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An intensive study of magnetic susceptibility anisotropy of amphibolite layers of the Thompson Belt, North Manitoba
315
Tectonophysics, 162 (1989) 315-329 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
An intensive study of magnetic susceptibility anisotropy of amphibolite layers of the Thompson Belt, North Manitoba G.W. PEARCE and F. FUETEN Tectonic Studies Group, Department of Ceolop,
~nive~~~t.~of Toronto, ~issis~u~g4
ht.
LSL lC6 ~Can~da~
(Received Suly 6, 1987; revised version accepted August 25, 1988)
Abstract Pearce, G.W. and Fueten, F., 1989. An intensive study of magnetic susceptibility anisotropy of amp~bolite the Thompson Belt, North Manitoba. Teclonophysics, 162: 315-329.
layers of
In an examination of the magnetic susceptibility anisotropy of folded amphibolite layers in intensly deformed gneisses at the Pipe 2 Mine, Thompson Belt, North Manitoba, the magnetic fabric was found to agree well in orientation with a fold axial planar foliation and the stretching lineation in the gneisses. The prolateness or oblateness of the magnetic fabric ellipsoid in this lithology depends primarily on the mineralogy of the sample and therefore cannot be used as a strain gauge. The magnetic fabric is made up of two components. One is a small, approximately isotropic part due to iron bearing siticates and perhaps isolated equant magnetite particles. The second is a variable anisotropic part, carried by of 2.018 f 0.008. This constant ratio represents elongated particles of magnetite and has constant ratio P = K,,, /K,i, a characteristic value of the magnetic anisotropy of these rocks and may be related to the strain in this area.
Introduction
Magnetic anisotropy has been employed as a tool in rock fabric analysis for many years (Hrouda, 1982). A number of studies (e.g., KLigfield et al., 1977, 1981; Rathore, 1979; Rathore and Henry, 1982) have examined magnetic anisotropy on a regional scale, but few (e.g., Goldstein, 1980) have examined variations over a small area. The present paper documents one such study in small deformed amphibolite layers, believed to be the metamorphosed equivalents of Molson dikes, at the Pipe 2 Mine, south of Thompson, Manitoba. The amp~bolite layers are enclosed in metasediments and gneisses of the Thompson Belt, which lies on the border between the Churchill and Superior Provinces and which records a complex tectonic history. The amphibolites are abundant across the belt and their magnetic fabric may constitute a very useful tool to help delineate the ~-1951/89/$03.50
0 1989 Elsevier Science Publishers B.V.
kinematic or strain history of the belt. Before this can be attempted, however, the amphibolites from within a small area need to be studied in order to know what importance can be attached to the directions of anisotropy, the amount of anisotropy and the relative prolateness of anisotropy. To assess these problems, amp~bo~ites from within a restricted area, in which the structure is reasonably well understood, were examined in detail. Regional setting
The Thompson Belt is a linear NE-SW trending belt at the border between the Churchill and Superior Provinces (Fig. 1) in north-central Manitoba. The basement lithologies in the belt are a series of felsic gneisses, with minor amounts of mafic and ultramafic material. These gneisses are considered to be the retrograded equivalent of the Pikwitonei granulites of the Superior Province
316
- Pipe 2 Mine Pik.
-
Pikwitonei
T.B.
-
Thompson
Belt
Fig. 1. Location of Thompson Belt and of Pipe 2 Mine.
PIPE
No2 SHAFT
@
-LEGENDSERqENTlNIZED
/
PERIDOTITE
(rn~nerol~zedin port)
QUARTZITE BIOTITE
GNEISS
AMPHIBOLE
GNEISS
PEGMATITE
eooo 5
gg --
PIT
OUTLINE
PIPE BIKE - OPEN PIT
SURFACE Scale0
Fig. 2. Plan map of Pipe 2 mine (reproduced from Coats et al.. 19’72). A-site area.
‘+
PLAN IO0
of southern sample area. B-site
400 Feet 110 MeR?rs
of northern sample
317
(Scoates et al., 1977; Weber and Scoates, 1978). At the western margin of the belt, they are overlain by a suite of metavolcanic and metasedimentary rocks called the Ospwagan Group by Scoates et al. (1977). Peredery et al. (1982) divided the sedimentary rocks into the Pipe and the Thompson assemblages. The gneisses are thought to have undergone several deformation events starting in the Archean (Green et al., 1985). Both gneisses and metasediments probably owe their current structural attitude to the Hudsonian orogeny, dated between 1.9 and 1.6 Ga in the Thompson Belt (Green et al., 1985). Preliminary results of ongoing structural studies in the Belt (Fueten et al., 1986; Bleeker, 1987) indicate that it has been a zone of widespread and intense shearing. The latest movement was in a direction which is presently vertical. A swarm of mafic to ultramafic dikes, known as the Molson Dike Swarm (Scoates and Macek, 1978) is located on the western side of the Superor Province. Recent U/Pb zircon dates of two dikes belonging to the Molson swarm yielded ages of 1883 f 2 Ma. (Heaman et al., 1986). The Molson dikes therefore appear to have been emplaced before or early in the Hudsonian orogeny. The dikes are fresh and undeformed in the Pikwitonei region but are metamorphosed in the Thompson Belt. The Pipe 2 Nickel Mine was operated by INCO Metals Company until 1984. The geology of this deposit has been discussed by Peredery et al. (1982) and Gale et al. (1982) (Fig. 2). It is located on the western side of the Thompson Belt, within 2 km of the Churchill-Superior boundary. The mine itself is located within a major tight fold plunging 80 o to the northeast, with a near vertical fold axial plane trending approximately 050” and opening towards the northeast. Minor folds are common on the eastern limb and affect basement gneisses as well as metasediments. From metamorphic assemblages the peak metamorphic temperatures at Pipe 2 Mine are estimated to have been 575 “-625 o C (Fueten et al., 1986). Deformation at Pipe 2 Mine is intense. Bedding and foliations (Fig. 3b) are near vertical. A strong, near vertical stretching lineation (Fig. 3a) is present on both limbs. Deformation on both limbs
PIPE 2 MINE LINEATIONS
/
position
63’/ 80’
0
PIPE 2 MINE FOLIATION
POLES
N
346
k
79.3
E
4.4
s
1.5
Peak: value position
Fig.
3. Orientation
data
for the Pipe
gneissic lineation,
2 mine.
63.2 320’/ 6’
(a) Poles
to
(b) gneissic foliation.
indicates a well defined vertical extension. The layering on the western limb is more disrupted than on the eastern limb, and some concentric structures in the western limb could be interpreted as steeply plunging sheath folds, suggesting the strain is higher here. Description lection
of amphibolite
layers and sample col-
In both the basement gneisses and the supracrustals, continuous amphibolite layers occur, most of which are thought to be metamorphosed Molson dikes. The width of these layers ranges from several centimetres to several metres. In some locations the layers may be sheared apart or boudinaged. Two mineral assemblages occur. The minerals within each assemblage, in order of decreasing abundance. are:
318
n
AMPHIBOLITE
LAYERS
LIMIT OF OUTCROP
Fig. 5. Small-scale map of south group of samples.
(1) hornblende, minerals,
magnetite
plagioclase, and minor
epidote amounts
group of quartz
(Fig. 4a); and (2) biotite, plagioclase, quartz, hornblende, epidote group minerals and minor amounts of mag-
The samples were oriented by a Brunton compass held on the top surface of the drilled out, in situ cores. The orientations
netite (Fig. 4b). There is more visible magnetite present in layers with assemblage 1 than 2 (Fig. 4a, b). Magnetite in
according
samples
Magnetic measurements
with
assemblage
1 occurs
as elongated
may be in error by as
much as 10” due to magnetic deflections. From one to four samples were cut from the cores to their length
for a total of 210 speci-
mens.
particles. These particles are interspersed among the other minerals and are aligned parallel with the fabric. Such large elongated particles are al-
The (AMS)
most absent from samples of assemblage 2, in which smaller isolated opaque grains, many of
commercial inductance bridge instrument (Sapphire Instruments) (Stupavski, 1984) and checked
which are equant, are present. The samples were collected
in a few cases with as 2.54 cm cores
using
anisotropy of of the samples
a cryogenic
magnetic susceptibility were measured using a
an inhouse-designed
magnetometer
system
(Sugiura
et al.,
drilled vertically (within 10”) from two localities: (1) just south of the lip of the pit (43 cores), and
1985). Since the samples are cylinders, measurements may be sensitive to the length-to-diameter
(2) just north of the pit (12 cores). Most of the southern samples were collected along a single layer which was tightly folded about a fold axial
ratio
plane trending about 055O (Fig. 5). Two sequences of seven samples each were collected across a single layer, one (No. 31-37) across a fold nose, the other (No. 38-44) across a limb. Based on mesoscopic appearance as well as on microscopic evidence, such as grain sizes and the amount of subgraining of quartz, the amount of deformation within each sample site appears to be quite similar.
