Oxygen-isotope geochemistry of the Scourian ...

Journal of the Geological Society Oxygen-isotope geochemistry of the Scourian complex, northwest Scotland I. CARTWRIGHT and J. W. VALLEY Journal of the Geological Society 1992; v. 149; p. 115-125 doi:10.1144/gsjgs.149.1.0115

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© 1992 Geological Society of London

Journal of ?he Geological Society, London, Vol. 149, 1992, pp. 115-125, 6 figs, 4 tables. Printed in Northern Ireland

Oxygen-isotope geochemistry of the Scourian complex, northwest Scotland I . C A R T W R I G H T ' & J. W . V A L L E Y ' 'Department of Earth Sciences and VIEPS, Monash University, Clayton, Victoria 3168, Australia 2Department of Geology and Geophysics, University of Wisconsin, Madison W I 53706, USA Abstract: The Scourian complex of northwest Scotland underwent granulite-facies metamorphism at 2.7 Ga. Granulite-facies orthogneisses have whole-rock6"O values similar to those of igneous rocks

of comparablecompositions(ultrabasicgneisses5.5-5.9%0,basicgneisses 6.4-6.8%0, tonaliticgneisses 9.5-9.7%0), and steep "0 gradients of up to 3%oin lOcm are recorded at two lithological contacts.These data suggest that large volumes of pervasive metamorphic fluids have not infiltrated the rocks, but do not preclude localized fluid-rock interaction. Scourian paragneisses have lower 6"O values (8.5-9.9%0)than typical metasediments, and may have undergone fluid-hosted isotopic exchange with the surrounding within tonalitic and basic gneisses have similar 6I8Ovalues to their hosts: these orthogneisses. Felsic sheets data, combined with petrological and geochemical information, suggest that the sheets were formed by local anatexis during granulite-facies metamorphism. Fluid infiltration during the 2.5 Ga amphibolitefacies Inverian retrogression caused homogenization of 6"O values of the Scourian gneisses. Unaltered tholeiitic Scourie dykes have low 6I8O values of 2.0%0. It is unlikely that heated, surface-derived fluids infiltrated the Scourian complex, and these dykes were probably emplaced as 1 0 w - 6 ~ ~magmas. 0

The Scourian complex forms the central region of the mainland Lewisian outcrop of northwest Scotland (Fig. la). Scourian rockspreservemainly Archaeanmetamorphic mineral assemblages and tectonic fabrics;by contrast, rocksin the adjacent Laxfordian complexes underwent extensive Proterozoic reworking. There have been several recent discussions of the geology of the Scourian complex (e.g. Park & Tarney 1987; Cartwright & Barnicoat 1987; Cartwright 1990), and only a brief summary is given here. TheScourian complex is dominated by tonaliticorthogneisses (75-80% of the total outcrop area),with subordinate volumes of basic and ultrabasic orthogneisses and metasediments. Theorthogneissprotoliths differentiated fromthe mantle at2.95 Ga, andall Scourian gneisses underwent granulite-facies metamorphism accompanied by intense subhorizontal deformation in the 2.7Ga Badcallian event. Peakmetamorphic conditions in the Scourie-Stoerarea(Fig. la) have been estimated from geothermometers, geobarometers, and phase equilibria as 1000 & 50 "C and in excess of 1.2 GPa. As in many granulite facies terrains, there has been considerable disagreement asto the originof the granulite-facies assemblagesinthe Scourian complex.Severalstudieshaveconcluded that the Scourian gneisses, with the exception of the ultrabasic lithologies, underwent anatexis during the Badcallian event (O'Hara & Yarwood 1978; Barnicoat 1983; Cartwright & Barnicoat, 1987; Cartwright 1988,1990; Cohen et al. 1991). By contrast, largely on the basis of trace-element data, other workers consider that anatexis didnot play amajor role in the evolution of the Scourian complex, and have proposedthattheanhydrous granulite-faciesassemblagesresulted from thewidespread influxof CO,-rich fluids (Tarney& Weaver 1987a; Park & Tarney 1987). Trondhjemitic and granitic felsic sheets occur within the tonalitic, basic and metasedimentary gneisses throughout the Scourian complex. These sheets locally compriseup to 40% of the outcrop (Cartwright 1988) and, from cross-cutting relationships with tectonic fabrics, are inferred to have been emplaced at or just prior to the peak of Badcallian metamorphism (Rollinson & Windley 1980; Cartwright & Barnicoat 1987). 115

Sheets within the metabasic and supracrustal lithologies are petrologicallydistinct from those thatcutthesurrounding tonalitic gneisses (Barnicoat 1983; Cartwright & Barnicoat 1987). The felsic sheets are considered by the proponents of anatexis to represent the products of local melting during prograde metamorphism. However, due to difficulties in reconciling aspectsof their trace-element geochemistrywith this origin (Park & Tarney 1987), alternativemodelsfortheir formation have been proposed. Rollinson & Windley (1980) concluded that these rocks were formed by fractional crystallization of theigneousprecursors of the tonalitic gneisses, while Rollinson & Fowler (1987) proposed that felsic sheets at Gruinard Bay were derived by melting of basic rocks outside the Scourian complex. FollowingtheBadcallianevent,theScouriancomplex underwent hydrous retrogression and shearing in the 2.5Ga Inverian event, at 550 f50 "Cand 0.5 & 0.2 GPa: the degree of retrogression varies markedly throughout the complex (Fig. la), and is most intense in the regionsthat contain abundant felsic sheets (Cartwright 1988). The Scourie dykes were intruded in two episodes at2.4 and 2.0 G a (Heaman & Tarney 1989; Waters et al. 1990). Ambient conditions during dykeemplacement were estimated as 500 50 "C and 0.5 0.1 GPa from the stability of olivine and plagioclase in picritic dykes and fromgarnet-bearing assemblages in dyke margins (Tarney & Weaver 1987b). Further minor retrogression and shearing occurred in the 1.9-1.7 Ga Laxfordian episode at 450 f 100 "C and 0.5 k 0.1 GPa, and the complex was exposed at the subTorridonian land surface at or before 1.0 Ca.

