Fluorite-bearing early and middle proterozoic granites, Okak Bay area, labrador: Geochronology, geochemistry and petrogenesis

Lithos, 28 (1992) 87-109 Elsevier Science Publishers B.V,, Amsterdam

87

Fluorite-bearing Early and Middle Prolerozoic g r an o tes, Okak Bay area, Labrador: Geochronology, geochemistry and petrogenesis t R.F. Emslie and W.D. Loveridge Geological SurvO' of Canada. 601 Booth Street° Ottawa. Ontario. Canada KIA OE8 (Received June 20, ! 991; revised and accepted January 3 i, 1992 )

LITHOS Emslie, R.F. and Lovendge, W.D., 1992. Fluorite-bearing Early and Middle Proterozoie graniles, Okak Bay area, Labrador: Geochronology, geochemistry and petrogenesis. Lithos, 28: 87-109. Three discrete age groups of relatively fractionated, fluorite-bearing, post-tectonic Proterozoic granitold rocks have been identified in the Okak Bay area of Labrador on the basis of U-Pb zircon geochronology. Two of the groups occur entirely within the Archean Nain Province; the third occurs largely witbAn t_he E=_r!yProrerez,,~ic Churchill Province (Torngat Orogen ). None of the three episodes ofgranitoid magmmism can be directly linked to orogenic acIivity in its immediate surroundings. Although all three granite groups are broadly similar, petrologically and chemically, to the fe!sic rocks of the anorogenic Elsonian { ~, 1450_ | 290 Ma) Nain P]utonic Suite (NPS), onty one ofthe dated units can be correlated confidently wRh Elsonian magmatism. The other two groups represent significantly older anorogenic or postorogenic magmatism. The oldest unit, the Wheeler Mountain granites of the Nain Province~ has yielded a U-Ph age of about 2135 (2134 + 3 ,~i a7 + 2 ) Ma. A major tectonothermal event of this age has not been previously identified in northern Labrador, and the origin ef the granites is uncertain. Three small intrusior,s on offsho.-e islands (White Bear. Saddle. Oping;viksuak ) in the Nain Province were intruded about 1'775 (1774_'2"~, 1776+~, 1774:~ ) Ma. These granites ma~ have been intruded past-kinematically into a stable Archean fordand representing a delayed effect of a preceding collisional event (Torngat Orogeny) between Nain and Churchill Province to the west. The youngest unit dated, at about 1318 ( 1319+2. 1316_+:~) Ma. is Umiakovik Lake batholith which crops out largely within the eastern Churchill Province, and is one of the largest granitoid intrusions of the NPS. The age confirms that it formed an integral pan of the Elsonian magmatic event, but both Umiakovik Lake batholith and nearby 1320 Ma Makhavinckh Lake p!uton are significantly older than some basic members of the NPS in the coasta! area to the southeast. Nd and Sr i~otopic data demonstrate that Archean prototitks formed significanl components of the soarce materiais for all three of these Proterozoic granitoid magmas. This source was supplemented by one or more Prolcrozoic episodes of lo~er crustal intrusion and underplating by juvenile mantle-derived magmas that occurred in the region.

In~mduetion T h i s m v e s l i g a t i o n p r e s e n t s new d a l a for three

aGeologicalSurveyofCanadaContributioe, No. 29091 Contribution No. 5 to IGCP Project --315 Correspondence to: P'. Emslie. Geological Survey of Canada, 601 Booth Steer, O~tawa, Ontario. Canada KI A 0EB.

groups of fluorite-bearing, postorogenic/anorogenie g r a n i t o i d i n t r u s i o n s in n o r t h e r n L a b r a d o r . T r a d i t i o n a l l y , most u n d c f o r m c d f l u o r i t e - b e a r i n g g r a n i t c s in ~,is 2tea h a v e been correlated wilh middie P r o t c r o z o i c E l s o n i a n ( a b o a t 1450 to 1290 M a ) a n o r o g e n i c m a g m a l i s m l h a t g e n e r a t e d the N a i n

Piumnic Suile ( N P S ). T h i s stud7 shows thal lwo o f the three g r a n i t o i d g r o u p s rcprcscnl previously an-

88

recognized early Proterozoic magmatic activity of similar character to Elsonian magmatism. The three groups of granites identified are: (i) Wheeler Mountain granites, (ii) offshore granites (White Bear, Saddle, Opingiviksuak islands) and (iii) the Umiakevik Lake batholith (Figs. 1,2 ). Clearly-defined examples of Proterozoic postorogenic granite magmatism have not been widely recognized in northern Labrador even though orogeny-related metamorphism and deformation of appropriate ages are known (for example, see Wardie et al., 1990). In the vicinity of Okak Bay several granitoid plutons intrude Archean rocks of the Nain Province and have physical characteristics consistent with postkinematic emplacement. These plutons are exposed on White Bear Island, Saddle Island and Opingiviksuak Island, and are potassic, high level, fluorite-bearing, small (25 kin: or less), massive granite intrusions wi~h strong magnetic signatures (Figs. 1,2). These had been considered to be of probable Archean age by Taylor (1979); he suggested, however, that tv,o of them (White Bear Island and Opingiviksuak Ishnd granites) might be as young as middle Proterozoic. Rb-Sr whole rock dating by Barton (1977) also suggested that Proterozoic ages were likely for the offshore granites, Small, fluorite-bearing granitic plutons in the vicinhy of Wheeler Mountain, north of Okak B a y , were inferred by Taylor (1979) to be part of the mid-Proterozoic Elsonian magmatic suite. Their proximity to the Elsonian Nain Piutoni~ Suite offered some support for that interpretation. The largest single, discrete fluorite-bearing granitoid studied is Umiakovik Lake batholith which outcrops west and southwest of the other groups, The batholith flanks the principal anorthosite massifofthe Nain Plutonic Suite (NPS) on the northwest (Fig. 1 ) and is one of several granitoid batholiths ascribed to Elsonian magmatism. Major rock units of monzodiorite, (fayalite)-pyroxene quartz monzonite and of biotite-hornblende granite have been identified in the northern part of the batholith (Emslie and Russell, 1988 ). This study was initiated primarily to test the conventional hypothesis, stated above, that all of the fl,aorite-bearing granitoids were related to the Elsonian magmatic event. Preliminary geochronological results indicated that such was not the case and provided incentive to characterize the granites in

R.F. EMSLIE AND W.D. LOVERIDGE

greater detail geochemically, mineraiogicaUy and isotopically.

Geology Granitoid rocks considered in this study occur in different geological settings. The Wheeler Mountain, White Bear Island, Saddle Island and Opingiviksuak Island granites are enclosed entirely within the Archean gneiss envelope of the Nain Province. Umiakovik Lake batholith lies chiefly within the Proterozoic Churchill (Rae) Province but its eastern margin intrudes rocks of the Nain Province. The Nain Province, a narrow strip along the northern and central Labrador coast (Fig. 1 ), in part contains some of the oldest rocks recognized in the Canadian Shield, 3800 to 3900 Ma (Bridgwater and Collerson, 1976; Schiotte et al., 1989; Collerson et al., 1991 ). It consists primarily of highly deformed quartzofeldspathic orthogneisses and lesser amounts of amphibolites and metasedimentary rocks. Similar gneisses occur within Churchill Province where they were strongly reworked during the early Proterozo,ic Torngat orogeny; they retain mesoscopic and isotopic vestiges of an Archean heritage (Ryan, 1990; Ryan et al., 1991 ). Archean gneisses and supracrustal rocks of the Nain Province were heavily intruded by Elsonian magmas under anorogenic conditions during the mid-Proterozoic (e.g. Emslie, 1980; Morse, 1982). Large parts of the western boundary and indeed some sections across the entire Nain Province have been virtually obliterated by Elsonian plutonism and are now occupied by rocks of the Nain igneous complex (Fig. 1 ). The magmatism designated as related to the Elsonian event by Stockwell (1964) was once regarded as orogenic but is now interpreted as primarily anorogenic in character. Although relatively few U-Pb zircon ages have been published for this region, an early study by Krogh and Davis (1973) suggested that Elsonian coastal plutonism may have been more than 100 m.y. younger than similar plutonic associations inland. Early P~'oterozoic (Aphebian) magmatism is exhibited in the basic lavas of the Mugford Group north of Okak Bay which, in a Rb-Sr whole rock study, yielded an age of 2321 + 210 Ma (STRbdecay constant of 1.42 × 10- t~yr- t, Barton 1975 ). Rocks of the metasedimentary Snyder Group (Fig. l )have

FLUORITE-BEARINGGRANITES, OKAK AREA, LABRADOR

~

Seal

89

Lu~ke G r o u p

[-p,D" D--~ P e r a Ik a lict • g r a n i t e pv - v o l c a n i c s (~

~'-

+o

NN b i - h b

granite +,

o

p x 1OI1 q t z m o n z o n i t e ,

.

om

o

= "~ c a: "~. mo

m+i)i?i

|eucotroctolite,

:,'i' +

'

trGctolite

FJowers B a y

.

.arp

.

:::i + +~

:;!

.-.::..

-',i

~G M:

;,~

++-

' ~

N A IN

'"--.

+

.:---:i( S ..

F

.

•-;:Yi:.-:"-."-". " " ' .... -:.-:+:i-.:i!::.:-:.:. •-::-....::....+. . . . . . ~ N

Km

.

.'.+ii

"

A r c he a n

50

.

,,

++ii:i.y I-.::::.:.N. ::+::::?:~.:iil--..-:.

Churchill Province Proterozoic and

J

, /

~ I~

ol ~northo,ite i.trusiva contacts

A

m

~.,.+,,

" :::i ::+:

monzocliorite ~leuconorite0 nor,i~e. anorthosite

_

: +++.' +

i

::::::::::::::::::::::

~

i::i~:i~::i~i

c

DAVIS INLET P

".::,,::::

.:..::::-:

::::iii~:

Lake

:!!:::: K

Kig|apait

MA

biakhavinekh

::.... .-'..-.-_:..-

MC

Michikamau

...:!::-:::::i~i.: i.

~HOPEDALr:

::""

_....+-..°" .,..., -,...,-:

...-.

N

Nain

N|

Newark

::-:i-------.-". .-..+.--..

NO

Notokwanon

S

Strange

u

U.',iakov+k

MG

Mugford Group

SG

Snyder

Island

-:.+-::::...-z~:~ . ,:,,,~:~

Lake

,

,~:+,,::

+,.-.+~.

..

..::::. .:...

~J . . . . . . . .

