Grenvillian magmatism in the eastern Grenville Province, Canada

Precambrian Research, 51 ( 1991 ) 315-336 Elsevier Science Publishers B.V., Amsterdam

315

Grenvillian magmatism in the eastern Grenville Province, Canada" Charles F. Gower a, Larry M. Heaman b, W. Dale Loveridge c, Urs Schiirer d. ~and Robert D. Tucker b a Newfoundland Department of Mines and Energy, P.O. Box 8700, St John "s, Nfld. A 1B 4J6, Canada b Department of Geology, Royal Ontario Museum, 100 Queen's Park, Toronto, Ont. M5S 2C6. Canada c GeologicalSurvey of Canada, 601 Booth St., Ottawa, Ont. KIA OE8, Canada o Departement des Sciences de la Terre et GEOTOP, Universitk du Qukbec ~ Montreal, C.P. 8888, Succ. A, Montreal P. Q., H3C 3P8, Canada (Received November 30, 1989; revised and accepted May 25, 1990)

ABSTRACT Gower, C.F., Heaman, L.M., Loveridge, W.D., Sch~irer, U. and Tucker, R.D., 1991. Grenvillian magmatism in the eastern Grenville Province, Canada. In: I. Haapala and K.C. Condie (Editors), Precambrian Granitoids--Petrogenesis, Geochemistry and Metallogeny. Precambrian Res., 51 : 315-336. U - P b age determinations on six newly identified examples of Grenvillian plutonism from eastern Labrador have yielded the following ages: Gilbert Bay granite 1132 + 7 Ma; Second Choice Lake pegmatite 1003 +_6 Ma; Southwest Pond granite 963 _+6 Ma; Chateau Pond granite 964_+ 2 Ma; Rivibre Bujeault headwaters quartz syenite 964_+ 5 Ma; Upper St Lewis River (west) granite 956 +_ 1 Ma. In addition, eight new K-Ar and Rb-Sr hornblende and biotite ages ranging from 953 Ma to 811 Ma are reported for three of these plutons. In conjunction with three previous U - P b determinations, it is concluded that the U - P b dates reflect two periods of Grenvillian plutonism that occurred in separate areas of eastern Labrador, and which were also characterized by distinct emplacement styles. North of the Mealy Mountains terrane boundary (Lake Melville terrane and its border regions with the Mealy Mountains and Hawke River terranes) plutonism occurred between ~ 1130 and 1080 Ma and consisted of sporadic, minor granitic intrusions. These intrusions had little structural effect on the host rocks and are inferred to have been emplaced at a high structural level, a model consistent with previous suggestions for tectonic stacking in the region. South of the Mealy Mountains terrane boundary (Mealy Mountains and Pinware terranes) plutonism was brief (966 to 956 Ma), but widespread. Typical magmatic products were circular (in plan) plutons, up to 20 km in diameter, having monzonite, quartz syenite and granite compositions, These plutons exerted marked structural influence on their host rocks and are interpreted to have been emplaced at intermediate structural levels. In a broader regional context, by utilizing a previously demonstrated correlation between positive magnetic anomalies and Grenvillian plutons together with reconnaissance geological mapping, a belt of Grenvillian plutons is inferred to exist across the southern half of the eastern Grenville Province. This zone of plutonism serves to emphasize a distinct difference between an exterior (northern) thrust belt and an interior (southern) magmatic belt. A similar bipartite division is apparent in other parts of the Grenville-Sveconorwegian Orogen. The mineral geochronological data probably do not reflect the cooling histories of individual plutons; alternative explanations include either a slow regional cooling event or the distal effects of younger plutonism/metamorphism to the south, as yet unidentified.

Introduction Until recently (Loveridge, 1986; Sch~irer et al., 1986; Gower and Loveridge, 1987; Schiirer and Gower, 1988) it was largely a matter of conjecture whether any magmatism of Gren-

villian age had occurred in the eastern Grenville Province (Gower and Owen, 1984). The U - P b zircon and titanite geochronological data that follow, document six newly identified examples of Grenvillian magmatic activity in southeastern Labrador. Including these ages,

"Geological Survey of Canada contribution 41689. ~Present address: Laboratoire de G6ochronologie, Sciences Physiques de la Terre et Institut de Physique du Globe, Universit6 de Paris VII, 2, place Jussieu, 75251 Paris Cedex 05 (France).

0301-9268/91/$03.50

© 1991 - - Elsevier Science Publishers B.V.

3 16

sufficient data are now available to assess the regional extent, timing and significance of Grenvillian plutonism in the eastern Grenville Province and to discuss this plutonism with reference to other parts of the GrenvillianSveconorwegian orogen. New Rb-Sr and K-Ar mineral data are also presented for three of the plutons allowing some discussion of thermal history subsequent to emplacement. GrenviUian magmatism in eastern Labrador

Previously documented examples Three examples of Grenvillian magmatism have been previously reported from the Grenville Province of eastern Labrador, all of which were dated using U - P b methods. These are the Southwest Brook granite (Sch~irer and Gower, 1988), the Beaver Brook microgranite (Sch~irer et al., 1986), and the herein-named Upper St Lewis River (east) monzonite (Gower and Loveridge, 1987 ). The Southwest Brook granite (Fig. 1 ) is situated within the Lake Melville terrane; it is about 14 km wide and consists of massive to weakly foliated, K-feldspar-rich granite containing abundant enclaves of earlier gneiss. At a few localities, where exposure is good, the enclaves are seen to be the dominant rock type; if this applies to the whole area, it may mean that the pluton depicted on the map is, in reality, a stockwork of Grenvillian vein material. Sch~irer and Gower (1988) reported a zircon age of 1079 + 6 Ma for the granite, pointing out that, ( 1 ) apart from migmatization events, this was the first evidence for significant volumes of acid magma having been generated during the Grenvillian orogenic cycle in eastern Labrador, and (2) the emplacement of the granite pre-dated final Grenvillian orogenesis in the area by about 50 Ma. In the same region, in the boundary zone between the Lake Melville and Hawke River terfanes, minor granitoid intrusions discordantly intrude granulite facies mylonitic rocks. The

{ . f : ( ~ ) W E R t l ~\1

mylonites were generated during deiormation associated with the assembly of the lithotectonic terranes and are one of several features that define this terrane boundary. One of the minor granite intrusions, the Beaver Brook microgranite dyke, has yielded a concordant U - P b monazite age of 1029_+ 2 Ma. Highly discordant zircons from the same sample, when regressed with the monazite, gave an upper intercept age of 1566_+ 13 Ma (Sch~irer et ak. 1986 ). Sch~irer et al. interpreted the data as indicating dyke emplacement during Grenvillian orogenesis and explained the discordant zircons as reflecting an inherited component. The third example of Grenvillian magmatism, the Upper St Lewis River (east) monzonite, is from the Pinware terrane, and has yielded a concordant zircon age of 966 _+3 Ma (Gower and Loveridge, 1987). This pluton is characterized by a particularly distinctive doughnut-shaped magnetic anomaly and the dating was carried out to test the hypothesis that the anomaly might correlate with a Grenvillian pluton (at the time Grenvillian plutonism was unknown in this part of the Grenville Province). In that instance the hypothesis proved correct and hence provided a new means for locating other Grenvillian plutons in this poorly m a p p e d region. The empirical link between magnetic anomalies and Grenvillian plutons was one factor used in selecting some of the samples for which ages are reported below.

Newly recognized examples q/ Grenvil/ian magmatism Six newly recognized examples of Grenvillian magmatism are reported here; the order of presentation is northeast to southwest, across the regional structural trend. Modal analyses and whole-rock geochemical data for the dated samples are given in Table 1. Selected traceelement characteristics of these plutons are illustrated in Fig. 2, which indicates that the plutons can be geochemically discriminated

317

GRENVILLIAN MAGMATISM IN THE EASTERN GRENVILLE PROVINCE

i NAIN

I 56°W

t / / ,~,,,Uv--,f~+ Fv.(~.

