Archean regional transpression and paleomagnetism in northwestern Ontario, Canada

117

Tectono~hysics, 220 (1993) 117-125 Elsevier Science Publishers B.V., Amsterdam

Archean regional transpression and paleomagnetism in northwestern Ontario, Canada G.J. l3orradailq T. Werner, J.F. Dehls and R.N. Spark ~~rtrn~nt

of Geology, Lake~ead Um.#e~i~, Thunder Bay?Ont. P7B 5EI, Canada

(Received May 5, 1992; revised version accepted August 20, 1992)

ABSTRACT Borradaile, G.J., Werner, T., Dehls, J.F. and Spark, R.N., 1993. Archean regional transpression northwestern Ontario, Canada. ‘Zeczonophysics, 220: 117-125.

and paleomagnetism

in

The Archean metamo~hic rocks of the Superior province of the Canadian Shield occur in litha~ogica~ly defined belts or subprovinces. The tectonically more stable interiors of belts possess consistent primary ~m~nents of magnetic remanenee. In the case of the Quetico belt, these stable directions are tightiy grouped about 005’/5Y with some minor dispersion and most were acquired during the cooling that followed syntectonic recrystallisation. This study examines the directions of primary remanence components for rocks along the margins of the Quetico belt, within 4 km of the strongly deformed vertical, ENE-trending boundaries. The boundaries are known to have experienced dextral transpression involving penetrative single-phase deformation which out-lasted metamorphism. Within a few kilometres of the belt boundaries, the primary remanence components are re-distributed along a vertical ENE-trending, great-circle girdle which is nearly parallel to the plane of transpressive shear and regional schistosity. It is suggested that the effects of transpression have mechanically deflected the components of primary remanence toward this plane.

It is not the purpose of this study to determine Archean paleopoles for northwestern Ontario: considerable progress has been made in that subject already (Dunlop, 1979, 1983, 1984a,b, 1985). Instead we wish to determine the effects of tectonic deformation on the directions of primary components of natural remanent ma~netisation (NRM) in zones of high regional strain. Dunlop’s studies concentrated on weakly deformed massive bodies of Archean igneous or metamorphic rocks located in the interiors of the belts or subprovinces (Card and Cieselski, 1986; Percival, 1989) of the Superior Province of the Canadian Shield. These subprovinces are litho-

Co~~~~~e to: Dr. G. Borradaile, ~epartmeot of Geology, Lakehead University, Thunder Bay, Ont. P7B 5E1, Canada. Fax: (807)-343-8023, tel.: (807>343-8328.

logically defined. According to Percival (1989) the Wabigoon volcanic belt remained active tectonically until 2702 Ma and the Shebandowan volcanic belt until 2692 Ma, with the intervening Quetico sedimentary basin receiving detritus until 2702 Ma. Closure of the Quetico sedimentary trough (sample area A in this study, Fig. 1) was complete by 2685 Ma. Younger sediments (including the Seine Group, sample area B in this study, Fig. 2) were deposited along fault-controlled depressions: these are stratigraphically younger than the Quetico sediments but the age difference cannot be resolved radiometrically (Davis et al., 1989). The margins of the Quetico metasedimentary belt are marked by low-grade, greenschist facies slates and greywacke which are dextrally sheared along an ENE-WSW vertical shear plane, and shortened in a N-S direction. This dextral transpression of the boundaries to the Quetico belt has been documented using conventional mi-

0040..1951/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

118

G.J.BORRADAILE

ETAL.

crostructures as well as magnetic fabrics both in northern Ontario and to the southwest in northern Minnesota tBorrada~le et al., 1988, Hudleston et al., 1988; Batter and Bidwell, 1990; Borradaile and Spark, 1991). Previous NRM data Dunlop (1979, 1983, 1984a,b, 1985) has measured the NRM for more than 450 samples from over 120 sites in northwestern Ontario. He recognised characteristic, primary NRM directions for

NRM primory directions f Dunlop 1979,1983, 1984a,b, 1985 i

Fig. 2. Stereographic plot of the NRM directions, obtained by Dunlop for a total of 246 samples, in northwestern Ontario (see Fig. 1 for sites).

