JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 8
PAGES 1551–1570
2002
A Mainly Crustal Origin for Tonalitic
Granitoid Rocks, Superior Province, Canada:
Implications for Late Archean
Tectonomagmatic Processes
JOSEPH. B. WHALEN1∗, JOHN A. PERCIVAL1, VICKI J. McNICOLL1
AND FREDERICK J. LONGSTAFFE2
1
GEOLOGICAL SURVEY OF CANADA, 601 BOOTH STREET, OTTAWA, ON, CANADA K1A OE8
2
DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF WESTERN ONTARIO, LONDON, ON, CANADA N6A 5B7
RECEIVED AUGUST 1, 2000; REVISED TYPESCRIPT ACCEPTED FEBRUARY 4, 2002
The central Wabigoon subprovince of the Superior Province, like
most plutonic domains within Archean cratons, is dominated by
granitoid rocks of the tonalite–trondhjemite–granodiorite (TTG)
series. Heterogeneous <2·83–2·74 Ga tonalite gneisses and foliated
tonalite to granodiorite units, emplaced at 2·722–2·709 Ga,
exhibit initial Nd values (–3·1 to +3·3) indicative of variable
input from light rare earth element enriched older (3·2–3·4 Ga)
crustal materials. Their 18O (VSMOW) range (+7·1 to
+8·9‰), which overlaps closely that of average upper Superior
Province crust, indicates input from high-18O crustal materials. The
preferred petrogenetic model for Wabigoon tonalitic rocks involves
partial melting of overthickened amphibolite-dominated lower-crustal
materials within a Cordilleran-type arc. Assimilation of >2·74
Ga tonalite gneiss crust by younger tonalite magmas was probably
an important process. Unlike the model of TTGs representing direct
partial melts of subducting slabs in an arc setting, this model
requires no direct tie to subduction. Careful re-evaluation of the
TTG classification is required for it can mistakingly ‘pigeon-hole’
temporally and genetically unrelated rocks and perhaps assume an
incorrect petrogenetic or tectonic model. An important role of crustal
recycling processes in central Wabigoon tonalite petrogenesis is in
keeping with evidence that supports substantial rates of continental
recycling as far back as the earliest Archean.
KEY WORDS:
Archean; crustal evolution; Nd–O isotopes; TTG
∗Corresponding author. Telephone: (613) 995-4972. Fax: (613) 9957997. E-mail: [email protected]
INTRODUCTION
Over the last decade, information fundamental to understanding and resolving many questions regarding Archean tectonic and crustal evolution has been obtained
from the plutonic record. Particularly controversial is
tonalite–trondhjemite–granodiorite (TTG) (Martin,
1986) or tonalite–trondhjemite–dacite (TTD) (Drummond & Defant, 1990) petrogenesis. If voluminous TTG
or TTD suites are direct products of slab melting in an
arc setting (see Martin, 1986, 1994, 1999; Drummond
& Defant, 1990), then they represent de facto evidence for
the operation of modern-type plate tectonic processes
throughout Archean time. Documentation of alterative
petrogenetic models for Archean TTG or TTD suites is
of particular relevance to Archean tectonic processes (see
de Wit, 1998; Hamilton, 1998; Smithies, 2000), as well
as processes of Archean continental crustal growth and
recycling (e.g. Ireland et al., 1994; de Wit, 1998; Hamilton,
1998; Henry et al., 1998, 2000; Nutman et al., 1999).
As in most Archean shield terrains, plutonic domains
of the Western Superior Province typically consist mainly
of TTG or TTD compositions, but also include more
mafic and more evolved granitoid compositions (Thurston et al., 1991; Henry et al., 1998; Stone, 1998). Wholerock geochemical and Nd–O isotopic data collected from
tonalitic to granodioritic plutonic rocks, plus possibly
consanguineous mafic intrusive rocks, within the central
Wabigoon subprovince plutonic domain (Fig. 1) are
 Oxford University Press 2002
JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 8
AUGUST 2002
Fig. 1. Location map showing the central Wabigoon region and flanking greenstone belts (after Sanborn-Barrie & Skulski, 1999); the area of
Fig. 2 is outlined. Also shown is highway 599 and the CNR railway.
employed herein to constrain mantle and crustal involvement in their petrogenesis and to evaluate their
tectonomagmatic implications.
REGIONAL GEOLOGICAL SETTING
Recent syntheses (Williams et al., 1992; Card & Poulsen,
1998) have regarded the linear volcanic–plutonic subprovinces of the western Superior Province as intraoceanic island arc terranes, juxtaposed during >2·7 Ga
collisional events. The granitoid rock dominated central
Wabigoon subprovince study area contains vestiges of
2·93–3·07 Ga crust and is postulated to be the basement
to bordering Neoarchean greenstone belts (Thurston &
Davis, 1985) (Fig. 1). In contrast, the eastern Wabigoon
subprovince contains mixed pre-2·8 Ga and Neoarchean
volcanic and plutonic rocks (Stott et al., 1998), and the
western Wabigoon consists mainly of juvenile Neoarchean greenstone belts and granites (Blackburn et al.,
1991; Henry et al., 1998). The study area contains a
complex record of regional intrusive and structural events
(Percival, 1998; Percival et al., 1999a, 1999b). The earlier
deformation phases (D1, D2) are preserved only sporadically in tonalite gneisses, having been largely overprinted (mainly by D3), whereas D3 and D4 are recorded
in all units. In general, D1 and D3 resulted in penetrative
fabrics, whereas D2 and D4 produced megascopic to mapscale folds. Geochronological constraints on deformation
events show that D1 and D2 occurred between <2·83 and
>2·715 Ga ( J. Brown & V. McNicoll, unpublished data,
2000), D3 occurred between <2·704 and >2·698 Ga, and
D4 occurred between <2·698 and >2·68 Ga (V. McNicoll,
unpublished data, 1999). Amphibolite-facies metamorphism accompanied D3 (Skulski et al., 1998).
The western edge of the central Wabigoon region is
defined by a <2·9 Ga volcanic-dominated continental
margin succession (Davis & Moore, 1991), which forms
the eastern part of the Sturgeon–Savant Lake greenstone
1552
WHALEN et al.
TONALITIC GRANITOID ROCKS, SUPERIOR PROVINCE
Fig. 2. Sketch map showing the geology of the central Wabigoon region in the Sturgeon–Obonga corridor (after Percival et al., 1999a, 1999b).
BRSZ, Brightsand River shear zone; GLSZ, Gunter Lake shear zone. Sample locations and geochronology sites are plotted with U–Pb zircon
results (in Ma); sites discussed in the text are identified with the letters A–D. Also shown are logging roads (dotted lines) and the CNR railway
(north end of map).
belt (SSLB in Fig. 1) (Sanborn-Barrie et al., 1998). The
western SSLB, a tectonically juxtaposed 2·775–2·718
Ga volcanic terrane, includes the 2·775 Ga Fourbay
asemblage, which consists mainly of pillowed basalt with
isotopically juvenile, oceanic plateau geochemical characteristics (Sanborn-Barrie & Skulski, 1999). The volcanic
terrane is overlain by a <2·704 Ga sedimentary overlap
sequence (Skulski et al., 1998). The Obonga Lake belt to
the east (Fig. 1) contains calc-alkaline volcanic rocks of
similar age (>2·73 Ga) to the SSLB rocks (Tomlinson
et al., 1996). Located immediately east of the area covered
by Fig. 1 is the central Onaman–Tashota belt of the
Wabigoon subprovince. This belt includes the >2·78–
2·765 Ga North Onaman sequence of basalt, dacite and
tonalite from which Tomlinson et al. (2000) reported
2·9–3·0 Ga Nd model ages and inherited zircons and
postulated a continental setting.
