The Michael Gabbro and other Mesoproterozoic ...

The Michael Gabbro and other esoproterozoic ithospheric probes in southern and central Labrador

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Ronald FeEmslie, Michael A. Hamilton, and Charles F. Gower

Abstract: The Michael Gabbro (1426 Ma) and the Shabogamo Gabbro (1459 Ma) represent two Iarge diabase intrusive episodes that affected the northwestern margin of the northeastern Grenville Province. Both have sustained variable effects of subsequent Grenvillian metamorphism. Other broadly contemporaneous magmatic activity in the region includes the formation of Elssnian anorthosite-granitoid complexes (1-46- 1.29 Ga) that intruded Churchill Province and Nain Province rocks to the north, the Harp dykes (1.27 Ga), and Mealy dykes ( - 1.25 Ga). Petrologic and geochemical data show that the Michael Gabbro and Shabogamo Gabbro are similar, with the latter displaying more cumulate rock characteristics, and the former having compositions closer to those of rapidly cooled magma. Both have compositions comparable to those of other continental diabases and to some continental flood basalts. Sr and Nd initial isotopic compositions of Michael Gabbro ( ~ ~ ~ ( 1 Ma) 4 2 6 = -4.7 to -6.0, 1,,(1426 Ma) = 8.7832 - 0 . 7 W ) and Shabogamo Gabbro (+,(I459 Ma) = -4.0 to -5.5, 1,,(1459 Ma) = 8.7020-0.7060) are alike and overlap, suggesting similar sources and processes of development. Comparisons with other nearly contemporary mafic suites in central and southern Labrador show that only the Mealy dykes have a distinctly more radiogenic isotopic signature. Paradoxically, other rnafic suites that intrude older basement rocks north of the Grenville Front have less enriched Nd signatures than do those that intrude younger basement rocks south of the front. The argument is made that the subcontinental lithospheric mantle, and not crustal contamination, played the most influential role in evoIution of the magmas. RCsumC : Le Gabbro de Michael (1426 Ma) et le Gabbro de Shabogamo (1459 Ma) reprbentent deux episodes d'intmsions de diabase majeurs qui ont affect6 la marge nord-ouest de la rigion nord-est de Ba Province de Grenville. Les deux unitis de gabbro ont subi les effets variCs du mCtamorphisme grenvillien subsequent. Une autre acthit6 magmatique grossikrement contemporaine a pris place dans la region, il s'agit de la formation des complexes dqanorthosite-granito'ide de 1'Elsonien (1,46- 1,29 Ga), qui ont pCn6trC les roches des provinces de Churchill et de Nain au nord, ainsi que les dykes de Harp (1,27 Ga) et les dykes de Mealy ( - 1,25 Ga). Les donnCes pCtrologique et gkochimique rCvklent que le Gabbro de Michael et le Gabbro de Shabogamo sont similaires, le dernier montre davantage 1es particularitCs des roches i cumulats, et le premier affiche des compositions qui sont plus proches de celles d'un magma refroidi rapidement. Les deux unitCs gabbroiques ont des compositions cornparables ?celles i d'autres diabases continentales et de certaines coulCes de basalte continentales. Les compositions initiales des isotopes de Sr et de Nd du Gabbro de Michael (+,(I426 Ma) = -4,7 i -6,8,1,,(1426 Ma) = 0,7032-0,7044) et du Gabbro de Shabogamo ( ~ ~ ~ ( 1 Ma) 4 5 9= -4,0 i -5,5,1,,(1459 Ma) = 0,7020 -8,7060) sont Cquivalentes et chevauchantes, par consdquent les sources magmatiques et le processus de dCveloppment sont similaires. Les comparaisons avec d'autres suites mafiques presque contemporaines, dans Be Labrador central et mCridiona1, montrent que les dykes de Mealy sont seuls i porter une signature isotopique radiogknique plus distinctive. Paradoxalement, les autres suites rnafiques qui pCnktrent le socle plus ancien au nord du Front de Grenville prksentent des signatures moins enrichies en Nd que celles qui font intrusions dans le socle plus jeune au sud du front. Nous soutenons l'opinion que c'est le manteau lithosphkrique subcontinental, et non pas une contamination crustale, qui a jouC Be r61e Ie plus important dans I'Cvolution de ces magmas. [Traduit par la rCdaction]

Introduction The lithospheric mantle (SCLM) has become recognized as an important source or interactive component in generation and transport of basic magmas from the mantle into the crust during nonorogenic continental magmatism. This 's whether the were plume gen-

R X Emslie' and M.A. Hamilton. Geological Survey of Canada, 681 Booth Street, Ottawa, ON KIA OE8, Canada. C.F. Gower. Newfoundland Department of Natural Resources, B.O. Box 87W, St. John's, NF A1B 456, Canada.