~m

GroSWa

c~ o 54°N

_oke Melville terrone )

~

[SOUTHWEST 1 ~

~. ~ J ~

~ A NITE~ _ ~ . ~ 11079.6z l ~ MeolyMountains L c ° - ~ I X terrane SECOND CHOICE I j # ~ LAKE PEGMATITE? ~ ca. i690z

~ terrone BEAVERBROOK1 c

~Hawke River

(

MICROGRANITE1 1566 * 13ZZ i029 * 2 m

LOO3 *6,

GRENVILLE

GRANbT~

~GRANITE 1962 * 3z I concordon~

IUPPERST LEWIS R ~ANITE 19--56"1z Labrador

GILBERT BAY !

ISOUTHWEST P O N D ;

PROVINCE

1132 ÷7/-6z ca.

482

[

j

I V E R ~ Pinwore terrone

. . ~9 ~ g ~ 2 ~

-

l E°nc°rdonf 52°N.

0 '

km

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~ S T LEWIS RIVER I(EAST) MONZONITE I

~

-

[ concoroont

i

~RIVIERE BU-JEA ~ I ~ ' ~ H E A D W A T E RS / Q ~ R T Z SYENITE |

11530*30z

i

Fig. 1. Geological terranes of southeast Labrador, showing sample locations a n d U - P b ages of dated G r e n v i l l i a n felsic intrusions. Dates are indicated as u p p e r i n t e r c e p t / l o w e r intercept; z = zircon, pn = monazite, t = titanite.

individually, but suggests that they cannot be assigned collectively to any one tectonic regime (cf. Pearce et al., 1984). U - P b data are reported in Table 2 and concordia diagrams presented in Fig. 3. Analytical techniques are summarized in Appendix I.

Gilbert Bay granite The Gilbert Bay granite (Wardle, 1976, 1977; Gower et al., 1987 ) is located within the Gilbert River shear belt, which forms the attenuated southeast part of the Lake Melville terrane between the Mealy Mountains and Hawke River terranes (Fig. 1). Gower et al.

( 1987 ) suggested that the pluton is transected by a fault that dextrally offsets the two halves of the pluton by about 2.5 km. In a pre-fault restoration, the pluton has an elliptical map pattern and measures roughly 4 by 3 kin. Its long axis is in a west-northwest direction, parallel to the regional trend. Rather than being a simple contact with the country rock, the margins of the pluton are marked by increased abundance of enclaves, passing outward into an agmatite. Minor granitoid intrusions related to the pluton occur up to 15 km westnorthwest and east-southeast of the pluton, but only up to about 3 km across the regional trend, suggesting that the pre-existing structure has

318

C,F, (it)WER E'I ~1.

TABLE 1 Whole-rock geochemical analyses a n d m o d a l analyses o f analyzed s a m p l e s CG86-688 Major e l e m e n t s (wt.%): SiO2 69.40 TiO2 0.38 A1203 14.67 Fe203 I. 11 FeO 1.42 MnO 0,04 MgO 0.48 CaO 1.36 Na20 3.70 KeO 5.50 P2Os 0.11 LOI 0.52 Total 98.69

CG85-495(

CG86-700

CG87-605

~ '(_;86-698

68.60 0,63 14.13 1.82 1.40 0.06 0.64 1.41 3.28 6, 13 0.20 0.48 98.78

70.95 0.54 13.72 1.24 0.87 0.07 0.51 0.71 3.27 6.34 0.13 0.58 98.93

~5.10 (/.80 15.05 2.84 t.88 O, t 3 1,09 2.00 4.07 4.88 0.46 1.22 99.52

6o, 75 0.44 16.70 1,42 0.97 0, 19 0~39 0.79 3.38 6.0(t !i.08 o.3 i 09.42

Not analyzed

Trace e l e m e n t s ( p p m ): F 708 V Cr 2 Cu 13 Zn 50 Ga 10 Rb 224 Sr 483 Y 14 Zr 277 Nb 18 Mo 3 Ba 1700 La 113 Ce 198 Pb 24 Th 33 IJ 3 Modal analyses (to nearest 0.5%): Quartz 25.5 plag 31.5 K-fs 33.5 Blot 6.5 Amph Apatite tr Fluorite tr Zircon tr Titanite Allanite tr Musc 1.5 Opaques 1.5

CG86-618

25 4 12 94 30 144 235 37 755 44 3 1343 203 492 28 20

18.0 21.0 58.0 0.5

13.0 27.0 51.5 4.5 0.5

tr . tr tr tr 0.5 1.0

555 2 3 81 10 210 195 73 47 50 3 2180 175 258 29 4 1

.

.

. tr 0.5 tr tr 2.5

.

33.0 26.5 34.5 2.5 tr . lr 1,5 0.5 1.5

1606 il -' z 166 33 160 41 ~ 75 5¢~ 60 "~ 3520 I t2 259 33 I1 ~

.

~7 , 2 i ,~i~ ,c, S3 f~7 25 ~5 9 1250 83 i47 i4 i ~d

19,0 43.0 2L0 7X) 1.5 f) 5

12,5 48.5 35.5 1.5 tr tr

t~

tr

~r

~r

.

~r -. I~5

tr t.5

Note. In m o d a l analyses trivial a m o u n t s o f s e c o n d a r y m i n e r a l s have been c o u n t e d as part o f the p h a s e they p s e u d o m o r p h . Abbreviations: P l a g = p l a g i o c l a s e ; K - f s = K - f e l d s p a r ; B i o t = b i o t i t e ; A m p h = a m p h i b o l e ; M u s c = m u s c o v i t e ; n d = n o t detected: t r = l e s s t h a n 0.5%.

319

G R E N V I L L I A N MAGMATISM IN THE EASTERN G R E N V I L L E PROVINCE

iO00

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1000

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0

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tO00

i

20

100

iO00

Y (log)

X ChateauPondgranite [] RiviereBujeaultheadwaters quartz syenite 2_ U. St Lewis~N)granite

Fig. 2. R b - ( Y + Nb) and N b - Y geochemical variation diagrams illustrating the specific compositional identity of the dated plutons on the basis of particular trace elements. Geochemical data for dated samples are presented in Table 1.

influenced the distribution of minor granitoid dykes related to the pluton. The granite is well exposed on the shores of Gilbert Bay, where it consists of light-pink- to buff-weathering, medium- to coarse-grained granite that is homogeneous and massive to weakly foliated. The granite intrudes K-feldspar megacrystic granitoid rocks (typically deformed to augen gneisses having mylonitic fabrics), sillimanite-bearing metasedimentary gneiss and net-veined amphibolite that represents the remnants of migmatized mafic dykes. These rocks have not been dated in the vicinity of the pluton, but from regional considerations are probably of Labradorian ( 1710 to 1620 Ma) age. Similar rocks occur as abundant, subrounded to angular xenoliths within the Gilbert Bay granite, from which it is clear that the deformation (including mylonitization) and migmatization that affects the host rocks pre-dated emplacement of the pluton. The pluton is intruded by unmetamorphosed, southeast-trending mafic dykes that compare closely with dykes in Sandwich Bay, one of which has

been dated at 327 + 13 Ma (K-Ar; Murthy et al., 1989). The only field evidence hinting at a Precambrian age for the pluton are closely spaced, north-northeast-trending joints that are parallel to, and possibly coeval with fractures occupied by Late Precambrian (Long Range) mafic dykes elsewhere in the region. The Long Range dykes have been dated at 615 _+2 Ma at Sandwich Bay (U-Pb, Kamo et al., 1989) and at 614+ 10 Ma in northwest Newfoundland (Ar-Ar, Stukas and Reynolds, 1974; recalculated using the decay constants of Steiger and J~iger, 1977 ). The granite comprises well-twinned, moderately sericitized and diffusely zoned plagioclase, undulose quartz, microcline (locally with relict perthitic textures), together with minor myrmekite, olive green biotite and muscovite. Accessory minerals are apatite, allanite, zircon, fluorite, and magnetite. Secondary minerals (mostly after biotite) include chlorite, white mica, rutile and hematite. The rocks lack evidence of post-emplacement deformation. The weak fabric seen in some outcrops, and