LITHOTECTONIC MAP UNITS
Granitoids of the central Wabigoon region were
subdivided in the field into generally foliated (S3),
homogeneous granitoid rock units and less abundant
tonalitic gneisses (Fig. 2). Crosscutting relationships
demonstrate that tonalitic gneisses are generally older
than foliated tonalitic rocks. However, several generations of foliated tonalitic rocks are present in the
region: (1) volumetrically minor (as recognized) foliated
tonalites within greenstone belts that have yielded ages
>2·9 Ga [e.g. 3·075 Ga Caribou Lake pluton (Davis
et al., 1988); 2·92 Ga southern Obonga tonalite
(Tomlinson et al., 1996)] which is indistinguishable in the
field from younger tonalites; (2) tonalite to granodiorite
bodies with ages in the 2·72–2·709 Ga range. In
addition, spatially associated mafic rafts and dykes were
identified that could be contemporaneous with these
tonalitic units. This study focuses on the pre-2·70 Ga
tonalitic and mafic plutonic units. Post-2·70 Ga highK plutonic units will be described elsewhere (Whalen
et al., in preparation). New U–Pb geochronological
data (see the Appendix for analytical techniques), which
provide further constraints on the ages of lithotectonic
map units, are tabulated in Table 1 and displayed in
concordia diagrams (Fig. 3a and b). Sites of combined
U–Pb zircon and Nd–O samples mentioned below are
1553
JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 8
AUGUST 2002
Another biotite gneiss from this area (site B), which
yielded an >2·69 Ga age for leucosome (U–Pb zircon)
and >2·68 Ga titanite ages, is interpreted as having a
>2·83 Ga emplacement age (V. McNicoll, unpublished
data, 1999). A tonalite gneiss sample collected to the
west (site C in Fig. 2) contains 2·72 Ga igneous zircons
(V. McNicoll, unpublished data, 2000), indicating a range
of ages for tonalite gneisses and illustrating the difficulty
of deducing intrusion age based on fabrics alone.
Tonalite to granodiorite (post-2·77 Ga to
>2·71 Ga)
Fig. 3. U–Pb concordia diagrams, with errors reported at the 2 level.
plotted in Fig. 2 and identified there and in Tables 1
and 2 by the letters A–D.
Tonalitic gneisses (pre-2·83–2·72 Ga)
Tonalitic gneisses occur as lenses and sheets up to tens of
kilometers long, and as smaller xenoliths within younger
plutonic units. Also, foliated to gneissic, coarse-grained
tonalite occurs as homogeneous sheets of 1–5 m scale
within compositionally heterogeneous tonalite gneiss.
Tonalite gneisses contain up to 20% mafic-poor felsic
layers, herein termed leucosome, that form layering of
millimeter to 1·5 cm scale parallel to a grain-scale foliation
(S1). Isoclinal F2 folds are locally overprinted by younger
generations of structures to form complex interference
patterns. Tonalite gneisses are fine- to medium-grained,
plagioclase–quartz–biotite ± hornblende rocks. Biotite
and poikilitic epidote aggregates commonly replace primary hornblende. Accessory minerals include titanite,
allanite, apatite, zircon and opaques.
A body of homogeneous strongly foliated tonalite
within tonalite gneiss (site A in Fig. 2 and Table 2)
contains igneous zircons of 2·774 ± 0·002 Ga and
metamorphic grains of 2·697 ± 0·002 Ga (Davis, 1989).
Homogeneous, foliated, medium- to coarse-grained
sheets of kilometer scale consisting of tonalite with lesser
granodiorite occur throughout the region (Fig. 2) (Percival
et al., 1999b). Mafic mineral contents range from 10 to
20%; hornblende plus biotite occur in samples with
<70 wt % SiO2 and biotite with rare hornblende at
higher silica contents. Ovoid dioritic enclaves (2–5 cm)
are relatively common. Large equant poikilitic epidote is
almost ubiquitous. Accessory minerals include titanite,
allanite, apatite, zircon and opaques.
Biotite tonalite was collected for U–Pb geochronology
south of the Brightsand River shear zone (site D in Fig.
2, Sample PBA98-724/WXP98-17). A linear regression
[mean square weighted deviation (MSWD) = 1·64]
through three multigrain zircon analyses has a lower
intercept at the origin and an upper intercept of 2·723
± 0·002 Ga, which is interpreted to be the crystallization
age of the rock (Fig. 3a, Table 1).
A biotite tonalite from central Seseganaga Lake (site E,
Sample PBA97-37) was also sampled. A linear regression
(MSWD = 1·45) through all five single and multigrain
zircon fractions analyzed has an upper intercept of 2·709
+0·004/–0·003 Ga, which is interpreted to be the age
of the tonalite (Fig. 3b, Table 1). Analysis of a multigrain
fraction of titanite (T2B) resulted in a concordant age of
2·680 ± 0·004 Ga (Fig. 3b, Table 1), interpreted as the
age of cooling below the closure temperature of titanite
(>650–600°C).
Mafic rafts, dykes and intrusions (pre-2·77
to >2·71 Ga)
Mafic plutonic lithologies within the area include fineto medium-grained diabase, gabbro, diorite and quartz
diorite. Hornblende, the predominant mafic mineral, is
commonly accompanied by biotite and opaques. Accessory minerals consist of titanite, apatite and zircon.
These mafic rocks include: (1) older (pre-2·77 Ga to 2·70
Ga?) amphibolite rafts within tonalite gneiss and post-D1
and D2 dykes cutting tonalite gneiss; (2) dykes cutting
foliated tonalite or granodiorite (syn- to post-2·71 Ga).
1554
WHALEN et al.
TONALITIC GRANITOID ROCKS, SUPERIOR PROVINCE
Table 1: U–Pb analytical data from central Wabigoon granitoid rocks
Fraction1
Pb2
Wt
U
(g)
(ppm) (ppm)
206
Pb/ Pb4
208
204
Pb3 (%)
(%)
Pb5
Ages (Ma, ±2)7
Radiogenic ratios (±1, %)6
206
238
Pb/
207
U
235
Pb/
207
U
206
Pb/
206
Pb
238
Pb/
207
Pb/
207
Pb/
U
235
U
206
Pb
Site A: Sample PBA98-724 (WXP98-17): biotite tonalite, Mountairy Lake (UTM: Zone 15, 680728E–5517116N)
A2 (5) el,c
2
122
71
3156
3
0·095
0·5246±0·11
13·576±0·11
0·18770±0·04
2719±5
2720±2
2722±1
C1 (9) eq,b
4
114
63
1828
9
0·051
0·5197±0·10
13·443±0·11
0·18763±0·05
2697±4
2711±2
2721±1
C2 (5) eq,b
4
132
72
1097
17
0·056
0·5120±0·14
13·211±0·16
0·18716±0·06
2665±6
2695±3
2717±2
Site B: Sample PBA97-37: foliated biotite granodiorite, central Seseganaga Lake (UTM: Zone 15, 692887E–5552923N)
A1 (1) st,c
3
107
59
2458
4
0·101
0·4998±0·11
12·704±0·12
0·18435±0·04
2613±5
2658±2
2692±1
B1 (6) el,c
2
282
154
1355
13
0·072
0·5054±0·09
12·867±0·11
0·18464±0·05
2637±4
2670±2
2695±2
D (7) p,c
3
176
98
2587
7
0·089
0·5077±0·09
12·951±0·11
0·18503±0·04
2647±4
2676±2
2699±1
E1 (1) el,i,c
6
51
30
2381
4
0·140
0·5086±0·11
12·986±0·12
0·18518±0·04
2651±5
2679±2
2700±1
E2 (1) el,i,c
9
67
38
3187
6
0·127
0·5104±0·10
13·036±0·11
0·18523±0·04
2658±4
2682±2
2700±1
T2B (45) a,b 266
54
42
494
956
0·578
0·5155±0·12
13·004±0·18
0·18297±0·12
2680±5
2680±5
2680±4
1
T, titanite; all other fractions are zircon. Number in parentheses is the number of grains in the analysis. Morphology:
el, elongate; p, prismatic; eq, equant; st, stubby prisms; a, anhedral. b, light brown; c, colourless; i, inclusions.
2
Radiogenic Pb.
3
Measured ratio corrected for spike and Pb fractionation.
4
Total common Pb in analysis corrected for fractionation and spike.
5
Radiogenic Pb.
6
Corrected for blank Pb and U and common Pb, errors quoted are 1 in percent.
7
Corrected for blank and common Pb, errors quoted are 2.
GEOCHEMISTRY
Major elements
Representative major and trace elements analyses
from 89 analyzed samples are presented in Table 2.
The complete dataset may be downloaded from the
Journal of Petrology Web site at http://www.petrology.
oupjournals.org. Sampling and analytical techniques are
described in the Appendix. It should be kept in mind
that both tonalitic unit sample groups were collected over
an area of 400 km2 (Fig. 2) and probably include samples
from multiple non-consanguineous intrusions.
