Unscrambling the stratigraphy of an Archean greenstone belt; a UPb geochronological study of the Favourable Lake belt, northwestern Ontario, Canada

Precambrian Research, 50 ( 1991 ) 201-220

201

Elsevier Science Publishers B.V., Amsterdam

Unscrambling the stratigraphy of an Archean greenstone belt: a U-Pb geochronological study of the Favourable Lake belt, northwestern Ontario, Canada F. Corfu a and L.D. Ayres b a Ontario Geological Survey, c/o Department of Geology, Royal Ontario Museum, 100 Queen "sPark, Toronto, Ont. M5S 2C6, Canada b Department of Geological Sciences, University of Manitoba, Winnipeg, Man. R3T 2N2, Canada (Received September 19, 1989; revised and accepted October 29, 1990)

ABSTRACT Corfu, F. and Ayres, L.D., 1991. Unscrambling the stratigraphy of an Archean greenstone belt: a U - P b geochronological study of the Favourable Lake belt, northwestern Ontario, Canada. Precambrian Res., 50:201-220. New zircon and baddeleyite U - P b ages show that the major units of the Archean Favourable Lake greenstone belt in the northern Superior Province were formed by episodic magmatism spanning more than 250 Ma and demonstrate that the original stratigraphy is disrupted by thrusts that juxtaposed older supracrustal rocks on top of younger ones. The oldest rocks of the area are a 2950 Ma gneissic tonalite and a 3000-2960 Ma granodiorite clast from a ca. 2725 Ma conglomerate. Five distinct volcanic (and sedimentary) groups formed during presumably short-lived episodes at 2925 Ma (I), 2870 Ma (II), 2858 Ma (III), ~<2734 Ma (IV), and 2725 Ma (V). The youngest group contains the thickest sedimentary unit, a turbiditic and alluvial-fluvial sequence. Compression caused thrusting that placed Groups I and V on top of IV, II1 on V, and II on III. The thrust complex was subsequently isoclinally folded. Compression was accompanied by major plutonism that emplaced the bulk of the bounding batholiths between 2732 and 2711 Ma ago. The late tectonic, Mo-mineralized Setting Net Lake Stock in the centre of the belt has an age of about 2708 Ma and was overprinted by younger hydrothermal events that produced monazite (2706 Ma), titanite (ca. 2695-2690 Ma) and rutile (the youngest rutile at ~<2657 Ma). Similar late hydrothermal pulses are recorded by secondary titanite elsewhere in the belt and within the batholiths. The protracted magrnatic evolution of the belt is typical of that observed in a number of greenstone belts of the northern Superior Province but is uncommon farther to the south. In contrast, the structural complications and out-of-sequence stratigraphy appear to be a quite common tectonic characteristic of greenstone belts of the whole Superior Province.

Introduction

Efforts to reconstruct Precambrian stratigraphic sequences are strongly hampered by the lack of fossil evidence and the general scarcity of reliable marker horizons. In addition, many sequences have been severely disrupted by deformation, metamorphism and intrusion. Thus, in the past, the geologist had to rely solely on carefully mapped facing relationships, structures and volcanological and sedimentological facies associations. While these methods can provide an accurate picture of lithol-

ogical relationships within a supracrustal sequence, they cannot by themselves resolve the absolute stratigraphy of such successions. Archean greenstone belts are generally characterized by apparently conformable relationships between distinct lithostratigraphic packages. In the absence of independent evidence, many of these conformable successions have historically been interpreted as representing simple stratigraphic successions. These stratigraphic interpretations were subsequently integrated into models describing the evolution of the belts. The Favourable Lake greenstone

202

belt represents one of the well documented case studies illustrating these relationships. Careful mapping in the 1960's and early 70's led to the recognition of 15 formations that were grouped into 5 cycles. These were interpreted as reflecting the progressive evolution of a suite of individual but onlapping volcanic complexes (Ayres, 1977). The geochronological study was undertaken to test the above model and to define an absolute chronostatigraphy of the belt. The results show that several of the lithological assemblages do not follow a simple stratigraphic order but are scrambled, with older units being juxtaposed on top of younger ones. The belt must therefore be viewed as a thrust-complex rather than a polycyclic sequence.

V. CORFU AND L.D. AYRES 95 °

940

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SACHIQo

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Geologic setting 0 L

The Favourable Lake greenstone belt is a relatively narrow, 160 km long, linear belt in the Sachigo Subprovince; it is generally less than 5 km wide, but at the southeast end the width increases to 13 km (Fig. 1). Stratigraphic and geochronologic work have been restricted to the southeast end of the belt where a 7.5 km thick, metavolcanic-metasedimentary sequence has been isoclinally folded and cut by numerous concordant and discordant faults (Fig. 2) (Ayres, 1970, 1972, 1974, 1977 ). Metamorphic grade ranges from low to mid-greenschist facies in the centre of the belt to amphibolite and hornblende hornfels facies at the margins adjacent to composite granitoid batholiths that define the present configuration of the greenstone belt (Ayres, 1978 ). The 15 formations that were identified (Ayres, 1977) include ultramafic, mafic, intermediate, and felsic metavolcanic units, metagreywacke-metasiltstone, metaconglomerate, and minor slate, marble, and iron formation (Table 1 ). Individual formations are discontinuous as a result of both depositional processes and later faults, and they are considerably thinner than those recognized in

-

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Fig. 1. Location of the Favourable Lake greenstone belt (lined pattern) in relationship to other Archean greenstone belts (black) and lithotectonic subprovinces in northwestern Ontario and adjacent parts of Manitoba. Unpatterned areas are granitoid plutons and gneisses. Approximate subprovince boundaries are shown by dotted lines; increase in spacing of dots reflects a larger degree of uncertainty in the boundary position. U - P b ages for named greenstone belts are summarized in Fig. 8.

many other Archean greenstone belts of the Canadian Shield (Thurston, 1986). The formations were originally interpreted in terms of 5 cycles numbered in ascending order, with cycle 1 interpreted to be the oldest and cycle 5 the youngest (Ayres, 1977). The geochronologic work indicates that the sequence of cycles, which was based on structural superposition, is incorrect and requires revision• In view of this re-interpretation, the term cycle is replaced by the more appropriate term " G r o u p " (Table 1 ). The sequence of formations within each group corresponds to that previously de-

203

UNSCRAMBLING THE STRATIGRAPHY OF AN ARCHEAN GREENSTONE BELT

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SYNCLINAL AXIS •

SAMPLE LOCATION

Fig. 2. Distribution of metavolcanic-metasedimentary groups in the southeastern part of the Favourable Lake greenstone belt (modified from Ayres, 1977 ) showing location ofisotopically analysed samples (A-J). Stratigraphy has been revised to reflect new U-Pb data. The upper sequence of Group I has not been isotopically dated but is probably only slightly younger than the lower sequence.

fined for the cycles, except for one minor modification (part of cycle 3 is now assigned to Group I that also includes cycle 2); the relationships between the various groups and the previous designation are shown in Table 1.