*

*

Petrology of the Scourian gneisses and Scourie dykes Unretrogressedgranulite-faciesorthogneissescontainclinoand orthopyroxene. Additionally, quartz and plagioclase are present in tonaliticgneisses, plagioclase and garnet arepresent in basic gneisses, and the ultrabasic gneisses contain olivine, spine1 and hornblende (Table 1). The metasediments include ironstone, calcsilicate and pelitic lithologies with more diverse granulite-faciesassemblages (e.g. Cartwright & Barnicoat

I. CARTWRIGHT & J . W . VALLEY

116

and picrites. Unaltered tholeiiticdykeshaveprimaryigneous plagioclase and clinopyroxene with minor quantities of magnetite,hornblende,quartz,orthopyroxene,ilmeniteand apatite. Garnet is commonly present at the margins of the & Weaver dykes, and locallyinthedykecentres(Tarney 19876). Unaltered tholeiiticdykeshavesubophitictextures with flow bandingand chilledzones at theirmargins. Where the dykes are cut by Laxfordian shear zones, hornblende replacespyroxene and a new tectonicfabric is developed. This paperpresents the results of an oxygen-isotope studyof rocks from two contrasting regions of the Scourian complex:a) theScouriearea,whichpreserveslargelyunretrogressed granulite-facies gneisses and little-altered Scourie dykes; and b) the Stoer area, which underwent intense Inverian retrogression. In particular we address the following three issues that have been the subject of recent debate: fluid-rock interaction during prograde and retrogrademetamorphism; the genesis of the felsic sheets; and the origin of the Scourie dykes.

Analytical procedures

45

Oxygen-isotoperatios of whole-rockandmineralseparateswere analysed at Madison using BrF, as the oxidizing reagent (Clayton & Mayeda 1963). Mineral separation was achieved by magnetic separation, HF dissolution, and hand picking. Purity of mineral separates was assessed optically and by XRD, and was typically > 95%. Values of 6I8O arereported in standard pennil (%o) notationrelativeto V-SMOW. 24 aliquots of NBS-28 quartz standard and 18 aliquots of W - M A P S laboratoryplagioclasestandardanalysedconcurrently with the samplesdiscussedinthispapergave 6"O values of 9.61 f 0.15%0 and 9.38 f 0.15%0 respectively.

1

44

Geodh Eanrulg

A

D

Oxygen-isotope geochemistry of unretrogressed granulite-facies gneisses

N

43 -

0

42

-

Geodh' nan Sgadan 13

14

15

16

Fig. l.(a) Generalized geological map of part of the mainland Lewisian complex showing the extent of the Scourian and Laxfordian gneisses and the Meal1 a' Chuna Mhor locality (D). (b) Location of Scourie Bay (A), Geodh Eanruig (B), and Geodh nan Sgadan (C).

1987; Table 1). In the retrogressed tonalitic and basic gneisses, hornblende and biotite replace the pyroxenes, and there is an increase in the albite content of plagioclase. The ultrabasic gneisses were converted to talc-tremolite-bearing assemblages during retrogression. The felsic sheetscomprise quartz and plagioclase, with K-feldspar present in the granitic lithologies. Maficminerals accountfor less than 10 modal% of these lithologies, andare typically hornblendeandbiotitebut locally pyroxene and garnet; muscovite is a common late mineral in these sheets. The Scourie dykes are dominantly quartz tholeiites (c. 80% of all dykes) with lesser volumes of olivine gabbros, norites,

Unretrogressedgranulite-facies gneisses were sampled from three localities near Scourie (Fig. Ib, Appendix A); at each locality, samples were collected from within a 20 m X 20m area. Each locality contains a representative suite of Scourian tonalitic, basic, ultrabasic, and metasedimentary gneisses and felsic sheets with well-exposed, unfaulted contacts. The ultrabasic and basic gneisses occur asindividual layers or pods within the tonalitic gneisses that are up to 10m wide, or as zoned ultrabasic-basic masses that are up to 20m wide. The metasediments form thin (< 1m wide) layerswithinthe tonaliticand basic gneisses. The felsic sheetsareaminor ( < S % ) component of theScourieregion and cut both the tonalitic and basic gneisses. Granulite-faciesorthogneisses(Table2 and Fig. 2) have whole-rock 8'0 values that are similar to igneous rocks of comparable composition(Taylor & Sheppard 1986): ultrabasicgneisses 5.5-5.9%0; basicgneisses 6.4-6.8%0; and tonalitic gneisses 9.5-9.7%0. By contrast, the thin metasedimentarylayers have lower 6"O values (8.3-9.9%0, Table 2) than those typical of metasedimentsof similar composition(6I8Oin excess of 10%0;e.g. Taylor & Sheppard 1986). The oxygen-isotope ratios shown in Fig. 2 are similar to those recorded by Harmon (1983) for undifferentiated Lewisian granulites (5-8'300). The minerals the inunretrogressed gneisses preserve hightemperature oxygen-isotopefractionations and there are no isotopic reversals (Fig. 2). Even minerals that are readily reset (e.g. plagioclase) do not show significant isotopic disequilibrium with coexisting phases.

