- Email: [email protected]
Proterozoic (1.85–1.75 Ga) igneous suites of the Western Churchill Province: granitoid and ultrapotassic magmatism in a reworked Archean hinterland
Precambrian Research 119 (2002) 73 /100 www.elsevier.com/locate/precamres
Proterozoic (1.85 1.75 Ga) igneous suites of the Western Churchill Province: granitoid and ultrapotassic magmatism in a reworked Archean hinterland /
T.D. Peterson a,, O. Van Breemen a, H. Sandeman b, B. Cousens c a
Natural Resources Canada, Geological Survey of Canada, 601 Booth Street, Ottawa, Canada K1A 0E8 b Canada-Nunavut Geoscience Office, P.O. Box 2319, Iqaluit, Canada XOA 0H0 c Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, Canada K1S 5B6 Received 1 April 2001; received in revised form 5 December 2001; accepted 19 March 2002
Abstract Paleoproterozoic igneous rocks in the Archean hinterland of the Paleoproterozoic Trans-Hudson orogen (THO) consist of voluminous late syn-orogenic to post-orogenic monzonite to granite (Hudson granitoids; :/1850 /1810 Ma), and contemporaneous ultrapotassic lamprophyre dykes and volcanic rocks (Dubawnt minettes) that are interbedded with alluvial fan and fluvial deposits (Baker Lake Group, lower Dubawnt Supergroup). They were followed at approximately 1750 Ma by rapakivi granite (Nueltin granite) and porphyritic rhyolite associated with aeolian sandstone (Pitz Formation, middle Dubawnt Supergroup). The tectonic cycle ended with the deposition of conglomerates and sandstones in a large sag basin (Thelon Formation, upper Dubawnt Supergroup, :/1.72 Ga). The Hudson granitoids, which are strongly concentrated northwest of the THO, were broadly synchronous with terminal collision between the Archean Churchill and Superior cratons and the development of NE-trending ductile structures in the Western Churchill Province (WCP) that may be related to tectonic escape to the northeast. They were emplaced at mid-crustal level and no volcanic equivalents are preserved. Fault-bounded basins containing the minette volcanic rocks are located farther west in a domain dominated more by brittle faulting. The Nueltin granites, emplaced during a period of active extensional faulting, are present in a band extending southwest from the minette basins toward a preserved remnant of the sag basin (the Athabasca basin). Hudson granitoids are largely absent from this band but reappear west of it, indicating a higher crustal level of exposure in a downdropped Nueltin ‘corridor’. The Nd isotope composition of the three suites is similar (minettes: o Nd,1830 Ma //5 to /11; Hudson granitoids: o Nd,1830 Ma //7 to /13.5; Nueltin suite: o Nd,1750 Ma //7 to /10.5), and they have late Archean model ages that match those of average Archean WCP rocks. The Hudson granitoids are rich in inherited Archean zircon, and both granitoid suites are interpreted as crustal melts. Some Nueltin granites and Pitz rhyolites are mingled with basalt, and the Nueltin suite fits a commonly cited model for rapakivi granite production, which postulates injection of basalt into extending, brittly faulted crust. The Hudson granitoids are similar to late syn- to post-orogenic plutons in numerous other collisional hinterlands, which are typically associated with ultrapotassic lamprophyres. The minettes, which have high mg# and bear mantle xenocrysts, must have
Corresponding author. Tel.: /1-613-992-3573; fax: /1-613-995-7997 E-mail address: [email protected] (T.D. Peterson). 0301-9268/02/$ - see front matter. Crown Copyright # 2002 Published by Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 ( 0 2 ) 0 0 1 1 8 - 3
74
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
a mantle source component, and their source region could have been subduction-enriched lithospheric mantle. However, their source had only slightly lower time-integrated LREE enrichment than did that of the granitoids, and the incompatible element signatures of the two suites are strikingly similar. The minette source region may have been in a zone of mixed crust and upper mantle, formed during a shortening event which resulted in crustal thickening and subsequent melting at mid-crustal layers to form the Hudson granitoid plutons. The generation and emplacement of minette melts may have been promoted by extension related to a combination of slab breakoff, gravitational collapse of thickened crust, and strike-slip faulting in the deforming hinterland. Subsequent anorogenic rapakivi granite-basalt activity may have been triggered by lithospheric mantle delamination. The hinterland tectonic cycle of the WCP was repeated in other large Archean terranes that were deformed during the early Proterozoic, but the igneous and sedimentary record is unusually complete in the WCP. Crown Copyright # 2002 Published by Elsevier Science B.V. All rights reserved. Keywords: Granite; Minette; Post-orogenic; Proterozoic; Rapakivi; Trans-Hudson
1. Introduction
1.1. Tectonic context The western portion of the Churchill structural province (WCP) of the Canadian Shield is a collage of Archean granite-greenstone and quartzofeldspathic gneiss domains, with Proterozoic
Fig. 1. Principal tectonic elements of North America, showing the location of Fig. 2. Adapted from Hoffman (1988). ThO, Thelon orogen.
supracrustal and intrusive rocks, that was trapped as the upper plate between two large Proterozoic orogens (Figs. 1 and 2) (Hoffman, 1988). To the west, the Thelon orogen, active at about 2.0 Ga, separates the WCP from the Archean Slave Province. To the southeast, the Trans-Hudson orogen (THO), active from about 1.9 /1.8 Ga, separates the WCP from the Archean Superior Province. Terminal collision between the Churchill and Superior Provinces in this portion of the THO occurred between 1.83 and 1.81 Ga (Orrell et al., 1999). The WCP underwent episodic Proterozoic reworking from 2.55 to 1.75 Ga, a fact first established from K /Ar and Rb /Sr geochronological studies (Loveridge et al., 1988) and since confirmed by metamorphic and igneous U /Pb geochronology (e.g. LeCheminant et al., 1987a; Peterson and van Breemen, 1999; Sanborn-Barrie, 1999; Stern and Berman, 2000). Within the study area (the southern half of Fig. 3, below 658 latitude) Archean rocks are dominated by approximately 2.8 /2.65 Ga granite-greenstone belts and voluminous intrusions of approximately 2.6 Ga diorite-to-granite, with lesser gneisses E/3.0 Ga. This paper focuses on the extrusive and intrusive igneous activity related to reworking of the WCP between 1.9 and 1.7 Ga. This activity was dominantly granitic, but includes extensive minette and minor basaltic volcanism. The work was part of a recent 5-year NATMAP multidisciplinary study by Natural Resources Canada (Geological Survey of Canada) and partners, on Proterozoic reworking and the Archean architecture of the WCP. Further results of the NATMAP study will appear
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
75
Fig. 2. Simplified geology of the WCP. Sup, Archean Superior Province. CWB, Chipewayan-Wathaman batholith ( :/1850 Ma), M, location of the Martin Group; K, location of the Kramanituar granulite complex. The Amer and Wager Bay shear zones are major faults with dextral strike-slip motions at approximately 1.85 Ga.
in future volumes of Precambrian Research. The Proterozoic igneous study used geochronological, isotopic, geochemical, and petrological data from an area of approximately 250 000 km2 in an attempt to delineate Archean and Proterozoic crustal domains, to refine knowledge of the differential uplift and metamorphic history of the region, and to constrain the source regions and heat sources of the igneous suites.
The geometry of the WCP, and the disposition, orientation, and shear sense of its largest Proterozoic faults, give a strong impression of shortening perpendicular to the strike of the orogens and of tectonic escape along a system of E- to NEtrending dextral shear zones active from about 1.9 to 1.83 Ga. Some of these, such as the Tyrrell shear zone (Fig. 3), included significant south side up movement (TerMeer et al., 2000; MacLachlan
76
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Fig. 3. The Dubawnt Supergroup, Hudson granitoids, and Nueltin granites in the study area. K, Kramanituar complex; A, diamondbearing Akluilaˆk minette dyke; TSZ, Tyrrell shear zone.
et al., 2000a). Prominent Proterozoic structural elements, including synclinal keels of pre-1.85 Ga sedimentary rocks (Aspler and Chiarenzelli, 1997), and mylonitic shear zones with transposed Archean structures (MacLachlan et al., 2000b), are typically NE-trending. Extension in the northeast segment of the WCP may have promoted the rapid uplift near 1.9 Ga of the Kramanituar granulite massif, exposed on the north shore of Chesterfield Inlet (Sanborn-Barrie, 1994). Small (:/10 m) granulite bodies which may have been uplifted during the early Proterozoic are also present along the trace of the northeast-trending Snowbird
tectonic zone (Tella and Eade, 1985), an ancient, kinematically complex fault zone that transects the southwestern WCP (Hanmer, 1997). There is evidence for a high-pressure (/1.0 GPa) amphibolite facies metamorphic event southwest of Chesterfield Inlet at about 1.9 Ga (Berman et al., 2000) but map-scale structures associated with this event have not been identified (Hanmer et al., in preparation). A collision on the western margin of the Slave Province reactivated faults in the Thelon orogen, resulting in brittle indentation of the relatively rigid Slave craton into the WCP along a conjugate
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
fault system that cuts 1.84 Ga intrusions in the Slave Province (Culshaw, 1991). Between the Thelon orogen and the region extensively intruded by Hudson granitoids, brittle downfaulting and crustal sagging produced terrigenous basins filled with volcanic rocks, conglomerates, and arenites of the approximately 1.84 /1.7 Ga Dubawnt Supergroup (Fig. 4) (Gall et al., 1992). Basin development was probably initiated at the east end of Baker Lake, where there is a thick succession of NW-facing pre-volcanic conglomerate and arenite containing cobbles of the uplifted Kramanituar granulites (Rainbird et al., 1999). Volcanic rocks in the lower Dubawnt Supergroup (Baker
Fig. 4. Idealized stratigraphic section for the Dubawnt Supergroup. Thicknesses of formations in the Baker Lake Group are variable and can be in excess of 2000 m; the upper Dubawnt Supergroup (Barrensland Group) has a maximum thickness of about 1500 m and the middle Dubawnt Supergroup (Wharton Group) about 1000 m. The lower, middle, and upper divisions are separated by unconformities.
77
Lake Group, Christopher Island Formation) consist of ultrapotassic lamprophyre (minette) lavas, breccias, and tuffs of both subaerial and subaqueous facies, overlain by alluvial fan and fluvial deposits (Kunwak Formation). These are separated by an angular unconformity from approximately 1.75 Ga rhyolite flows of the Pitz Formation (Wharton Group, middle Dubawnt Supergroup). An erosional unconformity, with intense chemical weathering, separates the Wharton Group from mostly flat-lying conglomerates and arenites of the Thelon Formation (Barrensland Group, upper Dubawnt Supergroup) which were deposited in a sag basin (Rainbird et al., 2001). The Thelon Formation is capped by thin, shallow marine dolostones. One large erosional remnant, the Thelon basin, overlies the intersection of the indentation fault systems. The full former extent of the sag basin is unknown, but the Athabasca basin (Fig. 2), which contains identical rocks, is thought to be another remnant (Gall, 1994). The WCP was compared to the Tibetan Plateau by Dewey and Burke (1973), who suggested that it represents a similar-styled continental hinterland, now mostly exposed at lower to mid-crustal levels. Peterson (1992) noted similarities between the volcanic styles, sedimentary facies, and basin morphologies of the Baker Lake Group and of extensional basins flanking the Tibetan Plateau in the Karakorum fault zone (Miller et al., 1999). The WCP has many features common to Archean domains located in the hinterlands of large 2.0 / 1.7 Ga orogens. Notable among these are the Nagssuqtoquidian domain of western Greenland, which includes reworked Archean rocks and very similar Proterozoic igneous suites (Skerjnaa, 1992; Kalsbeek and Nutman, 1996), and the Archean Saˆo Francisco craton of Brazil, which features ca. 2.0 Ga ultrapotassic intrusions (Rosa et al., 1999) adjacent to the terrigenous Espinhac¸o basin, which contains rocks similar to the middle and upper Dubawnt Supergroup (Uhlein et al., 1998). However, the WCP is exceptional for its volume and extent of hinterland igneous activity, and for the excellent preservation of Proterozoic strata which recorded the uplift history of the region.
78
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
1.2. Extent of Proterozoic igneous activity in the Western Churchill Province 1.2.1. Data sources The geochronological, geochemical, and Nd isotope data summarized here are from numerous sources. Copies of all data referred to in this paper may be obtained from the first author. We obtained samples to provide coverage throughout the area of Fig. 3 between 608 and 658 latitude. A relatively large number of samples was obtained in areas of new mapping, principally near the Kaminak greenstone belt and Chesterfield Inlet. Samples from areas with no nearby active mapping project were obtained from GSC archives and external sources. U/Pb analyses of zircon for 24 granitic rocks were obtained for the NATMAP project using the Geological Survey of Canada SHRIMP II (Stern, 1997). Preliminary results were presented by Peterson and van Breemen (1999) and a full report is in preparation (van Breemen et al., in preparation). Proterozoic ages in the range 1750 /1850 Ma were obtained either from zircon overgrowths on resorbed cores (Hudson suite) or from euhedral phenocrystic zircon (Nueltin suite). Ten previously published U /Pb (zircon) ages are also available (cited in Peterson and van Breemen, 1999). The Nd and Sm isotopic compositions of 17 granitoid samples were determined during the NATMAP study at GSC laboratories (The´riault, 1990) and Memorial University, St. John’s, Newfoundland. Previously published Nd /Sm analyses of Proterozoic granites and minettes are available from Duda´s et al. (1991) and Peterson et al. (1994). Additional analyses of minette dykes and lava flows were made for NATMAP (Cousens, 1999) and were interpreted by Cousens et al. (2001). Whole-rock analyses in Table 1 were made at Memorial University and the Geological Survey of Canada. Rare earth elements and Hf, Ta, Cs, U, Th, and Pb were analyzed by ICP-MS (Jenner et al., 1990), and other elements by XRF. Additional elemental analyses, used in the construction of plots but not listed in Table 1, are given by Blake (1980), Tamboso (1981), Booth (1983), LeCheminant et al. (1987b), Peterson et al. (1994), Cousens (1999), Peterson (in preparation).
1.2.2. Extent of igneous activity There were two pulses of igneous activity (Fig. 5). The first generated plutons of monzonite, granodiorite, and granite (the Hudson granitoids) between 1850 and 1810 Ma, with a peak near 1830 Ma, and with two outlying ages at about 1790 Ma. The Hudson plutons are strongly concentrated in a belt immediately west of Hudson Bay, but are found across nearly the entire study area. Regional mapping and geochronological studies (Henderson, 1983; Henderson and Roddick, 1990; LeCheminant et al., 1987a) indicate this belt extends northeastward, across the Wager Bay shear zone and the Penhryn fold belt to central Baffin Island, where post-orogenic granitoid plutons intruded Archean basement and early Proterozoic metasedimentary rocks at approximately 1.83 Ga (Corrigan et al., 2001). The area immediately north of Chesterfield Inlet has not been mapped in detail and the apparent absence of Hudson granitoids in that area may only reflect an absence of data. Hudson granitoids are mostly absent within the Kaminak greenstone belt, which was relatively isolated from Proterozoic thermal metamorphism (Hanmer et al., in preparation). South of the Kaminak belt, Hudson granitoids extend as far as the Wathaman-Chipewayan batholith, dated at 1850 Ma (Meyer et al., 1992) (Fig. 2). Outliers of the batholith to the north and west yield U /Pb ages of 1840/1810 Ma (Annesley et al., 1997). A large part of the WCP east of the Thelon basin was intruded by an ultrapotassic lamprophyre (minette) dyke swarm (Fig. 3, inset). In most cases, Dubawnt minette dykes cut Hudson plutons, but granite is observed truncating dykes south of Chesterfield Inlet (Tella et al., 1993). The Dubawnt minettes have proved difficult to date due to a dearth of zircon; however, imprecise U/Pb and K/Ar ages (summarized in Peterson and van Breemen, 1999) suggest eruption occurred no earlier than 1.85 Ga and no later than 1.80 Ga. Minette volcanism and deposition of redbeds of the Baker Lake Group occurred in fault-bounded basins on the western side of the WCP, away from the main concentration of Hudson intrusions. Minette dykes adjacent to the basins typically have idiosyncratic or bimodal strike distributions parallel to the local basin-bounding faults,
Table 1 Whole rock elemental and Nd /Sm isotope analyses of Proterozoic granites and minettes of the WCP. Sample
EA70-
EA70-
EA70-
EA78-
EA78-
FD78-
FD78-
P97-
86T-
98TX-
98TX-
98TX-
98TX-
98TX-
98TX-
98T-
98TX-
894
184
1057
64
128A
118
120
N50A
X107
T095
H095
H185
H053
J067
T151
X241
N067
MOSQ-1
Suite
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Lat
60802?
60847?
60803?
60822?
60823?
60810?
60819?
60811?
62845?
63814?
63845?
63814?
63845?
63843
63847?
63851?
63836?
63832?
62838?
Long
98846?
96853?
97842?
97809?
95809
94848?
95812?
95813?
95855?
90807?
93850?
93850?
93808?
93815?
93810?
93815?
94839?
938001?
102857?
SiO2
73.69
73.68
70.80
71.80
71.60
69.30
72.50
73.40
73.10
71.68
72.60
70.30
73.30
74.90
72.90
72.10
73.40
70.30
67.65
TiO2
0.14
0.17
0.23
0.16
0.21
0.33
0.18
0.17
0.24
0.35
0.17
0.34
0.12
0.17
0.27
0.21
0.16
0.37
0.53
Al2O3
15.79
15.72
15.00
14.70
14.60
15.70
14.50
14.50
14.40
15.94
14.20
14.80
14.60
14.10
14.20
14.30
14.40
14.50
15.91
Fe2O3
1.03
1.13
1.00
1.40
0.90
1.40
1.90
1.20
0.50
1.80
0.90
0.50
0.70
0.40
0.80
1.00
1.00
1.10
3.37
FeO
NA
NA
1.40
0.60
0.80
0.80
0.60
0.60
1.10
NA
0.60
1.20
0.50
0.70
1.00
0.90
0.40
1.30
NA
MnO
0.02
0.02
0.03
0.01
0.02
0.03
0.01
0.02
0.02
0.02
0.03
0.02
0.01
0.03
0.02
0.02
0.05
0.02
0.06
MgO
0.34
0.64
0.51
0.36
0.27
0.47
0.39
0.37
0.59
0.97
0.47
1.00
0.16
0.37
0.38
0.48
0.37
0.66
1.34
CaO
1.09
0.47
2.08
2.16
1.21
1.67
1.71
1.67
1.54
1.60
1.28
1.36
0.99
1.04
1.21
1.16
0.22
1.44
1.78
Na2O
3.78
2.88
4.20
3.80
3.50
4.10
4.00
3.60
4.40
3.59
3.80
3.40
4.10
3.60
3.40
3.60
3..9
3.10
3.40 5.80
K2O
5.19
7.33
4.11
4.18
5.95
5.85
3.97
4.67
3.23
5.26
5.33
5.93
5.28
4.95
5.41
5.35
5.16
6.01
P2O5
0.01
0.03
0.09
0.09
0.08
0.12
0.06
0.05
0.07
0.11
0.07
0.15
0.02
0.04
0.07
0.07
0.06
0.10
0.28
H2O
NA
NA
0.30
0.40
0.40
0.50
0.50
0.40
0.30
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
CO2
NA
NA
0.10
0.10
0.10
0.20
0.10
NA
NA
NA
NA
NA
NA
NA
NA
NA
F
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
1153
2408
839
889
NA
NA
NA
925
835
NA
Rb
171
276
180
130
190
220
100
210
110
229
319
339
204
406
311
310
269
229
215
Cs
NA
NA
1.2
0.8
0.6
1.3
0.1
0.7
2.0
NA
11.0
10.0
1.6
9.2
5.3
5.6
8.4
2.1
NA
Be
NA
NA
2.5
2.0
1.2
1.6
1.8
3.5
1.8
NA
5.2
6.4
5.0
4.8
3.7
3.2
4.9
1.6
NA
Sr
86
144
200
140
180
210
170
120
250
487
268
551
654
142
241
284
312
478
522
Ba
334
995
930
770
570
1100
1100
750
920
1812
580
1300
2200
550
910
1100
790
2000
2102
Sc
NA
NA
3.4
2.1
1.7
2.9
2.4
2.4
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
3.4
Y
5.3
1.6
15.0
15.0
7.9
18.0
28.0
34.0
9.7
2.3
7.8
9.0
7.0
12.0
9.0
3.7
8.0
15.0
33.8
Zr
104
217
270
230
290
380
230
180
180
257
141
261
166
153
242
231
166
326
584
Hf
NA
7.7
7.4
7.1
8.8
11.0
6.6
6.8
4.5
7.4
4.8
8.1
4.5
5.5
6.9
7.1
4.9
8.2
17.7
V
3
9
17
14
12
21
20
10
11
20
9
17
B/LD
B/LD
12
13
9
25
37
Nb
6.9
4.8
13.0
8.1
4.6
15.0
12.0
15.0
11.0
13.9
16.0
24.0
8.4
25.0
18.0
10.0
12.0
21.0
27.7
Ta
NA
0.21
0.66
0.31
0.21
1.10
0.55
1.10
1.60
1.24
1.30
2.60
0.40
2.50
1.30
0.30
1.30
3.00
1.87
Cr
9
6
13
14
10
16
12
11
20
11
12
37
B/LD
B/LD
B/LD
B/LD
B/LD
11
16
Co
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
29
B/LD
B/LD
7
B/LD
B/LD
B/LD
B/LD
B/LD
8
B/LD
Ni
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
18
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
Cu
B/LD
1
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
15
B/LD
46
B/LD
B/LD
B/LD
B/LD
48
B/LD
B/LD
Zn
2
6
56
29
34
39
13
34
29
23
33
31
10
49
52
40
37
46
38
La
25
86
81
50
137
155
63
54
45
167
48
82
51
54
120
100
52
180
185
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
EA7670
79
45
151
140
99
240
270
110
100
85
288
84
150
89
100
240
180
91
320
336
Pr
4.9
15.1
15
10
24
27
12
11
8.6
30.2
9.4
16
9.1
11
22
19
9.8
33
35.1
Nd
15
43
46
34
69
84
40
35
27
89
29
50
28
34
70
57
30
100
120
Sm
2.4
5.7
7.3
6.2
8.0
13.0
7.7
7.1
3.9
9.6
4.5
6.7
3.5
5.3
8.6
6.9
4.1
13.0
18.8
Eu
0.40
0.62
0.75
0.75
0.77
0.92
1.10
1.00
0.55
1.56
0.65
0.89
0.34
0.55
0.81
0.77
0.61
1.40
3.01
Gd
2.0
3.6
4.9
4.9
3.8
8.1
6.5
5.8
2.4
3.7
2.7
3.8
2.2
3.3
4.2
2.7
2.5
7.0
11.2
0.27
0.37
0.60
0.65
0.37
0.89
0.93
0.94
0.32
0.37
0.36
0.46
0.25
0.43
0.49
0.34
0.30
0.82
1.35
Dy
1.51
1.53
2.70
3.20
1.60
3.80
5.10
5.30
1.70
1.56
1.70
2.20
1.20
2.20
2.30
1.10
1.50
3.60
7.09
Ho
0.26
0.21
0.45
0.51
0.26
0.63
0.87
1.00
0.30
0.22
0.89
0.39
0.23
0.44
0.35
0.35
0.29
0.59
1.24
Er
0.70
0.53
1.10
1.10
0.64
1.40
2.10
2.60
0.74
0.56
0.36
0.99
0.51
1.10
0.89
0.17
0.74
1.40
3.55
Tm
0.10
0.07
0.14
0.15
0.09
0.20
0.31
0.41
0.12
0.07
0.15
0.16
0.09
0.19
0.13
0.05
0.13
0.22
0.46
Yb
0.66
0.42
0.88
0.84
0.57
1.20
1.80
2.40
0.76
0.47
0.97
0.95
0.60
1.20
0.71
0.34
0.82
1.30
2.66
Lu
0.10
0.08
0.14
0.13
0.10
0.20
0.28
0.37
0.12
0.05
0.15
0.16
0.09
0.19
0.12
0.08
0.14
0.19
0.39
U
8.5
5.6
5.0
2.8
6.1
6.3
11.0
5.7
1.9
8.3
8.1
16.0
6.4
12.0
5.2
8.1
16.0
12.0
6.4
Th
27.8
70.2
55
34
150
81
47
59
16
71.9
43.6
68
28
40
62
58.5
37
113
30.3
Pb
40
55
31
25
46
51
25
35
21
74
66
48
90
63
58
63
51
64
30
147Sm/
0.09820
0.08410
0.09357
0.10735
NA
0.08368
0.10752
0.10890
0.08310
0.06037
0.08890
NA
NA
NA
NA
NA
0.07810
0.07820
NA
0.51098
0.51068
0.51082
0.51103
NA
0.51060
0.51110
0.51111
0.51078
0.51064
0.51090
NA
NA
NA
NA
NA
0.51083
0.51077
NA
EA27-
P89-23
P88-
P89-303
P89/R37
P89-PZ
P94-
P00-33
P00-12
P96-
P96-1A
P96-
P97-92
56D-12
144Nd 143Nd/ 144Nd Sample
69
500G
124C
110
168A
P97-
P97-
P97-
P97-
P91/
C32
C178
T138A
T138B
LBIF
Suite
Nueltin
Nueltin
Nueltin
Nueltin
Nueltina
Nueltina
PNb
PNb
PNb
Minette
Minette
Minette
Minette
Martell
Minette
Minette
Minette
Minette
Minetteb
Lat
60803?