Concordant faults that form some of the boundaries between groups were originally thought to be relatively unimportant structures; these now appear to be major thrust faults that caused older groups to structurally

204

F. CORFU AND L.D. AYRES

TABLE 1 Stratigraphy of the Favourable Lake greenstone belt Group

Structural position ( cycle a )

Main lithologies

V

3

IV

1

III

4

II

5

1

3

Upper sedimentary sequence comprising a turbidite fan, F, felsic tuff; with minor intercalated felsic tuff, overlying alluvial-fluvial G, granodiorite clast in conglomerate conglomerate Lower sequence of pillowed basaltic flows Upper sequence of felsic domes, mafic flows, argillite, H, felsic pyroclastic rock sandstone, iron formation, marble Lower sequence of intermediate to felsic lava flows and pyroclastic rocks Basaltic flows; minor lenses of sandstone, conglomerate, E, felsic tuff and felsic tufts C, D, diorite Intermediate lava flows and pyroclastic rocks; intercalated sandstone, slate, iron formation Upper sequence of subaqueous basaltic and komatiitic flows and pyroclastic rocks Intermediate caldera-filling sequence B, intermediate flow Siltstone, sandstone, marble, ferruginous chert A, tonalite sill Basal sequence of subaqueous komatiites and basalts

2

Sample

Age (Ma)

2725 + 2 2960-3000

< 2734

2858+] 2870+_ 8

2924 ± 1.5 2926 ± 2

Note: All units metamorphosed to greenschist to locally amphibolite facies conditions; the prefix meta is omitted for convenience. a Cycle designation according to the original classification of Ayres ( 1977 ); cycle 1 to 5 were numbered in structurally ascending order. Cycle 3 is included mainly in Group V, but one pan is now assigned to Group I (Fig. 2 ).

overlie younger groups. The stratigraphic and tectonic revisions are described in more detail in a companion paper (Ayres and Corfu, 1991).

Previous geochronology U - P b geochronology of the bordering granitoid batholiths indicated an age of 2950 Ma for a gneissic tonalite enclave in the North Trout Lake Batholith on the northeast side of the greenstone belt (Krogh and Davis, 1971; Corfu et al., 1985 ). The enclave is enclosed in multiphase tonalitic to granodioritic units, the main phases of which crystallized between 2732 and 2711 Ma (Corfu et al., 1985). A single age from the multiphase Setting Net Lake Batholith on the southwest side of the greenstone belt indicated that at least one lithologically similar phase crystallized contemporaneously in both batholiths (Corfu et al., 1985 ). The epizonal Setting Net Lake Stock within the

greenstone belt yielded discordant zircon analyses indicating a minimum age of about 2640 Ma (Nunes and Ayres, 1982); it was accordingly considered to be a late tectonic pluton although this conclusion was not in accord with petrologic data that indicated considerable post-emplacement deformation and possible low-grade greenschist-facies metamorphism (Ayres et al., 1982 ). Two granitoid plutons in the north part of the Berens River subprovince, 10 and 50 km south of the greenstone belt, yielded identical crystallization ages of 2697_+2 Ma and 2696_+ 1 Ma, respectively (Corfu and Ayres, 1984). U - P b zircon data from the neighbouring North Spirit Lake greenstone belt to the southeast (Fig. 1 ) indicated the presence of ( 1 ) volcanic and plutonic rocks formed between 3023 and 2986 Ma, (2) volcanic and associated sedimentary rocks with a poorly defined isotopic age of 2950-2800 Ma, and (3) a younger volcanic sequence formed about 2740-2730 Ma (Corfu and Wood, 1986).

205

UNSCRAMBLING THE STRATIGRAPHY OF AN ARCHEAN GREENSTONE BELT TABLE 2 U-Pb data Fractions No.

Properties (a)

Concentrations wt. (mg)

U (ppm) (b)

Pb rad (ppm) (b)

Atomic ratios 2°Spb rad (mol%)

Age (Ma)

2~pb/2~pb

2o6pb/238U

2oTpb/23sU

2O7pb/2~pb

2OTpb/2O6pb

(d)

(e)

(e)

(e)

(e)

(c) A. Tonalitesill, G r o u p l ( A - 9 ) ( f ) : 1 z, +74, eu, A 2 z, +74, eu, A,! 3(g) z, +74, A 4(g) z, +74 5(g) z, nm3-m2, 74-44

0.061 0.148 0.614 1.128 0.789

111 110 182 198 395

75 75 123 127 251

12.3 12.4 13.1 12.7 15.3

15 800 21 900 27 800 8 630 6 100

0.5730_+21 0.5707_+27 0.5630_+28 0.5384-+27 0.5171_+26

16.786_+65 16.730_+82 16.415_+83 15.648_+79 14.911_+75

0.21247_+19 0.21261_+21 0.21146_+21 0.21080_+21 0.20914-+21

2924 2926 2917 2912 2899

B. Intermediate flow, Group l(N-80-3): 6 z, +74, eu-~, A 0.042 7 z, +74, A 0.051 8(g) z, +74 0.875 9(g) z, nm5-m3, +74 0.698

151 177 248 242

99 116 161 153

9.3 9.2 10.0 9.7

44 800 17 900 7 390 10 300

0.5731_+35 0.5702_+17 0.5588_+28 0.5484_+27

16.78_+11 16.694_+56 16.318_+82 15.994_+80

0.21233_+27 0.21236_+19 0.21180_+21 0.21152_+21

2923 2924 2919 2917

C. Diorite, GroupII(C-84-17): 10 z, 6, A 11 z, ~(-eu), A 12 b, ~,br, op, A 13 b, eu-~,br,op-trl

0.008 0.015 0.010 0.013

308 337 139 160

239 264 82 93

25.0 25.9 2.9 3.5

22 000 4 710 2 000 8 790

0.5602_+28 0.5605_+22 0.5535+32 0.5453_+17

15.871_+81 15.850_+65 15.470_+92 15.077_+51

0.20546_+20 0.20508_+23 0.20271_+27 0.20054_+22

2870 2867 2848 2831

D. Diorite, Group lI(N-80-8): 14 b, eu-~,br, op-trl 15(g) z, 74-44 16(g) z, nm7-m5, +74 17(g) z, nm9-m5, +74, A

0.039 0.768 0.574 0.722

74 517 830 808

43 381 625 609

3.3 28.8 31.2 31.2

7 520 17 300 7 900 11 100

0.5384_+18 0.5056_+26 0.5015_+25 0.5026_+25

14.747_+52 14.014_+71 13.753_+69 13.718_+69

0.19867_+19 0.20104_+20 0.19890_+20 0.19796_+20

2815 2835 2817 2810

E. Felsictuff, GroupllI(N-80-5): 18 z, 74-44, eu, br(-w), A 19 z, 74-44, eu, cl-br, A,! 20(g) z, +74 21(g) z, 74-44 22(g) z, n m3 -m2,+7 4

0.013 0.050 0.723 0.546 0.512

1012 1242 448 974 1086

633 799 279 585 652

8.4 11.4 9.4 10.9 10.1

35 500 27 700 12 900 4 960 3 640

0.5538_+20 0.5511_+24 0.5453_+28 0.5194_+26 0.5225_+26

15.506_+59 0.20306_+18 15.411-+69 0.20281_+27 15.157_+76 0.20161_+20 14.055_+70 0.19625_+20 14.388_+72 0.19971_+20