COMPLEX OXYGEN-ISOTOPES, SCOURIAN

117

Table 1. Mineral modes of gneisses from this study

Cpx

Ksp

Plg

Qtz

Opx

Hbl

Bt

Gnt

Sp Mt

Olv

Other*

Scourie Area: orthogneisses

Tonalitic Basic Ultrabasic

55-65 10-20 (1)

0-5

3 W O

15-20 (1) 5-10 10-15 45-60 10-15 20-35

(1)

Ilm

(1)

0-25 50-70

0-5

5-10

Zir, 0-5 0-5 0-5

Ap,

Scourie Area: metasediments

Calcsilicate Ironstone Pelite

30-35 20-30 25-30 3040

15-20 15-20 15-20

Sph, AP 3540 10-15

10-15

(R) (R)

(R)

0-5

15-20

Ky, Zir

15-20 15-20

Stoer Area: orthogneisses

Tonalitic Basic

50-60 15-25 0-5

0-10 3

w

(R)

15-20 50-70

5-10 0-5

(R)

0-5 0-5

Zir Ep,

(1) minor late phase; (R) relict phase; * present as less than 5% in some samples Mineral abbreviations: Ap, apatite; Bt, biotite; Cpx, clinopyroxene; Ep, epidote; Gnt, garnet; Hbl, hornblende; Ilm, ilmenite; Ksp, alkali feldspar; Ky, kyanite; Mt, magnetite; Oh, olivine; Opx, orthopyroxene; Plg, plagioclase; Qtz, quartz; Sph, sphene; Sp, spinel; Zir, zircon.

Scale of isotopic exchange and metamorphic fluids The degree of isotopic exchange between adjacent lithologies places important constraints on theextent of fluid-rock interaction.TheScourian orthogneisses probably preservepremetamorphic whole-rock 6I8Ovalues, and there is no evidence forhomogenization of oxygen-isotoperatios atthethree Scourie localities (Fig. 2). This suggests that these lithologies were not infiltrated by large volumes of pervasive fluid during metamorphism. The isotope datain Fig. 2 d o not, however, rule out smallscale fluid movements. The contacts between lithologies that had contrasting initial isotope ratios are sensitive indicators of small-scale fluid flow across, or along, the contacts (e.g. Bickle & Baker 1990; Cartwright & Valley 1990, 1991b). Figure 3a shows the well-exposed, unfaulted, lower contact of a 10m thick layer of unretrogressed basic gneiss against granulitefacies tonalitic gneisses at Geodh nan Sgadan (Fig. l). The basiclayerdips at c. 15 O to the N W , parallel to regional Badcallian foliation. There is little post-Badcallian folding in this area, and the basic sheet would probably have been dipping shallowly at the peak of metamorphism. If metamorphic fluids migrated through the Scourian crust, they would have

probably approached isotopic equilibrium with the tonalitic gneisses, as these are the dominant lithology. In general, net regional-scale fluid flow is most probablyvertical asa response to fluid buoyancy, and, if there had been significant fluid flow acrossthetonalitic-metabasicgneiss boundary, thiswould have almost certainly perturbed the isotopic ratiosof the metabasic gneisses close to the contact. Significant unidirectional fluid flow would probably have resulted in the formation of distinct isotopic fronts in the lithology into which the fluid migrated (e.g. Bickle & McKenzie 1987; Bickle & Baker 1990). Oxygen-isotope ratios were determined insix samples from a 300cm traverse across the tonalitic-basic gneiss contact in Fig. 3a (Table 3, Fig. 3b). There is a sharp gradient in 6I8O values of c. 3%oin lOcm at this boundary. Basic gneisses collected5-175cmabovethecontacthave 6I8O values of 5.8-6.2%0, similar to the range of 6I8O values of the basic gneisses inFig.2 (6.4-6.8%0). Thetonalitic gneisses from 5cm-125cm below the contact have 6"O values of 8.5-9.4%0 which are similar to those of tonalitic gneisses elsewhere in the complex (9.5-9.7%0, Fig. 2). There arealso marked differences in the 6I8Ovalues of plagioclase (c. 2.0%0)and clinopyroxene (c. 1.5%0) in the tonalitic and basic gneisses (Fig. 3b). Plagioclase in the basic gneisses is of similar composition (An,, to

Fig. 2. 0-isotope data for unretrogressed granulite-facies gneisses from near Scourie (data from Table 2). The 6I80 values of orthogneisses are similar to those of igneous rocks of comparable compositions and the minerals preserve high-temperature "0fractionations. The heterogeneous whole-rock oxygen-isotope ratios suggest that large volumes of pervasive metamorphic fluids have not infiltrated these rocks, either prior to, during, or after the Badcallian metamorphism. Inset shows expected mineral I8O fractionations at 700 "C, data from Bottinga & Javoy (1973, 1975), Matthews er ul. (1983), and Clayton et al. (1989).Plagioclase is An,.

118

I. CARTWRIGHT VALLEY & J . W.

Table 2. Oxvpen-isotope ratios of gneisses and felsic sheets-from the Scourie and Stoer areas.

gy

Sample

S1'0 (Too SMOW)

LOC. Bt

Hbl

Cpx

Olv WRPlg

A A

5.5

Qtz

Mt

Gnt

KY

Scourie urea: host gneisses

Ultrabasic 86007 860 1 1 Ultrabasic 86035

Ultrabasic

B

5.3

6.6 7.8 7.5 7.0 7.6 7.8

5.7 6.0 6.3 5.6 6.4

9.3 9.9

7.8 7.6 8.0 7.5 7.0

86008 86009 86037 86039 87107

Basic Basic Basic Basic Basic

A A B

6.4 6.6 6.5 6.7 6.8

86043 86044

Tonalitic Tonalitic

B B

9.7 9.5

10.9 12.0

B A B

8.3 9.2 9.9

12.0

10.0

10.2 9.7

C A B C

9.4 9.4 9.0 6.9 7.2 7.5

10.9 11.2 10.3 8.7 8.8 8.9

8.6 8.7 8.4 6.3 7.0 7.O

D D D D

8.4 8.5 9.2 9.3

12.7 11.6

10.9 9.3 9.0 9.3

D

9.0

11.0

8.2

Ironstone 86013 Pelite 86017 Calcsilicate 86049

B B

6.6 6.6

5.8 5.9

11.3

3.2 3.3

5.6

2.3 3.4 3.8 2.3

5.7 6.1 5.4

3.7

1.3 7.5

Scourie area: felsic sheets

85115 86004 86045 85110 89SC21 89SC22

Granite* Granite* Granite* Trondhjemitet Trondhjemitet Trondhjemitet

B

C

5.7 6.0

3.2

Stoer urea: host gneisses

84222 84223 83031 83018

Basic Basic Tonalitic Tonalitic

5.9 5.7

8.1 8.3 7.5 6.2

Stoer urea: felsic sheets

Granite*84060

Abbreviations as for Table 1 and WR, whole-rock. Localities: A, Scourie Bay; B, Geodh Eanruig; C, Geodh nan Sgadan; D, Meall a' Chuna Mhor (Appendix A, Fig. 1). * Felsic sheet within tonalitic gneiss; t Felsic sheet within basic gneiss.