63818?
63813?
63818?
62856?
glacial
62854?
99802?
99804?
63828?
62817?
62815?
62802?
63838?
62827?
62816?
62818?
62818?
63812?
Long
100812?
101812?
101813?
101805?
101829?
erratic
102809?
62828?
62827?
93856?
93847?
93855?
95821?
95845?
95800?
95843?
95847?
95847?
101813?
SiO2
76.84
75.70
76.80
71.30
63.70
75.70
56.46
51.13
51.80
45.27
42.90
50.69
48.83
46.39
59.57
44.68
46.84
51.14
57.70
TiO2
0.23
0.23
0.21
0.17
0.93
0.22
1.45
1.94
2.01
1.14
1.07
1.18
1.32
0.52
1.09
1.04
0.84
1.11
0.90
Al2O3
15.45
12.30
12.00
14.10
14.60
11.80
13.81
14.28
14.32
14.23
7.19
9.37
10.32
5.61
12.39
9.82
8.77
10.33
11.60
Fe2O3
2.54
0.50
0.60
1.80
5.80
0.30
8.46
10.69
11.62
9.01
12.53
7.56
7.09
8.05
5.17
6.81
6.93
7.53
8.70
FeO
0.00
0.80
0.50
0.10
0.20
1.50
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
0.40
MnO
0.03
0.02
0.01
0.01
0.07
0.02
0.14
0.12
0.14
0.35
0.26
0.13
0.11
0.14
0.08
0.09
0.10
0.14
0.07
MgO
0.25
0.21
0.19
0.39
0.86
0.28
5.10
3.79
2.87
6.70
17.03
11.93
10.91
22.38
5.89
12.40
12.65
9.87
5.91
CaO
1.17
0.71
0.55
0.19
2.01
0.61
6.21
7.52
7.36
8.28
11.87
10.09
8.74
10.58
5.00
9.37
10.05
8.03
1.60
Na2O
3.34
2.90
2.80
4.40
3.80
2.60
2.65
2.70
2.92
0.99
0.34
2.40
1.60
0.66
3.36
0.70
0.60
1.46
2.10
K2O
5.90
5.45
5.56
5.34
5.29
5.65
2.40
2.36
2.22
8.56
4.69
3.41
6.42
3.47
3.57
7.43
5.84
4.13
7.56
P2O5
0.02
0.04
0.03
0.13
0.31
0.04
0.80
1.80
1.52
2.78
2.07
1.49
1.79
0.55
1.45
2.16
1.44
1.55
0.72
H2O
NA
0.50
0.60
0.70
1.30
0.70
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
1.30
CO2
NA
0.10
0.10
0.10
1.20
0.10
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
0.10
F
NA
NA
2042
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
Rb
292
366
334
283
166
301
111
69
77
659
236
135
370
182
165
244
172
103
Cs
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
Be
NA
6.5
4.8
5.4
3.1
3.1
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
292 21.0
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Tb
80
Ce
Sr
94
107
73
223
391
66
751
533
609
1542
842
547
1381
578
637
3887
3701
872
1040
Ba
706
651
529
1058
1742
523
2480
1573
1636
9659
2571
4099
8875
2112
3183
10215
10697
4283
4003
Sc
NA
NA
NA
NA
NA
NA
NA
20.2
23.8
15.8
21.5
23.6
21.4
38.6
11.9
18.0
19.3
16.7
Y
78.0
35.0
36.0
7.0
36.0
49.0
23.7
50.0
57.5
44.7
27.1
20.5
23.9
10.7
20.8
56.8
30.8
21.6
47.0
Zr
312
297
245
201
457
315
315
259
294
169
816
700
777
57
915
512
360
773
924
Hf
9.4
NA
NA
NA
NA
NA
NA
6.5
7.5
3.9
19.5
17.9
19.9
1.6
NA
NA
NA
NA
V
NA
1
1
18
11
3
152
161
90
65
119
139
130
113
80
84
131
137
82
Nb
35.9
26.0
67.0
12.0
43.0
31.0
25.1
14.3
19.6
10.7
31.3
16.5
39.3
3.9
7.3
4.7
10.7
18.4
37.0
Ta
2.68
NA
NA
NA
NA
NA
NA
0.81
0.90
0.36
1.02
0.52
1.86
0.15
NA
NA
NA
NA
B/LD
4
6
11
14
7
260
7
B/LD
196
737
316
338
814
254
647
389
937
13
NA
2
3
4
13
4
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
28
Ni
B/LD
1
10
3
0
2
90
5
B/LD
93
420
262
238
292
165
517
332
252
31
Cu
B/LD
2
2
4
5
3
32
26
15
17
32
29
52
41
3
96
70
60
52
Zn
B/LD
26
21
19
68
36
107
64
69
171
85
45
44
29
86
76
46
64
140
La
172
75
77
41
99
120
NA
69
83
97
242
95
201
37
123
231
183
146
210
Ce
329
150
170
NA
NA
250
NA
150
182
202
472
189
377
80
232
446
360
282
340
Pr
39.1
NA
NA
NA
NA
NA
NA
19.4
23.3
25.5
57.2
21.8
44.5
9.7
NA
NA
NA
NA
Nd
128
67
62
NA
NA
98
NA
82
98
106
220
82
165
40
93
213
180
123
190
Sm
21.8
8.7
9.2
NA
NA
13.0
NA
16.4
19.3
22.1
34.3
13.3
25.5
8.1
14.4
38.7
28.1
19.0
34.0
Eu
1.31
1.10
0.90
NA
NA
1.20
NA
3.55
4.05
6.13
8.11
3.36
6.06
2.10
3.84
13.04
8.93
4.78
6.60
Gd
16.6
7.0
6.7
NA
NA
11.0
NA
13.3
15.5
18.3
19.0
8.3
13.7
5.2
9.0
27.9
18.1
11.4
21.0
Tb
2.55
NA
NA
NA
NA
NA
NA
1.67
1.96
2.11
1.80
0.92
1.30
0.54
NA
NA
NA
NA
Dy
14.70
6.80
6.00
NA
NA
9.50
NA
9.13
10.73
9.66
7.13
4.41
5.56
2.57
4.47
13.65
8.21
4.85
12.00
Ho
2.80
NA
NA
NA
NA
NA
NA
1.71
2.01
1.45
0.97
0.73
0.82
0.43
0.68
1.97
1.21
0.75
2.00
Er
8.03
NA
NA
NA
NA
NA
NA
4.80
5.63
3.30
2.15
1.95
2.01
1.13
1.57
4.28
2.55
1.70
4.30
Tm
1.20
NA
NA
NA
NA
NA
NA
0.59
0.70
0.33
0.22
0.23
0.23
0.14
0.18
0.47
0.32
0.23
0.71
Yb
7.26
4.30
3.80
0.60
2.90
5.50
NA
3.46
4.13
1.67
1.19
1.36
1.28
0.80
1.10
2.33
1.77
1.20
Lu
1.02
NA
NA
NA
NA
NA
NA
0.52
0.61
0.21
0.16
0.19
0.19
0.12
0.18
0.35
0.27
0.18
U
5.3
NA
NA
NA
NA
NA
NA
B/LD
B/LD
B/LD
8.2
1.3
8.0
B/LD
9.6
3.2
3.0
14.5
16.0
Th
30.6
NA
NA
NA
NA
NA
NA
9
8.1
B/LD
55
8
53.3
B/LD
52.1
29.9
21.7
63.1
220.0
Pb
37
NA
NA
NA
NA
NA
NA
B/LD
B/LD
B/LD
B/LD
B/LD
158
B/LD
22
129
67
17
147Sm/
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
0.11050
0.10150
0.09720
0.11565
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
0.51121
0.51112
0.51104
0.51109
144Nd 143Nd/
0.51132
0.51130
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Cr Co
144Nd
B/LD, below detection limit; NA, not analysed. If FeO is not analysed, then all Fe as Fe2O3. PNb/basaltic rocks (MacRae Lake dyke) associated with Nueltin suite. Other minettes are dykes. a Rhyolites from the Pitz Formation. b Lamproitic felsic minette lava (upper volcanic member of Christopher Island Formation).
81
82
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Fig. 5. Histogram of U /Pb zircon ages for Hudson and Nueltin granitoids. SHRIMP ages: van Breemen et al. (in preparation). Other ages compiled by Peterson and van Breemen (1999).
whereas east and north of the basins the dykes are typically east to east-southeast trending, at a high angle to regional ductile fabrics (Fig. 6). The easterly trends have been interpreted as reflecting an approximately E /W directed stress field related to the indentation of the Slave Province (Hoffman, 1988). The second pulse of igneous activity consisted of rhyolite volcanism (Pitz Formation) and emplacement of subvolcanic intrusions of porphyritic, locally rapakivi-textured granite (Nueltin granite). This activity was strongly concentrated within the older basins of the Baker Lake Group, and was associated with additional extensional faulting that dissected and tilted the basins (Hadlari and Rainbird, 2001). The rhyolites are restricted to the Baker Lake Group basin complex; beyond the basins, to the southwest, only plutons of Nueltin granite are present. Additional 1.76 Ga plutons have been identified beneath the Athabasca basin (Krogh and Clark, 1987) suggesting that a SW / NE band of approximately 1.76 Ga plutonism
extends through the southern WCP, subparallel to its bounding orogens.
2. Geology and petrology of the igneous suites
2.1. Hudson granitoids Except for the plutons near Wager Bay, which are megacrystic granites through monzodiorites with minor gabbro (LeCheminant et al., 1987a), the Hudson granitoids are remarkably uniform in their field relations and petrography. Many of the plutons are elongated NE/SW, reflecting selective emplacement along pre- to syn-magmatic structures. Within the plutons, the rocks are typically nonfoliated with a grain size of 1 /5 mm (Fig. 7a). The plutons commonly have transitional contacts with country rocks, and sill-like margins that bear an inherited foliation defined by dispersed biotite (Fig. 7b) and narrow enclaves of partly assimilated
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
83
Fig. 6. Rose diagrams of strikes of minette dykes. Note the tendency for dykes near basins of the Baker Lake Group to have NWtrending or bimodal strike directions, due to conjugate brittle faulting related to eastward indentation of the Thelon Province. M, Martell syenites (intrusive minette); A, Akluilaˆk diamondiferous minette dyke; Y, Yathkyed Lake; TSZ, Tyrrell shear zone; STZ, Snowbird tectonic zone.
wall rocks. Some appear to have been emplaced in an injection migmatite (Davidson, 1969); in other cases the wall rocks were competent enough to allow meter-scale dykes to form. Mafic inclusions from wall rocks are invariably coarsely recrystallized (mainly to hornblende) and may be locally disaggregated and mixed to produce heterogeneous monzodiorite. The plutons typically cut the youngest ductile structures, such as mylonite bands and stretching lineations, but are also partly affected by them (e.g. Relf et al., 1999). Gravity anomalies associated with all but the largest plutons are negligible, consistent with thin, silllike bodies. Regional thermobarometric studies (Berman et al., 2000) indicate that the Hudson granitoids between Baker Lake and the Kaminak greenstone belt were emplaced at pressures near 0.5 GPa (:/15/20 km depth). The Hudson
granitoids have no known volcanic equivalents, and no silicic ash layers have been recognized in any similar-aged Proterozoic sedimentary rocks. The rocks are subsolvus and are mainly equigranular, although weakly Kspar-porphyritic rocks have been noted near some pluton margins. Plagioclase and alkali feldspar are nonidiomorphic and present in approximately equal abundance, and myrmekite is commonly present. Biotite comprises about 5% of most samples. Magnetite, varying from euhedral to interstitial and irregular, is abundant and most Hudson plutons are associated with distinctive magnetic anomalies. Garnet is present as rare subhedral grains which could be phenocrysts, but which are probably from a restite. Zircons are mostly present as subhedral inclusions in biotite, and highly rounded inclusions in quartz or feldspar (Fig. 8a and b).
84
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Fig. 7. Polished slabs of Hudson granitoids. (a) Typical sample from a pluton center; note equigranular texture. (b) Typical sample from a pluton margin, with a weak relict foliation defined by biotite. The sample is not deformed.
Fig. 8. Photomicrographs of zircons from Hudson granitoids. (a) Inherited Archean zircon armored by biotite (crossed polars). (b) Inherited Archean zircon, resorbed and included in quartz.
The average Hudson granitoid is a slightly peraluminous granodiorite with K/Na :/0.95 and overall composition similar to silicic rocks in calcalkaline series (Table 1). In a normative feldspar plot (Fig. 9), nearly all the Hudson granitoids have between 10 and 20% An, with a broad range in Or content. In a normative quartz /feldspar diagram (Fig. 10), the Hudson granitoids cluster strongly around the 5 kbar H2O-saturated quartz /two feldspar eutectic for bulk compositions with plagioclase of An20. They have very low Y contents (average 11 ppm; Fig. 11) and high LREE/HREE ratios (average Ce/Yb /206) (Fig. 12). Europium anomalies are typically modest (average Eu/Eu / 0.53). Neodymium isotope compositions, with o Nd varying from /7.0 to /13.2, show that the granitoids had a source component with longterm LREE enrichment (Figs. 13 and 14). The initial Nd isotope composition of the Hudson granitoids is indistinguishable from that of large tracts of Archean terrane in the WCP (e.g. The´riault et al., 1994). Analyses of samples of a widespread suite of approximately 2.6 Ga granitic plutons (Duda´s et al., 1991; Peterson, in preparation) are used as a proxy for Archean basement in Fig. 13. The field relations, petrography, and composition of the Hudson granitoids suggest they were formed by emplacement of near-solidus magmas into thin, partly concordant bodies at mid-crustal levels. The equigranular textures indicate simultaneous crystallization of quartz and feldspars, and the absence of significant Eu anomalies indicates little differential movement of melt and solids. Hence, the melts probably crystallized over a small temperature interval, consistent with minimummelt compositions. The magmas likely formed in contact with garnet-bearing wall rocks, resulting in depleted Y and high Ce/Yb, and contained inherited xenocrysts of garnet, zircon, and ferromagnesian minerals, especially biotite. Given their low thermal inertia, the Hudson granitoid magmas likely migrated only a short vertical distance. There is no field or geochemical evidence for an association with isotopically depleted juvenile mafic magmas on a regional scale, although gabbroic rocks are present at Wager Bay.
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
85
Fig. 9. Normative (CIPW) feldspar plot (mole fraction) for the granitoid suites and the mafic rocks associated with Nueltin granites.
The Hudson granitoids have all the characteristics of plutons that are widely interpreted as early post-orogenic intrusions in collisional hinterlands. These characteristics include: in situ melting (or nearly so) with restitic zircon and isotopic compositions indistinguishable from the wall rocks (e.g. Paleoproterozoic granitoids of the Nagssugtoqidian orogen: Whitehouse et al., 1998); correlation in time and space with late ductile structures (e.g. late Svecofennian anatectic granites: Nironen, 1997); and trace element compositions (depleted Y and HREE) consistent with a garnet-bearing source region in the lower to middle crust. An additional common feature of these post-orogenic granitoids is an association with potassic lamprophyres (e.g. Rock and Hunter, 1987; Eklund et al., 1998) which is a dramatic feature of the Hudson granitoids.
2.2. Dubawnt minettes The Baker Lake Group, with an estimated volume of 40 000 km3 preserved in a complex of fault-bounded basins (Peterson, 1992), comprises the largest extrusive ultrapotassic province yet described. The associated minette dyke swarm is asymmetrically disposed: the western edges of the basin complex and the dyke swarm coincide at Dubawnt Lake, but the dyke swarm extends much further east, into the area densely intruded by Hudson granitoids (inset, Fig. 3) and also to the southwest as far as the Athabasca basin. Probable equivalents of the Baker Lake Group are exposed at the north edge of Lake Athabasca (the Martin Group: Donaldson, 1968; see Fig. 2), where minette dykes are also found in the underlying Archean basement.
86
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Fig. 10. Normative (CIPW) quartz /feldspar plot (weight fraction) for the granitoid suites and the mafic rocks associated with Nueltin granites. E is the quartz /2 feldspar eutectic in albite /sanidine /quartz, at 5 kbar, water saturated. With increasing An content, the eutectic migrates toward the quartz /sanidine join, and the grayed eutectic point is its approximate position for a bulk composition with plagioclase of An20 (estimated from Carmichael, 1963).
Small plutons of ultrapotassic, mafic syenite (Martell syenites) occur within and southeast of the most easterly Baker Lake Group basin (Fig. 6). They are in contact with Hudson granitoid plutons and intermediate rock types are present within the syenites, consistent with magma mixing. Hudson granitoids are mingled with amphibole-rich lamprophyric rocks (spessartites) in plutons exposed near Chesterfield Inlet (Sandeman et al., 2000). The volcanic rocks (Christopher Island Formation, Baker Lake Group) were mostly erupted as a felsic-mafic-felsic sequence directly over crystalline basement. In the area of Baker Lake, volcanic rocks conformably overlie conglomerate and arkose that record pre-volcanic faulting on the margins of an E /W trending basin, and periodic inward progradation of alluvial fans over a flat braidplain (Rainbird et al., 1999). At Dubawnt Lake, epiclastic turbidites interbedded with lava
flows, subaqueous flow breccias, and siltstones in sections up to 2.5 km thick suggest eruption centers developed within, or alongside, deep and narrow lakes (Peterson, in preparation). Postvolcanic conglomerates and arenites (Kunwak Formation) recorded continued uplift, locally combined with dextral strike-slip faulting, on the margins of some basins (Rainbird and Peterson, 1990). Most exposures of the Baker Lake Group are tilted at 208 /608, due to both syndepositional faulting and later faulting associated with rhyolite volcanism (Hadlari and Rainbird, 2001). The lower felsic lavas are porphyritic with phenocrysts of sanidine plus highly resorbed and oxidized olivine, phlogopite, and clinopyroxene. Partially melted crustal xenoliths (especially tonalites) are common and the rocks show clear signs of crustal contamination, such as elevated Si, Al, and Na/K and depletion in incompatible elements
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
87
Fig. 11. Y versus Zr for both granitoid suites, the Nueltin mafic rocks, and the Dubawnt minettes.
relative to the mafic minettes (Peterson, 1992). The upper felsic lavas are crystal-poor and relatively enriched in incompatible elements, with compositions similar to felsic lamproites (Mitchell and Bergman, 1991; see sample P91-LBIF, Table 1). They are interpreted as crystal fractionation products of the primary mafic minettes. The mafic minettes, which comprise the great majority of the dykes outside the basins, are highly porphyritic (phlogopite/clinopyroxene/ apatite9/olivine9/leucite) (Fig. 15) with a groundmass dominated by potassium feldspar with minor alkali amphibole (richterite, K-richterite, and riebeckite), magnetite, titanite, and primary carbonate. Many dykes contain 1 /4 cm ellipsoidal glimmerite xenoliths with curved and kink-banded mica, thought to represent phlogopite megacrysts and pegmatites battered against the wall rocks during ascent (Peterson and LeCheminant, 1993). These contain euhedral magnesiochromite and resorbed chrome diopside. Mantle spinels and garnets, picroilmenite, and chrome diopside have
all been separated from Dubawnt minette dykes and breccias at various localities (e.g. Chisholm, 1993; Kaminsky et al., 1998). Microdiamonds, with a peak in size distribution at 0.25 mm, are present in large numbers in the Akluilaˆk dyke (Fig. 6) (Kaminsky et al., 1998). The diamonds are rich in Type Ib-IaA nitrogen aggregates, with a total maximum N content of 8000 ppm, far in excess of normal mantle diamonds (Chinn and Kyser, 2000). Hydrogen contents are also unusually high, and the average carbon isotope composition (:/ /7 per mil) is significantly lower than most other diamonds. Chinn and Kyser, 2000 note that the nitrogen aggregates are comparable to microdiamonds in the ultrahigh pressure Kokchetav metamorphic massif, and speculate that the Akluilaˆk microdiamonds developed at low temperature (8608/990 8C) and may have nucleated metastably on hydrogen species at pressures less than the diamond stability field. The nitrogen aggregates can only have had a very short residence time in the mantle (:/1 Ma).
88
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Fig. 12. Rare earth elements for: (a) the Hudson granitoids and Dubawnt minettes, and (b) the Nueltin granites and an associated diabase dyke. Envelopes represent 9/1 standard deviation from the average.
The most primitive minettes are ultrapotassic, depleted in Ca, and highly magnesian: of 155 analyzed samples, 78 have MgO /8% with average CaO /6.3%, MgO /10.2% and Mg/(Mg/ Fetotal)/0.73 (the analyses in Table 1 are of minettes from the Kaminak greenstone belt, and have abnormally high CaO). They show strong LILE enrichment and very strong HFSE depletion, particularly in Ti, Nb, and Ta (Fig. 16a). The Dubawnt minettes have moderate Al2O3 contents (:/12%) that are typical of the ‘calc-alkaline minettes’ of Rock (1991), but their low Ca contents, high K and Rb, and some minerals (leucite, tetraferriphlogopite, K-richterite) are diagnostic of lamproites (Mitchell and Bergman, 1991). Statistical studies show the Dubawnt minettes most
closely resemble other ultrapotassic rocks that have been termed transitional minette-lamproites (e.g. sills in the Gondwana coalfields, India: Rock et al., 1992) and minettes from other continental orogenic hinterlands (e.g. the Bohemian Massif: Ne´mec, 1974). The MORB-normalized trace element profiles of the minettes are similar to those considered typical of subduction-zone magmas. The average minette is remarkably similar to the most enriched of the Hudson granitoids, except that the granitoids have relatively depleted Ba and P (Fig. 16a). The minettes all have strongly enriched Nd isotope compositions, with o Nd,1830 Ma averaging /8 and ranging from /5 to /11 (Fig. 14). No isotopic distinction is noted between mafic and felsic rocks, suggesting that the Nd isotopic composition is inherited from the source region and is not due to crustal contamination. Present-day Pb in a suite of minette dykes is variable, extending from 206 Pb/204Pb :/18.5 to highly nonradiogenic values with 206Pb/204Pb :/15 (Peterson et al., 1994). The most nonradiogenic samples have Pb similar to that of the most extreme lamproites (e.g. Sisimiut, Greenland: Nelson, 1989). The Dubawnt minettes present several petrogenetic conundrums. They have a subduction zone trace element signature, despite being situated in Archean crust hundreds of kilometers from any subduction zone of similar age; they must originate in mantle rocks, but in their incompatible element and Nd isotope compositions they closely resemble consanguinous crustally-derived granitoids; and they have both highly enriched incompatible element and highly depleted compatible element (high Mg/Fe, low Ca) signatures. These geochemical features, plus very nonradiogenic lead, are typical of lamproites, which are thought to originate in a layer of lithospheric upper mantle that was once depleted by extraction of Ca- and Fe-rich basaltic melt, and subsequently enriched in K and other LILE (Mitchell and Bergman, 1991). 2.3. Nueltin Granites and Pitz Formation, 1.76 Ga The Nueltin granites are strongly porphyritic, hypersolvus rocks with phenocrysts of quartz, high-T alkali feldspar, and plagioclase (Fig. 17).