(L/D)
(Porath
et al., 1966). To determine
the sensitivity of the Sapphire instrument in this regard, a number of samples were measured successively
after progressive
shortening.
For L/D
ratios between 0.8 and 1.0 (ideal ratio = 0.85) the anisotropy parameters proved to be insensitive ( f 5%) to L/D. All other samples were then cut to an L/D ratio of 0.90 _t 0.05. Each sample was measured at least twice, varying the experimental conditions such as different numbers of orientations or different sampling times. If there was more than one sample per core, each sample was
320 TABLE 1 Anisotropy of magnetic susceptibility (AMS) of Thomson Belt amphibolite dikes: results averaged to samples * Sample
Number
No.
of meas.
Principal magnetic susceptibility axes
Td
PI
(“)
(“)
Kti, (SI)
Avg.
maximal
intermediate
minimal
K
Td
PI
Kim
Td
PI
Gi.X
(“)
(“)
(W
(“)
(“)
(W
1
2
297.49
30.63
0.000811
202.07
2
1
144.4
5.4
0.000906
237.6
8.97 30.2
(W
0.000864
97.52
57.84
0.000880
0.000852
0.000991
45.2
59.1
0.001017
0.000971
3
2
311.6
20.65
0.000691
214.78
16.79
0.000767
88.54
62.74
0.000784
0.000748
4
5
320.5
10.3
0.000421
222.5
39.6
0.000464
61.5
49.1
0.000484
0.000457
5
4
301.8
26.48
0.000811
189.46
37.07
0.000833
58.02
41.54
0.000860
0.000835
6
3
274.7
39.06
0.000859
160.59
26.86
0.000892
46.9
38.23
0.00926
0.000892
7
4
157.14
38.13
0.000998
251.05
53.64
0.001104
49.43
31.61
0.001239
0.001114
8
4
283.47
60.53
0.000677
184.58
2.75
0.000701
91.44
33.49
0.000718
0.000699
9
4
344.22
9.45
0.000714
253.84
52.81
0.000747
81.34
36.88
0.000764
0.000742
10
4
149.12
5.57
0.000670
248.39
56.39
0.000711
55.57
32.86
0.000728
0.000703
11
6
332.94
4.22
0.000808
237.58
49.84
0.000856
66.44
40.21
0.000878
0.000847
12
8
342.83
1.28
0.010493
252.4
22.66
0.012877
76.89
66.77
0.015571
0.012980
13
4
345.15
0.37
0.004557
254.96
32.87
0.005224
76.21
56.86
0.006675
0.005485
14
3
342.67
8.6
0.009544
249.6
19.1
0.012263
95.79
68.78
0.015300
0.012369
16
3
324.47
4.99
0.046965
233.12
14.67
0.053372
73.17
74.61
0.090044
0.063460
17
6
330.59
13.09
0.040802
235.57
20.31
0.056154
91.35
65.43
0.081653
0.059536
18
4
321.18
5.31
0.000913
228.26
29.01
0.001038
60.31
60.46
0.001061
0.001004
19
5
171.72
7.14
0.008378
265.45
26.64
0.009529
74.13
62.09
0.014429
0.010779
20
4
302.13
16.78
0.005606
205.31
21.35
0.005928
66.55
62.69
0.007652
0.006395
21
4
315.42
8.95
0.001638
219.81
31.54
0.001943
59.57
56.98
0.002085
0.001889
22
4
333.67
5.2
0.004860
240.69
29.13
0.005707
72.97
60.23
0.007303
0.005957
23
4
304.8
7.03
0.002519
211.72
22.79
0.002647
51.19
65.88
0.003415
0.002860
24
3
331.54
9.84
0.024686
239.46
11.87
0.027984
100.35
74.37
0.048902
0.033857
25
5
323.74
6.52
0.017801
240.47
18.31
0.019771
59.19
71.58
0.034413
0.023995
26
2
243.35
14.66
0.0265
338.25
18.16
0.033
66.2
0.0575
0.039
27
5
198.61
30.15
0.000838
296.65
14.56
0.000876
49.61
56.63
0.000896
0.000870
116.7
28
2
324.5
6.1
0.008425
232.15
14.05
0.009891
86.44
73.1
0.0160
0.011438
29
4
340.46
6.34
0.02
247.56
25.84
0.027
82.83
63.63
0.042
0.029666
30
5
318.42
19.5
0.009705
220.15
22.1
0.011635
85.9
59.64
0.020620
0.013986
31
3
177.8
22.36
0.001023
280.49
27.91
0.001026
54.54
53.19
0.001088
0.001045 0.002897
32
2
29.25
2.3
0.002256
238.25
22.75
0.002719
64.8
67
0.003717
33
6
319.71
6.61
0.001389
224.55
26.4
0.001553
56.5
61.5
0.001918
0.001620
34
2
190.58
25.39
0.001000
289.06
17.05
0.001020
50.39
58.16
0.001060
0.001027
35
6
210.71
49.05
0.000997
313.06
10.36
0.001036
51.54
38.91
0.001073
0.001035
36
4
198.49
53.37
0.000993
302.38
10.01
0.001017
39.52
35.29
0.001058
0.001023
140.55
14.49
0.000847
39.45
37.14
0.000865
0.000843
7.02
1.59
0.16375
101.11
72.22
0.308750
0.20925
7.4
0.1285
86.69
64.53
0.233375
0.1585
83.66
63.89
0.0652
0.0452
50
0.001000
0.000965 0.004477
37
2
248.1
48.23
0.000817
38
4
276.53
17.5
0.15525
39
8
247.81
24.78
0.113625
341.34 310.67
40
5
214.96
17.55
0.0328
18.2
0.0376
41
4
270.38
28.89
0.000906
15.37
25.15
0.000989
139.26
42
4
259.88
22.76
0.003790
351.56
3.92
0.00404
91.1
66.87
0.005602
43
6
251.68
31.83
0.000942
358.14
23.46
0.001021
116.53
47.92
0.001035
0.000999
44
8
281.77
31.94
0.000728
11.09
0.52
0.000790
101.79
59.47
0.000800
0.000773
45
2
148.4
6.9
0.000936
38.75
1.5
0.001070
340.82
82.89
0.001113
0.001040
46
2
302.6
0.75
0.000714
212.37
11.35
0.000771
35.57
78.5
O.ooO786
0.000757 0.000615
47
2
317.26
10.76
0.000556
49.48
12.13
0.000642
188.04
72.74
0.000648
49
2
328.95
2.3
0.001558
59.35
11.4
0.001897
227.33
78.31
0.002229
0.001894
50
2
324.7
6.9
0.001519
55.95
10.3
0.001838
201.43
77.45
0.002164
0.001840
321
TABLE
1 (continued)
Sample
Number
No.
of meas.
Principal
magnetic
susceptibility
axes Avg.
maximal
intermediate
minimal Td
PI
K
Td
PI
K ml
Td
PI
K
(“)
(“1
CS’;;”
(“1
(“1
@I)
(“1
(“)
(S;)=
K @I)
51
2
301.96
15.66
0.000669
43.46
34.53
0.000761
191.52
50.89
0.000772
52
2
109.8
6.5
0.001598
18.0
15.25
0.00201
222.17
73.21
0.002267
0.001958
53
2
82.58
2.68
0.000441
34.01
0.000499
182.63
55.75
0.000519
0.000486
13.95
0.000667
174.63
68.04
0.000728
54.92
48.26
0.000756
0.000717
8.25
0.000811
228.45
47.24
0.000894
36.45
42.31
0.000904
0.000870
0.000860
197
7.
0.000967
313.68
74.5
0.001059
0.000962
54
4
309.36
56
4
133.6
57
2
105.3
* Td = trend,
PI = plunge,
13.6
346
K = length of susceptibility
axes (SI units).
measured. For each core the resulting axial direction sets were averaged by using a procedure suggested by Mark (1973) and provided as a computational algorithm by Sapphire Instruments. This algorithm treats each principal axis separately and the computed average principal axes will not be exactly orthogonal. The deviations from orthogonality of the axes are within expected margins of error for all samples and the computed statistical parameters appear to be acceptable. The mean scalar anisotropy parameters were obtained by simple averaging. The averaged data are presented in Table 1.
Magnetic susceptibilities
An average susceptibility was computed as (K,, + Kin, + K,,)/3, where K,,, Kin, and K,, are respectively the lengths of the maximum, intermediate and minimum principal susceptibility axes. The samples show a very large range of average susceptibilities (0.00046-0.21 SI units) (Fig. 6). Examination of a number of thin sections shows that high susceptibility is correlated with abundant hornblende (assemblage l), whereas samples with high biotite contents (assemblage 2) have low susceptibilities.