An6,) to thatin the tonalitic gneisses (An,, to An,,), indicating that the discontinuity in plagioclase 6I8O values does not result from differences in plagioclase composition (e.g. Clayton et al., 1989). Magnetite 6I8O values are similarin both the tonalitic and basic gneisses. However, the difference between the 6 l 8 0 values of plagioclase and magnetite, d(P1g-Mt), in the tonalitic gneisses is 5-7%0, whereas d(P1g-Mt) in the basic gneisses is 4-5%0. The similarity in magnetite 6I8Ovalues may be due tominerals in the tonalitic gneisses undergoing a greater degree of isotopic resetting during cooling than those in the basic gneisses (as discussed below). The mineral lsO/160 ratiosalmostcertainlyunderwent closed-system resetting during cooling from the peakof metamorphism (see below). Therefore, the discussion of oxygenisotope profiles will concentrate on whole-rock 6"O values. The 6l8O value of a basic rock that equilibrated with an infiltrating fluid in isotopic equilibrium with the tonalitic gneisses would be governed by themodal mineralogies of the two rock types and the isotopic fractionations among the constituent minerals. This would be the case regardless of the H,O-CO,

ratio of the infiltrating fluid. Applying the mineral I8O fractionations of Bottinga & Javoy (1973,1975), Matthewset al. (1983), and Clayton et al. (1989) indicates that, at lOOO"C, afluid infiltrating from tonaliticgneisses (comprising 10-20% quartz, 55-65% plagioclase, and 15-25T0 pyroxene) with a 6"O value of 9.5%0 into basic gneiss (30-40% plagioclase, 50-70% pyroxene, and 0-20% garnet) with an initial6 l 8 0 value of 6%0 would elevate the 6 I 8 0 value of the basic rock to 8.5-9.0%0. Infiltration at 600 "C would result in elevation of the 6"O of the basic gneiss to 8.0-8.5%0. If full equilibration between infiltrating fluid and therocks did not occur, the 6I8Ovalues of the basic gneiss would be elevatedto intermediate values, and the isotopic profiles would be less sharp (Blattner & Lassey 1989). If fluids flowed from the basic gneiss into the tonalitic gneiss, resetting of the 6I8Ovalues of the tonalitic gneisses adjacent to the metabasic gneisses would have occurred. The absence of any perturbation of 6 l 8 0 values at the tonalitic-basic gneiss contact implies that there has been negligible pervasive fluid flow across this boundary. The presence of a static, interconnected, fluid at this locality

Fig. 3. 0-isotope. profiles across two

3b.

3c. Basic Gneiss

Tonalitic Gneiss

Pod

i

‘onalitic 3neiss

I

.X

9

------m

l

0

100

L 10 0

20

100

10

Distance (cm)

well exposed unfaulted contacts at Geodh nan Sgadan (Table 3). (a) Photograph of well-exposed, unfaulted contact between basic and tonalitic gneisses (scale line is 1 m). Samples were collected adjacent to hammer. (b) 6’*0 profile across the contact shown in (a) (data from Table 3). (c) tiL8Oprofile across the contact between a 2.0m X 0.5 m ultrabasic pod and tonalitic gneissesc. 5 m from the metabasic-tonalitic gneiss contact in Fig. 3a (data from Table 3). In both cases the oxygen-isotope profiles have step-like discontinuities at the contacts which suggest that isotopic transport by fluid infiltration across the contacts or by diffusion through a fluid phase has not occurred. Symbols as for Fig. 2.

Table 3. Stable-isotope results from traverses at Geodh Ran Sgadan

logy

9.5

1l9

OXYGEN-ISOTOPES. SCOURIAN COMPLEX

Sample

6”O

(Cm)

WR

Qk

Plg

(%o

SMOW) HblCPX

Mt

5.4

2.2 2.9 3.2

Traverse from basic layer into tonalitic gneiss (Fig. 3a)

89SClO 89SC 12 89SC13 89SC14 89SC15 89SC16

Basic6.2 Basic5.8 Basic6.0 Tonalitic Tonalitic 8.5 9.4 Tonalitic

5.9

175 30 5 9.2

5

7.2

25 125

7.3

5.6 7.0

9.3

7.0 9.0 8.9

3.7 2.7

10.9

Traverse from ultrabasic pod into tonalitic gneiss

5

89SC27 89SC28 89SC29

Pod Pod 9.4 Tonalitic

5.1

20 10

Abbreviations as for Tables 1 and 2. Dist., distance from mutual Contact.

4.7 6.8

120

I . C A R T W R I GVALLEY H T & J . W.