Fig. 1. (a) Location of the area described and the principal subprovinces or belts of the Archean rocks of northwestern Ontario. (b) Map of the northwestern Ontario region showing sites for NRM studies. 1 = Shelley Lake granite (Dunlop, 1984a); 2 = Poohbah Lake alkaline complex, K remanence (Dunlop, 1985); 3 = P~hbah Lake atkaline complex, N remanence (Dunlop, 1985); 4 = Huronian granite, paragneiss, (Dunlop, 1979); 5 = McKenzie Lake diorite (Dunlop, 1979); 6 = Burchell Lake granite, (Dunlop, 1984b); 7 = Shebandowan gabbros, (Dunlop, 1979); 8 = Sapawe iron formation, (Duntop, 1979); 9 = Gulliver greenstone, (Dunlop, 1979); 10 = Ignace, Martiu gneisses (Dunlop, 1979); 11 = Wabigoon gabbro, (Dunlop, 1983); 12 = Watcomb granite, (Dunlop, 1979); A = Quetico metasediments, Kashabowie slates; B = Seine Group metasediments

over 246 samples from at least 12 distinct units in northwestern Ontario which are reviewed here (Fig. 3, Table 1). These are distant from areas of recognisable high strains and were mainly located in rigid igneous intrusions or massive, weakly strained bodies of metamo~hi~ rock (Table 3). The mean directions of primary remanence components determined by Dunlop for his 12 units are presented in Figure 2. With respect to the local deformation, most of these directions were considered to be of early post-kinematic age (sites 1, 2, 6, 8, 12) whereas others were considered to be pre-kinematic (7, 9, 11). Upon ~nsid~ration of the blocking temperatures and relaxation times for magnetite (Pullaiah et al., 19751, these remanences shouid date from the time at which the metamorphic rocks cooled from about 5OO”C,just below peak greenschist facies conditions. Thus the primary remanence data of Dunlop relate largely to metamorphic cooling (in some places to post-volcanic plutonism) which are inferred to range mostly from 2700 to 2650 Ma, using the high precision ages available subsequently to Percival (19891, and possibly as late as 2580 Ma from the data of Berger and York (1979) for unit 1 (Shelley Lake). The dominant grouping of characteristic remanence directions (Fig. 2) is approximately 005”/55” with a few scattered magnetisa~ tions in the ENE and WSW quadrants which were formerly difficult to interpret (Dunlop, 1979).

ARCHEAB

REGIONAL

TRANSPRESSION

AND

PALEOMAGNETISM

IN NORTHWESTERN

New remanence data Our study has focused on two small areas (which could be considered almost as large units in the traditional paleomagnetic sense) on either side of the Quetico subprovince. These areas (A and B in Fig. 1) are located on the highly strained, transpressed belt boundaries. Samples from area A include slates and greywacke of low greenschist facies and from area B the samples are of schistose arkosic sandstones below greenschist facies. These areas have been subject to > 65% shortening in the N-S direction and extensive dextral shear in a vertically oriented ENE-WSW shear plane (Spark, 1990; Dehls, 1992). Such units would be considered unsuitable for conventional paleomagnetic work because the high strain would be expected to have disturbed any pre-kinematic remanence. Even a cursory examination indicates that this is true. Without any demagnetisation treatments to isolate components, the raw NRM directions for just area A (Fig. 41, indicate that there is a tendency for the NRM directions to align in the direction of the regional shear (a vertical ENE-WSW plane). Of course, all of the rocks that we have studied have anisotropy of magnetic susceptibili~ (AMS)defined by an oblate ellipsoid

119

ONTARIO

which is essentially parallel to the transpressive shear plane. The intensity or degree of anisotropy is expressed by P’ (Jelinek, 1981). This parameter takes into account the contributions of all three principal values of susceptibility (k,, > kint > kmi,) but for ellipsoids of revolution such as in flattened schists it is close in value to the simple anisotropy parameter Ps = kmax/kmin. The frequency distribution of the anisotropy of low field susceptibility is given in Figure 3. It could be argued that the NRM directions (e.g., Fig. 4) are simply postkinematic overprints, deflected into the magnetic foliation plane of the specimens by the intrinsic magnetic anisotropy of the rocks. This has been addressed in a practical manner during our investigation of the anisotropy of remanent ma~etisation in these samples (after the specimens were used for NRM analysis). Anisotropy of saturation isothermal remanent magnetisation (ASIRM) is more important than AMSas an indicator of the reliability of the specimens to record accurately the direction of a magnetising field since the AMS fabric is dominated by the effects of strongly oriented paramagnetic silicates which do not carry remanence (Daly and Zinsser, 1973). For pilot group of specimens anisotropy of