On a major-element-based lithological classification
diagram (Fig. 4a) tonalite gneiss and foliated tonalite or
granodiorite units plot almost exclusively in the tonalite–
trondhjemite fields. Mafic intrusive samples plot in the
gabbro–diorite and quartz diorite fields. All granitoid
samples are metaluminous to slightly peraluminous; that
is, molar Al/[Al – (Na + K + Ca)] <1·1. Tonalite
gneiss and tonalite or granodiorite units are of low- to
medium-K composition (Fig. 4b), contain high Al2O3
contents (i.e. >15 wt % at 70% SiO2; Fig. 5a) and have
high Na/K (Fig. 6), all characteristics of TTG or TTD
suites (Martin, 1994; Drummond & Defant, 1990). Like
adakites, the tonalitic units range to more calcic compositions than TTGs of Martin (1994) (Fig. 6). However,
unlike adakites but similar to pre-3·0 Ga TTDs (Smithies,
2000), the tonalitic units lack compositions with mgnumbers >47 at SiO2 <65 wt % (Fig. 5b). On an AFM
diagram (not shown), tonalitic units plot in the calcalkaline field. Mafic samples are of medium-K (Fig. 4b),
plot in both the calc-alkaline and tholeiitic fields on an
AFM diagram (not shown) and exhibit mg-numbers of
30–65 (Fig. 5b).
Trace elements
In primordial-mantle-normalized plots (Fig. 7), all but a
few mafic intrusive samples exhibit consistent negative
Nb anomalies; most samples also exhibit negative Ti
anomalies. The magnitude and direction of Ba, Sr, Zr,
P and Eu anomalies are variable. Characteristic rare
earth element (REE) features of the tonalitic units include
the following: (1) a ‘fanning’ at the heavy REE (HREE)
end of the patterns, resulting in slope variation in REE
patterns; (2) although most patterns lack obvious Eu
anomalies, samples with both positive and negative anomalies are present; (3) rocks with higher silica contents
range to lower overall REE abundances than less siliceous
rocks and include more samples with Eu anomalies.
Except for the appearance of positive Ba and Sr anomalies
in a few samples, all tonalite gneiss samples exhibit
relatively similar patterns (Fig. 7a and b). The most
1555
SiO2
1556
99·5
Total
58
36
9
81
Sc
V
Cu
Pb
Zn
Ga
20
0·52
271
Sr
Tl
281
1·50
77
Ba
Cs
Rb
2·30
7
Ni
13
19
0·32
523
636
0·83
39
0·60
45
7
11
41
4
<2
0·03
21
n.d.
896
586
n.d.
97
n.d.
6
7
19
33
6
<2
25
100·4
0·4
0·12
1·35
5·17
3·73
1·43
21
0·44
353
364
1·30
57
1·10
97
8
17
48
9
<2
13
100·1
0·2
0·17
1·47
4·30
3·77
1·31
0·07
2·70
1·40
15·3
0·45
69·0
72
WXP98-
tn gn
0·2
0·14
1·13
4·40
4·13
1·08
0·04
2·20
1·10
15·8
0·40
69·4
18
0·47
481
438
5·90
64
1·30
53
7
9
47
7
<2
7
100·0
5
WXP98-
tn gn
21
0·45
264
297
1·30
68
0·90
56
10
7
36
5
<2
<6
99·6
0·2
0·10
1·71
4·50
3·17
0·98
0·05
2·20
0·90
15·3
0·40
70·1
55
WXP98-
tn gn
22
0·34
408
629
1·10
47
0·70
69
6
14
65
8
<2
17
100·0
0·5
0·17
1·47
4·40
5·05
1·90
0·06
3·10
1·60
16·8
0·62
64·3
59
WXP98-
tn
21
0·60
505
377
6·80
74
0·50
45
7
19
39
6
<2
22
99·4
0·3
0·11
1·15
5·10
4·28
1·26
0·04
2·00
0·70
17·0
0·33
67·1
21
WXP98-
tn
20
0·35
334
290
1·80
50
0·80
51
2
<5
43
7
<2
6
99·3
0·3
0·08
1·15
4·60
4·43
1·25
0·06
2·50
1·30
16·1
0·35
67·2
16
WXP98-
tn
20
n.d.
581
556
n.d.
56
n.d.
24
6
27
44
7
<2
14
100·4
0·4
0·19
1·63
5·20
3·68
1·20
0·04
2·42
0·81
16·8
0·60
67·4
259
PBA97-
tn
21
0·18
354
392
1·20
26
1·40
53
2
9
39
7
<2
9
99·9
0·2
0·09
0·95
4·60
4·51
1·27
0·03
2·40
1·30
16·1
0·34
68·1
17
WXP98-
tn
A
20
0·59
302
283
2·50
68
0·50
50
5
76
29
5
<2
6
99·2
0·3
0·08
1·07
4·70
3·63
0·93
0·04
2·00
1·00
15·5
0·29
69·7
93
WXP98-
tn
24
n.d.
595
335
n.d.
62
n.d.
16
11
7
24
9
<2
18
100·4
0·4
0·10
1·48
5·69
2·82
0·90
0·03
1·41
0·69
16·3
0·32
70·2
37
PBA97-
tn
B
17
0·69
195
723
2·60
92
1·40
75
16
10
25
5
<2
<6
99·0
0·2
0·08
2·35
4·00
2·77
0·63
0·04
2·20
0·40
14·2
0·30
71·8
77
WXP98-
tn
B – di
17
0·69
355
420
4·2
84
1·7
72
7
38
124
27
96
429
99·93
0·9
0·09
1·60
3·10
9·55
7·09
0·16
6·70
2·10
17·5
0·54
50·6
89
18
0·12
140
146
0·15
11
1·4
120
4
86
374
40
49
151
99·81
0·5
0·11
1·09
2·70
10·67
6·18
0·28
9·10
4·10
14·2
1·28
49·6
24
WXP98-
A – di
WXP98-
Group
Group
Mafic intrusions
NUMBER 8
Sn
39
13
Cr
99·7
0·3
0·11
1·38
4·90
0·04
2·02
0·56
17·2
0·39
68·0
372
PBA97-
tn gn
C
Tonalite or granodiorite
VOLUME 43
ppm
0·11
1·72
K 2O
0·3
4·10
LOI
4·03
4·05
CaO
Na2O
P 2O 5
1·23
0·06
1·93
MnO
MgO
0·90
2·00
1·30
3·20
16·6
0·35
Fe2O3
15·7
0·47
FeO
Al2O3
TiO2
67·9
48
37
66·6
WXP98-
WXP98-
Sample:
wt %
tn gn
D
Rock type: tn gn
Site:
Map unit: Tonalite gneiss
Table 2: Representative analyses from the central Wabigoon subprovince
JOURNAL OF PETROLOGY
AUGUST 2002
0·51
U
1557
705
233
F
Cl
110
339
138
0·04
0·17
0·03
0·76
21
<50
346
0·04
0·32
0·03
0·32
0·13
0·71
0·15
1·29
0·72
1·97
11·5
3·14
28·4
14·0
<0·02
2·57
3·6
110
3·4
2·1
372
PBA97-
tn gn
7·8
4·1
122
524
246
0·17
1·00
0·18
1·20
0·44
2·40
0·47
3·20
0·91
4·60
31·0
9·30
85·0
46·0
0·64
11·00
12·4
165
72
WXP98-
tn gn
4·7
3·9
92
446
131
0·11
0·70
0·11
0·72
0·29
1·50
0·30
2·10
0·93
2·80
18·0
5·10
48·0
24·0
0·44
3·90
6·7
159
5
WXP98-
tn gn
C
8·0
3·6
139
427
56
0·14
0·82
0·15
0·83
0·32
1·60
0·28
2·00
0·62
2·70
15·0
4·50
43·0
24·0
0·97
5·60
8·9
124
55
WXP98-
tn gn
6·5
4·4
147
513
113
0·15
0·88
0·15
1·00
0·43
2·20
0·43
3·00
0·93
3·90
22·0
6·00
49·0
25·0
0·37
2·40
10·6
181
59
WXP98-
tn
2·4
2·9
40
468
100
0·06
0·61
0·07
0·54
0·17
1·10
0·26
1·60
0·74
2·40
19·0
5·80
53·0
30·0
0·46
4·50
5·1
118
21
WXP98-
tn
3·9
3·2
56
311
76
0·14
1·10
0·14
1·00
0·36
1·80
0·37
2·00
0·76
2·00
11·0
2·80
24·0
13·0
0·66
1·90
9·3
106
16
WXP98-
tn
Tonalite or granodiorite
75
<50
213
0·08
0·51
0·08
0·58
0·23
1·25
0·26
2·16
0·95
3·85
24·4
7·06
63·6
35·6
<0·02
7·71
6·1
172
3·8
6·2
259
PBA97-
tn
2·8
3·2
50
331
100
0·18
1·20
0·17
1·20
0·41
2·00
0·41
2·40
0·85
2·20
8·1
1·70
12·0
6·0
0·47
0·24
11·2
122
17
WXP98-
tn
A
Gb, gabbro; tn, tonalite; gn, gneiss; qtz, quartz diorite; di, diorite; n.d., not determined; LOI, loss on ignition.