2851 2849 2839 2795 2824

74 85 79 103 896 454 937

51 57 52 66 546 294 641

12.2 10.7 10.1 11.6 11.9 17.2 21.5

9 900 19 700 5 890 13 100 5 390 3 740 11 600

0.5762_+19 0.5711_+30 0.5663_+20 0.5448_+23 0.5240_+15 0.5226_+16 0.5249_+21

16.937_+61 16.780_+90 16.416_+63 14.891_+64 13.714_+44 13.655_+45 13.683_+57

2930 2929 2907 2812 2741 2738 2734

2680 2981 2237 2299

1542 1695 1266 1271

7.0 7.3 7.0 7.9

83 000 87 900 2 290 9 660

0.5237_+30 0.5159_+24 0.5152_+26 0.4989_+25

13.561-+79 0.18780-+15 2723 13.325_+65 0.18731_+18 2719 13.326_+67 0.18761-+19 2721 12.814_+64 0.18628_+19 2710

V (N-80-7): 288 186 575 332 612 360 48 27

8.0 4.9 5.8 5.1

4 540 3 120 3 820 2 940

0.5680+26 0.5280_+26 0.5334_+27 0.5148_+23

16.998___78 15.245_+76 15.339_+77 12.978_+58

H. Felsic pyroclastic rock, Group IV (C-84-18 ): 23 z, an, A 0.107 24 z, an, A 0.169 25 z, eu, spr, A 0.081 26 z, eu, spr(-lpr),A 0.021 27 z, eu, lpr, A 0.005 28 z, eu, lpr, A 0.016 29 z, eu, lpr, A 0.009 F. Felsictuff, Group V (N-80-6): 30 z, eu, cl(-br),A 31 z, eu, cl-br, A,! 32(g) z, nm3-m2, +74 33(g) z, 74-44

0.034 0.091 0.757 0.689

G. Granodiofiticclastin con#omerate ofGroup 34 z, nm3-m2,-44, eu, lpr, A 0.004 35(g) z , + 7 4 0.327 36(g) z, nm2, 74-44, A 0.262 37 t, nm5-m3, cl-br, A 0.391

0.21318_+19 0.21311_+20 0.21024_+18 0.19825_+27 0.18983_+18 0.18952_+18 0.18906_+16

0.21702_+32 0.20942_+21 0.20856_+21 0.18286_+33

2959 2901 2894 2679

206

F. CORFU AND L.D. AYRES

TABLE 2 (continued) Fractions No.

Properties (a)

Concentrations

Atomic ratios

Age ( Ma )

wt. (nag)

U (ppm) (b)

Pb rad (ppm) (b)

-~°SPb tad (mol%) (e)

-'°6pb/2°4pb ~ P b / 2 ~ s U (d) (e)

-'"TPb/-'~ll (e)

-'°Tpb/xtl~Pb (e)

~c~TPb/Z°"Pb (e)

1. Setting Net Lake Stock ((?-84-19): 38 z, eu, spr, A 39 z, eu, spr, p , A ( l g r a i n ) 40 z, eu, lpr, A 41 z, eu, spr, br, A ( l g r a i n ) 42 z, eu, lpr, A 43 z, eu,p, lr, A ( l g r a i n ) 44 mo, an, y, lrl, A 45 mo, an, y, lrl-op, A 46 t, fr, b r - r , A 47 t, fr, b r - y , A 48 1. fr, cl, y , A 4t~ r, an, bl, o p ( - t r l ) , A 5(1 r, fr, bl, op, A 51 r, fr, rd-y, trl, A

0.071 0.007 0.007 0.020 0.003 0.006 0.030 0.029 0.036 0.185 0.085 0.358 0.346 0.131

120 125 222 1120 197 475 1118 1339 60 36 21 28 24 18

74 71 135 618 121 278 3886 4261 (36) 48 12 15 13 9.4

13.2 7.5 14.0 4.4 15.6 11.6 84.6 86.1 59.8 8.0 2.4 3.7 2.1

56200 3290 3880 43400 1 730 11400 42000 24400 1060 I 230 618 458 952 1 110

0.5219-+26 0.5195+28 0.5096+17 0.5186_+29 0.5085+30 0.5075+21 0.5204+21 0.4320_+22 (I.5163+31 0 . 5 1 4 3 + 18 0.5066+18 0.5101±18 0.5081±22 0.5063+22

13.677+71 13.424.+_81 13.130:~48 13.319+77 13.027+81 12.912+54 13.331+57 10.926:~56 13.244+93 13.107+54 12.889-+62 12.844+72 12.751+63 12.595+62

0.19004_+17 0.18740+45 0.18688+21 0.18626+_16 0.18580+_30 0.18449_+25 0.18580+16 0.18346_+24 0.18605_+57 0.18483_+27 0.18452+48 0.18264_+61 0.18202_+36 0.18042-+33

2743 2720 2715 2709 2705 2694 2705 2684 2708 2697 2694 2677 2671 2657

J. Selling Net Lake Stock (A-I I ): 52 z, e u - s p r + f r , A 53 z, eu, lpr, cr, A 54 z, eu, lpr, A 55 t. eu. br, A 56 1, fi',y-br

0.043 0.008 0.002 0.107 0.227

165 447 130 101 33

96 245 73 83 20

14.8 14.1 18.8 34.8 16.0

6650 7180 610 860 464

0.4870+27 0.4630+17 0.4452+44 0.5232+28 0.4991+14

12.408L75 11.540+45 11.40 +11 13.424~79 12.775+63

0.18480_+29 0.18079_+21 0.18576_+54 0.18610-+36 [}.18563_+60

2696 2660 2705 2708 2704

Noles: (a) z = z i r c o n (all zircons clear, transparent and non-magnetic unless otherwise indicated ): b = b a d d c [ e y i t e ; t = l i l a n i t c ; t o o = monazite: r = r u t i l e : nm, rn =non-magnetic, magnetic at indicated angle of lilt of Frantz isodynamic separator (at 1.6 A and 0 slope ): 74, 4 4 = s i z e in microns: b r = b r o w n : rd = r e d ; p = pink: w = white: y = yellow; b = black; cl = colourless or pale p i n k / b r o w n ~ y e l l o w , transparent: Irl =translucent; o p = o p a q u e ; c r = c r a c k s ; -X= abraded ( Krogh, 1982b ): ! = zircons not picked up on magnetic pin ( Krogh, 1982a ) ~fr = fragments; eu = euhedraL an = anhedral, subrounded: Ipr =long-prismatic (l/w: 4 - 1 0 ); spr = short-prismatic (I/w: I - 3 ): the first term of combined properties indicates the d o m i n a n t feature ( > 50%); parentheses indicate subordinate features ( < 20%). ( b ) Concentrations are known to + 0.5% for sample weights over 1 rag, about 1-2% tbr sample weights of 1.0-0.2 mg, about 5% for sample weights of about 0.1 mg and about 10% for sample weights of < 0.05 rag. (c) Relative to total radiogenic Pb. (d) Corrected for fractionalion and for spike. (e) Corrected for fi'actionation, spike, blank and initial c o m m o n Pb; error estimales refer to the last significant digits of the isotopic ratios and reflect reproducibility of standards, measurement errors and uncertainties in the c o m m o n Pb correction. ( f ) Laboratory reference number. (g) Unpublished analyses by P.D. Nunes.