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Corresponding author (e-rnail: [email protected]) .

Can. J. Ear& Sei. 34: 1566-1580 (1997)

erated or had their origins in the upper levels of the mantle. Mantle-derived basic magma may-have its source in the asthenosphere or in the lithospheric mantle, but in both cases, must be transported through the subcontinental mantle lithosphere to reach its final location in the crust. Considerable debate has arisen over the extent to which the lithesphere is capable of leaving its and isotopic imprint on different basic magmas and how that signatire can be distinguished from one caused by crustal assimilation. The subcontinental mantle lithosphere is believed, for the most part, to have a large-ion lithophile element (L1LE)depleted geochemical signature, the result of long-term partial melt extractions containing the elements that formed the continental crust. There are, however, examples in which the subcontinental lithosphere appears to have had a chondritic 63 1997 NRC Canada

Ernslie et al.


or an enriched signature, which has led to ambiguities in interpretation, and some innovative proposed solutions. Studies of continental flood basalts, in particular, have been an important influence on the development of ideas bearing on the nature of the SCLM (e.g., Carlson 1991; Arndt and Christiansen 1992; Arndt et d. 1993; Wooden et al. 1993; Shirey et al. 1994). It is widely believed that diabase dyke magmas are closely related in origin to continental flood basdts because of their comparabIe mineralogy, petrographic features, geochemistry, and isotopic compositions. In some cases, a spatial relationship between diabase dyke swarms and large-scale continental magmatism has been demonstrated, e.g., the Mackenzie dykes, the Muskox intrusion, and the Coppermine flood basalts. Continental basic magmatism in Labrador during the Middle Proterozoic is chiefly associated with two different assemblages that occur both north and south of the Grenville Front. In the first assemblage, basic members of the great anorthosite complexes demonstrate the involvement of considerable volumes of mantle-derived basic magma. Examples of these were intruded at 1-64Ga (Mealy Mountains), 1-461 -42 Ga (Harp Lake, Michikamau), 1.35 - 1.29 Ga (Nain Plutonic Suite), and 1.13 Ga (Atikonak River). The second major association includes sills and sheets at 1.46 - 1.43 Ga (Michael Gabbro, Shabogamo Gabbro), diabase dykes at 1.27 - 1.25 Ga (Mealy dykes, Harp dykes), and volcanic flows and sills at 1.24 Ga (Seal Lake Group, Nascaupi sills). The interval following the Labradorian orogeny with its related pstorogenic magmatism, and preceding the Grenvillian orogeny ( 1.60 - 1.28 Ga), was a period of relative crustal stability over most of Labrador. Accordingly, much of the magmatic activity is recognized as having been of anorogenic character. The Michael Gabbro is widely distributed along the northern margin of the Grenville Province at its eastern extremity, intruding mainly gneisses of Labradorian age. The Shabogamo Gabbro, although perhaps slightly older ( 25 -35 Ma, see Table 5), intrudes Labradorian and older rocks along the margin of the Grenville Province and displays many similarities to the Michael Gabbro. The emphasis of this study is to better characterize the Michael Gabbro geochemically and isotopically, and to make comparisons with available corresponding data from the Shabogamo Gabbro and other middle Mesoproterozoic basic rocks of the region. The ultimate objective is to draw some conclusions for the region concerning the nature and variability of the underlying subcontinental lithosphere and its petrological, geochemical, and tectonic evolution.

