4A+Grt4A+Cpx4A) (BSE mi rograph).

180 6. EMPLACEMENT INTO THE CRUST6.4.2 PyroxeniteRepresentative major element mineral ompositions of Grt- linopyroxenite, Grt-websterite and Grt-orthopyroxenite are presented in Tab. 6.2 .Grt- linopyroxenitePorphyro lasti (Grt2) and re rystallized mineral phases (Grt3, Cpx3) bothare not zoned and show minor variations at the rystal rims of less than ±0.7oxide wt%. Re rystallized Grt3 ontains less Cr2O3, less MgO, more FeO andin sample DS0384 also more CaO than porphyro lasti Grt2. The omposi-tional range of both porphyro lasti and re rystallized Grt2,3 is however small,Prp67−69Alm19−21Sps0.7−0.9Gau11−12 in DS0295 and Prp53−61Alm28−32Sps0.6Gau10−16in DS0384.Porphyro lasti Cpx2 and re rystallized Cpx3 in sample DS0295 are hemi allyidenti al. Rims of all Cpx2,3 have slightly higher FeO and Al2O3 and lower Na2O ompared to the ores. The hemistry of Cpx2,3 in both samples di�ers withhigher CaO (22.2�22.7wt%) and lower Al2O3 and Na2O in DS0295 (1.4�2.0wt%and 1.2�1.6wt%) than in DS0384 (19.1�19.2wt%, 3.8�4.0wt% and 3.0�3.2wt%,respe tively). Cr2O3 is low in all Cpx analysed (0.1wt%).Grt-websteriteCores of porphyro lasti (M2) and re rystallized (M3) minerals are not zoned. The omposition of rystal rims di�er to that of ores with less than ±0.5 oxide wt%.Rims of re rystallized Opx3 form an ex eption with steeply in reased Al2O3 and CaO omponents (Fig. 6.17). Cores of re rystallized Opx3 ontain extremely low plateausof Al2O3, 0.10�0.15wt%. The lowest on entrations o ur in the largest M3 rystals,1mm in width. Small Opx3 grains <0.2mm in width la k these plateaus and showinstead on ave Al�di�usion pro�les. Opx3 ores vary in hemistry between di�erentsamples in Mg# with 0.86�0.90 and CaO with 0.1�0.2wt%.Cores of porphyro lasti Grt2 and re rystallized Grt3,4 in a single sample showminor variation in CaO of less than ±1.3wt%. The ompositional range of allGrt2,3,4 ores is Prp55Alm27−30Sps1Gau14−17 (Mg# 0.65�0.67) for sample DS0288and Prp61−66Alm21−26Sps1Gau10−14 (Mg# 0.70�0.75) for the other three samples.All Grt2,3,4 analysed vary signi� antly in Cr2O3 between 0.8�2.9wt%.Cpx3,4 in all samples is ri h in CaO (22.5�24.0wt%) and low in Cr2O3 (0.4�0.5wt%). Al2O3 and Na2O are lower in Cpx3 of sample DS0288 (0.4wt% and0.3wt%) than in the other three samples (0.8�1.6wt% and 0.6�1.2wt%).−→Figure 6.17: Compositional pro�les of Al2O3 in re rystallized Opx3 from Grt-websterite. BSE-mi rographs on the left show granular to prismati Opx3 in onta t to and in textural equilibriumwith Cpx3 and Grt3. Lines indi ate the position of the pro�les (AB) shown on the right. Minimum on entrations of Al2O3 in Opx3 ores are sensitive to the grain size.

6.4 Mineral hemistry 181Opx3Cpx3Grt3A B

Opx3

Cpx3Grt3ABOpx3 Cpx3Grt3A B

Opx3

Cpx3

Grt3AB

182 6. EMPLACEMENT INTO THE CRUSTG r t � l i n o p y r o x e n i t eSample DS0295 DS0384Type porphyro lasti M2 re rystallized M3 por. M2 re rystallized M3Mineral Grt Grtr Cpx Cpxr Grt Grtr Cpx Cpxr Grt Grtr Grt Grtr Cpx Cpxrwt%SiO2 42.79 42.90 54.68 54.38 42.20 41.80 54.99 55.12 41.81 41.82 41.40 41.98 55.19 55.36TiO2 0.05 0.03 0.05 0.10 0.05 0.02 0.03 0.06 0.07 0.06 0.07 0.04 0.13 0.14Al2O3 23.02 23.03 2.06 2.18 22.64 23.16 1.87 1.97 22.60 22.45 22.07 22.42 4.10 4.64FeO 9.99 9.63 1.56 2.01 10.52 10.36 1.37 2.03 14.38 14.95 14.65 15.40 3.77 3.97MnO 0.42 0.44 0.02 0.00 0.40 0.32 0.00 0.04 0.30 0.29 0.30 0.31 0.00 0.02MgO 19.07 19.49 16.17 16.22 18.97 18.38 16.19 16.59 16.93 16.20 14.62 14.44 13.45 13.27CaO 4.70 4.62 22.24 22.47 4.31 4.51 22.70 22.30 3.89 4.19 6.14 5.43 19.24 19.13Na2O 0.00 0.01 1.58 1.30 0.02 0.01 1.40 1.23 0.00 0.03 0.05 0.02 3.17 3.04Cr2O3 0.33 0.35 0.09 0.12 0.09 0.11 0.10 0.10 0.17 0.17 0.09 0.08 0.07 0.09NiO 0.01 0.00 0.07 0.04 0.00 0.00 0.05 0.01 0.00 0.00 n.a. n.a. n.a. n.a.Total 100.39 100.50 98.53 98.82 99.20 98.67 98.71 99.45 100.15 100.15 99.38 100.11 99.12 99.66mol%Alm 19.8 18.8 � � 20.2 21.0 � � 28.4 30.0 29.7 31.8 � �Prp 67.4 68.6 � � 67.9 66.6 � � 61.0 58.6 53.5 53.2 � �Sps 0.8 0.9 � � 0.8 0.7 � � 0.6 0.6 0.6 0.6 � �Gau 11.9 11.7 � � 11.1 11.7 � � 10.1 10.9 16.2 14.4 � �atomi ratiosMg#×100 77.3 78.4 94.9 93.5 77.0 76.0 95.5 93.6 68.3 66.1 64.3 62.6 86.4 85.6G r t � w e b s t e r i t eSample DS0246 DS0288 DS03AOType por. M2 re rystallized M3 por. M2 re rystallized M3 por. M2 re rystallized M3Mineral Grt Grt Opx Cpx Ol Grt Grt Opx Cpx Grt Grt Opx Cpx wt%SiO2 41.58 41.43 57.80 54.56 40.65 40.55 41.26 57.62 54.94 41.96 42.97 58.4 55.47TiO2 0.00 0.04 0.00 0.06 0.02 0.05 0.06 0.00 0.03 0.02 0.05 0.01 0.05Al2O3 22.50 22.05 0.10 1.62 0.00 20.66 20.54 0.15 0.36 22.60 22.43 0.54 0.81FeO 12.97 12.84 7.11 2.30 11.87 13.78 14.29 9.19 2.00 10.59 10.77 6.76 1.21MnO 0.51 0.51 0.11 0.03 0.08 0.61 0.68 0.13 0.06 0.37 0.42 0.12 0.02MgO 16.84 17.26 34.61 15.99 47.40 14.81 14.76 32.14 16.94 18.44 17.75 34.12 17.02CaO 4.63 4.04 0.10 22.50 0.00 6.48 5.22 0.11 24.00 4.51 4.62 0.12 23.83Na2O 0.04 0.01 0.01 1.21 0.00 0.00 0.00 0.02 0.32 0.01 0.04 0.03 0.67Cr2O3 0.86 0.84 0.02 0.51 0.00 2.85 2.70 0.05 0.38 1.10 1.15 0.07 0.46NiO 0.00 n.a. n.a. n.a. 0.32 0.00 n.a. n.a. n.a. 0.00 n.a. n.a. n.a.Total 99.93 99.03 99.85 98.78 100.34 99.79 99.50 99.41 99.03 99.59 100.20 100.17 99.53mol%Alm 26.2 25.7 � � � 27.2 29.8 � � 21.3 22.1 � �Prp 60.7 62.7 � � � 54.5 54.8 � � 66.2 64.9 � �Sps 1.0 1.1 � � � 1.3 1.4 � � 0.7 0.9 � �Gau 12.0 10.5 � � � 17.1 13.9 � � 11.7 12.1 � �atomi ratiosMg#×100 69.8 70.9 89.7 92.5 87.7 66.7 64.8 86.2 93.8 75.6 74.6 90.0 96.2Table 6.2: Representative major element mineral ompositions in pyroxenite ( � ore, r � rim,l � lamellae, n.a. � not analysed, ∗ � long prismati Opx3 rystal).Websterite DS0246 has re rystallized Ol4 with Mg# of 0.88 . Sample DS0346 ontains e logiti mineral assemblages in a long prismati , an equigranular and a�ne grained mi rostru ture (Fig. 6.13(e)) with mineral phases similar in hemi al omposition. Minor di�eren es omprise higher Cr2O3 ontent and lower CaO on-tent in Grt3 ores (>1.5 and <5.0wt%) than in Grt3 rims and re rystallized Grt4 ores (<1.5 and >5.0wt%).