Group

Fig. 1. Regional geological setting. Principal intrusions associated with mid-Proterozoic E[sonian magmatism are identified. Mugford and Snyder Groups are Early Proterozoic supracrastal rocks in the Nain Province. Seal Lake Group is a middle Proterozoic supracrustai sequence in the Churchill (Rae) Province_ Heav~' rectangle encloses area of Fig. 2. a minimum age of 1804 +43 Ma based on Rb-Sr whole rock dating o f cross-cutting igneous Snyder breccia (Barton and Barton, 1975). The small offshore granite stocks investigated in

this study are mostly well-defined discordant intrusions commonly with well-defined positive aeromagnetic signatures (GSC Map 7452G; Fig. 2 ). Sireilar aeromagnetic anomalies occur offshore to the

90

R,F. EMSLIE AND W.D. LOVERIDGE

,

;-::

o~

,

•i

------

-.

EE88-096 W B

.'."

HiJ

::::,

" ..... ~..........~~~

t

~

.o

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iilii

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N

t

.

i!iE C 8 9 -

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.

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--,#l,i%~I

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====~==

' -','-",-';,-'A;" "v-, ,- - - ~,-, ---~"..-,,,,~ ~granitic

C

rocks

anorthositic

rocks

,,,

~

Proterozoic or r e w o r k e d Archean rocks (Rae Province)

[~

~rchean rocks Nain P r o v i n c e

(.-~

M

Mugford Group

~.)

prominent positive aeromagnetic

anomalies

Fig. 2. Geological sketch and isotopic sample location map of the area around Okak Bay. ll,'M - Wheeler Mountain, W B - White Bear Island, S - Saddle Island, O - Opingiviksuak Island, U L - Umi,kovik Lake batholith. Mugford Group is an early .r'roterozoic

supracrustal sequence.

south of the intrusions sampled and may signify the presence of related granites. Granitic- intrus;ons near and underlying Wheeler Mountain are similar in size to the offshore granite stocks but lack their well-defined aeromagnetic expression.

Wheeler Mountaingranite Where observed on and around Wheeler Mountain the granite is medium to coarse grained, pink to greyish pink, massive, locally rusty and deeply weathered. Small, interstitial, purple fluorite grains

are visible with a hand lens in biotite-rich outcrops. Hornblende is variable in amount and becomes equal to or even exceeds biotite in places.

Opingiviksuaklsl~ndgranite Approximately 12 km 2 of granitic rocks are exposed on Opingiviksuak island. Archean gneiss forms the wallrock and outcrops on the northern edge of the island. On the western margin of the island, a narrow (less than 100 m) strip of quartzbearing, fresh, coarse gabbro is exposed. Both of these units are intruded by granitic dykes, a light

FLUORITE-BEARINGGRANITES, OKAKAREA, LABRADOR

grey medium grained variety being common. Massive pinkish-grey, coarse grained biotite granite dominates the northern part of the island. The southern part of the island is underlain by large areas of medium grained, pale pink granite. Fluorite coatings are found locally on fracture surfaces and rare interstitial fluorite grains are present in the granite. Dark brown titanite is a prominent accessory mineral. Near the summit of the island some joint surfaces are distinctly reddened, the effect nat extending more than a few millimetres into the rock; some of the reddened surfaces are fluorite-coated, Near the Archean gneiss contact the granite is slightly more mafic and hornblende accompanies biotite in some assemblages. Weak sporadic fo~iation is present in the g,ranite within about 1O0 m of the gneiss contact,

Saddle lslandgranite Massive granitic rock underlies the entire island, It is typically medium grained, light grey biotite granite with accessory purple fluorite and deep brown to brownish-black titanite. Rare aplkic dykes up to 1.5 m thick cut the granite. Dark, amphibolitic Archean gneiss xenoliths are present throughout the granite; most are subangular and they have a restricted size range from a few cm up to about 30 cm. The xenoliths have sharply-defined margins an6 obvious interaction with the enclosing granite is ~fight.

White Bear Island granite The rock is a homogeneous, massive, pinkish-grey medium to coarse grained granite. Like the previous two granites described, this rock has abundant well-crystallized primary titanite and fluorite as accessory minerals,

Umiakovik Lake batholith This batholith and Mistastin batholith (Fig. 1 ) are the two major charnockite-granite batholiths produced by the Elsonian magmatic episode in central and northern Labrador. Many smaller intrusions with similar rock types intrude the anorthositic massifs of this region. A Rb-Sr whole rock isochron age of 1290 + 37 Ma, Sri = 0.7096 _+0.0013 (87Rb decay constant adjusted to 1.42 × 10- ~~yr- ~)

91

on rocks of the batholith has been published (Taylor, 1978). Two distinct major rock units have been identifled in the northeastern part of the batholith (Wheeler, 1960; Emslie and Russell, 1988) and sampled for geochronological studies; an older pyroxene-bearing (and in part fayalite-bearing)monzodiorite to quartz monzonite suite which shows considerable textural and some compositional vatiability is intruded by a younger, mainly homogeneous massive biotite-hornblende granite. Exposures of biotite-hornblende granite are characteristically light grey to pinkish grey whereas the charnockitic suite typically weathers deeply to a b,,ffto deep brown, often c~.~mbly surface. The charnockitic suite contains rocks that range from fine to medium grained monzodiorites to porp~yrkic and coarse grained equigranular pyroxene quartz monzonite. Fine to medium grained equigranular rocks are present at contacts with anorthositic rocks and with wallrock gneisses. Inhomogeneities in texture are present in many outcrops principally as variable grain sizes of both alkali feldspar phenocrysts and groundmass. At the eastern margin of the batholith, southeast of Umiakovik Lake, the dominant rock is medium grained diorite to monzodiorite with colour index averaging about 30. Small, rounded perthites, 0.5 to 1 cm across, comprise 5 to 20% of many rocks with a 2 to 3 mm matrix. These rounded perthites can be highly variable in amount even over a single outcrop. Pyroxene and opaque oxides are the common mafic minerals but traces of biotite and hornblende also may be present. In many places scattered dark plagioclase tablets up to 3 × 6 cm, but more cornmonly 2 to 3 cm long, become prominent and perthite ovoids become rare or absent. Passing westward from the contact, monzodiorite contains increasing amounts of perthite ovoids and quartz and is transitional into coarse grained pyroxene quartz monzonite. This transition is irregular and large patchy areas of fine to medium grained monzodiorite are present up several kilometers into the interior of the batholith.

Petrography, mineralogy, geochemistry Whole rock chemical analyses, norms and estimated mineral modes fi.w ~epresemative granitoid

EC87-119

monzodiorite 59.50 1.48 1 !.80 1.70 13.10 0. ! 9 0.53 5.89 2.50 1.99 0.44 0.50 0.20 0,07 99.89

16 5 17 24 17 220 24 2020 343 14 737 5q 43 98 65 15 4.0 14 ! 2.0 5.9 1.3 4.9

Cr Ni Co V Cu Zn Rb Ba Sr Nb Zr Y La Ce Nd Sm Eu Gd Dy Yb Be (La/Yb)N

8 2 6 10 3 39 149 726 108 il 354 37 84 170 71 10 1.3 8.2 6.8 3.1 1.6 18.3

granite 73.20 0.31 12.90 0.50 2.00 0.03 0.27 1.24 2.90 5.39 0.07 0.40 0.20 0.00 99.41

Umiakovik Lake

EC87-86

Rock type SiO2 TiOz A1203 Fe20.~ FeO MnO MgO CaO Na~O K20 P2Os H:O CO2 S Total

Samplename locality

Whole rock chemistry ofgranitoid samples

TABLE 1

4 3 2 0 3 12 173 332 95 49 172 26 41 97 33 6.1 1.2 3.8 4.0 4.1 8.1 6.2

granite 76.90 0.13 12.80 0.00 0.80 0.01 0.16 0.72 3.80 4.46 0.03 0.30 0.10 0.00 100.21

Saddle Island

EC87-143B

13 3 9 19 6 83 135 1834 392 38 634 85 160 340 160 24 5.3 18 i 5.0 8.1 6.0 12.2

granite 69.70 0.63 14.30 1.90 1.90 0.09 0.69 2.36 3.50 4,38 0.22 0.50 0~00 0.03 100.20

EC87-143C

EC87-141

granite 71.5:0 0.46 14.20 1.60 1.30 0.08 0.58 1.45 3.90 4.40 0.14 0.80 0.10 0.00 100.51

18 10 18 56 iO 100 84 |441 697 30 501 57 140 300 130 22 4.8 14 10.0 5.1 3.6 17.0

monzonite 61.90 1.20 15.80 3.|0 3.60 0. I ! i.58 4.23 4.40 2.84 0.46 0.60 0.10 0.06 99.98

Opingiviksiak Island

EC87-140A

Trace Elementsppm 7 10 3 2 3 8 0 20 4 4 38 !10 247 124 022 i402 126 286 92 65 185 452 44 44 74 120 150 250 56 92 10 15 1.6 2.7 6.2 8.9 6. ! 7.5 6.0 4.6 8.0 4.8 7.6 16.2

granite 73.80 0.12 13.80 0.40 0.70 0.08 0.19 0.88 3.90 4,78 0,02 0.50 0.10 0.00 99.27

EC87-139

12 4 !1 31 5 85 i!8 1569 335 55 517 59 120 270 120 20 3.6 12 10.0 5.8 4.6 12.8

granite 68.90 0.80 14.00 1.50 2.50 O. ~0 0.89 2.38 3.90 4.31 0.24 0.50 0.00 0.03 100.05

EC87-142

9 0 7 18 17 85 140 t4~ 300 46 540 61 130 270 120 20 3,9 14 !2 7,0 4,8 I 1.5

granite 7|.80 0.65 12.40 1.60 Z50 0.09 0.78 2.02 3.30 3.95 0.18 0.30 0.10 0~05 99,72

EE88-096 White Bear

EC89-357

6 0 i 0 27 I!0 130 iiOO 150 18 360 15 190 360 II0 I! 2.6 5,4 2.8 !,4 !.7 84.1

granite 73.70 0.21 12.90 0.40 1.80 0.04 0.21 0.86 3.70 5.14 0,02 0.30 0.20 0.00 99.48

3 0 0 0 1 41 130 850 i20 18 360 16 170 330 100 11 2,4 5.9 3.2 1.6 i,5 65.8

granite 76.00 0.20 12.20 0.80 1.00 0.04 0.19 0.04 3.50 4.86 0.03 0.30 0.10 0.00 100.06

Wheeler Mountain

EC89-354

g

m

t,o

15 50 p I0 IS p P 3 = = . .