0, 0, 0, 0,

-

100+200 100+200 100+200 100+200

10 11 10 10

18 26 42 15 53 59

12 8 9 16

29 31 65 58 56 54

57 22 23 49

5 25 8 8 5 9

37 9 12 22

15

10

15 5

4 15 18 15

Common Pb (pg)

i213 2110 2333 1772

1729 1397 9237 2902 16710 12980

10732 17159 13713 11908

15231 2182 4612 20114

91 9l

28403 6363 5004

4770 2895 6284 8660

2°6pb 204pb

(i. t5948 0.15928 0.15918 0.15876

0.21328 0.20782 0.18620 0.17917 0.17t86 0.16909

0.16107 0.16083 0.16080 0.16105

0.1616 0.1617 0.1613 0.1603

0.1645 0.1659

01727 0.1723 0 1721

~.5607 15580 1.5568 I 5529

2.5153 2.4205 2.0350 1.9120 1.7736 1,7255

1.581 1.579 1.58l 1.581

1.584 1.586 1583 1.578

1.642 i.661

1763 1.760 1.761

1.916 1.739 1.695 1.743

235U

23SU

0.1814 0.1691 0.1637 0.1679

2°Tpb

2°6pb

~L07098 007094 0.07093 0.07094

0.08554 0.08447 0.07926 0.07740 0.07484 0.07401

0.07221 0.07139 0.07142 0.07245

0.07106 0.07114 0.07120 0.07140

0.07230 0.07260

0.07405 0.07411 0.07421

0.07663 0.07461 0.07512 0.07526

2°Tpb 20epb

954 953 952 950

1246 L217 il01 1062 1022 1007

963 962 962 963

966 966 964 958

982 990

1027 1025 1024

1075 1007 977 1001

2°6pb 2ssU

Apparent ages ( M a )

100 + 200 etc. = mesh size All fractions a b n l d e d

0.1726 0.I636 0.1507 0.2010

0.1503 0. t590 0,1178 0.1367 0.1483 0,1402

0.1684 0.1659 0.1340 0.1300

0.3506 0.3534 0.3211 0.3241

1.9253 1.0523

0.1972 0.2t12 0. t995

0.1628 0.1606 0.2576 0.1954

2°SPb 20~spb

Atomic ratios

Notes: M = magnetic: N M = non-magnetic: 0, 10 = degree of tilt of Frantz isodynamic separator:

7(! 49 51 95

130 142 338 309 309 308

(CG86-698 and CG86-697B)

Upper St Lewis River (west) granite l C G 8 6 - 7 0 0 ) Zircon (r) NM, 0. 140~ 200 12i (s) NM, 0, -~100+ 140 181 It) NM, 0, + 100 167 {u) NM, 0. - 140+ 200 116

Rivi~ B~eaultheadwatersqua~zsyenite Zi~on (v) NM, 0, -200 (w) NM, 0. +200 (x) M, 0. +200 (y) M, 0, +200 (z) NM, 0, +200 (aa) NM. 0, - 140+200

NM, NM, NM, NM,

57 66 57 59

392 252 284 255

Chateau Zircon (n) (o) (p) (q)

Pond granite ( C G 8 7 - 6 0 5 )

50 65 68 67

142 16 68 141

Southwest P o n d granite ( C G 8 6 - 6 1 8 ) Zircon (e) NM, 0, + 100 (f) NM, 0, + 100 (g) NM, 0, + 100 (h) NM, 0. + 100 t0 13 13 13

14 I1

32 38

47 44 42 30

79 75 74

245 240 215 162

Pb (ppm)

418 397 393

18 6 6 52

Gilbe~ Bay granite ( C G 8 6 - 6 8 8 ) Zircon (a) M, 0, - 100+200 (b) M, 0, - 100+200 (c) M, 0, - 1 0 0 + 325 (d) NM, 0, - 100+200

U (ppm)

Concentrations

Second C h o i c e L a k e p e g m a t i t e ( C G 8 5 - 4 9 5 C ) Zircon (i) NM, 0, - 100+ 200 374 (j) NM. O. - 200 230 (k) NM, 0. - 100+ 140 221 Titanite (1) M, 10, -140 552 (m) M, 10, -140 117

(~g)

Weight

Sample No., mineral fraction a n d characteristics

TABLE 2 U - P b analytical results

~#55 954 953 952

1276 1249 il27 1085 L036 1018

963 962 963 963

964 965 964 962

086 994

1032 1031 1031

1087 1023 1007 1025

2°Tpb 235U

957 956 956 956

1328 1304 1179 I131 1064 1042

962 963 965 964

959 962 963 969

997 1003

I043 1044 1047

1112 1058 1072 1076

2°TPb 206pb

r-

C~

321

GRENVILLIAN MAGMATISM IN THE EASTERN GRENVILLE PROVINCE

,17E .21

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I 1.95

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1003

+/- G /4o

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ZeTpb/235U

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167

(C) ®~

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• IG2

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.157 ~

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948

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~pb/Z35U

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1.58

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156 '

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I 1.58

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l !68

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1.52

1458/2//M

0_

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,

i i 1.56

I

1.58

~

2j 1,GO

,

I .G2

(f)

tsmej

BUZEAUL Y

CC86-Gg8, 697B

.2s ~

,152 1.58

UPPER

k

ST.

c

LEWIS

RIVER

(WEST)

-7oo

1488 [358

.22

1258 /\

.20~

IZ8 1158

w w

/-30

Mu

958

\ \r

.IB

.IG 964 ÷ / - 5

,141.5 b

11.7

,

MO

2eTpb/~35 U

2e7 ~35 Pb/ U

I , , + , I , , I , , i 1.9 2L.-I 2~.3 21.5 2.7 21.9 3.1 31.3 3.5

Fig. 3. Concordia diagrams for Grenvillian felsic intrusions dated in this study. Analytical data are presented in Table 2.

322 which appears to be parallel to the borders of the intrusion, was interpreted to be related to pluton emplacement by Gower et al. ( 1987 ). The sample collected for dating contained two distinguishable phases in hand specimen, a slightly foliated dark-grey phase and a massive lighter-grey phase. The zircon grains isolated from this sample do not have a uniform habit, suggesting that more than one zircon population is present. In addition, colourless cores were identified in some grains. Four fractions of colourless abraded zircon prisms were analyzed. Three of the four fractions (a, c and d in Fig. 3a ) are seemingly collinear, and yield an upper intercept age of 1132 -+- 67 Ma with a lower intercept of 482 Ma; the probability of fit for these zircons is quite poor, however. At present it is not possible to offer a unique explanation for the fourth fraction (b), that plots to the left of the discordia line. The 1132 Ma date cannot be considered as rigorous for the time of emplacement of the Gilbert Bay pluton. It is, however, consistent with field data that the pluton is part of the Grenvillian granitoid magmatic activity in southeast Labrador.

Second Choice Lake pegmatite The pegmatite discordantly intrudes banded migmatitic gneiss, which occurs at the boundary between the Mealy Mountains and Lake Melville terranes. The sample was collected from the same outcrop as that from which Sch~irer et al, (1986) reported an upper intercept age of 1677 -15 + 16 Ma for the host gneiss. The lower intercept of the zircon discordia for the gneiss is anchored by concordant monazite that yielded a date of 1030 + 2 Ma, an age also indicated by discordant titanite. The pegmatite trends northeast and is less than 1 m wide. The mineral assemblage comprises sericitized, well-twinned plagioclase, quartz, microcline and accessory orange-brown biotite, titanite, apatite, allanite and opaque minerals. Zircon was not seen in thin section. Figure 3b shows the concordia diagram for