6.4 Mineral hemistry 183G r t � w e b s t e r i t e G r t � o r t h o p y r o x e n i t eSample DS0346 DS0429Type re rystallized M3 as porphyro lasts re rystallized M4 M2Mineral Grt Grtr Opx∗ Opx∗r Opx Cpx Grt Opx Cpx Grt Opx Cpx Cpxlwt%SiO2 42.04 41.83 57.70 57.51 57.35 54.70 41.37 57.34 54.60 42.41 58.77 54.81 54.31TiO2 0.01 0.02 0.03 0.01 0.01 0.05 0.03 0.02 0.07 0.03 0.03 0.07 0.04Al2O3 22.01 22.12 0.75 2.03 1.14 1.12 22.14 0.83 1.14 21.71 0.45 3.03 3.00FeO 10.73 10.56 6.49 6.49 6.30 1.39 10.76 6.35 1.53 8.04 4.01 1.05 1.07MnO 0.40 0.41 0.10 0.09 0.09 0.06 0.40 0.08 0.03 0.32 0.05 0.09 0.04MgO 17.43 17.30 34.37 34.27 34.23 16.74 17.38 34.34 16.89 19.74 36.70 15.27 15.39CaO 4.94 5.36 0.19 0.17 0.19 23.54 5.30 0.20 23.71 4.55 0.15 21.06 21.16Na2O 0.01 0.02 0.00 0.00 0.00 0.63 0.03 0.02 0.59 0.00 0.02 2.21 2.06Cr2O3 1.76 1.40 0.12 0.26 0.18 0.48 1.25 0.13 0.46 2.83 0.21 2.21 2.06NiO n.a. n.a. n.a. n.a. n.a. 0.01 0.00 0.04 0.06 0.00 0.11 0.04 0.02Total 99.32 99.03 99.74 100.82 99.48 98.72 98.66 99.34 99.08 99.64 100.48 99.83 99.16mol%Alm 22.1 21.7 � � � � 22.0 � � 16.3 � � �Prp 64.0 63.3 � � � � 63.3 � � 71.3 � � �Sps 0.8 0.9 � � � � 0.8 � � 0.7 � � �Gau 13.0 14.1 � � � � 13.9 � � 11.8 � � �atomi ratiosMg#×100 74.3 74.5 90.4 90.4 90.6 95.5 74.2 90.6 95.2 81.4 94.2 96.3 96.3Table 6.2: Continued.Garnet-orthopyroxeniteM2 phases in Grt-orthopyroxenite DS0429/U95 (Carswell, 1973) are ompositionally�at and uniform. Grt2 has Prp71Alm16Sps1Gau12 (Mg# 0.81). The Al2O3 on en-tration in Opx2 varies slightly over ore areas several mm in size (Fig. 6.18) andis with 0.45wt% lower than reported by Carswell (1973) based on wet- hemi alanalysis (0.66wt%). Opx2 rims in onta t to Grt2 have in reased Al2O3 ontent.Cpx2 grains and Cpx2 lamellae in Opx2 both are hemi ally identi al and ompara-bly ri h in CaO (21.1�21.2wt%), Cr2O3 (2.1�2.2wt%), Al2O3 (3.0wt%) and Na2O(2.1�2.2wt%).

Figure 6.18: Compositional pro�le of Al2O3 in a oarse Opx2 rystal in onta t to oroniti Grt2in Grt-orthopyroxenite DS0249. The Opx2 ore is ompositionally �at with minor variations.

184 6. EMPLACEMENT INTO THE CRUST6.5 P�T estimates6.5.1 GeothermobarometersP�T al ulations on mineral ompositions assume that the partition of elements be-tween di�erent mineral phases represents equilibrium onditions, whi h su essivelyare preserved in a metastable state. In ase of hemi al equilibrium, ompositionalpro�les a ross minerals will be �at and di�erent geothermobarometers applied on thesame assemblage should give similar results. Natural samples have often a omplexpetrologi history, whi h may be preserved in ompositionally zoned minerals. Su hminerals may re ord several `snapshots' of hemi al equilibration. Results from P�T al ulations on zoned and unzoned minerals will be geologi al meaningful if the as-semblage and the orresponding mineral phase ompositions an be unambiguouslyidenti�ed. This identi� ation will be ome di� ult if parts of older assemblages arepreserved as reli ts or if omplex zoning patterns o ur in only one mineral phase,for example M2 in the studied peridotite. Nevertheless, the outer ompositionalzoning in peridotiti Grt2 reveals a qualitative insight into the post-M2 evolution ofthe peridotite.The approa h below is to study the evolution of P and T in the Otrøy peridotiteon en losed pyroxenite. One reason is that most geothermobarometers for the ma� and ultrama� system involve the phase Grt, whi h o urs as re rystallized Grt3predominantly in pyroxenite, but is rare in peridotite. Peridotite samples identi�edto ontain re rystallized Grt3 involve Spl3 and Cpx3 (Fig. 6.4( )�(d)). This virtu-ally wehrliti CrCMAS system is less well studied in the Grt-peridotite stability �eld(Grütters et al., 2006), for whi h geothermobarometri alibrations are not avail-able. The pyroxenite is suitable for the study of the peak metamorphi evolutionin that the pyroxenite samples are fresh, have simple assemblages (e�e tively bi-and triminerali ) and the minerals are not zoned and they are Cr-undersaturated.Compositional results from EMP line s ans and from ore measurements on min-eral pairs and triplets presented in Tab. 6.2 have been applied on the followinggeothermobarometri alibrations:(1) Al�in�Opx/Grt barometer of Carswell and Harley (1990)(2) Al�in�Opx/Grt barometer of Brey and Köhler (1990) in onsideration of theTs hermaks substitution ((Mg,Fe)− Si−Al+2 ) in Opx from Carswell (1991)(3) Cr�in�Cpx/Grt barometer of Nimis and Taylor (2000)(4) Grt/Opx Fe�Mg partitioning thermometer of Carswell and Harley (1990)(5) Grt/Opx Fe�Mg partitioning thermometer of Brey and Köhler (1990)(6) Single En-in-Cpx thermometer of Nimis and Taylor (2000)(7) Two Pyx solvus thermometer of Brey and Köhler (1990)

6.5P�Testimates185

No. Type Calibration Sample DS0295 DS0384 DS0246 DS0288 DS0346 DS03AO DS0429G e o b a r o m e t e r ( G P a ) a t : 400 °C 1200 °C 400 °C 1200 °C 400 °C 1200 °C 400 °C 1200 °C 400 °C 1200 °C 400 °C 1200 °C 400 °C 1200 °C1 Al�in�Opx/Grt CaHa90 M2- ore � � � � � � 1.54 6.69M3- ore � � 2.16 8.60 2.00 7.67 0.74 5.19 0.96 5.81 �2 Al�in�Opx/Grt BrKo90 M2- ore � � � � � � 1.50 7.16M3- ore � � 2.41 10.13 0.44 11.21 0.59 5.63 0.94 6.23 �3 Cr�in�Cpx/Grt NiTa00 M2- ore (low CrCpx) � � � � � 1.94 4.26M3- ore (low CrCpx) (high NaCpx) (high NaCpx) 3.32 6.83 1.96 5.08 (high NaCpx) �M3-rim (low CrCpx) (high NaCpx) � � � � �G e o t h e r m o m e t e r ( ° C ) a t : 2GPa 6GPa 2GPa 6GPa 2GPa 6GPa 2GPa 6GPa 2GPa 6GPa 2GPa 6GPa 2GPa 6GPa4 Fe�Mg Grt/Opx CaHa90 M2- ore � � � � � � 707 915M3- ore � � 717 928 760 977 763 981 801 1029 �5 Fe�Mg Grt/Opx BrKo90 M2- ore � � � � � � 613 824M3- ore � � 627 842 661 884 667 891 718 954 �. 6 En�in�Cpx NiTa00 M2- ore (no Opx) (no Opx) � � � � 616 674M3- ore (no Opx) (no Opx) 635 694 698 761 650 711 625 684 �7 2 Pyx BrKo90 M2- ore � � � � � � 694 741M3- ore � � 660 713 670 721 668 716 623 667 �M4- ore � � � � 652 700 � �8 Ca�in�Opx BrKo90 M2- ore � � � � � � 711 860M3- ore � � 656 797 676 820 750 905 683 829 �M4- ore � � � � 758 914 � �9 Fe�Mg Grt/Cpx Po85 M2- ore 709 825 � � � � � 683 797M3- ore 650 760 894 1027 715 834 620 724 664 774 596 700 �10 Fe�Mg Grt/Cpx KR00 M2- ore 576 809 � � � � � 546 772M3- ore 516 735 792 1065 591 829 511 715 536 751 467 668 �Table 6.3: Geothermobarometri al ulations for a given T and P on the omposition of mineral pairs in Otrøy pyroxenite given in Table 6.2 .Bra kets indi ate mineral hemistries outside the respe tive alibration.

186 6. EMPLACEMENT INTO THE CRUST

Figure 6.19: Linearly interpolated P�T equilibria al ulated for ore ompositions of un-re rystallized and porphyro lasti M2 (dashed lines) and re rystallized M3 (solid lines) phasesin Otrøy pyroxenite. Shaded �elds outline estimated equilibria on re rystallized Grt3 and Opx3.