18.3q O,00 !190 21.,14 15.23 0,69 t).:~3 1.01 15.78 0.00 0.00 2.a9 2.84 1.03

Q C or ab an di he ca fs Ib lh mt il ap

8

. .

monzod~orite

EC87-119

. .

31.92 0.21 3.~,,7 ~ 24.83 5.76 0,00 O.O0 0.68 2,84 0.00 0.00 0,73 0,60 0.16

25 43 20 5 7 . . p P P p

gran~te

Umiakovik Lake

EC87-86

Quartz Kspar Piagioclasc Amphibole Biotite Olivine Pyroxenc Apatite Zircon O|mqucs Timnite nuorite Muscovite Garnet p = prescm

Rock type

Sample nanle locality

.

. .

.

35.50 0,48 26.43 32.21 3,38 0,00 0.00 0.4.0 i.28 0.00 0,00 0.00 0.25 0,07

p P P p p

33 33 32 . 2

gra~ite

.

.

.

Saddle Island

EC87-143B

Estimated modes and norms ofgranitoid samples

TABLE I (cont'd)

granite

EC87-139

EC87-141

granite

26,71 0.04 25.99 29.71 10.30 0.00 0.00 1,72 1,05 0.00 0.00 2,76 1.20 0.51

CIPW Norlns ,8,,,9 30,69 "~ "* 0.66 0.72 28.66 26,13 33.44 33.~3 4.29 6. ~0 0,(10 11330 0.00 0,00 0.48 1,45 0,92 (I.46 0.00 0.00 0.00 0.00 O. 59 2.33 0.23 ,',),88 0.05 0.33

1504 0,00 16.93 37,52 i 5,09 i.64 0,82 3.21 1.85 0.00 0.00 4.53 2,30 1.08

.

monzonite

Opingiviksiak Island

EC87-140A

Estimated modes oi'samples ( volume % ) 30 33 30 7 33 30 32 35 25 26 30 45 . . . I ') I 6 I0 . . . . . . . . . . p p p p P i: P P P l' P 2 p 2 3 p p p p 10 . . . p . . .

granite

EC87-143C

. .

.

.

23,55 O,OO ,~5 ,6 ,.~' 33,16 7.99 I,Ot 0.88 !.76 !,76 0.00 0,00 2,18 1.51] 0.56

p P P 2 =

28 35 27 p 8 .

granite

EC87-142

.

.

.

31.97 0.00 23,54 28,12 7,40 0,61 0,65 1.67 2,04 0.00 0.00 2.34 1.24 0,42

. p P P 2 p

~2 33 25 = 8

t~ranitc

.

.

EE88-OOv White Bear

EC89-357

,,9,6,~ ~ "~ O.~) 30.72 31,63 3.43 0.12 0,53 0.47 2,45 0.00 0,00 0.59 0.40 0,05

p P P p

32 35 28 ! 4

granite

34.89 0.00 28.84 29.71 3.22 0.24 0,40 0,36 0,71 0.00 0,00 I, 16 0.38 O07

p P P = p

30 45 20 3 2

granite

Wheeler Mountain

EC~9-354

{=.

0

0

t~

o

94

R.E EMSLIE AND W.D. LOVERIDGE

TABLE 2

Microprobe analyses ofbiotites, partial analyses of titanite and projected compositions of iimenites Sample name Locality

EC87-86

EC87-1 i 9

Rock type

Oping.

White Bear

EC87-143C

EC87-142

EE88-096

EC89-354

EC89-357

Oping. EC87-142

titanite Umiakovik lake

SiO2 TiO2 Al203 FeO* MnO MgO CaO BaO Na~O K20 P:O~ (H20) F CI O= F O = CI

Saddle

monzodiorite 33.99 3.80 t3.60 28.55 0.06 5.65 0.06 0.28 0.07 9.58

Offshore granites

Wheeler Mountain

granite

granite

granite

granite

granite

granite

granite

33.89 2.84 12.32 32.38 0.23 4.34 0.06 O. ! 3 0.07 8.56

38.68 1.25 12.91 18.32 1.02 ! 2.93 0.00 0.25 0.04 i 0.07

36.78 !. 18 14.08 23.90 1.07 8.37 0.06 0.07 0.64 9.70

38.27 2. ! 7 12.72 i 7.88 1.03 12.27 0.0 ! O. i 9 0.08 9.87

35.77 2.42 13.47 26.72 0.75 5.69 0.06 0.20 0.04 9.71

35.4 ! 2.14 15.13 28.08 OA3 4.38 0.01 0.36 0.08 9.83

Total

3.07 i.35 0.01 0.57 0.00 i 00.65

3,04 I. ! 8 0.20 0.5 0.05 99.79

2.83 2.23 0.09 0.94 0.02 101.57

3. !~; i.2 0.74 O.'.i ! 0.05 100.43

2.94 1.99 0.09 0.84 0.02 i 00.37

2.69 !.71 0.87 0.72 0.20 i 01.02

3.02 1.2 0.56 0.5 i 0. i 3 i 0 !.23

30036 30.3 I 3.53 2. ?8 0.23 0.08 27.54 0.07 0.01 0.07 1.75 0.01 0.74 0.00 96.00

Si IV AI IV Ti IV T Site AI Vl Ti V! Fe + 2 Mn Mg O site Ba Ca Na K A site O OH F CI

5.48 2.52 8.00 0.07 0.46 3.85 0.01 1.36 5,74 0.02 0.01 0.02 !.97 2.02 20.00 3.3 ! 0.69 0.00

5.60 2.40 0.0 ! 8.00 0.00 0.35 4.47 0,03 i.07 5.92 0.01 0,01 0.02 1.80 1.84 20.00 3.33 0.62 0.06

5.91 2.09 . 8.00 0.23 0.14 2.34 0,13 2.94 5.79 0.01 0.00 0.01 1.96 1.99 20.00 2.90 1.08 0.02

5.78 2.22

5.75 2.25

5.0~ 234

4.19 -

8.00 0.39 0.14 3. ! 4 0.14 !.96 5.78 0.00 0.0 ! 0.0 i 1.95 1.97 20.00 3.34 0.60 0.06

5.89 2. I I . 8.00 0.20 0.25 230 0.13 2.82 5.71 0.01 0.00 0.02 !.94 1.98 20.00 3.01 0.97 0.02

8.00 0.30 0.29 3, 59 O, lO !,36 5.66 0.01 0.01 0.01 i.99 2.03 20.00 2.89 0.87 0.24

8.00 0.51 0,26 3.75 0.06 1.04 5.62 0.02 0.00 0.03 2.00 2.05 20.00 3.24 0.61 0.15

0.57 3.15 0.32 0.03 0.02 4.07 0.02 0.00 20.00 0.76 0.00

Fe/( Fe + Mg)

0.74

0.81

0.44

0.62

0.45

0.72

0.78

-

'llm' 'Hem'

0.948 0.052

0.956 0.044

0.928 0,072

0.940 0.060

0.943 0.057

0.953 0.047

0.957 0.043

-

.

.

.

.

rocks are listed in Table 1. Detailed petrographic notes for specimens dated are given in the Appendix. Petrographic characteristics of the rock units are summarized below, Whole rock chemical analyses were performed by staff of the analytical chemical laboratories of the Geological Survey of Canada. Major elements were determined by wavelength dispersive XRF tech-

.

.

.

.

.

.

.

niques supplemented by ICP-ES and rapid wet chemical methods. Trace element analysis was done by ICP-ES on solutions prepared from I g of sample dissolved with acid and fusion procedures, then diluted. REE were determined by ICP-ES and ICPMS on solutions concentrated using ion exchange resin. Mineral analyses were carried out on a Cameca SX-50 instrument using wavelength disper-

95

FLUORITE-BEARING GRANITES, O K A K AREA, LABRADOR

sive techniques with 10 to 30 second counting times, Electron beam conditions of 15 kv, with sample currents 10 to 30 nanoamperes, were appropriate for the minerals analysed using a variety of natural and synthetic standards,

Petrography The Wheeler Mountain granites contain microcline microperthite with subordinate weakly normally-zoned plagioelase. Large strained and partly polygonalized quartz grains are present as well as small drop-like quartz inclusions in microperthite, Deep brown biotite is the chief marie mineral; it is locally strongly chloritized. Deep brownish-green hornblende is present in minor quantities. Large zircon grains are abundant and optically visible cores were observed in several large grains. Yellowishbrown a!lanite is abundant locally and fluorite, aparite and opaque minerals are sparsely disseminated, The offshore granites are medium to coarse grained, pink, pink-grey andgrey in co!our. Microcline micmper~hite, normally-zoned plagioclase and large quartz grains, locally highly strained, are the major minerals. Pale to medium olive-green biotite is the chief mafic silicate; small amounts of bluishgreen to olive-green hornblende are present locally. Coarse, well-crystallized, primary titanite is abundant in all samples. Prismatic zircon, apatite, fluorire and opaque oxides (dominantly magnetite) are common accessory minerals. One sample of Opingiviksuak Island granite (EC87°139) contains accessory muscovite and ~rne:, (Table 1 ). a range of Umiakovik Lake bathoiitL1 c,.,,~,alns "-" granitoid rock types including medium grained quartz monzodiorite, coarse grained pyroxene-fayalite quartz monzonite and biotite-hornblende granite. Relatively marie quartz monzodiorite conrains fayalite and ferroaugite as common marie silicates together with traces of brown hornblende. Sub-equant to rounded quartz grains, plagio,clase and rare large perthite grains constitute the felsic minerals. Apatite, large prismatic zircon and opaque oxides are the chief accessory minerals. Quartz monzodiorite grades into pyroxene-(fayalite) quartz monzonite with increasing amounts of perthite and quartz; deep greenish-brown hornblende and biotite become increasingly dominant over pyroxene and olivine. Zircon, apatite, and opaque oxides remain as typical accessory minerals. Coarse

grained biotite- hornblende granite that intrudes pyroxene quartz monzonite contains very deep brown biotite and dark olive to tan hornblende as its sole marie silicates. Large prismatic zircon, apatire, rare allanite and sparsely disseminated opaque oxides are typical accessory minerals in the granite; disseminated fluorite is sporadic in occurrence.