A nalytical procedure This paper presents results from an early stage of the study carried out by P. D. Nunes (Corfu et al., 1981 ), new analyses produced to refine the early data set, and data on newly collected samples. The procedure followed by P. D. Nunes for the early set of zircon analyses involved a HBrcolumn separation and is described by Nunes and Thurston (1980). These early analyses

(Table 2 ) have been corrected for blanks of 90 pg Pb and 50 pg U. The new zircon and baddeleyite analyses were performed with refined versions of the basic technique developed by Krogh (1973). These new data are corrected for blanks of 3-10 pg Pb and 1-5 pg U. Titanire was dissolved using HF ( + HNO3) in Savillex-vials, whereas rutile and monazite were dissolved in teflon bombs at 220°C in HF ( + HNO3) and in 6N HCI, respectively. Data for these minerals are corrected for blanks of

UNSCRAMBLING THE STRATIGRAPHY OF AN ARCHEAN GREENSTONE BELT

20 pg Pb and 5 pg U. Details of the analytical and data reduction procedures are given elsewhere (e.g. Corfu and Grunsky, 1987). Initial common Pb compositions were estimated from the Stacey and Kramers (1975) Pb evolution model. Intercept ages ofcollinear arrays were calculated using the procedure of Davis (1982). Decay constants are those of Jaffey et al. ( 1971 ) and recommended by the lUGS (Steiger and Jaeger, 1977). Errors are quoted at the 95% confidence level.

207

group. The intermediate flow occurs near the top of the caldera-filling sequence, and the tonalite is from a sill-like pluton intruded into the base of the sequence. Both samples contain relatively abundant, low-U, euhedral and homogeneous zircons, whose analyses yield well constrained and essentially indistinguishable upper intercept ages of 2926_+2 Ma for tonalite A and 2924_+ 1.5 Ma for intermediate flow B (Table 2, Fig. 3 ).

Diorite (C, D): Group H Samples and analytical data Data on eight samples representing five felsic to intermediate volcanic and subvolcanic units and one clast from a conglomerate are presented (Table 2). These represent a crosssection through the major stratigraphic sequences, but, because only zircon-bearing rocks are adequate for U - P b dating, it was not always possible to sample the most typical members of each sequence. Data are also presented for two samples from the Setting Net Lake Stock. The analytical work was carried out in two stages: early work done on bulk magnetic and/ or size fractions that yielded somewhat discordant analyses, was upgraded by the addition of more concordant analyses obtained from selections of clear, generally crack-free and abraded zircons (Krogh, 1982b; Davis et al., 1982). Most of the resulting ages are tightly controlled by these latter analyses and are essentially unaffected by the slight scatter disturbing some of the discordia lines (e.g. samples A and E, Fig. 3 ) that reflect multistage Pbloss patterns.

Tonalite (.4) and intermediate flow (B): Group I These samples are from a 1.5 km thick, intermediate, caldera-fiUing sequence developed in a subaqueous mafic to ultramafic flow sequence that is the dominant component of the

This poorly exposed, moderately foliated unit was sampled at sites about 5 m apart on the same outcrop. It is believed to be the lower part of the structurally uppermost group recognized by Ayres (1977), because its association with iron formation and high aeromagnetic expression are characteristic of this lithologically variable group (Ayres, 1974 ). At the sample site, the unit is separated from mafic flows assigned to Group V by a metagabbro sill. A sample collected from a lava flow elsewhere in the group did not yield any zircons. The two samples are both variably recrystallized, medium-grained, dioritic rocks containing 5 to 10% quartz that, in places, is in granophyric intergrowth with plagioclase. These features suggest that the sampled unit is a subvolcanic intrusion. Nearby, superficially similar outcrops, however, lack quartz and appear to contain small rock fragments; they were mapped as massive, intermediate tuff by Ayres (1974), who gives a chemical analysis. We conclude that these units are a complex of genetically related volcanic and intrusive rocks. Both samples contain relatively few, dominantly U-rich and heterogeneous zircons and small amounts of baddeleyite. The relatively high T h / U ratio of these zircons, as shown by the high proportion of 2°8pb (Table 2 ), and the occurrence of baddeleyite are typical characteristics of mafic rocks (e.g. Corfu and Andrews, 1987). The two most concordant analyses were obtained from abraded fractions of

208

F. C O R F U A N D L.D. AYRES

f

2O6pb

I"-e-GI

~~,

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5

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600 Mo

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I

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2888 2850

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1 5 6 0 Mo

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14.5

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15.5

I. ~1

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14.4

,

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14.9

Fig. 3. Concordia diagram with zircon and baddeleyite (# 12-14 ) data for tonalite (A) and intermediate flow (B) from Group I, diorite (C, D) from Group 1I and felsic tuff (E) from Group III. Ellipses in this and the following diagrams indicate the 2-sigma uncertainty. clear, homogeneous zircons which are mostly fragments of long-prismatic crystals (# 10, 11; Table 2; Fig. 3 ), whereas bulk zircon fractions 15-17 are distinctly discordant. Baddeleyite is locally zoned from darkbrown, opaque interiors to brown-yellow, translucent rims. The three fractions (# 12-14;

Table 2; Fig. 3 ) have unusually low U contents of 74-160 ppm, and unusually high degrees of discordance that could not be totally reduced by abrasion, probably because of the unfavourable, tabular shape and small size. They are collinear (63% fit) with the two concordant zircon analyses defining an upper inter-

UNSCRAMBLING THE STRATIGRAPHY OF AN ARCHEAN GREENSTONE BELT

cept age of 2870 + 8 / - 2 Ma, which is interpreted to be the time of crystallization of the diorite and synchronous eruption of the related volcanic rocks. The discordance pattern of the baddeleyite is rather remarkable considering that this mineral is generally much less susceptible to Pbloss than co-existing zircon and normally yields concordant or nearly concordant data (e.g. Krogh et al., 1987). Discordant baddeleyite can be produced, however, by the metamorphic reaction of baddeleyite to zircon (Davidson and van Breemen, 1988 ). Although no such reaction was observed in the present samples, the pronounced discordance likely reflects a metamorphic disturbance, probably during regional metamorphism associated with emplacement of nearby batholiths. The lower intercept age of 2090 Ma (Fig. 3) is probably geologically meaningless and the result of rotation of the original discordia line by a younger superimposed Pb-loss that also affected the unabraded zircon fractions ~ 15-17.

Felsic tuff(E): Group III This group is composed mainly of mafic flows with a thin, strongly deformed, capping felsic pyroclastic unit. The sample is from a tuffaceous layer in a 60 m thick, greywackesiltstone lens, intercalated with the mafic flows rather than the capping unit. It contains euhedral, mostly prismatic zircons. They are rich in U, but internally homogeneous and after abrasion yield nearly concordant analyses (~18, 19; Table 2; Fig. 3 ) that constrain the upper intercept age at 2858 + 5 / - 4 Ma.