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Average Susceptibility
Fig. 6. Frequency
histogram
0.000734
for average
susceptibilities
(SI units)
for Pipe 2 mine amphibolite
samples.
322
0.9
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 iJ Field CkOe)
0
X26
+
#39
0
#4
Fig. 7. Normalized ferromagnetic response curves for several samples. These curves are obtained from measurements of magnetic moment as a function of applied magnetic field. The ferromagnetic response is calculated by subtracting a parama~etic moment, which is itseff determined from the high field (z 3 kOe or 0.24 MA m-‘) total magnetic response (M) using a model: M = M,(l - c/H)
+ K,H
where MS, H, K, and E are, respectively, the saturation moment of the ferromagnetic minerals, the applied field, the susceptibility of the paramagnetic minerals and a constant expressing the approach to saturation of the ferromagnetic response.
a.3 0.2 0.1
0,-I200 Temperature
("C)
Fig. 8. Variation of magnetic response with temperature for sample No. 39 in an applied field of 3 kOe (0.24 MA m-t).
323
1961). This response suggests that only one magnetic mineral is present in significant quantity. To verify that the ma~etic mineral was indeed magnetite, the saturation moment of powdered material from one sample (No. 39) was examined as a function of temperature (Fig. 8). As saturation was very close to completion at 3 kOe (0.24 MA m-l), this field was chosen for the experiment so as to lessen the effect of the parama~etic response. The resulting curve shows the presence of only one magnetic mineral with a Curie point of 575”, close to that of pure magnetite (580 “-585 o C), (e.g. Nagata, 1961). Although other magnetic minerals such as hematite or pyrrhotite may be present in small quantities they should have a negliglible effect on the low field susceptibility as presented in this study.
The high susceptibility appears to be caused by the large highly elongated opaque particles visible in these samples. Magnetic mineralogy Other magnetic properties were examined for a number of selected samples to determine the mineral or minerals responsible for the magnetic susceptibility. Using a vibrating sample magnetometer, magnetic hysteresis curves were run on samples with high and low susceptibilities. The samples varied considerably in their saturation magnetizations in good agreement with their respective average susceptibilities. However the hysteresis curves match each other closely in shape (Fig. 7) if the linear response of paramagnetic iron minerals is removed. The curves show a smooth rise to a saturation at 2-3 kOe (0.16-0.24 MA m-i) and have little hysteresis (the ratio of saturation remanence to saturation moment varies between 0.02 and 0.11) Such behaviour is consistent with multidomain magnetite grains (> 0.5 pm). A spherical multidomain magnetite particle would give an almost reversible linear ma~et~ation curve with a distinct rollover to saturation at 47rJJ3 (approximately 2.0 kOe or 0.16 MA m-i) (Nagata,
Lengths of principal axes (eigenvaiues) and anisotropy Several attempts (e.g., Hirt et al., 1988) have been made to quantitavely correlate AMS with the amount of strain in sedimentary and metamorphic rocks. In this study the absolute amount of strain is not known since no usable strain-gauges are present. However, the deformation within each small area sampled appears to be similar; it is
2.2 cl
u
2.1-
0 cl
2-
00
a
0
1.9-
0
0
1.8 1.7 -
u D q
1.6-
1
1 0
I
I,,
0.02
0.04
I 0.06
I
I
0.08
,
,
I,,
0.1
0.12
Average Susceptibility
Fig. 9. Plot of Kmax/Kmin
(parameter
P) against
,
,
0.14
I, 0.16
(SI units)
average
susceptibility.
,
0.18
,
,
0.2
324 0.34 0.32 0.3 0.28 0.26 0.24 0.22 0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 6
Minimum Susceptibility
Fig. 10. Plot of K,,,
with high average
are also quite anisotropic
shows a good linear relationship; a regression yields a slope of 2.018 + 0.008 and an intercept of - 0.00106 -c_0.0016 with a correlation coefficient
susceptibilities
(Fig. 9) as measured
K,,,;,.
against
therefore a reasonable assumption that the amount of strain in each area should also be similar. The samples
IS1 units)
(R)
by
of 0.9991.
The excellence
of the regression
the ratio Kmax/Kmin, a parameter called degree of anisotropy, P (Nagata, 1961). Fig. 9 shows an
suggests a simple for the magnetic
increase
consisting of two components: an isotropic to weakly anisotropic component (P - 1) and an anisotropic component (P = 2). The slope can be
in P with average
susceptibility
up to an
apparent
saturation value of approximately 2. In is plotted against Kmi,. This graph Fig. 10, K,,, 0.9
mixing model (see Appendix 1) susceptibility of these samples
q
0.8
no_
0
0.1 0
B
no
1
00
-0.8 -0.9
0
0 -3.4
I
I
-3
I
I
-2.6
I
I
-2.2
1
,
-1.8
I
I
-1.4
I
logcaverage susceptibility)
Fig. 11. Plot of the Jelinek
T parameter
against
average
susceptibility.
I
-1
I
-0'.6
325
North Group Ma~mum Axes
Maximum Axes
Peak: value
28.7
position 72Ji 81J
North Group
South Group
Minimum Axes
Minimum Axes
N 43 k
11.8
E
3.7
s
1.2
Peak: value
18.3
position 328Y 8J
Fig. 12. Magnetic
axis orientation
data.
a. Maximum
axes, north
group.
d. Minimum
considered an averaged value for P that has been corrected for the isotropic component. The intercept measures the isotropic component, which may be (1) paramagnetic susceptibility of iron-bearing silicates or (2) ferromagnetic susceptibility of magnetic particles which are isolated and approximately equant. The effect of this isotropic susceptibility is to decrease the apparent degree of anisotropy without, as is discussed below, affecting directions of axes. In the present group of samples the anisotropic component dominates the isotropic component if the average susceptibility is greater than about 0.02 SI units. The observation that parameter P, measured for any one sample, is affected by the presence of
group;
b. Minimum
axes, north
group.
c. Maximum
axes, south
axes, south group.
isotropic magnetic material would suggest that caution should be exercised when using P or any similar parameter in comparisons of magnetic anisotropy with strain. Most rocks containing magnetite also contain iron-bearing silicates, which will contribute an isotropic component to the magnetic response. In our case Fig. 9 shows that approximately 20% of the samples have sufficient magnetite to overwhelm the isotropic component, whereas the regression in Fig. 10 demonstrates that the anisotropic component of all samples has a consistent anisotropic ratio of 2.0. This ratio may reflect the strain event but it may not have any simple correlation with the amount of strain. The magnitude and form of the anisotropy is
326
illustrated in Fig. 11 which plots the T parameter (Jelinek, 1981):
T = 2 ld Kint/‘Ktin)/ln( K-,,ax/&n ) - 1 against log(average susceptibility). T varies from - 1 for a perfectly prolate figure to + 1 for a perfectly oblate one. The two-component mixing model can be used to explain the shape of the anisotropy. Samples which are dominated by the weak component have oblate fabrics, while strongly magnetic samples have prolate fabrics. This is consistent with the above stated observation that weakly magnetic samples are biotite-rich samples while samples with high susceptibilities are dominated by hornblende. The prolateness or oblateness of a sample is therefore apparently controlled by its mineralogy, rather than the strain. Directions of principal axes (eigenvectors)
The directional data is summarized in Fig. 12. The directions of maximum axes are steeply plunging to the northeast in agreement with a lineation which is visible in a few of the drilled samples. It is also in very good agreement with
gneissic lineation data for Pipe 2 mine obtained by Fueten et al. (1986) and presented in Fig. 4b. If the samples are split into the northern and southern groups, we see a difference in the average directions. The directions of the maximum susceptibility axes for the southern samples cluster very well, plunging steeply to the northeast (trend 71’) plunge 59 O, R,, 5.3”). The northern samples show more scatter in the trends of the maximum directions but the plunges cluster well. The average trend of the northern group is about 208”. There is little difference in average plunge between the two sites. The variation of trend of the maximum magnetic axes between the groups corresponds to the variation of the orientation of the fold limbs. The minimum directions represent the poles to the magnetic foliation. As Fig. 12 illustrates, minimum directions sweep out a broad girdle. The directions of the minimum axes on average (trend 321”, plunge lo’, R,, 12”) agree very well with poles to gneissic foliations at Pipe 2 mine (average trend 320 O, average plunge 6 “). The lower contours in Fig. 12d continue through the third quadrant. This is the result of mixing between minimum and intermediate axes in samples which are perfectly prolate to within the margin of mea-
360 350 cl B ?I?