during metamorphismwould have resulted in the modification of the initial isotopic discontinuity by diffusion through the fluid, forming a sigmoidal profile. Quantitative modelling indicates that diffusion through a fluid atmetamorphic temresult in isotopic peratures with porositiesof 10-3 to 10d6 may exchange over distances of tens of centimetres in few a thousand years (Ganor et al. 1989; Cartwright & Valley 19916). The absence of diffusion profiles at the contact in Fig.3a indicates that any metamorphic fluids at thislocality were either present over relatively short timescales, or did not form an interconnected network. A 2 m X 0.5 m ultrabasic pod at Geodh nan Sgadan also has different oxygen-isotope ratiosto the surrounding tonalitic gneisses (Table 3, Fig. 3c). The ultrabasic pod containsc. 90% hornblendeand samples 5 and20cmfromthetonalitic gneisseshave 6I8O values of 5.1-4.7%0. By contrast, clinopyroxene in the tonalitic gneisses lOcm from thepod has a6"O value of 6.8%0, some 2%0 higher thanthehornblendedominated ultrabasic rock. At temperatures over 500 "C, the fractionation of "0between clinopyroxeneand hornblendeis less than 0.5%~ (Bottinga & Javoy 1973, 1975), indicating that this isotopic contrast does not result from mineralogical differences. These results again suggest that there has been no significant pervasive fluid flow across this contact . Overall, the oxygen-isotope data indicate that the orthogneisses at allthreelocalitiespreservelittle-altered,premetamorphic whole-rock 6I8O values. The isotopicheterogeneity of the granulites in the Scourie area,and the steep "0 gradients at lithological contacts, suggest that large volumes of pervasive fluids have not infiltrated these rocks prior to, during, or after the peak of Badcallian metamorphism. These of Cartwright & results are consistentwiththesuggestion Barnicoat (1987) that prograde metamorphismin the Scourian complex may have occurred largely under fluid-absent conditions.However,astherockssampledrepresentasmall volume of the granulites in the Scourie area, channelled or localized fluid movements in this region are not precluded by these data, nor is fluid infiltration elsewhere in the complex; further detailedstudies of the sort described above will be necessary to demonstrate thegenerality of these observations. The relatively low 6"O values of the metasediments (8.3-9.9%0) may be due to oxygen isotope exchange with the surrounding orthogneisses resulting from localized fluid flow or diffusion through a fluid, although thepossibility that these sedimentsoriginally had low 6I8O values cannot bediscounted.

Resetting of mineral isotopic ratios during cooling Following the peak of Badcallian metamorphism, the Scourian complex underwent extremely slow cooling (cooling rates of less than 10 "C/Ma, Barnicoat 1987). Cation-exchange geothermometry yields estimatesof c. 1000 "C for the peak of Badcallian metamorphism, similar to temperatures estimated from phaseequilibria (e.g. Cartwright 1990). However,the fractionation of "0 between clinopyroxene and plagioclase, quartz, magnetite, and olivine in the granulite-facies gneisses is larger than predicted atpeak-metamorphictemperatures (Fig. 4), suggesting that theyunderwentsomeisotopic exchange during cooling. Sincethe gneisses at Scouriepreservegranulite-facies lithologies, resetting of mineral 6I8O values would have most probably taken place in a closed system, rather than in the presence of infiltrating metamorphic fluids. The fact thatmin-

erals such as plagioclase, that are readily resetby fluid infiltration, do notshowsignificantisotopicdisequilibriumalso suggests closed-system behaviour (Gregory & Criss 1986). Ina closedsystem,closuretemperaturesforvolume diffusion of oxygen isotopes or cations between minerals may be determined from equation (23) of Dodson (1973):

where T, is the closure temperature, Q is the activation energy, D, is the pre-exponential diffusioncoefficient, R is the gas constant (8.314J/Wmol), a is the average grain radius, aT/at is the rate of temperature change (negative for cooling),and A is the diffusional anisotropy variable(55 for spherical geometry). The diffusion data foroxygen in albite, anorthite, quartz, magnetite, and diopside, and Mg-Fe in clinopyroxene and garnet from hydrothermal experiments and under 'dry' conditionsin a 0, or CO, atmosphere are summarized in Table 4. The grain radii in Table 4 correspondto the typical crystal size analysed in geothermometry studies (e.g. Cartwright & Barnicoat 1987) and the average grain size of the samples in this study. For cooling rates of 1-40 "C/Ma, the closure temperatures for oxygen diffusion under hydrothermalconditionsrange from 230 "C to 960 "C, and are generally much lowerthan the closure temperatures of Fe-Mg interdiffusion in garnet and clinopyroxene (Table 4). The Scourian gneisses have largely anhydrous mineral assemblages, and were metamorphosed at temperatures higher than the solidus temperaturesof most of theselithologies(Cartwright & Barnicoat 1987). Therefore, these rocks almost certainly did not contain a hydrous fluid phase during cooling from the peakof metamorphism. The dry diffusion data imply that diffusion ratesof oxygen in anorthite and diopside may be up to two orders of magnitude slower in anhydrous granulite-facies lithologiesthan under fluid-present conditions(Elphick et al. 1988; Connolly & Muehlenbachs 1988). However, closure temperaturesfor these minerals in the absence of H,O are 660-910 "C for cooling rates of less than 10 "C/Ma, still generally well below peak metamorphic temperatures. Mineral pairs in the tonalitic gneisses have larger oxygenisotope fractionations than those in the basic gneisses (Fig. 4). Thismay be due to the tonalitic gneisses havingasmaller averagegrain-size (1-2mm radius) than thebasic gneisses (2-3mm), resultinginthemineralsinthetonalitic gneisses undergoing a greater degree of resetting. The differences in oxygen and Fe-Mg diffusion coefficients has resulted in significant down-temperature resetting of the oxygen-isotope ratios of all minerals, whilst the cation geothermometers preserve high temperatures. Additionally, conventionaloxygen-isotopeanalysesrepresentaverage 6"O values of many whole crystals, whereas the geothermometry studies are based on electronmicroprobeanalyses of the centres of coarse crystals (e.g.Cartwright & Barnicoat 1987). If minerals are isotopically zoned, temperature estimates from these average 6I8O values will be lower than those that would be estimated from theisotopic ratios of mineral cores. Overall, these data indicate that oxygen-isotope geothermometry is of little use intheScourianlithologies.Additionally,asthe Scourian gneisses most probably cooled fromthe peak of metamorphism without a coexisting hydrous fluid phase, it is not be possible to use the fractionationof I8O between coexisting minerals to calculate cooling rates (cf. Farver 1989) using

adii

121

OXYGEN-ISOTOPES,SCOURIANCOMPLEX

b.

a.

7

8

9

10

6 1 8 0 (Plg)

6 1 8 0 (Qtz)

d.

C.