Felsic pyroclastic rock (H): Group I V Group IV, the thickest group, is a 3 km thick sequence of intermediate to felsic lava flows and pyroclastic rocks that is capped by a 1 km thick complex composed of sedimentary units, felsic domes, and mafic lava flows. Sample H is from a thickly bedded tuffaceous unit asso-

209

ciated with a domical plagioclase porphyry body near the top of the group. Three other samples collected from plagioclase- and locally quartz-phyric flows lower in the group did not yield any zircons. The sample contains an abundant but very complex zircon population with a variety of morphological and colour types. All of the analysed fractions were selected to represent particular zircon types and were abraded to yield data points that plot on or near the concordia curve (Table 2; Fig. 4). Two analyses of subrounded zircons (~23, 24) yield the oldest apparent age of 2930 Ma. A somewhat younger age of about 2907 Ma was obtained from a fraction of euhedral, short-prismatic zircons (#25), whereas a fraction of euhedral zircons with more variable length to width ratios (~26) has an apparent age of 2812 Ma. Finally, three analyses of euhedral, long-prismatic zircons (#27-29) yield 2°Tpb/2°6pb ages of 2741-2734 Ma. The long-prismatic zircons that yield the youngest ages, lack any resorption and are probably indigenous to the magma from which the tuff was generated. They are thus interpreted to most closely approximate the time of crystallization of the magma and the time of emplacement of the tuff. Since the variation in apparent age of these three analyses (~27-29) is outside the limits of error, these zircons probably contain small amounts of an older, inherited zircon component. Assuming the grains have not been affected by significant secondary Pb-loss, 2734 Ma appears to be a maximum age for the eruption of the tuff. The complex array of older ages (Fig. 4) is interpreted to reflect the admixture of abundant older xenocrystic zircons to the tuff. Clearly at least two units of different age are represented by the heterogeneous zircon population, although the apparent ages defined by the various fractions are probably mixed ages that do not necessarily date real events. The oldest zircons in the tuff must be 2930 Ma or older.

210

F. C O R F U A N D L D . AYRES

r293o Mot Felsic pyroclastic rock

---

fiB/// /

28 ~

•55

~

,~

,~/ 275~~ r o ] j .9

~ 25

!

,

subrounded

short-prismotJc

~uh~dro]~ ]on~-prlsmotic 14.4

I6.4

Fig. 4. Concordia diagram with zircon analyses for felsic pyroclastic rock (H) from Group IV. The subrounded and euhedral, short-prismatic zircon data reflect inheritancefrom sources as old as 2930 Ma. The long-prismaticzircons indicate an age of ~<2734Ma for the eruptive event. Contamination of the tuff was apparently due to processes in the magma chamber or during ascent of the melts rather than to posteruptive reworking. This is indicated by the absence of recognizable lithic or older mineral components, and the surface features of old grains which lack pitting and scouring marks typical of detrital zircons, but have smooth, partly irregular surfaces suggestive of magmatic resorption.

Felsic tuff(F) and granodiorite clast (G): Group V These two samples were both taken from the upper, geographically restricted, metasedimentary part of the group in the northwestern part of the belt where the group is best defined (Fig. 2 ). The metasediments overlie subaqueous mafic flows that are laterally more extensive and are the dominant component of the group. The felsic tuff is from a 3 m thick tuffaceous unit that occurs within a submarinefan, greywacke-siltstone member at the top of the group, whereas the granodiorite clast is

from the underlying, alluvial-fan conglomerate member (Gordanier, 1982). Unlike the other geochronologic samples from the greenstone belt which are metamorphosed to greenschist facies, the bed containing the sampled clast was metamorphosed to amphibolite facies. The clast is partly recrystallized, but the original, medium-grained, hypidiomorphicgranular texture can still be recognized. The zircons in felsic tuff (F) are euhedral, prismatic and mostly metamict reflecting U contents of 2000-3000 ppm (Table 2). Despite these high U contents, the analyses are concordant to only moderately discordant and define a discordia line with an upper intercept age of 2725+2 Ma (Fig. 5). All three analyses of zircons from granodiorite clast (G) are discordant and do not fit a discordia line (Table 2; Fig. 5 ). The most concordant analysis has a 2°7pb/2°6pb age of 2959 Ma, that is a likely minimum age for the clast. On the other hand, a best-fit line through the three analyses projects toward about 2990 Ma. Although no precise age can be obtained from these data, they suggest that the granodiorite pluton, from which the clast was derived,

211

U N S C R A M B L I N G T H E S T R A T I G R A P H Y O F A N A R C H E A N G R E E N S T O N E BELT

3000

zircon

Izzz5

Mol

2gSO-3DOO

/ //

2900 /--

268

/ / /

/

I I ii I

I

±3 Mo]/ I

36 I /

.50 ../33

35/l /

Ma

f65o •

481

/

Felsic tuff - F ,

I

~

13.0

I

Granodiorite clast-G J

I

13.G

14

16

Fig. 5. Concordia diagram with zircon and titanite (~37) data for felsic tuff (F) and granodiorite clast ( G ) , both from Group V.

formed sometime between 2960 and 3000 Ma ago. It was hoped that titanite would preserve the primary age of the granodiorite, but the concordant titanite analysis (~37; Table 2; Fig. 5 ) has a much younger age of 2679 ___3 Ma, that probably reflects a late, post-depositional, metamorphic or hydrothermal event.

Setting Net Lake Stock (I,J) Two samples (I,J; Figs. 2 and 6) were collected in the Setting Net Lake Stock, a small, epizonal, granodiorite-quartz monzonite pluton that contains a subeconomic, porphyrytype molybdenum deposit (Ayres et al., 1982 ). Sample J, first investigated by Nunes and Ayres (1982 ), is a moderately altered granite from the southern part of the stock, away from the molybdenite-quartz vein system; plagioclase is partly replaced by sericite, and biotite by chlorite. Sample I is a fine-grained porphyritic granite from the mineralized zone at the north end of the stock. It is more highly altered with most of the plagioclase replaced by albite, ser-

icite and epidote pseudomorphs, and the biotite partly replaced by chlorite and epidote. Earlier zircon data (Nunes and Ayres, 1982 ) yielded a minimum age of 2643 Ma from very discordant data. Attempts to refine the age by focussing on more selectively chosen zircon fractions (~52-54; Table 2; Fig. 7) from the sample (J) used by Nunes and Ayres (1982), considerably improved the degree of concordancy. Nevertheless, these analyses were not collinear and were still too discordant for a precise age determination. The zircon population in the highly altered sample (I) includes a complex mixture of grains with differing morphologies, colours, and degrees of internal homogeneity; some zircons also have anhedral cores overgrown by euhedral rims. Although an effort was made to select only zircons that were free of optically visible cores, it is apparent that inheritance was present in several of the analysed fractions. The most pronounced case of inheritance is shown by short-prismatic zircons of analysis ~38 that yields a 2°7pb/2°6pb age of 2743 Ma, much higher than apparent ages defined by the other

21 2

F. CORFU AND L.D. AYRES

L

~?S/

Per

O0

cent

alteration

~ - - - - ~ Molybdenite •

of

plagioclase

zone

Fault Sample location

93° 34'