340 330 - 0
0
0
z %
310 -
2 0 2
300 -
2 2
0 q
0 320 -
0
+H 00
0 0
n
0
+ 290 280 270 -
5 -2
260 -
s
250 -
'u 0
240 -
2
230 -
hE
220 -
0
210 0
200 100
I
I 120
I
I 140
I
I,, 160
I 180
,,,,,,I, 200 220
240
260
:
Local Trend of Dike
Fig. 13. Plot of trends of minimum axes (and two intermediate
axes) against local dike orientation
samples.
for a subgroup
of south group
327
Suscepdbiltdes avemgcd
toSamples
Sampte Position
a
+
maximum moment
0
intermediate moment
Susceptibilities averaaed to Sam&s
0.8
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -I I
1
I
1
I
4
5
I
6
SampIe Position b
0
cwve
Fig. 14. Variation of (a) logarithm of average susceptibility,
+
stnught
(b) Jelinek T parameter, versus position
across layer for fold nose and
straight limb.
surement error. The magnetic foliation therefore has a fold axial planar relationship to the Pipe 2 fold. The minimum directions do not depend on the local orientation of the amphibolite body as is illustrated in Fig. 13, in which the trends of the
central set of sites along the main dike of the southern group are plotted against the local orientation of the dike itself (in some samples the minimum and intermediate axes are so close in length that they fall within the error of measurement and may be interchanged, hence, in two
328
cases, the inte~ediate Fig. 13).
trends are also plotted in
Across-layer data
In order to assess the variability of magnetic fabric across a single layer, two sets of several samples were taken across a single amphibolite layer. One set was taken across a fold nose (No. 31-37), the other across a straight limb (No. 38-44). Figures 14a and 14b show the variations in average susceptibility and Jelinik T plotted as two traverses across the layer. One margin of both layers is more enriched in magnetite content, possibly as a result of metamorphic reactions. Other AMS parameters vary in a manner consistent with this variation in magnetite content. This would suggest that variations in AMS parameters across layers 10 cm-50 cm wide are equivalent to variations along layers on the scale of 10 m. A statistically accurate sample of the variations of the magnetic properties for these layers can, in these rocks, be obtained from limited outcrop. Conclusions
(1) The maximum axes of the magnetic ellipsoids in the amphibolite layers accurately record the lineation present in the accompanying, highly strained gneisses. This therefore provides a potentially useful tool to determine the shearing direction in those areas in which the stretching lineation is obscured or weakly developed. (2) Indi~du~ ~~rnurn axes of the magnetic ellipsoids are independent of the local trends of the layers from which the samples were taken. They do c&-respond well to the average of the poles of gneissic foliation for the Pipe 2 fold and form a axial planar fabric. The AMS fabric directions therefore track the large scale strain pattern and not the contemporaneous minor local folding. (3) The degree of prolateness or oblateness of the magnetic fabric varies greatly and is primarily a result of the mineralogy of the samples. Samples with a considerable amount of biotite have oblate magnetic fabrics, whereas those samples dominated by hornblende have prolate fabrics. Prolateness or oblateness of the magnetic fabric in this type of
lithology can therefore not be used as a strain gauge. (4) The parameter P(K,,/K,,) increases with increasing magnetic mineral content until a saturation value of P = 2 is reached. The measured value of P in any one sample is the result of mixing between an approximately isotropic component (P 2: l), presumably related to iron bearing silicates, and an anisotropic component with constant P (3 2) carried by elongated masses of magnetite. Since the amount of each component varies, a regression taken over a group of samples from the same area will give a better indication of the true magnetic anisotropy. It is suggested that the value P 1: 2 is in some way related to the magnitude of strain in the amphibolites. Alternatively, it may simply represent a saturation value of magnetic anisotropy for ~lphibolites under these conditions and many have no bearing on the total amount of strain in the rock. The constant value P of the anisotropic component warrants more study on a larger scale to evaluate its significance. (5) A single outcrop may well provide the variations in AMS parameters needed to estimate the anisotropic component of P and the average AMS fabric directions. The parameter P, derived from a large number of samples may reflect the strain within the sample area and needs to be investigated further.
Acknowledgments
The cooperation and help of the staff of INCO Metals Company, especially R.C. Sommerville, M. Toderian and D. Mundy were greatly appreciated. This study was funded partly by NSERC operating grants (Pearce and P.-Y.F. Robin) and partly by a G.S.C. contract to P.-Y.F. Robin under the Canada-Manitoba Mineral Development Agreement. The authors also acknowledge the help with measurements and computer programming by Laurel Sinclair, a summer NSERC undergraduate fellow. Two anonymous reviewers and members of the University of Toronto Tectonic Studies Group provided many helpful suggestions which greatly improved the paper.
329
Appendix 1
Goldstein,
A.G.,
mylonites
mixing model between a small constant isotropic component and a variable anisotropic component of susceptibility. Definitions: susceptibility of isotropic component = c; susceptibility of anisotropic component along ~nimum axis = m; susceptibility of anisotropic component along maximum axis = Am; thus, amsotropy constant of anisotropic component=A. Observed equation of regression line of maximum (Y) and minimum (X) susceptibilities: A
Y=sX$i
(1)
Model for X-+X’=m+c; Ami”c. Equation (1) becomes:
model for Y-, Y’=
1980. Magnetic
New England. Green,
Tectonophysics,
A.G., Hajnal,
evolution
trib. Mineral.
A.M.. Lowrie,
Hirt,
Formation
145: 177-189.
to geology
W.H.
deposits
Hence the slope provides an estimate of the anisotropy, while the y axis intercept yields an estimate of the susceptibility of the weak component.
Miner.
Re-
Coats,
C.J.A.,
Quirke,
and Cambell, of
the
T.T., Jr., Bell, C.K.,
F.H.A.,
Flinn
Manitoba,
1972. Geology
Flon,
Lynn
Lake
and
Shield:
Guideb.,
24th Int. Geol. Congr., F., Robin,
P.-Y.F.
mation
in the Thompson
report.
In: Current
areas,
Excursion
A31-C31,
pp. l-94.
and Pickering, Belt, central
Research,
Thompson
front of the western
Field
Montreal,
D.A. deposits
a progress
Part B. Geol. Surv. Can., Pap.
86-1B: 797-809. Gale.
G.H..
Protins,
R.C.,
Chomoby,
D., Mundy,
mineral
deposits
B.,
A., 1982.
setting
Thompson,
Snow Lake and Flinn Flon. Geol. Assoc. Can., 14.
the
J., Haystead,
D. and Walker,
Geological
Field Trip Guideb.,
of
for magnetic
and
Belt, Manitoba.
Geol.
Assoc.
In:
(Editors), Can..
Spec.
A.S.. 1966. The choice of
anisotropy
measurements
susceptibility
B., 1982. Comparison
of Scotland. Macek,
Geol. Surv. Manit..
anisotropy
of
in the
Wales and correlation
in Dalradian
and
3.50 pp.
and nickel
with
53: 83-97.
Henry,
fabrics
R.F.J.,
Tokyo.
Sci. Lett., 1: 92.
1979. Magnetic
R.F.J.
rocks
J. Struct. J.J.,
of strain
from
and
the southwest
Geol., 4: 373-384.
1978. MoIson
dyke
swarm.
Pap. 78-l.
Macek,
J. and Russell,
Belt Project.
Stupayski,
M.,
Susceptibility Sugiura,
at
Ruttan,
tometer. Weber.
1984.
Manit.
Miner.
Operating
Instrument.
N., Redman,
1985.
Somerville, N., Braun,
Deposits.
Tectonophysics,
Nickel
Naruzen,
slate belt of North
Hi~lands
Scoates,
data. including
Staff, 1982. Geology
F.D. and Cheam,
shape
Rathore, J.S.,
Scoates,
deforma-
Alps (France).
J.K..
1977. Thompson
Res. Div.. Geol.
Surv.,
Rep. Field Act. 1977, pp. 47-53.
M.E., 1986. DeforManitoba:
from the Maritime
of the Thompson
rocks. Earth Planet.
magnetic
Cranstone,
and mineral
and the Churchill-Superior
Precambrian Fueten,
H., Stacey,
speciment
Rathore,J.S.
W., 1981. Magnetic
and progressive
of axial orientation
Sulphide
Cambrian Belt. Manit.
in the Sud-
40: 287-308.
C.D. Spence and J.M. Franklin
Precambrian
Nickel
1977. Magnetic
indicator
Lowrie,
strain,
W.V. and Geological
strain.
I.W.D.,
Geol. Sot. Am. Bull., 84: 136991374.
Pap., 25: 165-209.
W., 1987. Thompson
fabric of rocks.
Sci. Lett., 55: 181-189.