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
89
Fig. 13. 143Nd/144Nd as a function of time, calculated from present-day Nd /Sm isotopic ratios. Data from Table 1 and published sources (see text). Depleted mantle is assumed to have present-day 147Sm/144Nd/0.1966, 143Nd/144Nd/0.512638.
Their contacts are chilled and discordant, and several of the plutons intruded their own volcanic edifices. Most are associated with prominent negative gravity anomalies, suggesting a significant thickness, and with intense radiometric anomalies due to high Th contents. The correlative rhyolites (Pitz Formation; LeCheminant et al., 1987b) are mostly porphyritic but some glassy domes are nearly crystal-free. These contain highly resorbed phenocrysts of quartz and anorthoclase, plus abundant clots of basaltic material consisting of plagioclase (An60 40), clinopyroxene, and magnetite, surrounded by quenched, variably Fe-contaminated melt (Fig. 18). The Nueltin granites can
be associated with aphyric or plagioclase-phyric mafic to intermediate dykes and gabbroic bodies, and were locally mingled with mafic melts, producing heterogeneous monzodiorites (LeCheminant et al., 1987b). The mafic dykes consist mainly of clinopyroxene and plagioclase with microphenocrysts of magnetite, and can be rich in quartz xenocrysts. A U /Pb (baddelyite) age of 17509/1 Ma was obtained for the diabasic McRae Lake dyke (located in Fig. 3), which is mingled at its south end with a Nueltin granite pluton (A. LeCheminant, unpublished data). The most common crystallization sequence in the rhyolites was quartz (polymorph uncertain),
90
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Fig. 14. Nd and Sr isotope compositions of the Dubawnt minettes and both granitoid suites, as epsilon values at the time of emplacement. Rb /Sr isotope ratios were not determined for the granitoids or the McRae Lake diabase. The wide range in Sr composition of the minettes is due to decoupling of Rb and Sr into silicate and carbonate phases, respectively, and subsequent lowgrade metamorphism, with the observed mean close in value to the interpreted true mean (Cousens, 1999).
Fig. 15. Photomicrograph of a typical high-Mg minette lava from the Dubawnt Lake area. C, clinopyroxene; P, phlogopite.
quartz/anorthoclase, quartz/plagioclase, quartz/plagioclase/sanidine (or late microcline). Biotite is an important late phase and forms 1 cm crystals in the coarsest samples. Zircon, which is euhedral and finely zoned, can be phenocrystic but usually occurs in pockets of late-crystallizing
minerals with apatite, fluorite, magnetite, biotite, and titanite. Rapakivi texture is widespread in the Nueltin granites, though not common. Myrmekite is absent but graphic quartz /potassium feldspar is very common in porphyry dykes. Some rhyolites are topaz-bearing (LeCheminant et al., 1987b). Plots of normative feldspar contents (Fig. 9) are consistent with significant fractionation of anorthite component. In a normative quartz /feldspar plot (Fig. 10) the Nueltin granites form an array that overlaps the Hudson granitoid cluster, but extends toward the quartz /sanidine join. Large exposures of the Pitz Formation are only located on the north side of the complex of Baker Lake Group basins, and small outliers to the southwest are only present inside the basins (Fig. 6). The northernmost Nueltin plutons are south of the large Pitz exposures, and plutons are present in a SSW-trending band that extends well beyond the basins, suggesting relative post-Nueltin uplift occurred toward the south. North of 608 latitude, the Nueltin granites and the Hudson granitoids are nearly mutually exclusive. They are commonly separated by known shear zones and brittle faults,
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
91
Fig. 17. Polished slabs of (a) Nueltin granite, with sanidine phenocrysts, and (b) Pitz Formation rhyolite with phenocrysts of quartz and sanidine. Note the mafic fragments (m) in the rhyolite, which are clots of basalt. Round objects on the rhyolite are air bubbles in water.
Fig. 16. MORB-normalized trace element diagrams, after Rock and Wheatley (1988). (a) Dubawnt minettes and Hudson granitoids; (b) Nueltin granites and basaltic rocks. Envelopes represent 9/1 standard deviation from the average.
so the map pattern is probably due to different exposure levels in the crust, with the mid-crustal Hudson plutons remaining hidden in a downdropped corridor that bears the Nueltin plutons. Although the Nueltin plutons are relatively restricted, nearly the entire WCP shows resetting of Rb /Sr and K /Ar ages at about 1.75 Ga (e.g. Loveridge et al., 1988). Compared to the Hudson granitoids, the Nueltin granites and Pitz rhyolites are more aluminous, more siliceous, and have higher average K/Na and lower normative An. They are not depleted in Y (Fig. 11) and have much lower Ce/Yb (average is 47). A prominent negative Eu anomaly is typical (Fig. 12). The range of o Nd in the Hudson granitoids extends to more negative values because some samples come from a block of old ( /3.0 Ga)
Fig. 18. Microgabbroic fragment in superheated rhyolite, Pitz Formation. A resorbed anorthoclase phenocryst is above the basalt. The rhyolite glass surrounding the fragment is darkened from Fe contamination.
92
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
crust south of the Kaminak greenstone belt (van Breemen et al., in preparation); if these are excluded, the two granitic suites cannot be distinguished with Nd isotopes. The basaltic rocks associated with the Nueltin granites are difficult to characterize geochemically due to fractional crystallization, and mixing with rhyolite magmas and xenocrysts (the highest MgO concentration recorded in these rocks is 5.1%), but they most closely resemble fractionated alkali basalt (Table 1) and are distinctive amongst igneous rocks of the region at this time for having K/Na B/1. Their incompatible element profiles have an overall shape similar to the granitoids and the minettes, but are displaced towards MORB (Fig. 12). The average Nd isotope composition of two samples of the McRae Lake diabase dyke, which is locally mingled with Nueltin granite, lies at the depleted edge of the range of Nueltin granite compositions (Figs. 13 and 14) and is within, though near the upper limit of, the Nd composition of the minettes. Source contributions from enriched mantle are therefore possible but since the diabase has low K/Na, major contributions from the same source as the minettes is not indicated. It would be difficult to distinguish this from mixing with crustal rocks or granite magma, and the unusually high Y content of the McRae Lake dyke (Fig. 11) indicates that mixing with Nueltin granite magmas did occur. The Nueltin granites, which formed discordant subvolcanic stocks and had a wide temperature interval of crystallization, represent larger and hotter magma bodies than those which formed the Hudson plutons. The presence of mingled basalt-granite, and of basaltic clots and resorbed phenocrysts in nearly superheated rhyolites, suggests the magmas were generated by intrusion of basalt into crust. The Nueltin granites are concentrated in areas affected by previous to synvolcanic brittle faulting. They therefore fit a commonly cited model for post- or anorogenic rapakivi granites, which postulates basaltic melts invading the lower crust along faults and ponding beneath subvolcanic plutons that are undergoing low-pressure fractional crystallization (e.g. Ra¨mo¨ and Haapala, 1990).
2.4. Discussion: sources and heat triggers for the igneous suites The broad extent of ca. 1.75 Ga thermal resetting in the WCP associated with the anorogenic Nueltin granites, and the evidence for a basaltic trigger, suggests that a large-scale mantle disturbance caused their formation. The approximately 100 million year time gap measured between intrusion of post-orogenic granitoids and anorogenic rapakivi granites matches that observed in many other approximately 2.0 /1.6 Ga Laurentian orogens (e.g. the Svecofennian orogen: Nironen, 1997; the Ketilidian orogen: Brown et al., 1992), which argues for a common mechanism for their generation that is related to evolution of the hinterland lithosphere. Rutile and titanite Pb /Pb and U /Pb ages southwest of the Athabasca basin in northern Saskatchewan cluster tightly near 1750 Ma, suggesting rapid uplift and denudation rates outside the Nueltin corridor at that time (Orrell et al., 1999). As noted above, the absence of midcrustal Hudson granitoid plutons within the Nueltin corridor indicates that it was relatively downdropped during or after emplacement of the Nueltin suite, particularly at its north end. A mechanism that combines local crustal extension with passive, relatively unfocussed mantle upwelling is required and we suggest that delamination of lithospheric mantle is the most likely cause. The resulting thermal anomaly would be at least as extensive as the foundered lithospheric mantle, with basalt injection occurring where the overlying crust is subject to brittle faulting. In the WCP, this faulting most easily developed where previous faulting related to the Slave-Churchill indentation at approximately 1.84 Ga had occurred. Magnetotelluric data from the Chesterfield Inlet area, which is well outside the Nueltin corridor, indicate that Archean lithospheric mantle in that area is preserved (Jones et al., 2000), and hence delamination could not have occurred everywhere in the study area. The heat source for the Hudson granitoids is not easily constrained. Minor gabbros associated with the plutons near Wager Bay might represent juvenile mafic magmas emplaced in the crust, but these are not present in other areas. Although a
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
significant amount of cogenetic mafic magma (the minettes, and lesser spessartites) was present in much of the WCP, these have no discernible component of juvenile mantle, which seemingly requires that melting occurred only in the lithosphere. Most of the Hudson granitoids occur outside the area intruded by minette dykes, so that the minettes cannot have been the sole melting trigger for the granitoids. Finally, the timing and mechanism of enrichment of the minette source region is in doubt, making it difficult to incorporate them with the granitoids into a common model. Although crustal thickening alone is probably capable of generating lower to mid-crustal minimum granitoid melts at approximately 800 8C (e.g. Gerdes et al., 2000), the high-Mg minettes likely have liquidus temperatures near 1100 8C (Esperanc¸a and Holloway, 1987) and probably require a convective heat source. A successful petrogenetic model for the minettes and granitoids should additionally incorporate those features that are present in comparable igneous suites from similar tectonic environments. To guide the discussion, we next examine these features by describing important examples.
3. Petrogenetic models for the Hudson granitoids and Dubawnt minettes 3.1. Comparison of the WCP to other Laurentian hinterlands Overlapping minette-calcalkaline granite (MCG) and basalt-rapakivi granite (BRG) associations are not unique to the Trans-Hudson hinterland in the early Proterozoic. Similar rocks of nearly identical age have been described from at least three other localities: Greenland, Brazil, and Baltica. These examples also include similarities in the sequences of crustal deformation and basin development, and all occur within the upper plates (hinterlands) of prominent 2.0 /1.6 Ga orogens. 3.1.1. Greenland Pink-weathering, equigranular granitoid plutons dated at near 1830 Ma are prominent in the northern Nagssuqtoqidian orogen (Kalsbeek and
93
Nutman, 1996). These intrusions, described as insitu melts, shortly post-date ductile deformation in Archean wall rocks. Although minette dykes or plutons have not been described in the immediate vicinity, they are present approximately 100 km to the north, at Oqaitsungui, near Disko Bugt (Skerjnaa, 1992). These rocks are petrographically and geochemically identical to the Dubawnt minettes, and also contain ellipsoidal glimmerite xenoliths. The Greenland minettes have not been dated by U/Pb methods but have a K/Ar age of 1750 Ma. Dykes of approximately 1.76 Ga, undeformed granite are present in the Nagssuqtoqidian domain (Whitehouse et al., 1998), but rapakivi plutons have not been described. South of the Nagssuqtoqidian, in the Ketilidian orogen, is a prominent rapakivi granite province, dated at 1750 Ma. These granites also cut ca. 1.83 Ga post-tectonic equigranular granitoids (Brown et al., 1992). Tectonic reconstructions of this part of Laurentia are made difficult by water and ice cover and complex geology; however, it is possible that the Nagssuqtoqidian domain is an extension of the Churchill Province. 3.1.2. Brazil The early Proterozoic WCP was strikingly similar to the Saˆo Francisco craton of Brazil during and shortly after the Trans-Amazonian orogeny (:/2.1 /1.8 Ga: Alkim and Marshak, 1998). Ultrapotassic plutons mingled with calcalkaline granitoids, emplaced at approximately 2.05 Ga in reworked Archean rocks (Rosa et al., 1999), crop out west of the terrigenous Espinhac¸o Basin, on the western margin of the craton. The lower Espinhac¸o Group consists of alluvial and fluvial sediments, interbedded with bimodal (rhyolite-basalt) volcanic rocks, deposited in basins formed by extension and block faulting, beginning at approximately 1.75 Ga (Uhlein et al., 1998; Martins-Neto, 2000). The rift facies rocks were overlain by fluvial and aeolian sandstones, capped by shallow marine deposits, that were deposited in a broader basin possibly related to thermal subsidence. The Espinhac¸o sequence is thus very similar in age and lithofacies to the middle and upper Dubawnt Supergroup. The minette volcanics of the lower Dubawnt Supergroup have no
94
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
equivalents in exposed portions of the Espinhac¸o basin but the high-K plutons indicate that ultrapotassic magmatism did occur shortly before. The ultrapotassic rocks have Nd /Sr isotopic compositions similar to average Dubawnt minette (Rosa et al., 2000). 3.1.3. Baltica A shoshonitic series from minette to calc-alkaline granite occurs in ca. 1.80 /1.77 Ga plutons in a 600 km-long belt from southern Finland to Russian Karelia, in the Fennoscandian orogen (Eklund et al., 1998). The plutons and dykes occur in a region that was heavily intruded by approximately 1.83 Ga, late to post-tectonic granitoids, and which coincides closely with an anorogenic 1.65 /1.55 Ga rapakivi granite province. The minettes are described as being identical in isotopic composition to the granitoids, and having overlapping trace element compositions. This igneous province differs from the examples cited above in that the intrusions are located in juvenile Proterozoic crust, and have depleted isotopic compositions. The time gap between MCG and BRG activity was also somewhat longer (200 myrs, rather than 100 myrs). 3.2. Phanerozoic examples Important examples of post-orogenic MCG associations in the Phanerozoic are too numerous to catalogue here. The minettes of the Variscan Bohemian Massif, which are geochemically nearly identical to the Dubawnt minettes (Ne´mec, 1974) are hybridized with granodioritic melts and have highly enriched Nd isotopic compositions (Gerdes et al., 1999). Ultrapotassic enclaves are prominent in the granodioritic Hercynian plutons of Corsica, which shortly post-date crustal thinning (Roberts and Ferre´, 1999). There are numerous other Hercynian examples of the association attributed to extensional thinning of previously thickened continental crust (e.g. western Alps: Bussy et al., 1999). In most cases the presence of high-Mg, highly incompatible-element enriched rocks is attributed to subduction enrichment of the upper mantle, but it is also commonly noted that
additional crustal contamination of the high-Mg magmas is indicated. Rock (1991) used the Caledonian (:/414 Ma) minettes of northern Britain as the type example of his MCG association. These are intimately associated with the Ross of Mull granitic pluton, and bear a striking geochemical resemblance to cogenetic granitic dykes (Rock and Hunter, 1987). To quote: ‘. . .lamprophyric magmatism. . .was intimately related to granitic magmatism in space, time, and composition’ (their italics). These examples emphasize the persistent geochemical similarity between continental hinterland minettes and cogenetic granites, regardless of their emplacement age or the age and previous history of their source region . 3.3. Comparison to the Tibetan Plateau Miller et al. (1999) have concisely summarized the occurrences of ultrapotassic and associated calc-alkaline silicic magmatism in the Tibetan plateau. Dated at approximately 15 /25 Ma, they postdate continent/continent collision by about 30 Ma. The calc-alkaline dacites and rhyolites contain ca. 400 Ma zircon xenocrysts and have a crustal origin. The ultrapotassic rocks (which petrographically are minettes, but which have some compositional characteristics of lamproites) have isotopic and trace element characteristics that imply significant source contributions from Archean mantle. However, contamination of this mantle by a crustal component is required (Miller et al. suggest subducted sediment). The authors suggest that exposure of a metasomatized lithospheric mantle layer to hot asthenosphere, by extension and thinning or slab break-off, is required to generate the compositions observed. More precise models for the formation of the Tibetan minette source region depend on the chosen mechanism for crustal thickening. Thickening may have occurred primarily by shortening, underthrusting of Indian lithosphere, or lateral inflow of Indian lithosphere (Treloar, 1997). With these scenarios, enrichment of a mantle root could occur by enrichment from subducted sediments or underthrust crustal rocks, or by imbrication of crust and mantle.
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
3.4. Tectonic models for MCG associations We suggest that the following features are salient characteristics of late crust-mantle evolution in large continental hinterlands: (1) nearsimultaneous, late to post-orogenic melting of deformed lithosphere in a lower to mid-crustal layer to generate calc-alkaline granitoids, and in a LILE-enriched layer of upper mantle to generate minettes. The melting is synchronous with or shortly postdates ductile extension, and does not involve significant contributions from asthenospheric melts; (2) regional low-grade thermal metamorphism, accompanied by local extension and emplacement of bimodal BRG, about 100 million years after the post-orogenic igneous suites; (3) broad crustal sagging leading to the formation of terrigenous, arenaceous basins capped with shallow marine chemogenic deposits, about 10 /50 myrs after bimodal volcanism. There being no indications of significant melting of convecting mantle in our study area, our preferred model for generation of the MCG granitoids is anatexis of lower to mid-crustal rocks in response to rising temperatures resulting from crustal thickening in the collisional hinterland. Direct evidence for an appropriate thickening event shortly before 1.85 Ga in the WCP is scant, but significant. Crustal thickening at ca. 1.9 Ga has been suggested to account for the high pressure metamorphism observed in granulitefacies rocks of the Kramanituar Complex (Sanborn-Barrie, 1999) and the amphibolite-facies supracrustal belt southwest of Baker Lake (Berman et al., 2000). Recent geochronologic evidence for ca. 1.9 Ga Barrovian metamorphism further to the east of this belt lends support to this hypothesis (Berman et al., 2002). The unusual microdiamonds of the Akluilaˆk dyke may reflect deep underthrusting of volatile-rich crust into the upper mantle in response to lithospheric shortening. Vertical movements on major faults at about 1.83 Ga, such as the Tyrrell shear zone, imply isostatic readjustment with relatively buoyant (thickened) crust to the east, towards the highest concentration of Hudson granitoid plutons. The mechanism of enrichment/melting of the minette source region has implications for the role
95
of the upper mantle in hinterland lithosphere evolution. At present, we cannot eliminate any of three models for the enrichment event, and the melting trigger remains speculative. A common element of all of these models, is that the minette source region began as a layer of subcontinental lithospheric upper mantle, depleted (by extraction of basalt magma) in incompatible elements resulting in mantle with low Ca and Al, and high Mg/ Fe. The principal incompatible element source of the minettes was enriched fluids or melts, which invaded the depleted peridotite and generated glimmerite veins that subsequently melted to produce ultrapotassic magma. Since the minettes display a clear subduction zone signature, with depleted HFSE, we presume that the enrichment was not caused by mafic silicate melts, but by K and H-rich fluids that had an ultimate source in crustal rocks. 3.4.1. Enrichment model 1: Proterozoic subduction The source region of the minettes may have been enriched during the Proterozoic by subduction of Archean continental sediment beneath the Thelon and THOs (Peterson et al., 1994), with a small time gap between enrichment and minette melt generation. A significant possible source of LILE-rich sediment was erosion of 2.6 Ga granitoid plutons, which has been correlated with a rapid increase in radiogenic Sr in seawater at about 1.8 Ga (Taylor and McLennan, 1985). The chief attraction of this hypothesis is its linkage with well-established tectonic events (i.e. two Proterozoic collisional orogenies), and the fact that it can incorporate examples (such as Baltica) where the possible sediment sources are in juvenile crust and the ultrapotassic rocks have depleted isotopic compositions. However, in the WCP this hypothesis is difficult to reconcile with the observations that the minettes are restricted to the interior of the hinterland, well away from the presumed sites of subduction, and that high-K rocks within the arc volcanic sequences of the THO have Nd compositions close to depleted mantle (Stern et al., 1995). It is reasonable to presume that a large volume of oceanic lithosphere was subducted beneath the central WCP, and a highly speculative alternative is that this accumulated in a megalith that subse-
96
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
quently released a pulse of LILE-enriched fluids or melts beneath the center of the WCP. 3.4.2. Enrichment model 2: Archean subduction The Nd/Sr isotopic data allow that LILE enrichment of lithospheric mantle may have occurred at about 2.8 /2.7 Ga (Cousens et al., 2001). The enrichment may have resulted from subduction during the assembly of the WCP in the late Archean. This hypothesis is supported by the presence of some pre-1.9 Ga mafic dykes and sills in the WCP that plot within the Nd evolution envelope of the minettes, and is attractive for its simplicity, testability, and possible application to other Archean hinterlands. Its principal difficulty is that, if enrichment occurred in the Archean, the distribution of the Dubawnt minettes should reflect to some degree the distribution of major Archean domains and structures in the WCP. However, the minette dyke swarm cuts across all such features, including greenstone belts that were generated at approximately 2.8 /2.7 Ga. There is also no evidence for ultrapotassic rocks in the WCP before 1.9 Ga, particularly in association with the 2.6 Ga granitoid suite, which is similar to the Hudson suite in its average composition and level of emplacement. 3.4.3. Enrichment model 3: crust/mantle mixing An alternative to subduction of sediment as a means to enrich the base of the lithospheric mantle, is direct underthrusting of continental crust or imbrication of crust and mantle wedges deep in the lithosphere. This model is attractive in that: (1) it directly links the generation of minettes to lithospheric thickening, which is generally presumed to play a role in the generation of the associated granitoids, and (2) explains the widelymade observation that the minettes and the granitoids have similar isotopic and trace element signatures. Its principal difficulty is that the model is difficult to test, as it depends on mapping, lithosphere imaging, and thermobarometric data at a level of detail that is not yet available in the WCP. Peridotite xenoliths in the minettes are rare and invariably strongly altered, but if unaltered ones can be identified in the minettes or other rocks,
then it may be possible to date metasomatic mineral assemblages in them. Further teleseismic and magnetotelluric imaging of the WCP lithosphere (e.g. Jones et al., 2000) across the boundary of the minette dyke swarm might reveal contrasts in the composition of the lower lithosphere, or the presence of major low-angle faults. Finally, a comprehensive study of the approximately 2.6 Ga granitoids of the WCP may provide useful information on regional variations in possible lithosphere enrichment in the Archean.