3

h

X

Q

2-

8 a

Fig. 4. Plots of 6180(Cpx) v. (a) 6180(Plg), (b) 6I80(Qtz), (c) 6180(Mt),(d) 6180(01v)for

Scourian granulites (data from Tables 2 and 3). These data indicate that oxygen-isotope ratioswere reset during cooling from the peakof metamorphism to 800 "C to 400 "C. The 0-isotope ratios of minerals within the tonalitic gneisses (average grain radius 1-2mm) appear to be more reset than those in basic and ultrabasic gneisses (average grain radius2-3 mm), probably due to the differences in grain size.

4

"2

3 8180

(Mt)

5

6 6'80

(Olv)

Table 4. Closure temperature calculations

Q

Grain Mineral Diffusing species

("C) at cooling rate ("C/Ma)* 40 1 10

P(GPa) Atm

Isotopic dtffusion 284 0 Quartz

0 0 0 0 0 0

temp. Closure D, Ref. Conditions Expt.

226Diopside 400Diopside 236Anorthite 85 Albite 89 Anorthite Magnetite

0.2 0.2 0.2 0.2 0.2 0.2 0.2

186

1000 0.5

37 h1

190 1.5X 10-6 6.0 1.0~10-~ 1.0~10-~ 2.5 X 10-9 3.5 X 10-6

0.1 460 0.1 730 1~10 0.1 0.2 0.1

0.1

H,O H,O - CO, ~

a b

0,

d

H,O H,O H,O

C

e f g

830 660 250 230 480

580510 930810 960910 770700 320300 300280 600530

Cation dzffusion

arnet Fe-Mg 1140 nopyroxene Fe-Mg

1060

0.023

Diffusion data: a, Giletti & Yund (1984); b, Farver (1989); c, Connolly & Muehlenbachs (1988); d, Elphick et al. (1988); e, Elphick et al. (1986); f, Giletti er al. (1978); g, Giletti & Hess (1988); h, Elphick er al. (1981); i, Brady & McCallister (1983). * Temperatures given to nearest 10 "C.

122

I. C A R T W R I G H T & J. W . V A L L E Y

thehydrothermal diffusion data.To performsuchcalculations, data for oxygen diffusionunder dry conditionsor in CO,saturated systems for all the major rock-forming minerals in these rocks would be required.

Anatexis at the peakof metamorphism: formation offelsic sheets There are two suites of felsic sheets in Table 2 and Fig. 2. The granitic sheets that cut the tonalitic gneisses 6I8O havevalues of 9.0-9.4%0. By contrast, thetrondhjemitic sheets that occur within the basic gneisses have 6I8O values of 6.9-7.5%0. Both suites have similar 6"O values to those of their tonalitic and basic hosts (9.5-9.7%0 and 6.4-6.8%0, respectively). Given the debate about the origins of these sheets (summarized above), thereare severalexplanations of theoxygen-isotope data, which need not be mutually exclusive. (1) The similar 6"O values of thefelsic sheets and their hosts may result from isotopic homogenizationdue to regionalfluidinfiltration.This would explain the oxygen-isotope data regardless of the origins of the sheets. (2) Partitioncoefficients for oxygen isotopes between tonalitic or basic gneisses and felsic melts at 800-1000 "C are close to unity (1.0 0.0005, Taylor & Sheppard 1986). If the sheets were formedby local anatexis during Badcallian metamorphism, theywouldinherit 6"O values that were similarto those of their host gneisses. (3) Rollinson & Fowler (1987) proposed that felsic sheets in the Lewisian at Gruinard Bay were derived by anatexis of rocks outside the complex. The sheets at Scourie may have a similar origin,and melting of lithologies with similar 6I8O values to the tonalitic or basic gneisses could produce melts with the oxygen-isotope ratios of one of the setsof felsic sheets in Fig.2. (4) Fractional crystallization of theigneousprecursorsofthetonalitic gneisses at high temperatures(Rollinson & Windley 1980) could have produced granite or trondhjemite melts with similar 6 l 8 0 values to the tonalitic gneisses. The felsic lithologiesin Table2 were sampled fromthe data in Figs 2and 3 centres of 0.5-5 m thick sheets. Because the suggest that no large-scale isotopic homogenization has taken place at any of the three localities, the similar 6l8O values of the felsic sheets and their hosts are unlikely to be the result of fluid infiltration. It is doubtful that the felsic sheets are the products of fractional crystallization, as they were intruded at c. 2.7 Ga, some 200Ma after the formation of the tonalitic gneisses. The felsic sheets in Fig. 2 may have been produced by anatexis of rocks outside the Scourian complex; however, it would be a coincidenceif these source rocks had similar isotopic ratios to those of the Scourian gneisses. Consequently, we consider it most likely that the felsic sheets at Scourie were formed by anatexis of their host lithologies.

Fluid infiltration during the Inverian retrogression The gneisses at Meall a' Chuna Mhor and the surrounding Stoer area (Fig. 1) underwent extensive Inverian retrogression at conditions of 600 50 "C and c. 0.5GPa. Retrogressed tonalitic gneisses from Mealla' Chuna Mhorhave 6I80 values of 9.2-9.3%0 (Table 2; Fig. 5 ) , that are similar to their granulite-facies counterparts (9.5-9.7%0); however,basicgneisses have 6'*0 values of 8.4-8.5%0, significantly higher than unretrogressedbasic gneisses (6.4-6.8%0). The retrogressedbasic lithologies contain relict garnets from the peak-metamorphic assemblages. These garnets have6I8O values of 5.7-5.9%0 that are significantly out ofisotopicequilibriumwithcoexisting

+

t

Basic

*

l *

!