A=s
sour. Div., Geol. Surv., Rep. Field Act. 1987: 171.
and
sediments
R.W. Hutchinson,
References
Surv.. 5: 37-82.
of magnetic
T., 1961. Rock Magnetism.
Porath,
Ontario.
of rocks and its appli-
Tectonophysics,
anisotropy,
Planet.
sulphide
or:
basin,
R.,
Geophys.
as a strain
D.M., 1973. Analysis
Peredery,
Kligfield,
with strain in
W. and Dalziel,
anisotropy
R., Owens,
Nagata,
W.S. and
79: 563-567.
R., Lowrie,
till fabrics.
Con-
anisotropy
anisotropy
and geophysics.
V., 1981. Characterization
susceptibility
Mark,
Proterozoic
Canada.
of the Sudbury
F., 1982. Magnetic
cation
Earth
Bleeker,
of magnetic
Tectonophysics, Hrouda,
for Early Manitoba.
W., Clendenen,
the Chelmsford
Kligfield,
sm+si/(l-s)+i
W.,
94: 82-89.
1988. The correlation
or:
- s) =
in northeastern
Petrol.,
tion in Permian
i/(1
Weber.
sill: constraints
the Fox River
susceptibility
Am +
T.E. and
and
crustal
c=sc+i
(3) To obtain A replace c in (2) by expression in (3):
116: 281-323.
M., Krogh,
zircon ages for the Molson dyke swarm
bury Basin, Ontario.
-S)
and western margin
and the north-central
1986. Precise U-Pb
Kligfield,
(2)
province
in Canada
Machado,
Tectonophysics,
For case of m = 0:
c=i/(l
province
States. Tectonophysics, L.M.,
Jelinek,
Am+c=sm+sc+i
66: 197-211.
model of the western Churchill United
Zone, southeastern
2. and Weber, W., 1985. An evolutionary
of the Superior Heaman.
anisotropyof
susceptibility
from the Lake Char Mylonite
J.D., Hyodo,
Superconducting
metamorphism
for SI-2
R.F.J.,
H. and Strangway, spinner
D.W., magne-
Inter., 41: 101-107. 1978. Archean
in the northwestern
along the Churchill-Superior Surv. Can., Pap. 78-10: 5-16.
Magnetic
Instruments.
second-gradient
Phys. Earth Planet.
W. and Scoates,
Manual
Sapphire
and proterozoic
Superior
boundary,
province
Manitoba.
and Geol.
Tectonophysics, 162 (1989) 315-329 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
An intensive study of magnetic susceptibility anisotropy of amphibolite layers of the Thompson Belt, North Manitoba G.W. PEARCE and F. FUETEN Tectonic Studies Group, Department of Ceolop,
~nive~~~t.~of Toronto, ~issis~u~g4
ht.
LSL lC6 ~Can~da~
(Received Suly 6, 1987; revised version accepted August 25, 1988)
Abstract Pearce, G.W. and Fueten, F., 1989. An intensive study of magnetic susceptibility anisotropy of amp~bolite the Thompson Belt, North Manitoba. Teclonophysics, 162: 315-329.
layers of
In an examination of the magnetic susceptibility anisotropy of folded amphibolite layers in intensly deformed gneisses at the Pipe 2 Mine, Thompson Belt, North Manitoba, the magnetic fabric was found to agree well in orientation with a fold axial planar foliation and the stretching lineation in the gneisses. The prolateness or oblateness of the magnetic fabric ellipsoid in this lithology depends primarily on the mineralogy of the sample and therefore cannot be used as a strain gauge. The magnetic fabric is made up of two components. One is a small, approximately isotropic part due to iron bearing siticates and perhaps isolated equant magnetite particles. The second is a variable anisotropic part, carried by of 2.018 f 0.008. This constant ratio represents elongated particles of magnetite and has constant ratio P = K,,, /K,i, a characteristic value of the magnetic anisotropy of these rocks and may be related to the strain in this area.
Introduction
Magnetic anisotropy has been employed as a tool in rock fabric analysis for many years (Hrouda, 1982). A number of studies (e.g., KLigfield et al., 1977, 1981; Rathore, 1979; Rathore and Henry, 1982) have examined magnetic anisotropy on a regional scale, but few (e.g., Goldstein, 1980) have examined variations over a small area. The present paper documents one such study in small deformed amphibolite layers, believed to be the metamorphosed equivalents of Molson dikes, at the Pipe 2 Mine, south of Thompson, Manitoba. The amp~bolite layers are enclosed in metasediments and gneisses of the Thompson Belt, which lies on the border between the Churchill and Superior Provinces and which records a complex tectonic history. The amphibolites are abundant across the belt and their magnetic fabric may constitute a very useful tool to help delineate the ~-1951/89/$03.50
0 1989 Elsevier Science Publishers B.V.
kinematic or strain history of the belt. Before this can be attempted, however, the amphibolites from within a small area need to be studied in order to know what importance can be attached to the directions of anisotropy, the amount of anisotropy and the relative prolateness of anisotropy. To assess these problems, amp~bo~ites from within a restricted area, in which the structure is reasonably well understood, were examined in detail. Regional setting
The Thompson Belt is a linear NE-SW trending belt at the border between the Churchill and Superior Provinces (Fig. 1) in north-central Manitoba. The basement lithologies in the belt are a series of felsic gneisses, with minor amounts of mafic and ultramafic material. These gneisses are considered to be the retrograded equivalent of the Pikwitonei granulites of the Superior Province
316
- Pipe 2 Mine Pik.
-
Pikwitonei
T.B.
-
Thompson
Belt
Fig. 1. Location of Thompson Belt and of Pipe 2 Mine.
PIPE
No2 SHAFT
@
-LEGENDSERqENTlNIZED
/
PERIDOTITE
(rn~nerol~zedin port)
QUARTZITE BIOTITE
GNEISS
AMPHIBOLE
GNEISS
PEGMATITE
eooo 5
gg --
PIT
OUTLINE
PIPE BIKE - OPEN PIT
SURFACE Scale0
Fig. 2. Plan map of Pipe 2 mine (reproduced from Coats et al.. 19’72). A-site area.
‘+
PLAN IO0
of southern sample area. B-site
400 Feet 110 MeR?rs
of northern sample
317
(Scoates et al., 1977; Weber and Scoates, 1978). At the western margin of the belt, they are overlain by a suite of metavolcanic and metasedimentary rocks called the Ospwagan Group by Scoates et al. (1977). Peredery et al. (1982) divided the sedimentary rocks into the Pipe and the Thompson assemblages. The gneisses are thought to have undergone several deformation events starting in the Archean (Green et al., 1985). Both gneisses and metasediments probably owe their current structural attitude to the Hudsonian orogeny, dated between 1.9 and 1.6 Ga in the Thompson Belt (Green et al., 1985). Preliminary results of ongoing structural studies in the Belt (Fueten et al., 1986; Bleeker, 1987) indicate that it has been a zone of widespread and intense shearing. The latest movement was in a direction which is presently vertical. A swarm of mafic to ultramafic dikes, known as the Molson Dike Swarm (Scoates and Macek, 1978) is located on the western side of the Superor Province. Recent U/Pb zircon dates of two dikes belonging to the Molson swarm yielded ages of 1883 f 2 Ma. (Heaman et al., 1986). The Molson dikes therefore appear to have been emplaced before or early in the Hudsonian orogeny. The dikes are fresh and undeformed in the Pikwitonei region but are metamorphosed in the Thompson Belt. The Pipe 2 Nickel Mine was operated by INCO Metals Company until 1984. The geology of this deposit has been discussed by Peredery et al. (1982) and Gale et al. (1982) (Fig. 2). It is located on the western side of the Thompson Belt, within 2 km of the Churchill-Superior boundary. The mine itself is located within a major tight fold plunging 80 o to the northeast, with a near vertical fold axial plane trending approximately 050” and opening towards the northeast. Minor folds are common on the eastern limb and affect basement gneisses as well as metasediments. From metamorphic assemblages the peak metamorphic temperatures at Pipe 2 Mine are estimated to have been 575 “-625 o C (Fueten et al., 1986). Deformation at Pipe 2 Mine is intense. Bedding and foliations (Fig. 3b) are near vertical. A strong, near vertical stretching lineation (Fig. 3a) is present on both limbs. Deformation on both limbs
PIPE 2 MINE LINEATIONS
/
position
63’/ 80’
0
PIPE 2 MINE FOLIATION
POLES
N
346
k
79.3
E
4.4
s
1.5
Peak: value position
Fig.
3. Orientation
data
for the Pipe
gneissic lineation,
2 mine.