4. Summary Paleoproterozoic granitoid magmatism in the WCP, a reworked Archean hinterland to two Paleoproterozoic orogenies, occurred in two pulses. Minimum crustal melts, rich in inherited xenocrysts, were emplaced at mid-crustal levels during and shortly after collision of the Superior and Churchill cratons at the THO, at approximately 1.83 Ga (Hudson granitoids). The resulting plutons mostly cross-cut late ductile features in mid-crustal rocks and appear similar in all respects to post-orogenic granitoids that are present in other Archean hinterlands with a similar history. Throughout an area of about 105 km2, the Hudson plutons are closely associated in space and time with ultrapotassic (mainly minette) dykes and volcanic rocks which have isotopic and trace element compositions similar to the granitoids; globally, this association has appeared repeatedly and often in both Archean and juvenile hinterlands to large orogens since about 2.0 Ga. Approximately 100 myrs later ( :/1.76 Ga), the WCP was subjected to widespread heating sufficient to reset K/Ar and Rb /Sr systems and, where older and contemporaneous brittle faulting was present, basalts were emplaced in the lower crust to generate crustal melts that ascended to the surface (Nueltin granites and Pitz Formation rhyolites). The plutonic rocks are locally rapakivi-textured and are identical in all respects to anorogenic rapakivi granite plutons that typically intruded other hinterlands about 100 /200 myrs after emplacement of MCG suites. Since minettes were not generated at that time, it may be that
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
their source was removed by delamination and foundering of the lithospheric upper mantle, permitting the generation of basalt melts by mantle upwelling. In the WCP, the rapakivi province defines a SW-trending corridor that coincides with the volcanic part of the minette province, and with brittle fault systems active at approximately 1.83 Ga and later. The anorogenic rhyolite volcanism was followed within 50 myrs by continent-scale sagging, perhaps resulting from cooling of the upper mantle and thermal subsidence, which produced a very large-scale terrigenous basin capped by shallow marine deposits (Thelon and Athabasca sandstones). This tripartite sequence, for which a sedimentary record is completely preserved in the Dubawnt Supergroup of the WCP, is also evident in the approximately 2.0 /1.8 Ga Trans-Amazonian orogeny of the Saˆo Francisco craton. The first two igneous phases are also preserved in numerous other hinterlands, suggesting that a single tectonic model could be inferred for many of them. Different scenarios have been proposed to explain the presence of the ultrapotassic rocks and subduction enrichment of the lithospheric mantle, shortly before terminal collision, is the most widely cited. However, this model is unsatisfactory in the WCP, primarily because the ultrapotassic province is located far from any Proterozoic subduction zone and potassic rocks in those zones have depleted isotopic compositions. Subduction enrichment of the lithospheric mantle could have occurred at about 2.7 Ga, with glimmerite veins melting at 1.85 Ga due to intense reworking of the lithosphere. A third model, which attempts to integrate the geochemical features of the granitoids and minettes with the shortening/thickening history of the hinterland, places the minette source region in a zone of tectonically mixed crust (principal source of LILE and other incompatible elements) and depleted upper lithospheric mantle (principal source of compatible elements). In all three models, the granitoids are derived from a shallower level in the lower to middle crust. The high mg# (commonly /0.8) and liquidus temperature of the minettes (:/1100 8C?) require a relatively intense heat source that may have
97
resulted from mantle upwelling after slab breakoff beneath the hinterland.
Acknowledgements The authors wish to thank all members of the Churchill NATMAP team for their data, ideas, and support. Richard Stern assisted with SHRIMP analyses. Many of the samples and maps incorporated in this project are from previous GSC projects, and we particularly note contributions by A. LeCheminant, A. Davidson, K. Eade, and S. Tella. This paper is a contribution to the Churchill NATMAP Project and to IGC Project #426 (Proterozoic Granites and Tectonic Processes). This is GSC Contribution 2001155.
References Akim, F.F., Marshak, S., 1998. Transamazonian Orogeny in the Southern Sa˜o Francisco Craton Region, Minas Gerias, Brazil: evidence for collision and collapse in the Quadrila´tero Ferrı´fero. Precambrian Res. 90, 29 /58. Annesley, I.R., Madore, C., Rupan, S. Krogh, T.E., 1997. U / Pb geochronology of thermotectonic events in the Wollaston Lake area, Wollaston Domain: a summary of 1994 / 1996 results. Summary of Investigations 1997, Sask. Geol. Surv. Misc. Report 97 /4, 162 /173. Aspler, L.B., Chiarenzelli, J.R., 1997. Archean and Proterozoic geology of the North Henik Lake area, district of Keewatin, Northwest Territories, Current Research 1997-C, Geol. Surv. Can., 145 /155. Berman, R.G., Ryan, J.J., Tella, S., Sanborn-Barrie, M., Stern, R., Aspler, L., Hanmer, S.K., Davis, W., 2000. The case of multiple metamorphic events in the Western Churchill Province: evidence from linked thermobarometric and insitu SHRIMP data, and jury deliberations. Geol. Ass. Canada Yearly Meeting 2000, Abstr. [CD-ROM]. Berman, R., Davis, W.J., Ryan, J.J., Tella, S., 2002. In-situ U/ Pb SHRIMP geochronology of Barrovian facies-series metasedimentary rocks in the Happy lake and Josephine River supracrustal belts: implications for the Paleoproterozoic architecture of the northern Hearne domain, Nunavut. Current Research, Part F, Geol. Surv. Canada Pap. 2002-F4, pp. 1 /14. Blake, D.H., 1980. Volcanic rocks of the Paleohelikian Dubawnt Group in the Baker Lake /Angikuni Lake area, District of Keewatin, N.W.T. Geol. Surv. Can. Bull. 309. Booth, G.W., 1983. The petrology and geochemistry of the Pamiutuq Lake batholith, Northwest Territories 1983. M.Sc. Thesis Univ. Toronto, Canada.
98
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Brown, P.E., Dempster, T.J., Harrison, T.N., Hutton, D.H.W., 1992. The rapakivi granites of S Greenland */crustal melting in response to extensional tectonics and magmatic underplating. Trans. R. Soc. Edinburgh Earth Sci. 83, 173 /178. Bussy, F., Hernandez, J., von Raumer, J., 1999. Bimodal magmatism as a consequence of the post-orogenic readjustment of the thickened Variscan continental lithosphere (Aigulle Rouges/Mont-Blanc massifs, Western Alps). In: Barbarin, B. (Ed.), The Origin of Granites and Related Rocks, Fourth Hutton Symposium Abstr. Document du BRGM 290, 205. Carmichael, I.S.E., 1963. The crystallization of feldspar in volcanic acid liquids. Q. J. Geol. Soc. London 119, 95 /131. Chinn, I., Kyser, K., 2000. Microdiamonds from the Thirsty Lake (Akluilaˆk) dyke, Northwest Territories Canada. J. Conf. Abstr. 5, 307. Chisholm, R., 1993. Diamond Drilling Report on the DUB 1/ 2 Mineral Claims, District of Keewatin, Northwest Territories. Taiga Consultants Ltd, Calgary, Canada. Corrigan, D. Scott, D.J., St-Onge, M.R., 2001. Geology of the northern margin of the Trans-Hudson Orogen (Foxe Fold Belt), central Baffin Island, Nunavut. Current Research 2001-C, Geol. Surv. Can. C23 [CD-ROM]. Cousens, B.L., 1999. Geochemistry and geologic setting of the Baker Lake Group, Angikuni Lake to Yathkyed Lake. Indian and Northern Affairs Canada, Geol. Div. Open File 1999 /05. Cousens, B.L., Aspler, L.B., Chiarenzelli, J.R., Donaldson, J.A., Sandeman, H., Peterson, T.D., LeCheminant, A.N., 2001. Enriched Archean lithospheric mantle beneath Western Churchill Province tapped during Paleoproterozoic orogenesis. Geology 29, 827 /830. Culshaw, N., 1991. Post-collisional oblique convergence along the Thelon Tectonic Zone, north of the Bathurst Fault, NWT, Canada. J. Struct. Geol. 13, 501 /516. Davidson, A., 1969. Precambrian geology, Kaminak Lake maparea, District of Keewatin. Geol. Surv. Can. Pap., 51 /69. Dewey, J.F., Burke, K.C.A., 1973. Tibetan, Variscan, and Precambrian basement reactivation: products of a continental collision. J. Geol. 81, 683 /692. Donaldson, J.A., 1968. Proterozoic sedimentary rocks of northern Saskatchewan. Geol. Surv. Can., Report of Activities, Pap. 68-1A. ¨ ., LeCheminant, A.N., Sullivan, R.W., 1991. Duda´s, F.O Reconnaisance Nd isotopic study of granitoid rocks from the Baker Lake region, District of Keewatin, N.W.T., and observations on analytical procedures. Radiogenic and Isotopic Studies: Report 4, Geol. Surv. Can. Pap. 90 /2, pp. 101 /112. Eklund, O., Konopelko, D., Rutanen, H., Fro¨jdo¨, S., Shebanov, A.D., 1998. 1.8 Ga Svecofennian post-collisional shoshonitic magmatism in the Fennoscandian shield. Lithos 45, 87 /108. Esperanc¸a, S., Holloway, J.R., 1987. On the origin of some mica-lamprophyres; experimental evidence from a mafic minette. Contrib. Miner. Petrol. 95, 207 /216.
Gall, Q., 1994. The Proterozoic Thelon Paleosol, Northwest Territories, Canada. Precambrian Res. 68, 115 /137. Gall, Q., Peterson, T.D., Donaldson, J.A., 1992. A proposed revision of early Proterozoic stratigraphy of the Thelon and Baker Lake basins, Northwest Territories. Current Research 1992-C, Geol. Surv. Canada, pp. 129 /137. Gerdes, A., Wo¨rner G., Finger F., 1999. Hybrid generation by mixing enriched mantle magmas with crustal melts: constraints from the Rasyenburg pluton, southern Bohemia. In: Barbarin, B. (Ed.), The Origin of Granites and Related Rocks, Fourth Hutton Symposium Abstr. vol. Document du BRGM 290, 22. Gerdes, A., Wo¨rner, G., Henk, A., 2000. Post-collisional granite generation and HT-LP metamorphism by radiogenic heating: the Variscan South Bohemian Batholith. J. Geol. Soc. Lond. 157, 577 /587. Hadlari, T., Rainbird, R.H., 2001. Volcano-sedimentary correlation and fault relationships in the Baker Lake sub-basin, Thirty mile lake area, Nunavut. Current Research 2001-C, Geol. Surv. Can. C10 [CD-ROM]. Hanmer, S., 1997. Geology of the Striding-Athabasca mylonite zone, northern Saskatchewan and southeastern District of Mackenzie, Northwest Territories. Geol. Surv. Can. Bull., 324. Hanmer, S.K., Maclachlan K., Aspler L.B., Berman, R.G., Davis, W.J., Rainbird, R.H., Zaleski, E., Pehrsson, S.J., Sandeman, H.A., Ryan, J.J., Jones, A.G., Peterson, T.D., Tella, S., Relf, C. Late Neoarchean and Proterozoic reworking of the Western Churchill Province, Canadian Shield, Nunavut. Precambrian Res., in preparation. Henderson, J.R., 1983. Structure and metamorphism of the Aphebian Penrhyn Group and its Archean basement complex in the Lyon Inlet area, Melville Peninsula, District of Keewatin. Geol. Surv. Can. Bull., 324. Henderson, J.R., Roddick, J.C., 1990. U /Pb age constraint on the Wager shear zone, District of Keewatin, N.W.T. Radiogenic Age and Isotopic Studies: Report 3, Geol. Surv. Can. Pap. 89-2, pp. 149 /152. Hoffman, P.F., 1988. United Plates of America, the birth of a craton: early Proterozoic assembly and growth of North America. Ann. Rev. Earth Sci. Lett. 16, 543 /603. Jenner, G.A., Longerich, H.P., Jackson, S.E., Fryer, B.J., 1990. ICP-MS-a powerful new tool for high precision trace element analysis in Earth sciences: evidence from analysis of selected USGS standards. Chem. Geol. 83, 133 /148. Jones, A.G., Snyder, D., Asudeh, I., White, D., Eaton, D., Clarke, G., 2000. Lithospheric architecture at the RaeHearne boundary revealed through magnetotelluric and seismic experiments. Geol. Ass. Canada Yearly Meeting 2000, Abstr. [CD-ROM]. Kalsbeek, F., Nutman, A.P., 1996. Anatomy of the Early Proterozoic Nagssugtoqidian orogen, West Greenland, explored by reconnaissance SHRIMP U /Pb zircon dating. Geology 24, 515 /518. Kaminsky, F.V., Sablukov, S.M., Sablukova, L.I., Shpanov, V.E., Zhuravlev, D.Z., 1998. Diamondiferous minette dykes from the Parker Lake area, N.W.T., Canada. Seventh
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100 International Kimberlite Conference Ext. Abstr., pp. 392 / 394. Krogh, T.E., Clark, L.A., 1987. Zircon dating of sub-Athabasca granitoid rocks, Saskatchewan. Geol. Ass. Canada Annual Meeting Abstr. 12, pp. 64. LeCheminant, A.N., Roddick, J.C., Tessier, A.C., Bethune, K.M., 1987a. Geology and U /Pb ages of early Proterozoic calc-alkaline plutons northwest of Wager Bay, District of Keewatin. Current Research, Part A, Geol. Surv. Canada Pap. 87-1A, pp. 773 /782. LeCheminant, A.N., Miller, A.R., LeCheminant, G.M., 1987b. Early Proterozoic alkaline igneous rocks, District of Keewatin, Canada: petrogenesis and mineralization. In: Pharaoh, T.C., Beckinsdale, R.D., Rickard, D. (Eds.), Geochemistry and Mineralization of Proterozoic Volcanic Suites. Geol. Soc. Spec. Publ. 33, pp. 219 /240. Loveridge, W.D., Eade, K.E., Sullivan, R.W., 1988. Geochronological studies of Precambrian rocks from the southern District of Keewatin. Geol. Surv. Can., 88 /188. MacLachlan, K., Relf, C., Davis, W.J., 2000a. U/Pb geochronological constraints on structures controlling distribution of tectonothermal domains, Yathkyed Lake area, Western Churchill Province. Geol. Ass. Canada Yearly Meeting 2000 Abstr. [CD-ROM]. MacLachlan, K., Hanmer, S., Davis, W.J., Berman, R.G., Ryan, J.J., Relf, C., Aspler, L.B. 2000b. Complex, protracted, Proterozoic reworking of the Western Churchill Province: the craton that wouldn’t grow up? Geol. Ass. Canada Yearly Meeting 2000 Abstr. [CD-ROM]. Martins-Neto, M.A., 2000. Tectonics and sedimentation in a Paleo/Mesoproterozoic rift-sag basin (Espinhac¸o basin, southeastern Brazil). Precambrian Res. 103, 147 /173. Meyer, M.T., Bickford, M.E., Lewry, J.F., 1992. The Wathaman Batholith; an early Proterozoic continental arc in the Trans-Hudson orogenic belt, Canada. Geol. Soc. Am. Bull. 104, 1073 /1085. Miller, C., Schuster, R., Klo¨tzi, U., Frank, W., Purtscheller, F., 1999. Post-collisional potassic and ultrapotassic magmatism in SW Tibet: geochemical and Sr /Nd /Pb /O isotopic constraints for source characteristics and petrogenesis. J. Petrol. 40, 1399 /1424. Mitchell, R.H., Bergman, S.C., 1991. Petrology of Lamproites. Plenum, New York. Nelson, D.R., 1989. Isotopic characteristics and petrogenesis of the lamproites and kimberlites of central West Greenland. Lithos 22, 265 /274. Ne´mec, D., 1974. Petrochemistry of the dyke rocks of the central Bohemian Pluton. Neues J. Miner. Monatsh. 5, 193 /209. Nironen, M., 1997. The Svecofennian Orogen: a tectonic model. Precambrian Res. 86, 21 /44. Orrell, S.E., Bickford, M.E., Lewry, J.F., 1999. Crustal evolution and age of thermotectonic reworking in the western hinterland of the Trans-Hudson Orogen, northern Saskatchewan. Precambrian Res. 95, 187 /223. Peterson, T.D., 1992. Early Proterozoic ultrapotassic volcanism of the Keewatin Hinterland, Canada. In: Meyer, H.O.A.,
99
Leonardos, O.H. (Eds.), Proceedings of the Fifth International Kimberlite Conference, vol. 1: Kimberlites, Related Rocks and Mantle Xenoliths. Companhia de Pesquisa de Recursos Minerais (Brası´lia), pp. 221 /235. Peterson, T.D. Geology of the Dubawnt Lake area, Nunavut. Geol. Surv. Canada Bull., in preparation. Peterson, T.D., LeCheminant, A.N., 1993. Glimmerite xenoliths in Early Proterozoic ultrapotassic rocks from the Churchill Province. Can. Miner. 31, 801 /819. Peterson, T.D., Esperanc¸a, S., LeCheminant, A.N., 1994. Geochemistry and origin of the Proterozoic ultrapotassic rocks of the Churchill Province Canada. Miner. Petrol. 51, 251 /276. Peterson, T.D., van Breemen, O., 1999. Review and progress report of Proterozoic granitoid rocks of the Western Churchill Province, Northwest Territories (Nunavut). Current Research 1999-C, Geol. Surv. Can., pp. 119 /127. Ra¨mo¨, O.T., Haapala, I., 1990. The rapakivi granites of eastern Fennoscandia: a review with insights into their origin in the light of new Sm-Nd isotopic data. In: Gower, C.F., Rivers, T., Ryan, B. (Eds.), Mid-Proterozoic Laurentia-Baltica. Geol. Ass. Can. Spec. Pap. 38, 401 /415. Rainbird, R.H., Peterson, T.D., 1990. Physical volcanology and sedimentology of lower Dubawnt Group strata, Dubawnt Lake, District of Keewatin, N.W.T. Current Research 1990C, Geol. Surv. Can., pp. 207 /217. Rainbird, R.H., Hadlari, T., Donaldson, J.A., 1999. Stratigraphy and paleogeography of the Paleoproterozoic Baker Lake Group in the eastern Baker Lake basin, Northwest Territories (Nunavut). Current Research 1999-E, Geol. Surv. Can., pp. 43 /53. Rainbird, R.H., Hadlari, T.T., Donaldson, J.A., 2001. Evolution of the Paleoproterozoic intracratonic Baker Lake and Thelon basins, Western Churchill Province, Canada, In: Cassidy, K.F., Dunphy, J.M., Van Kranendonk, M.J. (Eds.), Fourth International Archean Symposium, Perth, Australia, AGSO-Geoscience Australia Record 2001/37, pp. 259 /260. Relf, C., Maclachlan, K., Irwin, D., 1999. Tectonic assembly and Proterozoic reworking of the northern Yathkyed greenstone belt, Northwest Territories (Nunavut). Current Research 1999-C, Geol. Surv. Can., pp. 139 /146. Roberts, M.P., Ferre´, E.C., 1999. The succession of high-K calc-alkaline to alkaline granitic rocks in collisional belts tracks post-collisional crustal thinning. In: Barbarin B. (Ed.), The Origin of Granites and Related Rocks, Fourth Hutton Symposium Abstr. vol. Document du BRGM 290, 184. Rock, N.M.S., 1991. Lamprophyres. Blackie and Son Ltd, Glasgow. Rock, N.M.S., Griffin, B.J., Edgar, A.D., Paul, D.K., Hergt, J.M., 1992. A spectrum of potentially diamondiferous lamproites and minettes from the Jharia coalfield, eastern India. J. Volc. Geotherm. Res. 50, 55 /83. Rock, N.M.S., Hunter, R.H., 1987. Late Caledonian dykeswarms of northern Britain: spatial and temporal intimacy between lamprophyric and granitic magmatism around the
100
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Ross of Mull pluton, Inner Hebrides. Geol. Rundsch. 76, 805 /826. Rock, N.M.S., Wheatley, M., 1988. SPIDER: a Macintosh program to generate normalized multi-element ‘spidergrams’. Am. Miner. 73, 919 /921. Rosa, M.L.S., Conceic¸aˆo, H., Martin, H., Conceic¸aˆo, R.V., Nardi, L.V.S., Oberli, F., Meier, M., Paim, M., Barreto dos Santos, E., Perreira Santos, C.G., Oliveira, A.E., Leahy, G.A.S., 1999. Significance of Paleoproterozoic potassicultrapotassic syenitic provinces in the Bahia State, Brazil. In: Barbarin, B. (Ed.), The Origin of Granites and Related Rocks, Fourth Hutton Symposium Abstr. vol. Document du BRGM 290, 186. Rosa, M.L.S., Conceic¸aˆo, H., Oberli, F., Meier, M., Martin, H., Macambira, M.J.B., Conceic¸aˆo, R.V., Barreto dos Santos, E., Paim, M., Leahy, G.A.S., Leal, L.R.B., 2000. Geochronology (U /Pb/Pb /Pb) and isotopic signature (Rb /Sr/Sm /Nd) of the Paleoproterozoic Guanambi batholith, southwest Bahia state (NE Brazil). Rev. Bras. Geo. 30, 62 /65. Sanborn-Barrie, M., 1994. Structural investigation of highgrade rocks of the Kramanituar Complex, Baker Lake area, Northwest Territories. Current Research 1994-E, Geol. Surv. Can., pp. 75 /89. Sanborn-Barrie, M., 1999. Geochemistry of gabbroic rocks from the 1.9 Ga Kramanituar Complex and surrounding country rocks of the Archean Western Churchill Province, Nunavut. Current Research 1999-E, Geol. Surv. Can., pp. 75 /89. Sandeman, H.A., Lepage, L.D., Ryan, J.J., Tella, S., 2000. Field evidence for commingling between ca. 1830 Ma lamprophyric, monzonitic and monzogranitic magmas, MacQuoid-Gibson Lakes area, Kivalliq Region, Nunavut; Current Research, Geol. Surv. Can., 2000-C [CD-ROM]. Skerjnaa, L., 1992. A lamproite stock with ellipsoidal phlogopite nodules at Oqaitsu´nguit, Disko Bugt, central West Greenland. Rapp. Grønlands Geol. Unders. 154, 33 /47. Stern, R.A., 1997. The GSC sensitive high resolution ion microprobe (SHRIMP): analytical techniques of zircon U /Th /Pb age determinations and performance evaluation. Radiogenic Age and Isotope Studies: Report 10. Geol. Surv. Can., Current Res. 1994-F, pp. 69 /74. Stern, R.A., Berman, R.G., 2000. Monazite U /Pb and Th /Pb geochronology by ion microprobe, with an application to in situ dating of an Archean metasedimentary rock. Chem. Geol. 172, 113 /130. Stern, R.A., Syme, E.C., Bailes, A.H., Lucas, S.B., 1995. Paleoproterozoic (1.90 /1.86 Ga) arc volcanism in the Flin
Flon Belt, Trans-Hudson orogen, Canada. Contrib. Miner. Petrol. 119, 117 /141. Tamboso, A.W., 1981. Geology of the McRae Lake dyke, District of Keewatin, Northwest Territories. B.Sc. Thesis, Univ. Toronto, Canada. Taylor, W.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, London. Tella, S., Eade, K.E., 1985. Occurrence and possible tectonic significance of high-pressure granulite fragments in the Tulemalu fault zone, District of Keewatin, N.W.T. Canada. Can. J. Earth Sci. 23, 1950 /1962. Tella, S., Schau, M., Armitage, A.E., Loney, B.C., 1993. Precambrian geology and economic potential of the northeastern parts of Gibson Lake map area, District of Keewatin, Northwest Territories. Current Research Part C, Geol. Surv. Can. Pap. 93-1C, 197 /208. TerMeer, M., Berman, R.G., Relf, C., Davis, W.D., 2000. Tectonometamorphic history of the Nowyak complex, Nunavut, Canada. Geol. Ass. Canada Yearly Meeting 2000, Abstr. [CD-ROM]. The´riault, R.J., 1990. Methods for Rb /Sr and Sm /Nd isotopic analyses at the Geochronology Laboratory, Geological Survey of Canada. Radiogenic Age and Isotopic Studies: Report 3. Geol. Surv. Can. Pap. 89-2, pp. 3 /6. The´riault, R.J., Henderson, J.B., Roscoe, S.M., 1994. Nd isotopic evidence for early to mid-Archean crust from high grade gneisses in the Queen Maud Block and south of the McDonald Fault, Western Churchill Province, Northwest Territories. Radiogenic Age and Isotopic Studies: Report 8. Geol. Surv. Can. Pap. 1994-F, pp. 37 /42. Treloar, P.J., 1997. Thermal controls on early-Tertiary, shortlived, rapid regional metamorphism in the NW Himalaya, Pakistan. In: Touret, J.L.R., Austrheim, H. (Eds.), Collisional orogens, zones of active transfer between crust and mantle. Tectonophysics 273, 77 /104. Uhlein, A., Trompette, R.R., Egydio-Silva, M., 1998. Proterozoic rifting and closure, SE border of the Saˆo Francisco Craton Brazil. J. S. Am. Earth Sci. 11, 191 /203. van Breemen, O., Peterson, T.D., Sandeman, H.A. Geochronology and Nd isotope geochemistry of Proterozoic granitoids, and Archean domains of the Western Churchill Province. Can. J. Earth Sci., in preparation. Whitehouse, M.J., Kalsbeek, F., Nutman, A.P., 1998. Crustal growth and crustal recycling in the Nagssugtoqidian orogen of West Greenland: Constraints from radiogenic isotope systematics and U /Pb zircon geochronology. Precambrian Res. 91, 365 /381.