248
S
1·10
0·15
Yb
Lu
0·20
1·00
0·15
Er
0·43
0·08
1·90
0·36
Dy
Ho
Tm
0·09
2·70
0·43
Gd
Tb
1·20
0·61
3·60
0·89
9·4
2·90
28·0
17·0
0·19
0·87
1·6
126
Sm
25·0
7·80
66·0
2·6
3·1
Eu
Nd
Pr
Ce
37·0
8·10
Th
La
8·8
145
Y
Zr
8·6
4·8
48
37
Nb
WXP98-
WXP98-
Sample:
Hf
tn gn
D
Rock type: tn gn
Site:
Map unit: Tonalite gneiss
4·2
3·4
76
562
114
0·11
0·59
0·10
0·63
0·22
1·20
0·23
1·50
0·57
2·10
11·0
3·50
30·0
17·0
0·86
3·60
6·3
110
93
WXP98-
tn
4·3
2·8
28
<50
100
0·05
0·37
0·06
0·48
0·17
0·86
0·15
1·50
0·60
2·24
13·5
3·49
29·9
14·9
<0·02
2·84
4·4
95
37
PBA97-
tn
B
7·6
4·1
60
392
173
0·15
0·84
0·14
0·90
0·35
1·80
0·34
2·30
0·69
3·00
22·0
6·60
63·0
38·0
4·30
13·00
9·6
161
77
WXP98-
tn
B – di
3·5
2·0
145
787
518
0·19
1·10
0·20
1·20
0·46
2·30
0·39
2·30
0·72
2·60
12·0
3·30
26·0
13·0
0·34
1·50
11·1
67
89
5·5
2·1
408
748
668
0·46
3·20
0·48
3·00
1·10
4·90
0·77
4·10
1·10
2·90
9·5
2·10
13·0
5·5
0·39
0·58
29·1
82
24
WXP98-
A – di
WXP98-
Group
Group
Mafic intrusions
WHALEN et al.
TONALITIC GRANITOID ROCKS, SUPERIOR PROVINCE
JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 8
AUGUST 2002
Fig. 4. The various plutonic units plotted on the (a) Q–P major-element-based granitoid rock classification diagram of Debon & Le Fort (1982),
and (b) K2O vs SiO2 diagram, subdivisions of igneous suites based on K2O content from Le Maitre (1989).
dissimilar are two leucosome-rich samples (shown with
dashed lines in Fig. 7b), which have marked positive Sr,
Zr and Eu anomalies; mesosome-rich samples collected
from the same outcrops contain higher normalized element concentrations and lack such anomalies. These
features suggest that the ‘leucosome’ samples actually
represent felsic cumulates. Although they exhibit a greater
range in elemental abundance, patterns of tonalite or
granodiorite samples (Fig. 7c and d) overlap and closely
parallel those of tonalite gneisses, but a higher proportion
of samples has marked positive Sr and Zr anomalies.
Geochemical characteristics of these tonalitic rocks, including a limited range in and low Rb/Sr values (0·04–
0·4), no correlation between SiO2 and Rb/Sr (Fig. 5c),
high Sr contents and variable Eu anomalies (Fig. 8b and
c), indicate that these rocks are not products of significant
degrees of fractional crystallization, for in such compositions, plagioclase with quartz would be the predominant fractionated minerals.
On the basis of their normalized trace element patterns,
mafic intrusive samples have been divided into two
groups. Group A samples are large ion lithophile element
(LILE) and light rare earth element (LREE) depleted
relative to group B samples and also lack the pronounced
negative Nb and Zr and positive Sr exhibited by group
B samples (Fig. 7e and f ). Field relationships indicate
that both groups include older (>2·74 Ga) and younger
(>2·70 Ga) samples.
Elemental ratios with petrogenetic significance are
plotted in Figs. 5 and 8. Low Rb/Sr (<0·15) values,
elevated Sr/Y (>40), (La/Yb)CN (>12) and Sr (>300 ppm)
values, and (Eu/Eu∗)CN [1 have been suggested to be
characteristics of TTG or TTD suites, interpreted as
‘slab melts’ (see Martin, 1986; Drummond & Defant
1990). Although tonalite unit samples exhibit variations
that transect many of these TTG or TTD identification
criteria, the vast majority of samples do exhibit compositional characteristics of ‘slab melts’.
Nd and O isotopes
A combination of Nd and O isotopes were employed
because of the ability of Nd isotopes, in spite of subsequent
alteration or deformation, to constrain the crustal residence time of granitoid protoliths (see de Paulo, 1988)
1558
WHALEN et al.
TONALITIC GRANITOID ROCKS, SUPERIOR PROVINCE
Fig. 5. Harker variation diagrams for (a) Al2O3, (b) mg-number ][MgO/
(MgO + FeOtotal)], and (c) Rb/Sr for the different plutonic units. In
(a) the line dividing high- (>15 wt %) and low-Al tonalite suites at
70 wt % SiO2 is from Barker (1979); shown in (b) is the field of
Cenozoic adakite compositions with low SiO2 (<65 wt %) plus high
mg-number (>47) that is absent in pre-3·0 Ga TTG suites (from
Smithies, 2000). Also shown: (i) the data trend for a mantle or M-type
oceanic arc plutonic suite from New Britain (NB) [data of Whalen
(1985)]; (ii) average compositions from Drummond et al. (1996) for
various high-La felsic compositions, including post-Archean adakite
plus high-Al tonalite–trondhjemitic–dacite (n = 394) (PAd), Cenozoic
adakite (n = 140) (CAd), Archean high-Al tonalite–trondhjemitic–dacite
(n = 174) (TTD) and Andean central volcanic zone (n = 55) (CVZ),
all plotted with an × symbol.
and of O isotopes to distinguish granitoids that have
received input from supracrustal materials (see Longstaffe,
1979). Nd and O isotopic data from a subset of 14
tonalite samples are tabulated in Table 3. Mean TDM
ages for tonalite gneiss (2·9 ± 0·1 Ga) and tonalite or
granodiorite (2·9 ± 0·1 Ga) units and their Nd(2·7 Ga)
values, −0·4 ± 1·1 and +0·2 ± 1·4, respectively, are
essentially identical. Nd(T) values calculated at 2·774
and 2·712 for tonalite gneiss and tonalite or granodiorite
units, respectively, are plotted against age in Fig. 9, along
with comparison fields for potential crustal and mantle
source components from the Western Superior province.
On the basis of the estimated Nd range of the late
Archean depleted mantle (+2·8 to +3·2), most of the
plutonic rocks were derived, or received some input,
from LREE-enriched older crustal material, such as
model 3·2 Ga Wabigoon crustal material (Henry et al.,
1998) or Wabigoon 3·2–3·5 Ga Cariboo Lake tonalitic
crust (Tomlinson, 2000). Significant Nd(T) dispersion
within individual units [tonalite gneiss (3·8 epsilon units)
and tonalite or granodiorite (5·1)] indicates derivation
from heterogeneous sources or mixing of different components (see below). Nd data reported previously by
Tomlinson & Percival (2000) from tonalitic units in
proximal portions of the central Wabigoon plutonic domain (filled circles joined by lines in Fig. 9) overlap closely
results obtained in this study.
Variations in 18O (VSMOW) vs Nd(T) are presented
in Fig. 10, along with 18O ranges for potential Archean
source components. Unit ranges are tonalite gneiss +7·4
to +8·3‰, and tonalite or granodiorite +7·1 to +7·7‰,
the overall average 18O value being +7·5‰. The average 18O (quartz–whole rock) is 1·3‰ with a range
of 1·1–1·8‰ (Fig. 10, Table 3); these separations are
compatible with whole-rock 18O values that describe
magmatic equilibrium. Independent corroboration of this
is provided by zircon 18O results reported by King et
al. (1998) from Wabigoon tonalites, which, based on
18O (whole rock-zircon) values (Valley et al., 1994),
equate to an average whole-rock value of 7·3‰, with a
range of 6·7–8·2‰. Very similar whole-rock 18O and
18O (quartz–whole-rock) values to those found in this
study have been reported previously from Western
Superior tonalitic rocks (e.g. Longstaffe, 1979).
All units have 18O values >+7‰ and most overlap
the Superior Province upper-crust range. As noted above,
geochemical characteristics of these tonalitic rocks indicate that these elevated 18O values do not result from
high degrees of fractional crystallization of depleted
mantle- or mantle wedge-derived magmas. Rather, they
require input from higher 18O (infracrustal or supracrustal?) materials during their genesis (see Longstaffe et
al., 1981; Longstaffe & Gower, 1983; Sheppard, 1986).