63.2 320’/ 6’
(a) Poles
to
(b) gneissic foliation.
indicates a well defined vertical extension. The layering on the western limb is more disrupted than on the eastern limb, and some concentric structures in the western limb could be interpreted as steeply plunging sheath folds, suggesting the strain is higher here. Description lection
of amphibolite
layers and sample col-
In both the basement gneisses and the supracrustals, continuous amphibolite layers occur, most of which are thought to be metamorphosed Molson dikes. The width of these layers ranges from several centimetres to several metres. In some locations the layers may be sheared apart or boudinaged. Two mineral assemblages occur. The minerals within each assemblage, in order of decreasing abundance. are:
318
n
AMPHIBOLITE
LAYERS
LIMIT OF OUTCROP
Fig. 5. Small-scale map of south group of samples.
(1) hornblende, minerals,
magnetite
plagioclase, and minor
epidote amounts
group of quartz
(Fig. 4a); and (2) biotite, plagioclase, quartz, hornblende, epidote group minerals and minor amounts of mag-
The samples were oriented by a Brunton compass held on the top surface of the drilled out, in situ cores. The orientations
netite (Fig. 4b). There is more visible magnetite present in layers with assemblage 1 than 2 (Fig. 4a, b). Magnetite in
according
samples
Magnetic measurements
with
assemblage
1 occurs
as elongated
may be in error by as
much as 10” due to magnetic deflections. From one to four samples were cut from the cores to their length
for a total of 210 speci-
mens.
particles. These particles are interspersed among the other minerals and are aligned parallel with the fabric. Such large elongated particles are al-
The (AMS)
most absent from samples of assemblage 2, in which smaller isolated opaque grains, many of
commercial inductance bridge instrument (Sapphire Instruments) (Stupavski, 1984) and checked
which are equant, are present. The samples were collected
in a few cases with as 2.54 cm cores
using
anisotropy of of the samples
a cryogenic
magnetic susceptibility were measured using a
an inhouse-designed
magnetometer
system
(Sugiura
et al.,
drilled vertically (within 10”) from two localities: (1) just south of the lip of the pit (43 cores), and
1985). Since the samples are cylinders, measurements may be sensitive to the length-to-diameter
(2) just north of the pit (12 cores). Most of the southern samples were collected along a single layer which was tightly folded about a fold axial
ratio
plane trending about 055O (Fig. 5). Two sequences of seven samples each were collected across a single layer, one (No. 31-37) across a fold nose, the other (No. 38-44) across a limb. Based on mesoscopic appearance as well as on microscopic evidence, such as grain sizes and the amount of subgraining of quartz, the amount of deformation within each sample site appears to be quite similar.
(L/D)
(Porath
et al., 1966). To determine
the sensitivity of the Sapphire instrument in this regard, a number of samples were measured successively
after progressive
shortening.
For L/D
ratios between 0.8 and 1.0 (ideal ratio = 0.85) the anisotropy parameters proved to be insensitive ( f 5%) to L/D. All other samples were then cut to an L/D ratio of 0.90 _t 0.05. Each sample was measured at least twice, varying the experimental conditions such as different numbers of orientations or different sampling times. If there was more than one sample per core, each sample was
320 TABLE 1 Anisotropy of magnetic susceptibility (AMS) of Thomson Belt amphibolite dikes: results averaged to samples * Sample
Number
No.
of meas.
Principal magnetic susceptibility axes
Td
PI
(“)
(“)
Kti, (SI)
Avg.
maximal
intermediate
minimal
K
Td
PI
Kim
Td
PI
Gi.X
(“)
(“)
(W
(“)
(“)
(W
1
2
297.49
30.63
0.000811
202.07
2
1
144.4
5.4
0.000906
237.6
8.97 30.2
(W
0.000864
97.52
57.84
0.000880
0.000852
0.000991
45.2
59.1
0.001017
0.000971
3
2
311.6
20.65
0.000691
214.78
16.79
0.000767
88.54
62.74
0.000784
0.000748
4
5
320.5
10.3
0.000421
222.5
39.6
0.000464
61.5
49.1
0.000484
0.000457
5
4
301.8
26.48
0.000811
189.46
37.07
0.000833
58.02
41.54
0.000860
0.000835
6
3
274.7
39.06
0.000859
160.59
26.86
0.000892
46.9
38.23
0.00926
0.000892
7
4
157.14
38.13
0.000998
251.05
53.64
0.001104
49.43
31.61
0.001239
0.001114
8
4
283.47
60.53
0.000677
184.58
2.75
0.000701
91.44
33.49
0.000718
0.000699
9
4
344.22
9.45
0.000714
253.84
52.81
0.000747
81.34
36.88
0.000764
0.000742
10
4
149.12
5.57
0.000670
248.39
56.39
0.000711
55.57
32.86
0.000728
0.000703
11
6
332.94
4.22
0.000808
237.58
49.84
0.000856
66.44
40.21
0.000878
0.000847
12
8
342.83
1.28
0.010493
252.4
22.66
0.012877
76.89
66.77
0.015571
0.012980
13
4
345.15
0.37
0.004557
254.96
32.87
0.005224
76.21
56.86
0.006675
0.005485
14
3
342.67
8.6
0.009544
249.6
19.1
0.012263
95.79
68.78
0.015300
0.012369
16
3
324.47
4.99
0.046965
233.12
14.67
0.053372
73.17
74.61
0.090044
0.063460
17
6
330.59
13.09
0.040802
235.57
20.31
0.056154
91.35
65.43
0.081653
0.059536
18
4
321.18
5.31
0.000913
228.26
29.01
0.001038
60.31
60.46
0.001061
0.001004
19
5
171.72
7.14
0.008378
265.45
26.64
0.009529
74.13
62.09
0.014429
0.010779
20
4
302.13
16.78
0.005606
205.31
21.35
0.005928
66.55
62.69
0.007652
0.006395
21
4
315.42
8.95
0.001638
219.81
31.54
0.001943
59.57
56.98
0.002085
0.001889
22
4
333.67
5.2
0.004860
240.69
29.13
0.005707
72.97
60.23
0.007303
0.005957
23
4
304.8
7.03
0.002519
211.72
22.79
0.002647
51.19
65.88
0.003415
0.002860
24
3
331.54
9.84
0.024686
239.46
11.87
0.027984
100.35
74.37
0.048902
0.033857
25
5
323.74
6.52
0.017801
240.47
18.31
0.019771
59.19
71.58
0.034413
0.023995
26
2
243.35
14.66
0.0265
338.25
18.16
0.033
66.2
0.0575
0.039
27
5
198.61
30.15
0.000838
296.65
14.56
0.000876
49.61
56.63
0.000896
0.000870
116.7
28
2
324.5
6.1
0.008425
232.15
14.05
0.009891
86.44
73.1
0.0160
0.011438
29
4
340.46
6.34
0.02
247.56
25.84
0.027
82.83
63.63
0.042
0.029666
30
5
318.42
19.5
0.009705
220.15
22.1
0.011635
85.9
59.64
0.020620
0.013986
31
3
177.8
22.36
0.001023
280.49
27.91
0.001026
54.54
53.19
0.001088
0.001045 0.002897
32
2
29.25
2.3
0.002256
238.25
22.75
0.002719
64.8
67
0.003717
33
6
319.71
6.61
0.001389
224.55
26.4
0.001553
56.5
61.5
0.001918
0.001620
34
2
190.58
25.39
0.001000
289.06
17.05
0.001020
50.39
58.16
0.001060
0.001027
35
6
210.71
49.05
0.000997
313.06
10.36
0.001036
51.54
38.91
0.001073
0.001035
36
4
198.49
53.37
0.000993
302.38
10.01
0.001017
39.52
35.29
0.001058
0.001023
140.55
14.49
0.000847
39.45
37.14
0.000865
0.000843
7.02
1.59
0.16375
101.11
72.22
0.308750
0.20925
7.4
0.1285
86.69
64.53
0.233375
0.1585
83.66
63.89
0.0652
0.0452
50
0.001000
0.000965 0.004477
37
2
248.1
48.23
0.000817
38
4
276.53
17.5
0.15525
39
8
247.81
24.78
0.113625
341.34 310.67
40
5
214.96
17.55
0.0328
18.2
0.0376
41
4
270.38
28.89
0.000906
15.37
25.15
0.000989
139.26
42
4
259.88
22.76
0.003790
351.56
3.92
0.00404
91.1
66.87
0.005602
43
6
251.68
31.83
0.000942
358.14
23.46
0.001021
116.53
47.92
0.001035
0.000999
44
8
281.77
31.94
0.000728
11.09
0.52
0.000790
101.79
59.47
0.000800
0.000773
45
2
148.4
6.9
0.000936
38.75
1.5
0.001070
340.82
82.89
0.001113
0.001040
46
2
302.6
0.75
0.000714
212.37
11.35
0.000771
35.57
78.5
O.ooO786
0.000757 0.000615
47
2
317.26
10.76
0.000556
49.48
12.13
0.000642
188.04
72.74
0.000648
49
2
328.95
2.3
0.001558
59.35
11.4
0.001897
227.33
78.31
0.002229
0.001894
50
2
324.7
6.9
0.001519
55.95
10.3
0.001838
201.43
77.45
0.002164
0.001840
321
TABLE
1 (continued)
Sample
Number
No.
of meas.