C'F. (i(~WER ET 4 1

zircon and titanite from the CG85-495C pegmatite. Three zircon fractions (i, j, k ) were selected based on different grain characteristics ranging from small to large, equant to medium prismatic grains. Despite the variety of grains selected, all three fractions analyzed yield essentially identical U - P b ages lying between 1032 and 1024 Ma. Since the 2°Tpb/>6pb ages of these fractions ( 1047-1043 Ma) are substantially older than the ( 1030 Ma) monazite and titanite ages of the host gneiss, the presence of an inherited component in the pegmatite zircon is indicated. As all grains analyzed were euhedral, perfectly transparent and free of cracks and inclusions, the formation of these gem-quality zircons can unambiguously be ascribed to new-growth during pegmatite crystallization, and the small amounts of inherited radiogenic Pb detected in all three fractions must be contained in totally resorbed old cores. In large grains of lesser quality, zircon cores can be distinguished clearly under the microscope, consistent with the interpretation of an inherited zircon component. In addition to zircon, two titanite tractions ( 1, m ) from the pegmatite were analyzed. Both fractions consisted of transparent, euhedral to subhedral red-brown crystals, selected from the best-quality grains of the population. Both titanite analyses plot very close to concordia (Fig. 3b), having 2°Tpb/2°°pb ages of 997 and 1003 Ma, respectively. Since the U - P b ages overlap within the analytical error (at the 95% confidence interval), the titanite age from the 2°7pb/Z°6pb ratio of the most concordant fraction yields a best estimate of 1003 + 6 Ma for the age of intrusion. Compared to zircon, UPb analyses on titanite yield larger errors due to higher concentrations of c o m m o n Pb (2°6pb/2°4pb of only about 90 versus a few 1000 p p m in zircon). In order to minimize these errors, the isotopic composition of initial Pb was determined in leached K-feldspar from the pegmatite (U-free mineral phase; see Appendix I ).

GRENV1LLIAN MAGMATISM IN THE EASTERN GRENVILLE PROVINCE

Southwest Pond granite The Southwest Pond granite, first mapped by Gower et al. ( 1987 ), is situated in the Pinware terrane. It has a slightly elliptical outline (5.5 km by 4.5 km), and is elongate in a northeast direction. The pluton is hosted entirely by sillimanite-bearing metasedimentary gneiss, the protolith of which probably pre-dates Labradorian orogenesis (Gower et al., 1987 ). Forceful emplacement is indicated by deflection of the regional northwest-trending gneissosity into parallelism with the pluton margin. No country-rock enclaves have been observed within the intrusion. The pluton consists of pink- to buff-weathering, coarse-grained, homogeneous granite associated with a medium-grained granite and minor pegmatite. In places, the mediumgrained granite forms the whole outcrop, in which case its relationship to the coarse-grained granite is unknown. Elsewhere, similar medium-grained granite forms planar-sided dykes intrusive into the coarse-grained granite, as well as discordantly intruding the surrounding metasedimentary gneiss. The appearance and distribution of these peripheral dykes (not more than 11 km from the pluton) links them with the parent body. The coarse-grained granite comprises anhedral well-twinned plagioclase having sericitized cores, anhedral, weakly undulose quartz, microcline, green biotite, secondary chlorite, white mica and rutile (all replacing biotite), and accessory titanite, apatite, allanite, opaque minerals and zircon. The medium-grained granite has a similar mineral assemblage except that muscovite forms a minor primary phase, and primary biotite and accessory phases are much less abundant. Four colourless, transparent zircon fractions, selected from the least magnetic split, were analyzed from the Southwest Pond granite (CG86-618 ). The zircon morphology varies from prismatic (length:width=3:l) to needle-like (length:width = 12:1 ), with many grains containing abundant fluid and mineral

323

inclusions. Some turbid interior zones (cores?) with colourless transparent rims were identified in more magnetic splits, but such grains were excluded during selection. Three of the zircon fractions selected for analysis consisted of needles generally devoid of inclusions. The U - P b results for these fractions cluster near concordia and have an average 2°Tpb/2°6pb age of 962 +_3 Ma (e, f, g; Fig. 3c), which is interpreted as the age of emplacement of the granite. A fourth fraction (h) consisted of colourless, transparent prismatic grains, with some grains containing a few tiny inclusions. This fraction is slightly discordant and plots to the right of the zircon cluster, possibly reflecting the presence of some inherited component. Given that zircon core/overgrowth relationships have been recognized in thin section and in the mineral separates, an inherited zircon component in this fraction cannot be ruled out.

Chateau Pond granite The Chateau Pond granite is located in the Pinware terrane and is the largest Grenvillian pluton yet mapped in eastern Labrador (Gower et al., 1988). The pluton is nearly circular in plan, having a diameter of about 19 km. A north-northeast-trending, quartz-filled fault, presumed to be Late Precambrian, transects the pluton and has an apparent sinistral displacement of 1.5 kin. The Chateau Pond granite intrudes foliated, recrystallized granitoid rocks of uncertain age. As with the Southwest Pond granite, its emplacement has imparted a pronounced foliation to the surrounding rocks parallel to the pluton margin. Large rafts of foliated biotite granitoid rock are present in several parts of the Chateau Pond granite; these are interpreted to be xenoliths of surrounding or underlying rocks. The granite is pink to white weathering, coarse- to very coarse-grained, massive and homogeneous. The essential minerals are anhedral plagioclase, weakly undulose quartz, and K-feldspar. Plagioclase is poorly- to welltwinned, moderately sericitized and com-

324

monly has clear albitic borders locally associated with myrmekite. K-feldspar, which occurs in grains up to 3.5 cm across, consists partly of poorly exsolved, stringlet or patch perthite, and partly of well-twinned microcline. Olive green-brown biotite is the dominant mafic mineral, although relict hornblende, partially pseudomorphed to an orangebrown phyllosilicate, is present in most samples. Accessory minerals include titanite (a common, anhedral phase), rare allanite, apatite, ilmenite, pyrite and zircon. Chlorite and white mica are minor secondary minerals after biotite. Zircons from the coarse-grained Chateau Pond granite consist of well-faceted grains exhibiting a range of shapes from long, prismatic needles to stubby, equant prisms. Regardless of zircon morphology, no core component is visible when the grains are examined at high magnification ( × 100 ). Three of the four fractions selected for analysis consisted of colourless tips from long prismatic grains (n, o, q; Table 2) and the other fraction (p) comprised colourless, short, prismatic grains. All analyses overlap within error of concordia, yielding an average 2°7pb/2°6pb age of 964 + 2 Ma (Fig. 3d), which is interpreted as the emplacement age of the Chateau Pond granite.

Rivikre Bujeault headwaters quartz syenite The Rivi6re Bujeault headwaters quartz syenite is located within the Pinware terrane and forms a circular pluton about 15 km across. Only the eastern margin of the pluton has been mapped (at 1:100,000 scale ) but the outline of the pluton can be readily inferred from the fact that the pluton forms an area of high topographic relief and is associated with a well-defined positive magnetic anomaly (Gower and Loveridge, 1987 ). On its eastern side the granite intrudes deformed syenitic rocks of unknown age that have foliations parallel to the margin of the pluton. The Rivi6re Bujeault headwaters pluton

r . v . (IOWER ET M .

consists of medium- to coarse-grained, rustyto pink-weathering, massive to weakly foliated, homogeneous alkali feldspar quartz syenite and granite. The essential minerals are large grains of stringier perthite, lesser quartz and minor plagioclase. Plagioclase includes both poorly-twinned, sericitized grains and unaltered, well-twinned crystals that appear to be later than the sericitized variety. The most common mafic mineral is amphibole, occurring as large, green grains containing exsolved quartz, together with allanite and opaque mineral inclusions. Clinopyroxene forms a pale green, anhedral partly relict mineral, altered to an orange-brown phyllosilicate. Biotite occurs as orange-brown flakes in minor amounts. Accessory minerals include allanite, opaque mineral, apatite, rare titanite and zircon. Zircon grains from sample CG86-698 are clear, colourless to pale yellowish brown, euhedral to mostly anhedral, with no zoning or cores visible under the binocular microscope (although cores are visible in thin section), length:width= 1:3, and many grains are fractured. Inclusions are rare and are mostly bubble-shaped (fluid inclusions?). Two zircon fractions from sample CG86-698 were analyzed, resulting in two closely spaced but highly discordant data points (v, w; Table 2, Fig. 3e ). These points define a discordia line having upper and lower intercepts of 1478 and 9 t 9 Ma, respectively. The results obtained are provisionally attributed to the presence of two phases of zircon; cores of age -,- 1480 Ma within late-Grenvillian igneous zircon. No further work was carried out on zircon from CG86-698 because of the complexity of the U - P b system and the small grain size and concentrate size. The Pb content is sufficiently low, 29 to 32 ppm, such that the best grains were all consumed during the original two analyses. A second sample from Rivi~re Bujeault headwaters quartz syenite, CG86-697B, was collected from a location 2.5 km southwest of CG86-698. Zircon grains from CG86-697B are similar to those from CG86-698. Four frac-