Although tonalite gneiss and tonalite or granodiorite
units do not show any good correlations of Nd(T) or
18O (VSMOW) with each other (Fig. 10) or whole-rock
K2O and Nd (Fig. 11), tonalite or granodiorite unit
samples tend to exhibit lower Nd(T) and 18O values
with increasing K2O and Nd. Variations in Figs 10 and
11 could, largely or in part, reflect contrasts in source
materials and/or in the roles played by fractionation or
partial melting processes, or could primarily reflect mixing
processes. If variations relate primarily to mixing processes, then these plots provide little support for significant
depleted mantle contributions to tonalitic units but rather
suggest that mixing processes were volumetrically predominated by various crustal components, including 3·2
Ga or older low-K but LREE-enriched infracrustal
materials (WCM in Figs 10 and 11) and more juvenile
supracrustal materials (WSS in Figs 10 and 11). It should
1559
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NUMBER 8
AUGUST 2002
Fig. 6. Na–K–Ca diagram for the various plutonic units. The shaded field is that for the Archean tonalite–trondhjemitic–granodiorite (TTG)
association (from Martin, 1994); the calc-alkaline and trondhjemitic differentiation trends are from Barker & Arth (1976). Symbols, comparison
fields and averages are as defined in Fig. 5.
be noted that the field enclosing older tonalitic gneiss
unit samples and Superior Province upper crust (Figs 10
and 11) also encompasses much or all of the variation
within the younger tonalite or granodiorite unit, suggesting such crustal material as a possible source component (see below).
PETROGENESIS
Mafic rocks
Both mafic groups A and B (Fig. 7e and f ) include
samples that pre-date and overlap in age with the tonalitic
gneiss unit as well as samples that are roughly contemporaneous with or, slightly post-date, the tonalite–
granodiorite unit. Potentially, these mafic rocks could
help constrain the tectonic environment during which
tonalite units were generated and could also represent
samples of mantle-derived magmas that played an important role in tonalite petrogenesis. Other than group
A mafic samples exhibiting negligible to only slight negative Nb anomalies (Fig. 7e), the group A pattern resembles that of high-Al basalt from an oceanic arc
(see Fig. 13, below). The group B mafic pattern, being
somewhat more LILE enriched and HREE depleted,
occupies a position intermediate between that of calcalkaline andesite (CAA) and high-Al arc basalt. Therefore,
the mafic group B pattern and, to a lesser extent, the
group A pattern are compatible with these rocks being
mafic components of andesite–dacite–rhyolite (ADR) arctype suites. ADR suites have been interpreted, not uncontroversially, to form under relatively low-pressure,
dry melting and fractionation conditions via partial fusion
of the mantle wedge above the Benioff zone, fluxed
by hydrous fluids derived from subducted lithosphere
(Drummond et al., 1996).
Tonalitic units
Average mantle-normalized trace element patterns for
the tonalite gneiss and tonalite or granodiorite units (Fig.
12b) closely overlap or parallel the field for adakites
and high-Al tonalite–trondhjemite–dacite (TTD) suites.
Three petrogenetic models, which are not mutually exclusive, are evaluated below for the tonalitic units, followed by a discussion of the possible impacts of magma
modifying or evolutionary processes.
Model 1—partial melts of subducted
oceanic crust
In this and model 2, the geochemical characteristics of
TTD or TTG suites are interpreted as directly reflecting
partial melting of amphibolitic sources containing garnet.
According to Drummond et al. (1996), minimum P–T
conditions for slab melting leaving garnet amphibolite
residues are 1·5 GPa (50 km depth) and [800°C but
range up to 2·0 GPa (70 km depth) and 950°C for melt
with an eclogitic residue (Sen & Dunn, 1994). The ‘onestage’ slab melt model for TTG or TTD petrogenesis is
judged to be not generally applicable to central Wabigoon
tonalitic rocks for the following reasons:
(1) Nd isotopic data (Fig. 9) indicate variable and
often volumetrically significant input from older crustal
materials to these tonalitic rocks. On the basis of studies
of adakite petrogenesis in recent arcs (e.g. Yogodzinski
et al., 1995; Kepezhinskas et al., 1997), it seems unreasonable to presume that such volumetrically significant
1560
WHALEN et al.
TONALITIC GRANITOID ROCKS, SUPERIOR PROVINCE
Fig. 7. Primordial mantle-normalized extended-element plots for
samples from the various plutonic units. The normalizing values of
Taylor & McLennan (1985) have been used. In (b) leucosome-rich
samples are shown with dashed lines, and in (c) and (d) dotted lines
are used to distinguish rocks that differ significantly in their patterns.
crustal input could have been supplied via a slab-derived
sediment-melt component.
(2) Although tonalitic units of this study exhibit large
ranges in Cr/Ni (>0·4–0·9) and mg-numbers (19–54),
they, like pre-3·0 Ga Archean TTG series examined
by Smithies (2000), lack the high mg-number–low SiO2
compositions characteristic of Cenozoic adakites (Fig.
5b). According to Smithies (2000), this indicates that the
tonalitic magmas have not interacted with peridotite
Fig. 8. Plot of (a) (La/Yb)CN (chondrite normalized), (b) Sr ppm, and
(c) (Eu/Eu∗)CN vs Sr/Y for samples from tonalitic plutonic units; symbols
as in Fig. 4. Shown are fields for ‘slab melts’ and non-‘slab-melts’ based
on compositional criteria from Drummond & Defant (1990) and Defant
& Drummond (1990). Also shown are Rayleigh fractionation (FC) and
equilibrium partial melting (PM) vectors for removal of or separation
from residual single mineral phases; mineral weight percent values are
indicated beside the vectors and, on some, 1% or 10% subdivisions
are shown. Plg, plagioclase; Hbl, hornblende; Gnt, garnet; All, allanite.
The partition coefficients of Martin (1987) have been employed in the
calculations. The MORB composition is from Sun & McDonough
(1989).
(mantle wedge), an improbable feature for melts generated beneath the wedge from a subducting slab, even
if, as suggested by Martin (1994), flat subduction prevailed
in the Archean. These features also rule out tonalite
magma generation via a recycling process centred within
the mantle wedge, as has been postulated in adakite
petrogenesis (Yogodzinksi et al., 1995; Kepezhinskas et
al., 1997).
(3) Because of low-temperature sea-floor alteration,
the pillow basalt plus dyke layers of oceanic crust, as
sampled in ophiolites, exhibit highly variable 18O values
of +5 to +10‰ (Muehlenbachs, 1986). The very restricted variation in 18O values exhibited by the tonalite
units (Fig. 10) would seem incompatible with an origin
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JOURNAL OF PETROLOGY
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NUMBER 8
AUGUST 2002
Table 3: Sm–Nd and O isotopic data from the central Wabigoon Subprovince
Sample
SiO2
(wt %)
Age
18O‰
(Ma)
WR
18O‰
Nd(T)
Qtz
Nd
TDM
143
Nd/
(2·7 Ga)
(Ga)
144
Nd
±2 SD
147
Sm/
Nd
Sm
144
Nd
(ppm)
(ppm)
Tonalite gneiss
WXP98-37
66·6
2774
7·4
+2·63
+1·61
2·763
0·510828
8
0·0905
24·26
3·63
WXP98-48
67·9
2774
8·3
+0·59
−0·54
2·889
0·510529
7
0·0799
8·88
1·17
PBA97-372
68·0
2774
8·0
+0·49
−0·44
2·915
0·510918
9
0·1014
11·13
1·87
WXP98-72
69·0
2774
8·3
+0·45
−0·59
2·905
0·510693
7
0·0892
30·50
4·50
WXP98-5
69·4
2774∗
7·7, 7·6
−1·13
−2·09
3·024
0·510767
6
0·0976
17·97
2·90
WXP98-55
70·1
2774
7·5
+0·70
−0·23
2·898
0·510912 15
0·1005
14·96
2·70
+0·10
−0·05
2·885
0·510921
7
0·1005
14·95
2·48
8·8
Foliated tonalite or granodiorite
WXP98-59
64·3
2712
7·7
−0·99
−1·13
2·970
0·510922 10
0·1036
22·10
3·79
WXP98-21
67·1
2712
7·6
+1·04
+0·83
2·808
0·510628 11
0·0815
18·33
2·47
WXP98-16
67·2
2712
7·2
+2·43
+2·32
2·713
0·511523
11
0·1275
9·22
1·94
PBA97-259
67·4
2715∗
7·8
+0·28
+0·11
2·858
0·510699
8
0·0875
24·80
3·59
WXP98-17
68·1
2712
7·1
+0·34
+0·28
3·023
0·511978
5
0·1589
11·59
3·05
WXP98-93
69·7
2712
7·3
+0·56
+0·45
2·853
0·511000
15
0·1035
11·38
1·95
PBA97-37
70·2
2709∗
7·3
+0·68
+0·54
2·849
0·511048
4
0·1059
13·21
2·32
WXP98-77
71·8
2712
7·5
−3·14
−2·93
3·067
0·510630 12
0·0924
20·27
3·10
9·0
8·5
∗U–Pb zircon ages; other ages assumed (see text).