Km

Fig. 6. Map of the SettingNet Lake Stockshowingsample locations (I, J) in relationship to alteration and mineralization. analyses (Table 2 ). Inheritance is probably also indicated by the 2°7pb/E°6pb age of 2720 Ma for analysis ¢39, a single zircon that is morphologically similar to zircons used in fraction g38, except for a weak colouration. Long-prismatic zircons of analysis ~40, usually the grains containing the least amount of memory, have a 2°7pb/2°6pb age of 2715 Me, which is significantly higher than that defined by very similar zircons in fraction #42 (2705 Me) and also higher than the ages of coexisting titanite and monazite. The other two analyses were done on a large brown, U-rich zircon (~41) that yields an almost concordant analysis with a 2°Tpb/2°6pb age of 2709 Me, and a pink, relatively U-rich grain (~43) that yields a more discordant analysis with a 2°7pb/2°6pb age of 2694 Ma. For a more meaningful assessment of these data, it is useful to consider first the age infor-

mation given by coexisting titanite and monazite. The titanite occurs mainly as broken fragments and only subordinately in sample J as euhedral, crack-free, yellow and brown crystals. One analysis of brown, euhedral titanite (~55; Table 2; Fig. 7) from J yields a concordant data point with an age of 2708 Ma that is collinear with a discordant fraction of unabraded brown-yellow fragments (~56) from the same sample and with a nearly concordant analysis of abraded, brown-red fragments (¢46) from sample I. The discordia line has a probability of fit of 80% and an upper intercept age of 2708 _+3 Me. Two other analyses of brown to yellow (~47) and yellow to colourless titanite (#48) from the strongly altered sample I plot distinctly to the left of this line yielding 2°7pb/2°6pb ages of 2698 and 2694 Me, respectively. These relationships suggest that the clear titanite in I is a secondary component formed during late alteration a n d / o r low-grade metamorphism. Fraction ~47 was a mixture of old (brown) and new (clear)grains. Therefore, the clear grains of g48 provide the closest age estimate for the second titanite generation, but more analyses would be necessary to exactly establish the age of this component. Monazite was found only in the more altered sample I, where it forms mostly turbid grains. One fraction comprising translucent to slightly opaque, turbid and partly altered monazite (~¢45; Table 2; Fig. 7 ) yields an 18%-discordant data point, whereas a second fraction of grains, without traces of turbidity, (~44) is concordant. A line through the two analyses defines an upper intercept age of 2706 _+ 1.5 Ma which is slightly but not significantly younger than the titanite age of 2708 _+3 Me. A comm o n regression line of monazite and old-titanite data has a 50% probability of fit, but the resulting age of 2706 _+ 1.5 Ma is strongly controlled by the much more precise monazite analyses. Three analyses (#49-51; Table 2; Fig. 7) of rutile from sample I are nearly concordant, but show a large variation in 2°7pb/2°6pb ages from

213

U N S C R A M B L I N G T H E S T R A T I G R A P H Y O F AN A R C H E A N G R E E N S T O N E BELT

[

titomito

(In)

Imonazi t~:

L27o8 ±3 Mo I 2~,t~t~ GgO-2Gg5

2706

±1. S bi~

2 6 7 7 Mo

ChJ I Mo

2658

,tft, o

.4B

5 3 j l • /r @ Ft~ r ~•n c ~

e65 Mo/ lgl. 5

f45 I

]In~

oe.lng Net Lake Stock- I,J i

11.5

I

I

i

L

11.5

i

I

12.5

Fig. 7. Concordia diagram with data for zircon (right side), monazite and rutile (centre), and titanite (left side) from a moderately (I) and a strongly altered (J) sample of the Setting Net Lake Stock. Presumed magmatic titanite (titanite (m)) from both samples yields an age of 2708 + 3 Ma, which is considered the best estimate for the time of intrusion and Mo-mineralization of the stock. The zircons yield a complex pattern reflecting inheritance, magmatic crystallization and superimposed multi-stage Proterozoic to Recent Pb-loss. The monazite data probably reflect hydrothermal crystallization at 2706 + 1.5 Ma. A younger titanite component (titanite (h)) formed at about 2695-2690 Ma. Rutile data suggest two stages of hydrothermal formation: opaque rutile at/or before 2677 Ma and clear rutile at/or after 2657 Ma, or, alternatively, the presence of inherited, radiogenic Fe-Ti oxides in ~<2657 Ma ruffle.

2677 Ma for ~49 to 2671 Ma for ~50 and 2657 Ma for ~51. Although there is a sympathetic relationship between decreasing 2°7pb/2°6pb and increasing 2°6pb/2°4pb ratio, it is not possible to explain the apparent age variation as resulting from the incorporation of different amounts of the same but relatively radiogenic initial Pb. The youngest rutile is translucent and has the lowest U-content, whereas the older rutiles are largely opaque and are somewhat richer in U. This suggests that there may be more than one generation of rutile: an earlier one formed prior to 2677 Ma and a secondary one formed at or later than 2657 Ma. Alternatively, the dark rutile may still contain remnants of a parent mineral (e.g. ilmeno-magnetite?) with its own isotopic memory, and there may have been only one late episode of rutile growth.

Although difficult to interpret, the complex U - P b data pattern for the Setting Net Lake Stock appears to reflect the occurrence of a number of different processes.

( 1 ) Entrainment ofxenocrystic zircons in the magma. Inheritance is present in fractions g38 and #39, and probably ~40. (2) Magmatic crystallization. The zircon " analyses without presumed inheritance can be interpreted in terms of a common age of crystallization followed by multiple Pb-loss events including Pb-loss during the Proterozoic and more recent Pb-loss (e.g. Corfu and Ayres, 1984; Corfu et al., 1985 ). Sets of discordia lines through the various fractions converge toward the most concordant analysis #41, defining intercept ages of about 2710-2712 Ma. Although this may directly reflect the time of igneous crystallization, it could still be biased upward

2 14

by small amounts of inheritance and we are reluctant to put too much confidence on the zircon age. A more solid estimate for the time of emplacement of the stock is provided by the titanite age of 2708_+ 3 Ma. Titanite is not known to survive as a xenocrystic, restite phase in large magmatic bodies; therefore inheritance can be excluded. Conversely, a partial resetting of the titanite during metamorphism is relatively unlikely because the stock only experienced lowgrade metamorphism and because the same age is provided by titanite in the two differently altered samples. The occurrence of well preserved euhedral titanite in the least altered sample J also suggests that this is a primary magmatic phase. Monazite only occurs in the highly altered sample and we suspect that it formed during the hydrothermal alteration. The more precise monazite age of 2706_+ 1.5 Ma may therefore post-date the main crystallization event. On the other hand the less precise titanite age of 2708_+ 3 Ma is indistinguishable from the monazite age and it also coincides with the concordant analysis of the single, brown, U-rich zircon (~41). The age of 2708 _+3 Ma is therefore taken as the most reliable date for the intrusion of the stock. (3) Subsolidus crystallization and hydrothermal alteration. Following magmatic crystallization, widespread alteration and an associated molybdenite-quartz vein system developed in the northern part of the stock (Ayres et al., 1982). In the alteration zone, as represented by sample I, secondary titanite and rutile were formed during alteration of biotite and Fe-Ti oxide minerals. This alteration assemblage was overprinted by a low-grade, greenschist-facies, metamorphic event. Both post-crystallization processes apparently formed various generations of secondary monazite, titanite and rutile that, in part, coexist with primary magmatic equivalents. The younger ages demonstrate that these, presumably intermittent, secondary processes spanned a very long period of time and suggest that the