6.5 P�T estimates 187(8) Ca�in�Opx thermometer alibration of Brey and Köhler (1990)(9) Grt/Cpx Fe�Mg partitioning thermometer of Powell (1985)(10) Grt/Cpx Fe�Mg partitioning thermometer of Krogh Ravna (2000)All Fe has been regarded as divalent during the appli ation of the equations for Pand T given in the publi ations above on mineral pair ompositions in Table 6.2 .Results are shown in Tab. 6.3 and 6.4 and Fig. 6.19 .6.5.2 ResultsFig. 6.19 shows that P�T al ulations on ea h sample form two groups. Cpx-freegeothermometers indi ate generally higher T than those, whi h involve Cpx. Thelowest T with all Fe as Fe2+ result from the Fe�Mg partitioning between Grt andCpx and the En omponent in Cpx. Higher T are indi ated by the Ca omponentin Pyx. The highest T estimates are based on the Fe�Mg partitioning between Grtand Opx. Cal ulated P are similar for Al�in�Opx and Cr�in�Cpx barometers inthe re rystallized M3 assemblage in Grt-websterite DS0288, but di�er signi� antlyin the M2 assemblage of Grt-orthopyroxenite DS0429. It follows by disregardingthe Fe3+ omponent that only the Grt/Opx geothermobarometers enable reliableestimates for maximum onditions of metamorphi P and T (Table 6.4). These aresummarised in Fig. 6.20 .M2 in un-re rystallized samplesP�T estimates using the alibrations of Grt/Opx from Carswell and Harley (1990)yield generally systemati ally higher T (≤160 °C) and higher P (≤1.2GPa) thanthose of Brey and Köhler (1990). If the latter alibrations are regarded to be morereliable as they are based on an internally onsistent set of thermo- and barometers,then the un-re rystallized Grt-orthopyroxenite sample indi ates an equilibration atSample DS0429 DS0246 DS0288 DS03AO DS0346 DS0346Assemblage M2 M3 M3 M3 M3* M4AOpx grain size (µm) 4500 1050 570 220 500 . 50CalibrationCaHa90 T 825 964 984 922 854 822P 4.27 6.70 6.14 4.13 3.47 3.09BrKo90 T 676 868 870 793 696 670P 3.20 6.48 5.74 3.28 2.25 2.05Table 6.4: Cal ulated T (°C) and P (GPa) of oexisting Grt2,3,4A and Opx2,3,4A ore omposi-tions in Otrøy websterites (* � porphyro lasti M3).

188 6. EMPLACEMENT INTO THE CRUST oarse re rystallized(peak M3)mega rysts(M2) medium size re rystal.(partly re-equilibr. M3)�ne re rystallized/exsolutionlamellae (re-equilibrated M2,3,4A)

Figure 6.20: P�T diagrams showing al ulated estimates on Otrøy mantle ro ks and dedu edsubdu tion and exhumation path for the northwestern WGR. Large sympols, this study. Smallsymbols, literature data (spe i�ed below). Open symbols, un-re rystallized assemblages (M2).Filled symbols, re rystallized assemblages (M3,4A). Cross-hat hed areas delineate data from Otrøyin grain size subdivisions. (A) Squares, Otrøy pyroxenite along dedu ed paths of su essive equi-libration of mineral pairs after the alibrations of Brey and Köhler (1990) (dashed arrow) andCarswell and Harley (1990) (dotted arrow; Table 6.4). Open diamonds, inter rystalline P�T es-timates on garnetite (>3GPa; analyses in Chapter 3) and intra rystalline estimates on garnetiteand Grt- linopyroxenite (<3GPa; Chapter 2). Literature data of Otrøy and Fjørtoft websterite(�inders and upside-down triangles) and e logite (upright triangles) is shown for omparison andrepresents `best estimates' from ombinations of the Al�in�Opx barometer alibration of Brey andKöhler (1990) with di�erent thermometers (Brue kner et al., 2002; Carswell and Van Roermund,2005; Carswell et al., 2006). (B) The P�T data from the mega rysti M2 and the oarse re rys-tallized M3 assemblages suggest a prograde metamorphi path 1. The early retrograde evolutionforms a narrow hairpin bent path 2 in the Ti-Chu stability �eld. Further retrogression o urredalong path 3 proposed from basement ro k data (small diamonds, modi�ed after Carswell et al.(2003a)): M, Grt�Ky�gneisses on Fjørtoft (Terry et al., 2000b); S, shear zone assemblage in theHaram metagabbro (Terry et al., 2000b); R, Grt�Ky�gneiss on Sandsøya (Root et al., 2005); Z,Ky�e logite on Furøya (E.J. Ravna in Carswell et al. (2003a)). Numbers refer to sample numbers.Rea tion urve of Dia after Bundy (1980) and of Ti-Chu after Weiss (1997).moderate P, 3.2GPa, and ex eptionally low T, 676 °C. These estimates are mini-mum estimates taking into a ount that wet hemi al analyses on sample U95 fromthe same Grt-orthopyroxenite lens (Carswell, 1973) showed signi� ant amounts ofFe3+ to be present in Grt (9% of Fetotal) and Opx (14% of Fetotal). Mineral ore ompositions of un-re rystallized garnetite (Chapter 3) indi ate an equilibration atslightly higher P and . 95 ° higher T (3.55GPa and 769 °C for sample DS0297 and3.72GPa and 772 °C for sample DS0298). The appli ation of this higher T (770 °C)on the M2 omposition in the Grt-orthopyroxenite yields P of 3.83GPa, lose tothose derived from the garnetite samples. An equilibration of the un-re rystallizedM2 assemblage at . 3.6�3.8GPa and 770 °C an therefore be obtained.

6.6 Isotope geo hemistry 189M3 and M4A in re rystallized samplesGeothermobarometri estimates on re rystallized websterite assemblages de reasein metamorphi grade with de reasing grain size of the mineral pairs analysed, 6.5�2.0GPa and 870�670 °C for the alibration of Brey and Köhler (1990) (dashed arrowin Fig. 6.20). Estimates using the alibrations of Carswell and Harley (1990) yieldhigher P and higher T of 6.7�3.1GPa and 984�825 °C (dotted arrow in Fig. 6.20).P estimates depend strongly on the Al on entration in Opx. The largest, mm-s ale M3 grains ontain the lowest Al2O3 ore plateaus (0.10�0.15wt%) and indi atepeak metamorphi onditions of 5.7�6.5GPa and 870 °C, if the equations of Breyand Köhler (1990) are applied, and 6.1�6.7GPa and 964�984 °C using Carswell andHarley (1990). If the former alibration is favoured then the re rystallized assem-blage M3 preserves . 100 °C higher T and . 2.5GPa higher P ompared to theun-re rystallized assemblage M2.The M4A assemblage has the smallest re rystallized grain size, a few tens ofµm and indi ate 2.0GPa/670 °C and 3.1GPa/822 °C for Brey and Köhler (1990)and Carswell and Harley (1990), respe tively. These estimates are in the order ofthose from intra rystalline Opx2 needles, ≤20µm in size, in porphyro lasti Grt2(1.4�2.4GPa and 680�700 °C on Brey and Köhler (1990); Chapter 2). Moderatelyto steeply in reasing Al2O3 at the rims of Opx3 suggests partial re-equilibration bydi�usion during retrogression. Geothermobarometri estimates on mineral grainsless than . 0.2mm in size may therefore be of limited value for the derivation of aP�T path.The results from porphyro lasti M3 and the surrounding re rystallized M4A insample DS0346 are similar despite the variable rystal size (Table 6.4). This mayindi ate that the lo al re rystallization to M4A was a ompanied with a hemi alre-equilibration of porphyro lasti Opx3, onsistent with the presen e of a late �uidphase.6.6 Isotope geo hemistry6.6.1 Samples and te hniquesThe Sm�Nd isotopi system has been investigated in pure mineral separates andwhole ro k powders from four Otrøy pyroxenite samples to determine the time ofthe re rystallization: Grt- linopyroxenite DS0380 (Raudhaugene C near the edge)and DS0384 (Ugelvik W near the edge) and Grt-websterite DS0346 (Ugelvik W)and DS03AO (Ugelvik W near the edge). These samples were hosen and preparedbefore very low on entrations of Al2O3 in Opx3 were surprisingly re ognised inDS0246 and DS0288.I applied a new te hnique to sele t porphyro lasti Grt2 separately from re rys-tallized Grt3 and Cpx3 in ea h sample. Double polished ro k-slabs (12×18×0.3�0.45mm) were stu k to removable tape, in the entre of a strip of . 8 m length(S ot h® Magi � Tape Removable 811 with a 1 in h ore). This strip was put ba k

190 6. EMPLACEMENT INTO THE CRUST(a)(b) ( ) (d)Figure 6.21: Mineral separation te hnique. (a) Minor bending of the ro k slab (300�450 µmthi k) between two layers of removable tape auses ra king along grain boundaries. (b) The tapestrip with the broken slab was subsequently been �xed on a glass blo k fa ing the sti ky side. Theslab is ready to sele t Grt and Cpx with tweezers. ( ) Slab before and (d) after mineral sele tion.Slab size 12×18mm, sample DS03AO.onto the tape roll, so that the ro k slab was positioned in between two layers oftape (Fig. 6.21(a)). The ro k slab fra tured due to the bending of the tape-rolland some areful �nger-nail pressing. The grain bond broke preferably along grainboundaries and ra ks, su h that single grains be ame loose but still maintainedtheir original position in the mi rostru ture. Then the . 8 m long strip was are-fully removed from the tape rol and �xed on a glass blo k fa ing the sti ky sidewith the broken ro k slab (Fig. 6.21(b)). Subsequent hand-pi king, using a pairof tweezers and a light mi ros ope, enabled good separation between M2 and M3minerals (Fig. 6.21( )�(d)).Additional minerals were separated using heavy liquids in ollaboration withH.K. Brue kner from two pyroxenite samples from the Nogvadalen Grt-peridotitebody on Flemsøy Island (Nordøyane, Fig. 1.8). A Grt(red)- linopyroxenite (sampleFl99-26) is omposed of re rystallized Grt3 and Cpx3 and ontains Amp. Theseminerals were separated and analysed together with the whole ro k powder of thissample. Grt(purple)-porphyro lasts, whi h la k Pyx2 exsolution mi rostru tures,from a se ond Nogvadalen Grt-websterite were analysed and added to this data setfrom Flemsøy.Mineral separates were lea hed in hot HNO3 and a old solution of HCl andHF. Samples were spiked, dissolved and Sm and Nd were separated using standardte hniques (Appendix A.5.1). Sm�Nd isotopes were analysed by a 9 olle tor VGSe tor 54-30 TIMS at Lamont-Doherty Earth Observatory of Columbia University.The results are presented in Table 6.5 . Sm�Nd iso hrons were al ulated using theadd-in program `Isoplot/Ex3' for Mi rosoft Ex el from Ludwig (2003).