GRENV1LLIAN MAGMATISM IN THE EASTERN GRENVILLE PROVINCE

tions were hand-picked and analyzed. Three of the four data points (x, z, aa; Table 2, Fig. 3e) are collinear (probability of fit 23%; Davis, 1982) and define a chord having upper and lower intercepts of 153 a+21-,-2oMa and 9 6 5 + 4 Ma, respectively. These data points are also collinear with the two from CG86-698, the five having a 16% probability of fit to a chord having intercepts of 1517 + 12 Ma and 962 +4 Ma. Although these latter results agree within analytical uncertainty with those from the threepoint curve, it is possible that the inherited zircon in CG86-697B is not of precisely the same age as that in CG86-698, 2.5 km to the north. Accordingly, the preferred lower intercept age and uncertainty are adjusted to 964 + 5 Ma to span both values calculated above. This value is used in Fig. 1 and Tables 5 and 6. The age of the inherited zircon probably falls in the range 1560-1500 Ma. The data point for fraction y, which falls marginally below the chord established by the other five points, may either have experienced minor secondary Pb loss, or may contain a small component of inherited zircon older than 1550 Ma. Taken in conjunction with field evidence, the upper intercept age of ~ 1530 Ma is tentatively interpreted as the age of the zircon cores and the 964_+ 5 Ma lower intercept age correTABLE 3 K-Ar mineral age data Material

K (wl.%)

4°Ar rad. ( c c / g × 1 0 -4 )

4°Ar rad. 4°Ar tot.

Rivibre Bujeault headwaters quartz syenite (CG86-698): Hornblende 0,615 0.289 0.942 Biotite 7,07 3.448 0.977 (CG86-697B): Biotite 4.60 2.169 0.985

Age (Ma)

926+_ 16 953+_ 12 927+- 18

Upper St Lewis River (east) monzonite (CG84-195): Hornblende 0.892 0.431 0.982 946+- 13 Biotite 7.30 3.377 0.996 914+_ 13 Upper St Lewis River (west) granite (CG86-700): Biotite 7.58 3.492 0.995 Analysts: D. Bellerive, F.B. Quigg and R.J.G. Seguin.

911 .+_10

325 TABLE 4 Rb-Sr mineral age data for Rivibre Bujeault headwaters quartz syenite Sr (ppm)

87Rb

87Sr

86Sr

86Sr

Age (Ma)

(CG86-698): Biotite 430.6

7.69

202.9

3.2848

888 +_ 12

CG86-697B): Biotite 524.7 499.8

18.58 13.29

90.05 124.39

1.7534 2.1698

811 +- 9 822+_9

Material

Rb (ppm)

Analyst: R.J. Thdriault

sponds to growth of magmatic zircon during emplacement of the quartz syenite. The lower intercept date confirms other lines of reasoning that the pluton is late Grenvillian in age, namely (i) the rock lacks any indication of deformation, so it is clearly late- to post-tectonic, and (ii) it is characterized by a well-defined aeromagnetic anomaly, a feature also associated with the nearby Upper St Lewis River (west) monzonite and the Upper St Lewis River (east) granite, both of unequivocal Grenvillian age. The upper intercept may represent a vestige of a newly recognized earlier period ofgranitoid plutonism in the area, presently dated as having occurred between 1500 and 1470 Ma (Tucker and Gower, 1990, in prep.). As the dated 1500 to 1470 Ma rocks show clear evidence of mafic dyke injection and polyphase deformation, both of which are absent in the Rivi6re Bujeault headwaters quartz syenite, this further points to a Grenvillian emplacement age. K-Ar ages of 926 + 16 Ma on hornblende, and 953+ 12 Ma and 927+ 18 Ma on biotite (Table 3 ) were also obtained from the Rivi6re Bujeault headwaters quartz syenite (samples CG86-698 and CG86-697B). R b - S r analysis of biotite from the same samples yielded ages of 888_+ 12 Ma and 8 1 1 + 9 Ma (Table 4). A duplicate measurement from the same concentrate that gave the 811 Ma date, yielded an age of 822 z 9 Ma. These data are discussed later.

326

Upper St Lewis River (west) granite The Upper St Lewis River (west) granite is located within the westward extrapolation of the Pinware terrane. The pluton is estimated to be 10 km in diameter on the basis of its magnetic signature. The rock is a coarse-grained, massive, pink-weathering, homogeneous granite. Its felsic minerals are anhedral, moderately sericitized plagioclase, anhedral quartz and microcline. Biotite occurs as locally chloritized, buff-green flakes full of small zircon inclusions. Accessory minerals include titanite, apatite, opaque minerals (both oxide and sulphide ) and zircon. The zircon forms fairly large grains having euhedral cores. The U - P b results for four moderately abraded zircon fractions of the Upper St Lewis River ( w e s t ) g r a n i t e (CG86-700) are nearly concordant (r, s, t, u; Fig. 3f); three of the four error ellipses overlap concordia. Since the measured uranium contents are low (49 to 95 p p m ) , radiation damage has likely been minimal and it is assumed that the small a m o u n t of Pb loss indicated occurred at or near 0 Ma. The upper concordia intercept, therefore, has been calculated as 956_+ 1 Ma, the average of the four 2°7pb/2°6pb ages, and is interpreted to be the time of intrusion. A K-Ar biotite determination from the same sample yielded an age of 911 + 10 Ma (Table 3). Discussion

Regional setting The northern half of the eastern Grenville Province (taken as the region east of 68°W; Fig. 4) is now known (from recent 1:100,000 scale geological mapping and high-precision U Pb geochronology) to be almost entirely underlain by rocks having pre-Grenvillian protoliths. Most of this region is underlain by crust that was newly generated during the 1710-1620 Ma Labradorian orogeny (Sch~irer et al., 1986; Thomas et al., 1986; Sch~irer and Gower,

( ' . F (;~ ~WER E I M

1988). Pre-Labradorian pelitic to semipetitic gneisses (probably not older than 1750 Ma), were intruded by anorthosite-gabbronoritemonzonite complexes and a wide range of calcalkaline to alkali-calcic granitoid plutons. Subsequent crustal additions included the Michael-Shabogamo mafic intrusive suites emplaced at 1430 Ma, and anorogenic felsic magmatism at ~ 1300 Ma. The fundamental Grenvillian characteristic of the northern region is that it forms a collage of generally south-dipping, thrust-bound, iithotectonic terranes (Fig. 4), which achieved their final (but not necessarily initial) configuration during Grenvillian orogenesis I Gower and Owen, 1984; Rivers and Chown, 1986: Thomas et al., 1986; Wardle et al., 1988; Gower et al., 1988; Rivers et al., 1989; Connoily et al.. 1989). Grenvillian isotopic ages are almost entirely related to tectonic and metamorphic effects, and can be linked to the assembly of the thrust-bound lithotectonic terranes. For example, Sch/irer et al. ( 1986 ) and Sch~irer and Gower (1988) have demonstrated that the Groswater Bay and Lake Melville terranes in eastern Labrador are characterized by titanite ages of ~ 980 to 970 Ma and 1040 to 1030 Ma, respectively. Farther west, the Churchill Falls and Gagnon terranes appear to have titanite ages around 990 Ma (Thomas et al.. 1986: Connolly et al., 1989). In the structurally higher Hawke River, Mealy Mountains, Wilson Lake and Lac Joseph terranes farther south, Grenvillian high-grade metamorphic effects and Grenvillian U - P b isotopic ages are virtually absent over large areas (ScMrer and Gower, 1988; Connolly et al., 1989 ). From the point of view of this publication, however, a particularly noteworthy feature of the northern region is the lack of supracrustal or magmatic rocks of Grenvillian age. In the southern half of the eastern Grenville Province, this pattern appears to change, although it must be emphasized that the region is still rather poorly known (information being based mainly on 1:250,000 scale mapping). It