TABLE 1 Selected mean remanence directions for Archean formations in northwestern 1985)

Ontario (after Dunlop, 1979, 1983, 1984a, 1984b,

N*

DC)

If”)

k

CY,P)

References

Quetico Metasedimentary Belt 1 Shelley Lake granite Poohbah Lake alkaline complex 2 3 Poohbah Lake alkaline complex 4 Huronian granite

12/43 13/39 13135 ?/14

5.9 198.0 359.5 14.7

56.4 - 22.5 55.5 42.3

95.4 62 28 25.8

4.4 5.3 8.3 13.9

Dunlop, Dunlop, Dunlop, Dunlop,

Shebandowan volcanic-plutonic belt 5 McKenzie Lake granite 6 Bumhell Lake granite 7 Shebandowan gabbros

?/6 lo/36 ?/8

4.6 2.3 82.9

45.5 48.9 - 28.6

24.1 23.9 13.7

7.9 10.0 15.4

Dunlop, 1979 Dunlop, 1984b, BLl Dunlop, 1979

1/11

351.5 57.3 55.9 246 356.7

13.4 - 14.6 -8.7 12 55.6

6.1 8.6 10.0 19.5 16.3

20.0 18.5 25.3 10.5 11.6

Dunlop, Dunlop, Dunlop, Duniop, Dunlop,

No.

Formation

Wabigoon volcanic-phonic belt 8 Sapawe iron formation 9 Gulliver greenstone 10 Ignace, Martin gneisses 11 Wabigoon gabbro 12 Watcomb granite * N = No. of site/No.

?/9 ?/5 11,‘29 ?/ll

of samples; ? = unknown.

1984a, SLl 1985, R 1985, N 1979

1979 1979 1979 1983 1979

120

G.J. BOKRADAILE

I.0

I -4

1.8

2-Z

Fig. 3. ~iistograms of the frequency of occurrence of specimens with a given anisotropy degree (P’, Jelinek, 1981). AMS refers to data for anisotropy of low-field susceptibility (at 0.6 Oersted, 750 Hz) and ASIRM refers to data OIYthe anisotropy of saturation isothermal remanence acquired in induced fields of 600 mT.

remanence was determined at fields of 100, 200,. 1. ) 900 mT, and for a larger selection at fields of 300, 600 and 800 mT. It was found that ASIRM was reproducible in these rocks at 600 mT so that this was used for a11 subsequent specimens. This field should not be considered universally valid, of course; the presence of high coercivity carriers of remanence such as hematite or

goethite would require very high fields to achieve true saturation and meaningful estimates of anisotropy of remanence. In these rocks magnetite and pyrrhotite are the main carriers of remanence although there are some examples of higher coercivity minerals in some samples. The ASIRM intensity recorded by P’ (Fig. 3) has a modal value of approximately 1.2. However, during the course of determining the anisotropy of remanence, it was possible to magnetise the samples accurately in the directions required by the anisotropy analysis procedure (modified after Stupavsky, 1984, SI-2 computer program). Samples which could not be magnetised in a specified direction with a deflection of less than 8” were not used further in this study. Therefore, these practical considerations suggest that anisotropies of remanence with P’ G 1.25 (ASIRM in Fig. 3) are not sufficient to deflect magnetising fields more than 8” due to the preferred alignment of ferromagnetic grains. The problem of deflection of remanence from the direction of the magnetising field by the rock’s anisotropy has been well studied. Hargraves (1959) first recognised the problem; subsequently, Fuller (1963) and Uyeda et al. (1963) anaIyzed the subject more fully. Two effects can be of concern. The first is the shape effect of the magnetised body which causes a demagnetising field that influences the direction of the acquired remanence (e.g., Stacey and Banerjee, 1974). This shape effect is important in bodies of limited size such as dykes (Stran~ay, 1961). The second effect, the intrinsic anisotropy, is more important in our case where we consider the remanence within a large volume of highly schistose rock. Fuller (1963) and Uyeda et al. (1963) showed that there was a simple relationship between the inclination of the magnetising field (@,I, the rocks’ anisotropy in the same plane (let us assume the worst case with P3 = ~*~/~~j~), and the inclination of the deflected remanence (8,, ): tan 8,,