WR, whole rock; Qtz, quartz; TDM, depleted mantle model age [based on DePaolo (1988)].
via direct partial melting of the upper portion of a slab,
unless some type of bulk 18O homogenization process
was involved.
Model 2—crustal partial melts of
amphibolitic protoliths
Fig. 9. Nd–age (Ma) plot for tonalitic plutonic units with symbols as
in Fig. 4. The Nd(T) values were calculated at 2·774 and 2·712 for
tonalite gneiss and tonalite or granodiorite (tn/gd) units; some data
points have been shifted horizontally to reduce data overlap. Central
Wabigoon granitoid domain data of Tomlinson & Percival (2000),
assigned to units used in this study and Nd(T) values recalculated
accordingly, are also plotted (Χ joined by lines); rock type abbreviations
as in Table 3 plus anorthosite (an). Also shown are: (i) the age–Nd
evolutionary paths for tonalite gneiss crust and tonalite sample WXP9877 of this study; (ii) model 3·2 Ga Wabigoon continental crustal endmember (WCM) of Henry et al. (1998); (iii) 3·16 and 3·48 Ga Cariboo
Lake tonalite crust (CL Tn) [data of Tomlinson (2000)].
Variations of this model have been proposed recently for
various post-Archean occurrences of ‘TTG-like’ suites
(e.g. Barnes et al., 1996; Petford & Atherton, 1996; Wolde
et al., 1996; Beard, 1998; Tate & Johnson, 2000). In these
studies, estimates of crustal thickness required to generate
‘slab-like’ melts and garnet amphibolite or eclogite residues within the lower crust range from 27 km (Tate &
Johnson, 2000) to >40 km (Petford & Atherton, 1996),
with the lower end of the range involving wet melting
of metabasaltic crust. The availability of appropriate
amphibolitic protoliths within the lower crust is mainly
addressed by invoking recently underplated ( juvenile)
basaltic crust. Such magmatically underplated juvenile
sources could be equally viable protoliths for Archean
TTG magmas. An attractive alternative is provided by
the early Archean continental keel formation model
proposed by de Wit (1998). In this model, the early
continents formed via continuous under-stacking of slices
of hydrated, buoyant upper oceanic lithosphere from
1562
WHALEN et al.
TONALITIC GRANITOID ROCKS, SUPERIOR PROVINCE
Fig. 10. 18O‰ (VSMOW)–Nd(T) plot for tonalitic plutonic units
with symbols as in Fig. 4; Χ connected by tie lines show quartz 18O
data with coexisting whole-rock values. Potential crustal source and/
or end-member components shown are: (i) model 3·2 Ga Wabigoon
continental crustal material (WCM); (ii) sedimentary material (WSS).
The Nd(T) is from Henry et al. (1998). The 18O values for WCM are
from the Western Superior average crustal value (+7·8‰) (Shieh &
Schwarcz, 1978) plus the 1 SD (±0·5‰) value from Western Superior
granitoid rock zircon (King et al., 1998) based on the finding of Shieh
& Schwarcz (1978) that Archean province crustal and granitoid averages
had the equivalent 18O values. The 18O values for sedimentary
material are based on the average plus 1 SD of non-tuffaceous sediment
data of Longstaffe & Schwarcz (1977). Potential mantle-like components
are: depleted mantle (DM) and mantle wedge (MW) derived materials
[18O ranges based on Eiler et al. (2000)] and subducted slab [hydrated
oceanic crust 18O‰ values from Muehlenbachs (1986)].
which lower anhydrous ultramafic layers had been delaminated. A similar under-stacking model involves shallow to flat type subduction, which may have prevailed
during the Archean (Martin, 1994), combined with frequent accretion events involving oceanic plateau and arc
materials, resulting in subduction zone clogging followed
by outstepping. The P–T conditions deep in the underthrust stack would probably readily attain those proposed
in previously cited models for crustal TTG generation.
This model may be able to account for the differences
noted previously between ‘typical’ TTG (Martin, 1994),
adakites (Drummond et al., 1996; Martin, 1999) and
many tonalites of this study. The range to higher Ca and
Sr contents (Figs 6 and 8b) may indicate that some
central Wabigoon subprovince tonalites formed under
slightly greater pressure conditions than those for ‘typical’
Archean TTG. Variable HREE depletion (Fig. 7a–d)
could reflect mineralogical differences in the source, with
the least steep patterns representing melts from garnetpoor amphibolites at lower pressures and more HREEdepleted patterns reflecting increasing pressure and proportion of garnet in the source (see Fig. 8a). The presence
Fig. 11. K2O wt % (a, b) and Nd ppm (c, d) vs Nd(T) and 18O values
for tonalitic units; the tonalite gneiss field is outlined with a dotted line.
Symbols are as in Fig. 4. End-members WCM, DM and WSS have
isotopic compositions as in Fig. 10 and bulk compositions based
on upper Archean crust, primitive mantle and Archean graywacke
compositions, respectively, of Taylor & McLellan (1985). Shown are
modeled mixing lines between depleted mantle and hypothetical crustal
end-member, WCM (3·2 Ga Wabigoon crust), from Henry et al. (1998).
Fig. 12. Primordial mantle-normalized extended element plots for:
(a) averages for various high La/Yb suites or types: Archean Hi-Al
trondhjemite–tonalite–dacite (TTD), post-Archean adakite plus Hi-Al
TTD, Cenozoic adakite (Cen. Ad) and the central volcanic zone of the
Andean arc (CVZ), all from Drummond et al. (1996); (b) averages for
the various tonalitic units, as shown in Fig. 7. Normalizing values are
from Taylor & McLennan (1985).
of negative Eu anomalies in some samples could reflect
plagioclase in the source, whereas many tonalite or
granodiorite unit samples with positive or no Eu anomalies (Fig. 8c) accompanied by positive Sr anomalies
(Figs. 7c and d) could reflect melting at pressures above
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Fig. 13. Primordial mantle-normalized extended element plots for the
average mafic plutonic units, as shown in Fig. 7e and f. Also plotted
are representative volcanic rock compositions, including calc-alkaline
andesite (SiO2 = 58 wt %) (caa) ( Jockum & Jenner, 1994), oceanic
arc high-Al basalt [sample IA-10 from Basaltic Volcanism Study Project
(1981)] and oceanic arc gabbro (Whalen, 1985). Normalizing values
are from Taylor & McLennan (1985).
the plagioclase stability field (>15 kbar). Also, as argued
by Smithies (2000), the lack of dispersion to high mgnumbers at low silica levels in Archean TTG suites
indicates an absence of mantle wedge interaction and
supports a lower-crustal origin. If the tonalitic magmas
were all generated by partial melting of crustal amphibolitic sources, then it must be inferred that this source
occupied a significant crustal section, possibly from the
base of the crust to mid-crustal levels.
Melting of amphibolitic protoliths alone cannot explain
the enriched Nd isotopic characteristics of the tonalitic
units (Fig. 9), as non-LREE-enriched amphibolites would
remain isotopically juvenile, no matter how long they
resided in the lower crust. The importance of protoliths
consisting of interleaved metasedimentary and metavolcanic rocks has been recognized in some models of
Phanerozoic tonalite petrogenesis (e.g. Barnes et al., 1996).
Similarly, perhaps shallow subduction during the Archean would have facilitated underthrusting of older
crustal materials deposited within the trench, materials
which during steep subduction are scraped off to form
accretionary-wedge deposits. According to Hamilton
(1998), such wedge deposits have not been identified in
Archean cratons. Isotopically enriched crustal materials,
probably including sediments derived from old TTG
sources, imbricated within the underthrust stack of
oceanic material, could then have contributed to the
TTG magmas during the partial melting process. An
alternative or additional source for older crustal contributions to TTG magmas would be via contamination
during ascent through the crust [e.g. assimilation–
fractional crystallization (AFC) model of DePaolo (1981);
see below] During the lower- to mid-crustal melting
process, fluid exchange and O-isotope homogenization
NUMBER 8
AUGUST 2002
between interleaved amphibolites and sediments (see
Holk & Taylor, 1997) may have been capable of producing the restricted elevated 18O range exhibited by
the tonalitic units (Fig. 10).