T

0

Granite Sheet

L

WR

X

10 1

6

Hbl Bt 600" C

Tonalitic

Fig. 5. Variations in 6"O values for retrogressed gneisses and constituent minerals from Stoer (data from Table 2), bars show range of whole-rock 6I8O values of granulite-facies gneisses (Fig. 2). The 6"O values of the retrogressed tonalitic gneisses are similar to the those of the granulite-facies tonalitic gneisses; however, basic gneisses have higher 6"O values than their

unretrogressed counterparts. Relict granulite-facies garnets in the basic rocks have similar6"O values to the same minerals in the Scourie lithologies (Fig. 2), suggesting that the elevation in whole-rock 6"O values occurred during retrogression. These data are most consistent with the Inverian fluids being in isotopic equilibrium with the tonalitic gneisses. Inset shows "0 partitioning between minerals at 600 "C.

plagioclase and hornblende (Fig. 5 ) , but close to the rangeof garnet 6I8O values fromthe granulite-faciesbasic gneisses (5.4-6.1 %o). These data suggest that, prior to retrogression, the basic gneisses at Meall a' Chuna Mhor had6I8Ovalues similar to those of theirunretrogressedcounterparts at Scourie (c. 6.6%0), and that isotopic values were elevated during the Inverian event. Harmon (1983) also showed that retrogressed Lewisian gneisses have higher 6 l 8 0 values (in excess of 8%0) than the granulites (5-8%0). Since the whole-rock oxygen-isotope ratios of the tonalitic gneisses were apparently little modified by retrogression, the Inverian fluids wereprobably in equilibrium with the tonalitic gneisses. At 600 "C, a retrogressed basic gneiss containing 40% plagioclase and 60% amphibole, which had equilibrated via a fluid with the tonalitic gneisses ( 6"O Of 9.5%0), would have a 6I8O value of 8.0-8.5%0. The 6 l 8 0 values recorded from the basic lithologiesat Meall a' Chuna Mhor(8.4-8.5%0) are close to thesecalculated values, suggesting that theserockshave exchanged with fluids in isotopic equilibrium with the tonalitic gneisses. The basic samplesare fromwithin 10m of the base of a basic sheetthat is over 50m thick. If Inverian fluid movement was vertical, I8O fronts may be present at higherlevels in this body. A felsic sheet from within the tonalitic gneisses at Meall a' Chuna Mhor has a6 l 8 0 value of 9.0%0which, as at Scourie, is similar to those of its host gneisses (9.2-9.3%0). However, it is not possible to distinguish whether this similarity resultsfrom thissheetbeingformed by localanatexisofthetonalitic

O X Y G E N - I S O T O P E S , SCOURIAN COMPLEX

Fig. 6. Whole-rock and mineral 6 I 8 0 values from Scourie quartz

tholeiite dykes (from Cartwright & Valley 1991~).Unsheared dykes have homogeneous I O W - ~values ~ O and the constituent minerals have high-temperature 0-isotope fractionations. Low-"O values are not recorded in the surrounding gneisses (Fig. 2). These data are most consistent with the dykes being emplacedas 10w-'~Omagmas. Sheared dykes have higher6 l 8 0 values due to interaction of fluids within Laxfordian shear zones.

gneisses or from isotopic homogenization due to the influx of the Inverian fluids. Cartwright (1988) proposed that the Scourian felsic sheets represent melts formed during theBadcallian metamorphism, and thatfluids exsolvedfrom these meltsduring crystallization caused the Inverian retrogression.In the Stoer area, mostfelsic sheets occur within the tonalitic gneisses and were considered by Cartwright (1988) to have been derived from these gneisses. The felsic sheets and tonalitic gneisses have similar mineral assemblages, and if, as in the Scourie area, the felsic sheets at Stoer had similar initial 6 ' * 0 values to those of their hosts (Fig. 2), fluidsexsolved from thesemeltswouldhavebeen approximately inisotopicequilibriumwiththetonalitic gneisses. The oxygen-isotope data from the Stoer lithologies are consistent with the model of Cartwright (1988); however, they do notrule out thepossibility that theInverian fluidswere derived from sources atdepthandequilibrated withthe tonalitic gneisses en route tothecurrentlyexposedcrustal level.

123

textures. (3) The dykemineralspreservehigh-temperature oxygen-isotopefractionations(Fig. 6). By contrast, rocks which have interacted with surface-derived fluids in an open system usually show isotopic disequilibrium between minerals that reset readily, such as plagioclase, and those which have slower mineral-fluid isotope exchange rates, such as clinopyroxene, (Criss & Taylor 1986). (4) The dykes were intruded at depths of over 15 km, below the level in the crust where large volumes of surface-derived fluids can readily penetrate (Wood & Walther 1986). It is therefore considered more likely that the dykes were emplaced as low-I80 magmas. Rare-earth-element, large-ionlithophile, and radiogenic-isotopegeochemical data suggest that the dykes did not assimilate significant volumes of continental lithosphere (Tarney & Weaver 19876; Waters et al. 1990). Consequently, the oxygen-isotope data are best explained by derivation of thedykemagmasfroma "0depleted mantle source region.The ultimate source of the dyke magmas may have been a mantle region that contained relicts of subducted oceanic crust that underwent high-temperature seafloor alteration. If thiswas the case, it has significant implications for Lewisian tectonics. There is probably in excess of 11 000 km3 of quartz tholeiite dykes in the mainland Lewisian complex, which may represent only a fraction of the original volume of the Scourie dykes (Cartwright & Valley 1991~).If allthetholeiiticdykes were emplacedas l 0 w - ' ~ 0magmas, large volumes of hydrothermally-altered oceanic crust must have been subducted into the mantle during Archaean and Proterozoic times, probably requiring rapid plate generation and subduction rates. Retrogressed dykes within the Laxfordian shear zones have hornblende-plagioclase assemblages and new tectonic fabrics. These altered dykes have elevated 6 l 8 0 values of 5.6-6.4%0 (Fig. 6), suggesting that the fluids that infiltrated these dykes during shearing had undergone isotopic exchange with the surrounding gneisses.