Q UP (n-87) Fig. 4. The raw (i.e. uncleaned) NRM directions for 87 specimens from area B, the Seine Group metasediments, indicated in Figure 1.

ET AL.

= [tan %x 1/p,

The anisotropy degree (P’) depends on temperature. It could be a few percent higher during the acquisition of the~o-remanence (Uyeda et al., 1963). Therefore it seems reasonable to as-

ARCUEAN

REGIONAL

TRANSPRESSION

AND

PALEOMAGNETISM

sume that our values of anisotropy degree (P’ = PJ are valid (Fig. 3). Hrouda (1982, fig. 27) presented a convenient graphical representation of the deflections produced in this way. For P’ = 1.5, the rn~rn~ deflection is 10”. For P’ = 1.2 (the modal value in our study, see Fig. 31, the maximum deflection is less than 5”. Therefore we do not believe that the dispersion of remanences in the general plane of the regional schistosity is due to refraction of the magnetising field. Moreover, it stretches the imagination too far to suppose that the natural, ancient magnetising fields were conveniently oriented to deflect characteristic components into positions smeared all along the great circle defined by the rocks’ schistosity (Figs. 6a, 7b).

IN NORTHWESTERN

121

ONTARIO

(01 Area

B: JDl28

(l-i=251

(b) Area

A: RS6P2.1

W& up

(n=20)

Analysis of new remanence data For the two areas, A and B, situated on the transpressed belt boundaries we have been able to assemble a small number (36) of suitable specimens. These satisfied the constraints that they yielded stable characteristic remanence components and that during the determination of anisotropy of remanence it was possible to magnetise them in any given direction with deflections less than 8”. Primary remanence components were revealed by alternating field demagnetisation using a Sapphire Instruments SI-4 demagnetiser to a peak a-f. of 200 mT (2000 Oersteds) in steps between 1.5 and 10 mT. Many specimens were demagnetised by quite small maximum peak a.f. fields (70 mT) but a few showed harder remanences which were not entirely erased by 200 mT a.f. peak fields, Remanence was measured in a Molspin spinner magnetometer whose noise level prevented the study of many specimens with weak NRM’S. In most cases the samples possess a single or a simple two-component NRM as revealed by vector plots (Fig. 5). Characteristic vectors were determined for 7 specimens in area B and for 29 specimens in area A. The hard, stable components for these samples are illustrated in Figures 6 and 7. Stable components were read directly off Zijderveld (1967) plots in some cases whereas in

Fig. 5. Typical vector plots (Zijdervefd, 1967) for a specimen from area B (Seine Group metasediments) and area A (Quetico metased~ents). Most specimens in both areas had either simpie one or two component remanences, with some signal remaining to the maximum peak alternating field available to us (200 mT).

other cases they were determined by principal component analysis (Kirschvink, 1980). The notable feature of both areas is the corre-

NRM (area

primary

directions f n = 29 1

A)

Fig. 6. (a> Stable, characteristic components of NRM for specimens of Quetico metasediments from area A compared with the mean schistosity direction St. Characteristic components were determined by the methods of Kirschvink (1980). (b) The best fit great circle to the remanence data of (a) are compared with the mean AMS or magnetic fabric defined by a great circle (the magnetic foliation) and the maximum susceptibility k,,,. The mean stretching lineation CL,) is also shown.