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Middle Mesoproterozoic mafic magmatism in Labrador Mafic magmatic activity in Labrador during the middle Mesoproterozoic is chiefly associated with one of two types of continental igneous assemblages. The most widespread includes diabase dyke swarms and sheets, flood basalts, and mafic sills. Less broadly distributed, but nevertheless involving substantial magma volumes, are the anorthosite mangerite -charnockite -granite complexes that span an interval from early to late Mesoproterozoic. Table 1 summarizes the ages and briefly describes the principal representa-

tives of middle Mesoproterozoic mafic magmatism in Labrador; their distribution is shown in Fig. 1 . The Harp Lake complex and the Michikamau intrusion, lying north of the GrenvilIe Front, are temporally close to the Michael Gabbro and Shabogamo Gabbro in the Grenville Province. Olivine-bearing basic rocks dominate in both of the complexes, which is also suggestive of possible genetic links. The younger Nain Plutonic Suite has no prominent readily correlative contemporary mafic activity in the Grenville Province, but the Seal Lake Group lying at the margin of the Grenville Province, although slightly younger, may be related. From Table 1, it is apparent that at least two distinct episodes of magmatic activity are recognizable, at about 1.46- 1.43 Ga and 1.35 - 1.27 Ga; other activity about 1.25 - 1.23 Ga is less well defined. The 1.46- 1.43 Ga mafic magmatism had broad distribution north and south of the present Grenville Front. It is possible that the event was related to widespread basaltic underplating and lower crustal intrusion at that time, as inferred for the development of some anorthosite - mangerite - charnockite -granite complexes (e.g., Emslie and Stirling 1993; Emslie et al. 1994). The emphasis of this paper is on rocks of basaltic composition, so beyond pointing out the spatial and temporal ties to anorthosite -mangerite -charnockite -granite complexes in Labrador, the latter rocks will not be considered further. Mesoproterozoic mafic magmatism is widely distributed elsewhere in North America (exclusive of anorthositemangerite -charnockite -granite complexes). The Sudbury dykes, the Keweenawan Mid-Continent Rift, the Coppermine River basalts - Muskox intrusion - Mackenzie diabase dykes are prominent examples. In northern Europe, the Jotnian diabases of Finland and Sweden are similar in age to the Mackenzie dykes, the Harp dykes, and the Nain lowphosphorous (LP) dykes. Appendix Table A 1 summarizes chemical, age, and isotopic data for a selected range of continental mafic rocks with comparable ages and tectonic settings.

Michael Gabbro (and Shabogamo Gabbro) Field occurrence and petrography The Michael Gabbro outcrops from the northern edge of the Grenville Province to well south of Groswater Bay (Fig. 2). The rocks vary from aphanitic and fine grained in chilled margins and small dykes to medium and coarse grained in the interiors of thick dykes and sheets. Small plagioclase phenocrysts occur rarely, and microphenocrysts of olivine are even less common. The rocks exhibit a marked increase in metamorphic grade, from essentially unmetamorphosed assemblages to granulite-grade rocks from north to south. The Michael Gabbro has been described most recently by Emslie ( I 983), Gower (1986), and Gower and Erdmer (1988). The least metamorphosed assemblages contain olivine, plagioclase, and augite in subophitic textures; red-brown biotite, apatite, and opaque oxides are typical accessory minerals. Olivine composition ranges from FoT6 to Foql, primary plagioclase is mostly in the range Anqoto An6o, and metamorphic plagioclase is An2o to Anqo. Narrow double coronas on olivine are present at even the lowest grade samples in the north; these consist of an inner, fine prismatic hypersthene zone and an outer rim of pargasitic hornblende d 1997 NRC Canada

Can. J. Earth Sci. Vol. 34, 1997

Table 1. Principal middle Mesoproterozoic mafic magmatism in Labrador. Unit

Age (Gal

Occurrence

Referenceu

Comments


Grenville Province Mealy dykes

H .25

NE-trending dyke swarm

Michael Gabbro

1.43

Sheets, lenses, and sills

Skabogarno Gabbro

1.46

Sheets, lenses, and sills

Plagioclase An,,-,, zoned to An,,, - ~ ~ ;olivine olivine F o ~ ~ Fe-rich tholeiite, intrudes Mealy Mtns. anorthosites Plagioclase An4,-, olivine Fo4,-76; olivine tholeiite, intrudes mainly Labradorian gneisses Plagioclase , olivine Fo,-,~; olivine tholeiite, intrudes Archean, Early Proterozoic and Labradorian rocks north and south of the Grenville Front