Proterozoic (1.85 1.75 Ga) igneous suites of the Western Churchill Province: granitoid and ultrapotassic magmatism in a reworked Archean hinterland /
T.D. Peterson a,, O. Van Breemen a, H. Sandeman b, B. Cousens c a
Natural Resources Canada, Geological Survey of Canada, 601 Booth Street, Ottawa, Canada K1A 0E8 b Canada-Nunavut Geoscience Office, P.O. Box 2319, Iqaluit, Canada XOA 0H0 c Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, Canada K1S 5B6 Received 1 April 2001; received in revised form 5 December 2001; accepted 19 March 2002
Abstract Paleoproterozoic igneous rocks in the Archean hinterland of the Paleoproterozoic Trans-Hudson orogen (THO) consist of voluminous late syn-orogenic to post-orogenic monzonite to granite (Hudson granitoids; :/1850 /1810 Ma), and contemporaneous ultrapotassic lamprophyre dykes and volcanic rocks (Dubawnt minettes) that are interbedded with alluvial fan and fluvial deposits (Baker Lake Group, lower Dubawnt Supergroup). They were followed at approximately 1750 Ma by rapakivi granite (Nueltin granite) and porphyritic rhyolite associated with aeolian sandstone (Pitz Formation, middle Dubawnt Supergroup). The tectonic cycle ended with the deposition of conglomerates and sandstones in a large sag basin (Thelon Formation, upper Dubawnt Supergroup, :/1.72 Ga). The Hudson granitoids, which are strongly concentrated northwest of the THO, were broadly synchronous with terminal collision between the Archean Churchill and Superior cratons and the development of NE-trending ductile structures in the Western Churchill Province (WCP) that may be related to tectonic escape to the northeast. They were emplaced at mid-crustal level and no volcanic equivalents are preserved. Fault-bounded basins containing the minette volcanic rocks are located farther west in a domain dominated more by brittle faulting. The Nueltin granites, emplaced during a period of active extensional faulting, are present in a band extending southwest from the minette basins toward a preserved remnant of the sag basin (the Athabasca basin). Hudson granitoids are largely absent from this band but reappear west of it, indicating a higher crustal level of exposure in a downdropped Nueltin ‘corridor’. The Nd isotope composition of the three suites is similar (minettes: o Nd,1830 Ma //5 to /11; Hudson granitoids: o Nd,1830 Ma //7 to /13.5; Nueltin suite: o Nd,1750 Ma //7 to /10.5), and they have late Archean model ages that match those of average Archean WCP rocks. The Hudson granitoids are rich in inherited Archean zircon, and both granitoid suites are interpreted as crustal melts. Some Nueltin granites and Pitz rhyolites are mingled with basalt, and the Nueltin suite fits a commonly cited model for rapakivi granite production, which postulates injection of basalt into extending, brittly faulted crust. The Hudson granitoids are similar to late syn- to post-orogenic plutons in numerous other collisional hinterlands, which are typically associated with ultrapotassic lamprophyres. The minettes, which have high mg# and bear mantle xenocrysts, must have
Corresponding author. Tel.: /1-613-992-3573; fax: /1-613-995-7997 E-mail address: [email protected] (T.D. Peterson). 0301-9268/02/$ - see front matter. Crown Copyright # 2002 Published by Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 ( 0 2 ) 0 0 1 1 8 - 3
74
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
a mantle source component, and their source region could have been subduction-enriched lithospheric mantle. However, their source had only slightly lower time-integrated LREE enrichment than did that of the granitoids, and the incompatible element signatures of the two suites are strikingly similar. The minette source region may have been in a zone of mixed crust and upper mantle, formed during a shortening event which resulted in crustal thickening and subsequent melting at mid-crustal layers to form the Hudson granitoid plutons. The generation and emplacement of minette melts may have been promoted by extension related to a combination of slab breakoff, gravitational collapse of thickened crust, and strike-slip faulting in the deforming hinterland. Subsequent anorogenic rapakivi granite-basalt activity may have been triggered by lithospheric mantle delamination. The hinterland tectonic cycle of the WCP was repeated in other large Archean terranes that were deformed during the early Proterozoic, but the igneous and sedimentary record is unusually complete in the WCP. Crown Copyright # 2002 Published by Elsevier Science B.V. All rights reserved. Keywords: Granite; Minette; Post-orogenic; Proterozoic; Rapakivi; Trans-Hudson
1. Introduction
1.1. Tectonic context The western portion of the Churchill structural province (WCP) of the Canadian Shield is a collage of Archean granite-greenstone and quartzofeldspathic gneiss domains, with Proterozoic
Fig. 1. Principal tectonic elements of North America, showing the location of Fig. 2. Adapted from Hoffman (1988). ThO, Thelon orogen.
supracrustal and intrusive rocks, that was trapped as the upper plate between two large Proterozoic orogens (Figs. 1 and 2) (Hoffman, 1988). To the west, the Thelon orogen, active at about 2.0 Ga, separates the WCP from the Archean Slave Province. To the southeast, the Trans-Hudson orogen (THO), active from about 1.9 /1.8 Ga, separates the WCP from the Archean Superior Province. Terminal collision between the Churchill and Superior Provinces in this portion of the THO occurred between 1.83 and 1.81 Ga (Orrell et al., 1999). The WCP underwent episodic Proterozoic reworking from 2.55 to 1.75 Ga, a fact first established from K /Ar and Rb /Sr geochronological studies (Loveridge et al., 1988) and since confirmed by metamorphic and igneous U /Pb geochronology (e.g. LeCheminant et al., 1987a; Peterson and van Breemen, 1999; Sanborn-Barrie, 1999; Stern and Berman, 2000). Within the study area (the southern half of Fig. 3, below 658 latitude) Archean rocks are dominated by approximately 2.8 /2.65 Ga granite-greenstone belts and voluminous intrusions of approximately 2.6 Ga diorite-to-granite, with lesser gneisses E/3.0 Ga. This paper focuses on the extrusive and intrusive igneous activity related to reworking of the WCP between 1.9 and 1.7 Ga. This activity was dominantly granitic, but includes extensive minette and minor basaltic volcanism. The work was part of a recent 5-year NATMAP multidisciplinary study by Natural Resources Canada (Geological Survey of Canada) and partners, on Proterozoic reworking and the Archean architecture of the WCP. Further results of the NATMAP study will appear
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
75
Fig. 2. Simplified geology of the WCP. Sup, Archean Superior Province. CWB, Chipewayan-Wathaman batholith ( :/1850 Ma), M, location of the Martin Group; K, location of the Kramanituar granulite complex. The Amer and Wager Bay shear zones are major faults with dextral strike-slip motions at approximately 1.85 Ga.
in future volumes of Precambrian Research. The Proterozoic igneous study used geochronological, isotopic, geochemical, and petrological data from an area of approximately 250 000 km2 in an attempt to delineate Archean and Proterozoic crustal domains, to refine knowledge of the differential uplift and metamorphic history of the region, and to constrain the source regions and heat sources of the igneous suites.
The geometry of the WCP, and the disposition, orientation, and shear sense of its largest Proterozoic faults, give a strong impression of shortening perpendicular to the strike of the orogens and of tectonic escape along a system of E- to NEtrending dextral shear zones active from about 1.9 to 1.83 Ga. Some of these, such as the Tyrrell shear zone (Fig. 3), included significant south side up movement (TerMeer et al., 2000; MacLachlan
76
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Fig. 3. The Dubawnt Supergroup, Hudson granitoids, and Nueltin granites in the study area. K, Kramanituar complex; A, diamondbearing Akluilaˆk minette dyke; TSZ, Tyrrell shear zone.
et al., 2000a). Prominent Proterozoic structural elements, including synclinal keels of pre-1.85 Ga sedimentary rocks (Aspler and Chiarenzelli, 1997), and mylonitic shear zones with transposed Archean structures (MacLachlan et al., 2000b), are typically NE-trending. Extension in the northeast segment of the WCP may have promoted the rapid uplift near 1.9 Ga of the Kramanituar granulite massif, exposed on the north shore of Chesterfield Inlet (Sanborn-Barrie, 1994). Small (:/10 m) granulite bodies which may have been uplifted during the early Proterozoic are also present along the trace of the northeast-trending Snowbird
tectonic zone (Tella and Eade, 1985), an ancient, kinematically complex fault zone that transects the southwestern WCP (Hanmer, 1997). There is evidence for a high-pressure (/1.0 GPa) amphibolite facies metamorphic event southwest of Chesterfield Inlet at about 1.9 Ga (Berman et al., 2000) but map-scale structures associated with this event have not been identified (Hanmer et al., in preparation). A collision on the western margin of the Slave Province reactivated faults in the Thelon orogen, resulting in brittle indentation of the relatively rigid Slave craton into the WCP along a conjugate
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
fault system that cuts 1.84 Ga intrusions in the Slave Province (Culshaw, 1991). Between the Thelon orogen and the region extensively intruded by Hudson granitoids, brittle downfaulting and crustal sagging produced terrigenous basins filled with volcanic rocks, conglomerates, and arenites of the approximately 1.84 /1.7 Ga Dubawnt Supergroup (Fig. 4) (Gall et al., 1992). Basin development was probably initiated at the east end of Baker Lake, where there is a thick succession of NW-facing pre-volcanic conglomerate and arenite containing cobbles of the uplifted Kramanituar granulites (Rainbird et al., 1999). Volcanic rocks in the lower Dubawnt Supergroup (Baker
Fig. 4. Idealized stratigraphic section for the Dubawnt Supergroup. Thicknesses of formations in the Baker Lake Group are variable and can be in excess of 2000 m; the upper Dubawnt Supergroup (Barrensland Group) has a maximum thickness of about 1500 m and the middle Dubawnt Supergroup (Wharton Group) about 1000 m. The lower, middle, and upper divisions are separated by unconformities.
77
Lake Group, Christopher Island Formation) consist of ultrapotassic lamprophyre (minette) lavas, breccias, and tuffs of both subaerial and subaqueous facies, overlain by alluvial fan and fluvial deposits (Kunwak Formation). These are separated by an angular unconformity from approximately 1.75 Ga rhyolite flows of the Pitz Formation (Wharton Group, middle Dubawnt Supergroup). An erosional unconformity, with intense chemical weathering, separates the Wharton Group from mostly flat-lying conglomerates and arenites of the Thelon Formation (Barrensland Group, upper Dubawnt Supergroup) which were deposited in a sag basin (Rainbird et al., 2001). The Thelon Formation is capped by thin, shallow marine dolostones. One large erosional remnant, the Thelon basin, overlies the intersection of the indentation fault systems. The full former extent of the sag basin is unknown, but the Athabasca basin (Fig. 2), which contains identical rocks, is thought to be another remnant (Gall, 1994). The WCP was compared to the Tibetan Plateau by Dewey and Burke (1973), who suggested that it represents a similar-styled continental hinterland, now mostly exposed at lower to mid-crustal levels. Peterson (1992) noted similarities between the volcanic styles, sedimentary facies, and basin morphologies of the Baker Lake Group and of extensional basins flanking the Tibetan Plateau in the Karakorum fault zone (Miller et al., 1999). The WCP has many features common to Archean domains located in the hinterlands of large 2.0 / 1.7 Ga orogens. Notable among these are the Nagssuqtoquidian domain of western Greenland, which includes reworked Archean rocks and very similar Proterozoic igneous suites (Skerjnaa, 1992; Kalsbeek and Nutman, 1996), and the Archean Saˆo Francisco craton of Brazil, which features ca. 2.0 Ga ultrapotassic intrusions (Rosa et al., 1999) adjacent to the terrigenous Espinhac¸o basin, which contains rocks similar to the middle and upper Dubawnt Supergroup (Uhlein et al., 1998). However, the WCP is exceptional for its volume and extent of hinterland igneous activity, and for the excellent preservation of Proterozoic strata which recorded the uplift history of the region.
78
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
1.2. Extent of Proterozoic igneous activity in the Western Churchill Province 1.2.1. Data sources The geochronological, geochemical, and Nd isotope data summarized here are from numerous sources. Copies of all data referred to in this paper may be obtained from the first author. We obtained samples to provide coverage throughout the area of Fig. 3 between 608 and 658 latitude. A relatively large number of samples was obtained in areas of new mapping, principally near the Kaminak greenstone belt and Chesterfield Inlet. Samples from areas with no nearby active mapping project were obtained from GSC archives and external sources. U/Pb analyses of zircon for 24 granitic rocks were obtained for the NATMAP project using the Geological Survey of Canada SHRIMP II (Stern, 1997). Preliminary results were presented by Peterson and van Breemen (1999) and a full report is in preparation (van Breemen et al., in preparation). Proterozoic ages in the range 1750 /1850 Ma were obtained either from zircon overgrowths on resorbed cores (Hudson suite) or from euhedral phenocrystic zircon (Nueltin suite). Ten previously published U /Pb (zircon) ages are also available (cited in Peterson and van Breemen, 1999). The Nd and Sm isotopic compositions of 17 granitoid samples were determined during the NATMAP study at GSC laboratories (The´riault, 1990) and Memorial University, St. John’s, Newfoundland. Previously published Nd /Sm analyses of Proterozoic granites and minettes are available from Duda´s et al. (1991) and Peterson et al. (1994). Additional analyses of minette dykes and lava flows were made for NATMAP (Cousens, 1999) and were interpreted by Cousens et al. (2001). Whole-rock analyses in Table 1 were made at Memorial University and the Geological Survey of Canada. Rare earth elements and Hf, Ta, Cs, U, Th, and Pb were analyzed by ICP-MS (Jenner et al., 1990), and other elements by XRF. Additional elemental analyses, used in the construction of plots but not listed in Table 1, are given by Blake (1980), Tamboso (1981), Booth (1983), LeCheminant et al. (1987b), Peterson et al. (1994), Cousens (1999), Peterson (in preparation).
1.2.2. Extent of igneous activity There were two pulses of igneous activity (Fig. 5). The first generated plutons of monzonite, granodiorite, and granite (the Hudson granitoids) between 1850 and 1810 Ma, with a peak near 1830 Ma, and with two outlying ages at about 1790 Ma. The Hudson plutons are strongly concentrated in a belt immediately west of Hudson Bay, but are found across nearly the entire study area. Regional mapping and geochronological studies (Henderson, 1983; Henderson and Roddick, 1990; LeCheminant et al., 1987a) indicate this belt extends northeastward, across the Wager Bay shear zone and the Penhryn fold belt to central Baffin Island, where post-orogenic granitoid plutons intruded Archean basement and early Proterozoic metasedimentary rocks at approximately 1.83 Ga (Corrigan et al., 2001). The area immediately north of Chesterfield Inlet has not been mapped in detail and the apparent absence of Hudson granitoids in that area may only reflect an absence of data. Hudson granitoids are mostly absent within the Kaminak greenstone belt, which was relatively isolated from Proterozoic thermal metamorphism (Hanmer et al., in preparation). South of the Kaminak belt, Hudson granitoids extend as far as the Wathaman-Chipewayan batholith, dated at 1850 Ma (Meyer et al., 1992) (Fig. 2). Outliers of the batholith to the north and west yield U /Pb ages of 1840/1810 Ma (Annesley et al., 1997). A large part of the WCP east of the Thelon basin was intruded by an ultrapotassic lamprophyre (minette) dyke swarm (Fig. 3, inset). In most cases, Dubawnt minette dykes cut Hudson plutons, but granite is observed truncating dykes south of Chesterfield Inlet (Tella et al., 1993). The Dubawnt minettes have proved difficult to date due to a dearth of zircon; however, imprecise U/Pb and K/Ar ages (summarized in Peterson and van Breemen, 1999) suggest eruption occurred no earlier than 1.85 Ga and no later than 1.80 Ga. Minette volcanism and deposition of redbeds of the Baker Lake Group occurred in fault-bounded basins on the western side of the WCP, away from the main concentration of Hudson intrusions. Minette dykes adjacent to the basins typically have idiosyncratic or bimodal strike distributions parallel to the local basin-bounding faults,
Table 1 Whole rock elemental and Nd /Sm isotope analyses of Proterozoic granites and minettes of the WCP. Sample
EA70-
EA70-
EA70-
EA78-
EA78-
FD78-
FD78-
P97-
86T-
98TX-
98TX-
98TX-
98TX-
98TX-
98TX-
98T-
98TX-
894
184
1057
64
128A
118
120
N50A
X107
T095
H095
H185
H053
J067
T151
X241
N067
MOSQ-1
Suite
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Hudson
Lat
60802?
60847?
60803?
60822?
60823?
60810?
60819?
60811?
62845?
63814?
63845?
63814?
63845?
63843
63847?
63851?
63836?
63832?
62838?
Long
98846?
96853?
97842?
97809?
95809
94848?
95812?
95813?
95855?
90807?
93850?
93850?
93808?
93815?
93810?
93815?
94839?
938001?
102857?
SiO2
73.69
73.68
70.80
71.80
71.60
69.30
72.50
73.40
73.10
71.68
72.60
70.30
73.30
74.90
72.90
72.10
73.40
70.30
67.65
TiO2
0.14
0.17
0.23
0.16
0.21
0.33
0.18
0.17
0.24
0.35
0.17
0.34
0.12
0.17
0.27
0.21
0.16
0.37
0.53
Al2O3
15.79
15.72
15.00
14.70
14.60
15.70
14.50
14.50
14.40
15.94
14.20
14.80
14.60
14.10
14.20
14.30
14.40
14.50
15.91
Fe2O3
1.03
1.13
1.00
1.40
0.90
1.40
1.90
1.20
0.50
1.80
0.90
0.50
0.70
0.40
0.80
1.00
1.00
1.10
3.37
FeO
NA
NA
1.40
0.60
0.80
0.80
0.60
0.60
1.10
NA
0.60
1.20
0.50
0.70
1.00
0.90
0.40
1.30
NA
MnO
0.02
0.02
0.03
0.01
0.02
0.03
0.01
0.02
0.02
0.02
0.03
0.02
0.01
0.03
0.02
0.02
0.05
0.02
0.06
MgO
0.34
0.64
0.51
0.36
0.27
0.47
0.39
0.37
0.59
0.97
0.47
1.00
0.16
0.37
0.38
0.48
0.37
0.66
1.34
CaO
1.09
0.47
2.08
2.16
1.21
1.67
1.71
1.67
1.54
1.60
1.28
1.36
0.99
1.04
1.21
1.16
0.22
1.44
1.78
Na2O
3.78
2.88
4.20
3.80
3.50
4.10
4.00
3.60
4.40
3.59
3.80
3.40
4.10
3.60
3.40
3.60
3..9
3.10
3.40 5.80
K2O
5.19
7.33
4.11
4.18
5.95
5.85
3.97
4.67
3.23
5.26
5.33
5.93
5.28
4.95
5.41
5.35
5.16
6.01
P2O5
0.01
0.03
0.09
0.09
0.08
0.12
0.06
0.05
0.07
0.11
0.07
0.15
0.02
0.04
0.07
0.07
0.06
0.10
0.28
H2O
NA
NA
0.30
0.40
0.40
0.50
0.50
0.40
0.30
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
CO2
NA
NA
0.10
0.10
0.10
0.20
0.10
NA
NA
NA
NA
NA
NA
NA
NA
NA
F
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
1153
2408
839
889
NA
NA
NA
925
835
NA
Rb
171
276
180
130
190
220
100
210
110
229
319
339
204
406
311
310
269
229
215
Cs
NA
NA
1.2
0.8
0.6
1.3
0.1
0.7
2.0
NA
11.0
10.0
1.6
9.2
5.3
5.6
8.4
2.1
NA
Be
NA
NA
2.5
2.0
1.2
1.6
1.8
3.5
1.8
NA
5.2
6.4
5.0
4.8
3.7
3.2
4.9
1.6
NA
Sr
86
144
200
140
180
210
170
120
250
487
268
551
654
142
241
284
312
478
522
Ba
334
995
930
770
570
1100
1100
750
920
1812
580
1300
2200
550
910
1100
790
2000
2102
Sc
NA
NA
3.4
2.1
1.7
2.9
2.4
2.4
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
3.4
Y
5.3
1.6
15.0
15.0
7.9
18.0
28.0
34.0
9.7
2.3
7.8
9.0
7.0
12.0
9.0
3.7
8.0
15.0
33.8
Zr
104
217
270
230
290
380
230
180
180
257
141
261
166
153
242
231
166
326
584
Hf
NA
7.7
7.4
7.1
8.8
11.0
6.6
6.8
4.5
7.4
4.8
8.1
4.5
5.5
6.9
7.1
4.9
8.2
17.7
V
3
9
17
14
12
21
20
10
11
20
9
17
B/LD
B/LD
12
13
9
25
37
Nb
6.9
4.8
13.0
8.1
4.6
15.0
12.0
15.0
11.0
13.9
16.0
24.0
8.4
25.0
18.0
10.0
12.0
21.0
27.7
Ta
NA
0.21
0.66
0.31
0.21
1.10
0.55
1.10
1.60
1.24
1.30
2.60
0.40
2.50
1.30
0.30
1.30
3.00
1.87
Cr
9
6
13
14
10
16
12
11
20
11
12
37
B/LD
B/LD
B/LD
B/LD
B/LD
11
16
Co
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
29
B/LD
B/LD
7
B/LD
B/LD
B/LD
B/LD
B/LD
8
B/LD
Ni
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
18
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
Cu
B/LD
1
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
B/LD
15
B/LD
46
B/LD
B/LD
B/LD
B/LD
48
B/LD
B/LD
Zn
2
6
56
29
34
39
13
34
29
23
33
31
10
49
52
40
37
46
38
La
25
86
81
50
137
155
63
54
45
167
48
82
51
54
120
100
52
180
185
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
EA7670
79
45
151
140
99
240
270
110
100
85
288
84
150
89
100
240
180
91
320
336
Pr
4.9
15.1
15
10
24
27
12
11
8.6
30.2
9.4
16
9.1
11
22
19
9.8
33
35.1
Nd
15
43
46
34
69
84
40
35
27
89
29
50
28
34
70
57
30
100
120
Sm
2.4
5.7
7.3
6.2
8.0
13.0
7.7
7.1
3.9
9.6
4.5
6.7
3.5
5.3
8.6
6.9
4.1
13.0
18.8
Eu
0.40
0.62
0.75
0.75
0.77
0.92
1.10
1.00
0.55
1.56
0.65
0.89
0.34
0.55
0.81
0.77
0.61
1.40
3.01
Gd
2.0
3.6
4.9
4.9
3.8
8.1
6.5
5.8
2.4
3.7
2.7
3.8
2.2
3.3
4.2
2.7
2.5
7.0
11.2
0.27
0.37
0.60
0.65
0.37
0.89
0.93
0.94
0.32
0.37
0.36
0.46
0.25
0.43
0.49
0.34
0.30
0.82
1.35
Dy
1.51
1.53
2.70
3.20
1.60
3.80
5.10
5.30
1.70
1.56
1.70
2.20
1.20
2.20
2.30
1.10
1.50
3.60
7.09
Ho
0.26
0.21
0.45
0.51
0.26
0.63
0.87
1.00
0.30
0.22
0.89
0.39
0.23
0.44
0.35
0.35
0.29
0.59
1.24
Er
0.70
0.53
1.10
1.10
0.64
1.40
2.10
2.60
0.74
0.56
0.36
0.99
0.51
1.10
0.89
0.17
0.74
1.40
3.55
Tm
0.10
0.07
0.14
0.15
0.09
0.20
0.31
0.41
0.12
0.07
0.15
0.16
0.09
0.19
0.13
0.05
0.13
0.22
0.46
Yb
0.66
0.42
0.88
0.84
0.57
1.20
1.80
2.40
0.76
0.47
0.97
0.95
0.60
1.20
0.71
0.34
0.82
1.30
2.66
Lu
0.10
0.08
0.14
0.13
0.10
0.20
0.28
0.37
0.12
0.05
0.15
0.16
0.09
0.19
0.12
0.08
0.14
0.19
0.39
U
8.5
5.6
5.0
2.8
6.1
6.3
11.0
5.7
1.9
8.3
8.1
16.0
6.4
12.0
5.2
8.1
16.0
12.0
6.4
Th
27.8
70.2
55
34
150
81
47
59
16
71.9
43.6
68
28
40
62
58.5
37
113
30.3
Pb
40
55
31
25
46
51
25
35
21
74
66
48
90
63
58
63
51
64
30
147Sm/
0.09820
0.08410
0.09357
0.10735
NA
0.08368
0.10752
0.10890
0.08310
0.06037
0.08890
NA
NA
NA
NA
NA
0.07810
0.07820
NA
0.51098
0.51068
0.51082
0.51103
NA
0.51060
0.51110
0.51111
0.51078
0.51064
0.51090
NA
NA
NA
NA
NA
0.51083
0.51077
NA
EA27-
P89-23
P88-
P89-303
P89/R37
P89-PZ
P94-
P00-33
P00-12
P96-
P96-1A
P96-
P97-92
56D-12
144Nd 143Nd/ 144Nd Sample
69
500G
124C
110
168A
P97-
P97-
P97-
P97-
P91/
C32
C178
T138A
T138B
LBIF
Suite
Nueltin
Nueltin
Nueltin
Nueltin
Nueltina
Nueltina
PNb
PNb
PNb
Minette
Minette
Minette
Minette
Martell
Minette
Minette
Minette
Minette
Minetteb
Lat
60803?