327

G R E N V I L L I A N MAGMAT1SM IN T H E EASTERN G R E N V I L L E P R O V I N C E

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Pos~tqve rnagnehc onor~olies possibly rel•fed to GrenvdllOn granitoqd rocks ,

Late P r e c a m b r , a n - Phonerozolo rocks

956t I ~ 5

Empl~cement age of Grenvlllion intrusion

~ Time of lest hLgh grade me~arnor phism {generalized;

Fig. 4. Major geological features of the eastern Grenville Province. The distribution of Grenvillian plutons is based on an interpretation that combines geochronological data with geophysical criteria and reconnaissance geological mapping. The map also illustrates a bipartite division between an exterior (northern) thrust belt and an interior (southern) magmatic belt characterized by extensive Grenvillian magmatism. The exterior thrust belt is divisible further into an outer parautochthonous zone and an inner allochthonous region (cf. Rivers et al., 1989). All dates indicated are U - P b determinations, except those indicated by (R), which are Rb-Sr whole-rock dates.

seems that pre-Labradorian supracrustal rocks are less extensive, the amount ofgranitoid plutonic crust is vastly increased, and younger supracrustal rocks are present. The younger supracrustal rocks are represented by the Wakeham Supergroup (Bourne et al., 1977; Martignole et al., 1987 ), a rhyolite of which has been dated at 1271 + - - 313 Ma (Loveridge, 1986) Although some of the granitoid crust is of Labradorian age (Loveridge, 1986), it is diluted by plutonism at --~ 1500 to 1470 Ma (Tucker

and Gower, in prep. ) and at ~ 1130 to 950 Ma (Gower and Loveridge, 1987; this paper).

Timing of Grenvillian magmatism in the eastern Grenville Province Tables 3, 4 and 5 summarize geochronological data that can be interpreted as defining the period of Grenvillian magmatism and subsequent thermal history in the eastern Grenville Province and Long Range Inlier in western Newfoundland. These data, taken at face value,

328

indicate that Grenvillian plutonism occurred between ~ 1130 Ma and 950 Ma, followed by cooling over the next 50 million years. The oldest date involves some interpretational uncertainty but, nevertheless, this time interval is the best currently available estimate for the duration of Grenvillian plutonism in the eastern Grenville Province. The distribution of ages (Figs. 1 and 4) suggests that the northeast boundary of the Mealy Mountains terrane divides the Grenville Province in eastern Labrador into two regions affected by Grenvillian plutonism at different times. The Lake Melville terrane and the margins of adjacent terranes were affected by an older period of Grenvillian plutonism between ~ 1130 and 1000 Ma, whereas the Pinware and southeast Mealy Mountains terranes experienced a short-lived and younger period of Grenvillian magmatism, having ages between 966 and 956 Ma. In addition, the Mealy Mountain terrane boundary separates regions affected by different styles of Grenvillian magmatism. To the northeast, Grenvillian magmatism consisted of minor felsic intrusions (especially true if the Southwest Pond granite is a stockwork intrusion) that exerted little influence on the structure of their host rocks. The Gilbert Bay intrusion is a partial exception to this generalization in that it forms an elliptical pluton discordant to its surroundings. On the other hand, it does have agmatitic margins and it shows no indication of having modified regional structures. Moreover, its regional structural position within the Gilbert River shear belt (the full width of which could be regarded as having tectonic affinity with the southeast end of the Mealy Mountains terrane boundary ) is somewhat equivocal. South of the Gilbert River shear belt, all the Grenvillian intrusions are nearly circular in plan, and regional foliations in the country rock are parallel with the margins of the plutons. Differences in the age and style of pluton emplacement suggest that Grenvillian tectonic

c,F. (J~)WER ET AI,

regimes on each side of this boundary were rather different, such that Grenvillian plutonism was earlier, more sporadic, and representative of a higher structural level in the north, and younger, briefer, more extensive, and representative of a deeper level in the south. The suggestion that the Grenvillian plutonism was at a higher level in the north, is consistent with the tectonic relationships envisaged by Sch/irer and Gower ( 1988 ) for tectonic stacking in this region. In association with a deeper level of Grenvillian tectonism in the south, one would predict widespread Grenvillian U - P b titanite ages in the host rocks to the Grenvillian plutons. Evaluation of the latter suggestion awaits further data.

Constraints on the timing ( f Grenvillian deformation Since the Gilbert Bay granite discordantly truncates mylonitic fabrics in its host rock, accepting the date for the pluton implies that deformation related to the formation of the Gilbert River shear belt occurred prior to 1132 Ma. There are no other m i n i m u m age constraints on the timing of shear belt deformation and it is conceivable that it took place during Labradorian orogenesis. The 1079 Ma age for the Southwest Brook granite provides a m i n i m u m age at that locality for the gneissic fabric in the host rock that is truncated by the granite. Regional relationships suggest that the fabric is Labradorian, however, hence offering no time constraints for the start of Grenvillian deformation. In contrast, the end of Grenvillian deformation is much better known. The pegmatite at Second Choice Lake (1003+ 6 Ma) and the microgranite at Beaver Brook ( 1029 _+2 Ma) are both planar-sided dykes discordantly intruding gneissic fabrics that were partly produced during transposition of the Mealy Mountains and the Hawke River terranes with respect to the Lake Melville terrane. These ages and field relationships are consistent with the

GRENVILLIAN MAGMATISM IN THE EASTERN GRENVILLE PROVINCE

findings of Sch~irer et al. (1986) and Sch~irer and Gower (1988) that the major pulse of Grenvillian m e t a m o r p h i s m in the Lake Melville terrane ended between 1040 to 1030 Ma, at which time closure occurred in the U - P b system in titanite and monazite. Lack of deformation in the granitic plutons in the Pinware terrane indicates that Grenvillian deformation had ceased in this region prior to 966 Ma. If the 1003 Ma age for the Second Choice Lake pegmatite is accepted as its time of emplacement, then it would have been intruded between two events, recorded by titanite closures at 1040 to 1030 Ma and 980 to 970 Ma in the Lake Melville and Groswater Bay terranes, respectively (Sch~irer et al., 1986). Titanite closure in these terranes is inferred to be related to uplift following pulses of tectonom e t a m o r p h i s m and thrusting (Sch~irer and Gower, 1988), and it is possible that the pegmatite was emplaced during a period of kinematic relaxation between these pulses. Although the 1003 Ma time of emplacement of this intrusion differs from that of titanite closure in eastern Labrador, it is close to the time of titanite closure in central and western Labrador (see earlier). Similarly the dates from Grenvillian granitoid intrusions in the Pinware terrane are similar to the time of isotopic closure of titanite in the Groswater Bay terrane. The relative spatial positions of the various terranes during coeval activity are not known, but the possibility of some form of cause-and-effect linkage between thrusting in one part of the eastern Grenville Province and plutonism elsewhere in the same region seems quite likely.

Regional extrapolations Eastern Grenville Province Figure 4 attempts to interpret areas that may be underlain by as yet undated Grenvillian plutons in the eastern Grenville Province and the Long Range Inlier. The distribution is

329

based on: ( 1 ) correlation of magnetic anomalies with Grenvillian plutons (as outlined earlier, but adding the proviso that undated granites of other ages, nevertheless, might be included); (2) the mapping of Gower et al. (1985, 1987, 1988) in eastern Labrador; and (3) mapping by Owen and Erdmer ( 1988 ) in the Long R a n ~ Inlier. This database is coupled with additional interpretation based on the geological compilation maps of Quebec and Labrador, which were compiled from reconnaissance 1:250,000 mapping. Outside eastern Labrador, geochronological data have been published (Table 5) for intrusions in the Lac du Morhiban area ( U - P b ; Loveridge, 1986), for several plutons from the Long Range Inlier (Rb-Sr, whole-rock pooled data; Pringle et al., 1971 ), and for the Turgeon Lake granite on the Quebec north shore (Fowler and Doig, 1983 ). In addition, Erdmer ( 1986 ) has cited a U - P b zircon age pooled for several Grenvillian granitoid plutons within the Long Range Inlier, but full data are presently unpublished. Although definitive data are rather scattered, we feel that sufficient evidence exists to argue that this region represents a distinctive part of the eastern Grenville Province (at least 300 km wide and up to 1000 km long) that is, in particular, characterized by abundant magmatism of Grenvillian age. Developing a concept proposed by Owen and Erdmer (1990), the northern and southern regions of the eastern Grenville Province are termed here the Exterior Thrust Belt and the Interior Magmatic Belt on Fig. 4, and an approximate boundary drawn between them, based on geophysical criteria. It should be further noted that the Exterior Thrust Belt can be subdivided into parauthochthonous and allochthonous zones (cf. Rivers et al., 1989). These two zones last experienced high-grade m e t a m o r p h i s m (as recorded by their titanite/ monazite signatures; cf. Fig. 4) during Grenvillian and Labradorian orogenesis, respectively.