6.6 Isotope geo hemistry 1916.6.2 ResultsThe re rystallized M3 Grt�Cpx separates and whole ro ks from Otrøy and FlemsøyGrt- linopyroxenite form iso hron relationships (Fig. 6.22). Sample DS0380 yieldsan unpre ise age of 431±89Ma (3 points). The other two samples preserve pre iseages of 423.8±7.1Ma in DS0384 (3 points) and of 434.0±3.2Ma in Fl99-26 (5 points,in luding porphyro lasti Grt(purple)). Re rystallized M3(±4A) assemblages in thetwo studied Grt-websterite samples do not form iso hron relationships indi atingthat the isotopi system of these samples did not re-equilibrate ompletely duringthe formation of M3 (Fig. 6.23). An alternative for sample DS0346 is that the M3isotopi ratios were signi� antly disturbed during the formation of M4A.Sample Separate Sm Nd 147Sm/144Nd 143Nd/144NdDS0380 wr 1.25 4.01 0.188 0.512513±27Cpx3 2.22 8.33 0.161 0.512491±27Grt3(red) 0.408 0.197 1.25 0.515549±33Grt2(red) 0.550 0.248 1.34 0.517311±33DS0384 wr 1.00 3.11 0.195 0.512859±28Cpx3 1.64 9.25 0.107 0.512635±27Grt3(red) 0.675 0.468 0.871 0.514748±28Grt2(red) 0.590 0.413 0.864 0.515135±27DS0346 wr 0.0669 0.0869 0.466 0.518812±27Cpx3±Cpx4A±Opx4A(A) 0.114 0.172 0.401 0.518326±114Cpx3±Cpx4A±Opx4A(B) 0.106 0.0989 0.649 0.518958±33Grt3(purple) 0.0482 0.0189 1.55 0.519948±45Grt2(purple) 0.0585 0.0184 1.92 0.524488±51DS03AO wr 0.0662 0.100 0.400 0.517285±35Cpx3(A) 0.126 0.182 0.421 0.518805±40Cpx3(B) 0.118 0.160 0.447 0.519280±35Cpx3(C) 0.189 0.238 0.478 0.516284±32Grt3(A)(purple) 0.0358 0.0255 0.848 0.515638±87Grt3(B)(purple) 0.0267 0.0128 1.26 0.517864±44Fl99-26* wr 0.0420 0.152 0.167 0.512274±27Cpx3 1.20 5.82 0.125 0.512159±25Grt3(red) 0.350 0.218 0.969 0.514541±23Amp 2.14 8.87 0.146 0.512197±23Grt(purple) 0.227 0.126 1.33 0.515573±24Table 6.5: Sm and Nd data from whole ro k powders (wr) and mineral separates of re rystallizedM3(±4) phases (Grt3, Cpx3, Cpx3±Cpx4A±Opx4A) and porphyro lasti M2 phases (Grt2 ontain-ing Pyx2 exsolution lamellae) from Otrøy and Flemsøy Grt-pyroxenite. Letters in bra kets (A, B,C) refer to analyses of di�erent mineral separates. Two mineral separates from the Nogvadalenperidotite (porphyro lasti Grt(purple) and Amp) re ord an equilibration with the M3 assemblage(Fig. 6.22). Con entrations in ppm. Errors are 2σ. * � separated and analysed by H.K. Brue kner.

192 6. EMPLACEMENT INTO THE CRUST

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0.5115

0.5165

0.5155

0.5145

0.5135

0.5125

Grt-clinopyroxeniteDS0380

Grt3

wrCpx3

Age = 431 ± 89 Ma

Initial143

Nd/144

Nd = 0.51201 ± 0.00042MSWD = 8.6

0.0 0.2 0.4 0.6 0.8 1.0

0.5120

0.5150

0.5140

0.5130

Age = 423.8 ± 7.1 Ma

Initial143

Nd/144

Nd = 0.512329 ± 0.000024MSWD = 1.09

Grt-clinopyroxeniteDS0384

wr

Cpx3

Grt3

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Age = 434.0 ± 3.2 Ma

Initial143

Nd/144

Nd = 0.511794 ± 0.000016MSWD = 0.78

Cpx3

Grt-clinopyroxeniteFl99-26

0.5110

0.5160

0.5150

0.5140

0.5130

0.5120 Ampwr

Grt(red)3

Grt(purple)

143

144

Nd

/N

d143

144

Nd

/N

d143

144

Nd

/N

d

147 144Sm/ NdFigure 6.22: Sm�Nd iso hron relationships of re rystallized assemblages in Grt- linopyroxenitefrom Otrøy (3 points) and Flemsøy (5 points, in luding porphyro lasti Grt(purple)). Error barsare 2σ.

6.6 Isotope geo hemistry 193

0.511

0.513

0.515

0.517

0.519

0.0 0.4 0.8 1.2 1.6

Grt-clinopyroxeniteDS0380

Grt2

wrAge = 635 ± 6 Ma

Initial143

Nd/144

Nd = 0.511729 ± 0.000031

Grt-websteriteDS0346

Age = 594 ± 6 Ma

Initial143

Nd/144

Nd = 0.516999 ± 0.000040

Grt2

wr

0.517

0.519

0.521

0.523

0.525

0.0 0.4 1.2 1.6 2.00.8

0.0 0.1 0.2 0.4 0.50.30.508

0.511

0.514

0.517

0.520

Grt-pyroxenite5 wr

DS03AO

DS0346

DS0384DS0380

Fl99-26

Age = 3333 ± 190 Ma

Initial143

Nd/144

Nd = 0.50851 ± 0.00039MSWD = 55

0.2 0.6 0.8 1.2 1.41.00.40.515

0.516

0.518

0.519

0.520

0.517

Grt (B)3

Grt3(A)

Grt-websteriteDS03AO

wr

Cpx (A)3

Cpx (B)3

Cpx (C)3

0.0 0.4 0.8 1.6 2.01.20.517

0.519

0.521

0.523

0.525

Grt-websteriteDS0346

Grt3

Grt2

Cpx ±Cpx ±Opx3 A4 4A(A)

Cpx (B)3±Cpx ±Opx4A 4Awr

0.512

0.513

0.514

0.515

0.516

wr

Grt2

Grt-clinopyroxeniteDS0384

Age = 519 ± 9 Ma

Initial143

Nd/144

Nd = 0.512196 ± 0.000037

0.0 0.2 0.6 0.8 1.00.4

143

144

Nd/

Nd

147 144Sm/ Nd

147 144Sm/ Nd

143

144

Nd/

Nd

143

144

Nd/

Nd

Figure 6.23: Sm�Nd isotope data in Otrøy and Flemsøy Grt-pyroxenite; error bars are 2σ. Left:2 point iso hrons of whole ro k and porphyro lasti Grt2, whi h ontains exsolution lamellae ofPyx2, indi ate a pre-Caledonian origin for the assemblage M2. Right: An `error hron' of 5 wholero k ratios demonstrates a mid-Ar haean origin for the di�erent types of Grt-pyroxenite. Lettersin bra kets (A, B, C) refer to analyses of several M3(±4A) mineral separates from sample DS03AOand DS0346 and show that the re rystallized phases in these two samples are not in equilibrium.

194 6. EMPLACEMENT INTO THE CRUSTTwo-point iso hrons of porphyro lasti Grt2, whi h ontains intra rystallinePyx2, and the orresponding whole ro ks indi ate in three samples pre-Caledonianages, 635±6Ma (DS0380), 519±9Ma (DS0384) and 594±6Ma (DS0346). Althoughthese ages are likely to be mixed ages, they indi ate a pre-Caledonian origin of theporphyro lasti M2 assemblage. The isotopi ratios of all �ve pyroxenite whole ro ksamples form an `error hron' of 3333±190Ma.6.7 Dis ussionThe formation of S andian re rystallized mineral assemblages (M3,4A,4B,5), the pres-en e of porphyro lasti minerals (M2), boudins, tight to iso linal folds and the larges ale folding of fold axes re ord intensive and multistage deformation and re rys-tallization in peridotite and en losed pyroxenite at Otrøy. In the following, thepresented data will be dis ussed hronologi ally.6.7.1 Meso- and mi rostru turesPre-S andianThe oldest information preserved is an `error hron' formed by isotopi ratios of�ve pyroxenite whole ro ks, 3.33±0.19Ga. This age is as old as the melting eventre orded in the Otrøy peridotite, 3.1Ga (Chapter 4) and in agreement with meltdepletion data from other peridotite bodies in the WGR, 3.3�2.7Ga (Brue kneret al., 2002; Beyer et al., 2004; Lapen et al., 2005). The 3.33±0.19Ga age doesnot support models for Grt- rystallization during a mid-Proterozoi peridotite re-fertilization event as re ently proposed for the WGR (Beyer et al., 2004). Instead,the similar age information preserved in di�erent types of Grt-pyroxenite and hostGrt-peridotite on Otrøy and (possibly) Flemsøy suggests a shared evolution of thesemantle ro ks sin e the mid-Ar haean. An alternative omplex model, whi h in- ludes the te toni juxtaposition of Grt-pyroxenite with Ol-Grt-pyroxenite and Grt-peridotite during the Proterozoi , has re ently been proposed for mantle fragmentsfrom Sandvik, southern WGR (Lapen et al., 2005). This model does not apply toOtrøy and Flemsøy showing that di�erent types of pyroxenite, garnetite (whi h isasso iated with mega rysti orthopyroxenite (Carswell and Van Roermund, 2005))and peridotite are similar in the age of their origin and similar in that they ontainPyx2 exsolution lamellae in Grt2, whi h is proposed to be pre-Caledonian in origin(Chapter 3).M2 mega ryst assemblages in isolated lenses, boudins and nodules of Grt-orthopyroxenite and garnetite on Otrøy and Fjørtoft are internally slightlydeformed in that these minerals show rystal latti e distortion in the form ofundulose extin tion in thin se tions under XPL (sample DS0429, Fig. 6.14(a)) andfolded M2 exsolution lamellae (Van Roermund et al., 2002). Although the outershape of these mega rystal ro ks is lenti ular, they preserve internally an overall