63818?
63813?
63818?
62856?
glacial
62854?
99802?
99804?
63828?
62817?
62815?
62802?
63838?
62827?
62816?
62818?
62818?
63812?
Long
100812?
101812?
101813?
101805?
101829?
erratic
102809?
62828?
62827?
93856?
93847?
93855?
95821?
95845?
95800?
95843?
95847?
95847?
101813?
SiO2
76.84
75.70
76.80
71.30
63.70
75.70
56.46
51.13
51.80
45.27
42.90
50.69
48.83
46.39
59.57
44.68
46.84
51.14
57.70
TiO2
0.23
0.23
0.21
0.17
0.93
0.22
1.45
1.94
2.01
1.14
1.07
1.18
1.32
0.52
1.09
1.04
0.84
1.11
0.90
Al2O3
15.45
12.30
12.00
14.10
14.60
11.80
13.81
14.28
14.32
14.23
7.19
9.37
10.32
5.61
12.39
9.82
8.77
10.33
11.60
Fe2O3
2.54
0.50
0.60
1.80
5.80
0.30
8.46
10.69
11.62
9.01
12.53
7.56
7.09
8.05
5.17
6.81
6.93
7.53
8.70
FeO
0.00
0.80
0.50
0.10
0.20
1.50
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
0.40
MnO
0.03
0.02
0.01
0.01
0.07
0.02
0.14
0.12
0.14
0.35
0.26
0.13
0.11
0.14
0.08
0.09
0.10
0.14
0.07
MgO
0.25
0.21
0.19
0.39
0.86
0.28
5.10
3.79
2.87
6.70
17.03
11.93
10.91
22.38
5.89
12.40
12.65
9.87
5.91
CaO
1.17
0.71
0.55
0.19
2.01
0.61
6.21
7.52
7.36
8.28
11.87
10.09
8.74
10.58
5.00
9.37
10.05
8.03
1.60
Na2O
3.34
2.90
2.80
4.40
3.80
2.60
2.65
2.70
2.92
0.99
0.34
2.40
1.60
0.66
3.36
0.70
0.60
1.46
2.10
K2O
5.90
5.45
5.56
5.34
5.29
5.65
2.40
2.36
2.22
8.56
4.69
3.41
6.42
3.47
3.57
7.43
5.84
4.13
7.56
P2O5
0.02
0.04
0.03
0.13
0.31
0.04
0.80
1.80
1.52
2.78
2.07
1.49
1.79
0.55
1.45
2.16
1.44
1.55
0.72
H2O
NA
0.50
0.60
0.70
1.30
0.70
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
1.30
CO2
NA
0.10
0.10
0.10
1.20
0.10
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
0.10
F
NA
NA
2042
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
Rb
292
366
334
283
166
301
111
69
77
659
236
135
370
182
165
244
172
103
Cs
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
Be
NA
6.5
4.8
5.4
3.1
3.1
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
292 21.0
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Tb
80
Ce
Sr
94
107
73
223
391
66
751
533
609
1542
842
547
1381
578
637
3887
3701
872
1040
Ba
706
651
529
1058
1742
523
2480
1573
1636
9659
2571
4099
8875
2112
3183
10215
10697
4283
4003
Sc
NA
NA
NA
NA
NA
NA
NA
20.2
23.8
15.8
21.5
23.6
21.4
38.6
11.9
18.0
19.3
16.7
Y
78.0
35.0
36.0
7.0
36.0
49.0
23.7
50.0
57.5
44.7
27.1
20.5
23.9
10.7
20.8
56.8
30.8
21.6
47.0
Zr
312
297
245
201
457
315
315
259
294
169
816
700
777
57
915
512
360
773
924
Hf
9.4
NA
NA
NA
NA
NA
NA
6.5
7.5
3.9
19.5
17.9
19.9
1.6
NA
NA
NA
NA
V
NA
1
1
18
11
3
152
161
90
65
119
139
130
113
80
84
131
137
82
Nb
35.9
26.0
67.0
12.0
43.0
31.0
25.1
14.3
19.6
10.7
31.3
16.5
39.3
3.9
7.3
4.7
10.7
18.4
37.0
Ta
2.68
NA
NA
NA
NA
NA
NA
0.81
0.90
0.36
1.02
0.52
1.86
0.15
NA
NA
NA
NA
B/LD
4
6
11
14
7
260
7
B/LD
196
737
316
338
814
254
647
389
937
13
NA
2
3
4
13
4
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
28
Ni
B/LD
1
10
3
0
2
90
5
B/LD
93
420
262
238
292
165
517
332
252
31
Cu
B/LD
2
2
4
5
3
32
26
15
17
32
29
52
41
3
96
70
60
52
Zn
B/LD
26
21
19
68
36
107
64
69
171
85
45
44
29
86
76
46
64
140
La
172
75
77
41
99
120
NA
69
83
97
242
95
201
37
123
231
183
146
210
Ce
329
150
170
NA
NA
250
NA
150
182
202
472
189
377
80
232
446
360
282
340
Pr
39.1
NA
NA
NA
NA
NA
NA
19.4
23.3
25.5
57.2
21.8
44.5
9.7
NA
NA
NA
NA
Nd
128
67
62
NA
NA
98
NA
82
98
106
220
82
165
40
93
213
180
123
190
Sm
21.8
8.7
9.2
NA
NA
13.0
NA
16.4
19.3
22.1
34.3
13.3
25.5
8.1
14.4
38.7
28.1
19.0
34.0
Eu
1.31
1.10
0.90
NA
NA
1.20
NA
3.55
4.05
6.13
8.11
3.36
6.06
2.10
3.84
13.04
8.93
4.78
6.60
Gd
16.6
7.0
6.7
NA
NA
11.0
NA
13.3
15.5
18.3
19.0
8.3
13.7
5.2
9.0
27.9
18.1
11.4
21.0
Tb
2.55
NA
NA
NA
NA
NA
NA
1.67
1.96
2.11
1.80
0.92
1.30
0.54
NA
NA
NA
NA
Dy
14.70
6.80
6.00
NA
NA
9.50
NA
9.13
10.73
9.66
7.13
4.41
5.56
2.57
4.47
13.65
8.21
4.85
12.00
Ho
2.80
NA
NA
NA
NA
NA
NA
1.71
2.01
1.45
0.97
0.73
0.82
0.43
0.68
1.97
1.21
0.75
2.00
Er
8.03
NA
NA
NA
NA
NA
NA
4.80
5.63
3.30
2.15
1.95
2.01
1.13
1.57
4.28
2.55
1.70
4.30
Tm
1.20
NA
NA
NA
NA
NA
NA
0.59
0.70
0.33
0.22
0.23
0.23
0.14
0.18
0.47
0.32
0.23
0.71
Yb
7.26
4.30
3.80
0.60
2.90
5.50
NA
3.46
4.13
1.67
1.19
1.36
1.28
0.80
1.10
2.33
1.77
1.20
Lu
1.02
NA
NA
NA
NA
NA
NA
0.52
0.61
0.21
0.16
0.19
0.19
0.12
0.18
0.35
0.27
0.18
U
5.3
NA
NA
NA
NA
NA
NA
B/LD
B/LD
B/LD
8.2
1.3
8.0
B/LD
9.6
3.2
3.0
14.5
16.0
Th
30.6
NA
NA
NA
NA
NA
NA
9
8.1
B/LD
55
8
53.3
B/LD
52.1
29.9
21.7
63.1
220.0
Pb
37
NA
NA
NA
NA
NA
NA
B/LD
B/LD
B/LD
B/LD
B/LD
158
B/LD
22
129
67
17
147Sm/
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
0.11050
0.10150
0.09720
0.11565
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
0.51121
0.51112
0.51104
0.51109
144Nd 143Nd/
0.51132
0.51130
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Cr Co
144Nd
B/LD, below detection limit; NA, not analysed. If FeO is not analysed, then all Fe as Fe2O3. PNb/basaltic rocks (MacRae Lake dyke) associated with Nueltin suite. Other minettes are dykes. a Rhyolites from the Pitz Formation. b Lamproitic felsic minette lava (upper volcanic member of Christopher Island Formation).
81
82
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Fig. 5. Histogram of U /Pb zircon ages for Hudson and Nueltin granitoids. SHRIMP ages: van Breemen et al. (in preparation). Other ages compiled by Peterson and van Breemen (1999).
whereas east and north of the basins the dykes are typically east to east-southeast trending, at a high angle to regional ductile fabrics (Fig. 6). The easterly trends have been interpreted as reflecting an approximately E /W directed stress field related to the indentation of the Slave Province (Hoffman, 1988). The second pulse of igneous activity consisted of rhyolite volcanism (Pitz Formation) and emplacement of subvolcanic intrusions of porphyritic, locally rapakivi-textured granite (Nueltin granite). This activity was strongly concentrated within the older basins of the Baker Lake Group, and was associated with additional extensional faulting that dissected and tilted the basins (Hadlari and Rainbird, 2001). The rhyolites are restricted to the Baker Lake Group basin complex; beyond the basins, to the southwest, only plutons of Nueltin granite are present. Additional 1.76 Ga plutons have been identified beneath the Athabasca basin (Krogh and Clark, 1987) suggesting that a SW / NE band of approximately 1.76 Ga plutonism
extends through the southern WCP, subparallel to its bounding orogens.
2. Geology and petrology of the igneous suites
2.1. Hudson granitoids Except for the plutons near Wager Bay, which are megacrystic granites through monzodiorites with minor gabbro (LeCheminant et al., 1987a), the Hudson granitoids are remarkably uniform in their field relations and petrography. Many of the plutons are elongated NE/SW, reflecting selective emplacement along pre- to syn-magmatic structures. Within the plutons, the rocks are typically nonfoliated with a grain size of 1 /5 mm (Fig. 7a). The plutons commonly have transitional contacts with country rocks, and sill-like margins that bear an inherited foliation defined by dispersed biotite (Fig. 7b) and narrow enclaves of partly assimilated
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
83
Fig. 6. Rose diagrams of strikes of minette dykes. Note the tendency for dykes near basins of the Baker Lake Group to have NWtrending or bimodal strike directions, due to conjugate brittle faulting related to eastward indentation of the Thelon Province. M, Martell syenites (intrusive minette); A, Akluilaˆk diamondiferous minette dyke; Y, Yathkyed Lake; TSZ, Tyrrell shear zone; STZ, Snowbird tectonic zone.
wall rocks. Some appear to have been emplaced in an injection migmatite (Davidson, 1969); in other cases the wall rocks were competent enough to allow meter-scale dykes to form. Mafic inclusions from wall rocks are invariably coarsely recrystallized (mainly to hornblende) and may be locally disaggregated and mixed to produce heterogeneous monzodiorite. The plutons typically cut the youngest ductile structures, such as mylonite bands and stretching lineations, but are also partly affected by them (e.g. Relf et al., 1999). Gravity anomalies associated with all but the largest plutons are negligible, consistent with thin, silllike bodies. Regional thermobarometric studies (Berman et al., 2000) indicate that the Hudson granitoids between Baker Lake and the Kaminak greenstone belt were emplaced at pressures near 0.5 GPa (:/15/20 km depth). The Hudson
granitoids have no known volcanic equivalents, and no silicic ash layers have been recognized in any similar-aged Proterozoic sedimentary rocks. The rocks are subsolvus and are mainly equigranular, although weakly Kspar-porphyritic rocks have been noted near some pluton margins. Plagioclase and alkali feldspar are nonidiomorphic and present in approximately equal abundance, and myrmekite is commonly present. Biotite comprises about 5% of most samples. Magnetite, varying from euhedral to interstitial and irregular, is abundant and most Hudson plutons are associated with distinctive magnetic anomalies. Garnet is present as rare subhedral grains which could be phenocrysts, but which are probably from a restite. Zircons are mostly present as subhedral inclusions in biotite, and highly rounded inclusions in quartz or feldspar (Fig. 8a and b).
84
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Fig. 7. Polished slabs of Hudson granitoids. (a) Typical sample from a pluton center; note equigranular texture. (b) Typical sample from a pluton margin, with a weak relict foliation defined by biotite. The sample is not deformed.
Fig. 8. Photomicrographs of zircons from Hudson granitoids. (a) Inherited Archean zircon armored by biotite (crossed polars). (b) Inherited Archean zircon, resorbed and included in quartz.
The average Hudson granitoid is a slightly peraluminous granodiorite with K/Na :/0.95 and overall composition similar to silicic rocks in calcalkaline series (Table 1). In a normative feldspar plot (Fig. 9), nearly all the Hudson granitoids have between 10 and 20% An, with a broad range in Or content. In a normative quartz /feldspar diagram (Fig. 10), the Hudson granitoids cluster strongly around the 5 kbar H2O-saturated quartz /two feldspar eutectic for bulk compositions with plagioclase of An20. They have very low Y contents (average 11 ppm; Fig. 11) and high LREE/HREE ratios (average Ce/Yb /206) (Fig. 12). Europium anomalies are typically modest (average Eu/Eu / 0.53). Neodymium isotope compositions, with o Nd varying from /7.0 to /13.2, show that the granitoids had a source component with longterm LREE enrichment (Figs. 13 and 14). The initial Nd isotope composition of the Hudson granitoids is indistinguishable from that of large tracts of Archean terrane in the WCP (e.g. The´riault et al., 1994). Analyses of samples of a widespread suite of approximately 2.6 Ga granitic plutons (Duda´s et al., 1991; Peterson, in preparation) are used as a proxy for Archean basement in Fig. 13. The field relations, petrography, and composition of the Hudson granitoids suggest they were formed by emplacement of near-solidus magmas into thin, partly concordant bodies at mid-crustal levels. The equigranular textures indicate simultaneous crystallization of quartz and feldspars, and the absence of significant Eu anomalies indicates little differential movement of melt and solids. Hence, the melts probably crystallized over a small temperature interval, consistent with minimummelt compositions. The magmas likely formed in contact with garnet-bearing wall rocks, resulting in depleted Y and high Ce/Yb, and contained inherited xenocrysts of garnet, zircon, and ferromagnesian minerals, especially biotite. Given their low thermal inertia, the Hudson granitoid magmas likely migrated only a short vertical distance. There is no field or geochemical evidence for an association with isotopically depleted juvenile mafic magmas on a regional scale, although gabbroic rocks are present at Wager Bay.
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
85
Fig. 9. Normative (CIPW) feldspar plot (mole fraction) for the granitoid suites and the mafic rocks associated with Nueltin granites.
The Hudson granitoids have all the characteristics of plutons that are widely interpreted as early post-orogenic intrusions in collisional hinterlands. These characteristics include: in situ melting (or nearly so) with restitic zircon and isotopic compositions indistinguishable from the wall rocks (e.g. Paleoproterozoic granitoids of the Nagssugtoqidian orogen: Whitehouse et al., 1998); correlation in time and space with late ductile structures (e.g. late Svecofennian anatectic granites: Nironen, 1997); and trace element compositions (depleted Y and HREE) consistent with a garnet-bearing source region in the lower to middle crust. An additional common feature of these post-orogenic granitoids is an association with potassic lamprophyres (e.g. Rock and Hunter, 1987; Eklund et al., 1998) which is a dramatic feature of the Hudson granitoids.
2.2. Dubawnt minettes The Baker Lake Group, with an estimated volume of 40 000 km3 preserved in a complex of fault-bounded basins (Peterson, 1992), comprises the largest extrusive ultrapotassic province yet described. The associated minette dyke swarm is asymmetrically disposed: the western edges of the basin complex and the dyke swarm coincide at Dubawnt Lake, but the dyke swarm extends much further east, into the area densely intruded by Hudson granitoids (inset, Fig. 3) and also to the southwest as far as the Athabasca basin. Probable equivalents of the Baker Lake Group are exposed at the north edge of Lake Athabasca (the Martin Group: Donaldson, 1968; see Fig. 2), where minette dykes are also found in the underlying Archean basement.
86
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Fig. 10. Normative (CIPW) quartz /feldspar plot (weight fraction) for the granitoid suites and the mafic rocks associated with Nueltin granites. E is the quartz /2 feldspar eutectic in albite /sanidine /quartz, at 5 kbar, water saturated. With increasing An content, the eutectic migrates toward the quartz /sanidine join, and the grayed eutectic point is its approximate position for a bulk composition with plagioclase of An20 (estimated from Carmichael, 1963).
Small plutons of ultrapotassic, mafic syenite (Martell syenites) occur within and southeast of the most easterly Baker Lake Group basin (Fig. 6). They are in contact with Hudson granitoid plutons and intermediate rock types are present within the syenites, consistent with magma mixing. Hudson granitoids are mingled with amphibole-rich lamprophyric rocks (spessartites) in plutons exposed near Chesterfield Inlet (Sandeman et al., 2000). The volcanic rocks (Christopher Island Formation, Baker Lake Group) were mostly erupted as a felsic-mafic-felsic sequence directly over crystalline basement. In the area of Baker Lake, volcanic rocks conformably overlie conglomerate and arkose that record pre-volcanic faulting on the margins of an E /W trending basin, and periodic inward progradation of alluvial fans over a flat braidplain (Rainbird et al., 1999). At Dubawnt Lake, epiclastic turbidites interbedded with lava
flows, subaqueous flow breccias, and siltstones in sections up to 2.5 km thick suggest eruption centers developed within, or alongside, deep and narrow lakes (Peterson, in preparation). Postvolcanic conglomerates and arenites (Kunwak Formation) recorded continued uplift, locally combined with dextral strike-slip faulting, on the margins of some basins (Rainbird and Peterson, 1990). Most exposures of the Baker Lake Group are tilted at 208 /608, due to both syndepositional faulting and later faulting associated with rhyolite volcanism (Hadlari and Rainbird, 2001). The lower felsic lavas are porphyritic with phenocrysts of sanidine plus highly resorbed and oxidized olivine, phlogopite, and clinopyroxene. Partially melted crustal xenoliths (especially tonalites) are common and the rocks show clear signs of crustal contamination, such as elevated Si, Al, and Na/K and depletion in incompatible elements
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
87
Fig. 11. Y versus Zr for both granitoid suites, the Nueltin mafic rocks, and the Dubawnt minettes.
relative to the mafic minettes (Peterson, 1992). The upper felsic lavas are crystal-poor and relatively enriched in incompatible elements, with compositions similar to felsic lamproites (Mitchell and Bergman, 1991; see sample P91-LBIF, Table 1). They are interpreted as crystal fractionation products of the primary mafic minettes. The mafic minettes, which comprise the great majority of the dykes outside the basins, are highly porphyritic (phlogopite/clinopyroxene/ apatite9/olivine9/leucite) (Fig. 15) with a groundmass dominated by potassium feldspar with minor alkali amphibole (richterite, K-richterite, and riebeckite), magnetite, titanite, and primary carbonate. Many dykes contain 1 /4 cm ellipsoidal glimmerite xenoliths with curved and kink-banded mica, thought to represent phlogopite megacrysts and pegmatites battered against the wall rocks during ascent (Peterson and LeCheminant, 1993). These contain euhedral magnesiochromite and resorbed chrome diopside. Mantle spinels and garnets, picroilmenite, and chrome diopside have
all been separated from Dubawnt minette dykes and breccias at various localities (e.g. Chisholm, 1993; Kaminsky et al., 1998). Microdiamonds, with a peak in size distribution at 0.25 mm, are present in large numbers in the Akluilaˆk dyke (Fig. 6) (Kaminsky et al., 1998). The diamonds are rich in Type Ib-IaA nitrogen aggregates, with a total maximum N content of 8000 ppm, far in excess of normal mantle diamonds (Chinn and Kyser, 2000). Hydrogen contents are also unusually high, and the average carbon isotope composition (:/ /7 per mil) is significantly lower than most other diamonds. Chinn and Kyser, 2000 note that the nitrogen aggregates are comparable to microdiamonds in the ultrahigh pressure Kokchetav metamorphic massif, and speculate that the Akluilaˆk microdiamonds developed at low temperature (8608/990 8C) and may have nucleated metastably on hydrogen species at pressures less than the diamond stability field. The nitrogen aggregates can only have had a very short residence time in the mantle (:/1 Ma).
88
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Fig. 12. Rare earth elements for: (a) the Hudson granitoids and Dubawnt minettes, and (b) the Nueltin granites and an associated diabase dyke. Envelopes represent 9/1 standard deviation from the average.
The most primitive minettes are ultrapotassic, depleted in Ca, and highly magnesian: of 155 analyzed samples, 78 have MgO /8% with average CaO /6.3%, MgO /10.2% and Mg/(Mg/ Fetotal)/0.73 (the analyses in Table 1 are of minettes from the Kaminak greenstone belt, and have abnormally high CaO). They show strong LILE enrichment and very strong HFSE depletion, particularly in Ti, Nb, and Ta (Fig. 16a). The Dubawnt minettes have moderate Al2O3 contents (:/12%) that are typical of the ‘calc-alkaline minettes’ of Rock (1991), but their low Ca contents, high K and Rb, and some minerals (leucite, tetraferriphlogopite, K-richterite) are diagnostic of lamproites (Mitchell and Bergman, 1991). Statistical studies show the Dubawnt minettes most
closely resemble other ultrapotassic rocks that have been termed transitional minette-lamproites (e.g. sills in the Gondwana coalfields, India: Rock et al., 1992) and minettes from other continental orogenic hinterlands (e.g. the Bohemian Massif: Ne´mec, 1974). The MORB-normalized trace element profiles of the minettes are similar to those considered typical of subduction-zone magmas. The average minette is remarkably similar to the most enriched of the Hudson granitoids, except that the granitoids have relatively depleted Ba and P (Fig. 16a). The minettes all have strongly enriched Nd isotope compositions, with o Nd,1830 Ma averaging /8 and ranging from /5 to /11 (Fig. 14). No isotopic distinction is noted between mafic and felsic rocks, suggesting that the Nd isotopic composition is inherited from the source region and is not due to crustal contamination. Present-day Pb in a suite of minette dykes is variable, extending from 206 Pb/204Pb :/18.5 to highly nonradiogenic values with 206Pb/204Pb :/15 (Peterson et al., 1994). The most nonradiogenic samples have Pb similar to that of the most extreme lamproites (e.g. Sisimiut, Greenland: Nelson, 1989). The Dubawnt minettes present several petrogenetic conundrums. They have a subduction zone trace element signature, despite being situated in Archean crust hundreds of kilometers from any subduction zone of similar age; they must originate in mantle rocks, but in their incompatible element and Nd isotope compositions they closely resemble consanguinous crustally-derived granitoids; and they have both highly enriched incompatible element and highly depleted compatible element (high Mg/Fe, low Ca) signatures. These geochemical features, plus very nonradiogenic lead, are typical of lamproites, which are thought to originate in a layer of lithospheric upper mantle that was once depleted by extraction of Ca- and Fe-rich basaltic melt, and subsequently enriched in K and other LILE (Mitchell and Bergman, 1991). 2.3. Nueltin Granites and Pitz Formation, 1.76 Ga The Nueltin granites are strongly porphyritic, hypersolvus rocks with phenocrysts of quartz, high-T alkali feldspar, and plagioclase (Fig. 17).