F. CORFU AND L.D. AYRES

mineralized portion of the stock remained accessible to fluids penetrating from depth, probably generated by late Archean metamorphism and magmatism in the lower crust of the Sachigo Subprovince (Heaman et al., 1986; Mezger et al., 1989). In summary, intrusion of the Setting Net Lake Stock at 2708 + 3 Ma was followed by an extended period punctuated by episodes of lowgrade metamorphism and/or hydrothermal activity that formed monazite at 2706 Z 1.5 Ma, a secondary titanite generation at around 2695-2690 Ma, and rutile at or after 2657 Ma and possibly also an early generation prior to 2677 Ma. The age of 2708 z 3 Ma, which includes intrusion and presumably Mo-mineralization of the stock, is essentially indistinguishable from the age of 2 7 1 1 z 2 Ma previously established for the youngest, widespread, granodioritic phases of the surrounding batholiths (Corfu et al., 1985 ), pointing to a genetic link between these magmatic processes. Discussion

Magmatic evolution The U - P b data presented above, in conjunction with previously published ages on the bounding granitoid batholiths (Corfu and Ayres, 1984; Corfu et al., 1985), document a tectonic-magmatic evolution that spanned more than 250 Ma (Fig. 8 ). The oldest unit recognized to date in the Favourable Lake area is a 2950_+ 5 Ma old, metamorphosed, gneissic tonalite enclave within the North Trout Lake Batholith (Corfu et al., 1985 ). Relationships between the enclave and younger supracrustal rocks have been obliterated by intrusion of the surrounding 27322711 Ma old granitoid phases that form the bulk of the batholith. The tonalite was probably intruded into older, but unrecognized, supracrustal units (Hillary and Ayres, 1980), possibly similar to those preserved in the North

215

UNSCRAMBLING THE STRATIGRAPHY OF AN ARCHEAN GREENSTONE BELT

_j

t~

09 q~

t~

•

~

¢-

c

0

--

~

~

0

_~

U_

m

Z

o D

a) rr

•

2700 •

Se I

IS

•

"1 II

t~ 2800

v

I,

l

I ts

+ to

>-

=s

Volcanic unit Subvolcanic pluton

Is~t • ~. A

Sandstone

0

Plutonic c l a s t

•

L a r g e granitoid pluton

$

Metamorphic age within large granitoid pluton

2900 Se •

3000

l

/x 0

t"

Fig. 8. Compilation of U-Pb ages from several Archean greenstone belts in northwestern Ontario and adjacent Manitoba (Fig. 1 ). Vertical lines indicate error limits to ages; where no error bars are shown, the error is normally less than the size of the symbol. Symbols connected by a vertical line represent poorly constrained ages. Sources of data: Island Lake-Turek et al. (1986); Favourable Lake--Corfu and Ayres (1984), Corfu et al. (1985), this paper; Berens River--Corfu and Ayres (1984), Corfu and Wood (1986); North Spirit Lake--Corfu and Wood ( 1986 ); Red Lake--Corfu and Wallace (1986), Corfu and Andrews ( 1987 ); and Uchi--Nunes and Thurston (1980).

Spirit Lake greenstone belt, about 50 km to the southeast (Fig. 1 ). The North Spirit Lake sequence includes felsic volcanic units dated at 3023 + 2 Ma, and sedimentary rocks that contain tonalitic clasts and detrital zircons ranging in age from 3001 ___3 Ma to 2986 + 3 / - 2 Ma (Corfu and Wood, 1986 ). Based on the enclave and on clasts in conglomerate in both the Favourable Lake and North Spirit Lake areas, the old plutons must have been areally extensive, ranging in composition from tonalite to granodiorite and in age from 3000 to 2950 Ma. During the youngest recognized volcanic event, the old plutons must have been exposed near the present Favourable Lake greenstone belt in order to provide the 2960-3000 Ma, granodioritic clast (sample G) in conglomerate of

the 2725___2 Ma supracrustal units (Gordanier, 1982). The oldest supracrustal sequence documented in the Favourable Lake greenstone belt is Group I, the upper, caldera-filling part of which yielded two ages of 2926_+2 and 2924 _+ 1.5 Ma. These two overlapping ages indicate that extrusive and intrusive events within the caldera are coeval, as suggested by Buck ( 1978 ). The caldera developed in an undated, subaqueous, mafic to ultramafic shield volcano that has a maximum preserved thickness of at least 2 km (Ayres, 1977); the present base of the shield is interpreted to be a thrust fault (Ayres and Corfu, 1991 ), and the original thickness of the shield was probably considerably greater than that now exposed. The

216

mafic-ultramafic shield is separated from the caldera sequence and an associated intermediate pyroclastic cone by a thin sedimentary formation that contains marble and may represent a volcanic hiatus between mafic-ultramafic and intermediate volcanism. Therefore the lower mafic-ultramafic part of Group I could be somewhat older than the dated upper part. A distinctly younger period of magmatism at 2870-2850 Ma led to the development of Groups II and III. As previously discussed, the age of 2870 + 8 / - 2 Ma for a dioritic intrusion near the base of Group II, probably also represent the age of volcanism in this group. A somewhat younger age of 2858 + 5 / - 4 Ma was obtained for felsic tuff in the mainly basaltic Group III, that structurally underlies the older Group II. The youngest and most intense period of magmatism occurred between about 2734 and 2708 Ma ago and probably represent a sequence of distinct magmatic pulses. It includes development of Groups IV and V and emplacement of the bulk of the external batholiths and internal plutons. The age of Group IV, 2734 Ma or younger, is not well constrained because of problems with inherited older zircon. The dated unit is from the uppermost part of the group, a > 4 km thick sequence restricted to the east end of the greenstone belt and composed of intermediate to felsic flows and pyroclastic rocks that appear to represent a stratovolcano (Ayres, 1977). Modern stratovolcanoes develop rapidly (Crandell et al., 1975), and, consequently, the ~<2734 Ma age is considered to be the m a x i m u m age of the entire group, the base of which has been removed by emplacement of younger granitoid batholiths. The occurrence of xenocrystic zircons indicates that this volcano was erupted through older felsic to intermediate units, some of which were as old as 2930 Ma, the oldest apparent age of the inherited zircon component. Development of Group IV appears to have been synchronous with in-

F. C O R F U

AND

L.D. AYRES

trusion of the earliest widespread, tonalitic phase of the composite North Trout Lake Batholith at 2732 + 2 Ma (Corfu et al., 1985 ). The youngest volcanic rocks formed at • . m 2 7 2 5 + 2 Ma during deposition of the thick sedimentary succession of Group V. Felsic to intermediate volcanism was probably a major source of detritus in the sedimentary sequence (Gordanier, 1982). In-situ felsic units representing the source volcano were not originally recognized in the greenstone belt, although the new ages indicate that felsic units at the top of Group IV are a potential source; they have the right composition and were apparently subaerial (Ayres, 1977). The subaqueous mafic flows that underlie the sedimentary sequence and form the bulk of Group V, have not been dated and may be slightly or considerably older than the dated tuffaceous unit. Minor pulses of plutonism within the North Trout Lake Batholith at about 2716 were succeeded by the emplacement of the youngest widespread, granodioritic phase of the batholith at 2711 _+2 Ma; the same age was obtained for a similar phase in the Setting Net Lake Batholith on the southwest (Corfu et al., 1985). No volcanic units equivalent to the youngest plutonic phases have been recognized. Intrusion of the Setting Net Lake Stock represents one of the final stages in the magmatic and tectonic evolution of the belt. The complex mineralogical relationships and the ages of titanite, monazite and rutile indicate renewed pulses of hydrothermal activity long after the major plutonic/hydrothermal event that emplaced and mineralized the stock at about 2708_+3 Ma. Younger titanite ages (2690-2670 Ma) in clast G and in several intrusions within the North Trout Lake Batholith appear to reflect similar late low-grade events (Corfu et al., 1985).