122

G.J. BORRADAILE

NRM

prmary

(oreo

Fig. 7. (a) The mean foliation

defined

magnetisation sured

.

dlrectaons (n= 7 )

down

B1

magnetic

foliation

by the anisotropy

(ASIRM)

compared

CAMS)

in the field for the Seine Group

the

characteristic

NRM

the Seine

for some

specimens

ments (area

from

remanent

schistosity

metasediments

B, Fig. 1). (b) The stable primary

fabric development. The foliations defined by the AMS ellipsoid and by the ellipsoid of anisotropy of isothermal remanent magnetisation (ASIRM) are slightly anticlockwise with respect to the best-fit plane for the directions of the primary remanence components (Fig. 7a). This is explained in terms of progressive transpression with the sequential inception of rock schistosity S,, followed by the AMS fabric and then the ASIRM fabric (Borradaile and Dehls, in press).

and the mean

of isothermal with

ET AL.

components Group

mea-

Discussion

(area of

metasedi-

B, Fig. 1) and the best fit great circle.

spondence between the scatter-pattern of characteristic remanence directions and the petrofabric of the tectonites in each case. For area A the directions agree closely with the mean attitude of the rocks’ schistosity, S, (Fig. 6a). This is very close to the best-fit plane through the distribution of primary remanences (Fig. 6b). It is close in orientation to the AMS “foliation” or /~,,,,,-k~,~ plane, controlled by the paramagnetic subfabric of chlorite grains, but slightly offset from it. This offset has been explained previously (Borradaile and Spark, 1991) in terms of the kinematic history which produced very consistent, tightly grouped orientations of schistosity and stretching lineation but chronologically distinct subfabrics of minerals with different magnetic properties. The fabric lineations from stretched grains in the field CL, is the mean of tightly grouped stretching lineations) and the mean lineation (K,,) of the AMS fabric are also shown. For area B, fewer data are available, but here also the best-fit plane through the characteristic remanence directions (Fig. 7b) is close to the orientation of the rocks’ schistosity. A detailed study of the remanence of samples from within plastically deformed cobbles in a conglomerate in area B revealed the same pattern: primary remanence components scattered in the schistosity plane. Thus the remanence has been changed in orientation during some part of the tectonites’

The undisturbed, Archean, primary components of remanence for the region examined are known to group closely around 005/5S’ (Fig. 2 after Dunlop, 1979, 1983, 1984a,b, 1985) with some scatter in the ENE and WSW quadrants. Here we have concentrated on rocks within 4 km of the transpressional belt boundaries to the Quetico subprovince. Such sheared rocks would normally be avoided in paleomagnetic studies but we believe that we have detected stable characteristic remanences in 36 samples from the two areas on in the low-grade rocks on the southern (A) and northern (B) margins of the subprovince. This coincidence of primary components of remanence and petrofabric is too close to be fortuitous (Figs. 6 and 71. A simplified sketch of the distributions of characteristic magnetisation directions with position in the regional “belt” structure is shown in Figure 8. There are two possibilities for the smeared distributions of remanence parallel to schistosity along the belt boundaries. One is that the rocks’ anisotropies are so great that post-tectonic magnetic fields have been deflected towards parallelism with the rocks’ magnetic foliation planes. Previous theoretical studies indicate that our rocks are not sufficiently anisotropic to cause a problem. Moreover, we found it possible to magnetise our samples accurately in any given direction indicating that deflection of remanence is of little concern here. We favour the second possibility, that the primary remanence components have been rotated by the single phase of penetrative deformation into the shear plane which is close to the S,

ARCI-IEAN

REGIONAL

TRANSPRESSION

AND

PALEOMAGNETISM

N .

A’

@

ktl

dr. I N

VI

I

N

7-

WABIGOON BELT

WE TIC0

SHEBANDOWAN

BELT

r-----v I

BELT

”

Fig. 8. Schematic diagram illustrating the distributions of characteristic magnetisation directions on stereograms, for different locations across the lithological belts in northwestern Ontario.