Churchill-Ndn provinces Seal Lake Group

1.25 - 1.24

Continental flood basalts and associated dolerite sills

Harp dykes

1.27

NE-trending dyke swarm

Nain LP dykes

H .28

E -W-trending dyke swarm

Nascaupi dolerite sills: plagimlase An,,-,,, olivine F O , , - ~ ~ olivine ; thsleiitic to transitional; age refers to dolerite sills and is a minimum for the lavas Plagioclase An oIivine Fo4 dykes intrude anorthosites of the Harp Lake complex , Olivine F o ~ ~ dykes - ~ ~intrude ; the Nain Plutonic Suite Entire complex underlies about 20000 km2; least fractionated rwks are troctolitic Complex underlies about 1 0 0 0 km2; least fractionated rocks are troctolitic Underlies about 2500 km2; least fractionated rocks are troctolitic

,+,,

Nain Plutonic Suite

1.34- 1.29

AMCG complex with associated troctslite, norite, and olivine gabbro

Harp Lake complex

1.45

Michikamau intrusion

1.46

AMCG complex with associated troctolite and norite AMCG complex with associated troctolite and orite

,-,,;

5

6

7 , 8, 9

10 10

N o h : Mineral data: Baragar (1981), Emslie (1983), Emslie et al. (1984), Meyers and Emslie (1977), Rivers and Mengel (1988), Wiebe (1985). AMCS, anorthosite -mangerite-charnockite -granite. "Age references: 1, Hamilton and Ernslie (1997); 2, Schiirer et al. (1986); 3, Connelly and Heaman (1993); 4, Rorner et 91. (1995); 5, Cadman et a]. (1993); 6, Carlson et al. (1993); 7, ErnsIie and Loveridge (1992); 8, Hamilton et aB. (1994); 9, Connelly and Ryan (1994); 10, Krsgh and Davis (1973).

with probable very fine spinel. Toward the south the coronas become thicker and more prominent, and clinopyroxene can be observed intergrown with hornblende in the outer coronas. Garnet forms a partial outer corona rim in some assemblages and also occurs sporadically, rimming opaque oxides. Even though relict subophitic texture can locally be discerned in southemost samples, olivine disappears, having been entirely reacted out to form hypersthene and clinopyroxene; coarse garnet becomes prominent and thin corundum platelets appear in plagioclase. Corona structures are largely obliterated by extensive recrystallization at the highest grades. Farther to the west, the Shabogamo Gabbro also occupies the northeastern marginal part of the Grenville Province and adjoining Churchill Province, similarly displaying increasing metamorphic grades from north to south. Unlike the Michael Gabbro, the Shabogmo Gabbro outcrops on both sides of the Grenville Front, being relatively pristine in outcrops north of the front. The rocks have been described by Brooks et al. (1981), and a recent study of the metamorphic assem-

blages by Indares (1993) indicated maximum metamorphic pressures of 16 kbar (1 kbar = 18Q MPa) at 700- 800°C.

Sample set and analytical methods From a total of 70 samples of Michael Gabbro, for which data summarizing major and trace elements were reported by Gower et al. (1990), 13 were chosen from 12 localities (Fig. 2) to investigate isotopicalHy in more detail. These spanned the range of rare earth element (REE) concentrations and Mg#s described by the authors. It was assumed, because of the remarkably regular and consistent behaviour of the REEs, that samples exhibiting the entire REE concentration range would also reflect fractionation of the melts in other respects. Samples C 0 8 1-6343 and CG84- 172E, from the same locality, provide an indication of within-site data variability; the former was analyzed for REE, and the crystallization age of the latter was 1426 f 6 Ma (U-Pb, zircon, Scharer et al. 1986). Major elements and Ba, Sr, Cr, Cu, Zn, Ni, Mo, and Pb were obtained by atomic absorption spectrophotometry at the 8 1997 NRC Canada

Emslie et al.


Fig. 1. Map showing distribution of diabasis intrusions and other Mesoprsterozoic igneous assemblages, central Labrador.

Fig. 2. Detailed map of the distribution of Michael Gabbro.

Newfoundland Department of Natural Resources, as was FeZf and loss-on-ignition, Other trace elements were determined by X-ray fluorescence (XRF) spectroscoy at Memorial University of Newfoundland. Analytical procedures are summarized by Wagenbauer et al. (19831, and precision and accuracy are discussed by Hayes (1994). REE analyses were obtained by inductively coupled plasma - emission spectroscopy (ICP-ES) methods (Gower et al. 1998).