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
89
Fig. 13. 143Nd/144Nd as a function of time, calculated from present-day Nd /Sm isotopic ratios. Data from Table 1 and published sources (see text). Depleted mantle is assumed to have present-day 147Sm/144Nd/0.1966, 143Nd/144Nd/0.512638.
Their contacts are chilled and discordant, and several of the plutons intruded their own volcanic edifices. Most are associated with prominent negative gravity anomalies, suggesting a significant thickness, and with intense radiometric anomalies due to high Th contents. The correlative rhyolites (Pitz Formation; LeCheminant et al., 1987b) are mostly porphyritic but some glassy domes are nearly crystal-free. These contain highly resorbed phenocrysts of quartz and anorthoclase, plus abundant clots of basaltic material consisting of plagioclase (An60 40), clinopyroxene, and magnetite, surrounded by quenched, variably Fe-contaminated melt (Fig. 18). The Nueltin granites can
be associated with aphyric or plagioclase-phyric mafic to intermediate dykes and gabbroic bodies, and were locally mingled with mafic melts, producing heterogeneous monzodiorites (LeCheminant et al., 1987b). The mafic dykes consist mainly of clinopyroxene and plagioclase with microphenocrysts of magnetite, and can be rich in quartz xenocrysts. A U /Pb (baddelyite) age of 17509/1 Ma was obtained for the diabasic McRae Lake dyke (located in Fig. 3), which is mingled at its south end with a Nueltin granite pluton (A. LeCheminant, unpublished data). The most common crystallization sequence in the rhyolites was quartz (polymorph uncertain),
90
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Fig. 14. Nd and Sr isotope compositions of the Dubawnt minettes and both granitoid suites, as epsilon values at the time of emplacement. Rb /Sr isotope ratios were not determined for the granitoids or the McRae Lake diabase. The wide range in Sr composition of the minettes is due to decoupling of Rb and Sr into silicate and carbonate phases, respectively, and subsequent lowgrade metamorphism, with the observed mean close in value to the interpreted true mean (Cousens, 1999).
Fig. 15. Photomicrograph of a typical high-Mg minette lava from the Dubawnt Lake area. C, clinopyroxene; P, phlogopite.
quartz/anorthoclase, quartz/plagioclase, quartz/plagioclase/sanidine (or late microcline). Biotite is an important late phase and forms 1 cm crystals in the coarsest samples. Zircon, which is euhedral and finely zoned, can be phenocrystic but usually occurs in pockets of late-crystallizing
minerals with apatite, fluorite, magnetite, biotite, and titanite. Rapakivi texture is widespread in the Nueltin granites, though not common. Myrmekite is absent but graphic quartz /potassium feldspar is very common in porphyry dykes. Some rhyolites are topaz-bearing (LeCheminant et al., 1987b). Plots of normative feldspar contents (Fig. 9) are consistent with significant fractionation of anorthite component. In a normative quartz /feldspar plot (Fig. 10) the Nueltin granites form an array that overlaps the Hudson granitoid cluster, but extends toward the quartz /sanidine join. Large exposures of the Pitz Formation are only located on the north side of the complex of Baker Lake Group basins, and small outliers to the southwest are only present inside the basins (Fig. 6). The northernmost Nueltin plutons are south of the large Pitz exposures, and plutons are present in a SSW-trending band that extends well beyond the basins, suggesting relative post-Nueltin uplift occurred toward the south. North of 608 latitude, the Nueltin granites and the Hudson granitoids are nearly mutually exclusive. They are commonly separated by known shear zones and brittle faults,
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
91
Fig. 17. Polished slabs of (a) Nueltin granite, with sanidine phenocrysts, and (b) Pitz Formation rhyolite with phenocrysts of quartz and sanidine. Note the mafic fragments (m) in the rhyolite, which are clots of basalt. Round objects on the rhyolite are air bubbles in water.
Fig. 16. MORB-normalized trace element diagrams, after Rock and Wheatley (1988). (a) Dubawnt minettes and Hudson granitoids; (b) Nueltin granites and basaltic rocks. Envelopes represent 9/1 standard deviation from the average.
so the map pattern is probably due to different exposure levels in the crust, with the mid-crustal Hudson plutons remaining hidden in a downdropped corridor that bears the Nueltin plutons. Although the Nueltin plutons are relatively restricted, nearly the entire WCP shows resetting of Rb /Sr and K /Ar ages at about 1.75 Ga (e.g. Loveridge et al., 1988). Compared to the Hudson granitoids, the Nueltin granites and Pitz rhyolites are more aluminous, more siliceous, and have higher average K/Na and lower normative An. They are not depleted in Y (Fig. 11) and have much lower Ce/Yb (average is 47). A prominent negative Eu anomaly is typical (Fig. 12). The range of o Nd in the Hudson granitoids extends to more negative values because some samples come from a block of old ( /3.0 Ga)
Fig. 18. Microgabbroic fragment in superheated rhyolite, Pitz Formation. A resorbed anorthoclase phenocryst is above the basalt. The rhyolite glass surrounding the fragment is darkened from Fe contamination.
92
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
crust south of the Kaminak greenstone belt (van Breemen et al., in preparation); if these are excluded, the two granitic suites cannot be distinguished with Nd isotopes. The basaltic rocks associated with the Nueltin granites are difficult to characterize geochemically due to fractional crystallization, and mixing with rhyolite magmas and xenocrysts (the highest MgO concentration recorded in these rocks is 5.1%), but they most closely resemble fractionated alkali basalt (Table 1) and are distinctive amongst igneous rocks of the region at this time for having K/Na B/1. Their incompatible element profiles have an overall shape similar to the granitoids and the minettes, but are displaced towards MORB (Fig. 12). The average Nd isotope composition of two samples of the McRae Lake diabase dyke, which is locally mingled with Nueltin granite, lies at the depleted edge of the range of Nueltin granite compositions (Figs. 13 and 14) and is within, though near the upper limit of, the Nd composition of the minettes. Source contributions from enriched mantle are therefore possible but since the diabase has low K/Na, major contributions from the same source as the minettes is not indicated. It would be difficult to distinguish this from mixing with crustal rocks or granite magma, and the unusually high Y content of the McRae Lake dyke (Fig. 11) indicates that mixing with Nueltin granite magmas did occur. The Nueltin granites, which formed discordant subvolcanic stocks and had a wide temperature interval of crystallization, represent larger and hotter magma bodies than those which formed the Hudson plutons. The presence of mingled basalt-granite, and of basaltic clots and resorbed phenocrysts in nearly superheated rhyolites, suggests the magmas were generated by intrusion of basalt into crust. The Nueltin granites are concentrated in areas affected by previous to synvolcanic brittle faulting. They therefore fit a commonly cited model for post- or anorogenic rapakivi granites, which postulates basaltic melts invading the lower crust along faults and ponding beneath subvolcanic plutons that are undergoing low-pressure fractional crystallization (e.g. Ra¨mo¨ and Haapala, 1990).
2.4. Discussion: sources and heat triggers for the igneous suites The broad extent of ca. 1.75 Ga thermal resetting in the WCP associated with the anorogenic Nueltin granites, and the evidence for a basaltic trigger, suggests that a large-scale mantle disturbance caused their formation. The approximately 100 million year time gap measured between intrusion of post-orogenic granitoids and anorogenic rapakivi granites matches that observed in many other approximately 2.0 /1.6 Ga Laurentian orogens (e.g. the Svecofennian orogen: Nironen, 1997; the Ketilidian orogen: Brown et al., 1992), which argues for a common mechanism for their generation that is related to evolution of the hinterland lithosphere. Rutile and titanite Pb /Pb and U /Pb ages southwest of the Athabasca basin in northern Saskatchewan cluster tightly near 1750 Ma, suggesting rapid uplift and denudation rates outside the Nueltin corridor at that time (Orrell et al., 1999). As noted above, the absence of midcrustal Hudson granitoid plutons within the Nueltin corridor indicates that it was relatively downdropped during or after emplacement of the Nueltin suite, particularly at its north end. A mechanism that combines local crustal extension with passive, relatively unfocussed mantle upwelling is required and we suggest that delamination of lithospheric mantle is the most likely cause. The resulting thermal anomaly would be at least as extensive as the foundered lithospheric mantle, with basalt injection occurring where the overlying crust is subject to brittle faulting. In the WCP, this faulting most easily developed where previous faulting related to the Slave-Churchill indentation at approximately 1.84 Ga had occurred. Magnetotelluric data from the Chesterfield Inlet area, which is well outside the Nueltin corridor, indicate that Archean lithospheric mantle in that area is preserved (Jones et al., 2000), and hence delamination could not have occurred everywhere in the study area. The heat source for the Hudson granitoids is not easily constrained. Minor gabbros associated with the plutons near Wager Bay might represent juvenile mafic magmas emplaced in the crust, but these are not present in other areas. Although a
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
significant amount of cogenetic mafic magma (the minettes, and lesser spessartites) was present in much of the WCP, these have no discernible component of juvenile mantle, which seemingly requires that melting occurred only in the lithosphere. Most of the Hudson granitoids occur outside the area intruded by minette dykes, so that the minettes cannot have been the sole melting trigger for the granitoids. Finally, the timing and mechanism of enrichment of the minette source region is in doubt, making it difficult to incorporate them with the granitoids into a common model. Although crustal thickening alone is probably capable of generating lower to mid-crustal minimum granitoid melts at approximately 800 8C (e.g. Gerdes et al., 2000), the high-Mg minettes likely have liquidus temperatures near 1100 8C (Esperanc¸a and Holloway, 1987) and probably require a convective heat source. A successful petrogenetic model for the minettes and granitoids should additionally incorporate those features that are present in comparable igneous suites from similar tectonic environments. To guide the discussion, we next examine these features by describing important examples.
3. Petrogenetic models for the Hudson granitoids and Dubawnt minettes 3.1. Comparison of the WCP to other Laurentian hinterlands Overlapping minette-calcalkaline granite (MCG) and basalt-rapakivi granite (BRG) associations are not unique to the Trans-Hudson hinterland in the early Proterozoic. Similar rocks of nearly identical age have been described from at least three other localities: Greenland, Brazil, and Baltica. These examples also include similarities in the sequences of crustal deformation and basin development, and all occur within the upper plates (hinterlands) of prominent 2.0 /1.6 Ga orogens. 3.1.1. Greenland Pink-weathering, equigranular granitoid plutons dated at near 1830 Ma are prominent in the northern Nagssuqtoqidian orogen (Kalsbeek and
93
Nutman, 1996). These intrusions, described as insitu melts, shortly post-date ductile deformation in Archean wall rocks. Although minette dykes or plutons have not been described in the immediate vicinity, they are present approximately 100 km to the north, at Oqaitsungui, near Disko Bugt (Skerjnaa, 1992). These rocks are petrographically and geochemically identical to the Dubawnt minettes, and also contain ellipsoidal glimmerite xenoliths. The Greenland minettes have not been dated by U/Pb methods but have a K/Ar age of 1750 Ma. Dykes of approximately 1.76 Ga, undeformed granite are present in the Nagssuqtoqidian domain (Whitehouse et al., 1998), but rapakivi plutons have not been described. South of the Nagssuqtoqidian, in the Ketilidian orogen, is a prominent rapakivi granite province, dated at 1750 Ma. These granites also cut ca. 1.83 Ga post-tectonic equigranular granitoids (Brown et al., 1992). Tectonic reconstructions of this part of Laurentia are made difficult by water and ice cover and complex geology; however, it is possible that the Nagssuqtoqidian domain is an extension of the Churchill Province. 3.1.2. Brazil The early Proterozoic WCP was strikingly similar to the Saˆo Francisco craton of Brazil during and shortly after the Trans-Amazonian orogeny (:/2.1 /1.8 Ga: Alkim and Marshak, 1998). Ultrapotassic plutons mingled with calcalkaline granitoids, emplaced at approximately 2.05 Ga in reworked Archean rocks (Rosa et al., 1999), crop out west of the terrigenous Espinhac¸o Basin, on the western margin of the craton. The lower Espinhac¸o Group consists of alluvial and fluvial sediments, interbedded with bimodal (rhyolite-basalt) volcanic rocks, deposited in basins formed by extension and block faulting, beginning at approximately 1.75 Ga (Uhlein et al., 1998; Martins-Neto, 2000). The rift facies rocks were overlain by fluvial and aeolian sandstones, capped by shallow marine deposits, that were deposited in a broader basin possibly related to thermal subsidence. The Espinhac¸o sequence is thus very similar in age and lithofacies to the middle and upper Dubawnt Supergroup. The minette volcanics of the lower Dubawnt Supergroup have no
94
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
equivalents in exposed portions of the Espinhac¸o basin but the high-K plutons indicate that ultrapotassic magmatism did occur shortly before. The ultrapotassic rocks have Nd /Sr isotopic compositions similar to average Dubawnt minette (Rosa et al., 2000). 3.1.3. Baltica A shoshonitic series from minette to calc-alkaline granite occurs in ca. 1.80 /1.77 Ga plutons in a 600 km-long belt from southern Finland to Russian Karelia, in the Fennoscandian orogen (Eklund et al., 1998). The plutons and dykes occur in a region that was heavily intruded by approximately 1.83 Ga, late to post-tectonic granitoids, and which coincides closely with an anorogenic 1.65 /1.55 Ga rapakivi granite province. The minettes are described as being identical in isotopic composition to the granitoids, and having overlapping trace element compositions. This igneous province differs from the examples cited above in that the intrusions are located in juvenile Proterozoic crust, and have depleted isotopic compositions. The time gap between MCG and BRG activity was also somewhat longer (200 myrs, rather than 100 myrs). 3.2. Phanerozoic examples Important examples of post-orogenic MCG associations in the Phanerozoic are too numerous to catalogue here. The minettes of the Variscan Bohemian Massif, which are geochemically nearly identical to the Dubawnt minettes (Ne´mec, 1974) are hybridized with granodioritic melts and have highly enriched Nd isotopic compositions (Gerdes et al., 1999). Ultrapotassic enclaves are prominent in the granodioritic Hercynian plutons of Corsica, which shortly post-date crustal thinning (Roberts and Ferre´, 1999). There are numerous other Hercynian examples of the association attributed to extensional thinning of previously thickened continental crust (e.g. western Alps: Bussy et al., 1999). In most cases the presence of high-Mg, highly incompatible-element enriched rocks is attributed to subduction enrichment of the upper mantle, but it is also commonly noted that
additional crustal contamination of the high-Mg magmas is indicated. Rock (1991) used the Caledonian (:/414 Ma) minettes of northern Britain as the type example of his MCG association. These are intimately associated with the Ross of Mull granitic pluton, and bear a striking geochemical resemblance to cogenetic granitic dykes (Rock and Hunter, 1987). To quote: ‘. . .lamprophyric magmatism. . .was intimately related to granitic magmatism in space, time, and composition’ (their italics). These examples emphasize the persistent geochemical similarity between continental hinterland minettes and cogenetic granites, regardless of their emplacement age or the age and previous history of their source region . 3.3. Comparison to the Tibetan Plateau Miller et al. (1999) have concisely summarized the occurrences of ultrapotassic and associated calc-alkaline silicic magmatism in the Tibetan plateau. Dated at approximately 15 /25 Ma, they postdate continent/continent collision by about 30 Ma. The calc-alkaline dacites and rhyolites contain ca. 400 Ma zircon xenocrysts and have a crustal origin. The ultrapotassic rocks (which petrographically are minettes, but which have some compositional characteristics of lamproites) have isotopic and trace element characteristics that imply significant source contributions from Archean mantle. However, contamination of this mantle by a crustal component is required (Miller et al. suggest subducted sediment). The authors suggest that exposure of a metasomatized lithospheric mantle layer to hot asthenosphere, by extension and thinning or slab break-off, is required to generate the compositions observed. More precise models for the formation of the Tibetan minette source region depend on the chosen mechanism for crustal thickening. Thickening may have occurred primarily by shortening, underthrusting of Indian lithosphere, or lateral inflow of Indian lithosphere (Treloar, 1997). With these scenarios, enrichment of a mantle root could occur by enrichment from subducted sediments or underthrust crustal rocks, or by imbrication of crust and mantle.
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
3.4. Tectonic models for MCG associations We suggest that the following features are salient characteristics of late crust-mantle evolution in large continental hinterlands: (1) nearsimultaneous, late to post-orogenic melting of deformed lithosphere in a lower to mid-crustal layer to generate calc-alkaline granitoids, and in a LILE-enriched layer of upper mantle to generate minettes. The melting is synchronous with or shortly postdates ductile extension, and does not involve significant contributions from asthenospheric melts; (2) regional low-grade thermal metamorphism, accompanied by local extension and emplacement of bimodal BRG, about 100 million years after the post-orogenic igneous suites; (3) broad crustal sagging leading to the formation of terrigenous, arenaceous basins capped with shallow marine chemogenic deposits, about 10 /50 myrs after bimodal volcanism. There being no indications of significant melting of convecting mantle in our study area, our preferred model for generation of the MCG granitoids is anatexis of lower to mid-crustal rocks in response to rising temperatures resulting from crustal thickening in the collisional hinterland. Direct evidence for an appropriate thickening event shortly before 1.85 Ga in the WCP is scant, but significant. Crustal thickening at ca. 1.9 Ga has been suggested to account for the high pressure metamorphism observed in granulitefacies rocks of the Kramanituar Complex (Sanborn-Barrie, 1999) and the amphibolite-facies supracrustal belt southwest of Baker Lake (Berman et al., 2000). Recent geochronologic evidence for ca. 1.9 Ga Barrovian metamorphism further to the east of this belt lends support to this hypothesis (Berman et al., 2002). The unusual microdiamonds of the Akluilaˆk dyke may reflect deep underthrusting of volatile-rich crust into the upper mantle in response to lithospheric shortening. Vertical movements on major faults at about 1.83 Ga, such as the Tyrrell shear zone, imply isostatic readjustment with relatively buoyant (thickened) crust to the east, towards the highest concentration of Hudson granitoid plutons. The mechanism of enrichment/melting of the minette source region has implications for the role
95
of the upper mantle in hinterland lithosphere evolution. At present, we cannot eliminate any of three models for the enrichment event, and the melting trigger remains speculative. A common element of all of these models, is that the minette source region began as a layer of subcontinental lithospheric upper mantle, depleted (by extraction of basalt magma) in incompatible elements resulting in mantle with low Ca and Al, and high Mg/ Fe. The principal incompatible element source of the minettes was enriched fluids or melts, which invaded the depleted peridotite and generated glimmerite veins that subsequently melted to produce ultrapotassic magma. Since the minettes display a clear subduction zone signature, with depleted HFSE, we presume that the enrichment was not caused by mafic silicate melts, but by K and H-rich fluids that had an ultimate source in crustal rocks. 3.4.1. Enrichment model 1: Proterozoic subduction The source region of the minettes may have been enriched during the Proterozoic by subduction of Archean continental sediment beneath the Thelon and THOs (Peterson et al., 1994), with a small time gap between enrichment and minette melt generation. A significant possible source of LILE-rich sediment was erosion of 2.6 Ga granitoid plutons, which has been correlated with a rapid increase in radiogenic Sr in seawater at about 1.8 Ga (Taylor and McLennan, 1985). The chief attraction of this hypothesis is its linkage with well-established tectonic events (i.e. two Proterozoic collisional orogenies), and the fact that it can incorporate examples (such as Baltica) where the possible sediment sources are in juvenile crust and the ultrapotassic rocks have depleted isotopic compositions. However, in the WCP this hypothesis is difficult to reconcile with the observations that the minettes are restricted to the interior of the hinterland, well away from the presumed sites of subduction, and that high-K rocks within the arc volcanic sequences of the THO have Nd compositions close to depleted mantle (Stern et al., 1995). It is reasonable to presume that a large volume of oceanic lithosphere was subducted beneath the central WCP, and a highly speculative alternative is that this accumulated in a megalith that subse-
96
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
quently released a pulse of LILE-enriched fluids or melts beneath the center of the WCP. 3.4.2. Enrichment model 2: Archean subduction The Nd/Sr isotopic data allow that LILE enrichment of lithospheric mantle may have occurred at about 2.8 /2.7 Ga (Cousens et al., 2001). The enrichment may have resulted from subduction during the assembly of the WCP in the late Archean. This hypothesis is supported by the presence of some pre-1.9 Ga mafic dykes and sills in the WCP that plot within the Nd evolution envelope of the minettes, and is attractive for its simplicity, testability, and possible application to other Archean hinterlands. Its principal difficulty is that, if enrichment occurred in the Archean, the distribution of the Dubawnt minettes should reflect to some degree the distribution of major Archean domains and structures in the WCP. However, the minette dyke swarm cuts across all such features, including greenstone belts that were generated at approximately 2.8 /2.7 Ga. There is also no evidence for ultrapotassic rocks in the WCP before 1.9 Ga, particularly in association with the 2.6 Ga granitoid suite, which is similar to the Hudson suite in its average composition and level of emplacement. 3.4.3. Enrichment model 3: crust/mantle mixing An alternative to subduction of sediment as a means to enrich the base of the lithospheric mantle, is direct underthrusting of continental crust or imbrication of crust and mantle wedges deep in the lithosphere. This model is attractive in that: (1) it directly links the generation of minettes to lithospheric thickening, which is generally presumed to play a role in the generation of the associated granitoids, and (2) explains the widelymade observation that the minettes and the granitoids have similar isotopic and trace element signatures. Its principal difficulty is that the model is difficult to test, as it depends on mapping, lithosphere imaging, and thermobarometric data at a level of detail that is not yet available in the WCP. Peridotite xenoliths in the minettes are rare and invariably strongly altered, but if unaltered ones can be identified in the minettes or other rocks,
then it may be possible to date metasomatic mineral assemblages in them. Further teleseismic and magnetotelluric imaging of the WCP lithosphere (e.g. Jones et al., 2000) across the boundary of the minette dyke swarm might reveal contrasts in the composition of the lower lithosphere, or the presence of major low-angle faults. Finally, a comprehensive study of the approximately 2.6 Ga granitoids of the WCP may provide useful information on regional variations in possible lithosphere enrichment in the Archean.