schistosity (vertical with an ENE-WSW strike). The rotation of remanence may be a function of the rotation of ferromagnetic grains which passively carried their remanence or it may occur as a result of intragranular modification of the remanence. The sensitivity of remanent magnetisation to stress has been known for a long time (e.g., Carmichael, 1968a,b; Nagata, 1970; Stacey and Banerjee, 1974). However, both grain rotation during deformation and the largely later intragranular processes of rotation of remanence (by domain wall movement or spin-moment rotation) may have occurred as overlapping events. What is the regional significance of tectonic deformation on the orientation of pre-tectonic remanence? Within 4 km of the belt boundary the effects of deformation dominate, as our study areas A and B illustrate (Figs. lb, 6, 7). But how far afield may the remanences be disturbed ? If we examine the directions derived by Dunlop (Fig. 21, it may be noted that even at some distance from the belt boundaries (units 9, 10, 11) there is a tendency for some primary components of remanence to scatter along the ENE-WSW vertical plane of shear. It may be possible that there was some rotation of remanence up to 10 km from the sheared belt boundaries (Fig. 8). Finally, we address the question of the timing

fN NORTHWESTERN

ONTARIO

123

of the remanence acquisition and its rotation. One might favour the association of the remanence rotation with the penetrative, schistosityforming and cleavage-forming episode (S,). Indeed, Dunlop (1979) recognised the synkinematic/synmetamorphic age of the oldest NRM component in this region. However, it is now known that the deformation episode was long and that the transpressive event outlasted the formation of the cleavage: it was responsible for the rotation of the embryonic cleavage even before the growth of chlorite (Borradaile and Spark, 1992) which preceeded the mineralogical stabilisation of the remanence-carrying iron oxides. Next, in most cases the primary components of remanence were acquired sometime during the cooling after peak metamorphism during which the transpressive stress regime persisted. Since the dominant remanence carrying mineral in these rocks is magnetite, with a Curie temperature of 580°C primary remanence components would be established during early post metamorphic cooling down to about 350°C. Dextral transpression continued after metamorphism, rotating the remanence near the highly strained belt boundaries. Continued dextral transpression eventually generated the transcurrent faults in the region, some of which partly follow belt boundaries, such as the Quetico fault with 200 km of dextral displacement (Kennedy, 1984). It is difficult to bracket the ages of the acquisition of primary remanence and of its rotation without some high precision technique for dating the various structure-forming and mineral-forming events. Nevertheless, it is instructive to realise that the primary remanences were acquired during a relatively long cooling history following peak metamorphism. In the highly strained parts of this region the remanence directions were then mechanically dispersed by long-lasting stressfields which had dominated much of the earlier tectonic history. Percival’s review (1989) included radiometric data not available to Dunlop (1979, 1983, 1984a,b, 1985): this showed that the Quetico trough had received all its sediment by 2700 Ma and that the penetrative-fabric forming aspects of transpression had probably ceased by 2685 Ma. We believe

124

G.J. BORRADAILE

that our components of primary remanence were sheared during the latter part of the episode that caused the tectonic fabrics (schistosity or cleavage, and stretching lineation), and subsequently by the long-acting transpressive stress-regime, as the rocks cooled from peak greenschist facies conditions. Dunlop’s remanence directions (Fig. 2) may span a greater range, probably from 2700 to 2580 Ma as they include some truly posttectonic examples. The time-interval spanned by the acquisition of the primary remanence components of Dunlop and in this paper is at least 50 Ma; part way through this interval the Shebandowan/ Quetico/ Wabigoon subprovinces had docked. The subprovince boundaries were in an ideal, almost E-W, orientation for us to detect significant latitudinal drift of the crust. However, this may have been accomplished early on (before 2685 Ma) because drift rates were so fast; Berger and York (1979) estimated > 10 cm/year. Alternatively, any late component of closure of the Quetico basin may have been too small to detect paleomagnetically. (The minimum latitudinal drift that could be detected paleomagnetically is approximately 300 km). The tight clustering of Dunlop’s primary remanence directions (145 samples, Fig. 2) confirms the cratonic stability of the interior of the Quetico belt at that time.

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G.W. and York, D., 1979. ““Ar-“Ar

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We thank David Dunlop and Rob van der Voo for constructive and helpful reviews of this manuscript. G.B. thanks NSERC (Canada) for an operating research grant which financed this research. The facilities of the Rock Physics Laboratory at Lakehead University were provided by grants to G.B. from NSERC, BILD (Ontario), Bickell Foundation (Toronto), Noranda, and American Barrick Resources.

Can.

of multi-

Shelley

1933-1941.

head Univ., Thunder

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ARCHEAN

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