Model 2 differs from model 1 in that, in the absence
of a subducting slab, it lacks a conveyor belt supply
and removal process for source materials from which
15–30 wt % partial melts can be extracted. Where TTG
magmatism is episodic, as it is within the Paleozoic
fringing arc system preserved in the Klamath Mountains,
it can reflect separate tectonic events during which amphibolitic crust was underthrust and partially melted
(Barnes et al., 1996). If TTG magmatism is long lived, a
continuing process of melt-depleted protolith removal
and fertile amphibolitic material underthrusting may be
required. As garnet-rich residues of TTG magma genesis
would be denser than surrounding lower-crustal or underlying upper-mantle rocks, negative buoyancy would cause
bodies of even moderate size to sink through typical
middle and lower crust and into the mantle at rates of
several kilometers per million years (Arndt & Goldstein,
1989; Glazner, 1994). Decoupling of residues could be
facilitated by shallow-dipping tectonic contacts, continuing subhorizontal underthrusting, and a lack of cohesiveness between adjacent under-stacked slices. These
processes could accommodate the removal of the large
volumes of dense residues needed to balance the production of upper-crustal TTG suites within Archean
cratons, calculated by Ireland et al. (1994) to amount to
1·5–3 times the volume of the entire crust and 5–7 times
the volume of mafic lower crust currently present in
Archean cratons.
Model 3—melting of tonalitic crust
Experimental studies (e.g. Rutter & Wyllie, 1988; Skjerlie
et al., 1993; Singh & Johannes, 1996) have demonstrated
that fluid-absent partial melting of tonalitic compositions
under lower- to middle-crustal conditions ( P =
10–20 kbar and T = 850–1000°C) can generate liquids
that, with increasing P and rising T, vary from granitic
to tonalitic in composition. In these studies, melt fractions
of >20–35 vol. % were obtained from protoliths containing >20% hydrous minerals. If magmas extracted
from TTG protoliths entrain with them restite (mainly
plagioclase, quartz, pyroxene and garnet), then their bulk
composition will increasingly approximate or ‘image’ that
of the protolith [compare the restite model of Chappell
et al. (1987)]. Heat for partial melting could be supplied
either by crustal thickening or, as is more likely, heat
transfer, without significant mass transfer, from injection
or underplating of mantle- or mantle wedge-derived
mafic magma (see Huppert & Sparks, 1988), of which
mafic group A and B rocks may represent samples.
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TONALITIC GRANITOID ROCKS, SUPERIOR PROVINCE
However, the earliest TTG crust would need to have been
generated via some other process and TTG petrogenesis
could not be attributed to one encompassing model. This
model can account for both the bulk geochemical and
Nd–O isotopic features of tonalitic units because melt–
restite mixtures would mirror their protoliths isotopically,
i.e. isotopic compositional ranges (Figs 10 and 11) would
result from isotopically diverse or heterogeneous sources,
not mixing. Also, the close geochemical and isotopic
correspondences between pre-existing tonalitic crust (see
tonalite gneiss range and fields in Figs 10 and 11) and
younger tonalite or granodiorite units is consistent with
bulk recycling of this material.
Magma-modifying processes
The first two models have a difficulty in easily accounting
for the Nd–O isotopic signatures of the tonalitic units
(Figs 10 and 11) and the significant dispersion that these
rocks show in the geochemical criteria used to identify
‘slab-melts’ (Fig. 8). Modification of TTG-type magmas
during melting, assimilation, fractional crystallization, or
AFC (see DePaolo, 1981) processes could help explain
these features. If, as seems likely, pre-existing TTG crust
is assimilated by younger tonalitic magmas, significant
volumes can be processed, accompanied by modification
of magma Nd and O isotopic signatures, without greatly
changing bulk geochemical characteristics. Another
magma-modifying option is that, during partial melting
driven by heat supplied from mafic magma, heat transfer
is accompanied by mass transfer and bulk assimilation
(see Huppert & Sparks, 1988). The resultant magma will
geochemically and isotopically diverge from restite +
partial melt mixtures according to the mass and composition of magma transferred.
Trace element modeling presented in Fig. 8 illustrates
the effects that various minerals could produce during
both fractional crystallization and partial melting. Average MORB is also plotted to help illustrate bulk assimilation effects. In general, most geochemical dispersion
shown by tonalitic units in ‘slab-melt characteristics’
could probably be accounted for by crystal fractionation
and/or AFC processes from an initial magma with TTGtype characteristics. Partial melting to produce TTGtype magmas, however, appears to require partial melting
of amphibolitic protoliths with residual garnet (see Martin, 1987, 1994, 1999) and/or partial melting of TTG
crust. Fractionation of allanite, a common accessory
mineral in tonalitic units, plus hornblende, may be able
to help explain the positive Eu anomalies, accompanied
by elevated Sr/Y values, in some tonalite samples (Fig.
8c), possibly negating the need for plagioclase-free eclogitic residues. Additional factors that could help explain
the variations in Fig. 8 might be variability of parental
magmas as a result of heterogeneous sources and variable
degrees of partial melting.
Discussion
It is not easy to rule out any of the above options totally,
let alone suggest a single generally applicable model for
Archean TTG petrogenesis. This provides an important
insight into TTG genesis for, like granites (senso stricto)
(e.g. Whalen et al., 1987; Sylvester, 1994), multiple processes can lead to formation of similar composition products. Hence, each example needs to be critically evaluated
within its individual overall context. Nevertheless, model
1, the ‘single-stage’ slab melt model, would appear to
be the least tenable model for Wabigoon subprovince
tonalitic units. As these tonalitic units clearly require a
significant, if not predominant, crustal role, the magmas
were probably mainly generated within the lower crust
via melting of amphibolitic sources (model 2) and/or
melting of TTG crust (model 3).
In many regards, petrogenesis of Wabigoon subprovince tonalitic units may have been very similar
to models proposed recently for post-Archean ‘TTG’
occurrences. These invoke a number of tectonic scenarios:
(1) arc-rifting and mafic magma intrusion, inducing hybridization and melting of mafic arc crust (Beard, 1998);
(2) ophiolite underthrusting, crustal overthickening and
emplacement of mafic magmas inducing melting of basaltic ophiolitic crust (Barnes et al., 1996); (3) basaltic
underplate addition as a result of lithospheric delamination followed by underplate partial melting within
a dynamically thickening crustal environment (Petford &
Atherton, 1996); (4) mafic underplating, arc–arc collision,
lithosphere overthickening and lower-crustal underplate
melting facilitated by continual mafic magma intraplating
(Tate & Johnson, 2000). Common features of these
models include a continental arc setting, no direct link
to slab subduction but rather a link to overthickening of
mafic crust, and magma modification via magma-mixing,
fractionation and/or AFC process. Thus, although Wabigoon TTG petrogenesis probably does not directly
reflect contemporaneous slab melting, it probably involved multistage granitoid generation processes similar
to those documented within Mesozoic to Cenozoic continental margin arcs (e.g. Brandon & Smith, 1994; Miller
& Wooden, 1994; Barnes et al., 1996; Petford & Atherton,
1996). Such arcs are underlain by thick crustal sections
that facilitate voluminous granitoid batholith formation,
in contrast to the thinner crustal substrate beneath primitive oceanic arcs that are characterized by only small
scattered plutons (e.g. Mason & McDonald, 1978;
Whalen, 1985).
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NUMBER 8
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CONSTRAINTS ON TECTONIC
EVOLUTION OF THE WABIGOON
SUBPROVINCE
Discrete magmatic episodes at 2·93, 2·77 and 2·72–2·67
Ga are recorded within the study area. The oldest, poorly
geologically constrained TTG-type magmatic event, represented by 2·93 Ga tonalite south of the Obonga belt
(Fig. 2), was not examined in this study. The second
magmatic episode, represented by the heterogeneous
tonalite gneiss unit, includes >2·83, 2·774 and 2·723 Ga
components. As >2·77 Ga and <2·73 Ga components
are indistinguishable, except by geochronology, their
relative volumetric importance and distribution are not
known. Discussions above favoured formation of both
>2·77 and <2·73 Ga TTG magmatic episodes within a
continental arc-type setting, via deep crustal melting
of amphibolitic sources and/or anatexis of pre-existing
(>2·93 Ga?) TTG crust. Mafic rocks that may overlap
in age with the >2·77 Ga tonalite gneiss component
exhibit both fairly primitive tholeiitic non-arc to arc-type
and more evolved arc-type characteristics (Fig. 13). The
>2·77 Ga TGG and mafic magmatism may be related to
both the 2·78–2·765 Ga continental-type North Onaman
sequence in the central Onaman–Tashota belt, eastern
Wabigoon (Tomlinson et al., 2000) and the similar age
(2·775 Ga) juvenile oceanic plateau-like Fourbay assemblage of the SSLB (Fig. 1) (Sanborn-Barrie & Skulski,
1999). These sequences may be related within a plumedriven igneous province that produced basalts in the
oceanic regime, and basalt–dacite–tonalite through mafic
under- and intra-plating, accompanied by anatexis of
2·93 Ga TTG crust, within the continental margin (Fig.