6.7 Dis ussion 195mineral mi rostru ture, whi h originated by exsolution and la ks eviden e for apenetrative re rystallization (M3) as evident in layered pyroxenite (Carswell, 1973;Van Roermund and Drury, 1998; Van Roermund et al., 2000b, 2001,; Chapter 2,3; Fig. 6.4(b), 6.11(b) and 6.13). Both, layered re rystallized pyroxenite andmega rystal un-re rystallized pyroxenite preserve the same intra rystalline Pyx2exsolution mi rostru ture in Grt2 (Fig. 6.11(a), 6.13( ) and 6.14( ); Carswelland Van Roermund (2005)). It is therefore reasonable to assume that the majordeformation event, whi h formed M3, postdates the exsolution of Pyx2 lamellaein the M2 phases. A pre-S andian origin of M2 is supported by the followingarguments:� Three Sm�Nd 2-point iso hron ages (Otrøy pyroxenite whole ro k and por-phyro lasti Grt2 ontaining intra rystalline Pyx2 lamellae) of >500Ma are onsistent with a pre-Caledonian origin for un-re rystallized pre ursor pyrox-enite, whi h ontained majoriti Grt (Mj1) (Fig. 6.23).� P�T estimates on the mega rysti assemblage (3.5�3.7GPa and 770 °C) aremu h lower than peak metamorphi onditions during M3 (5.7�6.5GPa and870 °C). Based on the mineral hemistry it is illogi al to assume that relativelylow-P�T Grt2 preserves exsolution mi rostru tures after Mj formed during M3,whereas high-metamorphi Grt3 la ks this mi rostru ture.� The mineral hemistry of porphyro lasti Grt2 in peridotite shows a meta-morphi zoning at the outermost Grt2 rims, whi h an be related to theCaledonian prograde metamorphism (Fig. 6.16). Grt2 ores ontrasts witha pre-Caledonian hemistry as evident from the Sm�Nd isotope data, P�Testimates and the metasomati history (Chapter 5). It follows that di�usionwas not e� ient enough to equilibrate porphyro lasti Grt2 ores during theCaledonian and hen e annot explain a super-sili i omponent in Grt2 oresformed during the Caledonian. In ontrast, m-sized unzoned Grt2 re ordsthe freshest Pyx2 exolution mi rostru ture (Fig. 2.1(a) and 4.6(b)).� The majoriti pre ursor of Grt2+Pyx2 ( orresponding to a solid solutionof 1.5±0.2 and 0.7±0.1 vol.% Pyx2 lamellae in Grt2 for garnetite andGrt- linopyroxenite respe tively, Chapter 2) is not stable at the ratoni geotherm at 3.5�3.7GPa (Fig. 2.17). A syn-orogeni exsolution of M2 fromM1 has re ently been proposed (Carswell and Van Roermund, 2005), butrequires the metastability of Mj1 for billions of years at lithospheri depth.In addition, syn-orogeni formation of Pyx2 lamellae in Grt2 is not onsistentwith a HT formation of Pyx2 lamellae at ≥1300 °C (Chapter 3).It it therefore di� ult to argue for a Caledonian origin for Pyx2 lamellae in Grt2.Pyx3 lamellae in Grt3 ould have theoreti ally been formed post-peak S andian,be ause experimental data suggests . 1�2% of Pyx to be present in solid solutionwith Grt at the M3 peak metamorphi onditions (Fig. 2.17). Su h lamellae havenot been observed in Grt3.

196 6. EMPLACEMENT INTO THE CRUSTThe mesostru tural spatial relationship of the mega ryst assemblages beforethe onset of the major peridotite deformation (M3) is unknown. Similar intra-and inter- rystalline mineral mi rostru tures in M2 of re rystallized pyroxenite,un-re rystallized pyroxenite and garnetite and the marginal o urren e of Grtmega rysts at a mega rysti orthopyroxenite lens at Bardane (Fjørtoft) suggestsa similar history and lose asso iation of the di�erent mega rysts before most ofthem be ame disrupted and re rystallized together with the peridotite during theCaledonian orogeny (Carswell and Van Roermund, 2005).Early/peak S andianRe rystallized Grt3 and Pyx3 in pyroxenite and re rystallized Grt3 asso iated withSpl3, Cpx3 and Ol3 in peridotite show that intensive deformation of the mantlefragments o urred within the e logite-fa ies. Following the arguments given above,this re rystallization destroyed earlier exsolution mi rostru tures in M2 phases.The peridotite has a ompositional layering S2, whi h is de�ned by variable min-eral modes. This ompositional layering is tight to iso linally folded, in whi h theorientation of re rystallized M3 assemblages and disrupted M2 porphyro lasts formthe axial plane foliation S3 with S3 oriented (sub-)parallel to S2. The e logite-fa iesfolds have mineral lineations L3 and fold axes FA3, whi h are oriented sub-parallelto the fold hinges. Similarly sub-parallel, but shallow dipping e logite-fa ies L andFA have been reported from the Drøsdal e logite in the southern WGR (Foremanet al., 2005). Sub-parallel e logite-fa ies stru tures at Drøsdal are interpreted tohave formed in a onstri tional strain �eld during an early stage of the S andianexhumation. The sub-parallel e logite-fa ies M3 stru tures on Otrøy di�er in thatthey re ord peak UHP metamorphi onditions and early S andian ages. It followsthat L3 and FA3 formed during prograde metamorphism, i.e. during burial of themantle fragments from hanging wall lithospheri depth levels of `peridotite sam-pling' (3.6�3.8GPa, M2) down to the maximum depth of re orded ontinental platesubdu tion (5.7�6.5GPa, peak M3). This implies that peridotite deformation o - urred substantially after their empla ement into the subdu ting rustal ro ks. Theearly S andian re rystallization from M2 to M3 learly demonstrates that orogeni peridotite was empla ed into rustal basement during ontinent� ontinent ollisionas re ently proposed by Brue kner (1998).Mineral-whole ro k 5-point and 3-point iso hron relationships indi ate re rystal-lization ages of 434.0±3.2Ma, 431±89Ma and 423.8±7.1Ma, whi h overlap withinerror at 430.9Ma. These re rystallization ages are signi� antly older than urrent es-timates for the S andian peak metamorphism in the WGR, whi h is thought to haveo urred between . 410�400Ma (Terry et al., 2000a; Carswell et al., 2003a). The . 430Ma age is loser to a previous unpre ise Grt�Cpx iso hron age of 437±58Mafrom an Otrøy pyroxenite (Jamtveit et al., 1991) and is in agreement with re entEMP-dating of Mnz in rustal gneiss from Otrøy (∼430Ma, H.L.M. van Roermundpers. om., 2006). Other early-S andian metamorphi ages have been reported fromWGR e logite in both HP and UHP domains at Raudberg, 423±12Ma (Gri�n

6.7 Dis ussion 197and Brue kner, 1985), Grytting, 422±10Ma (re-determined, H.K. Brue kner, pers. om., 2006), Verpeneset, 419.5±4.3Ma (Kylander-Clark et al., ress,in press) and fromthe Lindås-Nappe/Bergen Ar s, 422±10Ma and 423±4Ma (Glodny et al., 2002;Bingen et al., 2004). If these early-S andian ages are true and not mixed ages thenthe timing, the te toni model and the preservation of the S andian metamorphi re ord needs to be re- onsidered.Post-peak S andianM3 related stru tures (FA3, L3, S3 sub-parallel to S2) di�er systemati ally withinand in between both peridotite bodies. A shared rotation pole axis κRU suggeststhat both peridotite bodies represent a single mantle fragment, whi h be ame post-M3 non- ylindri ally re-folded in a km-s ale synform (Fig. 6.7(d)). A orrespondingsimple, non- ylindri al fold is a oni al fold with κRU the entral axis of the moder-ately in lined one. The one is inferred from the stru tural data and has a entralaxis with moderately plunge towards ESE and has the tip dire ted towards WNW.The distan e between the limbs of su h a oni al ESE-in lined synform broadens to-wards ESE in the horizontal se tional plane (Fig. 6.8(b)). This is in agreement withthe out rop orientation of both peridotite bodies in that their longest axes form anangle, whi h opens towards ESE (by disregarding Raudhaugene E2, Fig. 6.3). Thee logite-fa ies re rystallization M4A re orded in sample DS0346 ould be relatedto this deformation event. If true, then the non- ylindri al km-s ale open fold-ing indi ates that the deformation regime hanged in the rustal slab after the peakmetamorphism, possibly related to strain partitioning between gneiss and peridotiteduring exhumation.A petrographi al support that both peridotite bodies are related to ea h otheris given by the frequent o urren e of Grt(red)- linopyroxenite at the northern edgeof Raudhaugene C and at the southern edge of Ugelvik W (En losures 1 and 2).An alternative to the proposed non- ylindri al synform is that the Raudhaugeneand Ugelvik peridotite bodies represent two boudins. In this ase, the orrelationbetween M3 stru tures and the out rop shape would be o-in idental. The originby boudinage requires a separate explanation for the lo allized re rystallization ofM3 to M4A in pyroxenite. A possibility is that this re rystallization in pyroxenite(e logite-fa ies) was driven by �uids, similar to the lo al �uid-driven re rystallizationof porphyro lasti Grt2 to Kelyphite4A in peridotite (Grt-Spl-peridotite stability�eld, Fig. 6.9).The variation in orientation of FA3 and S2 in the Raudhaugene peridotite pre-serves a se ond stage of re-folding, whi h e�e ted predominantly the limbs of theperidotite body on a km-s ale and may have led to its res ent shape (Fig. 6.3).This folding event is des ribed by steeply plunging rotation pole axes (πSA,B, rpFA,Fig. 6.7(a)�6.7(b) and 6.8( )), whi h are similar in orientation to steeply plungingFA5 in the proximal gneiss (Fig. 6.2(a)). Therefore, the km-s ale late folding ofthe Raudhaugene peridotite body may have o urred either simultaneously withthe formation of penetrative and steeply plunging lineations (M5) in the proximal