Mineral chemistry Granite samples from the offshore islands all contain abundant coarse, primary titanite and have mor~ magnesian biotites than granites of the other, two groups. Only one granite sample, from Oping!lviksuak Island, contains large flakes of primarry muscovite (EC87-139, Table 1 ). Microprobe on, Iyses of biotites from all dated samples are listed i,a Table 2 and plotted in Fig. 3. Biotites from the off:. shore island granites have consistently lowec Fe~ ( F e + Mg), AI and Ti and tend to have higher MI~ and F, than the other two groups. Biotites from annite

~.o ~ - - - , - - - - ~ .

•,

.

u.

0.6

,

% ~ ~o o & ® 0

oa ~, ~ o ~

sideroDhyllite

•

o

a

o4 m ® , o

o.2 o.o . . . . 2o 22 oh=ogopito

.

. 2.4

Mfshore g r a n i t e s U~=ako~,k W,ee~e, urn. " blotites, other Nain comDlex granitoids ' 2.6 2.8 ,.o eastoaite

AI ~v Fig. 3 Biotite compositions in the granite samples. Biotites shown as associated with other granitoid occurrences of the Nain Plutonic Suite it.elude localities at Tunungayualok Island, Voisey Bay, Zoar, and Ukpaume and are plotted for comparison. Note that Ai'~ remains relatively high in Nain complex granitoids but is significantly lower in Wheeler Mountain and offshore granites as is Fe/(i:c+Mg). Slrucrural formulae are calculated on 24(O,OH,F,Ci), OH = 4.0- (F + Ci ) and all Fe is taken as Fe: +.

R.F.EMSLIE ANDW.DLOVERIDGE .

96

Wheeler Mountain granites are notably more Cl-enriched than the others. llmenite compositions from the same samples are expressed as projected end members "llm" and

O /A~

/"

"Hem" (Lindsleyet al., 1990) and listed in Table

2. Ilmenite is very rare in the offshore island granites and, where present, is notably enriched in Mn (from 8.6 wt.% MnO in EC87-142 to 24.0 wt.% MnO in EC87-143c). Ilmenites in Wheeler Mountain granites are also enriched in MnO but less so (6.0 to 7.5 wt.% MnO). Slightly but consistently higher "Hem" levels in ilmenites of offshore island granites are in accord with their more oxidized, magnetite-rich assemblages. A partial analysis of titanite from the granite on Opingiviksuak Island is also given in Table 2; the missing components are mainly light rare earth elements which were not determined. Perhaps the most notable feature of the composition is the high F content, in accord with high F observed in biotite and the presence of modal fluorite. Microprobe partial analyses (unpublished) profiling a large apatite grain in White Bear Island granite EE88-096 revealed a thick outer shell enriched by a factor of about 3 in LREE over a welldefined core. The core of a large zircon in Wheeler Mountain granite EC89-354 is enriched in LREE by a factor of 3 to 4 relative to the rim. Zircon cores and rims in EC87-119 from Umiakovik Lake batholith on the other hand, have essentially constant LREE compositions with no evidence of zoning, These results, although incomplete, have significant implications bearing on the origins of the respective magmas. The apatite LREE zoning in EE88-096 clearly indicates the presence of a distinct core, either inherited from source materials, relict from a much earlier stage of fractionation, or from hybridization processes. The REE data for Wheeler Mountain EC89-354 zircon also supports the presence of cores; however, in this case the indication is that the zircon core came from an already very highly fractionated source. Lack of evidence for distinct cores in apatite and zircon from Umiakovik Lake batholith is in accord with high magma tempera,ires indicated by H20-poor, pyroxene-bearing, early members of that complex,

Whole rock chemistry Excluding monzodiorite from Umiakovik Lake batholith and monzonite from Opingiviksuak Is-

~A

•

"~

Umaakov~k

* Wheolor urn.

m.A ~ l ~ h o r a

gran'tas

/,, .i~_ ,~ Ab

Or ® m j ®

Ab

.

.

.

.

.

Or

Fig. 4. Wholerockcompositionsplottedon normativeQ-abor and an-ab-or projections. Offshore granites includesWhite Bear Island,Saddle Island, and OpingiviksuakIslandgranite ~amples.

land, the remainder of the samples have granite compositions (Table 1, Fig. 4). The major element chemistry of the granites for the most part shows only small but nonetheless distinctive variations. Both Umiakovik Lake and Wheeler Mountain granites are, for the most part, less aluminous, lower in Fe3+/Fe 3+ +Fe 2+, TiO2, MgO, CaO and Na20, and higher in K20 than the offshore granites. The trace element chemistry also has several distinctive aspects. Nb, for example, is consistently higher in the offshore granites by a factor oftwo to four than in either Umiakovik or Wheeler Mountain granites. Sr, Zr, Y and Yb on average are lower in Umiakovik and Wheeler Mountain granites than in the offshore granites. Chondrite-normalized REE plots (Fig. 5a,b) show small negative Eu anomalies in all samples except those of Wheeler Mountain granites which have no such anomalies. The Wheeler Mounrain granites also display steeper negative slopes than those of the other granitoid groups [ (La/Yb)N is 66 to 84 fox Wheeler Mountain samples cornpared to 5 to 18 for the other two granite groups, Table l].

97

FLUORITE-BEARING GRANITES, OKAK AREA, LABRADOR

1000

=

- ~

_. ~-

®

,

,

,

;

,

"i

,

:

,

i

!

T--'--'I-'---I~=

W h e e l e r Mtn. and Umiakovik L. g r a n i t e s

1 ~ . . . _---~--~ ~

-

=

"E 100 0

~

0 ¢) 10

o

- -

W n

EC-89-354

--

W g

EC89-357

--

U @ EC87-119 U o

1

1000

/

~

'

EC87-86

a

!

I

|

I

|

I

I

I

t

i

Ce

Pr

Nd

Pm

Sm

Eu

Gd

T~

Dy

Ho

r'

,

,

l

~

t

_

f

_

~

l

,

i

,

-

_

1__.~.

I

La

Er

1 ~.

Tm

f ....

i

L__.

Yb

Lu

-

r

-

~

White B e a r Is., S a d d l e Is. and

-= S

®

•E

100

_----

C c-

rO

°

J~ o

O nr"

10

1

I

=__ ~

S

a

EC87-143C

O

•

EC87-141

&

EE88-096

O

a

EC87-142

O

~

EC87-140A

0

o

EC87-139

WB

i

:

La

Ce

J ~ l

Pr

Nd

"-'=--

b I

~

l

I

Pm

Sm

Eu

Gd

~ _ _ L

Tb

Dy

i

t

Ho

Er

J___

Tm

I

Yb

-

l _ _

Lu

Fig. 5. Chondrite-normalized (Evensen et al., 1978~ REE patternsfor rocks investigated. (a) WheelerMountain (W) and Umiakovik Lake ( U} samples; (b) samples of the offshore granites, White Bear ( WBL Saddle (S) and Opingiviksuak (O).

The offshore granites are for the most part higher in incompatible elements such as I ~ , Y and Be than Umiakovik or Wheeler Mtn granites (Table 1 ). This suggests that source materials for the offshore granires contained a relatively undepleted or enriched component not presen, in the source for the V q'~:c!er Mountain granite. Much greater degrees of ,Tdfial melting represented by Umiakovik granitoid magmas would have tended to mask the presence of a sma|! undep!eted or enriched component,

Geochronology

Analytical,echniques Zircon results presented in this report were meas~ red in 1988-1991 using methods described by Pa~Ssh et alo (1987). Zircon from EC87-86 and EC8'i. 119 (Umiakovik Lake batholi,~h ) wa~ analyzed ~: 1988; other samples were analyzed in 1989199| u~ing a more sensitive technique for mass spectrometry of uranium. This technique, which provides a factor of ten increase in sensitivity over

98

the previous double filament technique, entails loading U as a phosphate on a single degassed rhenium filament using the silica gel-phosphoric acid method, and measuring the U O f spectrum. A 23~U235U ( 1 : 1 ) tracer is used, permitting correction for mass fractionation. Ion beams are collected in the static mode with separate collectors positioned to receive the three masses 265,267 and 270 (23~UO2, 23sUO2 and 23sUO2). Corrections are made for isobaric interference due to ~70 and ~sO. The method of Davis ( 1982 ) is used for linear regression of U Pb data sets and for calculating probabilities of fit of regressed data. Procedures for K-Ar dating are described by Roddick and Souther ( 1987 ) with error evaluation similar to the method of Roddick ( 1987 ). Methods for Rb-Sr and Sm-Nd isotopic investigations are presented by Theriault (1990).

Results U-Pb zircon analytical data are listed in Table 3 and displayed in Figs. 6 (a) to 6 (f). K-At analytical data for hornblendes and biotJtes investigated are given in Table 4. U-Pb zircon and K-Ar biotite and hornblende dates determined in this investigation are summarized in Table 5.

Wheeler Mountaingranites EC89-354: Zircon crystals are euhedral, short (length/breadth is 1 : 1 to 1:3), colourless to dark pink with variaJle amounts of bubble and other inclusions. Most grains were unbroken and no cores or zoning were noted. The clearest, most colourless and inclusion-free grains were selected for analysis and strongly abraded, Three fractions were analyzed (Fig. 6a) yielding a collinear set of data points (probability of fit 72.0%), one of which is only 0.6% discordant. Calculated intercept ages are 2137 + 2 and 1048 + 5 Ma. U contents are low, ranging from 28 to 47 ppm with the highest U fractions being most Oiscordant. The upper intercept age, 2137 + 2 Ma, is interpreted as the age of emplacement of the granite at Wheeler Mountain based on the simple zircon morphology and uncomplicated U-Pb systematics, EC89-357: Zircon crystals are euhedral and range in habit from equant to elongate (4.1), and from colourless and clear to pink and cloudy. Most grains

R.F. EMSLIE AND W.D. LOVERIDGE

were unbroken, inclusions were relatively few, and mostly bubble-shaped. No cores or zoning were noted during hand picking but rare large cores were visible in thin section. Three fractions were selected for analysis (Fig. 6b), all were strongly abraded: (2a) the most clear and coiourless, largest ( + 1 4 9 pro), elongate, euhedral or broken grains with the fewest inclusions; (2b) the most clear and coloudess bipyramidal equant grains with the fewest inclusions; (2c) sireliar to 2a but smaller ( - 149 + 74 pm size range). The ti~ree fractions yielded a collinear set of data points with point 2b being concordant, and 2a and 2c less than 1% discordant. Calculated upper and lower intercept ages are 2133 + 4 / - 1 Ma and - 244 Ma. The lower intercept age is poorly defined due to clustering of the U.-Pb data points near concordia. In contrast to the results for EC89-354 discussed above, a 0 Ma lower intercept age provides a much better fit (86.6% probability) to the data than does 1048 Ma (4.8% probability). When the lower intercept of the chord through the three data points is tied to the origin (0 Ma), the calculated upper intercept age changes slightly to 2134 +3/-1 Ma. This is interpreted as the age of crystallization of granite sample EC89-357 and is in agreement with the result f,~r EC89-354 within analytical uncertainty. Measured U contents in the EC89-357 zircon fractions range from 44 to 72 ppm, about 50% higher than in zircon from EC89-354. Nonetheless, EC89357 zircon shows only very minor, recent Pb loss compared with up to 29% Pb loss from EC89-354 zircon at 1048 Ma. K-Ar ages on biotite from EC89357 and EC89-354 are 1671 + 23 and i 823 + 1~ !Via respectively (Tables 4,5). These results give no support for a significant metamorphic event at about 1950 Ma; therefore the 1048 Ma lower intercept age for EC89-354 zircon is interpreted as a time of Pb loss, associated with accumulated radiation damage within the zircon, triggered by minor environmental factors. Due to the simple, almost undisturbed U-Pb systematics of EC89-357 zircon, the cores seen in thin section ~re assumed to be ofessential!y the same age as the overgrowths. This is probably also the case for EC89-354 zircon, since conditions sufficiently intense to initiate new zircon growth ai 1048 Ma would almost certainly have reset the K-Ar biotite system.