Isotopic compositions and Wb, Sr, Sm, and Nd concentrations for all samples were made on single 200 mg aliquots of powder, after spiking with mixed g7R$-@Sr and I4%m 14$Nd tracers and dissolution in an HF-HNQ3 mixture. Following conventional cation-exchange chromatography, Sm and Nd were further separated using a modified version of the HDEHP-Teflonm powder method described by Richard et d. (1976). Procedural blanks were < '100pg for O 1997 NRC Canada

Can. J. Earth Sci. Vof. 34, 1997 Table 2. Major and trace element analyses of selected Michael Gabbro samples.

CG81627B

CG81634

CG82023

CG79677

CG79658

CG80580

CG80561

CG82035

186 158 87 16

94 69 208 33

128 213 117 23

136 71 121 22

100 213 140 23

CG83234

CG83246

CG83437

CG83462

CG84172~

Si02 TiOz A1203


Fe203

FeO MnO MgO CaO Na20 K2O B2°~

LO1 Total

Cr Ni V Cu Pb Zn Mo NbINb* Ba/Ba* SrISr* K Wb

Ba Sr Ga Li Nb Zr Ti Y

Th U La Ce Pr Nd S~I ELI Gd DY

HQ Er Yb Lu

F

56 48 168 30

100 94 298 48

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111 4 0.22 2.25 0.47 10128 30 69 1 357 18 17 12 192 15 707 40 1 0.2 25.30 61.17 7.38 33.84 6.86 2.20 7.15 6.15 1.30 3.64 3.23 0.50 441

158 4 0.26 2.93 0.42 7305 19 606 279 20 11 12 178 18 465 38 2 0.3 23.64 54.32 6.58 31.17 6.46 2.07 6.74 6.01 1.25 3.45 3.14 0.47 568

39 34 252 31

-

146 4 0.19 1.08 0.55 11041 35 376 337 20 10 12 215 14 808 41 13 0.8 30.29 66.62 8.03 36.04 7.51 2.19 7.59 6.64 1.43 3.87 3.43 0.53 530

88 3 0.28 2.46 1.40 3819 7 226 379 18 5 6 74 5755 13 -

0.2 10.28 22.57 2.79 12.39 2.37 1.03 2.42 2.69 0.45 1.25 1.11 0.17 156

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106 5 0.27 1.19 0.91 5230 14 1?8 363 20 8 8 111 11 990 21 2 0.3 14.50 33.56 4.09 19.10 3.82 1.45 3.92 3.54 0.73 2.84 1.85 0.28 289

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72 5 0.70 2.31 1.52 4732 11 293 464 22 8 9 83 7973 17

102 3 0.31 1.98 0.94 4317 9 217 333 15 6 8 101 7614 17

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0.2 11.91 26.61 3.28 14.87 2.86 1.23 2.92 2.54 0.54 1.50 1.31 0.19 235

0.2 12.43 28.22 3.41 15.90 3.13 1.19 3.22 2.82 0.60 1.67 1.54 0.24 215

-

0.3 13.21 30.44 3.75 17.14 3.31 1.25 3.37 2.99 0.63 1.64 1.58 0.24 174

110 4 0.40 2.28 1.04 4068 10 255 328 16 8 7 98 8813 17

61 101 145 27 1 107 6 0.64 2.97 0.78 4815 13 413 409 19 5 11 134 10 911 29 5 0.2 20.34 45.15 5.50 24.47 4.75 1.68 4.90 4.15 0.87 2.33 2.15 0.34 304

116 75 89 16 1 73 5 0.32 2.53 1.47 3985 11 294 449 23 7 4 76 6774 12 2 0.2 12.65 27.41 3.32 14.92 2.76 1.23 2.89 2.42 0.51 1.48 1.25 0.20 220

72 138 188 31 1 127 5 0.40 1.63 0.68 5894 22 331 305 21 15 8 162 9652 36 7 0.8 22.70 50.09 6.19 27.21 5.61 1.82 5.90 5.22 1.11 3.08 2.79 0.43 356

74 68 2 15 41 1 146 4 0.46 2.87 0.41 7388 25 692 348 28 14 13 225 16 846 47 2 0.5 33.83 73.47 9.09 41.12 8.03 2.46 7.98 6.85 1.46 4.03 3.57 0.55 759

73 49 202 43 -

154 5 0.62 2.94 0.36 9214 18 670 325 26 13 14 247 21 042 58 -

03 18.00 72.00 -

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785

Notes: Major elements in weight percent; trace elements in parts per million.

Nd and