198 6. EMPLACEMENT INTO THE CRUST+

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(a)320°

(b)320°

( )Figure 6.24: Lower hemisphere, equal area proje tion of sele ted stru tural data from Otrøy.(a) Poles to amphibolite-fa ies S6 in distal gneiss (+) and best �t great ir le (solid line) withrotation pole axis at 255/01 (⋆). (b) FA5 in proximal gneiss interpreted to be e logite-fa ies(N) and ontours (dotted lines) with density maximum at 254/66 (⋆⋆). Rotation of the densitymaximum along a small ir le (dashed line with solid arrow) around the rotation axis (⋆ in (a))towards horizontal indi ates azimuth of 320 °. ( ) E logite-fa ies L3 in the entral subdomain of theRaudhaugene peridotite body (◦) with mean ve tor at 278/64 (⋆⋆). Small ir le rotation as in (b).gneiss or alternatively during late orogen-parallel deformation (M6). Strong retro-gression and strong bending of peridotite and pyroxenite in the eastern subdomainsof Raudhaugene (Fig. 6.6(d)) show that the deformation of the limbs ontinued inthe amphibolite-fa ies.The systemati in orientation of lineations in gneiss with distan e to peridotitesuggests that the steeply plunging lineations (M5) in the gneiss formed undere logite-fa ies onditions and be ame re-oriented to shallow plunging lineations(M6) with in reasing distan e to the peridotite bodies by late orogen-paralleldeformation in the amphibolite-fa ies (Fig. 6.2( )). In this respe t, the steeplyplunging M5 stru tures in the proximal gneiss represent stati ally re rystallizedreli ts of earlier e logite-fa ies assemblages. This is supported by a higher frequen yof e logite and retro-e logite in the proximal than distal gneiss (Fig. 1.10), whi hsuggests a omparably less intense retrogression and amphibolite-fa ies deformationin the former.Linear M5 stru tures in the Otrøy gneiss (FA5, L5), whi h vary in plunge fromsteep to shallow, are similar to stru tures in the rustal ro ks at Nordøyane de-s ribed by Terry and Robinson (2004). These authors showed that a relative platemotion between Balti a and Laurentia an be re onstru ted, if early lineations anbe rotated out of late amphibolite-fa ies stru tures. If the same method is appliedon both the steep FA5 in proximal gneisses and the steep L3 from the entral sub-domain in the Raudhaugene peridotite, then a relative motion ve tor for Balti awith respe t to the present day referen e frame of 320 ° an be obtained for a hor-izontal plate movement (Fig. 6.24). The azimuth of the dire tion obtained for theplate movement hanges westwards depending on the in lination of the subdu tedBalti plate. These northwestward oriented estimates are identi al to those from

6.7 Dis ussion 199

Figure 6.25: Cartoon of Otrøy peridotite in surrounding gneiss that summarises the majorstru tural information to four deformation and re rystallization stages: 1. Prograde e logite-fa iestight to iso linal folding of peridotite with L3± ‖ FA3 (orogen-normal); 2. (Possible) retrogradenon- ylindri al folding of peridotite on a km-s ale around a moderate ESE plunging axis κRU(a ompanied by �uids in the e logite fa ies?); 3. Amphibolite-fa ies re rystallization of earlierstru tures in the gneiss; 4. Late amphibolite-fa ies deformation (orogen-parallel) of gneiss (L6)asso iated with re-orientation of earlier steep stru tures. � � Grt(red)-pyroxenite,♦ � Grt(purple)-pyroxenite.Terry and Robinson (2004) (320 ° for a quasi-horizontal plate divergen e), althoughthe steep lineations from Nordøyane appear to be post-peak S andian with respe tto both age and metamorphi grade and may therefore represent the dire tion of(early) exhumation. Hen e, the Otrøy data may indi ate that the onvergent platemotion between Balti a and Laurentia (represented by L3, Fig. 6.24( )) and theexhumation of UHP metamorphosed ro ks (FA5, Fig. 6.24(b)) o urred along thesame orogen-normal motion ve tor, in agreement with data from Torsvik (1998)(320 ° obtained from palaeomagneti plate re onstru tions).The stru tural information is attributed to four su essive deformation and re- rystallization stages in Fig. 6.25:1. Prograde peridotite deformation o urred within the subdu ted rustal slabin the e logite-fa ies, led to boudinage, fragmentation and re rystallisation(M3) of previous, partly mega rystal M2 assemblages and aused the tight to

200 6. EMPLACEMENT INTO THE CRUSTiso linally folded ompositional layering in the peridotite with orogen-normal(NW�SE oriented) L3 subparallel to FA3. Re rystallized assemblages preserveages of 431+3/−7 Ma.2. Lo al retrograde re rystallization o urred in the e logite-fa ies and in theGrt-Spl-peridotite stability �eld and was probably a ompanied by �uids. Ret-rograde deformation ould have non- ylindri ally folded the early-S andianpeak-metamorphosed mantle fragment at a km-s ale.3. Amphibolite-fa ies re rystallization of early steep stru tures in gneiss.4. Late amphibolite-fa ies deformation had a minor e�e t on the peridotite bod-ies, but re-folded previous stru tures espe ially at the limbs of the Raudhau-gene peridotite at a km-s ale. The gneiss was folded intensely with L6 and FA6subhorizontal WSW�ENE in trend, onsistent with orogen-parallel extension.Earlier, steep stru tures be ame reoriented.6.7.2 P�T pathThermometri and barometri estimates, based on the partition of elements betweenadja ent minerals, are subje t to several un ertainties so a areful interpretation isne essary to obtain reliable PT information. A serious un ertainty is that al ulatedT and P depend on ea h other with higher T obtained if higher P are used andvi e versa. Fig. 6.20 shows that di�erent alibrations of the same thermobarometersre�e t this e�e t. The qualitative P�T information derived from the alibrationsof Carswell and Harley (1990) and of Brey and Köhler (1990) are similar. Bothsuggest a hairpin bent metamorphi evolution of peridotite during re rystallization,although the equations of Carswell and Harley (1990) yield systemati ally higher Tof ≤160 °C and higher P of ≤1.2GPa than those of Brey and Köhler (1990). P�T estimates derived from alibrations of Brey and Köhler (1990) are preferred forthe interpretation below for three reasons: the thermobarometri alibrations wereperformed at a onsistent set of di�erent thermometer and barometer ex hangeme hanisms between the 4 major mineral phases in Grt-peridotite (Ol, Opx, Cpx,Grt); results are in line with P�T estimates in WGR gneiss; the onservative es-timates using Brey and Köhler (1990) do not require partial melting of the gneissduring peak metamorphism.Pre-S andianPrevious mineral- hemi al P�T estimates on mega ryst assemblages from Otrøy in- lude 3.6�4.0GPa and 800±50 °C on Grt-orthopyroxenite (Carswell, 1973; Brue k-ner et al., 2002) and 3.2±0.2GPa and 805±40 °C on garnetite (Van Roermund andDrury, 1998). Similar estimates of 3.4�4.1GPa and 840�900 °C have been al ulatedon Grt-orthopyroxenite from Bardane/Fjørtoft (Van Roermund et al., 2002). These