99

FLUORITE-BEARING GRANITES. OKAK AREA. LABRADOR TABLE 3 U-Pb isotopic data for zircon Fraction size

Wt. mg

U ppm

Pb* ppm

206Pb 204Pb

Pbc Pg

208Pb %

206Pb 238U

207Pb 235U

207Pb 206Pb

Apl}arew~t Age (Ma) 206/238 207/206

EC89-354 Wheeler Mountain la+105NI 0.090 28 Ib+i05N! 0.084 41 !c-105+74N1 ,,.0-,9 47

13 19 lg

5913 2032 3642

I0 39 13

16.5 16.2 14.7

0.3917_+0.09% 0.3902±0.09% 0.3311_+0.09%

7.170+0.10% 7.133±0.11% 5.659±0.10%

0,13275~0,03%

0.13260±0.05% 0.12398+0.04%

2130.9 2123,6 1843.6

2134.7±1.1 2132.7±1.7 2014.2+!.3

EC89-357 Wheeler Mountain 2a+ 149N2 0.058 44 2b-lO5+74N2 0.020 65 2c-149+74N2 0.037 72

20 29 32

1442 1240 3972

41 25 16

13.2 1~.7 15.0

0.3914±0.09% 0.3923_+0.10% 0.3902+0.09%

7.161 +0.12% 7.174_+0.13% 7.138+0.10%

0.13268±0,06% 0.13264±0.06% 0.13269+0°03%

2129.4 2133.3 2123.5

2133.8+2.2 2133.24_'2.2 2133,9+ !.!

26 36 15 43

9 13 6 16

257 3216 839 740

26 31 31 26

21.2 18.2 tO.0 ~9.6

0.3046_+0.59% 0.3161±0.10% 0.3159+0.24% 0.3128_+0.23%

4.565+0.68% 4.728+_0.12% 4.723+0.28% 4.674+0.24%

0.10871±0.30% 0.10849-+0°04% 0.10845_+0.11% 0.10837_+0.16%

1714.0 1770.6 1769.7 1754.3

1"/77.9±11.1 1774.2+1.5 1773.5_+4.0 1772.2_+5.9

FE88-096 W~ite Bear Island 4a+105N! 0.015 73 4b-!05+74N! 0.062 40 4c-74N1 0.028 32

25 15 12

869 5326 862

24 9 20

13.1 17.,t I8.2

0.3116.+_0.10% 4.648_+0.14% 0.3161_+.0.09% 4.730_+0.10% 0.3135±0.12% 4.680+0.15%

OI0818_+0.08% 1 7 4 8 . 5 0.10851 _+0.04% 1 7 7 0 . 9 0.10827+0.08% 1 7 5 7 . 9

1769.0+3.0 1774.5+ 1.4 1770,5+3.1

14348 25 4150 93 1663 54 J1166 25 6525 35 4488 21

13.3 150 14.5 14.6 12.5 12.6

0.3162+__0.08% 0.3151+0.08% 0.3129_+0.09% 0.3083_*0.08% 0.3!71 __.0.08% 0.3!75_+0.09%

4.738_+0.10% 4.708__.0.10% 4.665 ÷,0.12% 4.604-+0.10% 4.746_+0.10% 4.750_+0.10%

0.10867±0.03% 0.10838_+0.04% 0.10812_+_0.06% 0.10820+__0.03% 0.10854-+0.03% 0.10851 -+0.03%

1771.2 1765.6 1755.2 1732.5 1775.7 !777.4

1777.3+ 1.0 1772.3_+ i,3 1768.1-+2.2 1770.8_+1.1 1775.0-+!.i 1774.5+!.!

EC87-143c Saddle Island 3a-149+!O5NO 0.013 3n-149+105 0.146 NO 0.089 3c-105+74N0 0.022 3 d - 105+74 N0

EC87-142 Opingiviksuak Island 5a+ 149N0 0_145 129 5b-149+105 0.199 104 NO 0.042 116 5c-105+74N0 0.125 119 5d-74+62N0 0.081 140 5e+149N0 0.036 135 5 f - 149+ !05 NO

45 37 40 4I 49 47

EC87-86 Umiakovik Batholith 6a+149N0 0.102 30 6b+149N0 0.053 26 6 c - 1 4 9 + 1 0 5 N 0 0.066 30 6d-105+74NO 0.0"~.~ 77

9 7 8 ~

!151 785 938 ~'~a

38 25 3~ "~.~

24.2 24.3 23.9 "~,~o

0.2271~-0.11% 0.2270-+0.13% 0 .?'~7 . . . . ~÷r~' . u.~7% ,'~'~'~',-,-n 1 ~

2.668*__0.14% 0.08523±0.07% 2.663-+0.15% 0.08510 ~ 0.07% 2.668+0.19% 0.08517±0.08% ~¢'7+,~, 0.08519*0g~

1319.1 1318.5 1319.8 131' '

1320.8-+2.8 _..7 1317.7+'~ 1319.3+~.0 -i,~, ) 1 9 . or -I- ..t..~""~

EC~7-119 Umiakovic Batholith 7a-149+lO5NO 0.063 42 7b-105N0 0.015 48 7c+105N! 0.046 54

11 12 14

1863 616 1859

20 17 19

16.1 ; 6.4 ~7.1

9.2255+0.10% 0.2258-+0.~6% 0.2257+_0.09%

2.642_+0.11% ,..6,~0,0..0~o "~ " + "~ ~' 2.645-+0.11%

1310.7 131~,3 1311.7

1315.4'__.11.9 1311.3+3.5 1315.9_+i.7

0.08500+0.05% 0.08481+0.09% 0.08502+0.04%

All zircon fractions were abraded: size fractions are 4isled in m~: N = nonmagnetic at given tilt a~g~e: Fb*= radiogenic lead; :°~Pb/"°~Pb, spike subtracted: Pbc=common lead: errors are one s:andard error of ~he n~ean in % excepl 207Pb/206P~ age errors ~hich are two standard errors h, Ma.

Offshore ::ranites EC87.143c. Saddle Island graniw: Zircon crystals are colourless to pale ~,ellow, euhedrat, generally elongate (length/breadth ratio of most cD'stals is 3" I to 5" 1 ), with abundant inclusions. Most grains i n t h e - 149 +105 micron size range are broken but many in the smaller size fractions are intact. Euhedral zoning is present in some grains; no cores were seen. All fractions were slronglyabraded.

The U contents of these zircon fractions, 15 to a3 ppm, are among the lowest measured in this study. It is not surprising that two ofthe four data points overlap coneordia within analytical uncertainty: the 2,.pb/:O~,Pbages of these fractions are 1774.2__+ 1.5 and 1773.5 ± 4.0 Ma. A chord through the four points yie{ds intercept ages 17741~ Ma and - 7 7 Ma. The upper intercept age does not change when the cho~d is forced through the origin (0 Ma ). The euhedral zoning and simple morphology

100

R.F. EMSLIE AND W.D. LOVERIDGE

TABLE 4 K-Ar age determinations, biotite and hornblende Sample

Mineral

K, wt% + 1 s.c.

Rad. 4°At cm~/g ( × l0 -~)

% A1mos. ~"Af

Age ± 2 s.c. Ma

WM EC89-354 WM EC89-357 S EC87-143c WB EE88-096 O EC87-142 U EC87-.119 U EC87-I i9

biotite biotite biotite biotite biotite hornblende biotile

5.55 + 0.41 7.44 + 0.89 7.82+ i.08 7.83 + 0.35 7.77+0.55 0.547+0.72 7.01 +0.82

6802 7954 8857 8782 7700 414.7 5108

O,a 0.4 0.3 0.5 0.3 1.8 0.2

! 823 _+ 17 ! 671 + 23 1734+27 ! 724 + 15 1589-17 1322+ 18 1302+ 19

WM-Wheeler Mtn., S-Saddle Island, WB-White Bear Island, O-Opingiviksuak Island, U-Umiakovik Late balho!ith.

TABLE5 Summary of U-Pb zirct~,n ages and K-Ar dates for hornblendes and biotites U-Pb zircon

(Ma)

UmiakovikLakebatholi'th EC87-86 EC87-119

monzodiorite bi-hbgranite

1319 +_2 1316+2/-3

K-Ar

CMa)

1322+ 18, hb; 1302± 19,

hi. S~dd:eisianU EC87-143c granite White Bear lsland EE88-096 granite Opingiviksuak Island EC87-142 granite Wheeler Mtn. EC89-354 granite EC89-357 granite

1774+2/-I 1734+27.bi. ! 776 + 4 / - 2 1724 ± 15, bi.