Implications for stratigraphy and tectonism Based on the apparent conformable relationships between most groups and the consis-

UNSCRAMBLING THE STRATIGRAPHY OF AN ARCHEAN GREENSTONE BELT

tent facing relationships, the greenstone belt had been interpreted as representing a normal, upward younging stratigraphic sequence formed by the progressive evolution and onlapping of a succession of volcanic complexes (Ayres, 1977). The new U - P b data show that in reality the apparent stratigraphic succession is the result of structural juxtaposition, and the previously presented stratigraphy requires revision (Ayres and Corfu, 1991 ). Group IV, with an age of 42734 Ma, is the structurally lowest unit. It is overlain by the much older (2925 Ma) Group I, which is part of a thrust sheet that also includes the apparently comformably overlying, basaltic and sedimentary, 2725 Ma Group V. These units are overlain by a second thrust sheet that comprises the 2858 Ma Group III. The structurally uppermost 2870 Ma Group II apparently represents a third thrust sheet. Thrusting occurred after the development of the 2725_+2 Ma sedimentary sequence. The direction and magnitude of thrusting, discussed in more detail by Ayres and Corfu ( 1991 ), remain somewhat speculative but appear to have been the product of a general north-south compressive regime. Thrusting was followed by the development of major, east- to southeast-trending isoclinal folds that were subsequently overprinted by zoned regional metamorphism (Ayres, 1978 ). All these orogenic events appear to be temporally and genetically related to the emplacement of the major batholiths.

Regional perspective The structural complexity defined by the geological and geochronological work done in the Favourable Lake greenstone belt, is comparable with the picture gradually emerging in other greenstone belts of the Superior Province. Out-of-sequence stratigraphy caused by thrusting has been documented, for example, in the Uchi Subprovince (Corfu and Stott, 1989) and in the Wabigoon Subprovince

217

(Davis et al., 1988, 1989). Another common feature is the occurrence of thick, mainly turbiditic sedimentary sequences, as one of the latest supracrustal assemblages in these greenstone belts (e.g. Porcupine Group of the Abitibi belt)~ Turbiditic sequences in the large metasedimentary terrains such as the Quetico Subprovince, also appear to have formed during the terminal stages of volcanism in adjacent greenstone terrains (Davis et al., 1990). The Quetico Subprovince has been interpreted as an accretionary prism related to active subduction (Percival and Williams, 1989 ). In the Favourable Lake belt the relationships between the turbidites and the other supracrustal units were previously masked by the fact that the sedimentary assemblage (Group V, 2725 Ma) occupied a relatively low structural position in the belt. The overall time span of magmatism recognized in the Favourable Lake area, between about 3000 and 2700 Ma, compares with that documented in most other greenstone belts of the northern Superior Province (Fig. 8). The ages and the stratigraphic data suggest that this part of northwestern Ontario was the site of long-lived but pulsating volcanism, spanning more than 300 Ma. As in many Cenozoic volcanic regions, individual volcanoes probably had life spans of only a few million years, but new volcanoes continued to develop and were locally superimposed on older volcanoes to build up a complex volcanic sequence (Williams and McBirney, 1979 ). Individual events appear to be relatively short-lived, suggesting that volcanism was intermittent and localized, although some of the discrete events can be correlated between different greenstone belts. The major magma production event was between 2750 and 2700 Ma when most of the granitoid batholiths crystallized; this event coincides with widespread tectonism. Conclusions

The southeast end of the Archean Favourable Lake greenstone belt comprises five groups

218

that formed during three main periods of volcanism at about 2925 Ma (Group I), about 2870-2858 Ma (Groups II and III) and about 2734-2725 Ma (Groups IV and V). Based on facing directions and the apparently conformable contacts, this sequence had originally been interpreted as representing a polycyclic succession. The ages, however, do not agree with this interpretation and show that the present configuration of the belt is the result of thrusting of older sequences on top of younger ones. Three of the group boundaries appear to be thrust faults (Ayres and Corfu, 1991 ). The oldest rocks of the belt are represented by a 2950 Ma gneissic tonalite enclave in the North Trout Lake Batholith on the northeast side of the greenstone belt, and by a 3000-2960 Ma granodioritic clast in a ca. 2725 Ma old conglomerate of Group V. No volcanic rocks of this age were found in the greenstone belt, but volcanic units as old as 3023 Ma are found in the North Spirit Lake greenstone belt, about 50 km to the southeast. Most of the granitoid rocks, with ages of 2732 to 2708 Ma, were formed during the youngest magmatism of the belt. Emplacement of significant parts of the North Trout Lake Batholith, accompanied the compressive regime that first stacked the thrust slices in the greenstone belt, subsequently produced upright syn- and antiformal folds and finally contributed the heat that metamorphosed the folded greenstone sequences. The 2708_+ 3 Ma Setting Net Lake Stock, which was formed in the waning stages of this tectono-magmatic period, experienced a protracted, younger hydrothermal history during which various generations of monazite, titanite and rutile formed. This hydrothermal history is probably unrelated to the original syn-/late magmatic mineralization. The protracted magmatic history recorded in the Favourable Lake and the nearby North Spirit Lake greenstone belts, can also be recognized in the Uchi Lake and Red Lake greenstone belts 150 km to the south (Figs. 1, 8). Each magmatic event appears to represent rel-

F. C O R F U AND L.D. AYRES

atively short-lived volcanoes that progressively built up volcanic-plutonic complexes in this part of the Superior Province. As in many other terrains of the Superior Province, major turbitite sequences formed near the end of the volcanic evolution. Major compression led to thrusting and folding of the supracrustal assemblages and was associated with the widespread plutonism that concluded the orogeny.

Acknowledgements This project is part of a geochronological program carried out by the Ontario Geological Survey at the Jack Satterly Laboratory of the Royal Ontario Museum. We would like to thank P.D. Nunes for permission to use his unpublished data and to carry out additional work on the initial sample set. We also wish to thank T.E. Krogh for his interest and support, P.C. Thurston and H. Wallace for input in delineating the project and assistance in sampling, J. Hodgson and I. Nicklin for mineral separation, and B. Podstawskyj for mass spectrometer maintenance. Comments by D.W. Davis and two journal reviewers are gratefully appreciated. This paper is published with permission of the Director, Ontario Geological Survey.

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