6.7 Dis ussion 201estimates are based on di�erent thermobarometers and di�erent alibrations result-ing in a relatively large T range of . 100 °C, identi al to the size of the T rangeof the interpreted P�T path shown in Fig. 6.20 (B). It implies that the obtainableP�T information an be re�ned only if one set of thermobarometri alibrations isused.P�T estimates on the Grt-orthopyroxenite samples DS0429 yield 3.2GPa and676 °C (Brey and Köhler, 1990) with all Fe regarded as divalent. The ompa-rably low T are probably not representative as it has been shown that signi�- ant amounts of trivalent Fe is present in another sample (U95) derived from thesame Grt-orthopyroxenite lens (Carswell, 1973). The garnetite samples DS0297 andDS0298 yield 3.5�3.7GPa and 770 °C (Chapter 3), whi h are dominantly ontrolledby Al2O3 on entrations in Opx ores of approximately 0.45wt%. These P�T es-timates are onsistent with previous results on mega ryst assemblages in Otrøyperidotites (Van Roermund and Drury, 1998; Brue kner et al., 2002). In addition,the mega ryst assemblages equilibrated at P�T onditions, whi h are typi al for a old ratoni geotherm as al ulated for the Ar haean Balti plate (Kukkonen andPeltonen, 1999), whi h in turn is onsistent with the mid-Ar haean origin of allpyroxenite types as indi ated by a 5-point whole ro k `error hron' of 3.33±0.19Ga(MSWD=55Ma, Fig. 6.23).Early/peak S andianEstimates for peak metamorphi onditions on re rystallized assemblages are basedon the Al2O3 on entration in Opx. Al2O3 in Opx is sensitive to the Opx grain sizebe ause of late di�usional ex hange rea tions with Grt (Fig. 6.17). Nevertheless, re- rystallized Opx3 preserves lower Al2O3 on entrations than un-re rystallized Opx2asso iated with mega rysts. This is onsistent with a prograde deformation.Re ently, Carswell et al. (2006) reported 0.29wt% Al2O3 in Opx3 from a re rys-tallized Grt-websterite from Ugelvik (sample U292-L1), whi h yield 5.0GPa and860 °C using several thermobarometri alibrations. P�T estimates on this andother re rystallized samples from Otrøy and Fjørtoft are onsistent with P�T es-timates on some samples from this study in that the literature shown in Fig. 6.20 lusters in between the retrograde part of the hairpin bent paths de�ned from thisstudy by the alibrations of Brey and Köhler (1990) and Carswell and Harley (1990).Ex eptionally low Al2O3 ore plateaus of 0.10�0.15wt% in re rystallized Opx3 ofsample DS0246 and DS0288 from the Midsundvatnet and Raudhaugene peridotitebodies demonstrate peak metamorphi onditions of 5.7�6.5GPa and 870 °C. Themaximum P estimate of 6.48GPa orresponds to lithospheri depth of 200 km, ifthe Preliminary Referen e Earth Model (PREM) of Anderson (1989) is used for the onversion (P (kbar) = −3.263 + 0.34032× d, with 50 ≤ d ≤ 400 the depth in km).These estimates on M3 demonstrate that the UHP metamorphi history on Otrøyextends deep into the Dia stability �eld.Comparable, but slightly lower P�T estimates have re ently been reported from

202 6. EMPLACEMENT INTO THE CRUSTa newly dis overed Fe�Ti type Grt-peridotite at Svartberget (Vrijmoed et al., 2006).This peridotite body ontains Grt-websterite veins with 0.16wt% Al2O3 in Opx(∼5.5Ga, ∼800 °C derived from several thermobarometers) interpreted to be Cale-donian in origin, onsistent with the Otrøy data presented here.Symple ti intergrowths of Ol4, Ilm4 and minor Rt4 form part of the re rystal-lized assemblage in Grt- linopyroxenite (Chapter 2, Fig. 6.12). This symple tite isinterpreted as a break-down produ t of Ti-Chu3 and is not deformed. It followsthat the re rystallization o urred in the Ti-Chu stability �eld, whi h is lose to there orded peak metamorphism in Fig. 6.20 (Weiss, 1997). This gives an independenteviden e that major re rystallization and folding of the Otrøy peridotite o urred lose to peak metamorphi onditions. The sour e for water bound in Ti-Chu3 maybe related to the S andian subdu tion zone.The outer rims of porphyro lasti Grt2 in Spl-Grt-peridotite preserve a steep hemi al zoning of in reasing Mg# and de reasing Cr2O3 (Fig. 6.15 and 6.16). Theinner rims are interpreted to represent part of long term equilibration pro�les (Chap-ter 5). The steep gradients at the outer rims suggest to have formed either in anopen system (di�usion after hanged hemi al onditions) or alternatively in a losedsystem (di�usion after hanged P�T or by growth of se ondary Grt). Changed hemi al onditions in an open system may be ex luded as they would most likelyhave lowered the Mg#. In the other ase ( losed system), low Cr2O3 Grt in a Cr�ri h Spl-Grt-perdotite suggests to have formed during prograde metamorphism inthe Spl-Grt-peridotite stability �eld, when Spl requires higher Cr# to stabilize inasso iation with Grt, onsistent with the S andian subdu tion. In this ase, theouter Grt2 rim zoning may have formed in response to one subdu tion event.Post-peak S andianPorphyro lasti Grt2 is lo ally repla ed by Kelyphite4A ( omposed predominantly ofSpl4A, Grt4A and Cpx4A), whi h uts the ompositional Grt2 zoning (Fig. 6.16(b)).This suggests that Kelyphite4A formed along the retrograde path within the Spl-Grt-peridotite stability �eld. Moderate average Cr# in Spl4A (0.40) ontrasts tohigh Cr# in oarse mm-s ale Spl2 (0.45�0.68, Fig. 5.10). This supports a post-peak metamorphi rystallization of Kelyphite4A. The undulose onta t relationshipof Kelyphite4A with porphyro lasti Grt2 suggests the presen e of a �uid phase(Fig. 6.9(e)), whi h may be derived from the subdu tion zone.Sample DS0346 ontains a �ne grained e logite fa ies assemblage (M4A) andporphyro lasts omposed of re rystallized (M3) grain aggregates (Fig. 6.13(e)). Bothindi ate a se ond e logite-fa ies re rystallization, whi h ould possibly be orrelatedwith the se ond deformation event (Fig. 6.25). Meaningful P�T estimated annotbe obtained from the �ne grained assemblage M4A, be ause late di�usional ex hangeme hanisms overprinted initial mineral- hemi al element partitions.

6.7 Dis ussion 2036.7.3 Geodynami interpretationA geodynami model for the S andian orogeny in the entral part of the orogenrequires the need to ful�l the following onstraints from this study:� P�T onditions before peridotite deformation of 3.5�3.7GPa (115±3 kmdepth) and 770 °C� peak metamorphism during re rystallization at 6.5GPa (200 km) and 870 °C� e logite-fa ies re rystallization at 431+3/−7 Ma in the Ti-Chu stability �eld� e logite-fa ies re rystallization during orogen-normal (NW�SE oriented)plate motion� peridotite empla ement from Ar haean SCLM with a old geotherm into the rustal basement of Balti a� lo allised retrograde �uid in�ltration in the Spl-Grt-peridotite stability �eld� late amphibolite-fa ies re rystallization during orogen-parallel deformationThe simplest model is that the peridotite bodies were `sampled' from the SCLMat depth of 115 km during the subdu tion of the Balti ontinental plate marginunderneath Laurentia (Brue kner, 1998). The major deformation of the peridotite ulminated with intensive folding and re rystallization of peridotite and pyroxeniteat metamorphi onditions orresponding to 200 km of subdu ted depth. The peakmetamorphi estimates preserved in the re rystallized mantle fragments may be orrelated to minimum depth estimates for the subdu tion of the ontinental platemargin if the peridotite deformation stopped on the prograde path. Mega ryst sam-ples lagged behind hemi al equilibration, but rims of some porphyro lasti Grt2preserve a hemi al zoning interpreted as prograde. The mantle fragments mayhave stopped to deform by two reasons. Either the ompeten e ontrast in reasedbetween peridotite and gneiss that led to strain partitioning during subdu tion,probably asso iated with heating of the ontinental gneiss. Alternatively, the defor-mation of both gneiss and mantle fragments stopped simultaneously, whi h is likelyto have o urred at the ulmination point. For both ases, the major deformationin the mantle fragments stopped at the early S andian, approximately at 431Ma.Growing eviden e for similar early S andian ages preserved in other WGR peridotitebodies and Otrøy gneiss and the reprodu ibility of the . 431Ma age on iso hronsfrom di�erent peridotite bodies on Otrøy and Flemsøy suggest that this age is atrue age not asso iated with isotope mixing or isotope dis-equilibrium. The timebefore subdu ted ontinental rust rea hes depth levels of 200 km an be estimatedfrom the onvergen e rate and the subdu tion angle. Conservative estimates froman assumed relatively fast and onstant subdu tion rate of 20mm/a yields for ahigh subdu tion angle of 60 ° 11.5Ma and for 45 ° 14.1Ma. This suggests that the ontinental plate ollision between Balti a and Laurentia in the southern part ofthe northern Iapetus may have initiated between approximately 445�440Ma. This

204 6. EMPLACEMENT INTO THE CRUST

Figure 6.26: Depth�time plot for UHP ro ks in the WGR with the preferred exhumation pathdeliniated (dashed arrow). M, U�Th�Pb mean Mnz EMP ages and P estimates on Grt�Ky�gneissfrom Fjørtoft (Terry et al., 2000a,b); O, Sm�Nd mineral�whole ro k iso hron ages and P estimateson re rystallized Grt-pyroxenite from Otrøy and Flemsøy (this study); R, 40Ar/39Ar Hbl oolingages and P estimate on a Ky�assemblage and on a Sil�overgrowth in Grt�Ky�gneiss from Sandsøya(Root et al., 2005); T; U�Pb titanite (+ Zrn) ooling ages and P estimates on WGR orthogneiss(Tu ker et al., 1987); Z, U�Pb Zrn age and minimum P estimate for Coe in lusions in bi-minerali e logite from Hareidlandet (Carswell et al., 2003b). ontrasts with the known S andian history in the WGR believed to have startedwith ontinental subdu tion at ∼425Ma and to have ulminated in an UHP meta-morphism at ∼405Ma (Gri�n and Brue kner, 1980, 1985; Mørk and Mearns, 1986;Terry et al., 2000a,b; Carswell et al., 2003a,b).An alternative simple model is the ollision of the Balti plate margin with ami ro- ontinent during the late stage of the Iapetus losure before the �nal olli-sion with Laurentia. This appears unlikely by taking into a ount that the mi ro- ontinent would have had to be thi k, old and old as typi al for Ar haean ratons.An alternative more omplex model involves two ontinental subdu tion zonesbehind ea h other separated by an intra ratoni fault as re ently suggested byBrue kner and Van Roermund (2004). Following this s enario, �rst the outer partof the Balti plate margin (Lindås�Nappe, Middle Allo hthone) was subdu ted un-derneath Laurentia. Subsequently an intra ratoni fault developed in the forelandof the Balti plate. Continued onvergen e aused subdu tion of the WGR alongthe intra ratoni fault with the former Balti margin situated in between the WGRand Laurentia. It follows from the early re rystallization ages of the Otrøy andFlemsøy peridotite bodies, that the surrounding gneiss forms part of the earliermetamorphosed Balti plate margin (Lindås�Nappe, Middle Allo hthone) and doesnot form part of the WGR. Supporting eviden es for su h a Middle Allo hthonous