1774+2/-11589_*i7, bi. 2 ! 37 ± 2 1823 ± 17, bi. 2134+ 3 / - ! 1671 + 23, bi.

identify these as magmatic zircons. Accordingly, the upper intercept age 1-)7a+2, Ma is interpreted as the age of crystallization of the granite, EE88-096, White Bear Island granite: Zircon crystals are well terminated, euhedral, colourless, flat and stubby (length/breadth of most crystals is 1"1 to 3:1 ). Inclusions and bubbles are present in most grains. The three fractions analyzed were selected to minimize inclusions, and were strongly abraded. U contents are low but variable (32 to 73 ppm) with the smaller analyzed size fractions containing the least U. The three data points are collinear (65% probability of fit) and define a chord with intercept ages 177h+4,,-2and 637 Ma (Fig. 6d). The most concordant data point is only 0.5% discordant, Based on the simple zircon morphology and the uncomplicated U-Pt, systematics, the age 177 ~+4 ,,-2

Ma is interpreted as the age of crystallization of the White Bear Island granite. .~'C87-142, Opingiviksuak Island granite: Zircon crystals are euhedrai, mostly short and stubby (length/breadth 1.5 to 2), colourless to pale pink with some bubble inclusions. Marly grains were broken; no zoning or cores were seen. The most colourless, inclusion-free grains were picked in t w o steps; fractions 5a to 5d were moderalely abraded, fractions 5e and 5f were very strongly abraded. The results from fractions 5e and 5f (Fig. 6e) are concordant within analytical uncertainty, point f lying slightly above concordia. The measured 2 ° T p b / 2°6pb ages are 1775.0+1.1 and 1774.5+1.1 Ma respectively. Data points 5b, 5c and 5d form a linear trend with t h o s e f r o m f r a c t i o n s 5 e a n d 5f, with point 5a falling to the right of this trend. Points 5b,5c,5e,5f and ~ ,~ ,~h, 5b,.,d,.,,.,5f are s,.parat,.,: collinear within analytical uncertainty but the five points 5b,Sc,5d,5e,5fare not (probabilities of fit 42.2, 11.5 and 0.03%, respectively). Using Davis' (1982) model for nonlinear data, a regression line may be fitted to the five data points 5b,5c,5d,5e,5f (Fig. 6e). Intercept ages are 1774 +2 and 528 Ma reflecting the same span in upp¢~ intercept age as the uncertainties associated with the twe 2°Tpb/2°6pb ages. The U contents of EC8 7-142 zircons are the highest measured in this study, ! 04 to 140 ppm. The U Pb systematics are also the most complex. Frae,,ion 5a appears to contain a small component of inherited Pb and yields a 2°TpD/2°6pb age of ! 777.3 + 1.0 Ma, marginally higher than those ofconcordan) data points 5e and 5f. The lack of collinearity of data points 5b, 5c, and 5d with points 5e and 5fsuggests either the presence of minor inherited I-o in these

FLUORITE-BEARINGGRANITES,OKAKAREA,LABRADOR

101

EC89-354

EcBg-357

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Fig. 6. U-Pb concordia diagrams for zircon: ( a ) EC80-354. Wheeler M o u n t a i n granite ( b ) EC80-357. Wheek'r Mountain granite (c) EC87-143c. Saddle Island granile (d) EE88-O96. While Bear Island granite (e) EC87-~ 42, Opi~giviksu:~k Island granite (f) EC87-86, fayalite quartz monzodiorhe (shaded symbols ) arid EC87-! 19. biolile-hornb|ende granite (open symbols), both from Umiakovik Lake balhoiith. Er~'or ellipses indicate two sig;P,a uncertainties.

102

more discordant fractions, or more than one period of Pb loss. Considering the equivalence in measured age and the strong similaritie.~" in petrography and chemistry, it is most probable that the granites of Opingiviksuak, Saddle and Wk,,ite Bear Islands were emplaced at essentially the same time and from a common source. Thus, by an,~logy'with the other two granites, the age of 1774_+~ Ma ;~ interpreted as the age of emplacement of the granite on Opingiviksuak Island. Biotite in Opingiviksuak Island granite has a KAr age of 1589 + 17 Ma suggesting that some sort of disturbance affected the system subsequent to crystallization. On the other hand, biotites from Saddle Island and White Bear Island, with K-Ar ages of 1734 + 27 Ma and 1724 + 15 Ma do not seem to have been similarly disturbed. These ages are in relatively good agreement with one another, and marginally younger than the zircon crystallization ages of about 1775 Ma.

Umiakovik Lake batholith Two major igneous components of the Umiakovik Lake batholith, fayalite quartz monzodiorite and younger biotite hornblende granite, were sampled for U-Pb determination on zircon. Zircon morphologies of the two units are similar, grains are clear, colourless, euhedral with length/breadth ratios of 2 to 3, tabular to prismatic, without cores or visible zoning under transmitted light. Bubble inclusions are prevalent, black speck inclusions were also noted in the monzodiorite zircons. EC87-86, fayalite quartz monzodiorite: Four fractions were selected and strongly abraded. Three of the four resultant data points are concordant, the fourth is slightly discordant (Fig. 6f). The average of the four 2°7pb/2°6pb ages is 1319 + 2 Ma. Based on the simple morphology and uncomplicated U Pb systematics, these results are interpreted as providing the age of igneous crystallization of the fayalite quartz monzodiorite. Measured U contents are sufficiently low, about 30 ppm, that only the smallest grain size fraction analyzed shows some minor Pb loss. EC87-119. biotite-hornblende granite: The three fractions selected were strongly abraded. The two more precise analyses, on fractions 7a and 7c, yielded 2°7pb/2°6pb ages ! 315.4 + 1.9 and

R.F. EMSLIE AND W.D. LOVERIDGE

1316.0 + 1.7 Ma (Table 3 ). U contents of 42 to 54 ppm, although almost twice those in EC87-86 zircon from monzodiorite, are still relatively low. Assuming minor recent Pb loss (less than 0.5%) these 2°Tpb/2°6Pb ages date the age of crystallization of the zircon (e.g. if the Pb loss occurred at ca. 300 Ma, the estimated age of crystallization would increase by only 1 Ma). Fraction 7b, which has a lower 2°TPb/2°~Pb age of 1311.3 + 3.5 Ma (Table 3 ) was less satisfactory analytically. It contained only 0.2 ng Pb of which 91.5% was radiogenic compared with about 0.7 ng Pb for fractions 7a and 7c of which 97% was radiogenic. Based o~ the analyses of fractions 7a and 7c, an age of 1316_+] Ma is adopted for the emplacement of the biotite-hornblende granite component of the Umiakovik Lake batholith. The overall uncertainty assigned is derived from the span of the analytical uncertainties in 2°7pb/2°6pbages of fractions 7a and 7c and overlaps that of fraction 7b. This result is marginally lower than the age of 1319 + 2 Ma preseated above for the fayalite quartz monzodiorite component, in agreement with field relationships. Both the hornblende and biotite K-Ar dates ( 1322 + 18 Ma and 1302 + 19 Ma respectively, Table 4) obtained for Umiakovik Lake biotite-hornblende granite are in agreement, within analytical uncertainty, with the zircon age of 1316_+2 Ma from the same sample.

Sm-Nd and Rb-Sr isotopic studies Whole rock Sm-Nd and Rb-Sr isotopic data for the seven samples dated by U-Pb zircon have been obtained by R. Theriau!t and are given in Table 6 together with calculated depleted mantle Nd model ages ( ToM, DePaolo 1981 ). Initial Nd ratios (calculated as end) and initial Sr ratios are based on the U - P b zircon crystallization ages and all samples lie in the range ENd= -- 7.5 to -- 11.0, Sri=0.7019 to 0.7090. Initial 87Sr/86Sr ratios calculated for the samples exhibit marked increase with decreasing age. TDM model ages of the analyzed granites in the present study extend only from 2.4 to 2.7 Ga despite the large range of crystallization ages represented from 1.32 to 2.14 Ga. All of the granites have undergone within-crust fractionation to some degree but even the most fractionated do not show sig-

FLUORITE-BEARING GRANITES, OKAK AREA, LABRADOR

103

TABLE 6 Sm= Nd and R b-Sr isotopic data for analyzed samples TABLE 6a Sm-Nd isotopic data

WM EC89,354 WM EC89-357 O EC87-142 S EC87-143c WB EE88-096 U EC87-86 U EC87-119

Sm ppm

Nd ppm

147Sm/ 144Nd

! 43Nd/ 144Nd

U-Pb age Ga

t~,,,m, I

(N,t,

TDM

11,12 I !,65 16.83 24.14 20.30 13.80 10.80

113.2 106.9 !13.3 155.0 129.5 64.97 66.80

0.05937 0°06584 0.08978 0.09410 0.09473 0.12836 0.09774

0.510314+6 0.510383 + 7 0,511012+6 0.510975_+8 0.511048 4_4 0,511652_+7 0.511220± ! I

2.14 2.14 i.77 i.77 1.78 1.32 1.32

-0,70 -0.66 -0.54 -0.$2 - 0.52 -0.35 -0.50

-.7.7 - 8. I -7.5 -9.2 - 7.8 -7.7 - i 1.0

2,69 2.74 2,49 2.63 2.55 2.48 2.39

Assumed present day CHUR has 143Nd/144Nd=0,512638, 147Sm/144Nd=O.1966; Nd isotopic compositions are normalized to 146Nd/ 144Nd = 0.7219. Repeated analysis of Ames Nd standard yielded a ! 43Nd/144Nd = 0.512151 ( __5, 2 sigma mean ) corresponding to Lajolla 143Nd/ ! 44Nd = 0.5 ! ! 866. ! 47Sm/144Nd, expressed as f S m / N d , is pt.ecise to 0.3%. Total procedure blanks are less than 200 pg for Nd and less than 80 pg for Sm. WM-Wheeler Mtn., O-Opingiviksuak Island, Saddle Island, WB-White Beat. Island, U-Umiakovik TABLE 6b

Rb-St. isotopic data

WM EC89-354 WM EC89-357 O EC87-142 S EC87-143c WB EE88-096 U EC87-86 U EC87-119

Rb ppm

Sr ppm

87Rb/86St.