330

( I : . t;{ ~WER E I A I

FABLE 5 Summary o f ages for Grenvillian felsic magmatism in the eastern Grenville Province (arranged in order of interpreted emplacement age I t inil

Material ~+

Uppcr intercept

Lower intercept

Reference

zn z~r zu zir

I 132 !/, 1079 ~ 6 1079~ 17 1042 !i~

ca. 482 ca. 393 ca. 62

This study ( t t e a m a n ) Schfircr and Gowcr { 1988 ) Loveridge (1986) Erdmcr ( 1986 }

zir, tin zir zu zir zu zir zir

1566 :!713 ca. 1690 993 :+ 3 966 ): 3 964 +: 2 1530 ~ 30 962 ~ 3 950 ~ 1

1(129 :k 2 1003 _+6 1842 ~0 concordant concordant 964+ 5 concordant concordant

Sch~irer et al. ( 198¢~ This study ( SchMer ! Lo~ eridge ( 1986 ) Gower and Loveridge ( 1987 } This study (Tuckcr~ This study (Lovm idge) This study ( H e a m a n 1 This study ( Loveridge )

Age

Initial ratio

RetL,rence

I 130 + t)ti

0.7066 + 0.0034

Pringle el al. ( 197 !

~)48 :~ 23

0.7091 2:0.0043

Fowler and D o i g ( 1!)83 )

- P b a~es:

Gilberl Bay granite Southwest Brook granite Romaine River monzogranite Long Range lnlier granite to granodiorite b Beaver Brook microgranite dykc Second Choice Lake pegmatite Lac la Oalissoniere monzogranite /}pper St Lewis River (east) monzonite ('hateau Pond granite Rivibre Bujeault headwaters qtz syenitc Southwest Pond granite 1 Jpper St Lewis River (west) granile

zlr,

nl n Z

R/,-,S)" whole-rock axes:

Portland Creek Pond and Hawke's Bay granites ( Lake Michel pluton o f Bostock el al., 1983 ) b Turgeon Lake granite zir = zircon; mnz = monazitc: tin = titanite. h Pooled data from more than one pluton.

Grenville and Sveconorwegian provinces Extrapolation can be made beyond the c o n f i n e s of the eastern Grenville Province (Fig. 5 ), as regional geophysical data suggest that this distinct part of the Grenville Province continues to the southwest. As drawn, it diverges from the Grenville front in the central Grenville Province--a configuration that correlates with the southeastward bulge of the older Superior Province cratonic nucleus under the northern part of the central Grenville Province. The northern limit of extensive Grenvillian plutonism in the southwestern part of the Grenville Province appears to be the northern margin of the Central Metasedimentary Belt. A similar bipartite division to that proposed above for the eastern Grenville Province, also applies to the Sveconorwegian Province. The eastern part of the Sveconorwegian Province

(the Southwest Swedish Gneiss Belt) consists of thrust-bound terranes almost entirely composed of pre-Sveconorwegian rocks, having protolith ages between 1700 and 1600 Ma (e.g.. Lindh, 1987). In contrast, the Norwegian part of the Sveconorwegian Province includes a wide range of plutons emplaced during the Sveconorwegian orogenic cycle (Gafil and Gorbatschev, 1987). Although it appears that Grenvillian plutonism occurred along the full length of the Interior Magmatic Belt (including the western part of the Sveconorwegian Orogenic Belt ), age data suggest that the timing of post-tectonic plutonism varied, being progressively younger from west to east. In the southwest part of the Grenville Province, post-tectonic plutonism ranges in age from 1090 to 1076 Ma (Heaman and Krogh, 1985; Corriveau et al., 1988: Davis and

331

GRENVILLIAN MAGMATISM IN THE EASTERN GRENVILLE PROVINCE

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~

~ J ~ .

Fig. 5. E x t r a p o l a t i o n o f t h e b i p a r t i t e d i v i s i o n i n f e r r e d for t h e e a s t e r n G r e n v i l l e P r o v i n c e (cf. Fig. 4) to the r e m a i n d e r o f the G r e n v i l l e - S v e c o n o r w e g i a n O r o g e n , T h e Baltic Shield is p o s i t i o n e d with respect to L a u r e n t i a in a p o s t - 1 0 1 0 M a orie n t a t i o n ( m o d i f i e d after Stearn a n d Piper, 1984). T h e d i v i s i o n s are s e p a r a t e d by a d a s h e d line. T h e c o n t i n u o u s line with the ticks on o n e side is t h e a l l o c h t h o n o u s - p a r a u t h o c h t h o n o u s b o u n d a r y o f R i v e r s et al. ( 1 9 8 9 ) in C a n a d a , a n d the m y lonite zone in S c a n d i n a v i a . T h e black areas are p o s s i b l e late- to p o s t - G r e n v i l l i a n g r a n i t o i d p l u t o n s i n f e r r e d f r o m c o m b i n e d geological a n d g e o p h y s i c a l data.

Bartlett, 1988; Lumbers et al., 1990), in eastern Labrador final plutonism occurred between 966 and 956 Ma, and in the Sveconorwegian Orogenic Belt post-tectonic plutonism may have continued until 850 Ma (Falkum, 1985; Eliasson and SchiSberg, 1989). The broad temporal relationships between granite plutonism in the south and thrusting farther north, argues in favour of a syn-collisional origin for the Grenvillian magmatism, a contention consistent with inter-continent tectonic models. Rotation of the Baltic Shield rel-

ative to Laurentia and mutual collision along their (previous) "southern" sides has been proposed on paleomagnetic grounds (Stearn and Piper, 1984 and references therein) and supported by geological reasoning (Gower and Owen, 1984; Ga~il and Gorbatschev, 1987). Stearn and Piper (1984) have reasoned that most of this rotation was accomplished between 1100 Ma and 1010 Ma, which means that Grenvillian magmatism in the eastern Grenville Province would have a syn- to postcollisional tectonic setting.

('.t: ~i~ :AER E1 ',1

332

Cooling~resetting and upl~/t Geochronological data have been obtained by more than one method from three of the six plutons studied, and are summarized in Table 6. Closure temperatures are influenced by many factors, such as deformation (Wayne and Sinha, 1988), fluid activity (Harrison and Watson, 1983) and rate of cooling (Harrison et al., 1985), making quantative statements concerning rates of cooling difficult to constrain. The age data presented in Table 6 are internally consistent, however, except for the K-Ar ages (discussed below) obtained from the Rivibre Bujeault headwaters quartz syenite. In particular, the ~ 50 Ma difference between the U - P b zircon and the K-Ar biotite ages of the Upper St Lewis (west) granite and the Rivibre Bujeault headwaters quartz syenite is noteworthy. As these two plutons differ considerably in size, it would seem unlikely that the similar contrast between U - P b zircon and K-Ar biotite ages reflects the individual thermal histories of each pluton. Alternative explanations are that it is due to regional cooling and uplift of intermediate-level plutons, or the distal expression of younger plutonism to the south (as yet unidentified, or in the western Sveconorwegian Orogenic Belt, which would have been in close proximity at the time )~ FABLE 6 ( o m p a r i s o n of U - P b , K - A r and R b - S r geochronological data Sample No.