6.7 Dis ussion 205origin for the Otrøy and Flemsøy gneiss has not been reported.A variation of the model with the intra ratoni fault is that the ages of ∼431Marepresent partly re-equilibrated and therefore meaningless ages. In this ase, theAr haean peridotite on Otrøy (and probably Flemsøy) have been folded, re rystal-lized and embedded into the WGR basement gneiss during the S andian orogeny,but did not fully re-equilibrate. This is unlikely as some of the iso hrons are wellde�ned.The simplest model of peridotite empla ement and metamorphism during a sin-gle ontinental subdu tion event after Brue kner (1998) is preferred to explain thenew data presented. This predi ts that the Otrøy peridotite is derived from Lau-rentian Ar haean SCLM beneath Greenland. A simple ollision requires reasonablearguments for a longer lasting subdu tion of buoyant ontinental lithosphere, whi htends to melt the longer it stays at upper mantle onditions. A prolonged subdu tionhistory an be produ ed by the following fa tors:1. The geotherm of an Ar haean hangingwall is signi� antly older than that ofa `normal' Proterozoi or Palaeozoi hangingwall.2. Pre eded subdu tion of o eani lithosphere ooled the Ar haean hangingwallto even lower T.3. Subdu tion of ontinental lithosphere deep into the Dia stability �eld (200 kmdepth) will tend to slow subdu tion rates so that a subdu tion�exhumation y le in reases in time with in reasing maximum subdu tion depth.4. Slab break�o� is a ontinous pro ess that may require several millions of yearsto pro eed along a ollision front of several hundreds of km (Wortel and Spak-man, 2000).5. Peak P�T estimates from the Otrøy Grt-pyroxenite are at the lower edge ofwet solidi for rustal ro ks (Stern et al., 1975; Ni hols et al., 1994; Kesselet al., 2005). Dehydration of the slab may have prevented the gneiss frommelting during the prograde evolution. Gneiss migmatisation in adja ent areasis reported to have o urred mainly during de ompression (Labrousse et al.,2002).Figure 6.26 shows the orresponding preferred de ompression path for UHPro ks in the WGR with time. The position of the estimates from this study(O) are based on the assumption that the re rystallization ages preserved in Grt- linopyroxenite (431�424Ma; from Raudhaugene, Ugelvik and Nogvadalen) orre-late with the peak metamorphi onditions in re rystallized Grt-websterite (notdated; from Raudhaugene and Midsundvatnet). P -estimates for the Coe-bearingbi-minerali e logite at Hareidlandet are shown as minimum P based on the stabil-ity of Coe at 800 °C (Hemingway et al., 1998). The reason is that P�T estimatesinterpreted to apply at the Hareidlandet e logite are derived from e logite at a dif-ferent lo ality, Furøya (Carswell et al., 2003a). Data of Root et al. (2005) is shown

206 6. EMPLACEMENT INTO THE CRUST(a) WGR peridotites � Carswell (1986) Pre ambrian CaledonianStageI StageII StageIII StageIV StageV StageVI StageVIIOlivinenearpervasivedu tiledeform. extensivesheard

eformation strain-freede ompression limiteddeformat

iondeform.inhighstrainzones latestagebrittle

fra turingOrthopyroxeneClinopyroxeneSpinelGarnetAmphiboleChloriteTal Serpentine(b) WGR peridotites � Van Roermund et al. (2000b, 2002); Brue kner et al. (2002)Pre ambrian CaledonianStageI StageII StageIII StageIV StageV StageVI StageVIIOlivinenearpervasivedu tiledeform. extensivesheard

eformation strain-freede ompression limiteddeformat

iondeformationinhighstrainzones latestagebrittle

fra turingOrthopyroxeneClinopyroxeneSpinelGarnet MjDiamondAmphiboleChloriteTal Serpentine( ) Peridotites from the Nordøyane�Otrøy UHP provin e � this studyPre ambrian CaledonianStage0 StageI StageII A StageII B StageIII StageIV A IV B StageV StageVI StageVIIOlivinemantlede ompression

lowerlithospheregrowth+ ooling lithospherethinn

ingintensivedu tiledeformation strain-freede om

pression limiteddeformation

deformationinhighstrainzones latestagebrittle

fra turingOrthopyroxeneClinopyroxene ?Spinel (Cr#) ∼60 ∼70Garnet (%Pyx in ss) ∼20 1�2 ∼0 0 fTi-Clinohumite, DiamondAmphiboleChloriteTal SerpentineStru ture S2 L3‖FA3 (FA4A?) FA6Assemblage M0 M1 M2 M2* M3 M4AM4B M5 M6 M7Pressure (GPa) ≥11.5 5 5 3.5�3.7 6.5 >2Temperature (°C) ≥1800 ∼1700 ≥1300 770 870 >670Age (Ma) >3100 3100 ≤3100 ? 431 ?

6.8 Con lusion 207for omparison (R). These authors reported retrograde Hbl 40Ar/39Ar ooling agesof 402.1±2.9(4.2)Ma and 402.7±8.8(9.3)Ma (un ertainty in bra kets in lude errorsin de ay onstant) obtained from Aspøy and Enge (NE of Otrøy) and two meta-morphi equilibria in meta-pelite from Sandsøya (SW of Otrøy; 1.7�1.2GPa and650�800 °C with favours for 1.7�1.2GPa and . 750 °C based on a Ky-bearing as-semblage and for 4.7±0.3 kbar and . 750 °C based on a Sil-overgrowth assemblage).The Hbl-ages were asso iated with both P estimates in a model of rapid exhumationof WGR (U)HP units from 32 kbar to . 5 kbar, proposed to have o urred in a fewMa after 410�407 and before 403�402Ma ago. Errors have not been dis ussed intheir model. The more pre ise age of 402.1Ma ombined with the error of ±4.2Made�nes a large almost separate �eld in Figure 6.26 .The delineated exhumation path in Fig. 6.26 suggests for the exhumation of theUHP metamorphosed slab gradually in reasing exhumation rates from the point of ulmination until on�ning P of . 1GPa. Further exhumation ontinued slowly asreported earlier (Terry et al., 2000a; Carswell et al., 2003a).A te tono-metamorphi evolution s heme for WGR peridotite has �rst beenproposed by Carswell (1986) and has been modi�ed several times by several authors.Table 6.27 presents an overview of the modi� ations and an updated version of thiss heme based on the data in this thesis.6.8 Con lusionThe new data presented in this hapter demonstrates the peridotite empla ementfrom an Ar haean hangingwall into basement gneiss of the Balti plate marginduring prograde subdu tion. Re rystallization of the mantle fragments started atlithospheri depth of . 115 km and destroyed older exsolution mi rostru tures inminerals. Grt-pyroxenite in Grt-peridotite from Otrøy and Flemsøy broaden thetime range and in rease the metamorphi grade of UHP metamorphism re orded←−Figure 6.27: Proposed te tono-metamorphi evolution of Mg�Cr type Grt-peridotite in westernNorway. (a)�(b) Previous studies. ( ) Synthesized diagram from this study: � oarse re rystal-lized; f � �ne re rystallized; M0 � in ludes Mj0 stable at Transition Zone depth; M1 � in ludesthe formation of the inter rystalline mi rostru ture during de ompression (Mj0→Mj1+HT-Pyx1);M2 � in ludes the formation of the intra rystalline mi rostru ture by ooling (Mj1→Grt2+Pyx2);M2* � hemi al re-equilibration of mineral phases in the inter- and intra rystalline mi rostru -ture towards the geotherm after lithospheri thinning; M3 � (early) S andian peak UHP meta-morphi re rystallization; M4A � S andian post-peak UHP metamorphi e logite-fa ies repla e-ment of high-Cr-Grt2→Grt4A+Spl4A+Cpx4A (Kelyphite4A) and re rystallization of low-Cr-Grt3+Cpx3+Opx3→Grt4A+Cpx4A+Opx4A interpreted to be linked with �uid in�ltration andeventually be linked with inferred large s ale folding FA4A of the peridotite during S andianexhumation; M4B � break-down of Grt2,3,4A→ Spl4B+Cpx4B+Opx4B (Kelyphite4B) during strain-free S andian exhumation in the Spl-peridotite stability �eld; M6 � amphibolite-fa ies km-s alere-folding of earlier linear and planar stru tures in the peridotite around FA6 interpreted to belinked with penetrative deformation of the surrounding gneiss.

208 6. EMPLACEMENT INTO THE CRUSTin the WGR. Subdu tion of the Balti plate margin an be tra ed deep into theDia stability �eld to lithospheri depths of 200 km (6.5GPa and 870 °C). Sm�Ndre rystallization iso hron ages indi ate that major deformation and folding of theperidotite o urred during the early-S andian at 431+3/−7 Ma. A single subdu tionevent is envisaged as the simplest model, whi h requires the subdu ted ontinentalplate to be stable in the upper mantle for a few tens of millions of years. Exhumationrates in reased gradually until UHP metamorphosed ro ks were de ompressed to . 1GPa. An additional 5�whole ro k `error hron' shows that di�erent types ofpyroxenite are mid-Ar haean in origin, 3.33±0.19Ga.The results demonstrate that orogeni Grt-peridotite an be used as a metamor-phi tra er for ontinent� ontinent plate ollisions at P�T onditions, that appearto ex eed the re ordable limits in gneiss and e logite.