87Sr/86Sr

U-Pb age Ga

87St./86St. initial

87Sr/86St. 1.32 Ga

124.0 ! 2 !.2 118.7 136.8 130.3 23.04 146.0

140.3 110.4 301.0 359.9 265.3 327.9 103.3

2.577 3,207 !.143 1.|02 !.425 0.2034 4.122

O.7814 ! x 2 0.80146 + 2 0.73207+2 0.73248_+ 3 0.74066 + 2 0.71285 + 3 0.78396_+3

2. ! 4 2. i 4 i.77 1.77 1.78 1.32 !.32

O.70 ! 90 0.70252 0.70297 0.70442 0.70418 0.70901 0.70598

O.73265 0.74078 0.71044 0.71163 0.71370 0.70901 0.70598

St" isotopic compositions are normalized to 88Sr/86St.= 8.37521. Repeated analysis of NBS.987 yielded 87Sr/86Sr= 0.710255 ( + / - 20, 2 sigma mean). Total procedure blanks are less than 130 pg for Sr and less than 60 pg for Rb. WM-Wheeler Mtn., O-Opingiviksuak Island, Saddle Island, WB-White Bear island, U-Umtakovik

nificant LREE depletion nor strong negative Eu anomalies which characterize very highly fractionated granites and can lead to unreliable TDM model ages (Pimentel and Charnley, 1991 ). If anything, there might be a tendency for data fr:~m all of these granite samples to yield results that slightly underestimate the mean ToM model ages of their sources due 1o the expected decrease in Sm/Nd in partial melts relative to residues, The Sm-Nd data clearly imply that Archean source materials played a significant role in derivation of magmas for all granitoid groups. With the exception of sample EC87-119, there is an overall progressi e increase in.~,,;N,~ from older to younger granite groups {.fs~/Nd= [( m47Smg4"~Nd)/ 0 . ! 9 6 6 ) ] - 1 } This might reflect successive withdrawal ofpartial melt fractions from a source cornmon to allgranite groups. Both older granite groups

are small in volume and presumably represent small degrees of partial melting of crustal sources. The older Wheeler Mountain granite has REE pattern~ with the steepest negative slopes (Figs. 5a, 4b) and most negative ~m/Nd (Table 6 ). The offshore granires have intermediate chondrite-normalized REE slopes and fsm/nd values. Samples from Umiakovik Lake batholith have still less steep REE slopes and least negative ~,,/sd (especially EC87-86); sample EC87-119 is strongly fractionated and representatire of a relatively small volume of the batholith (Emslie and Russell, 1988 ). The mean./sm/N,j of 73 samples of Umiakovik Lake granitoids is - 0.40 _+0.02 {calculated as.~m/Nd = [ (Sm/Nd~,¢k) / 0 . 3 2 5 ] - 1. uncertainty stated as two standard errots of the mean}. Umiakovik Lake batholifll is at least two orders of magnitude greater in volume than either of the older granite groups and imphes vet3~

104 much greater degrees of partial melting of the crustal source. Granites of the two older groups may have contributed to the source materials for Umiakovik Lake batholith magmas but the small apparent volumes of the former imply minimal contributions, if any. The data appear consistent with succe~sive partial melting events that operated on a common source. If that source were at least partly Archean crust, as seems likely, TDM model ages suggest admixtures of Proterozoic materials to the Archean source, possibly increasing somewhat in successively younger granites. If the mean age of the older crustal source were middle Archean or older, larger proportions of Proterozoic components would be indicated. In any case, intrusion of early and/or middle Proterozoic basic magmas into lower crustal source regions could satisfy the requirements. In Fig. 7 the band shown for average Umiakovik Lake granitoids withfsm/Nd= --0.40+0.02 hasa less steep slope than the older granites. DePaolo (1988) has noted an apparent secular change involving increase in Am/Nd of successively ~,ounger crustal granitic rocks from worldwide continental basement regions with model ages from 3.6 to 1.4 Ga. The apparent increase infsmmd observed in the three Proterozoic granitic suites investigated here is in accord with that observation. Explanations discussed by DePaolo include a IoWf~m/Nd in Archean crust that increases largely through reprocessing due to different melt-residue partitioniing of REE. A plot Of~NdVS. initial 875r/86Sr (both calculated at 1.3 Ga) for all granite samples is shown in Fig. 8. The isotopic data range for Archean Kiyuktok gneisses (comprising > 3.6 Ga components variably mixed with 2.8 to 3.0 Ga components; Collerson et al., 1989 ) sampled in the Nain Province about 100 km north of Okak Bay, is i~dicated as two standard errors about the mean. Simple two-component bulk mixing curves (Langmuir et al., 1978) are shown between an assumed juvenile crustal cornponent with bulk earth isotopic values (eNd=0, 87Sr/a6Sr= 0.7030, containing I0 ppm Nd, and 300 ppm Sr) and a component similar to average Kiyuktok gneiss ( 17.7 ppm Nd, 269.5 ppm Sr and isotopic compositions within the Kiyuktok range arbitrarily chosen so that the mixing envelope would enclose all of the granites). Support for assuming bulk earth isotopic values for a juvenile component comes from the apparent anorogenic character cr~he

R.F. EMSLIEAND W.D. LOVERIDGE

!.3 Ga Eisonian magmatism. Not more than about 20% of source material like Kiyuktok gneiss is indicated to have contributed to the Umiakovik batholith. This contribution would be increased somewhat ifthejuvenile component had depleted mantle isotopic characteristics but still would not likely exceed about 25%. Offshore granites and Wheeler Mountain granitespresumablyhad relatively greater amounts of a Kiyuktok gneiss-like source at the respective times of their formation. The exercise illustrates that, despite apparent Archean TDM source ages, the Archean component may constitute substantially less than the total of the source materials.

Discussion The granite samples investigated in this study fall into three clearly defined age groups at 2135, 1775 and 1318 Ma. All ofthe intrusions are discrete, discordant plutons of post-kinematic character and have substantial similarities in mineralogy and bulk chemistry. The Wheeler Mountain granites at 2135 Ma form the most enigmatic group. This is chiefly because details of early Proterozoic history of the region are still imperfectly known and there is little existing chronologic-tectonic framework within which to view the granites. Crystallization of the Wheeler Mountain granites closely follows the time of a proposed mantle enrichment event at about 2.3 +0.1 Ga suggested by Hamilton and Morse (1988) from consideration of Sm-Nd data from Elsonian anorthositic whole rocks. The chemistry and mineralogy of the granites could be construed as evidence the magmas formed in an extensional, asthenosphererelated event. The offshore granites (White Bear, Saddle and Opingiviksuak islands)intruded following a major tectonic episode during which Nain and Churchill Province blocks were juxtaposed, possibly as a result of continental collision (e.g. Van Kranendonk, 1990; Van Kranendonk and Ermanovics, 1990). An obvious possibility is that formation of the offshore granites was the result of crustal partial melting consequent upon tectonic overthickening of the crust. Sinistrai transpressional shearing (Abloviak Zone, Fig. 2 ) followed by east-directed thrusting as a result of continent-continent collision describes the tectonic regime rel~ted to the Torngat orogeny

FLUORiTE-BEARING GRANITES, OKAK AREA, LABRAD()R

8

~

t

!

"

105

!

I

I

""

I

I-

---T

/

-------________~ 4 -

CHUR

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I

~a

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/ / -t6 f =~o

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/

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i

~ //

.o

~

~.4

i

__i

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~.s

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Age

t

t ~ t _ _ _ _ J 2.6

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(Ga)

Fig. 7, ~Na vs. crystallization ages of whole rock samples. Shaded band is caF'ulated for/Sm/Na = - 0 . 4 0 _ 0.02, determined from

the average of 73 samples from Un',iakov=k Lake batholith (see text). DM i~ ;.he model depleted mantle curve after DcPaolo ( 198| ). CHUR is chondrhic uniform reservoir.

,_n this region according to Van Kranendonk (1990). Over~hrust movements indicate west over east at the U-Pb, ~ z~rcon . ~oges ,-u~~2-E'Ej .lo ~_~..-\ ~ ,, o ~ , o r e ( l z z Ga) I \ ® , , - \ ,o = wheo~e, Mt~ t2 ~4 Ga~ •- -20 i * ~ ' - " " ~ _ t z " I " ~ i ii'.i".i/I ' i oko ~i~ [k -ao ........ 0

I~ x, ,o 1\~2°

~

~

'

t

-4o L 7o0

: z2o

' z4o

....... 76o

' ~ J 78o

8oo

(87Sr186sr), 3o Ga

Fig. 8. (Na vs. s~'Sr/S6Sr,both calculated at 1.30 Ga. Two component bulk mixing curves for a juvenile crustal component with Archean Kiyuktok gneiss. Subdivisions on mixing curves are inpercent,

present expomre level, which, if applicable to the crust as a whole in this region, imply that Nain Province Archean cratonic rocks were overridden by rocks of tile Torngat Orogen and may form part or most ofthe lower crust over substantial areas west of the present Nain Province. Archean crust is therefore favoured on tectonic as well as isotopic grounds as important source material for the granites of this study. Consideration of the preliminary dating of tectonic events in the region by Bertrand el ai. ( 1990 ) also favours the offshore granites as possible exampies of post-collisio~ml granite formation. The timing of early Proterozoic regional events outlined by Bertrand et al. ( ! 990 ) for north-central kabrador indicates major tectonic activity and metamor-

106

phism between about 1860and 1825 Ma with uplift extending to about 1786 Ma. Potential analogues for the offshore granites are the Tertiary Himalayan leucogranites, now recognized a: prt~!otype collision-related granites (Debox et ~!., 1986; Sylvester, 1989; Crawford and Windley, 1990). Chemically, the offshore granites bear closest resemblance to the type-b leucogranites (associated with mantle magmas) of Crawford and Windley (1990) and the Opingiviksuak Island himodal granite-ga~r'~ association also argues against simple collision-related crustal thickening followed by anatectic melting as suggested for their type-a model. The Manaslu and related Tertiary leucogranites of the High Himalaya have been considered the quintessence of post-collision granites formed by anatexis ofparagneisses (LeFort, 1 9 8 1 ; Debon et al., 1986). These latter granites are rooderately to strongly peraluminous (S-type), ieucocratic, F-bearing and for the most part two-mica granites. They intruded some 20 to 40 Ma after collision of India and Eurasia and the consequent onset of crustal thickening by thrusting ( Debon et al., t986). Unlike Himalayan leucogranites, the offshore granites studied here are only marginally peraluminous, with some samples having normative corundum contents up to 0.7 wt.%. In contrast, both Wheeler Mountain and Umiakovik Lake granitoids are distinctly metaluminous, One of the most distinctive properties of the off. shore granites relative to the other granitoids investigated in this study lies in their magnetic expression. This is clearly shown on aeromagnetic Maps 7452G and 7453G (Geological Survey of Canada, 1983a,b) where strong gradients near well-defined magnetic highs occur over Opingiviksuak, Saddle and White Bear islands (Fig. 2). By contrast, much smaller and more dispersed positive anomalies occur over Wheeler Mountain granites. The feeble magnetism of younger Elsonian granitoids is excellently displayed on aeromagnetic Map 7453G where Umiakovik Lake batholith is magnet|cally weak and almost featureless over nearly its entire extent. An obvious petrographic feature of the offshore granites is the presence of abundant primary titanite, a rare mineral in typical Elsonian granites. Since reduced mineral assemblages bearing fayalite, ferrous-rich pyroxenes, hornblendes and biotites are typical of EIsonian granitoids and are associated with ilmenites and ulvospinel-rich spinels, substan-