Principal
magnetic
susceptibility
axes Avg.
maximal
intermediate
minimal Td
PI
K
Td
PI
K ml
Td
PI
K
(“)
(“1
CS’;;”
(“1
(“1
@I)
(“1
(“)
(S;)=
K @I)
51
2
301.96
15.66
0.000669
43.46
34.53
0.000761
191.52
50.89
0.000772
52
2
109.8
6.5
0.001598
18.0
15.25
0.00201
222.17
73.21
0.002267
0.001958
53
2
82.58
2.68
0.000441
34.01
0.000499
182.63
55.75
0.000519
0.000486
13.95
0.000667
174.63
68.04
0.000728
54.92
48.26
0.000756
0.000717
8.25
0.000811
228.45
47.24
0.000894
36.45
42.31
0.000904
0.000870
0.000860
197
7.
0.000967
313.68
74.5
0.001059
0.000962
54
4
309.36
56
4
133.6
57
2
105.3
* Td = trend,
PI = plunge,
13.6
346
K = length of susceptibility
axes (SI units).
measured. For each core the resulting axial direction sets were averaged by using a procedure suggested by Mark (1973) and provided as a computational algorithm by Sapphire Instruments. This algorithm treats each principal axis separately and the computed average principal axes will not be exactly orthogonal. The deviations from orthogonality of the axes are within expected margins of error for all samples and the computed statistical parameters appear to be acceptable. The mean scalar anisotropy parameters were obtained by simple averaging. The averaged data are presented in Table 1.
Magnetic susceptibilities
An average susceptibility was computed as (K,, + Kin, + K,,)/3, where K,,, Kin, and K,, are respectively the lengths of the maximum, intermediate and minimum principal susceptibility axes. The samples show a very large range of average susceptibilities (0.00046-0.21 SI units) (Fig. 6). Examination of a number of thin sections shows that high susceptibility is correlated with abundant hornblende (assemblage l), whereas samples with high biotite contents (assemblage 2) have low susceptibilities.
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Average Susceptibility
Fig. 6. Frequency
histogram
0.000734
for average
susceptibilities
(SI units)
for Pipe 2 mine amphibolite
samples.
322
0.9
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 iJ Field CkOe)
0
X26
+
#39
0
#4
Fig. 7. Normalized ferromagnetic response curves for several samples. These curves are obtained from measurements of magnetic moment as a function of applied magnetic field. The ferromagnetic response is calculated by subtracting a parama~etic moment, which is itseff determined from the high field (z 3 kOe or 0.24 MA m-‘) total magnetic response (M) using a model: M = M,(l - c/H)
+ K,H
where MS, H, K, and E are, respectively, the saturation moment of the ferromagnetic minerals, the applied field, the susceptibility of the paramagnetic minerals and a constant expressing the approach to saturation of the ferromagnetic response.
a.3 0.2 0.1
0,-I200 Temperature
("C)
Fig. 8. Variation of magnetic response with temperature for sample No. 39 in an applied field of 3 kOe (0.24 MA m-t).
323
1961). This response suggests that only one magnetic mineral is present in significant quantity. To verify that the ma~etic mineral was indeed magnetite, the saturation moment of powdered material from one sample (No. 39) was examined as a function of temperature (Fig. 8). As saturation was very close to completion at 3 kOe (0.24 MA m-l), this field was chosen for the experiment so as to lessen the effect of the parama~etic response. The resulting curve shows the presence of only one magnetic mineral with a Curie point of 575”, close to that of pure magnetite (580 “-585 o C), (e.g. Nagata, 1961). Although other magnetic minerals such as hematite or pyrrhotite may be present in small quantities they should have a negliglible effect on the low field susceptibility as presented in this study.
The high susceptibility appears to be caused by the large highly elongated opaque particles visible in these samples. Magnetic mineralogy Other magnetic properties were examined for a number of selected samples to determine the mineral or minerals responsible for the magnetic susceptibility. Using a vibrating sample magnetometer, magnetic hysteresis curves were run on samples with high and low susceptibilities. The samples varied considerably in their saturation magnetizations in good agreement with their respective average susceptibilities. However the hysteresis curves match each other closely in shape (Fig. 7) if the linear response of paramagnetic iron minerals is removed. The curves show a smooth rise to a saturation at 2-3 kOe (0.16-0.24 MA m-i) and have little hysteresis (the ratio of saturation remanence to saturation moment varies between 0.02 and 0.11) Such behaviour is consistent with multidomain magnetite grains (> 0.5 pm). A spherical multidomain magnetite particle would give an almost reversible linear ma~et~ation curve with a distinct rollover to saturation at 47rJJ3 (approximately 2.0 kOe or 0.16 MA m-i) (Nagata,
Lengths of principal axes (eigenvaiues) and anisotropy Several attempts (e.g., Hirt et al., 1988) have been made to quantitavely correlate AMS with the amount of strain in sedimentary and metamorphic rocks. In this study the absolute amount of strain is not known since no usable strain-gauges are present. However, the deformation within each small area sampled appears to be similar; it is
2.2 cl
u
2.1-
0 cl
2-
00
a
0
1.9-
0
0
1.8 1.7 -
u D q
1.6-
1
1 0
I
I,,
0.02
0.04
I 0.06
I
I
0.08
,
,
I,,
0.1
0.12
Average Susceptibility
Fig. 9. Plot of Kmax/Kmin
(parameter
P) against
,
,
0.14
I, 0.16
(SI units)
average
susceptibility.
,
0.18
,
,
0.2
324 0.34 0.32 0.3 0.28 0.26 0.24 0.22 0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 6
Minimum Susceptibility
Fig. 10. Plot of K,,,
with high average
are also quite anisotropic
shows a good linear relationship; a regression yields a slope of 2.018 + 0.008 and an intercept of - 0.00106 -c_0.0016 with a correlation coefficient
susceptibilities
(Fig. 9) as measured
K,,,;,.
against
therefore a reasonable assumption that the amount of strain in each area should also be similar. The samples
IS1 units)
(R)
by
of 0.9991.
The excellence
of the regression
the ratio Kmax/Kmin, a parameter called degree of anisotropy, P (Nagata, 1961). Fig. 9 shows an
suggests a simple for the magnetic
increase
consisting of two components: an isotropic to weakly anisotropic component (P - 1) and an anisotropic component (P = 2). The slope can be
in P with average
susceptibility
up to an
apparent
saturation value of approximately 2. In is plotted against Kmi,. This graph Fig. 10, K,,, 0.9
mixing model (see Appendix 1) susceptibility of these samples
q
0.8
no_
0
0.1 0
B
no
1
00
-0.8 -0.9
0
0 -3.4
I
I
-3
I
I
-2.6
I
I
-2.2
1
,
-1.8
I
I
-1.4
I
logcaverage susceptibility)
Fig. 11. Plot of the Jelinek
T parameter
against
average
susceptibility.
I
-1
I
-0'.6
325
North Group Ma~mum Axes
Maximum Axes
Peak: value
28.7
position 72Ji 81J
North Group
South Group
Minimum Axes
Minimum Axes
N 43 k
11.8
E
3.7
s
1.2
Peak: value
18.3
position 328Y 8J
Fig. 12. Magnetic
axis orientation
data.
a. Maximum
axes, north
group.
d. Minimum
considered an averaged value for P that has been corrected for the isotropic component. The intercept measures the isotropic component, which may be (1) paramagnetic susceptibility of iron-bearing silicates or (2) ferromagnetic susceptibility of magnetic particles which are isolated and approximately equant. The effect of this isotropic susceptibility is to decrease the apparent degree of anisotropy without, as is discussed below, affecting directions of axes. In the present group of samples the anisotropic component dominates the isotropic component if the average susceptibility is greater than about 0.02 SI units. The observation that parameter P, measured for any one sample, is affected by the presence of
group;
b. Minimum
axes, north
group.
c. Maximum
axes, south
axes, south group.