4. Summary Paleoproterozoic granitoid magmatism in the WCP, a reworked Archean hinterland to two Paleoproterozoic orogenies, occurred in two pulses. Minimum crustal melts, rich in inherited xenocrysts, were emplaced at mid-crustal levels during and shortly after collision of the Superior and Churchill cratons at the THO, at approximately 1.83 Ga (Hudson granitoids). The resulting plutons mostly cross-cut late ductile features in mid-crustal rocks and appear similar in all respects to post-orogenic granitoids that are present in other Archean hinterlands with a similar history. Throughout an area of about 105 km2, the Hudson plutons are closely associated in space and time with ultrapotassic (mainly minette) dykes and volcanic rocks which have isotopic and trace element compositions similar to the granitoids; globally, this association has appeared repeatedly and often in both Archean and juvenile hinterlands to large orogens since about 2.0 Ga. Approximately 100 myrs later ( :/1.76 Ga), the WCP was subjected to widespread heating sufficient to reset K/Ar and Rb /Sr systems and, where older and contemporaneous brittle faulting was present, basalts were emplaced in the lower crust to generate crustal melts that ascended to the surface (Nueltin granites and Pitz Formation rhyolites). The plutonic rocks are locally rapakivi-textured and are identical in all respects to anorogenic rapakivi granite plutons that typically intruded other hinterlands about 100 /200 myrs after emplacement of MCG suites. Since minettes were not generated at that time, it may be that
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
their source was removed by delamination and foundering of the lithospheric upper mantle, permitting the generation of basalt melts by mantle upwelling. In the WCP, the rapakivi province defines a SW-trending corridor that coincides with the volcanic part of the minette province, and with brittle fault systems active at approximately 1.83 Ga and later. The anorogenic rhyolite volcanism was followed within 50 myrs by continent-scale sagging, perhaps resulting from cooling of the upper mantle and thermal subsidence, which produced a very large-scale terrigenous basin capped by shallow marine deposits (Thelon and Athabasca sandstones). This tripartite sequence, for which a sedimentary record is completely preserved in the Dubawnt Supergroup of the WCP, is also evident in the approximately 2.0 /1.8 Ga Trans-Amazonian orogeny of the Saˆo Francisco craton. The first two igneous phases are also preserved in numerous other hinterlands, suggesting that a single tectonic model could be inferred for many of them. Different scenarios have been proposed to explain the presence of the ultrapotassic rocks and subduction enrichment of the lithospheric mantle, shortly before terminal collision, is the most widely cited. However, this model is unsatisfactory in the WCP, primarily because the ultrapotassic province is located far from any Proterozoic subduction zone and potassic rocks in those zones have depleted isotopic compositions. Subduction enrichment of the lithospheric mantle could have occurred at about 2.7 Ga, with glimmerite veins melting at 1.85 Ga due to intense reworking of the lithosphere. A third model, which attempts to integrate the geochemical features of the granitoids and minettes with the shortening/thickening history of the hinterland, places the minette source region in a zone of tectonically mixed crust (principal source of LILE and other incompatible elements) and depleted upper lithospheric mantle (principal source of compatible elements). In all three models, the granitoids are derived from a shallower level in the lower to middle crust. The high mg# (commonly /0.8) and liquidus temperature of the minettes (:/1100 8C?) require a relatively intense heat source that may have
97
resulted from mantle upwelling after slab breakoff beneath the hinterland.
Acknowledgements The authors wish to thank all members of the Churchill NATMAP team for their data, ideas, and support. Richard Stern assisted with SHRIMP analyses. Many of the samples and maps incorporated in this project are from previous GSC projects, and we particularly note contributions by A. LeCheminant, A. Davidson, K. Eade, and S. Tella. This paper is a contribution to the Churchill NATMAP Project and to IGC Project #426 (Proterozoic Granites and Tectonic Processes). This is GSC Contribution 2001155.
References Akim, F.F., Marshak, S., 1998. Transamazonian Orogeny in the Southern Sa˜o Francisco Craton Region, Minas Gerias, Brazil: evidence for collision and collapse in the Quadrila´tero Ferrı´fero. Precambrian Res. 90, 29 /58. Annesley, I.R., Madore, C., Rupan, S. Krogh, T.E., 1997. U / Pb geochronology of thermotectonic events in the Wollaston Lake area, Wollaston Domain: a summary of 1994 / 1996 results. Summary of Investigations 1997, Sask. Geol. Surv. Misc. Report 97 /4, 162 /173. Aspler, L.B., Chiarenzelli, J.R., 1997. Archean and Proterozoic geology of the North Henik Lake area, district of Keewatin, Northwest Territories, Current Research 1997-C, Geol. Surv. Can., 145 /155. Berman, R.G., Ryan, J.J., Tella, S., Sanborn-Barrie, M., Stern, R., Aspler, L., Hanmer, S.K., Davis, W., 2000. The case of multiple metamorphic events in the Western Churchill Province: evidence from linked thermobarometric and insitu SHRIMP data, and jury deliberations. Geol. Ass. Canada Yearly Meeting 2000, Abstr. [CD-ROM]. Berman, R., Davis, W.J., Ryan, J.J., Tella, S., 2002. In-situ U/ Pb SHRIMP geochronology of Barrovian facies-series metasedimentary rocks in the Happy lake and Josephine River supracrustal belts: implications for the Paleoproterozoic architecture of the northern Hearne domain, Nunavut. Current Research, Part F, Geol. Surv. Canada Pap. 2002-F4, pp. 1 /14. Blake, D.H., 1980. Volcanic rocks of the Paleohelikian Dubawnt Group in the Baker Lake /Angikuni Lake area, District of Keewatin, N.W.T. Geol. Surv. Can. Bull. 309. Booth, G.W., 1983. The petrology and geochemistry of the Pamiutuq Lake batholith, Northwest Territories 1983. M.Sc. Thesis Univ. Toronto, Canada.
98
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Brown, P.E., Dempster, T.J., Harrison, T.N., Hutton, D.H.W., 1992. The rapakivi granites of S Greenland */crustal melting in response to extensional tectonics and magmatic underplating. Trans. R. Soc. Edinburgh Earth Sci. 83, 173 /178. Bussy, F., Hernandez, J., von Raumer, J., 1999. Bimodal magmatism as a consequence of the post-orogenic readjustment of the thickened Variscan continental lithosphere (Aigulle Rouges/Mont-Blanc massifs, Western Alps). In: Barbarin, B. (Ed.), The Origin of Granites and Related Rocks, Fourth Hutton Symposium Abstr. Document du BRGM 290, 205. Carmichael, I.S.E., 1963. The crystallization of feldspar in volcanic acid liquids. Q. J. Geol. Soc. London 119, 95 /131. Chinn, I., Kyser, K., 2000. Microdiamonds from the Thirsty Lake (Akluilaˆk) dyke, Northwest Territories Canada. J. Conf. Abstr. 5, 307. Chisholm, R., 1993. Diamond Drilling Report on the DUB 1/ 2 Mineral Claims, District of Keewatin, Northwest Territories. Taiga Consultants Ltd, Calgary, Canada. Corrigan, D. Scott, D.J., St-Onge, M.R., 2001. Geology of the northern margin of the Trans-Hudson Orogen (Foxe Fold Belt), central Baffin Island, Nunavut. Current Research 2001-C, Geol. Surv. Can. C23 [CD-ROM]. Cousens, B.L., 1999. Geochemistry and geologic setting of the Baker Lake Group, Angikuni Lake to Yathkyed Lake. Indian and Northern Affairs Canada, Geol. Div. Open File 1999 /05. Cousens, B.L., Aspler, L.B., Chiarenzelli, J.R., Donaldson, J.A., Sandeman, H., Peterson, T.D., LeCheminant, A.N., 2001. Enriched Archean lithospheric mantle beneath Western Churchill Province tapped during Paleoproterozoic orogenesis. Geology 29, 827 /830. Culshaw, N., 1991. Post-collisional oblique convergence along the Thelon Tectonic Zone, north of the Bathurst Fault, NWT, Canada. J. Struct. Geol. 13, 501 /516. Davidson, A., 1969. Precambrian geology, Kaminak Lake maparea, District of Keewatin. Geol. Surv. Can. Pap., 51 /69. Dewey, J.F., Burke, K.C.A., 1973. Tibetan, Variscan, and Precambrian basement reactivation: products of a continental collision. J. Geol. 81, 683 /692. Donaldson, J.A., 1968. Proterozoic sedimentary rocks of northern Saskatchewan. Geol. Surv. Can., Report of Activities, Pap. 68-1A. ¨ ., LeCheminant, A.N., Sullivan, R.W., 1991. Duda´s, F.O Reconnaisance Nd isotopic study of granitoid rocks from the Baker Lake region, District of Keewatin, N.W.T., and observations on analytical procedures. Radiogenic and Isotopic Studies: Report 4, Geol. Surv. Can. Pap. 90 /2, pp. 101 /112. Eklund, O., Konopelko, D., Rutanen, H., Fro¨jdo¨, S., Shebanov, A.D., 1998. 1.8 Ga Svecofennian post-collisional shoshonitic magmatism in the Fennoscandian shield. Lithos 45, 87 /108. Esperanc¸a, S., Holloway, J.R., 1987. On the origin of some mica-lamprophyres; experimental evidence from a mafic minette. Contrib. Miner. Petrol. 95, 207 /216.
Gall, Q., 1994. The Proterozoic Thelon Paleosol, Northwest Territories, Canada. Precambrian Res. 68, 115 /137. Gall, Q., Peterson, T.D., Donaldson, J.A., 1992. A proposed revision of early Proterozoic stratigraphy of the Thelon and Baker Lake basins, Northwest Territories. Current Research 1992-C, Geol. Surv. Canada, pp. 129 /137. Gerdes, A., Wo¨rner G., Finger F., 1999. Hybrid generation by mixing enriched mantle magmas with crustal melts: constraints from the Rasyenburg pluton, southern Bohemia. In: Barbarin, B. (Ed.), The Origin of Granites and Related Rocks, Fourth Hutton Symposium Abstr. vol. Document du BRGM 290, 22. Gerdes, A., Wo¨rner, G., Henk, A., 2000. Post-collisional granite generation and HT-LP metamorphism by radiogenic heating: the Variscan South Bohemian Batholith. J. Geol. Soc. Lond. 157, 577 /587. Hadlari, T., Rainbird, R.H., 2001. Volcano-sedimentary correlation and fault relationships in the Baker Lake sub-basin, Thirty mile lake area, Nunavut. Current Research 2001-C, Geol. Surv. Can. C10 [CD-ROM]. Hanmer, S., 1997. Geology of the Striding-Athabasca mylonite zone, northern Saskatchewan and southeastern District of Mackenzie, Northwest Territories. Geol. Surv. Can. Bull., 324. Hanmer, S.K., Maclachlan K., Aspler L.B., Berman, R.G., Davis, W.J., Rainbird, R.H., Zaleski, E., Pehrsson, S.J., Sandeman, H.A., Ryan, J.J., Jones, A.G., Peterson, T.D., Tella, S., Relf, C. Late Neoarchean and Proterozoic reworking of the Western Churchill Province, Canadian Shield, Nunavut. Precambrian Res., in preparation. Henderson, J.R., 1983. Structure and metamorphism of the Aphebian Penrhyn Group and its Archean basement complex in the Lyon Inlet area, Melville Peninsula, District of Keewatin. Geol. Surv. Can. Bull., 324. Henderson, J.R., Roddick, J.C., 1990. U /Pb age constraint on the Wager shear zone, District of Keewatin, N.W.T. Radiogenic Age and Isotopic Studies: Report 3, Geol. Surv. Can. Pap. 89-2, pp. 149 /152. Hoffman, P.F., 1988. United Plates of America, the birth of a craton: early Proterozoic assembly and growth of North America. Ann. Rev. Earth Sci. Lett. 16, 543 /603. Jenner, G.A., Longerich, H.P., Jackson, S.E., Fryer, B.J., 1990. ICP-MS-a powerful new tool for high precision trace element analysis in Earth sciences: evidence from analysis of selected USGS standards. Chem. Geol. 83, 133 /148. Jones, A.G., Snyder, D., Asudeh, I., White, D., Eaton, D., Clarke, G., 2000. Lithospheric architecture at the RaeHearne boundary revealed through magnetotelluric and seismic experiments. Geol. Ass. Canada Yearly Meeting 2000, Abstr. [CD-ROM]. Kalsbeek, F., Nutman, A.P., 1996. Anatomy of the Early Proterozoic Nagssugtoqidian orogen, West Greenland, explored by reconnaissance SHRIMP U /Pb zircon dating. Geology 24, 515 /518. Kaminsky, F.V., Sablukov, S.M., Sablukova, L.I., Shpanov, V.E., Zhuravlev, D.Z., 1998. Diamondiferous minette dykes from the Parker Lake area, N.W.T., Canada. Seventh
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100 International Kimberlite Conference Ext. Abstr., pp. 392 / 394. Krogh, T.E., Clark, L.A., 1987. Zircon dating of sub-Athabasca granitoid rocks, Saskatchewan. Geol. Ass. Canada Annual Meeting Abstr. 12, pp. 64. LeCheminant, A.N., Roddick, J.C., Tessier, A.C., Bethune, K.M., 1987a. Geology and U /Pb ages of early Proterozoic calc-alkaline plutons northwest of Wager Bay, District of Keewatin. Current Research, Part A, Geol. Surv. Canada Pap. 87-1A, pp. 773 /782. LeCheminant, A.N., Miller, A.R., LeCheminant, G.M., 1987b. Early Proterozoic alkaline igneous rocks, District of Keewatin, Canada: petrogenesis and mineralization. In: Pharaoh, T.C., Beckinsdale, R.D., Rickard, D. (Eds.), Geochemistry and Mineralization of Proterozoic Volcanic Suites. Geol. Soc. Spec. Publ. 33, pp. 219 /240. Loveridge, W.D., Eade, K.E., Sullivan, R.W., 1988. Geochronological studies of Precambrian rocks from the southern District of Keewatin. Geol. Surv. Can., 88 /188. MacLachlan, K., Relf, C., Davis, W.J., 2000a. U/Pb geochronological constraints on structures controlling distribution of tectonothermal domains, Yathkyed Lake area, Western Churchill Province. Geol. Ass. Canada Yearly Meeting 2000 Abstr. [CD-ROM]. MacLachlan, K., Hanmer, S., Davis, W.J., Berman, R.G., Ryan, J.J., Relf, C., Aspler, L.B. 2000b. Complex, protracted, Proterozoic reworking of the Western Churchill Province: the craton that wouldn’t grow up? Geol. Ass. Canada Yearly Meeting 2000 Abstr. [CD-ROM]. Martins-Neto, M.A., 2000. Tectonics and sedimentation in a Paleo/Mesoproterozoic rift-sag basin (Espinhac¸o basin, southeastern Brazil). Precambrian Res. 103, 147 /173. Meyer, M.T., Bickford, M.E., Lewry, J.F., 1992. The Wathaman Batholith; an early Proterozoic continental arc in the Trans-Hudson orogenic belt, Canada. Geol. Soc. Am. Bull. 104, 1073 /1085. Miller, C., Schuster, R., Klo¨tzi, U., Frank, W., Purtscheller, F., 1999. Post-collisional potassic and ultrapotassic magmatism in SW Tibet: geochemical and Sr /Nd /Pb /O isotopic constraints for source characteristics and petrogenesis. J. Petrol. 40, 1399 /1424. Mitchell, R.H., Bergman, S.C., 1991. Petrology of Lamproites. Plenum, New York. Nelson, D.R., 1989. Isotopic characteristics and petrogenesis of the lamproites and kimberlites of central West Greenland. Lithos 22, 265 /274. Ne´mec, D., 1974. Petrochemistry of the dyke rocks of the central Bohemian Pluton. Neues J. Miner. Monatsh. 5, 193 /209. Nironen, M., 1997. The Svecofennian Orogen: a tectonic model. Precambrian Res. 86, 21 /44. Orrell, S.E., Bickford, M.E., Lewry, J.F., 1999. Crustal evolution and age of thermotectonic reworking in the western hinterland of the Trans-Hudson Orogen, northern Saskatchewan. Precambrian Res. 95, 187 /223. Peterson, T.D., 1992. Early Proterozoic ultrapotassic volcanism of the Keewatin Hinterland, Canada. In: Meyer, H.O.A.,
99
Leonardos, O.H. (Eds.), Proceedings of the Fifth International Kimberlite Conference, vol. 1: Kimberlites, Related Rocks and Mantle Xenoliths. Companhia de Pesquisa de Recursos Minerais (Brası´lia), pp. 221 /235. Peterson, T.D. Geology of the Dubawnt Lake area, Nunavut. Geol. Surv. Canada Bull., in preparation. Peterson, T.D., LeCheminant, A.N., 1993. Glimmerite xenoliths in Early Proterozoic ultrapotassic rocks from the Churchill Province. Can. Miner. 31, 801 /819. Peterson, T.D., Esperanc¸a, S., LeCheminant, A.N., 1994. Geochemistry and origin of the Proterozoic ultrapotassic rocks of the Churchill Province Canada. Miner. Petrol. 51, 251 /276. Peterson, T.D., van Breemen, O., 1999. Review and progress report of Proterozoic granitoid rocks of the Western Churchill Province, Northwest Territories (Nunavut). Current Research 1999-C, Geol. Surv. Can., pp. 119 /127. Ra¨mo¨, O.T., Haapala, I., 1990. The rapakivi granites of eastern Fennoscandia: a review with insights into their origin in the light of new Sm-Nd isotopic data. In: Gower, C.F., Rivers, T., Ryan, B. (Eds.), Mid-Proterozoic Laurentia-Baltica. Geol. Ass. Can. Spec. Pap. 38, 401 /415. Rainbird, R.H., Peterson, T.D., 1990. Physical volcanology and sedimentology of lower Dubawnt Group strata, Dubawnt Lake, District of Keewatin, N.W.T. Current Research 1990C, Geol. Surv. Can., pp. 207 /217. Rainbird, R.H., Hadlari, T., Donaldson, J.A., 1999. Stratigraphy and paleogeography of the Paleoproterozoic Baker Lake Group in the eastern Baker Lake basin, Northwest Territories (Nunavut). Current Research 1999-E, Geol. Surv. Can., pp. 43 /53. Rainbird, R.H., Hadlari, T.T., Donaldson, J.A., 2001. Evolution of the Paleoproterozoic intracratonic Baker Lake and Thelon basins, Western Churchill Province, Canada, In: Cassidy, K.F., Dunphy, J.M., Van Kranendonk, M.J. (Eds.), Fourth International Archean Symposium, Perth, Australia, AGSO-Geoscience Australia Record 2001/37, pp. 259 /260. Relf, C., Maclachlan, K., Irwin, D., 1999. Tectonic assembly and Proterozoic reworking of the northern Yathkyed greenstone belt, Northwest Territories (Nunavut). Current Research 1999-C, Geol. Surv. Can., pp. 139 /146. Roberts, M.P., Ferre´, E.C., 1999. The succession of high-K calc-alkaline to alkaline granitic rocks in collisional belts tracks post-collisional crustal thinning. In: Barbarin B. (Ed.), The Origin of Granites and Related Rocks, Fourth Hutton Symposium Abstr. vol. Document du BRGM 290, 184. Rock, N.M.S., 1991. Lamprophyres. Blackie and Son Ltd, Glasgow. Rock, N.M.S., Griffin, B.J., Edgar, A.D., Paul, D.K., Hergt, J.M., 1992. A spectrum of potentially diamondiferous lamproites and minettes from the Jharia coalfield, eastern India. J. Volc. Geotherm. Res. 50, 55 /83. Rock, N.M.S., Hunter, R.H., 1987. Late Caledonian dykeswarms of northern Britain: spatial and temporal intimacy between lamprophyric and granitic magmatism around the
100
T.D. Peterson et al. / Precambrian Research 119 (2002) 73 /100
Ross of Mull pluton, Inner Hebrides. Geol. Rundsch. 76, 805 /826. Rock, N.M.S., Wheatley, M., 1988. SPIDER: a Macintosh program to generate normalized multi-element ‘spidergrams’. Am. Miner. 73, 919 /921. Rosa, M.L.S., Conceic¸aˆo, H., Martin, H., Conceic¸aˆo, R.V., Nardi, L.V.S., Oberli, F., Meier, M., Paim, M., Barreto dos Santos, E., Perreira Santos, C.G., Oliveira, A.E., Leahy, G.A.S., 1999. Significance of Paleoproterozoic potassicultrapotassic syenitic provinces in the Bahia State, Brazil. In: Barbarin, B. (Ed.), The Origin of Granites and Related Rocks, Fourth Hutton Symposium Abstr. vol. Document du BRGM 290, 186. Rosa, M.L.S., Conceic¸aˆo, H., Oberli, F., Meier, M., Martin, H., Macambira, M.J.B., Conceic¸aˆo, R.V., Barreto dos Santos, E., Paim, M., Leahy, G.A.S., Leal, L.R.B., 2000. Geochronology (U /Pb/Pb /Pb) and isotopic signature (Rb /Sr/Sm /Nd) of the Paleoproterozoic Guanambi batholith, southwest Bahia state (NE Brazil). Rev. Bras. Geo. 30, 62 /65. Sanborn-Barrie, M., 1994. Structural investigation of highgrade rocks of the Kramanituar Complex, Baker Lake area, Northwest Territories. Current Research 1994-E, Geol. Surv. Can., pp. 75 /89. Sanborn-Barrie, M., 1999. Geochemistry of gabbroic rocks from the 1.9 Ga Kramanituar Complex and surrounding country rocks of the Archean Western Churchill Province, Nunavut. Current Research 1999-E, Geol. Surv. Can., pp. 75 /89. Sandeman, H.A., Lepage, L.D., Ryan, J.J., Tella, S., 2000. Field evidence for commingling between ca. 1830 Ma lamprophyric, monzonitic and monzogranitic magmas, MacQuoid-Gibson Lakes area, Kivalliq Region, Nunavut; Current Research, Geol. Surv. Can., 2000-C [CD-ROM]. Skerjnaa, L., 1992. A lamproite stock with ellipsoidal phlogopite nodules at Oqaitsu´nguit, Disko Bugt, central West Greenland. Rapp. Grønlands Geol. Unders. 154, 33 /47. Stern, R.A., 1997. The GSC sensitive high resolution ion microprobe (SHRIMP): analytical techniques of zircon U /Th /Pb age determinations and performance evaluation. Radiogenic Age and Isotope Studies: Report 10. Geol. Surv. Can., Current Res. 1994-F, pp. 69 /74. Stern, R.A., Berman, R.G., 2000. Monazite U /Pb and Th /Pb geochronology by ion microprobe, with an application to in situ dating of an Archean metasedimentary rock. Chem. Geol. 172, 113 /130. Stern, R.A., Syme, E.C., Bailes, A.H., Lucas, S.B., 1995. Paleoproterozoic (1.90 /1.86 Ga) arc volcanism in the Flin
Flon Belt, Trans-Hudson orogen, Canada. Contrib. Miner. Petrol. 119, 117 /141. Tamboso, A.W., 1981. Geology of the McRae Lake dyke, District of Keewatin, Northwest Territories. B.Sc. Thesis, Univ. Toronto, Canada. Taylor, W.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, London. Tella, S., Eade, K.E., 1985. Occurrence and possible tectonic significance of high-pressure granulite fragments in the Tulemalu fault zone, District of Keewatin, N.W.T. Canada. Can. J. Earth Sci. 23, 1950 /1962. Tella, S., Schau, M., Armitage, A.E., Loney, B.C., 1993. Precambrian geology and economic potential of the northeastern parts of Gibson Lake map area, District of Keewatin, Northwest Territories. Current Research Part C, Geol. Surv. Can. Pap. 93-1C, 197 /208. TerMeer, M., Berman, R.G., Relf, C., Davis, W.D., 2000. Tectonometamorphic history of the Nowyak complex, Nunavut, Canada. Geol. Ass. Canada Yearly Meeting 2000, Abstr. [CD-ROM]. The´riault, R.J., 1990. Methods for Rb /Sr and Sm /Nd isotopic analyses at the Geochronology Laboratory, Geological Survey of Canada. Radiogenic Age and Isotopic Studies: Report 3. Geol. Surv. Can. Pap. 89-2, pp. 3 /6. The´riault, R.J., Henderson, J.B., Roscoe, S.M., 1994. Nd isotopic evidence for early to mid-Archean crust from high grade gneisses in the Queen Maud Block and south of the McDonald Fault, Western Churchill Province, Northwest Territories. Radiogenic Age and Isotopic Studies: Report 8. Geol. Surv. Can. Pap. 1994-F, pp. 37 /42. Treloar, P.J., 1997. Thermal controls on early-Tertiary, shortlived, rapid regional metamorphism in the NW Himalaya, Pakistan. In: Touret, J.L.R., Austrheim, H. (Eds.), Collisional orogens, zones of active transfer between crust and mantle. Tectonophysics 273, 77 /104. Uhlein, A., Trompette, R.R., Egydio-Silva, M., 1998. Proterozoic rifting and closure, SE border of the Saˆo Francisco Craton Brazil. J. S. Am. Earth Sci. 11, 191 /203. van Breemen, O., Peterson, T.D., Sandeman, H.A. Geochronology and Nd isotope geochemistry of Proterozoic granitoids, and Archean domains of the Western Churchill Province. Can. J. Earth Sci., in preparation. Whitehouse, M.J., Kalsbeek, F., Nutman, A.P., 1998. Crustal growth and crustal recycling in the Nagssugtoqidian orogen of West Greenland: Constraints from radiogenic isotope systematics and U /Pb zircon geochronology. Precambrian Res. 91, 365 /381.