R.F. EMSLIE AND W.D~ LOVERIDGE

tiai differences in redox conditions seem likely. Wones' (1989) assessment of the assemblage titanite+magnetite+quartz in granites as being a relatively oxidized one is likely applicable here and would account readily for the strong differences in magnetic intensity between the offshore (strong) and Elsonian (weak) granites. Elsonian magmatism associaaed with the Nain Plutonic Suite, repr~,sented here by Umiakovik Lake batholith, exhibits significantly older crystallization ages than U-Pb zircon ages previously reported from the eastern part of the Nain Plutonic Suite ( 1295 Ma, granite, Krogh and Davis, 1973; 1305 +_5 Ma, granite, troctolite, Simmons et al., 1986; 1305 + 10 Ma, pyroxene granite, Simmons and Simmons, 1987). A recent U-Pb zircon age of 1322+ i Ma (Ryan et al., 1991 ) for Makhavinekh Lake pluton south and east of Umiakovik Lake batholith is similar to the 1319+2 Ma age presented for the Umiakovik Lake batholith in this report. These dates from the western flanks of the Nain complex imply that magmatism began and ceased earlier there than it did in the east.lncreasing objections have been raised to the proposiLion that residual granulitic sources are preferred for so-called Atype granite magmas (Anderson and Bender, 1989; Creaser et al., 1991; Emslie, 1991 ). One principal objection is that large volumes of K-rich mal~mas would be unlikely to be extractable from such Ksparand biotite-depleted residues. Nevertheless, a more precise definition of "residual" may be in order considering that at least two of the thre~ partial melting events here involved "residues" from prior episodes of melt extraction. Successive partial melting episodes of a source p~rotoliththat produced a sequence of reduced (Wheeler Mountain granites), more oxidized (offshore granites), then again reduced (Umiakovik Lake batholith) melts requires explanation. If source regions control the redox condition of granitic magmas when formed, as widely believed (Wones, 1989; Carmiehael, 1991 ) the conditions under which the three magma batches formed or developed clearly must have differed. A likely scenario is that the more reduced magmas (in particular Umiakovik Lake batholith) derive from the lower crust, whereas the offshore granite magmas developed through partial melting at higher levels from more oxidized protoliths.

FLUORITE-BEARINGGRANITES.OKAKAREA~LABRADOR

Summary and conclusions

At least three granitoid groups in the vicinity of Okak Bay on the Labrador coast, although having a number of petrographic similarities, are shown to have been emplaced in three episodes spanning more than 800 Ma. The oldest group of Wheeler Mountain granites is about 2135 (2134_~,+s 2137 + 2) Ma old, and represents a late early Proterozoic postorogenic or anorogenic suite not previously recognized in central Labrador. The next younger group, "the offshore granites" including those exposed on White Bear, Saddle, and Opingi+, viksuak Islands, intruded about 1775 ( 1774_i, 77,~+4 +2 t .,,_ 2, ! 774_ ~) Ma. These are clearly postectonic granites, possibly related to tectonic thickening caused by an earlier continental collision stage of the Torngat Orogeny. The youngest and most voluminous granitoids are :hose of Umiakovik Lake batholith, yielding crystallization ages about 1318 ( 1319+2, 1316+2/.3) Ma. These rocks are widely regarded as anorogenic in character and closely related to development of th~ Nain anorthositic complex. Whole rock major element chemistry of all granitoids is rather similar but significant differences in trace element concentrations, mineral assemblages, and mineral chemistry are apparent. Wheeler M o u n t a i n a n d U m i a k o v i k granitoids h a v e iron-enriched biotites [ F e / ( F e + M g ) = 0 . 7 2 to 0.81 ]

whereas biotites from Ihe offshore island granites are less enriched (0.44 to 0.62 ). Less iron-rich biotile, the presence of coarse primary titanite and elevated hematite in solid solution in flmemite are all indicative of substantially more oxidized states of the offshore islands granite m a g m a s .

Nd and Sr isotopic data indicate that all of the granitic rocks fall in a limited range Of~N,d between about - 7 . 5 and - l 1.0 at the respective times of their crystallization. Depleted mantle model ages (TOM) range from only 2.4 to 2.7 Ga despite lhe wide range of crystallization ages from 2137 to 1 3 1 6 Ma. Sm-Nd data and REE patterns support deftvalion of all granites from successive partial melting events in Archean crustal source rocks which, however, have had imporlant additions of younger crustal components at least once and probably two or more times during the Proterozoic. The addilions most likely took place by intr,asion of basaltic

I0 7

magmas into the lower crust and by crustal underplating which promoted crustal anaIexis.

Acknowledgements: Reg Theriault carried out the Rb-Sr and Sm-Nd isotopic analyses. John Stirling provided assistance and advice with microprobe work. Deborah Lemkow picked the zircon fractions and drafted the figures. We are indebted to lngo Ermanovics for dohating sample EE88-096, and to Mike Hamilton for drawing to our attention the fact that E.P. Wheeler 2nd had previously recorded the presence of a biorite-hornblende granite pluton in the northern part ofUmiakovikLakebatholith (oral communication to R.F. Emslie, July 1989). Ingo Ermanovics, Mike Hamilton, Tony LeCheminant9 E.R. Neumann, Chris Roddick, Bruce Ryan, Reg Theriault, Otto van Breemen and Mike Villeneuve kindly provided vaiuable constructive comments on the manuscript. Bruce Ryan is thanked for an especially thorough review.

References Anderson, J.L. and Bender, E.E., 1989. Nature and origin of Proterozoic A-type granitic magmatism in the southwestern United States ofAmerica. Lithos, 23: 19-52. Bartom J.M., Jr.. 1975. The Mugford Group volcanics of Labrador: age, geochemistry, and tectonic setting. Can. J.

Earth Sci.. i2: ! 196-1208.

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J. Earth Sci.. 12: 361-370. Barton, J.M.Jr., 1977. Rb-Sr ages and tectonic setting of some granite intrusions, coastal Labrador. Can. J. Earth Sci., 14:

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Appendix

61 : 24'w.

EC87-143c - SE side of Saddle lsiand, high ground. 57 ~ 36'N,

Locations and petrographic noles on specimens dated.

|t'heeler Mmmtam gramws EC89,354 from small knoll av southeast end of pond iramediately NE of sum,--.;, , f Wheeler Mountain. 57: 34N. 62 ~ i9'W. Coarse grained massive, salmon pink to pink-grey granite. Microcline microperthite and weak! ,-zoned plagioclase accompany large, strained to partly polygonalized quartz. Additional quartz is present as small, rounded, drop-like grains enclosed in large feldspar grains. Deep brown biotite, locally highly chloritized, is the chief marie mineral. Large, abundant, short prismatic zircon, apalite, allanite, fluorite and rare opaque oxides are the accessory' minerals. Most z~rcon~ are stubby (length: breadth = ! : 1 to 3: l ), dear. prismatic c~sla[s but one large blocky zircon grain contains a welbdefined core. -

EC89-357 - sample taken a~ rockfall on hillside belo~ peak about 3 km N of Siugak Brook. 57: 37.5 N, 62 : 23.7' W. Coarse grained grey-pink massive hornblende-biolite granite. Microcline microperthite is the dominant feldspar: not more than about 10% plagioclase is present. Most quartz is highly strained but recrystallized domains are smal~ and localized. Zircon is abundant and opticaliy visible cores are present in some large grains. Pale to deep olive-brown biotite is slightly more abundant than deep brownish-green hornblende. Deep brown to yellowish-brown allanite is abundant, Large fluorite grains occur sporadically disseminaled Iogerber with a little apatite and opaque oxides,

Offshore gramtes EC87-142 - near shore. S side of Lady Bight Harbour, ()pingiviksuak Island. 57 ~ 26.2'N, 61 : 32.5"W. The rock is a medium grained massive, pink-grey g:anite, Quartz, strained to sutured, microcline microperthi~e, and plagioclase are the main minerals. Pale to medium o[i,,c green biotite is accompanied by traces ofbluish-greez~ to dark olive hornblende. Zircon, large abundam, well formed titanite

The Saddle Island sample is a medium grained, massive, pink to pink-grey biotite granite, locally fluorhe-bearin~. Thin ,~ections show that strained quartz, ~n~crocline microperthite, and normally-zoned plagioe!ase arc tile principal miner~!s. Pale brownish-green to olive biotite is the marie silicate and a sho~vs only minon local chloritic alteration. Primary welli,brmed titanite crystals are abundant. Small prismatic zir~:ons. allanite, apatile, opaque oxides and fuorite are other accessor3~minerals. EE88-096 - White Bear Island, in bay abouy i km northwest of White Point. 57 ~ 54'N, 61 : 40'W. Medium to coarse grained grey massive granite. Very simliar in appearance to Opingiviksuak granite. Microcline microperthite, markedly zoned plagioclase and large quartz grains form most of the cock. Light to medium olive biotite is the main marie silicate and chlor/fization is minor. Large wellforme4 weakly pleochroic primaq' titanite crystals are prominent. Prismatic small zircons and apalite are abundant and are accompamed by small amounts of fluorite and opaque oxides.

L'mtako~lk Lake katkolitfi EC87-86 - stream bedjust below outlet of small lake marked l145+elevation on the topographic map. 57: 19'N, 62+" 37.5'W. Medium grained quartz fa~alite monzodiorite. A relatively marie rock with colour index near 20, fayalitc and ferroaugite are the principal marie minerals, with only traces of olive brown hornblende. Sub-equant to rounded quartz grains are weakly strained, Plagioclase is the dominant feldspar but rare la,ge penhite grains occur. Apatite, large well-formed zircon, and opaoue oxides arc fl+e chtefaccessors minerals. EC87- l 19 - top of #romontors' overlooking Umiakovik Lake from the north. 5 7 24'N, 62: 50.5'W. Coarse grained grey to pinkish biotite-hornblende granite. Large subequant slrained quartz, microcline micropertllite and patchy-zoned plagioclase are the essential minerah. Vet5 deel~ bro~vw~biotile and dark olive Io lax hornblende arc the marie ~tlicales. Large prismatic zircon is abundant. ,:3 t~ ai?ao tile: rare aHanite and scaucred opaque oxide grains compk'te the complememofaccesso~ ~minerals.