U-Pb zircon

K-Ar hornblende

K-Ar biotite

Rb-Sr biotite

U p p e r St Lewis River (east) m o n z o n i t e : CG84-195 966±3 9 4 6 ± 13 914+13 Riviere Bujeault h e a d w a t e r s quartz syenite: CG86-698 964+5 a 926.+_ 16 953+12 C G 8 6 - 6 9 7 B 964_+5 " 917-~:18

U p p e r St Lewis River (west) granite: CG86-700 956+ 1

8 8 8 ± 12 811± 9 822 ± 9

911 + 10

~' T h e age reported c o m b i n e s data from s a m p l e s C G 8 6 - 6 9 8 and ('G86-697B.

The preferred interpretation for the particularly anomalous 953+ 12 Ma age obtained from biotite of sample CG86-698 from the Rivibre Bujeault headwaters quartz syenite, is that it is due to the presence of excess ~*"Ar. By the same argument it is likely that biotite from sample CG86-697B, which yielded a K-Ar age of 927 + 18 Ma, also contains some inherited excess 4°Ar. The source of the excess 4~Ar is inferred to be a pre-Grenvillian component in the pluton, the existence of which is clearly demonstrated by U - P b zircon data. This reasoning is supported by two turther observations, namely: ( 1 ) the other two plutons in the area from which K - A t "normal" biotite dates have been obtained show no such pre-Grenvillian component on the basis of their U - P b data: and (2) a similar example of an anomalousl~ old K-Ar biotite age in a rock that does have a pre-Grenvillian component, was reported b? Loveridge ( 1986; sample BRA-76-182 ) from the Lac du Morhiban area, 300 km to the wesL For both that rock and the example discussed here, Rb-Sr biotite ages are younger and record disturbances to the Rb-Sr system up to about 150 million years after Grenvillian plutonism. Conclusions

U - P b zircon and titanite geochronological data indicate two periods of Grenvillian felsic magmatism in eastern Labrador. In addition to being characterized by different ages, these two periods of magmatism occur in different areas and have different styles. The older period occurred between ~ 1130 and 1000 Ma and was confined to areas northeast of the Mealy Mountains terrane boundary. The products of this magmatism are minor, sporadic, discordant intrusions that are typical of high structural levels. The younger period of magmatism has ages between 966 and 956 Ma and is only found southwest of the Mealy Mountains terrane boundary. In this region, the magmatism resulted in well-defined, interme-

333

(;RENVILLIAN MAGMATISM IN THE EASTERN GRENVILLE PROVINCE

diate-level plutons, the emplacement of which exerted a marked structural influence on the surrounding rocks. Combined geochronological and field evidence indicates that a major mylonite-generating transposition event in the southeast Gilbert River shear belt predated ~ 1130 Ma; it may have occurred during Labradorian orogenesis. Final transposition at the borders of the Lake Melville terrane had ceased by 1000 Ma. However, tectonism continued after this time (until ~ 970 Ma) in the northernmost eastern Grenville Province (Sch~irer et al., 1986) and plutonism was active in the Pinware terrane until ~950 Ma. Following emplacement of these younger granitoid rocks, Rb-Sr and K Ar mineral data document a thermal history extending for a further 150 million years. Utilizing a previously established correlation between Grenvillian plutons and positive magnetic anomalies (taken in conjunction with earlier mapping), the distribution of widespread Grenvillian magmatism is extrapolated across the southern half of the eastern Grenville Province; this, in turn, leads to the inference that there is a bipartite division between belts of Grenvillian thrusting (in the north) and Grenvillian magmatism (in the south) in this region. It is emphasized that this spatial division in no way denies the undoubted temporal, and probable tectonic, linkages between the two regions. We note that such a division also characterizes other parts of the Grenville Province and also the Sveconorwegian Orogenic Belt, but that time of widespread, posttectonic Grenvillian plutonism differs, being older in the southwest Grenville Province and younger in the Sveconorwegian Orogenic Belt.

Acknowledgements We extend thanks to D. Bellerive, F.B. Quigg and R.J.G. Seguin for K - A r measurements, and R.J. Th6riault for Rb-Sr analyses. We also thank the two journal reviewers, R.F. Emslie and E.G. Nisbet for their comments. This pa-

per is published with the permission of the Assistant Deputy Minister, Geological Survey Branch, Newfoundland Department of Mines and Energy.

Appendix I. Analytical techniques The separation of zircon and titanite from 20-kg or larger total-rock samples was accomplished using standard heavy liquid and magnetic separation techniques. The U - P b results presented in Table 2 represent a combination of data obtained at three institutions: Royal Ontario Museum (ROM), Geological Survey of Canada (GSC) and University of Quebec in Montreal (UQAM). Unless otherwise indicated, relevant information from each laboratory is presented in the order listed above. The dissolution of zircon and isolation of U and Pb from zircon in all laboratories generally follows the procedure of Krogh ( 1973 ) and an outline of modifications to this procedure is given by Parrish et al. ( 1987 ) and Scb~irer et al. (1988). The extraction of U and Pb from titanite and Kfeldspar follows closely the HBr method described by Corfu and Stott (1986). At the ROM and UQAM laboratories, U and Pb were loaded together as phosphates onto degassed, single rhenium filaments following the Si-gel technique (Cameron et al., 1969), whereas at the GSC laboratory U and Pb were analyzed by this technique but on separate beads. U and Pb isotopic data were acquired utilizing VG354, Finnigan MAT-261 and VG-Sector mass spectrometers, respectively; all data were corrected for fractionation based on results obtained for NBS standards. For example, Pb isotopic data obtained using a Faraday collector have been corrected using the following constants +0.1%/ainu, +0.09%/ainu, +0.09%/ainu, respectively. The results in Table 2 have also been corrected for the current analytical blanks (in picograms) forU (2, 2, I ) and Pb ( 10, 14, 10) measured in each laboratory. All error ellipses in Fig. 3 correspond to two-sigma errors, but most of these represent blanket errors with only the maximum errors cited here for the U / P b (0.5%, 0.4%, 0.5%) and 2°TPb/2°61'b (0.1%, 0.1%, 0.1%) ratios. The precision of some analyses, however, has been calculated by propagating all sources of uncertainty for an individual analysis, and in these cases the associated error can be visually evaluated by the size of the error ellipse shown in Fig. 3. The initial common Pb isotopic composition used to correct the litanite analysis (17.631; 15.554: 37.007) was determined on K-feldspar from the same sample and the uncertainlies in both the U / P b and the 2°~pb/2°6pbratios for this analysis are estimated to be 0.5%. Discordia lines and intercept ages were calculated using the regression program of Davis ( 1982 ) and errors associated with the intercept ages are quoted at two-sigma. For samples where all the data cluster near or on concordia.

334 an average of the 2°7pb/2°6pb ages is reported as the best estimate for the age of the emplacement, and error estimation is based on the measured range in 2°7pb/2°6pb ratios. The isotopic composition of U (137.88) and the decay constants used in this study for 238U ( 1.55125X l0 ~0 yr "-1), 235U (9.8485X 10 -myr- ]) and 4°K (0.581X t0 H~ and 4.962X 10 -~° yr - j ) are those determined by Jaffey et al. ( 1971 ; for uranium) and recommended by Steiger and J~iger (1977). The experimental procedures lor K-Ar dating at the GSC are described by Roddick and Souther (1987) with error evaluation similar to the method ofRoddick ( t 987 ). Analytical methods for Rb-Sr age measurement are described by Th6riault (1990). For the Rb-Sr biotite ages, an initial 878r/S6Sr of 0.7100_+ 0.0050 is assumed. Whole-rock major- and trace-element analyses were carried out on a portion of the same samples as that used for isotopic analysis. Major elements, V, Cr, Cu, Zn, Rb, Sr, Ba, and Pb were analyzed by atomic absorption spectrophotometry; Ga, Y, Zr, Nb, Mo, La, Ce, Th were determined by inductively coupled plasma emission spectroscopy; F was obtained by ion-selective electrode analysis using a digital ion-analyser; and U was determined by neutron activation analysis. Details of procedures are given by Wagenbauer et al. ( 1983 ).

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('.F.(I(~WER

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