14a).
On the basis of available geochronology, a third TTGtype magmatic event extended at least from 2·722 to
2·709 Ga. Between this and the >2·77 Ga TTG magmatism, there was an intervening non-magmatic period
and two deformation events (D1 and D2). This interval
may reflect an accretion or docking event, possibly involving some of the juvenile oceanic and arc materials
within the southern portion of the SSLB. The widespread
and voluminous 2·722–2·709 Ga tonalitic magmatism,
and geochemical evidence for its generation by complex
interaction of mantle- and crust-derived materials, suggest
a continental arc setting (Fig. 14b). This is analogous
with modern Cordilleran environments (see Parada,
1990; Miller & Wooden, 1994), which include examples
of ‘TTG-like’ magmatism (e.g. Barnes et al., 1996; Petford
& Atherton, 1996; Beard, 1998; Tate & Johnson, 2000).
Supporting such an environment are the primitive to
evolved calc-alkaline arc-type compositions exhibited by
mafic rocks (Fig. 13), which overlap in age with this third
TTG event.
Fig. 14. Schematic cross-sections illustrating temporal evolution of the
central Wabigoon subprovince and possible linkages with oceanic
assemblages of the western Wabigoon. (a) Impingement of a plume
head at 2775 Ma produces juvenile oceanic plateau magmatism in the
western Wabigoon and induces melting of amphibolitic crust to produce
tonalites in the central Wabigoon. (b) Arc magmatism in the central
Wabigoon from >2722 to 2709 Ma yields TTG plutons from thick
mafic lower arc crust.
IMPLICATIONS FOR GRANITOID
PETROGENETIC PROCESSES IN THE
ARCHEAN
Evidence has been presented that crustal recycling played
a significant role in the petrogenesis of Neoarchean
TTG or TTD composition plutonic units in the central
Wabigoon subprovince. It has been suggested that TTG
or TTD ‘slab-melt signatures’ can also be formed directly
by melting of appropriate lower- to middle-crustal
sources, including amphibolites and pre-existing TTG
crust. There is no need for TTG or TTD formation to
be directly tied to subduction. This is contrary to popular
models of Archean granitoid petrogenesis, in particular
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TONALITIC GRANITOID ROCKS, SUPERIOR PROVINCE
that voluminous TTG or TTD suites represent direct
products of slab melting in an arc setting (see Martin,
1986, 1994, 1999; Drummond & Defant, 1990). Clearly,
the relevance of the ‘TTG or TTD’ classification for
Archean granitoid suites should be re-evaluated. Designation of voluminous spatially associated tonalitic to
granodioritic rocks found within most Archean cratons
as belonging to ‘TTG or TTD suites’ may conveniently,
but mistakingly, ‘pigeon-hole’ temporally and genetically
unrelated rocks and perhaps assume an incorrect petrogenetic or tectonic model. It should be borne in mind
that there is a paucity of well-documented post-Archean
examples of direct derivation of voluminous granitoid
magmas from either the depleted mantle or other mantle
sources or precursors. Therefore, its seems intuitively
correct that, unless good evidence to the contrary is
provided, granitoid petrogenesis within Archean cratons
should not be interpreted any differently than in postArchean terranes. In these terranes, it is generally recognized that arc plutonism, particularly within continental arcs, involves crustal recycling (frequently
juvenile crust) through processes such as mafic magma
influx, hybridization and anatexis of hybridized crust
followed by further magma evolution via AFC and
MASH processes (e.g. Brandon & Smith, 1994; Miller
& Wooden, 1994; Whalen et al., 1997, 1998). This
conclusion is in keeping with a significant body of evidence that supports substantial rates of continental crustal
recycling as far back as the earliest Archean (see de Wit,
1998).
ACKNOWLEDGEMENTS
Visits and discussions with the Western Superior NATMAP team are appreciated. D. Davis shared unpublished
geochronological information. West Caribou Air Services
(Savant Lake) provided safe and efficient fixed-wing support. F.J.L. thanks Kim Law and Li Huang for assistance
in the stable isotope laboratory, and NSERC for financial
support. W. Davis and M. Hamilton provided helpful
critical input on earlier versions of this paper. Journal
reviews by C. Barnes, C. Frost, H. Rollinson and H.
Smithies greatly improved the manuscript. This paper is
Geological Survey of Canada Contribution 2001059.
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VOLUME 43
APPENDIX: SAMPLING AND
ANALYTICAL TECHNIQUES
About 4–5 kg of unweathered chips were collected for
geochemistry in addition to a hand specimen. In some
tonalite gneisses, where the scale of layering allowed,
multiple compositional components were sampled. Many
mafic raft or dyke samples were collected from exposures
from which tonalite gneiss and tonalite or granodiorite
units were sampled.
The U–Pb analytical methods utilized in this study
have been outlined by Parrish et al. (1987) and Davis et
al. (1997). Heavy mineral concentrates were prepared by
standard crushing, grinding, Wilfley table, and heavy
liquid techniques. Mineral separates were sorted by magnetic susceptibility using a FrantzTM isodynamic separator.
Multigrain and single-grain zircon fractions analyzed
were very strongly air abraded following the method of
Krogh (1982). Multigrain titanite fractions were also
lightly air abraded. Treatment of analytical errors follows
Roddick (1987), with regression analysis modified after
York (1969). U–Pb analytical results are presented in
Table 1, where errors on the ages are reported at the
2 level, and displayed in the concordia plots (Fig. 3a–c).
U–Pb sample locations are plotted in Fig. 2 and UTM
coordinates are listed in Table 1.
Whole-rock major elements (Table 2) were analyzed
on fused glass disks by X-ray fluorescence spectroscopy
(XRF) at the Geological Survey of Canada (GSC). FeO
was determined by dichromate titration and F contents
by ion electrode at the GSC. Trace element data (Table
2) were obtained using a combination of ICPES and
ICP-MS techniques at the GSC.
Sm/Nd isotopic separations were carried out at the
GSC by the senior author following the method of
Thériault (1990). Sample powders, spiked with mixed
148
Nd–149Sm and 84Sr–87Rb solutions, were dissolved in
an HF–HNO3 mixture. Separation of REE was performed by standard cation exchange chromatography.
NUMBER 8
AUGUST 2002
Separation of Sm and Nd from other REE followed
HDEHP [di(2-ethylhexyl)orthophosphoric acid]–Teflon
powder chromatography. Total procedure blanks were
approximately 0·3 pg for Nd and 0·2 pg for Sm. Mass
analysis was carried out on a MAT-261 solid source mass
spectrometer in static multi-collection mode for Nd and
Sm. Nd isotopic compositions were normalized to 146Nd/
144
Nd = 0·7219. Repeated measurements of an AMES
Nd solution yielded 143Nd/144Nd = 0·512194 ± 22 (2
SD). All 143Nd/144Nd ratios were corrected to La Jolla
143
Nd/144Nd = 0·511860. 147Sm/144Nd are reproducible
to 0·5%. In addition to reporting measured 143Nd/144Nd
ratios in Table 3, Nd isotopic data are reported as epsilon
values (Nd), which measure deviation in 143Nd/144Nd
between sample and chondritic uniform reservoir
(CHUR) at a specified time (DePaolo, 1988). Depletedmantle Nd model ages (TDM) are based on DePaolo
(1988).
Oxygen isotopic analyses were performed at the University of Western Ontario (UWO) using an Optima
dual-inlet mass spectrometer (Table 3). Oxygen was
extracted from whole-rock powders and hand-picked
quartz separates using the ClF3 method of Borthwick &
Harmon (1982), and quantitatively converted to CO2
over red-hot graphite. The oxygen-isotope data are presented in the normal -notation relative to VSMOW
(Vienna Standard Mean Ocean Water) (Baertschi, 1976;
Coplen, 1994). An oxygen-isotope CO2–H2O fractionation factor of 1·0412 at 25°C has been employed
in these calculations to calibrate the mass spectrometric
reference gas. An average 18O value of +11·5 ±
0·2‰ was obtained for 23 measurements of the UWO
laboratory quartz standard analyzed over the period of
this study. This corresponds to a 18O value of +9·65‰
for NBS-28. Measured 18O values for NBS-28 and
NBS-30 during this study were +9·66‰ and +5·18‰,
respectively.
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