Conclusions

Unretrogressed granulite-facies gneisses from the Scourie area have varied whole-rock 6 l 8 0 values. The orthogneisses have oxygen-isotope ratios similarto those of igneous rocksof comparable compositions,and steep "Ogradients ofc. 3%0in lOcm are present at the contact between tonalitic and basic granulite gneisses (Fig. 3). These data imply that large volumes of pervasive fluid did not infiltrate across lithological boundaries in the Scourie granulites prior to, during, or after the peak of Badcallianmetamorphism.However,asthe dataare only from three outcrops, they do not preclude localized or chanOxygen-isotope geochemistry of the Scourie dykes nelled fluid migration in this area and do not constrain fluid movement elsewhere in the Scourian complex. Nor do these The Scourie dykes in the Scourie area intrude granulite-facies gneisseswith sharpdiscordantcontactsand preservelittledata exclude strictly layer-parallel flow in these rocks. More altered igneous mineral assemblages. Unaltered quartzextensive sampling may reveal evidence for polyphase, tholeiitedykes fromthe Scourieareahave localized fluid infiltration events similar to those recorded in 6"O valuesof other granulite facies terrains, such as the Adirondack Mounc. 2.0%0 (Cartwright & Valley 1991a; Fig. 6), 3-4%0 lower than tains (e.g. Valley et al. 1990; Cartwright & Valley 1990, 19916) those of typical unaltered mafic igneous rocks (e.g. Taylor & or Ontario (Dunn & Valley, 1991). The 6"O values of ScourSheppard 1986). Igneous rocks with such low 6I80 values are generally interpreted as having interacted with heated, surface- ian metasediments are lower than those typically recorded in derived fluidssoon after intrusion. However, this is unlikely to metasedimentary rocks. These lithologies may have undergone isotopic exchange with the surrounding orthogneisses, resultexplain the low 6I8O values of the Scourie dykes for several ing from localized fluid infiltration or diffusion through afluid. reasons. (1) Theadjacent gneissespreserveunretrogressed Overall,the oxygen isotopedatasupportthe evolutionary granulite-facies mineral assemblages, and no low 6 ' * 0 values modelsfortheLewisiancomplex of O'Hara & Yarwood are recorded fromthese lithologiesto within 1 cmof the dykes. (1978), Cartwright & Barnicoat (1987), and Cartwright (1990). (2) The dykessampledpreserveigneousmineralogies and

124

I. CARTWRIGHT & J . W. VALLEY

These authors concluded that prograde granulite-facies metamorphism occurred under low fluidhock ratios, without the introduction of large volumes of externally-derived fluid. Whilemany of thegranulite-faciesgneisses appearto preservelittle-modifiedinitialwhole-rock 6I8O values,the oxygen-isotope ratios of the minerals in these rocks were reset during extremelyslowcooling from the peak of metamorI 8 0 diffusionbetween phism. Theclosuretemperaturesfor mineralsinslowly-cooledhigh-grade metamorphicterrains, such as the Scourian complex, are significantly lower than the temperatures at which diffusion of cations (e.g. Fe and Mg) ceases. The low closure temperatures restricts use the of oxygenisotope thermometry in these rocks, and the lack of suitable diffusion data precludes their use for calculating cooling rates. Granitic sheets within the tonalitic gneisses, and trondhjemitic sheets from the basic gneisses at Scourie have similar 6l8Ovalues to those of their hosts. Thesedata, together with the petrology, major-element geochemistry and relative age of the sheets, are best explained by the felsic lithologies being formed by local anatexis at the peak of metamorphism (e.g. O'Hara & Yarwood 1978; Barnicoat 1983; Cartwright & Barnicoat 1987; Cartwright 1988, 1990). However, the oxygenisotope data do notpreclude other origins forfelsic lithologies elsewherein the complex,suchas theproposalthat felsic sheets at GruinardBay were formed by melting of basic rocks outside the complex (Rollinson & Fowler 1987). The fluids that caused the Inverian retrogression in the Stoer area were in isotopic equilibrium with the tonalitic gneisses, and a degree of isotopic homogenization occurred during this event. Cartwright (1988) concluded that the felsic sheets were formed by localmeltingduringBadcallianmetamorphism, and thatfluids exsolvedfrom these melts during crystallization causedtheInverianretrogression. The oxygen-isotope data support this idea, but do not rule out the possibility that the fluids were derived from sources below the current exposure level and equilibrated with the tonalitic gneisses during ascent. The tholeiitic Scourie dykes have uniform low6I8Ovalues. The depth of dyke intrusion, unretrogressed mineral assemI80 blages of dyke andcountry rocks,high-temperature fractionations between the dyke minerals, and heterogeneous 6I80values of the Scourian gneisses are most consistent with the intrusion of these dykes as primary low- 6"O magmas. Trace-element and radiogenic isotope data indicate that the dyke magmas did not assimilate significant volumes of I 8 0 depleted continental crust. The data are best explainedby the dyke magmas being derivedfrom a I80-depleted mantle source region and, if this is the case, imply that subduction of large altered oceanic crust was volumes of hydrothermally occurring prior to 2.0Ga. We thank K. Baker for help with the isotope analyses, C. Graham, T. Weaver, and an anonymous reviewer for helpful comments, D. and Gelt. This study was supported by NATO Postdoctoral Fellowship GT8/F/86/GS/11 NERC grant GR9/16, and ARC grant20.142.042 (to I. C.) and NSF grants EAR8548102, EAR 8945101, and GRI grant 5086-260-1425 (to J. W. V.). A. Cohen provided some of the Scourie samples.

Appendix A: sample localities Gridreferences are fromtheOrdnanceSurvey.Namesappear on 1:25000 and 1: 10000 or 10 560 Ordnance Survey topographic maps. A North side of Scourie Bay [NC 1524521. 30m wide inlet in coast, exposures studied are between low water mark and head of inlet. B: Geodh Eanruig, [NC 142 4421. Rocks exposed along indented rocky foreshore above high-water mark.

C: Geodh nan Sgadan WC 146 4171. Rocks exposed on rocky coastal

strip at base of broad gully close to high-water mark. D: SW slopes of Meall a' Chuna Mhor [NC 0842671. 200m-high hill

with intermediate exposure.

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Received 21 March 1991; revised typescript accepted 2 August 1991