isotropic magnetic material would suggest that caution should be exercised when using P or any similar parameter in comparisons of magnetic anisotropy with strain. Most rocks containing magnetite also contain iron-bearing silicates, which will contribute an isotropic component to the magnetic response. In our case Fig. 9 shows that approximately 20% of the samples have sufficient magnetite to overwhelm the isotropic component, whereas the regression in Fig. 10 demonstrates that the anisotropic component of all samples has a consistent anisotropic ratio of 2.0. This ratio may reflect the strain event but it may not have any simple correlation with the amount of strain. The magnitude and form of the anisotropy is
326
illustrated in Fig. 11 which plots the T parameter (Jelinek, 1981):
T = 2 ld Kint/‘Ktin)/ln( K-,,ax/&n ) - 1 against log(average susceptibility). T varies from - 1 for a perfectly prolate figure to + 1 for a perfectly oblate one. The two-component mixing model can be used to explain the shape of the anisotropy. Samples which are dominated by the weak component have oblate fabrics, while strongly magnetic samples have prolate fabrics. This is consistent with the above stated observation that weakly magnetic samples are biotite-rich samples while samples with high susceptibilities are dominated by hornblende. The prolateness or oblateness of a sample is therefore apparently controlled by its mineralogy, rather than the strain. Directions of principal axes (eigenvectors)
The directional data is summarized in Fig. 12. The directions of maximum axes are steeply plunging to the northeast in agreement with a lineation which is visible in a few of the drilled samples. It is also in very good agreement with
gneissic lineation data for Pipe 2 mine obtained by Fueten et al. (1986) and presented in Fig. 4b. If the samples are split into the northern and southern groups, we see a difference in the average directions. The directions of the maximum susceptibility axes for the southern samples cluster very well, plunging steeply to the northeast (trend 71’) plunge 59 O, R,, 5.3”). The northern samples show more scatter in the trends of the maximum directions but the plunges cluster well. The average trend of the northern group is about 208”. There is little difference in average plunge between the two sites. The variation of trend of the maximum magnetic axes between the groups corresponds to the variation of the orientation of the fold limbs. The minimum directions represent the poles to the magnetic foliation. As Fig. 12 illustrates, minimum directions sweep out a broad girdle. The directions of the minimum axes on average (trend 321”, plunge lo’, R,, 12”) agree very well with poles to gneissic foliations at Pipe 2 mine (average trend 320 O, average plunge 6 “). The lower contours in Fig. 12d continue through the third quadrant. This is the result of mixing between minimum and intermediate axes in samples which are perfectly prolate to within the margin of mea-
360 350 cl B ?I?
340 330 - 0
0
0
z %
310 -
2 0 2
300 -
2 2
0 q
0 320 -
0
+H 00
0 0
n
0
+ 290 280 270 -
5 -2
260 -
s
250 -
'u 0
240 -
2
230 -
hE
220 -
0
210 0
200 100
I
I 120
I
I 140
I
I,, 160
I 180
,,,,,,I, 200 220
240
260
:
Local Trend of Dike
Fig. 13. Plot of trends of minimum axes (and two intermediate
axes) against local dike orientation
samples.
for a subgroup
of south group
327
Suscepdbiltdes avemgcd
toSamples
Sampte Position
a
+
maximum moment
0
intermediate moment
Susceptibilities averaaed to Sam&s
0.8
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -I I
1
I
1
I
4
5
I
6
SampIe Position b
0
cwve
Fig. 14. Variation of (a) logarithm of average susceptibility,
+
stnught
(b) Jelinek T parameter, versus position
across layer for fold nose and
straight limb.
surement error. The magnetic foliation therefore has a fold axial planar relationship to the Pipe 2 fold. The minimum directions do not depend on the local orientation of the amphibolite body as is illustrated in Fig. 13, in which the trends of the
central set of sites along the main dike of the southern group are plotted against the local orientation of the dike itself (in some samples the minimum and intermediate axes are so close in length that they fall within the error of measurement and may be interchanged, hence, in two
328
cases, the inte~ediate Fig. 13).
trends are also plotted in
Across-layer data
In order to assess the variability of magnetic fabric across a single layer, two sets of several samples were taken across a single amphibolite layer. One set was taken across a fold nose (No. 31-37), the other across a straight limb (No. 38-44). Figures 14a and 14b show the variations in average susceptibility and Jelinik T plotted as two traverses across the layer. One margin of both layers is more enriched in magnetite content, possibly as a result of metamorphic reactions. Other AMS parameters vary in a manner consistent with this variation in magnetite content. This would suggest that variations in AMS parameters across layers 10 cm-50 cm wide are equivalent to variations along layers on the scale of 10 m. A statistically accurate sample of the variations of the magnetic properties for these layers can, in these rocks, be obtained from limited outcrop. Conclusions
(1) The maximum axes of the magnetic ellipsoids in the amphibolite layers accurately record the lineation present in the accompanying, highly strained gneisses. This therefore provides a potentially useful tool to determine the shearing direction in those areas in which the stretching lineation is obscured or weakly developed. (2) Indi~du~ ~~rnurn axes of the magnetic ellipsoids are independent of the local trends of the layers from which the samples were taken. They do c&-respond well to the average of the poles of gneissic foliation for the Pipe 2 fold and form a axial planar fabric. The AMS fabric directions therefore track the large scale strain pattern and not the contemporaneous minor local folding. (3) The degree of prolateness or oblateness of the magnetic fabric varies greatly and is primarily a result of the mineralogy of the samples. Samples with a considerable amount of biotite have oblate magnetic fabrics, whereas those samples dominated by hornblende have prolate fabrics. Prolateness or oblateness of the magnetic fabric in this type of
lithology can therefore not be used as a strain gauge. (4) The parameter P(K,,/K,,) increases with increasing magnetic mineral content until a saturation value of P = 2 is reached. The measured value of P in any one sample is the result of mixing between an approximately isotropic component (P 2: l), presumably related to iron bearing silicates, and an anisotropic component with constant P (3 2) carried by elongated masses of magnetite. Since the amount of each component varies, a regression taken over a group of samples from the same area will give a better indication of the true magnetic anisotropy. It is suggested that the value P 1: 2 is in some way related to the magnitude of strain in the amphibolites. Alternatively, it may simply represent a saturation value of magnetic anisotropy for ~lphibolites under these conditions and many have no bearing on the total amount of strain in the rock. The constant value P of the anisotropic component warrants more study on a larger scale to evaluate its significance. (5) A single outcrop may well provide the variations in AMS parameters needed to estimate the anisotropic component of P and the average AMS fabric directions. The parameter P, derived from a large number of samples may reflect the strain within the sample area and needs to be investigated further.
Acknowledgments
The cooperation and help of the staff of INCO Metals Company, especially R.C. Sommerville, M. Toderian and D. Mundy were greatly appreciated. This study was funded partly by NSERC operating grants (Pearce and P.-Y.F. Robin) and partly by a G.S.C. contract to P.-Y.F. Robin under the Canada-Manitoba Mineral Development Agreement. The authors also acknowledge the help with measurements and computer programming by Laurel Sinclair, a summer NSERC undergraduate fellow. Two anonymous reviewers and members of the University of Toronto Tectonic Studies Group provided many helpful suggestions which greatly improved the paper.
329
Appendix 1
Goldstein,
A.G.,
mylonites
mixing model between a small constant isotropic component and a variable anisotropic component of susceptibility. Definitions: susceptibility of isotropic component = c; susceptibility of anisotropic component along ~nimum axis = m; susceptibility of anisotropic component along maximum axis = Am; thus, amsotropy constant of anisotropic component=A. Observed equation of regression line of maximum (Y) and minimum (X) susceptibilities: A
Y=sX$i
(1)
Model for X-+X’=m+c; Ami”c. Equation (1) becomes:
model for Y-, Y’=
1980. Magnetic
New England. Green,
Tectonophysics,
A.G., Hajnal,
evolution
trib. Mineral.
A.M.. Lowrie,
Hirt,
Formation
145: 177-189.
to geology
W.H.
deposits
Hence the slope provides an estimate of the anisotropy, while the y axis intercept yields an estimate of the susceptibility of the weak component.
Miner.
Re-
Coats,
C.J.A.,
Quirke,
and Cambell, of
the
T.T., Jr., Bell, C.K.,
F.H.A.,
Flinn
Manitoba,
1972. Geology
Flon,
Lynn
Lake
and
Shield:
Guideb.,
24th Int. Geol. Congr., F., Robin,
P.-Y.F.
mation
in the Thompson
report.
In: Current
areas,
Excursion
A31-C31,
pp. l-94.
and Pickering, Belt, central
Research,
Thompson
front of the western
Field
Montreal,
D.A. deposits
a progress
Part B. Geol. Surv. Can., Pap.
86-1B: 797-809. Gale.
G.H..
Protins,
R.C.,
Chomoby,
D., Mundy,
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In:
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A.S.. 1966. The choice of
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B., 1982. Comparison
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anisotropy
of
in the
Wales and correlation
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R.F.J.,
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Macek,
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Stupayski,
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N., Redman,
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Rathore,J.S.
W., 1981. Magnetic
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